optimization of wwtp excellent)

Wastewater Treatment Plant Optimization — November 2003 1

Wastewater Treatment PlantOptimization

This document is the fifth in a series of bestpractices that deal with buried linear infrastructureas well as end of pipe treatment and managementissues. For titles of other best practices in this andother series, please refer to www.infraguide.ca.

5

WA

STEW

ATE

R T

REA

TMEN

T PL

AN

T O

PTIM

IZA

TIO

N

Stor

m a

nd W

aste

wat

er

National Guide toSustainable Municipal

Infrastructure

2 Wastewater Treatment Plant Optimization — November 2003

Wastewater Treatment Plant Optimization

Issue No. 1.0

Publication Date: November 2003

© 2003 Federation of Canadian Municipalities and National Research Council

ISBN 1–897094–32–9

The contents of this publication are presented in good faith and are intended as general

guidance on matters of interest only. The publisher, the authors and the organizations to

which the authors belong make no representations or warranties, either express or implied,

as to the completeness or accuracy of the contents. All information is presented on the

condition that the persons receiving it will make their own determinations as to the

suitability of using the information for their own purposes and on the understanding that the

information is not a substitute for specific technical or professional advice or services. In no

event will the publisher, the authors or the organizations to which the authors belong, be

responsible or liable for damages of any nature or kind whatsoever resulting from the use

of, or reliance on, the contents of this publication.

Why Canada Needs InfraGuide

Canadian municipalities spend $12 to $15 billion

annually on infrastructure but it never seems to be

enough. Existing infrastructure is ageing while demand

grows for more and better roads, and improved water

and sewer systems responding both to higher

standards of safety, health and environmental

protection as well as population growth. The solution

is to change the way we plan,

design and manage

infrastructure. Only by doing

so can municipalities meet

new demands within a

fiscally responsible and

environmentally sustainable framework, while

preserving our quality of life.

This is what the National Guide to Sustainable

Municipal Infrastructure (InfraGuide) seeks to

accomplish.

In 2001, the federal government, through its

Infrastructure Canada Program (IC) and the National

Research Council (NRC), joined forces with the

Federation of Canadian Municipalities (FCM) to create

the National Guide to Sustainable Municipal

Infrastructure (InfraGuide). InfraGuide is both a new,

national network of people and a growing collection of

published best practice documents for use by decision

makers and technical personnel in the public and

private sectors. Based on Canadian experience and

research, the reports set out the best practices to

support sustainable municipal infrastructure decisions

and actions in six key areas: municipal roads and

sidewalks, potable water, storm and wastewater,

decision making and investment planning,

environmental protocols, and transit. The best

practices are available on-line and in hard copy.

A Knowledge Network of Excellence

InfraGuide´s creation is made possible through

$12.5 million from Infrastructure Canada, in-kind

contributions from various facets of the industry,

technical resources, the collaborative effort of

municipal practitioners, researchers and other

experts, and a host of volunteers throughout the

country. By gathering and synthesizing the best

Canadian experience and

knowledge, InfraGuide

helps municipalities get the

maximum return on every

dollar they spend on

infrastructure—while

being mindful of the social and environmental

implications of their decisions.

Volunteer technical committees and working

groups—with the assistance of consultants and other

stakeholders—are responsible for the research and

publication of the best practices. This is a system of

shared knowledge, shared responsibility and shared

benefits. We urge you to become a part of the

InfraGuide Network of Excellence. Whether you are

a municipal plant operator, a planner or a municipal

councillor, your input is critical to the quality of

our work.

Please join us.

Contact InfraGuide toll-free at 11--886666--333300--33335500 or visit

our Web site at www.infraguide.ca for more

information. We look forward to working with you.

Introduction

InfraGuide –

Innovations and

Best Practices


INTRODUCTION

InfraGuide – Innovations and Best Practices


The InfraGuide Best Practices Focus

TransitUrbanization places pressure on an eroding,ageing infrastructure, and raises concerns aboutdeclining air and water quality. Transit systemscontribute to reducing traffic gridlock andimproving road safety. Transit best practicesaddress the need to improve supply, influencedemand and make operational improvementswith the least environmental impact, whilemeeting social and business needs.

Storm and WastewaterAgeing buried infrastructure, diminishing financial resources, stricterlegislation for effluents, increasing public awareness of environmentalimpacts due to wastewater and contaminated stormwater are challengesthat municipalities have to deal with. Events such as water contaminationin Walkerton and North Battleford, as well as the recent CEPAclassification of ammonia, road salt and chlorinated organics as toxicsubstances, have raised the bar for municipalities. Storm and wastewaterbest practices deal with buried linear infrastructure as well as end of pipetreatment and management issues. Examples include ways to control andreduce inflow and infiltration; how to secure relevant and consistent datasets; how to inspect and assess condition and performance of collectionssystems; treatment plant optimization; and management of biosolids.

Decision Making and InvestmentPlanning Elected officials and senior municipaladministrators need a framework for articulating the value of infrastructure planningand maintenance, while balancing social,environmental and economic factors. Decision-making and investment planning best practicestransform complex and technical material intonon-technical principles and guidelines fordecision making, and facilitate the realizationof adequate funding over the life cycle of theinfrastructure. Examples include protocols fordetermining costs and benefits associated with desired levels of service; and strategicbenchmarks, indicators or reference points for investment policy and planning decisions.

Potable WaterPotable water best practices address variousapproaches to enhance a municipality’s or waterutility’s ability to manage drinking water deliveryin a way that ensures public health and safety atbest value and on a sustainable basis. Issues suchas water accountability, water use and loss,deterioration and inspection of distributionsystems, renewal planning and technologies forrehabilitation of potable water systems and waterquality in the distribution systems are examined.

Municipal Roads and SidewalksSound decision making and preventive maintenance are essential to managingmunicipal pavement infrastructure cost effectively. Municipal roads andsidewalks best practices address two priorities: front-end planning and decisionmaking to identify and manage pavement infrastructures as a component of theinfrastructure system; and a preventive approach to slow the deterioration ofexisting roadways. Example topics include timely preventative maintenance ofmunicipal roads; construction and rehabilitation of utility boxes; and progressiveimprovement of asphalt and concrete pavement repair practices.

Environmental Protocols Environmental protocols focus on the interactionof natural systems and their effects on humanquality of life in relation to municipalinfrastructure delivery. Environmental elementsand systems include land (including flora), water,air (including noise and light) and soil. Examplepractices include how to factor in environmentalconsiderations in establishing the desired levelof municipal infrastructure service; anddefinition of local environmental conditions,challenges and opportunities with respect tomunicipal infrastructure.

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7

Executive Summary . . . . . . . . . . . . . . . . . . . . . . 9

1. General . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .11

1.2 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

1.3 Health and Safety . . . . . . . . . . . . . . . . . . . . . .11

1.4 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . .15

2.2 Expected Benefits of WWTP Optimization . . . . . . . . . . . . . . . . . . . .15

2.2.1 Improved Plant Performance,Reliability, Flexibility, and Efficiency . . . . . . . . . . . . . . . . . . .15

2.2.2 Reduced Capital Costs of Expansion/Upgrading . . . . . . . . . . . . .16

2.2.3 Reduced Operating Costs . . . . . . . . .16

2.2.4 Improved Operating Practices . . . . .16

3. Work Description . . . . . . . . . . . . . . . . . . . . 17

3.1 Elements of a WWTP Optimization Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

3.2 Establish Objectives . . . . . . . . . . . . . . . . . . . .19

3.3 Plant Evaluation Tools . . . . . . . . . . . . . . . . . .19

3.3.1 Self-Assessment Report . . . . . . . . . .20

3.3.2 Historical Data Review . . . . . . . . . . .20

3.3.3 Unit Process Capacity Chart . . . . . . .21

3.3.4 Sludge Accountability Analysis . . . .22

3.3.5 Benchmarking Operating Costs and Staffing . . . . . . . . . . . . . . . . . . . . .22

3.3.6 Flow Meter Assessment . . . . . . . . . .22

3.3.7 Continuous Monitoring . . . . . . . . . . .23

3.3.8 Off-Line Monitoring . . . . . . . . . . . . . .24

3.4 Process Analysis Tools . . . . . . . . . . . . . . . . .24

3.4.1 Aeration System Capacity and Efficiency Analysis . . . . . . . . . . . . . . .24

3.4.2 Hydraulic Modelling . . . . . . . . . . . . . .25

3.4.3 Analysis of Recycle Streams . . . . . .25

3.4.4 Stress Testing . . . . . . . . . . . . . . . . . . .25

3.4.5 Clarifier Hydraulic Tests . . . . . . . . . .27

3.4.6 Other Clarifier Diagnostics Tests . . .27

3.4.7 Mixing Tests . . . . . . . . . . . . . . . . . . . . .27

3.4.8 Process Modelling and Simulation .28

3.5 Optimization Approaches . . . . . . . . . . . . . . .28

3.5.1 Improved Operations and Maintenance . . . . . . . . . . . . . . . . . . . .28

3.5.2 Instrumentation, Control and Automation . . . . . . . . . . . . . . . . . . . . . .29

3.5.3 Treatment Process Modifications . .31

3.5.4 Achieving Resource Cost Savings . .32

3.6 Document Benefits . . . . . . . . . . . . . . . . . . . .34

3.7 Optimization Task Flow Sheet . . . . . . . . . . .34

4. Applications and Limitations . . . . . . . . . . 35

4.1 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . .35

4.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

Appendix A: Selected Case Histories. . . . . . 37

Appendix B: Optimization OpportunitiesThrough Process Modifications . . . . . . . . . . 43

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

TABLESTable 3–1: Performance limiting factors at a WWTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Table 3–2: Examples of historical data review impacts on subsequent optimization tasks . . . . 20

Table 3–3: On-Line process variables . . . . . . . . . 23

Table 3–4: Summary of typical unit process design parameters and evaluation criteria . . . . 26

Table 3–5: Automation applications at WWTPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Table 3–6: Potential treatment process optimization approaches . . . . . . . . . . . . . . . . . . . . 31

FIGURESFigure 3–1: Elements of WWTP optimization. . . 17

Figure 3–2: Example of a process capacity chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 3–3: Representation of optimization task flow sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Table of Contents


TABLE OF CONTENTS

The dedication of individuals who volunteered their

time and expertise in the interest of the National Guideto Sustainable Municipal Infrastructure (InfraGuide) isacknowledged and much appreciated.

This umbrella best practice for wastewater treatment

plant optimization was developed by stakeholders

from Canadian municipalities and specialists from

across Canada based on information from a scan of

municipal practices and an extensive literature

review. The following members of InfraGuide’s Storm

and Wastewater Technical Committee provided

guidance and direction in the development of this

document. They were assisted by the Guide

Directorate staff and by XCG Consultants Ltd.

John Hodgson, ChairCity of Edmonton, Alberta

André AubinCity of Montréal, Quebec

Richard BoninCity of Québec, Quebec

David CalamCity of Regina, Saskatchewan

Kulvinder DhillonProvince of Nova Scotia, Halifax, Nova Scotia

Tom FieldDelcan Corporation, Vancouver, British Columbia

Wayne GreenCity of Toronto, Ontario

Claude OuimetteOMI Canada Inc., Fort Saskatchewan, Alberta

Peter SetoNational Water Research Institute, Environment Canada, Burlington, Ontario

Timothy A. TooleTown of Midland, Ontario

Bilgin BuberogluTechnical Advisor, NRC

In addition, the Storm and Wastewater Technical

Committee would like to thank the following

individuals and institution for their participation in

working groups, peer reviews, and support.

Susheel K. AroraMunicipality of the County of Colchester, Nova Scotia

Vince CorkeryCity of Edmonton, Alberta

Paul DoCity of Calgary, Alberta

Graeme FarisRegional District of Comox-Strathcona, British Columbia

André MarsanCentre d’Épuration Rive-Sud de Longueuil, Quebec

Gaétan MorinRoche Ltée, Groupe-Conseil, Quebec

Mark RupkeCity of Toronto, Ontario

Peter SetoNational Water Research Institute, Environment Canada, Burlington, Ontario

James Arnott Ontario Ministry of the Environment, Hull, Quebec

Tony HoMinistry of Environment, Toronto, Ontario

Debbie MaceyCanadian Association for Environmental AnalyticalLaboratories (CAEAL), Ottawa, Ontario

Vince Pileggi Ontario Ministry of the Environment, Toronto, Ontario

Serge ThériaultDepartment of Environment and Local Government,New Brunswick

A. Warren WilsonWPC Solutions Inc., Calgary, Alberta

Great Lakes Sustainability Fund, Environment Canada

Acknowledgements


ACKNOWLEDGEMENTS


This and other best practices could not have been

developed without the leadership and guidance of

the Project Steering Committee and the Technical

Steering Committee of the National Guide toSustainable Municipal Infrastructure (InfraGuide),whose memberships are as follows:

Project Steering Committee:Mike Badham, ChairCity Councillor, Regina, Saskatchewan

Stuart BriesePortage la Prairie, Manitoba

Bill CrowtherCity of Toronto, Ontario

Jim D’OrazioGreater Toronto Sewer and Watermain Contractors Association, Ontario

Derm FlynnMayor, Appleton, Newfoundland and Labrador

David GeneralCambridge Bay, Nunavut

Ralph HaasUniversity of Waterloo, Ontario

Barb HarrisWhitehorse, Yukon

Robert HiltonOffice of Infrastructure, Ottawa, Ontario

Joan LougheedCity Councillor, Burlington, OntarioStakeholder Liaison Representative

Saeed MirzaMcGill University, Montréal, Quebec

René MorencyRégie des installations olympiquesMontréal, Quebec

Lee NaussCity Councillor, Lunenburg, Nova Scotia

Ric RobertshawRegion of Halton, Ontario

Dave RudbergCity of Vancouver, British Columbia

Van SimonsonCity of Saskatoon, Saskatchewan

Basile StewartMayor, Summerside, Prince Edward Island

Serge ThériaultDepartment of Environment and Local Government,New Brunswick

Alec WatersAlberta Transportation, Edmonton, Alberta

Wally WellsDillon Consulting Ltd., Toronto, Ontario

Technical Steering Committee:Don BrynildsenCity of Vancouver, British Columbia

Al CepasCity of Edmonton, Alberta

Andrew CowanCity of Winnipeg, Manitoba

Tim DennisCity of Toronto, Ontario

Kulvinder DhillonProvince of Nova Scotia, Halifax, Nova Scotia

Wayne GreenCity of Toronto, Ontario

John HodgsonCity of Edmonton, Alberta

Bob LorimerLorimer & Associates, Whitehorse, Yukon

Betty Matthews-MaloneHaldimand County, Ontario

Umendra MitalCity of Surrey, British Columbia

Anne-Marie ParentCouncillor, City of Montréal, Quebec

Piero SalvoWSA Trenchless Consultants Inc., Ontario

Mike SheflinFormer CAO, Regional Municipality of Ottawa-Carleton, Ontario

Konrad SiuCity of Edmonton, Alberta

Carl YatesHalifax Regional Water Commission, Nova Scotia

Founding Member:Canadian Public Works Association (CPWA)

Acknowledgements

Wastewater treatment plants (WWTPs) aretypically designed to conservative designguidelines and are operated based on historicpractices. Generally, experience has shownthat such facilities often have considerableadditional capacity that can be realizedthrough optimization. Improvements in effluentquality and reductions in operating costs canalso be realized. This best practice providesan overview of the approach that should betaken to optimize an existing WWTP. It alsodescribes a set of tools that can be used toachieve the specific objectives of anoptimization program. By applying this bestpractice, the capacity of the existinginfrastructure can be maximized, theperformance of the works enhanced, and theoperating and maintenance costs reduced.

WWTP optimization should become anoperating philosophy for the municipality thatis championed by management, supported bycouncil and staff at all levels, and has theoverall objective of continuous improvement.The best practice for WWTP optimizationincludes the following elements.

■ Establish the objectives of optimization.

■ Evaluate the WWTP to establish orbenchmark conditions, prioritizeopportunities for optimization, anddetermine performance or capacity limitingfactors.

■ Identify and implement operational orprocess changes to address performanceor capacity limiting factors.

■ Conduct follow-up monitoring to documentthe benefits.

A WWTP optimization program is iterative, andclear objectives should be established beforeeach iteration. Depending on the objectivesestablished, the outcome of WWTP optimizationmay include any or all of the following:

■ an increase in the capacity of the existingworks without the major capital costsassociated with a plant expansion;

■ an improvement in process without themajor capital costs associated with a plantupgrade; and

■ a reduction in operating costs through moreefficient use of power, chemicals, or labour.

This best practice provides WWTP ownersand operators with a description of some ofthe state-of-the-art tools available to evaluateand optimize their WWTP and the individualunit processes that comprise it, such as:

■ oxygen transfer testing;

■ hydraulic modelling;

■ clarifier hydraulic testing;

■ stress testing; and

■ process modelling and simulation.

Available tools to optimize through improvedoperations and maintenance practices;instrumentation, control, and automation; andprocess modifications are described in thedocument, along with opportunities to achieveresource cost savings.

A key element of the WWTP optimization bestpractice that is often ignored is the follow-upmonitoring needed to document the level ofsuccess achieved. Communication of thebenefits of the optimization program isessential to build support for future initiatives.This support is the key to ensuring the iterativeprocess of optimization is sustained and anenvironment conducive to optimization isfostered within the municipality.

As a guide to conducting WWTP optimization,a step-wise approach is illustrated thatsuggests the type of testing that could be doneto meet various optimization objectives.

Executive Summary


EXECUTIVE SUMMARY

1.1 Introduction

Wastewater treatment plants (WWTPs) havetraditionally been designed to conservativedesign guidelines and standards that weredeveloped based on historic design practices.Procedures are often passed from operator to operator without consideration for newapproaches that might improve performanceor reduce costs. Generally, experience hasshown that WWTPs often have considerableadditional capacity beyond the rated capacitythat was assigned at design. Furthermore,improvements in performance and reductionsin operating costs can often be achievedthrough optimization approaches.

This best practice provides an overview ofan iterative approach to optimization of anexisting WWTP that will allow theowner/operator to maximize the capacity ofthe existing infrastructure, enhance theperformance of the facility, and reduce theoperational costs.

1.2 Scope

This best practice has been developed by theNational Guide to Sustainable MunicipalInfrastructure: Innovation and Best Practices.It is one of more than 50 aspects identified bythe Guide’s Storm and Wastewater TechnicalCommittee relating to linear infrastructure,wastewater treatment, customer interaction,and receiving water issues.

This best practice applies to the optimizationof municipal wastewater treatment plants.WWTP optimization is considered to be a step-wise process that results in the maximum useof the existing infrastructure at a competitiveoperating cost consistent with principles ofsustainability. Depending on the objectives ofthe optimization program, the outcomes mayinclude any or all of the following:

■ increasing the capacity of the existingworks without the major capital costsassociated with a plant expansion;

■ improving process performance without themajor capital costs associated with a plantupgrade; and

■ reducing operating costs through moreefficient use of power, chemicals, or labour.

This best practice covers the processing ofthe most common liquid and sludge treatmentprocesses that typically comprise a WWTP.The liquid treatment processes includepreliminary, primary, secondary, and tertiarytreatment, and the disinfection of the treatedeffluent. The sludge treatment processesinclude thickening, dewatering and digestion(aerobic and anaerobic). Management of thebiosolids stream produced by the WWTP is not addressed in this best practice. A bestpractice for biosolids management has beendeveloped by the National Guide toSustainable Municipal Infrastructure:Innovation and Best Practices. The reader isreferred to that best practice for informationspecific to biosolids management.

1.3 Health and Safety

Some of the test procedures described in thisbest practice involve using hazardouschemicals or working in hazardous areas of aWWTP around electrical and mechanicalequipment. Appropriate safety measuresshould be taken before undertaking any of thetesting described, including reference tomanufacturer’s safety data sheets (MSDSs) onchemicals that might be used during testingand adherence to occupational health andsafety standards.

1. General

1.1 Introduction

1.2 Scope

1.3 Health and Safety


1. General


1.4 Glossary

Biochemical oxygen demand (BOD) —The quantity of oxygen consumed, usuallyexpressed in mg/L, during the biochemicaloxidation of organic matter over a specifiedtime period (i.e., five day BOD or BOD5) at atemperature of 20ºC.

Biological nutrient removal (BNR) —Processes that remove nitrogen and/orphosphorus by biological rather than chemicalor physical means.

Chemical oxygen demand (COD) — Thequantity of oxygen required in the chemicaloxidation of organic matter under standardlaboratory procedures, expressed in mg/L.

Dissolved oxygen (DO) — The concentrationof oxygen dissolved in water usuallyexpressed in mg/L. Dissolved oxygen isimportant for aerobic (“with air”) biologicaltreatment. An adequate DO concentration in a wastewater effluent is important for theaquatic life in the receiving stream or river.

Endogenous Oxygen Demand — Oxygendemand for the basic respiration of the micro-organisms, independent from the currentwastewater loading.

Food-to-micro-organism ratio (F/M) —The ratio of the influent mass loading (usuallyexpressed in kg/d) of BOD or COD to the massof volatile suspended solids concentration in awastewater treatment aeration tank. The unitsof F/M are typically d-1.

Hydraulic retention time (HRT) — A measureof the length of time a volume of liquid isretained in a tank or vessel, calculated bydividing the tank or vessel volume (L) by theliquid flowrate (L/d) and is presented in eitherdays or hours.

Inflow and infiltration (I/I) — Inflow is waterentering the sanitary sewer during wetweather events from such sources as roofleaders, foundation drains, manhole covers orstorm sewer interconnections. Infiltration is

water entering the sanitary sewer system fromthe ground through defective pipes, pipejoints, connections, or manhole walls.

Mixed liquor suspended solids (MLSS) —The concentration of dry solids in mg/L ofmixed liquor biomass in the aeration tank of a suspended growth (activated sludge orextended aeration) WWTP.

Oxidation-reduction potential (ORP) —A measure of the net potential of all oxidantsand reducing agents in a solution usuallyexpressed in mvolts.

Return activated sludge (RAS) — That portionof the activated sludge separated from themixed liquor in the secondary settlementtanks, which is returned to the aeration tanks.

Sequencing batch reactors (SBR) —A treatment process characterized by theinterruption of flow to the reactor during thesedimentation and decanting phase oftreatment.

Sludge loading rate (SLR) — The mass loadingrate in kg/d of mixed liquor suspended solids(MLSS) per unit area of the secondary clarifier.It is typically expressed as kg/m2.d

Sludge volume index (SVI) — A measure ofthe settling characteristics of biomass definedas the volume in mL occupied by 1 g of settledsludge after settling for 30 minutes in a settlingcolumn, typically a 1 litre graduated cylinder.SVI is usually expressed in mL/g.

Solids retention time (SRT) — A measure ofthe theoretical length of time the averageparticle of mixed liquor suspended solids hasbeen retained in the biological reactor sectionof the treatment plant. It is usually presentedin days, and is also referred as mean cellresidence time (MCRT) or sludge age.

Specific Oxygen Uptake Rate (SOUR) —Also known as the oxygen consumption orrespiration rate, is defined as the milligram of oxygen consumed per gram of volatilesuspended solids (VSS) per hour.

1. General

1.4 Glossary

Step-feed aeration — A modification ofconventional plug-flow process in which thesettled wastewater is introduced at severalpoints in the aeration tank to equalize F/Mratio, thus lowering peak oxygen demand.

Stirred Sludge Volume Index (SSVI) —A measure to determine the settling propertiesof an activated sludge. It is expressed in mL/g.

Supervisory control and data acquisition(SCADA) — A computer-monitored sensing,alarm, response, control, and data acquisitionsystem used in WWTPs to monitor theiroperations.

Total Kjeldahl nitrogen (TKN) — The sum ofthe organic and ammonia nitrogen in a watersample usually expressed in mg/L.

Total Phosphorus (TP) — Total amount ofphosphorus present in the wastewater (orwater) either in soluble or insoluble forms, inorganic and inorganic (orthophosphates,metaphosphates or polyphosphate)compounds, expressed in mg/L.

Total suspended solids (TSS) — Solidspresent in a water sample that are retainedon the filter paper after filtering the sample,usually expressed in mg/L.

Volatile Suspended Solids (VSS) — The amountof total suspended solids burned off at 550 ± 50°Cexpressed normally as mg/L. It indicates thebiomass content of the mixed liquor.

Waste activated sludge (WAS) — The excessportion of the activated sludge separated fromthe biological treatment process.

1. General

1.4 Glossary


2.1 Background

In the 1980s and early 1990s, WWTPoptimization first gained recognition as a cost-effective way to achieve improved performance,reduce costs, and maximize the use of existinginfrastructure. Early efforts at optimization in theUnited States were initiated by the recognitionthat considerable capital dollars had been spenton new facilities, but these facilities were notperforming to expectations. The CompositeCorrection Program (CCP) was developed toidentify the major causes of poor performancein these plants (Water Pollution ControlFederation, 1985).

Rising energy prices in the 1980s led to a focuson energy conservation in WWTPs throughoptimization techniques. The process audit wasdeveloped based on work undertaken at theTillsonburg, Ontario WWTP, primarily as a meansto reduce process energy use at these facilities(Speirs and Stephenson, 1985). Experience withthe tool showed it could also be applied toevaluate plant capacity and identifyopportunities to obtain additional capacity in anexisting works at lower capital costs.

Case histories showing substantial capital andoperating cost savings as a result ofoptimization of WWTPs began to appear in thetechnical literature. Guidance manuals wereprepared describing the benefits of, andavailable approaches for, WWTP optimization(WEAO, 1996). By the mid-1990s, WWTPoptimization had become a well-establishedpractice. In some jurisdictions, optimization ofthe existing works became a prerequisite forobtaining grants for plant expansion.

Specific goals of WWTP optimization mayinclude any or all of the following:

■ improved plant performance, reliability,flexibility, and efficiency;

■ reduced capital costs of expansion orupgrading;

■ reduced operating costs associated withenergy use, chemical use, and labour; and

■ improved operating practices.

2.2 Expected Benefits of WWTP Optimization

2.2.1 Improved Plant Performance, Reliability, Flexibility, and Efficiency

Applying this best practice will result inimproved plant performance, and reduce therisk of noncompliance with either effluentquality requirements or biosolids qualityregulations.

The Regional Municipality of Halton, ownerand operator of the Burlington Skyway WWTP,used the Composite Correction Program (CCP)as an optimization tool, together with otheroptimization tools, to improve the performanceof this facility significantly. This was inresponse to a need to achieve enhancedeffluent quality requirements. The CCPapproach is described in Section 3.5 of thisbest practice. Through a comprehensiveperformance evaluation (CPE), non-technicalor management and human resources relatedlimitations are identified as performancelimiting factors. As a result of improvementsachieved during the follow-up comprehensivetechnical assistance (CTA), significantimprovements in plant performance wereachieved, allowing the plant to attain bothphosphorus and ammonia limits notconsidered achievable without major capitalexpenditures. At the same time, substantialsavings in capital costs for future plantexpansion were deferred as a result of theadditional capacity realized at the plant. Thetotal savings in capital costs were estimatedat about $50 million. A more detailed casehistory of the Burlington Skyway WWTPoptimization project accomplishments ispresented in Appendix A (Case History 1).

2. Rationale

2.1 Background

2.2 Expected Benefits of

WWTP Optimization


2. Rationale

In somejurisdictions,optimization of the existing works became a prerequisite forobtaining grants for plantexpansion.


2. Rationale

2.2 Expected Benefits of

WWTP Optimization

2.2.2 Reduced Capital Costs of Expansion/Upgrading

Through WWTP optimization, significantcapital cost savings can be realized bymaximizing the capability and capacity of theexisting infrastructure.

The Regional Municipality of Waterloo, ownerand operator of the Ayr WWTP, was able to re-rate this facility from a nominal rated capacityof 1,181 m3/d to a new rated capacity of 1,500m3/d after applying some of the optimizationtools described in this best practice. A historicdata review and production of a processcapacity chart identified additional availablecapacity in the major unit processescomprising the packaged extended aerationplant. This review also questioned theaccuracy of the plant flow metering. Stresstesting of the secondary clarifiers, oxygentransfer testing, and biological simulationmodelling were used to confirm the findings of the desktop evaluation. As a result, theregulator issued a new certificate of approvalfor the plant for the increased capacity andwith more stringent effluent limits for ammoniaand phosphorus, allowing further developmentin the community. The 27 percent increase incapacity was realized after minor upgrades tothe aeration system, the raw sewage pumpingstation, and the return sludge pumping system.No new aeration or clarification tankage wasrequired to allow the increased capacity. Amore detailed case history of the Ayr WWTPproject is presented in Appendix A (CaseHistory 2). Another example of optimizationleading to reduced capital costs for expansionof the Montréal WWTP is also presented inAppendix A (Case History 4).

2.2.3 Reduced Operating Costs

The operating costs associated with energyuse, chemical use, and labour can be reducedthrough WWTP optimization.

A demonstration of optimized aeration modeoperation was conducted at the TillsonburgWWTP to determine the impact of on-offaeration on energy costs. The plantconfiguration allowed for a direct comparisonof parallel activated sludge aeration basins(also known as bioreactors) operated in the on-off mode and in the conventionalcontinuous aeration mode. Aeration energysavings of between 16 and 26 percent wereachieved at the plant depending on whetherone of the two aeration cells or both aerationcells were cycled. Operation in the on-offmode also resulted in denitrification at the plant, reducing the total nitrogenconcentration in the plant effluent. A moredetailed case history of the on-off aerationdemonstration is presented in Appendix A(Case History 3). Another example ofoptimization resulting in reduced chemicalcosts at the Montréal WWTP is also includedin Appendix A (Case History 4).

2.2.4 Improved Operating Practices

Improved operating practices will result inbenefits in all the areas outlined above.

An enhanced understanding of thefundamentals of sewage treatment processesthrough operator training and appropriateapplication of these concepts to processcontrol will improve plant performance andreliability, and allow operating staff torecognize opportunities to reduce costs.Through the use of techniques likecomprehensive technical assistance (CTA), itis possible to transfer the knowledge and skillsthat will lead to sustained WWTP optimizationand continuous improvement, as illustrated bythe optimization work undertaken at theBurlington Skyway WWTP.

An enhancedunderstanding ofthe fundamentals

of sewagetreatment

processesthrough operator

training andappropriate

application ofthese concepts to

process controlwill improve plantperformance and

reliability.


3.1 Elements of a WWTP Optimization Program

WWTP optimization is not a “one shot” projectconducted by a contractor on behalf of theWWTP owner. Rather, it is an operatingphilosophy that is sustained with the overallobjective of continuous improvement. Someof the tools used to optimize the WWTPdescribed in this best practice can beundertaken by a contractor on behalf of themunicipality, but the overall optimizationprogram must be championed by themunicipality and supported by staff at all levelsof the organization. The elements of the bestpractice for WWTP optimization apply to anysize or type of treatment plant; however, thetools used may vary. Those applied at a smallWWTP may be different than those applied ata larger WWTP, because the costs and thepotential return from some approaches maynot be justified at smaller facilities.

The best practice for WWTP optimizationincludes the following elements.

■ Establish the objectives of optimization.

■ Evaluate the WWTP to establish thebaseline or benchmark conditions, prioritizeopportunities for optimization, anddetermine performance or capacity limitingfactors.

■ Identify and implement operational orprocess changes to address performanceor capacity limiting factors.

■ Conduct follow-up monitoring to documentthe benefits.

The level of improvement achieved and thebenefits realized from implementing this bestpractice will depend on the starting point.Initial corrective measures may be needed to bring operating staff to a basic level ofknowledge and plant performance to anacceptable level. Subsequently, furtherenhancement to the performance of thefacility can be targeted. Thus, the process is iterative, and clear objectives should beestablished before each iteration.

The Composite Correction Program (CCP)was developed by the U.S. EnvironmentalProtection Agency (EPA, 1985) to identifyfactors that prevent a WWTP from achievingcompliance with its effluent requirements andto mitigate operational problems at suchfacilities. Through the CCP, the problemscausing poor plant performance can beresolved with minimal capital expenditure. Theapproach has been modified for application atCanadian WWTPs (MOEE, 1996).

3. Work Description

3.1 Elements of a

WWTP Optimization

Program

Figure 3–1

Elements of WWTP

optimization

WWTP is anoperatingphilosophy that issustained withthe overallobjective of continuousimprovement.

3. Work Description

Figure 3–1: Elements of WWTP optimization

The CCP is a two-step process that follows a fairly rigorous format. The first stage, theComprehensive Performance Evaluation (CPE),is conducted to evaluate the potential of theWWTP to achieve the desired performancelevels. The evaluation focuses on four majorareas: plant design, operation, maintenance,and administration. During the evaluation,performance limiting factors, typically five tofifteen, are identified and prioritized. Some of the factors that can limit performance orcapacity are identified in Table 3–1.

The methodology of conducting a CPE can be summarized as follows.

■ Identify performance limiting factors.

■ Prioritize performance limiting factors.

■ Assess the approach to improveperformance.

■ Produce a CPE report.

Based on the results from the evaluation, the WWTP is classified as capable (Type 1),marginal (Type 2), or not capable (Type 3), interms of its ability to achieve compliance at its current flow. The causes of the problemsare identified and grouped into three prioritycategories.

■ Priority A factors have a major effect onplant performance on a continuous basis.

■ Priority B factors have a major effect onplant performance on a periodic basis, or a minor effect on a continuous bases.

■ Priority C factors have a minor effect onplant performance.


3. Work Description

3.1 Elements of a

WWTP Optimization

Program

Table 3–1

Performance Limiting

Factors at a WWTP

Table 3–1: Performance Limiting Factors at a WWTP

Category Factors

Operation

Design

Maintenance ■ Scheduling and recording

■ Equipment malfunction

■ Availability of equipment

■ Skilled manpower

■ Age of equipment

■ Knowledge/training of staff

Administration ■ Level of staffing

■ Support from administrative bodies

■ Financial

■ Policies

■ Record keeping

■ Operator training

■ Process monitoring

■ Sludge wasting and disposal

■ Knowledge of operating staff

■ Manual and technical support

■ Availability of equipmentProperchemical selection and use

■ Hydraulic load

■ Organic load

■ Oxygen transfer

■ Inflow and infiltration (I/I)

■ Instrumentation and control (I&C)

■ Industrial load

■ Lack of flexibility

■ Sludge treatment capacity

■ Sludge storage capacity

■ Sludge disposal capacity

■ Process equipment

■ Non-modular design

■ Configuration of process tankage

To achieve long-term performanceimprovements, all the factors contributingto poor performance at a facility must beaddressed in the next stage of optimization.

The second stage of the CCP, termedComprehensive Technical Assistance (CTA),is normally undertaken at a Type 1 or Type 2WWTP and involves systematically addressingthe performance limiting factors identified inthe CPE that do not involve capital works. A major component of the CTA is hands-onoperator training and support to implementprocess control techniques and standardoperating procedures (SOPs) to improveprocess performance. In addition,empowerment of operating staff in prioritysetting and problem-solving skills isaccomplished with the result that performanceis improved. WEAO has published guidancemanuals comprising an instructor’s manualand a student workbook (“Training Operatorson Problem Solving Skills”). The manuals canbe obtained by contacting WEAO [email protected].

Within the context of this best practice, theCPE phase of the CCP would be considered tobe a plant evaluation tool (refer to Section 3.3)and generally involves such components as ahistorical data review (Section 3.3.2) and unitprocess capacity charts (Section 3.3.3).However, the CPE also includes a broaderevaluation of administrative factors that canlimit plant performance.

The CTA phase of the Composite CorrectionProgram is the actual optimization phase and isdiscussed in this best practice in Section 3.5.1.

3.2 Establish Objectives

The tools used for WWTP optimization willdepend on whether the objectives are:

■ reduced energy costs;

■ reduced chemical costs;

■ improved reliability by eliminatingoperational problems and upsets;

■ improved effluent quality;

■ improved biosolids quality;

■ increased treatment plant capacity;

■ reduced labour costs;

■ reduced sludge production or biosolidsmanagement costs;

■ reduced capital costs for plant upgradingor expansion; or

■ reduced odour production.

Clear objectives should be established anddocumented before WWTP optimization isinitiated. The objectives may be qualitative (i.e.,fewer upsets, fewer effluent exceedances) orquantitative (15 percent reduction in energycosts, 25 percent increase in plant capacity).This will allow the success of the measurestaken to be compared to the objectives.

3.3 Plant Evaluation Tools

During the WWTP evaluation stage, performanceis evaluated, capacity limiting factors areidentified and prioritized, and the approach tooptimizing the WWTP is developed. In doing thisevaluation, various tools can be used.


3. Work Description

3.2 Establish Objectives

3.3 Plant Evaluation Tools

3.3.1 Self-Assessment Report

A self-assessment report, prepared by aqualified operational staff, allows the WWTPto evaluate its performance, and identify andprioritize areas for optimization by collectinginformation on the condition, quality, andcapacity of the treatment system.

The report should be done annually andrepresents a report card on the facility formunicipal managers and councillors. Thereport is used to evaluate the status of:

■ effluent compliance and plant performance;

■ plant capacity (current and five-yearprojections);

■ combined sewer overflows and plantbypasses;

■ biosolids handling, storage, and disposal;

■ effluent sampling and analysis;

■ equipment maintenance;

■ operator training and certification; and

■ budgets for current operation andmaintenance, as well as for future facilityreplacement and growth.

A sample self-assessment report has beendeveloped by the Ontario Ministry of theEnvironment (MOE) based on the model whichhas been successfully used for many years bythe Wisconsin State’s Department of NaturalResources. The report can be obtained fromthe MOE Web site <http://www.ene.gov.on.ca/>.

3.3.2 Historical Data Review

A historical data review is an essentialcomponent of the evaluation stage of a WWTPoptimization program. The review defines the current loadings on the facility, theperformance of each unit process, and thekey process operating parameters. It alsoidentifies data gaps that need to be filledthrough additional monitoring and can be usedto determine the representativeness of thehistorical data.

The detailed historical data review can alsobe used to redefine the project. Table 3–2provides examples of possible impacts of the historical data review on subsequentoptimization tasks.


3. Work Description

3.3 Plant Evaluation

Tools

Table 3–2

Examples of historical

data review impacts on

subsequent optimization

tasks

Table 3–2: Examples of historical data review impacts on subsequent optimization tasks

Source: Adapted from WEAO (1996).

Historical Data Review Finding Impact on Optimization TasksMass balance cannot be completed Obtain required information

Mass balance does not close within 15 percent Complete flow meter assessment and/or review of off-line sampling accuracy

Effluent BOD5 and/or nitrogenous compoundsconcentrations exceed criteria

Conduct aeration capacity analysis

Return stream flows/concentrations not available Include sampling of recycle streams in off-linemonitoring program

Effluent SS higher than design values Conduct stress tests and hydraulic analysis (dye tests)to determine capacity and performance limits

3. Work Description


Tools

Figure 3–2

Examples of a process

capacity chart


3.3.3 Unit Process Capacity Chart

One outcome of a historical data review is aprocess capacity chart based on the results ofa process capacity assessment of the key unittreatment processes at the WWTP. Theprocess capacity chart is used to identifybottlenecks that need to be addressed toincrease the capacity of the facility. The unitprocess capacity chart should cover both theliquid and sludge treatment processes as

either could be the limiting factor inmaximizing overall WWTP capacity. Figure 3–2provides an example of a process capacitychart. Full-scale stress testing is oftenperformed following a process capacityassessment to confirm the capacity suggestedby this analysis, especially for borderlinecases. It should be noted that the chart isbased on typical design guidelines orstandards which are often conservative.

Figure 3–2: Example of a process capacity chart

Source: XCG Consultants Ltd. (2002).

3. Work Description


Tools


3.3.4 Sludge Accountability Analysis

An output of the historic data review is asludge accountability analysis. This isbasically a solids mass balance across unitprocesses (e.g., clarifiers) or the overall plantto account for solids within the treatmentprocess. In general, mass balances will notclose exactly. A discrepancy from about 10 to15 percent is considered acceptable; however,a discrepancy of more than 15 percentindicates the need for further assessment toresolve the cause of the inconsistency. Thecommon sources of discrepancies in solidsmass balance analysis include:

■ non-representative samples (analyticalaccuracy, sampling techniques);

■ inaccurate flow monitoring;

■ the impact of periodic recycle streams (theboundaries of the balance must be clearlydefined and all inputs/outputs must beaccounted for in the mass balance); and

■ assumptions made concerningaccumulations.

A sludge accountability analysis should beperformed on a routine basis by plantoperating staff to verify the accuracy of flowmeasurements and analytical data.

3.3.5 Benchmarking Operating Costs and Staffing

If an objective of the optimization program is to reduce operating costs, the historicaloperating and maintenance costs for thefacility should be compared to those of othersimilar plants of similar size. This will identifythe magnitude of the cost reductionopportunity for energy, chemicals, sludgedisposal, and labour that represent the largestcomponents of the operating costs.Benchmarking information for resource costs(energy, chemicals, water) is available in theGuide to Resource Conservation and CostSavings Opportunities in the Water andWastewater Sector (MOEE, 1997). Moredetailed benchmarking data are available inBenchmarking Wastewater Operations:Collection, Treatment and BiosolidsManagement (WERF, 1997).

3.3.6 Flow Meter Assessment

Field evaluation and calibration of plant flowmeters are important since evaluation ofhistorical data and unit process capacity isbased on the assumption that recorded flowsare representative of the historical plantoperation. As a first step in any evaluation, aphysical inspection should be done to confirmthat the flow meters are installed according tosound engineering practices. Any questionsregarding flow meter installation and flow data should be verified before proceedingfurther with other investigations. Sludgeaccountability imbalance can be an indicatorof inaccurate flow meters.

A number of different methods can be used inflow metering assessment and calibration,including:

■ recording run times on pumps andestimating flow based on the pump capacityor pump curve;

■ injecting a tracer material into the flowstream at a constant and known rateupstream of the flow meter and determiningthe concentration of the tracer in samplescollected downstream;

■ drawing down the liquid level in a basin ortank and filling it back up while recordingthe meter reading;

■ flow measurement from a redundant meterto calibrate a suspected meter over a rangeof flow; and

■ hydraulic modelling to develop the headversus flow relationship for non-standardflume and weir installations.

As a first step inany evaluation, a

physicalinspection should

be done toconfirm that theflow meters are

installedaccording to

soundengineering

practices.

3. Work Description


Tools

Figure 3–3

On-Line process variables


3.3.7 Continuous Monitoring

Typical data collection at a WWTP involves acombination of grab and composite sampling.This type of sampling will not identify dynamicconditions occurring in the plant. On-linecontinuous monitoring involves the use oftemporarily or permanently installedinstrumentation to measure the processloading and performance parameters, and adata acquisition system to collect real-timeprocess data. The real-time process dataallow for the identification of various dynamicrelationships in the plant, such as:

■ the impact of hydraulic surges on processperformance;

■ floc shear caused by extreme variation inprocess air flow;

■ effluent quality deterioration caused bydiurnal loadings;

■ return activated sludge concentrationvariations and;

■ process upsets or instabilities caused byreturn streams from sidestream solidsprocesses such as digester supernatant or biosolids dewatering.

On-line monitoring data have also been usedto identify potential energy and chemicalsavings at WWTPs. Primary clarifier sludge,return activated sludge (RAS) and wasteactivated sludge (WAS) flows are useful on-line process variables, and are important forsolids accountability. Measurement of theflows of internal recycle streams such asdigester supernatant, dewatering filtrate orcentrate, and thickener overflow is alsobeneficial. Table 3–3 identifies some processvariables typically measured with on-lineinstrumentation (WEAO, 1996).

Table 3–3: On-Line process variables

Category Measurements Process flow rates ■ Influent/effluent wastewater

■ Primary clarifier sludge

■ RAS

■ WAS

■ Biosolids flow

■ Process air flow

■ Chemical metering rates

Process variables ■ MLSS concentration

■ RAS/WAS suspended solidsconcentration

■ Dissolved oxygen concentration

■ Effluent suspended solids concentration

■ Sludge blanket height

■ pH

■ Ammonia-nitrogen

■ Nitrite/nitrate-nitrogen

■ Orthophosphate

■ Conductivity

■ UV transmissivity

Measurement ofthe flows ofinternal recyclestreams such asdigestersupernatant,dewatering filtrateor centrate, andthickener overflowis also beneficial.

3. Work Description


Tools

3.4 Process Analysis

Tools


Wherever possible, on-line monitoring isencouraged, because of the benefit real timedata provide to operating staff; however, it isrecognized that on-line monitoring may not befeasible for some WWTPs, depending on sizeand available resources. These plants are stillencouraged to monitor their operation byconducting sampling and analysis on a regularbasis. A sampling and analysis scheduleshould be developed, including a list of theparameters to be analyzed daily or weekly.For example, mixed liquor and effluentsuspended solids concentrations can beanalyzed by obtaining daily grab or compositesamples. For parameters that do not changerapidly, such as sludge quality data (e.g.,solids concentrations in digested sludge),sampling can be performed on a daily orweekly basis. Grab samples can also beobtained to monitor variations in processparameters throughout the day. It is goodpractice to conduct on-line monitoring ofthose parameters with more rapid fluctuationssuch as dissolved oxygen concentrations inaeration basins (also referred to asbioreactors), or process flows.

3.3.8 Off-Line Monitoring

Off-line monitoring is conducted to supplementplant historical data, or to obtain data nothistorically collected at the plant but importantfor plant evaluation purposes. These mayinclude analytical parameters or internalstreams within the plant not routinelymonitored by plant staff.

Microscopic examination of the biological masscan be performed to determine the general stateof the system, and to identify potential problemssuch as bulking sludge due to filamentousorganisms. Jar testing is generally performed toevaluate and optimize coagulant or chemicaladdition to wastewater for improved settleabilityor precipitation of some element in thewastewater (e.g., phosphorus removal).Additional laboratory/field tests can beperformed to assess the performance of a

particular unit process such as settling columntests, dissolved oxygen monitoring and profiling,oxygen uptake rate, sludge volume index (SVI),and sludge blanket monitoring.

3.4 Process Analysis Tools

Various tests can be used to optimize aWWTP. These process analysis tools are usedto identify cost-effective ways to increaseplant capacity and meet more stringenteffluent limits or improve biosolids quality,without major capital works. They can also beapplied at those facilities identified by a CPE tobe incapable of meeting compliance limits atthe current flow due to design deficiencies.

This toolbox of tests is often referred to as a“process audit.” When applied at a WWTP, theaudit can lead to an optimized facility in termsof capacity, operating cost, and performance.The Water Environment Association of Ontario(WEAO) has published the Guidance Manualfor Sewage Treatment Plant Liquid TrainProcess Audits (WEAO, 1996), an invaluableresource for any WWTP owner/operatorembarking on a WWTP optimization program.Unfortunately, no similar guidance manual hasbeen developed as yet that is specific tosludge treatment unit processes.

3.4.1 Aeration System Capacity and Efficiency Analysis

Aeration is one of the most fundamental andcostly processes in aerobic biologicalwastewater treatment, representing as muchas 75 percent of total plant energy use.Inadequate oxygen transfer may result inthe deterioration of effluent quality due toinsufficient oxygen to meet the biologicaloxygen demand and the endogenous oxygendemand of the biological mass. Aerationsystem capacity analysis is conducted toevaluate the aeration system capacity, and to identify opportunities for energy savings.

Aeration is oneof the most

fundamental andcostly processes

in aerobicbiological

wastewatertreatment,

representing asmuch as 75

percent of totalplant energy use.


The two most common techniques for testingin-situ oxygen transfer efficiency (OTE) are theoff-gas analysis and hydrogen peroxide tests.The results of these tests are used to comparethe existing aeration capacity with currentand future (or potential) oxygen demands.This comparison is then used to evaluate thecapacity of an aeration system for increasedloadings and further treatment capabilities(e.g., nitrification), and to evaluate the energysaving potential for a plant. For more informationon oxygen transfer testing and test protocols,readers are referred to American Society ofCivil Engineers Standard Guidelines for In-Process Oxygen Transfer Testing (ASCE, 1997).

3.4.2 Hydraulic Modelling

Hydraulic modelling involves developing thehead loss versus discharge relationships forthe hydraulic control sections and performingbackwater calculations for the open channelsections between control sections. Thecalibrated hydraulic model can be used to:

■ determine the hydraulic capacity of anexisting facility;

■ identify hydraulic bottlenecks andinvestigate alternative strategies forreducing the hydraulic limitations identified;

■ determine flow imbalances and investigatemethods of improving the flow distributionbetween parallel unit processes; and

■ determine velocity gradients and identifyoptimum locations for chemical addition.

3.4.3 Analysis of Recycle Streams

Sludge treatment recycle streams are oftenresponsible for problems in the liquid train of a WWTP. These streams can increase theorganic loading by five to fifty percent,depending on the type and number of solidstreatment processes used. The following arepossible solutions to minimize or eliminate theimpact of sludge handling recycle streams onthe liquid train.

■ Modify the solids handling processes toimprove the quality of the recycle streams.

■ Change the timing, return rate or return pointof the recycle streams to minimize the impact.

■ Modify the liquid train to handle the recyclestreams.

■ Provide separate treatment for the solidsrecycle streams.

Analysis of recycle streams from sludgeprocessing (digester supernatant, dewateringcentrate, or filtrate) can also provide anindication that these processes would benefitfrom optimization.

3.4.4 Stress Testing

Stress testing is conducted to identify the loading rate at which the processperformance approaches the design value.Diurnal and/or wet weather flow increasesmay be used to stress unit processes that areaffected by hydraulics, such as clarifiers.Hydraulic, organic and solids loading rates to the unit processes can be increased byvarying the number of units in service, biasingthe flow to the test unit.

Stress testing is generally not conducted untilprocess failure occurs due to the potentialimplications on compliance. Prior to undertakingstress testing, a plan should be developed thatidentifies the possible impacts of stressing aspecific unit process, the monitoring that willbe done to evaluate the performance of theprocess, and the steps that will be taken if itappears that process failure is imminent. Theneed to inform the pertinent regulatory agencyabout the test must also be considered.

3. Work Description


Tools


Table 3–4 summarizes typical unit processdesign parameters and evaluation criteria thatwould be applied during a stress test.

Stress testing is seldom conducted on sludgedigestion processes due to the long responsetime to changing conditions and the longrecovery time if stress testing results in aprocess upset.

3. Work Description


Tools

Table 3–4

Summary of typical unit

process design parameters

and evaluation critieriaTable 3–4: Summary of typical unit process design parameters and evaluation criteria

Unit Process Design Parameter Evaluation Criteria

Primary clarifier ■ Surface overflow rate ■ Removal efficiencies

■ Sludge blanket depth

■ Real detention time

Secondary clarifier ■ Surface overflow rate

■ Solids loading rate

■ Effluent quality criteria

■ Sludge blanket depth

Activated sludge(Including aeration)

■ HRT/SRT

■ Organic/nitrogenous loading rate

■ F/M ratio

■ Recycle ratio

■ Effluent quality

■ Dissolved oxygen concentration

■ SVI/SSVI

■ SOUR

Effluent filter ■ Hydraulic and solids loading rate ■ Effluent quality

■ Head loss

■ Backwash solids concentration

Disinfection(chlorination/UV)

■ Cl2 dosage

■ Retention time

■ Effluent solids

■ UV dosage/transmissivity

■ Residual Cl2

■ Bacterial concentrations (total/fecal coliform, E. coli)

Sludge Thickening andDewatering

■ Hydraulic and solids loading rate

■ Chemical dosage (if applicable)

■ Sludge concentration

■ Recycle stream quality

Sludge Digestion(Aerobic or Anaerobic)

■ Hydraulic retention time

■ Solids retention time

■ Gas production (anaerobic digestion)

■ Volatile solids destruction

■ Pathogen destruction

■ Supernatant quality

■ Biosolids concentration

Source: Adapted from WEAO (1996).


3.4.5 Clarifier Hydraulic Tests

Clarifier hydraulic tests are conducted toevaluate the hydraulic characteristics within aclarifier and to determine possible methods toincrease the hydraulic capacity of the clarifier.The clarifier dye test, also called the CrosbyDye Test, is a qualitative test that uses dye totest the hydraulic flow pattern of clarifiers(Crosby, 1987). The test has two components:the dispersion test and the flow pattern/solidsdistribution test.

The dispersion test involves an instantaneousinjection of tracer upstream of the clarifier andsampling the effluent over a period of time.The test is used to determine the actualhydraulic residence time, estimate the degreeof hydraulic short circuiting, and determinesampling times for the flow pattern test.

The flow pattern/solids distribution testinvolves injecting dye continuously at aconstant rate into the flow entering theclarifier. Samples are then collected atmultiple depths and locations in the body ofthe clarifier to provide “snapshots” of themovement of dye. TSS concentrations aremonitored at each position and depth. Flowpattern tests are used to evaluate the spatialdistribution of flow through the clarifierincluding the location of dead zones, densitycurrents, and the possible effect of bafflearrangements.

Sophisticated hydrodynamic models can alsobe used to simulate the hydraulic patterns inclarifiers, and to assess the effect of variousphysical modifications to the clarifier (inletbaffles, weir baffles, etc.) on clarifierperfomance or to predict the impact of highflows, high solids loading rates or poorsettleability. Two-dimensional and complexthree-dimensional models have beensuccessfully used to improve clarifierperformance (Ekama et al., 1994).

3.4.6 Other Clarifier Diagnostics Tests

While SVI and SSVI are the most commontools used to determine the settleability ofbiological sludges, other diagnostic tools suchas State Point Analysis (Keinath, 1985) andDispersed Suspended Solids (DSS)/Flocculated Suspended Solids (FSS) testing(Wahlberg et al, 1995) can provide insight intothe causes of poor secondary clarifierperformance. State Point Analysis (SPA) willprovide information on whether a clarifier isoperating in an overloaded condition anddirection on operational steps that can betaken to eliminate the problem. DSS/FSStesting will indicate whether poor secondaryclarifier performance is related to poor solidsflocculation or poor clarifier hydraulics.

3.4.7 Mixing Tests

Mixing tests are conducted to evaluate thehydraulic characteristics of unit process tankswhen mixing problems are suspected, and arealso used to evaluate mixing equipment,equipment layout, and geometry. The resultsof the mixing test can be used to:

■ identify hydraulic short circuiting;

■ define mixing characteristics;

■ identify dead zones within the fluid volume;

■ evaluate the effectiveness of bafflingarrangements; and

■ determine the predominant flow patternswithin the unit process.

Mixing tests are particularly valuable indigestion tanks because scum, grit, and othermaterials can accumulate causing a loss ofactive reactor volume and short-circuiting.Improving mixing can often result in increasedvolatile solids destruction and improvedbiosolids quality.

While fluorescent dyes can be used effectivelyin mixing tests in clarifiers or chlorine contactchambers, lithium chloride is the preferredtracer in digesters. Test procedures and dataanalysis methods are outlined in Monteith andStephenson, 1984.

3. Work Description


Tools


3.4.8 Process Modelling and Simulation

Process models are efficient tools to determineoptimum operating conditions. This can includehydraulic retention time (HRT) and solidsretention time (SRT), and the capacity of thesystem to meet specified performance criteria.Process models are available for many of thecommon biological processes, such as activatedsludge, extended aeration, sequencing batchreactors (SBRs), rotating biological contactors(RBCs), and trickling filters.

Process modelling and dynamic processsimulation can be used for:

■ process capacity estimation;

■ bottleneck identification;

■ hydraulic load change analysis;

■ optimization of aeration system operation;

■ optimization of sludge recycle and wastage;

■ optimization of the operational sequence ofSBR systems;

■ bypass impact reduction;

■ evaluation of alternate design strategies;

■ management of wet-weather flow;

■ sludge production estimation; and

■ design of reactor configurations forbiological nutrient removal (BNR).

A dynamic model simulates variationsthroughout the diurnal and seasonal cyclesand tracks the effects of these variations onprocess performance. Process simulationmodelling is also employed to establish thecapacity of biological components of thewastewater treatment plant and model theeffects of process changes on plant capacityor performance. Recent work has focused onlinking dynamic models with supervisorycontrol and data acquisition (SCADA) andlaboratory information management systems(LIMSs) to further improve the accuracy andvalue of their predictions (Irrinki et al., 2002).

3.5 Optimization Approaches

3.5.1 Improved Operations and Maintenance

Improved process control procedures tailoredfor the particular WWTP can both improveprocess performance and save money. Aprocess control testing schedule to monitorcontrol parameters, including but not limited tosludge settling, sludge mass, sludge wasting,sludge return concentration and flow, volatilesolids destruction in digesters, dewateringperformance, and aeration basin dissolvedoxygen should be established as a first step inWWTP optimization. On-the-job training shouldalso be provided for the operators in specificprocess control sampling and testingrequirements, as well as process controlcalculations.

Formalizing record keeping will generallyimprove maintenance practices. The followingfour-step procedure is suggested fordeveloping a maintenance record keepingsystem:

■ Inventory all equipment.

■ Gather manufacturers’ maintenanceinformation and schedules on all equipment.

■ Complete equipment information summarysheets for all equipment.

■ Develop a time-based preventivemaintenance schedule.

The list of equipment should be updated whenequipment is added or removed from thefacility. The maintenance schedule shouldinclude daily, weekly, monthly, quarterly, semi-annual, and annual checklists of requiredmaintenance tasks.

For larger facilities, a computerizedmaintenance management system (CMMS)can cost effectively optimize the maintenancefunction. Through information technology (IT),process control information, SCADA, CMMS,laboratory data and other information can belinked so all key information is available tostaff on-line and in real time.

3. Work Description


Tools

3.5 Optimization

Approaches

A staffing plan (Daigger and Buttz, 1992)should be developed to determine if a facilityis properly staffed. Benchmarking informationis available from various sources to assessstaffing needs (WERF, 1997).

Staff training can help improve plantperformance (as it relates to poor operationalpractices), address safety issues, and improvestaff morale. Staff training should recognizethat on-site training is the most effective wayto develop an operator’s capability to applywastewater treatment concepts properly toprocess control. Operating personnel shouldalso be encouraged to improve sewagetreatment understanding through budgetsupport for off-site training and certification.Comprehensive technical assistance (CTA) is asystematic approach to eliminate those factorsthat inhibit performance in existing WWTPs.CTA facilitators work with plant operators andmanagers to develop process control activitiesand to transfer skills and knowledge.

3.5.2 Instrumentation, Control, And Automation

Opportunities to reduce costs and improveoperational performance and reliability arepotentially available through the on-lineinstrumentation and/or automation ofwastewater treatment operations. By addingprocess measurements, the operator also hasmore information on which to base judgmentsand implement control decisions. Efficientoperation can be maintained using automatedcontrols. Optimization of processes throughthe use of on-line measurement and feedbackcontrol can significantly reduce the amount of

chemical, energy, and water use as well asreduce the production of waste residualsrequiring treatment and disposal (WEF, 1997).Higher savings potential occurs in facilitieswith high variability in wastewater quality andflow. Examples of best practice automationapplications are summarized in Table 3–5.

Instrumentation and Control (I&C) at a WWTPcan provide information to the operator onthe status of equipment, provide real time

measurements of process parameters, allowfor automatic control of equipment (e.g.,turning equipment on and off), and signalalarm conditions. Various parts of the I&Csystems can be upgraded. For example,primary elements can be upgraded by addingprocess measurements, and control hardwareand software can be upgraded by addingalarms that automatically switch to a backupwhen equipment fails. The overall processcontrol system can be improved for WWTPswith outdated I&C systems. In emergencies,automatic controllers can switch to a backup.All critical control functions should have amanual control backup. Proper staffingsupport to calibrate and maintaininstrumentation is critical to attain the benefits provided by automation.

Automated process control strategies forspecific unit processes are discussed in detailin the WEF special publication AutomatedProcess Control Strategies (WEF, 1997) and inthe recent WERF report Sensing and ControlSystems: A Review of Municipal and IndustrialExperiences (WERF, 2002).


3. Work Description

3.5 Optimization

Approaches

Operatingpersonnel shouldalso beencouraged toimprove sewagetreatmentunderstandingthrough budgetsupport for off-sitetraining andcertification.


3. Work Description

3.5 Optimization

Approaches

Table 3–5

Automation applications

at WWTPs

Table 3–5: Automation applications at WWTPs

Source: WERF (2002).

Process/Unit Application

Preliminary treatment ■ Automatic screen cleaning based on head loss, total flow treated and/or timers

Primary and chemicallyenhanced primarytreatment

■ Flow proportional chemical dosage control

■ On-line effluent suspended solids/turbidity monitoring

■ Automated sludge density control of sludge pumping

■ Automated sludge blanket height control of sludge pumping

Biological treatment ■ On-line respirometry

■ On-line measurement of BOD load

■ Automated sludge age (SRT) control

■ Automated biological sludge wasting control

■ Automated ORP control in the control of biological nutrient removal processes

■ On-line measurement of MLSS concentration

■ On-line dissolved oxygen monitoring and control

■ On-line measurements of NH3-N, NOx-N and PO4-P concentrations

Secondary clarifiers ■ On-line effluent TSS or turbidity analysis

Tertiary filters ■ On-line monitoring of turbidity and/or phosphorus concentration

■ On-line monitoring of head loss

Aeration system ■ Automated blower control based on on-line dissolved oxygen sensors

■ On-off aeration control

■ Variable speed control of mechanical aerators

Disinfection

(i) Chlorination/dechlorination

(ii) UV irradiation

■ Flow proportional chemical dosage

■ Automated chlorine residual control

■ Automated ORP control

■ UV intensity monitoring and control

■ Flow pacing of UV lamps

■ Initiation of automatic self-cleaning

Sludgethickening/dewatering

■ Automatic flow pacing of chemical addition

■ Automatic mass dosage control of chemical addition

■ Automatic monitoring of solids content of liquid stream

■ Automatic chemical dosage control based on flocculation properties

Digestion ■ Automated control of sludge distribution between multiple reactors based on flowor solids mass load

■ On-line monitoring of supernatant quality


3.5.3 Treatment Process Modifications

A variety of modifications are possibledepending on the unit process underconsideration and the specific performancelimiting factor identified during the plantevaluation stage. Table 3–6 summarizes, on aunit process by unit process basis, some ofthe optimization opportunities that could beconsidered to increase capacity, improveefficiency, or reduce the costs associatedwith chemical or energy use.

More detailed discussions of how these andother optimization opportunities might beimplemented in each specific unit process areprovided in Appendix B. Readers should referto Appendix B for a discussion of potentialapproaches to optimize the particular unitprocesses that make up their WWTP, or forthose unit processes that have been identifiedduring the plant evaluation stage to limitperformance or reduce overall plant capacity.

3. Work Description

3.5 Optimization

Approaches

Table 3–6

Potential treatment process

optimization approaches

Table 3–6: Potential treatment process optimization approaches

Note: More detailed discussion of these optimization approaches is provided in Appendix B

Process Optimization Approach

Plant hydraulics ■ Eliminate surges due to pump station operation

■ I/I control

■ System storage and real-time control

Preliminary treatment ■ Upgrade screens and improve control

■ Improve hydraulics in grit tanks

■ Improve grit removal and handling

Primary treatment

Biological treatment

Secondary clarifiers

Tertiary filtration ■ Optimize chemical use

■ Optimize backwash

Disinfection ■ Improve mixing

■ Implement automatic control

Sludge Thickening/Dewatering

■ Optimize chemical dosage or chemical type

■ Manage primary sludge and WAS separately

Aerobic Digestion

Anaerobic Digestion

■ Improve process flexibility

■ Optimize BOD5 removal

■ Optimize nitrification

■ Implement BNR

■ Optimize oxygen transfer

■ Implement step feed

■ Implement foam/scum controlmeasures

■ Optimize chemical use

■ Improve hydraulics

■ Improve scum/sludge removal

■ Eliminate co-settling of wasteactivated sludge

■ Improve flow splitting

■ Eliminate hydraulic surges

■ Improve hydraulic patterns

■ Control sludge bulking

■ Improve RAS/WAS flexibility

■ Optimize oxygen transfer

■ Optimize settling to increase sludgethickness or improve supernatantquality

■ Improve mixing

■ Increase raw sludge concentration

■ Improve mixing

■ Increase temperature to improvevolatile solids destruction

■ Improve load distribution betweenmultiple tanks

■ Increase raw sludge concentration

■ Use biogas for energy value

3.5.4 Achieving Resource Cost Savings

Energy usage in wastewater treatment can bea major portion of the annual operating costs.Much of the information presented in this sub-section is adapted from the Guide to ResourceConservation and Cost Savings Opportunitiesin the Water and Wastewater Sector (MOEE,1997). Readers are referred to this documentfor more detail on opportunities for resourcecost savings in WWTPs.

High Efficiency Motors/Variable Speed Drives

Many facilities operate using inefficient pumpsand motors designed and installed years agowhen system constraints and requirementswere very different than today. Motorefficiencies are now much higher than whatwas available even 10 years ago. As a result,significant energy savings can be realized byreplacing old motors in existing equipment.Using variable speed drives, facilities canoptimize pump operation by matching energyrequirements with pumping requirements.

The most attractive lifecycle payback occurswhen existing motors need replacement, andhigh-efficiency motors or variable speeddrives are appropriate for that application.It must also be noted that relative cost ofmaintenance and replacement of variablespeed drives (variable frequency drives (VFDs)in most cases) needs to be considered in theevaluation of payback expected from suchdevices. Higher energy savings will also occurin facilities with high peak demand ratios thatare pumping outside of the efficient range ofthe existing pumps. The plant operator shouldensure that pumps operate at the mostefficient point on their operating curve.

Off-Peak Operation

During peak demand periods, energy demandand consumption charges may be higher thanduring off-peak demand periods. Wherepossible, moving the operation of existingprocesses to off-peak periods can significantlyreduce energy costs. Shifting demands to off-peak periods requires operational changesonly (i.e., no capital investment) and, as aresult, payback can be immediate. Although

the energy cost is reduced, the amount ofenergy used during off-peak operation is notalways reduced. Energy use reductions willonly be achieved if the operating ranges ofthe process equipment are better suited forlower intensity/longer duration operationsimplemented by transferring operation tooff-peak periods.

This technique is applicable throughout afacility. The potential benefit varies with thetype of process, available storage and thedesign of the specific facility under review.It should be noted that small plants do notnecessarily have hydro demand meters or off-peak rates available. Therefore, this techniquewill not offer any savings in energy use or costin these instances.

Flow Measurement

Accurate flow metering equipment forwastewater flows, sludge flows, effluent andbackwash water, and chemical dosing ratesensures optimized resource usage withsignificant effects on chemical usage, filterruns, backwashes, and sludge productionrates. For example, if flow measurement isinaccurate in a flow-paced disinfectionprocess, then unnecessary wastage of energyand chemical use can occur by overpumpingand overchlorinating.

Biological Treatment System

When nitrification is not required, controllingsolids retention time and/or reducing dissolvedoxygen levels will reduce oxygenrequirements significantly, which can reducerun time for mechanical aerators or blowers,resulting in reduced energy use, preventingunnecessary nitrification. The addition ofcoagulant during primary treatment improvesthe removal of particulate matter beforeaeration. This reduces aeration energyconsumption. Although energy use is reduced,chemical use and primary sludge productionare increased during primary treatment, andtrade-offs must be investigated.


3. Work Description

3.5 Optimization

Approaches

The mostattractive

lifecycle paybackoccurs when

existing motorsneed

replacement, andhigh-efficiency

motors orvariable speed

drives areappropriate for

that application.


3. Work Description

3.5 Optimization

Approaches

By switching to an on/off aeration mode,blowers or mechanical aerators can beoperated for short periods (e.g., 30 minutes) andthen shut down for equal or smaller periods.This reduces energy usage significantly. Thisapproach should not be used for aerationsystems that would foul if the air supply is shutoff (i.e., ceramic, fine pore, or some coarsebubble diffusers). Aeration devices will need tobe retrofitted with some form of ramp startingequipment to protect them from the wearassociated with an increased number of start-ups. Soft start devices can be used to reducethe peak demand.

By optimizing the solids retention time (SRT),biomass production can be reduced resultingin a reduction in energy use required forhandling and disposal. There may be anenergy increase for aeration at a higher SRT.

Fine pore aeration systems produce smallerair bubbles, which provide better oxygentransfer efficiency compared to coarse bubblesystems. Improved oxygen transfer reducesthe amount of air blowers must supply and,therefore, reduces energy consumption byblowers. Energy savings potentials rangingfrom nine to forty percent can be achievedwith fine pore systems (EPRI, 1996). Additionalcleaning is sometimes required with fine poresystems to eliminate problems with clogging;however, the associated costs are minimal.

Anoxic reactors will recover bound oxygenfrom nitrate, reducing the oxygen inputrequirement for blowers in downstreamaeration basins. Although additional pumpingto recirculate flow will be required, thesignificant reduction in blower use canprovide for net energy savings.

Excessive power use by blowers or aeratorscan be eliminated by monitoring dissolvedoxygen within the aeration basins, andmanually or automatically controlling thenumber of blowers and air flow rates.

By providing anaerobic/aerobic environmentsto increase the biological uptake ofphosphorus, significant reductions in chemicaluse can be achieved. In some cases, the needfor chemical input may be eliminated,however, the need for increased processcontrol and operator knowledge will increase.

Backup Generators

Most treatment facilities have backupgenerators to provide power duringemergencies. They are not normally usedexcept for testing and as part of routinemaintenance procedures. By operating thesegenerators during peak periods, electricalenergy use reductions and significant electricalenergy cost savings can be achieved. Air qualityrequirements and the costs of fuel for backupgenerators may limit this application in somefacilities. This scenario will only provideworthwhile savings for facilities with lowgenerator operating costs and high peakdemand ratios and rate structures.

Effluent Water Use

In wastewater treatment facilities, potablewater may be used in a number of processesfor backwashing, rinsing, chemical makeup,foam control, and odour control. By replacingthe use of potable water with treated effluentwater, significant savings in water costs canbe achieved. Effluent water use depends onthe level of treatment, and is generally limitedto usage as process water. The effluent watershould be disinfected with chlorine to protectoperator health.

Biogas Utilization

The methane contained in the biogasproduced by anaerobic digestion can be usedto replace natural gas for digester or spaceheating. In large facilities, generation ofelectrical power using biogas can have afavourable payback time.

Energy savingspotentials rangingfrom nine to fortypercent can beachieved withfine pore systems (EPRI, 1996).

Resource Costs

There are a number of opportunities to reducethe costs of resources, although the specificresource use is not reduced.

■ Negotiate utility bills to reduce electricalenergy and gas costs. For example,improving a plant’s time to come off-lineduring peak periods or in emergencies canassist in negotiating lower energy chargerates.

■ Combine/separate utility bills betweenplants and pumping stations to reduceenergy and gas costs. For example,combining plant energy costs with zonepumping station energy costs can reduceenergy charge rates.

■ Combine chemical purchasing with otherplants or industries to increase shipmentsizes and reduce unit costs.

3.6 Document Benefits

Following completion of a WWTP optimizationprogram, it is important to ensure that anevaluation of the benefits of optimization iscompleted and the benefits are documented.This assessment should compare theobjectives established for the project (e.g.,capacity gain of 30 percent) to the actualoutcome of the optimization program and thereturn on investment (i.e., savings realizedcompared to program costs).

Communication of the benefits of theoptimization program to the decision makers in the municipality and plant operators isessential to maintain benefits gained by theoptimization initiative and to build support forfuture initiatives. This support is key to ensurethe iterative process of optimization issustained and an environment conducive to optimization is fostered.

3.7 Optimization Task Flow Sheet

Figure 3–3 illustrates a WWTP optimizationtask flow sheet as a guide to the approachthat might be used to achieve variousobjectives. It is important to note that manyoptimization programs have multipleobjectives, and different approaches can beused to achieve the same objectives. Forexample, during the CTA phase of a CCP, anyof the process analysis tools described in thisbest practice could be used to evaluate aparticular unit process that appears to limit theperformance of a facility. Similarly, operatortraining can be undertaken in a manner similarto that done in a CTA even if the CCP approachto optimization has not been formally applied.Any of the process analysis tools, such asoxygen transfer testing or simulationmodelling, can identify opportunities foroperating cost reduction.

It is essential to recognize the importance of the plant evaluation stage to the success of theoptimization program. This stage will establishthe validity of the historic data that are the basisfor determining the performance capabilitiesand capacity of the facility. This stage will alsoestablish the benchmarks against which thebenefits of subsequent optimization steps canbe measured. If the data are suspect due topoor sludge accountability or poor flow meterinstallation, additional monitoring at this stage isimportant to ensure subsequent work is basedon sound knowledge of the plant’s capabilitiesand limitations.

The plant evaluation stage will also establishthe approach and work plan that willsubsequently be implemented to optimize theplant and meet the optimization objectivespreviously established. The task flow sheet(Figure 3–3) suggests the type of tests oroptimization tools that might be used toachieve specific objectives. Again, it isemphasized that this is merely a guide. Theapproaches used must be tailored to meet theoverall objectives and will vary depending onthe size and type of plant being optimized, theresources available, and the capabilities of the plant staff to conduct the specific tests.


3. Work Description

3.5 Optimization

Approaches

3.6 Document Benefits

3.7 Optimization Task

Flow Sheet


4. Applications and Limitations

4.1 Optimization Task

Flow Sheet

4.2 Applications and

Limitations

Figure 3–3

Representation of

optimization task flow

sheet

4.1 Applications

The elements of the best practice for WWTPoptimization apply to any size or type oftreatment plant. The tools that might beapplied at a small WWTP may be differentthan those that would be applied at a largerWWTP, because the costs and the potentialreturn from some approaches may not bejustified at smaller facilities.

4.2 Limitations

This best practice covers most common liquidand sludge treatment processes, includingsuch processes as preliminary, primary,secondary, and tertiary treatment, and thedisinfection of the treated effluent andthickening, dewatering, and digestion (aerobicand anaerobic) of sludge. Optimization of moresophisticated and complex sludge treatmentprocesses, such as incinerators, dryers, and

pelletizers is not included in this best practice.Management of biosolids produced at theWWTP is also not addressed in this bestpractice. A best practice for biosolidsmanagement has also been developed by theNational Guide to Sustainable MunicipalInfrastructure: Innovation and Best Practices.The reader is referred to that best practice for information on biosolids management.

This best practice focuses primarily onoptimization of mechanical WWTPs rather thanlagoon-based systems, although aspects of thebest practice that relate to operator trainingare equally applicable to all types and sizes of

WWTPs. A best practice for operation andmaintenance of lagoons will be developed bythe National Guide to Municipal Infrastructure:Innovation and Best Practices. The reader isreferred to that best practice for information on optimization of lagoon-based systems.

4. Applications and Limitations

Figure 3–3: Representation of optimization task flow sheet


A.1 Case History 1 – Burlington Skyway WWTP (Wheeler and Hegg, 1999)

The Burlington Skyway WWTP is the largesttreatment facility in the Regional Municipalityof Halton. Treated effluent from the plant isdischarged to Hamilton Harbour. HamiltonHarbour has been designated as one of sevenCanadian areas of concern in the Great Lakesby the International Joint Commission.Stringent effluent limits (TP = 0.3 mg/L,ammonia = 5.6 mg/L, and TSS = 10 mg/L) havebeen initially targeted for discharge from theBurlington Skyway facility to alleviateeutrophication and toxicity in HamiltonHarbour. To meet stringent effluent limits, theRegional Municipality of Halton implemented aformal optimization program in 1995. The goalswere to maximize the hydraulic capability ofthe existing infrastructure while meetingperformance requirements, and to empowerstaff with skills and initiative to implementactivities to maintain the targetedperformance levels economically.

The Burlington Skyway plant is a conventionalactivated sludge facility with a nominal designcapacity of 93,000 m3/day, and serves bothindustrial and residential dischargers. The maincomponents of the liquid train treatmentprocesses include preliminary treatment,primary settling, conventional activated sludgesecondary treatment, and disinfection. The dualpoint addition of ferric chloride (to primaryclarifier influent and to secondary clarifierinfluent) is employed for phosphorus control.The solids treatment train includes dissolved airflotation (DAF) waste activated sludge thickeningand mesophilic anaerobic digestion. The primarydigesters are equipped with gas mixing. The off-gas is used in gas-fired boilers, and excess gasis stored on-site. Digested sludge is hauled tothe regional biosolids handling facility beforeland application.

The optimization tool used at the BurlingtonSkyway WWTP was the Composite CorrectionProgram (CCP). The CPE identified non-technical or management and humanresource-related limitations to be majorperformance limiting factors, includinginadequate communication between operatorsand managers, a lack of understanding offacility needs, inadequate application ofoperational concepts, and inadequate plantcoverage to respond to high flow events. Theapproach to resolving non-technical issueswas to address these in conjunction withaddressing technical limitation, based on therealization that any improvements in effluentquality may not be sustained if these non-technical issues are not resolved. Enhancedcommunication and properly applying prioritysetting and problem-solving skills wereemphasized during the CTA phase of the CCP.

Other optimization efforts undertaken at thefacility during the CTA included:

■ evaluation of the optimum polymer dosageand dosage control (December 1996 toFebruary 1997);

■ pilot scale evaluation of spiral blademechanism to enhance clarifier sludgeremoval efficiency for improved nitrification(September 1997 to March 1998);

■ retrofitting the remaining clarifiers withspiral blade mechanism (summer of 1998);

■ optimizing the removal mechanism (rake tipspeed of 305 cm (10 feet/min) (summer of1999); and

■ reactivation of the existing Dissolved AirFloatation (DAF) unit (March 1999).

A. Selected Case Histories

A.1 Case History 1 –

Burlington Skyway

WWTP (Wheeler

and Hegg, 1999)

Appendix A: Selected Case Histories

The approach to resolving non-technicalissues was toaddress these in conjunction with addressingtechnicallimitation, basedon the realizationthat anyimprovements ineffluent qualitymay not besustained if thesenon-technicalissues are notresolved.

In terms of operational control, improvedoperator application of process controltechniques at all the major unit processes was emphasized, with priority given to thesecondary unit process. Improved solidsinventory control consisted of daily samplingand testing to monitor sludge mass in aerationbasins (also known as bioreactors) andsecondary clarifiers, return activated sludge(RAS) underflow lines, as well as in theprimary clarifiers to maintain stable removal.

The ongoing optimization efforts at theBurlington Skyway WWTP resulted in asubstantial improvement in the plantperformance in terms of phosphorusremoval. The reduction in the phosphorusloading to Hamilton Harbour achieved as aresult of the optimization efforts is illustratedin Figure A–1. In addition, the plant was ableto achieve nitrification without major capitalexpenditure, meeting the targets establishedfor Hamilton Harbour.

Maximizing the operational skills through theCTA, in conjunction with the other optimizationwork done at the plant, resulted in a level ofperformance not considered achievablebefore optimization. It had been estimated that $33 million in capital upgrades would beneeded to achieve nitrification at the plant.

In addition, additional capacity was foundat the plant to defer $17 million in plantexpansion costs. Based on the successachieved, the Region has expanded itsoptimization program to include all itswastewater and water treatment facilities.




Burlington Skyway

WWTP (Wheeler

and Hegg, 1999)

Figure A–1

Total phosphorus average

loading – Burlington

Skyway WWTP effluent

Figure A–1: Total phosphorus average loading – Burlington Skyway WWTP effluent

Source: WEFTEC (1999)

A.2 Case History 2 – Ayr WWTP (XCG, 2000b)

The Ayr WWTP serves the Town of Ayr inthe Regional Municipality of Waterloo. It is

a circular, package extended aeration plantconstructed in 1978 with a design capacityof 1,181 m3/d. The treatment processes at theplant include coarse screening, grit channels,aeration using fine pore membrane diffusersthat had been installed as a retrofit for energysavings, secondary clarification, ferricchloride addition for phosphorus removal, and chlorine disinfection. Excess sludge isaerobically digested and hauled either directlyto land for utilization or to a regional sludgestorage facility for interim storage.

By 2000, the plant was operating at about 89 percent of its design capacity and furthergrowth in the community was restricted due toservicing constraints. To determine if additionalcapacity was available in the existing facility,an optimization and re-rating study wascommissioned to define the maximum capacityof the plant, identify any processes that wouldneed to be upgraded, and collect adequate datato support an application to increase the ratedcapacity of the facility.

A detailed historic review and processcapacity analysis indicated there waspotential to increase the rated capacity of theplant from 1,181 m3/d to 1,500 m3/d. At the sametime, the regulatory agency imposed morestringent phosphorus removal limits on theplant and included a requirement to nitrifyyear-round and produce a non-toxic effluentafter disinfection.

Process testing at the plant to confirm theresults of the plant evaluation phase includedoxygen transfer testing to determine the abilityof the existing aeration hardware to achievenitrification, clarifier stress testing, and

biological process simulation modelling. Inaddition, the evaluation phase suggested theplant flow meter accuracy was questionable.Therefore, flow meter calibration testing wasalso undertaken to confirm the validity of thehistoric flow and loading data.

The process testing demonstrated thatupgrades to the RAS pumping system, whichused air lift pumps that lacked controllabilityand operated at a high return rate, would beneeded to ensure adequate clarificationcapacity was available. Increased oxygentransfer capacity would also be required tosustain the nitrification needed to meet thenew effluent limits for ammonia. Upgrades to the raw sewage pump station and a newflow metering station were also included.To meet the non-toxic effluent requirementimposed on the plant, UV disinfection wasinstalled to eliminate the toxic chlorineresiduals associated with the originalchlorine disinfection.

The estimated cost to achieve a 27 percentincrease in capacity from 1,181 m3/d to 1,500 m3/d was $450,000. Of this total, morethan half was associated with the installationof UV disinfection that was a requirement of the new certificate of approval and notspecifically required to achieve additionalcapacity. The cost estimates to upgrade theplant are summarized in Table A–1. To achievethe higher capacity, no new tankageconstruction was needed except for the UVdisinfection system. The equivalent cost ofachieving the additional capacity was lessthan $700 per m3/d of capacity, exclusive of thecost of the UV disinfection installation. Equallyimportant to the low upgrade cost, additionaldevelopment in the community was allowedwithout extensive delays normally associatedwith major treatment plant construction.



A.2 Case History 2 – Ayr

WWTP (XCG, 2000b)

These upgrades have now been implementedand a new Certificate of Approval issued witha re-rated capacity of 1,500 m3/d

A.3 Case History 3 – Tillsonburg WWTP (Phagoo et al., 1996)

Nitrification in an activated sludge plant canresult in a significant increase in energy coststo provide the additional oxygen required bythe nitrifying bacteria to oxidize ammonia tonitrate. Denitrification, the reduction of nitrateto nitrogen gas under anoxic conditions, canrecover some of the bound oxygen present inthe nitrate. Typically, denitrification occurs in aseparate mixed reactor that is maintained atdissolved oxygen concentrations approachingzero to allow the process to occur. Thus,implementing denitrification can require asignificant capital investment.

In an optimized approach, aerators can becycled on and off within the same tank toprovide the oxygen supply needed to achieve

nitrification and to then recover the boundoxygen under non-aerated (anoxic) conditions.This optimized approach can reduce theoverall energy costs while at the same timeproducing an effluent lower in total nitrogenconcentrations.

A demonstration of on-off aeration wasconducted at the Tillsonburg, Ontario WWTPto determine the possible energy savings andthe impact on plant performance. TheTillsonburg WWTP is a conventional activatedsludge plant with a design capacity of 8,200 m3/d. It was ideally suited for thedemonstration since it contains two parallel,identical treatment trains (primary clarifiers,aeration basins and secondary clarifiers).Aeration and mixing in the biological reactorsare provided by coarse bubble diffusers, andair is supplied by fixed and variable speedpositive displacement blowers. Aeration DO concentrations are monitored andautomatically controlled.



A.2 Case History 2 – Ayr

WWTP (XCG, 2000b)

Table A–1

Summary of Upgrades and

Costs to Re-Rate the Ayr.

WWTP to 1,500 m3/d


Tillsonburg WWTP

(Phagoo et al, 1996)

Table A–1: Summary of Upgrades and Costs to Re-Rate the Ayr WWTP to 1,500 m3/d

Process Description EstimatedCapital Cost ($)

Raw wastewater pumping ■ Two raw lift pumps, each with 4,000 m3/d capacity

■ Pumps could either be two-speed or installed with onevariable frequency drive with switch gear to allow it to beused for either raw lift pump

80,000

RAS pumping ■ Self-priming centrifugal pumps

■ Firm capacity for 1,500 m3/d

55,000

Oxygenation capacity ■ Add two new 900 m3/h positive displacement blowers

■ Provide additional aeration basin diffusers

75,000

Disinfection ■ Upgrade to UV disinfection

■ Separate contact chamber with approximately 40 lowpressure lamps

■ New flow metering station consisting of rectangular weirand ultrasonic sensor

240,000

Total ■ Upgrades to provide 1,500 m3/d capacity 450,000

One of the two parallel trains was retrofitted toallow on-off aeration. Each train consisted oftwo aeration basins in series and the retrofitallowed either one or both basins to beoperated in the on-off mode. The controlsystem allowed the on and off cycle times tobe varied and included air bursts during the offcycle of varying frequency and duration toprovide mixing in the reactor. The two trainswere then operated under comparativeloading conditions in summer and winter toestablish the energy savings achieved and the impact, if any, on effluent quality.

The plant operated in the on-off aerationmode achieved poorer nitrification than theplant operated with continuous aeration;however, the test plant (on-off aeration) hadlower total nitrogen concentrations than thecontrol plant (continuous aeration). Theimprovement in total nitrogen removal was 39 percent when one of the two aerationbasins was cycled and 67 percent when bothbasins were cycled. The aeration savingswere 16 percent when air supply to one of thetwo tanks was cycled and 26 percent whenthe air supply to both tanks was cycled. Withone tank cycled, about nine percent of thesavings was due to the recovery of boundoxygen during denitrification while theremaining seven percent resulted fromimproved oxygen transfer efficiency due tothe lower dissolved oxygen present in thetank when the air on cycle was initiated.Similarly, when both tanks were cycled, about20 percent of the 26 percent aeration savingsresulted from the recovered of bound oxygenduring denitrification.

A.4 Case History 4 – Montréal WWTP(Forest, 2003)

The Wastewater Treatment Plant (WWTP) ofMontréal is the largest treatment plant in theProvince of Quebec, treating all wastewaterfrom the Island of Montréal with its 1.8 millioninhabitants since 1995. It is an enhancedprimary (physical chemical) treatment plantwith a maximum capacity of 88 m3/s (7,600MLD) and a dry weather flow capacity of 25m3/s (2,160 MLD). There are two interceptors,one on each shore of the island, whichintercept all the wastewater outfalls and directthe flow by gravity to the WWTP. Treatment atthe WWTP comprises addition of a metal salt(ferric chloride or alum) plus an anionicpolymer prior to clarification.

The north interceptor was the first to be putinto operation, in 1984. At that time, there were14 rectangular primary clarifiers, 91 m long x30 m wide x 4.6 m deep. When the southinterceptor was connected, it was anticipatedthat the flow would double, necessitatingconstruction of an additional 14 clarifiers. Priorto the expansion, studies were undertaken bythe Lasalle Hydraulic Laboratory in Quebec tosimulate the hydraulic patterns in a modelledclarifier and identify ways to modify it toincrease its capacity. Based on the modelling,two existing clarifiers were modified to allowtesting at full scale. Modifications included theaddition of a special vertical screen across thefull width at the clarifier entrance to avoid flowshort-circuiting, changing effluent horizontalcollectors at the clarifier exit from 60 cm to 76 cm diameter pipes with the addition of avent to increase flow capacity, and installationof exit holes on these collectors at the bottomto avoid scum suction. Full scale tests weredone in 1991 using two types of coagulant forphosphorus removal, ferric chloride and alum,and results proved conclusive. The hydrauliccapacity with the modified clarifiers increasedfrom 3.5 to 5.0 m3/s.



Tillsonburg WWTP

(Phagoo et al, 1996)


Montréal WWTP

(Forest, 2003)


All existing clarifiers were then retrofitted withthese modifications at a total cost of $1 million.As a result of the optimization work, it wasnecessary to construct only 7 additionalclarifiers instead of 14 as originally planned.The construction cost was $37 million. Thesavings resulting from the optimization of the clarifier capacity was estimated atapproximately $36 million.

Subsequently, optimization work at theMontréal WWTP focussed on reducingchemical costs. In 1994, the metallic salt (ferricchloride or alum) was added at the entranceof each of the 14 grit chambers. Based onlaboratory jar testing, the coagulant dosagewas increased during the time period whenloads to the WWTP were higher and reducedwhen loads had declined. In 1994, coagulantcosts represented $4 million per year on anoperating budget of $38 million.

In order to reduce costs, a processoptimization study was conducted in 1994. As an initial step, the coagulant dosing pointwas moved from the entrance to each of 14 grit tanks to a point at the front of the barscreens and air injection was added toimprove mixing at the dosage point. Thischange eliminated problems with unequalchemical dosages to individual grit chambersand remedied dosage control problems.

Secondly, analysis of venturi scrubber watercollecting particles (fly ash) in combustion gasin the incinerator air treatment system wasfound to contain significant concentrations ofphosphorus (about 0.2 mg/L). This scrubberwater recycle stream was discharged into theWWTP effluent at a flow rate of about 160 L/s,raising phosphorus concentrations in the planteffluent. In order to achieve the objective of0.5 mg/L total phosphorus in the plant effluent,it was necessary to operate at higherchemical dosages. A modification was madein the piping to return this venturi scrubberwater back to the front of the plant so thatit would be treated along with the rawwastewater for phosphorus removal.

After these modifications were complete, anautomated chemical dosage control systemwas installed as part of the implementation ofa plant-wide SCADA system. On-line turbidityand phosphorus analyzers were installed inthe plant to measure the characteristics ofraw wastewater in the North and Southinterceptors, the effluent from a settlingcolumn that simulates in five minutes theperformance of the full scale clarifiers, andthe effluent water at the WWTP discharge.The SCADA system sets initially the chemicaldosage based on the raw wastewater flowrate and characteristics as measured by theon-line analyzers. The dosage is then adjustedbased on the output of the on-line analyzersmonitoring the simulated effluent from thesettling column, and later fine-tuned based onthe on-line analyzers monitoring the treatedeffluent from the plant.

With all these modifications, an estimatedchemical dosing reduction of about 40 percentwas realized. At 2002 chemical coagulantcosts, this represent an annual saving of about$3 million per year on a total 2002 plantoperating budget of $46 million.



Montréal WWTP

(Forest, 2003)


B.1 Plant Hydraulics

Rapid changes in hydraulic load are causedby such things as intermittent pumped flows,the dormitory nature of the community, orcombined sewage systems. Excessivevariations in flow and load can affect theperformance of the whole plant. Theseissues can be addressed by operationalmodifications such as:

■ using a recycle system for variable flowcontrol;

■ using multiple smaller constant speed pumps;

■ replacing the constant speed pumps withvariable speed pumps or screw pumps;

■ operation of the constant speed pumps inan influent pump station at a lower flow rateover a longer period of time;

■ use of step feed and contact stabilizationmodes to alleviate the impact of excessiveinflow and infiltration (I/I) on suspendedgrowth treatment plants;

■ returning digester supernatant or otherconcentrated streams during low flow periods;

■ pump speed controllers and wet well levelcontrollers set to minimize the number ofpump starts and stops; and

■ providing system storage and real-time control.

Inflow and infiltration (I/I) can be majorsources of flow in wastewater systems. This impacts on system performance due toincreased flow through the system and higherdemand requirements from pumping stations.By implementing I/I reduction programs,wastewater flows requiring treatment canbe significantly reduced resulting in lowertreatment requirements and consequentresource (chemical and energy) savings. A best practice for I/I control/reduction hasbeen developed by the National Guide toSustainable Municipal Infrastructure:Innovation and Best Practices.

B.2 Preliminary Treatment

B.2.1 Screening

Inadequate screening can limit plantperformance and capacity, and greatlyincrease O&M requirements. Although manysmall WWTPs still use manually cleanedscreens, all plants should consider upgradingto automatically cleaned screens. Screencleaning should be automated based on headloss and operating time. Adding bypass linesaround screens for maintenance can alsoincrease process flexibility.

B.2.2 Grit Removal

The installation of longitudinal or transversebaffles or modifying air flow in aerated grittanks can improve performance. If gritproblems are occurring in a plant and the gritchamber appears to be adequately designed,the problem may be with the grit removalsystem. Components such as pumps, chainand flight conveyors, screw conveyors, orbucket elevators may be inadequatelydesigned, installed, or maintained.

B.3 Primary Treatment

The following modifications should beconsidered to improve the process efficiencyof primary treatment.

■ Add coagulants to an undersized primaryclarifier, or clarifier with high surface overflowrates (in excess of 40 to 60 m3/m2.d).

■ Relocate internal recycle or WAS flows.

■ Improve flow splitting and control.

■ Improve scum and sludge removal throughautomation.

B. Optimization Opportunities Through Process Modifications

B.1 Plant Hydraulics

B.2 Preliminary

Treatment



Appendix B: Optimization Opportunities Through Process Modifications

B.3.1 Reduce Chemical Usage

By improving the feed rate control and mixing ofchemicals at the point of addition, reducedchemical use will be achieved. There are manytechniques and products available to improvechemical addition and mixing. Among others,they include in-line flash mixers or high velocitymixing systems. Jar tests should be performedon a routine basis to determine the optimumchemical dosage and dosing procedure.

High velocity mixing systems (HVMS) can beused to replace the injector, injector pump,diffuser, mechanical mixer, filter, and strainerof traditional induction systems. The HVMSoperate with a propeller that injects thechemicals into the process stream at highvelocities for better mixing. As a result of thebetter mixing achieved with HVMS, significantchemical use reduction can be achieved.

B.3.2 Reduce Pre-Precipitation Chemicals for Phosphorus Removal

Chemicals can be added for phosphorusremoval at either the primary or secondarylevel of treatment. Generally, chemicals aremore efficient for phosphorus removal whenadded to secondary treatment, and chemicaluse savings can be achieved. Chemicals maystill be used in primary treatment to enhancebiological nutrient removal (BOD5) removal insome facilities, reducing energy use in thebiological system and secondary sludgeproduction. Multi-point chemical additionresults in the lowest chemical usage andsludge production when low effluentphosphorus concentrations must be achieved.

B.4 Biological Treatment

B.4.1 Inadequate Process Flexibility

If inadequate process flexibility is limiting thebiological treatment process performance orcapacity, piping and valving can be installedso aeration basins can be operated in thecomplete mix mode, the plug flow mode, thestep feed mode, or the contact stabilizationmode depending on flows, loads, and othercritical conditions.

Process equipment can be installed toincrease process flexibility. This includes:

■ the piping necessary to isolate individualtanks or processes;

■ variable speed aerators or blowers in theaeration basin(s);

■ variable speed sludge pumps for return andwaste sludge flow; and

■ chemical feed systems to improve settlingcharacteristics.

B.4.2 Nitrification

Ammonia, chloramines, and chlorinatedmunicipal effluents are considered to be toxicsubstances under the Canadian EnvironmentalProtection Act (CEPA). Many new permits nowinclude ammonia limits. Nitrification is thebiological conversion of ammonia into nitrate.Alkalinity control is important in activatedsludge systems designed for nitrification. If insufficient alkalinity is present during theconversion of ammonia to nitrate, the pH of thesystem drops, and nitrification may becomeinhibited. An adequate alkalinity adjustmentsystem must be in place to provide a residualalkalinity of 50 mg/L for aeration and 150 mg/Lfor high-purity oxygen systems (EPA, 1982).



B.4 Biological

Treatment


B.4.3 Biological Nutrient Removal Processes

BNR processes improve the nutrient removalcapability of the WWTP and may also result in other benefits, such as improved sludgesettlement, reduced sludge production,reduced process alkalinity consumption, andreduced process oxygen requirements. Thepotential reduction in plant capacity fromimplementing BNR needs to be considered.

A wide variety of BNR process configurationsare available. The process configurationselected must consider the effluent limits to beachieved and the current configuration of thebioreactors. It is also possible to create therequired anaerobic and/or anoxic zones byinstalling baffles in the existing tankage ifsufficient reactor volume and hydraulic gradientare available. Installation of mixing equipmentand reconfiguration of aeration system andrecycle pumping capabilities may be requireddepending on the BNR process selected.

B.4.4 Oxygen Transfer System

If a WWTP is experiencing inadequate oxygentransfer or if energy costs associated with theaeration system are to be minimized, methodsof reducing the organic loading should beinvestigated before major modifications aremade. Operational steps, such as cleaningdiffusers or removing rag accumulation onsurface mechanical aerators should also bepursued. If these measures do not improve theoxygen transfer capacity of the system, thefollowing modifications can be considered.

■ Install additional blowers to address anoxygen deficiency in a diffused aerationsystem if higher flow per diffuser isacceptable.

■ Upgrade the diffused air system byreplacing a mechanical system with adiffused air system, or replacing a lowefficiency diffused aeration system with ahigher efficiency system.

■ Upgrade the mechanical aerator byrefurbishing the old aerator cones, modifyingaerator submergence, and operating allaerators at a higher rotational speed.

■ Rearrange the aerator or diffuser spacing to remove dead zones and improve mixing.

■ Increase the horsepower of existingblowers or mechanical aerators.

■ Install baffles or mechanical mixing devicesto improve basin mixing.

■ Install/check air filters on the intake sideof blowers.

■ Supplement aeration systems withadditional diffusers, or by alternative means.

■ Inspect/maintain/repair the diffusers anddelivery piping.

If the existing system must be upgraded orreplaced as part of the plant upgrade, thefollowing list outlines the best practice toupgrade an existing oxygen transfer system.

■ Examine the condition of the existingoxygen transfer system.

■ Determine the efficiency of the existingsystem through oxygen transfer testing.

■ Calculate an estimate of existing systemcapacity, based on the efficiency of theexisting system.

■ Estimate the efficiency of alternative oxygentransfer systems.

■ Determine whether evaluation of upgradealternatives is necessary and desirable.

■ Evaluate alternatives and select the mostdesirable alternative.

■ Evaluate options for implementing theselected alternative.

■ Implement oxygen transfer systemimprovements.

■ Install an automatic dissolved oxygensystem to vary air input according to thebasin dissolved oxygen level to reduceenergy consumption.



B.4 Biological

Treatment

B.4.5 Cold Climate Operation

Cold wastewater temperatures result indecreased microbial activity and lowertreatment efficiencies. To prevent freezingproblems and minimize the effect of coldtemperatures on biological treatmentefficiency, covers can be placed over opentanks, and an earthen berm can beconstructed to insulate above-ground tanks.The principles discussed for optimization inthis best practice are applicable to WWTPs in any climatic condition.

B.5 Secondary Clarifiers

B.5.1 Clarifier Modifications

Modifications that have proven effective inimproving the performance and capacity ofclarifiers at existing wastewater treatmentplants include the following (Daigger andButtz, 1992).

■ Influent flow splitting can be implementedwhen the full capacity of existingclarification units is not used due to anunequal and uncontrolled flow split. Severaltechniques are available, including flowsplitting using multiple weirs, or orificeswith a flow meter and flow control valve onthe influent to each treatment unit.Hydraulic analysis is required to verify thatadequate head is available and to design an effective system.

■ Rapid flow variations are generated when a constant speed pump either turns on orturns off. Variable speed pumping can beimplemented to smooth out and control flowvariations. One method of variable speedpumping is to provide adjustable speedpumps with the number of pumps and theirspeed determined by fluid level in anupstream wet well. Constant speed pumpscan also be coupled with recycle of pumpedflow in excess of the influent flow back tothe pump wet well. It is noted thatimplementation of a variable speed pumpingsystem can increase the mechanicalcomplexity of the plant and result inincreased O&M costs.

■ An appropriately sized flocwell can beincluded in the clarifier to minimize theoccurrence of dispersed suspended solidsin the effluent.

■ Inlet baffles can be used to dissipate energycontained in the influent flow, and todistribute flow for uniform entry into theclarifier. For circular clarifiers, a ring bafflesupported off the sludge collectionmechanisms has also proven useful indissipating inlet energy and disrupting thedensity current. Outlet baffles are useful todirect high solids streams away from theclarifier effluent withdrawal point. Twotypes of effluent baffles are commonly used:McKinney baffle, which is horizontal inorientation and located just below theeffluent weir, and the Stamford baffle, whichis oriented at a 45 degree angle and isgenerally placed lower on the clarifiersidewall.

■ Tube or plate settlers act as shallowclarifiers and improve the performance ofexisting clarifiers by increasing the effectivearea for clarification. The hydraulic flowpattern within the clarifier can also bepartially modified to improve performance.Tube settlers are not effective for sludgethickening.

■ Separate WAS and RAS pumps with flowmeters provide the flexibility to optimizeeach function.

■ Polymer can be added to enhance settlingcharacteristics of sludge.

■ Implementation of rapid sludge withdrawalsystems can reduce sludge blanket levels inclarifiers, preventing blanket washout athigh flows.

■ If the ability of sludge to settle is a cause of reduced clarifier capacity, theimplementation of a selector zone toenhance settlement should be considered.

Before adding clarifiers at high capital cost,these optimization measures should bethoroughly investigated.

Wahlberg (1998) has developed a protocol that can be used to optimize clarifiers.



B.4 Biological

Treatment

B.5 Secondary

Clarifiers

B.5.2 Excessive Clarifier Hydraulic Currents

Dye testing can be used to identify excessivehydraulic currents. Modifications used tocorrect hydraulic current problems includeinlet modifications to achieve both horizontaland vertical distribution of the incoming flowacross the entire cross-sectional area, whileminimizing short circuiting and turbulence bythe addition of inlet or outlet baffles, or weirrelocation/addition and blanking off cornerweirs. If short circuiting or a sludge densitycurrent is observed, baffling should beprovided to prevent short circuiting and poorsolids removal. Baffles and flow deflectors canalso provide equal flow distribution across thewidth of the clarifier.

B.5.3 Sludge Bulking Control

A common misconception associated with the performance of clarifiers in a suspendedgrowth system is that solids loss is the resultof a clarifier failure, when, in fact, it is oftendue to poor sludge settling characteristics.The presence of excessive quantities offilamentous micro-organisms can cause apoorly settled biomass. By improving thesettling characteristics of the sludge, themixed liquor suspended solids (MLSS)concentration that can be maintained in thesystem is increased, which allows an increasein the organic loading on the system, resultingin the opportunity to increase plant capacitywithout increasing the basin volume. For anitrifying system, an increased MLSSconcentration allows nitrification to beaccomplished at shorter HRTs. Alternatively,higher hydraulic loadings can be applied to thesecondary clarifiers. Several sludge bulkingcontrol measures are available including:

■ chlorinating the return activated sludge ormixed liquor in the reactor;

■ modification of the environmentalconditions (e.g., addition of nutrients,including nitrogen, phosphorus, anddissolved oxygen);

■ introduction of an organic loading gradientthrough addition of a selector to thesuspended growth system;

■ implement selective wasting to removefoam/scum-causing microorganisms fromthe system;

■ remove impediments to the free passage of foam/scum through the bioreactor/secondary clarifier system to a point wherethe foam/scum can be eliminated from thesystem; and

■ discontinue the practice of co-settling ofwaste activated sludge in the primarytreatment system.

Microscopic examinations should beperformed routinely to monitor biomass forsludge bulking due to filamentous organisms.The methods are described in Manual on theCauses and Control of Activated SludgeBulking, Foaming, and Other SolidsSeparations Problems (Lewis Publishers, 2003)and in Dynamic Corporation (USEPA,1987),along with options to control sludge bulking.

B.5.4 Inadequate Return Sludge and WasteSludge Flexibility

According to Assessment of Factors Affectingthe Performance of Ontario Sewage TreatmentFacilities (XCG, 1992), the lack ofinstrumentation to measure return sludge andwaste sludge flow rates was the most seriouslimitation at small WWTPs with air lift sludgereturn systems. Without the knowledge ofthese flow rates, it is difficult to adjust forchanges in flow or settling characteristics, or to control solids inventory in the plant.


B.5 Secondary

Clarifiers


Return sludge flow is used to control thedistribution of sludge between the aerationbasin (also referred as bioreactor) and theclarifier. Return sludge flexibility is importantto address adverse process conditions on atimely basis. If insufficient or inflexible sludgereturn pumping capacity is limiting the plantperformance, auxiliary sludge pumping andpiping can be added or, alternatively, theimpeller and/or motor size of the existingsludge pumps can be increased. Possiblemodification to improve RAS flexibility include:

■ recycling flow around a constant speedpump;

■ using pumps with adjustable speed drives;

■ installing time clocks to control valveoperations (airlift pumps);

■ using multiple pumps for RAS pumping; and

■ providing continuous flow measurementcapability.

By adjusting return sludge rates, a facility canmaintain optimal sludge blanket levels in thesecondary clarifier. This reduces RAS pumpingrates and energy use.

To increase sludge wasting flexibility, separatewaste and return sludge pumps can beprovided to optimize each function. In small tomedium-sized plants, positive displacementpumps are typically the most appropriate.Variable speed drives, timers, or a combinationof both can provide the needed flexibility.

The waste sludge removed is typicallydirected to a sludge treatment facility, such asthickening, digestion, and dewatering, beforefinal disposal. The operation of the biologicalprocess should not have to be modified,because of limitations of the sludge wasting,treatment, and disposal facilities.

B.6 Tertiary Filtration

Granular media filtration has been used tocontrol suspended solids and phosphorusdischarges from WWTPs. A variety of factorsaffect the filter performance, including the sizeand nature of the particles to be removed,filtration rate, media size and type, and beddepth. The effluent quality is a function of theupstream biological treatment process, theuse of chemical pre-treatment prior tofiltration, and the filter itself. Enhancedremoval of TP and TSS can be achievedthrough polymer addition and/or dual pointcoagulant application (i.e., application ofcoagulant prior to settling and to thesecondary effluent prior to tertiary filtration).

By performing backwashing during off-peakhours, energy costs associated with operatingpumps will be reduced due to lower unitcharge rates; however, there will be noreduction in energy or water use. The ability toperform off-peak filter cleaning is influencedby the available storage and effluentconcentrations. Alternately, elevatedbackwashing storage can be used to storeduring low demand periods for use duringpeak periods.

B.7 Disinfection

A number of processes can be used fordisinfection. The most common one ischlorination. As chloramines and chlorinatedmunicipal effluents are considered to be toxicsubstances under the Canadian EnvironmentalProtection Act (CEPA) (CWWA, 2003), whenchlorine is used, it is frequently necessary toremove excess chlorine through the use of adechlorinating agent once acceptable levelsof pathogen reduction have been achieved.Disinfection by ultraviolet irradiation hasbecome popular in recent years, because of lowoperating costs to achieve a non-toxic effluent.



B.5 Secondary

Clarifiers

B.6 Tertiary Filtration

B.7 Disinfection

A variety offactors affect the

filter performance,including the sizeand nature of the

particles to beremoved, filtration

rate, media sizeand type, and bed

depth.

B.7.1 Chlorination/Dechlorination

Flow proportional and/or residual chlorinecontrol of the chemical addition to meetrequirements will prevent excessive chemicaluse. This will reduce chemical use duringperiods of the day when flows are lower orchemical requirements are not high. Byimproving mixing, the effectiveness of thechemical addition is maintained with reducedchemical input.

Other factors that can impact theeffectiveness of the disinfection include shortcircuiting, the applied chlorine dosage, andlength of contact time. A dye tracer study can be used to identify the extent of shortcircuiting and the ratio of actual to theoreticalcontact time. Baffling can be installed tofacilitate plug flow. Initial mixing should bevery rapid and thorough. Diffusers can berelocated to a location with more turbulence.Some options for improving mixing includesupplemental mixers or high velocity mixingsystems. The required dosage will varydepending on water quality, mixing conditions,temperature, pH, contact time, and the levelof disinfection required. The amount ofdechlorination agent depends on the appliedchlorine dosage.

Flexibility in chlorination/dechlorinationprocesses can be enhanced by providingmultiple contact chambers and a chemicaladdition system, as well as piping and valvingnecessary to isolate a contact chamber and/orchemical feed systems (e.g., chlorinator) formaintenance purposes.

B.7.2 Ultraviolet Irradiation

Inadequate maintenance and cleaning canreduce UV system performance. Ultraviolettubes and lamps must be cleaned frequentlyand lamps replaced on a regular basis tomaintain a high level of radiation intensitytransferred to the wastewater. The use of ironsalts in the process for phosphorus removalcan increase the frequency of lamp cleaningin manually cleaned systems since theresidual iron in the effluent can deposit on thelamp sleeve. If this is problematic, alum couldbe used instead of iron salts. Weirs and bafflescan be used in the UV reactors to distributethe flow evenly through UV reactors and lampspacing can be adjusted.

B.8 Sludge Thickening and Dewatering

The optimization goal for sludge thickeningand dewatering processes, whether by gravityor mechanical means (dissolved air flotation,gravity belt, rotary drums, centrifuges, beltfilters or filter presses) is to obtain maximumsludge concentrations at maximum hydraulicloadings while achieving satisfactory solidscapture and minimizing chemical dosage.Regardless of the unit process applied, jartesting is essential to ensure that the properchemical is used at the optimum dosage. The frequency of jar testing depends on thevariability of the sludge being processed.

Chemical dosage control based on solids massloading to the thickening unit is also important.Thickening processes handling WAS that canvary significantly and quickly in strengthrequire more frequent adjustment of chemicaldosages than dewatering processes handlingdigested sludges from a well mixed, longretention time digester. Automation ofchemical feed systems in sludge thickeningand dewatering has been shown to bebeneficial in terms of reducing chemicaldosages, improving capture, and producingmore consistent cake and centrate/filtratequality (WERF, 2001); however, maintaining theinstrumentation required to automate theseprocesses is time-consuming.


B.7 Disinfection

B.8 Sludge Thickening

and Dewatering


Sludge thickening and dewatering processesare very sensitive to variations in flow andmass loading. Providing equalization facilitiesupstream of thickening and dewateringprocesses to minimize variations in feedstrength and flow will result in improvedperformance.

B.9 Sludge Digestion

Anaerobic digesters commonly are poorlymixed (Monteith and Stephenson, 1981) dueto the accumulation of scum, grit and othermaterial and relatively low energy inputs.Improving the mixing in the digesters throughretrofitting mechanical mixers for gas mixingor cleaning the digester to remove theaccumulated material will often significantlyimprove digester performance. Althoughaerobic digesters are more intensively mixedin order to ensure that adequate oxygen isavailable to the micro-organisms, heavy gritand scum can also accumulate in thesereactors, reducing available reactor volume.Tracer tests should be done to evaluate themixing characteristics in both anaerobic andaerobic digestion tanks and to assess thebenefits of upgraded mixing.

Poor flow or mass loading distribution amongmultiple digestion tanks can overload orunderload some reactors. Automation of feedcycles and on-line monitoring of raw sludgeconcentrations from settling tanks orthickeners will prevent hydraulic overloadingassociated with pumping thin sludge.

The methane-forming bacteria in anaerobicdigesters are very temperature sensitive.Temperature variations of more than 0.5 to 1.0°C should be avoided and automatedtemperature control is preferred. Frequentpumping of raw sludge into the digester insmall volumes prevents the temperaturechanges associated with the addition of largevolumes of cold sludge.

The rate of bacterial activity in aerobicdigestion processes slows significantly at low temperatures and almost stops attemperatures below 10°C (WEF, 1990). At lower temperatures, it is important to providelonger solids retention times in the process toachieve adequate volatile solids destructionand sludge stabilization.

Operation of both aerobic and anaerobicdigesters at thermophilic temperatures (50 to60°C) results in greater volatile solidsdestruction and increased pathogen reductionat shorter retention times. However, operatingexisting mesophilic reactors at thermophilictemperatures requires significant upgrades toexisting works and is not considered to bewithin the scope of optimization. Similarly,there are a number of new, innovative sludgetreatment processes (WERF, 1998) thatproduce a better quality biosolids stream andshould be considered in the design of new orexpanded facilities, but are outside the scopeof optimization of existing works.


B.8 Sludge Thickening

and Dewatering

B.9 Sludge Digestion


ASCE (American Society of Civil Engineers), 1997.

ASCE Standard Guidelines for In-Process OxygenTransfer Testing. Reston, Virginia.

CWWA (Canadian Water and Wastewater

Association), 2003. “Memorandum: Environment

Canada – Plans for Pollution Prevention Planning for

Municipal Wastewater Effluents – Consultation

Workshops.”Ottawa, Ontario.

<http://www.cwwa.ca/mwwe.htm>.

Cathcart, W.W. and T.E. Leonik, 1995. “Retrofitting a

1980’s Wastewater Treatment Facility with a 1990’s

Programmable Logic Controller System – A Case

Study.” WEF Specialty Conference Series Proc.Minneapolis, Minnesota.

Crosby, R.M., 1987. The Flow Pattern Test for ClarifierHydraulic Analysis. Crosby and Associates, Inc.

Winter Park, Florida.

Daigger, G.T. and J.A.Buttz, 1992. UpgradingWastewater Treatment Plants. Water QualityManagement Library. Vol. 2. Technomic Publishing Co.

Inc. Lancaster.

Dynamic Corporation, 1987. “Summary Report – The

Causes and Control of Activated Sludge Bulking and

Foaming.” Prepared for U.S. Environmental Protection

Agency, July. Washington, DC

Ekama, G.A., J.L. Barnard, F.W.Gunthert, P. Krebs, J.A.

McCorquodale, D.S. Parker, and E.J. Wahlberg.

“Secondary Settling Tanks: Theory, Modelling, Design

and Operation,” IAWQ Scientific and Technical Report

No. 6, 1997. ISBN 1 900222 03 5. London, UK

EPA (Environmental Protection Agency), 1979a.

Evaluation of Operation and Maintenance FactorsLimiting Biological Wastewater Treatment PlantPerformance. EPA-600/2-79-078. Washington, DC.

____ 1979. Evaluation of Operation and MaintenanceFactors Limiting Municipal Wastewater TreatmentPlant Performance, Phase II, EPA-600/2-80-129.

____, 1982. Handbook for Identification and Correctionof Typical Design Deficiencies at MunicipalWastewater Treatment Facilities, EPA-625/6-82-007.

____, 1984. Handbook: Improving POTW PerformanceUsing the Composite Correction Program Approach,EPA/625/6-84-008.

____, 1985. Handbook for Improving POTWPerformance, Water Pollution Control Federation.

____, 1989. Handbook Retrofitting POTWs, EPA-625/6-

89-020.

EPRI (Electric Power Research Institute), 1996. Waterand Wastewater Industries: Characteristics andEnergy Management Opportunities, EPRI’s Community

Environmental Center. Palo Alto, California.

Forest, R., Ville de Montréal, Personal

Communication, 2003.

Irrinki, S., Foess J., and Dudley J., 2002. “Plant

Optimization Tools in the New Millennium.” Poster.

Proc. Water Environ. Fed. 75th Annu. Conf. Exposition[CD-ROM], Chicago, Illinois.

Jenkins, D., G.R. Michael, T.D. Glen, 2003. “Manual

on the Causes and Control of Activated Sludge

Bulking, Foaming, and Other Solids Separation

Problems” 3rd edition, Lewis Publishers (available

through CRC Press, Ottawa, Ontario).

Keinath, T.M., 1985, “Operational Dynamics of

Secondary Clarifiers,” Journal – Water Pollution

Control Federation, 57(7), 770. Alexandria, Virginia.

MOEE (Ministry of Environment and Energy),

Ontario, 1996. Managers Guide to SewageTreatment Plant Optimization.

____, 1997. Guide to Resource Conservation andCost Savings Opportunities in the Water andWastewater Sector.

Monteith, H.D. and Stephenson J.P., 1981. “Mixing

Efficiencies in Full-Scale Anaerobic Digesters by

Tracer Methods,” J.Wat. Pollut. Con. Fed., 53(1), 78-84.

Phagoo, D.R., M. Jones, and A. Ho, 1996.

“Demonstration of On/Off Aeration at a Municipal STP

in Ontario for Denitrification and Energy Savings:

Status Report.” Presented at the 25th Annual WEAO

Technical Symposium, Toronto, Ontario.

References


References

Rowland, I. and R. Strongman, 2000. “Southern Water

Faces the Small Works Challenge.” Wat. Sci. Tech.

Vol. 41(1): 33-39.

Spiers, G.W. and J.P. Stephenson, 1985. “Operational

Audit Assists Plant Selection for Demonstration of

Energy Savings and Improved Control,” and

“Instrumentation and Control for Water and

Wastewater Treatment and Transport Systems.”

Proceedings of the 4th IAWPRC Workshop. Edited by

R.A.R. Drake. Houston, TX and Denver, CO. April 27

to May 4, 1985.

Wahlberg, E.J., 1998, “Clarifier Capacity Assessment

and Optimization Protocol.” Presented at the WERF

Workshop on Formulating a Research Program for

Debottlenecking, Optimizing and Rerating Existing

Wastewater Treatment Plants.

Wahlberg, E.J., D.T. Merritt, and D.S. Parker, 1995.

“Troubleshooting Activated Sludge Secondary Clarifier

Performance Using Simple Diagnostic Tools,”

Proceedings of the 68th Annual WEF Conference and

Exposition, Miami, Fa.

WEAO (Water Environment Association of Ontario),

1996. Guidance Manual for Sewage Treatment PlantLiquid Train Process Audits. Milton, Ontario.

____ Manual, 1996. Training Operators on ProblemSolving Skills. Milton, Ontario.

WEF (Water Environment Federation), 1990. “Operation

of Municipal Wastewater Treatment Plants, Manual of

Practice #11.” Alexandria, VA.

____, 1995. “Automating to Improve Water Quality.”

WEF Specialty Conference Series Proceedings.Minneapolis, Minnesota, June.

WEF, 1997a. Automated Process Control Strategies.

A Special Publication. Alexandria, VA.

____, 1997b. Benchmarking Wastewater Operations:Collection Treatment and Biosolids Management,Report 96-CTS-ST. Alexandria, VA.

WERF, 1998. “Biosolids Management: Assessment of

Innovative Processes,” Project Number 96-REM-1.

Alexandria, VA.

____, 2001. “Thickening and Dewatering Processes:

How to Evaluate and Implement an Automation

Package,” Project Number 98–REM-3. Alexandria, VA.

Wheeler, G.P. and B.A. Hegg, 1999. “Upgrading Existing

Secondary Clarifiers to Enhance Process

Controllability to Support Nitrification.” WEFTEC.

XCG Consultants Ltd., 1992. Assessment of FactorsAffecting the Performance of Ontario SewageTreatment Facilities. Ontario

____, 2000a. Optimization and Capacity AssessmentStudy of the Duffin Creek WPCP.

____, 2000b. “Ayr Wastewater Treatment Plant

Process Review.” Report submitted to the Regional

Municipality of Waterloo, January.

____, 2002. “Assessment of the Potential Capacity of

the Orangeville WPCP.” Technical Memorandum No. 1.

XCG Consultants Ltd. and Hydromantis Inc., 2000.

“Class EA Project File Ayr Wastewater Treatment

Capacity Increase.” Draft.

References


optimization of wwtp excellent)

Documents