Combined Schemes; Pros & Cons,or, “when to use”? Christos Anastasiou, PhDDepartment of Civil & Environmental EngineeringFaculty of Engineering & Architecture

MEDAWARE

Outline

“Definition” of a “Combined Scheme”

Combination of Treatment Methods

Co-treatment of Wastewaters

Centralized vs. Decentralized Systems

Treatment Technologies (advantages/disadvantages)

What is a “combined scheme”?

Combined Sewerage Systems (Combined Overflow Systems)?

Combination of Unit Processes and Treatment Methods?

Combination of Waste Streams (Co-treatment)?

Centralized vs. Decentralized Treatment Systems?

Sanitary sewer systems may either exist separate from storm-water sewers, or a single set of pipes may be used to carry both types of wastewater in a combined sewer system.

Old technology (prior 1960’s) – Combined systems are no longer built, but still exist in many communities. Combined sewer systems typically bypass part of the flow during

periods of high runoff.

What is a “combined scheme”?Combined Overflow Systems

Conventional Treatment Unit Processes & Methods

Physical/Chemical ProcessesDilution, screening, mixing, flocculation,

sedimentation, flotation, aeration, filtration, precipitation, coagulation, chemical oxidation, chemical stabilization.

Biological MethodsSuspended (e.g. AS, oxidation ditch, SBR) &

attached growth (e.g. trickling filters, RBC) aerobic, anaerobic suspended & attached growth (e.g. CSTR, PFR, UASB, packed reactors).

Advanced Treatment Unit Processes & Methods

Filtration processes (i.e. depth, surface, micro & ultrafiltration, and reverse osmosis)

Electrodialysis Adsorption Gas Stripping Ion Exchange Advanced Oxidation Processes - chemical Distillation

Why do we need to use Advanced Treatment?

To achieve further organics and TSS treatment to meet more stringent standards or to allow for better disinfection

To remove more nutrients (beyond what conventional methods allow) To remove specific organic and inorganic constituents

Basically, Wastewater Constituents:

Conventional, non-conventional, and emerging Conventional Treatment for Conventional Constituents

ANDAdvanced Treatment for Non-conventional Constituents

(Emerging constituents removal occurs in both but not well quantified)

1 = <25% removal of influent concentration2 = 25-50%3 = >50%

Potential for Contaminant Removalof various Unit Processes and Operations

Constituent Primary AS BNRTrickling

FilterCoag.

Floc-Sed.ActivatedCarbon

Filtration(after AS)

RO

BOD/COD 2 3 3 3 3 3 2 3

TSS 3 3 3 3 3 3 3 3

Nitrogen 2 1 3 1 2 2 3

Phosphorus 1 2 3 3 3 3 3

Alkalinity 2 2 3

TotalColiform

3 3 1 3 3

TDS 3

Turbidity 2 3 3 2 3 3 3 3

TOC 2 3 3 2 3 3 2 3

Combination of Unit Processesor Treatment Methods

Why Combine Methods? Complete abatement of pollutants cannot be achieved

by a single process for certain waste streams

Example: Combined Aerobic Treatment Processes (i.e.

Coupling of Trickling Filter and Activated Sludge treatments) are often used to Upgrade an existing AS system Reduce the strength of WW, if we have a combination of industrial

& domestic waste To protect a nitrification AS process from toxic or inhibitory

substances

Necessity of Combination of Treatment MethodsExample: Olive Oil Wastewater

Some characteristics of the wastewater: high organics, high BOD:N:P ratio, low pH, high conductivity, high polyphenols – toxic, high SS)

TreatmentCapital Cost

($/m3day)Energy required

(kWh/m3)Drawbacks

Combustion 5,000 678Destruction of recoverable

organics

Single effect distillation 11,000 750Post-treatment of distillate

(~2.5Kg COD/m3day)

Activated Sludge 20,000 30Dilution water. Nutrients

Addition. Sludge Disposal

Trickling Filters 10,000 15 (as Activated Sludge)

Anaerobic Degradation 4,000 <1Dilution water.Long start-up

Necessity of Combination of Treatment MethodsExample: Animal Waste

Barham Farm (4000 swine), North Carolina, USA Ambient Temperature Anaerobic Digester (with energy production);

Nitrification Tanks; Old Anaerobic Lagoon (denitrification); Greenhouses.

Necessity of Combination of Treatment MethodsExample: Animal Waste

Anaerobic Digester

Nitrification Tanks

GreenhousesSprayfield

Necessity of Combination of Treatment Methodsmore examples

Slaughterhouse waste Landfill Leachate etc.

Nevertheless,The simplest possible method is still desirable!!

What is a “combined scheme”?Combination of Waste Streams (co-treatment)

Why combine waste streams?Convenience, for example

Quantity of waste streams (economies of scale)

Proximity of waste sources to one another

Enhancement of treatment, for exampleto balance the pHTo add necessary nutrients

What is a “combined scheme”?Combination of Waste Streams (co-treatment)

ExamplesAnimal Waste & Organic Solid WasteMunicipal Wastewater & Landfill LeachateMunicipal Wastewater & Organic Solid WasteVarious Industrial Wastewaters

Example: Ypsonas Industrial Effluent Treatment Plant (Limassol, Cyprus) – wastewater received from

Potato chip factoryAluminum processingTextile factoryAnd others…

What is a “combined scheme”?Centralized vs. Decentralized Treatment Systems

Current “conventional” practice: Design of larger treatment systems (>3500 m3/day)

Capture of economies of scale

However, small or rural communities have different characteristics and needs Bringing wastewater from many small sources to one

single location for treatment may not always be the best option.

Decentralized Treatment SystemsWHERE to consider (according to USEPA)? Where the operation and management of existing onsite systems must be

improved Where individual onsite systems are failing and the community cannot afford

the cost of a conventional wastewater management system Where the community or facility is remote from existing sewers Where localized water reuse opportunities are available Where fresh water for domestic supply is in short supply Where existing wastewater treatment plant capacity is limited and financing is

not available for expansion Where, for environmental reasons, the quantity of effluent discharged to the

environment must be limited Where the expansion of the existing wastewater collection and treatment

facilities would involve unnecessary disruption to the community Where the site or environmental conditions that require further wastewater

treatment or exportation of wastewater are isolated to certain areas Where residential density is sparse Where regionalization would require political annexation that would be

unacceptable to the community Where specific wastewater constituents are treated or altered more

appropriately at the point of generation

Decentralized Treatment SystemsWHAT to consider?

Reliable, stable and robust process

Simple operation & management

Minimal or no need of chemicals

Minimal or no need of external power supply

Local availability of spares

Typical wastewater treatment options for small and decentralized systems

Wetland in Thessaloniki, Greece

Aerated lagoon system, USA

Schematic of Septic Tank

Technologies for the Treatment of WastewaterSome final words…

Each situation is different and needs to be given dual consideration, different alternatives exist for each system

from small scale households to large scale centralized one.

Nevertheless, there is a clear trend toward intensive treatment technologies (a trend that is exacerbated by the preference of engineering consultants and contractors).

Instead, more attention should be given, where appropriate, to properly designed lower-cost, simpler to operate

processes as well as to decentralized technologies. These should be adopted depending on the influent wastewater

and on the desired effluent quality.

Also, whenever feasible, a reuse component should be included for all new wastewater treatment projects

END OF PRESENTATION

Constituents in Reclaimed Water Conventional (measured in mg/L; used in designing conventional WWTPs)

TSS BOD; COD TOC Nitrogen (Ammonia; Nitrate; Nitrite) Phosphorus Microorganisms: Bacteria; Viruses ; Protozoan cysts & oocysts

Non-conventional (to be removed or reduced by advanced treatment processes) Refractory organics VOC Surfactants Metals TDS

Emerging (measured in μg/L; long-term health concerns possible; not easy to remove) Pharmaceuticals Antibiotics (veterinary & human) Home-care, industrial, and household products Hormones (steroids) and Endocrine Disrupters