eea - sludge treatment and disposal

Management Approaches and Experiences

Sludge Treatment and Disposal

By ISWAs Working Group on Sewage & Waterworks SludgeAlbrecht R. Bresters, The Netherlands

Isabelle Coulomb, FranceBela Deak, Hungary

Bernhard Matter, SwitzerlandAlice Saabye, DenmarkLudivico Spinosa, Italy

dne . Utvik, Norway

Lise Uhre, Project Co-ordinatorInternational Solid Waste Association

Paolo Meozzi, Project ManagerEuropean Environment Agency

Environmental Issues Series no. 7


1997 European Environment Agency

LEGAL NOTICENeither the European Commission, the European EnvironmentAgency, the International Solid Waste Association, nor any person orcompany acting on their behalf is responsible for the use which maybe made of the information in this publication. The contents of thispublication do not necessarily reflect the official opinions of theEuropean Community, its institutions, or the international organisa-tions and individual countries involved in preparing this report. Thedesignations employed and the presentation of material in thispublication do not imply the expression of any opinion whatsoeveron the part of the European Community or the European Environ-ment Agency concerning the legal status of any country, territory,city or area or its authorities.

ALL RIGHTS RESERVEDNo part of this publication may be reproduced in any form or by anymeans electronic or mechanical, including photocopying, recordingor by any information storage retrieval system without the permissionin writing from the copyright holder and publisher.

Cover and layout:Folkmann Design &Promotion

ISBN 87-90402-05-7

European EnvironmentAgencyKongens Nytorv 6DK-1050 Copenhagen KDenmarkTel. (+45) 33 36 71 00Fax (+45) 33 36 71 99E-mail: [email protected] page: http://www.eea.eu.int

ISWALderstrde 9, 2.DK-1201 Copenhagen KDenmarkTel. (+45) 33 91 44 91Fax (+45) 33 91 91 88E-mail: [email protected]

1. Introduction ............................................................................. 9

2. Background ............................................................................ 10

3. Sludge characterisation ......................................................... 13

3.1. Introduction ..................................................................... 13

3.2. Characterisation Parameters ........................................... 13

3.2.1. Agricultural Use ............................................................ 14

3.2.2. Composting .................................................................. 14

3.2.3. Incineration ................................................................... 14

3.2.4. Landfilling ..................................................................... 14

3.3. Stabilisation ..................................................................... 14

3.4. Solid-Liquid Separation ................................................... 15

3.5. Concluding Remarks ........................................................ 15

4. Transportation and storage .................................................. 17

4.1. Introduction ..................................................................... 17

4.2. Storage ............................................................................ 17

4.2.1. Operation Description .................................................. 17

4.2.2. Equipment .................................................................... 17

4.3. Transportation .................................................................. 18

4.3.1. Operation Description .................................................. 18

4.3.2. Equipment .................................................................... 19


5. Agricultural Use ..................................................................... 20

5.1. Conditions for Agricultural Use ....................................... 20

5.1.1. Sludge Types and Sludge Quality ................................. 20

5.1.2. Legislation ..................................................................... 20

5.2. Description of Agricultural Use ....................................... 20

5.3. Advantages/Disadvantages of Agricultural Use .............. 21

5.4. Financial Estimate ............................................................ 21

6. Composting ............................................................................ 22

6.1. The Influence of Variable Conditions

in Defining the Composting Process ...................................... 22

Table of contents


6.2. General Description of the Process

of Composting and Plant Conception Factors ....................... 23

6.3. Advantages/Disadvantages of Composting ........................

Sludges ................................................................................... 24

6.4. Financial Estimates .......................................................... 25


7. Drying ..................................................................................... 27

7.1. Introduction ..................................................................... 27

7.2. Conditions for Drying of Sludge

- Amounts and Drying Rates ................................................... 27

7.2.1. Quality .......................................................................... 28

7.3. Drying and Dryers ............................................................ 28

7.3.1. Indirect Dryers .............................................................. 29

7.3.2. Direct Dryers ................................................................. 29

7.4. Advantages/Disadvantages of Drying ............................. 30


8. Incineration (Vitrification, Co-incineration) .......................... 31

8.1. Designated Sludge Incinerators -

a General Description ............................................................. 31

8.2. Co-incineration - a General Description .......................... 33

8.3. Advantages and Disadvantages ...................................... 34



9. Landfilling ............................................................................... 36

9.1. Conditions for Application .............................................. 36

9.1.1. The Character and Treatment of Sludge

before Final Disposal on Landfill ............................................ 36

9.1.2. Regulations Governing the Construction

of Sludge Disposal Sites ......................................................... 37

9.2. Practical Solutions ............................................................ 37

9.2.1. Mono-deposits .............................................................. 38

9.2.2. Mixed Deposits ............................................................. 38

9.3. Evaluation of Cost Efficiency ........................................... 41

4

510. New Technologies (Gasification, Wet Oxidation) ................ 42

10.1. Gasification of Sludge.................................................... 42

10.1.1. The Gasification Process ............................................. 42

10.2. Wet Oxidation ............................................................... 42

10.3. Plasma Pyrolysis ............................................................. 42

11. Environmental Impact Assessments ..................................... 44

11.1. General .......................................................................... 44

11.2. Set-up of the EIA ........................................................... 44

11.3. Procedure ...................................................................... 45

12. How to Decide on Sludge Disposal ...................................... 46

12.1. Legal Boundaries ........................................................... 46

12.2. Costs .............................................................................. 46

12.3. Decision Tree ................................................................. 46

13. Appendices ............................................................................ 49

13.1. Legislation on Waste Water Sludge

as per 1 October 1996............................................................ 49

13.1.1. EU Countries ............................................................... 49

13.1.2. USA ............................................................................. 50

13.2. Plant Utilisation of Nutrients in Sludge ......................... 50

13.2.1. Nitrogen...................................................................... 50

13.2.2. Phosphorus ................................................................. 50

13.3. Basic Characterisation ................................................... 50

13.3.1. Particle Size Distribution ............................................. 51

13.3.2. Water Retention.......................................................... 51

13.3.3. Rheological Properties ............................................... 52

14. References .............................................................................. 53

14.1. Sludge Characterisation ................................................ 53

14.2. Storage and Transportation ........................................... 53

14.3. Agricultural Use ............................................................. 53

14.4. New Technologies ......................................................... 54

14.5. Basic Characterisation ................................................... 54


Figures and tables

6

Figure 2.1. Percentage of the population served

by Waste Water Treatment Facilities in 1990 ............. 10

Figure 2.2. Sludge Generation in member states before and

after the Urban Waste Water Treatment Directive..... 11

Figure 2.3. Different methods of pre-treatment of sludge .......... 11

Figure 2.4. Treatment and disposal routes for sludge ................. 12

Table 3.1. Disposal and use operations parameters .................. 13

Figure 4.1. Volume to area ratio for storing same volumes

of different sludges ................................................... 18

Figure 6.1. Sludge composting plant of Tortona (Italy):

example of a turned pile composting system with

forced aeration in corridor silos ................................. 24

Figure 6.2. Sludge composting plant of La Roche sur Foron

(France): example of an in-vessel composting system25

Figure 7.1. General flow sheet for a drying process .................... 27

Figure 7.2. Example of a disc dryer ............................................. 28

Figure 8.1. Sludge incineration plant of Dordrecht (Netherlands) 31

Figure 8.2. Process scheme of the sludge incineration plant of

Dordrecht (the Netherlands). Courtesy: Lurgi ........... 32

Figure 8.3. Injectors of sludges used in the incineration plant

of Monaco (France) ................................................... 33

Figure 8.4. Process scheme of the incineration plant of Monaco 34

Figure 9.1. Disposal of sewage sludge and solid

waste at mixed deposit ............................................. 39

Figure 9.2. Sewage sludge landfill disposal................................. 40

Table 9.1. EU-Directives on waste ............................................. 41

Figure 10.1. Sludge gasification - flow sheet.

Courtesy of Schelde Milieutechnologie B.V. .............. 43

Table 12.1. Treatment costs in DEMper tonne of dry substance.. 46

Figure 12.1. Sludge Treatment Decision Tree ................................ 47

Table 12.2. Comments on key words in figure 12.1 ..................... 48

Table 12.3. Explanation of numbers

contained in the decision tree ................................... 48

Table 14.1. Utilisation of nitrogen from

different types of sludge ........................................... 51

Table 14.2. Utilisation of phosphorus and potassium

from waste water sludge ........................................... 51

7Foreword

Wastewater treatment plants are becoming more advanced and more common world wide.In recent years, significant progress has been made in several countries to curb water pollu-tion from municipal wastewater. Almost the entire population of Denmark is now served bywastewater treatment facilities and more than 90% of the population in Sweden, the Nether-lands and Luxembourg is served by treatment plants. A problem that needs to be consideredcarefully is the efficient and environmentally sound management of the sludge generated bythese plants. When processing sludge it is critical not to fall into the trap of simply redirect-ing the pollution that originally affected water, to other media, such as soil and air. This canhappen through the use of inappropriate technologies or by applying disposal approachesunsuitable for the local conditions.

This report describes various methods of sludge treatment and disposal: it is intended forthose charged with the task of determining the fate of sludge, providing them with theinformation necessary to select among action alternatives. The report has been developedthanks to co-operation between the European Environment Agency (EEA) and the Interna-tional Solid Waste Association (the Working Group on Sewage and Waterworks Sludge.) Thiscollaboration has allowed not only the analysis, condensation and assessment of the qualityof this information in an efficient and comprehensive manner but also the organisation ofthis document in the structured presentation found here.

One of programmes of the EEA is to develop documents to address environmental issuesthat may support more efficient implementation of environmental policies. This responds totwo main objectives: the pooling of existing information on different techniques withoutpromoting or supporting specific ones; and the dissemination of relevant facts and figures topromote the wise management of natural resources. We believe that this publication whichaims to give a balance between technical and practical information is a significant steptowards meeting these objectives. The EEA is aware of the controversy raised by incineration,which has to be considered very carefully, but to respect the original idea of the programmewe had to assure that the full range of techniques used today were covered. The final choiceis left to the users of this information.

The reader will find a discussion on sludge characterisation by physical, chemical andbiological parameters, described by Ludivico Spinosa, who also developed the work ontransportation and storage. Agricultural use of sludge was prepared by Alice Saabye, which isfollowed by a section on composting of sludge by Isabelle Coulomb. Drying techniques areillustrated in Chapter Seven by dne . Utvik. A chapter on incineration including vitrifica-tion and co-incineration was prepared by Isabelle Coulomb. A section on accumulation andlandfilling of sludge was prepared by Bela Deak. The report ends with a look at new tech-nologies and environmental impact assessments, also presenting a tree for sludge disposal.Legislation related to sludge treatment and disposal is given in the Appendix.

We would like to thank all contributors and reviewers of this document who have co-oper-ated to produce this important report.

Alice Saabye Domingo Jimnez-BeltrnChair Executive DirectorInternational Solid Waste European Environment Agency (EEA)Association (ISWA)Working Group onSewage and Waterworks Sludge

1. Introduction

This booklet gives a general perspective ofthe alternative methods for handling anddisposing sewage waste water sludge. Itprovides an overview of the different meth-ods for processing and disposing sludgegenerated by waste water treatment plants.

The booklet is intended to be a helpinghand to both technical experts as well aspoliticians, public officials, decision makersand others who need to see things in a

broader perspective when faced with thechoice between alternative methods ofhandling and disposal of sludge.

This booklet has been written by members ofthe ISWA Working Group on Sewage &Waterworks Sludge.

9


2. Background

The percentage of the population served bysewage waste water treatment facilities variesconsiderably among the countries of the EU.The differences among thirteen membercountries of the Organisation for EconomicCo-operation and Development (OECD) areillustrated in figure 2.1.

For some countries an improvement in thenumber, size and efficiency of the treatmentplants would have significant positive im-pacts on the quality and health of aquaticenvironments.

Dry weight per capita production of sewagesludge resulting from primary and secondarytreatment is about 90 grams per day perperson, which is more or less the same in allEU countries where municipal communitiesare served by two stage physical, mechanicaland biological processing plants. After theimplementation of the Urban Waste WaterTreatment Directive (UWWTD) (91/271/EEC), the vast majority of the EU 15 popula-tion will be served by sewage treatmentfacilities by the year 2005. This is expected toresult in the generation of about 10.7 milliontons (dry weight) of sewage sludge every year

Percentage of thepopulation served byWaste Water TreatmentFacilities in 1990Source: The WorldResource Institute WorldResources, 1994-95Oxford University Press,Oxford, 1994, p. 331

Aus

tria

Den

mar

k

Finl

and

Fran

ce

Ger

man

y(F

ed.

Rep

.)

Ger

man

y(D

em.

Rep

)

UK

Swed

en

Spai

n

Po

rtug

al

Net

herl

and

s

Luxe

nbo

urg

Ital

y

100

90

80

70

60

50

40

30

20

10

0

Per

cent

age

Figure 2.1.

or about a 38% increase from the 7.7 milliontons produced today. This cumulative per-centage is distributed among the memberstates as shown in figure 2.2.

Information on the methods and approachesused by different member states for the finaldisposal of sewage sludge is still uncertain. Insome European countries the main practiceis landfilling (50 to 75%), while the rest isdisposed in agricultural fields as soil condi-tioner/fertiliser (25 to 35%) or other recy-cling outlets (e.g. parks, land restoration andlandscaping).

Agricultural use of raw sludge or other com-posting practices is encouraged by nationalauthorities as the best way for recycling,while incineration is considered the worst.Directive 86/278/EEC on Sewage Sludge inAgriculture requires, however, that no-onemay permit the use of sewage sludge onagricultural land unless specific require-ments are fulfilled. The Directive aims atavoiding the accumulation of toxic sub-stances, especially heavy metals, that mightreach excessive levels in the soil after anumber of applications.

10

Figure 2.3.

Dewatering DewateringDewateringDewateringDewatering

Aerobic/thermophpretreatment

Pasteurization

Wet-composting

Anaerobicstabilization



Raw Sludge (thickened)

Thermaldewatering

Thermaldewatering

Stabilized and hygienic sludge

Retention time

12 days

15 days

12 days

7 days

4 hour

Different methods of pre-treatment of sludge

Sea dumping is now prohibited, andlandfilling will also be regulated if the newLandfill Directive is passed and the removalof the organic content is made mandatory.The final disposal of sewage sludge remainstherefore an unsolved problem; both be-

cause the amount generated will increaseand also because agriculture can absorb onlya limited number of field applications.

The problem could be particularly difficultfor the 10 accession countries (Bulgaria,

Sludge Generation inmember states before and

after the Urban WasteWater Treatment DirectiveSource: European WasteWater Group, EU WasteWater Strategy: Obser-

vation From the Euro-pean Waste Water

Group, direct communi-cation to ISWA, London,

October 10, 1997

Aus

tria

Den

mar

k

Finl

and

Fran

ce UK

Swed

en

Spai

n

Po

rtug

al

Net

herl

and

s

Luxe

nbo

urg

Ital

y

Qua

ntit

y in

dry

so

lids/

tons

/per

ann

um

1500000

4000000

2000000

2500000

3000000

3500000

1000000

500000

0

Bel

giu

m

Ger

man

y

Gre

ece

(No

rway

)

Irel

and

CurrentAfter UWWT Directive

Figure 2.2.

11Background


Czech Republic, Estonia, Hungary, Latvia,Lithuania, Poland, Romania, Slovakia andSlovenia) which will have to transpose theUWWTD and phase in plant treatment anddisposal of about 4 million tons of sewagesludge generated every year.

Plant treatment of sludge generated byanaerobic or aerobic digestion, mechanicalde-watering and incineration, usually ac-counts for up to 50% of the total investmentin the complete facility. Plant treatment candramatically reduce the size of the final andmore complex disposal problem. Thereforeit is important to consider the full range of

alternatives for sludge handling and disposalwhen planning sewage management strate-gies.

An overview of the pre-treatment methodsadopted for transforming and concentratingraw and wet sludge is presented in figure 2.3.

In some countries, thin sludge is still utiliseddirectly as fertiliser in agriculture. However,an increasing part of thin sludge is nowprocessed. The methods used most fre-quently for treatment and disposal are shownin Figure 2.4.

Treatment and disposalroutes for sludge

Figure 2.4.

Composting

Agriculture

Sludge incineration

Gasification

Drying Waste incineration Landfill Reuse

Digested sludge

Concentration Dewatering

Combined heat and power plant

Activated sludge Wet oxidation

12

3. Sludge CharacterisationBy Ludovico Spinosa1

3.1. Introduction

Sewage sludges exhibit wide variations intheir properties depending on origin andprevious treatment, but their characterisa-tion based on history only gives qualitativeinformation. Many parameters have there-fore been introduced and tests developed tomeasure specific properties of sludge inrelation to particular methods of treatment.

Some of them have been adopted as nationalstandard methods, others as internationalones, but essential differences between themremain. For this reason, a vast standardisa-tion activity aiming at defining Europeanstandard methods is currently in progress inEurope, under the action called CEN/TC308/WG1.

Conventional characterisation parameterscan be grouped in physical, chemical andbiological parameters:

physical parameters give general infor-mation on sludge processability andhandlability;

chemical parameters are relevant to thepresence of nutrients and toxic/danger-ous compounds, so they become neces-sary in the case of utilisation in agricul-ture;

biological parameters give informationon microbial activity and organic mat-ter/pathogens presence, thus allowingthe safety of use to be evaluated.

A list of parameters which can be used, andtheir relevance to treatment and disposalsteps, is presented in table 3.1.

3.2. Characterisation Parameters

The sludge characteristics which are impor-

13

1 C.N.R. Water ResearchInstitute, Via F. De Blasio5, 70123 Bari, Italy

ParameterTemperature x x x x x xDensity x x xRheological prop. x x x x x xSettleability x x xSolids concentr. x x x x x x x x x x x x xVolatile solids x x x x x x x x xDigestability xpH x x x x x xVolatile acids xFats and oils x x xHeavy metals x x x x xNutrients x x x xParticle size x x xCST x xSpec. resistance x xCompressibility xCentrifugability xCalorific value xLeachability xMicrobiol. prop. x x x x

Sed

imen

tati

on

aero

bic

anae

rob

ic

chem

ical

ther

mal

Thic

keni

ng

Dew

ater

ing

Dry

ing

Tran

spo

rtat

ion

Land

fillin

g

Co

mp

ost

ing

Ag

ricu

ltur

e

Inci

nera

tio

n

Method of Treatment and DisposalDisposal and use

operations parameters

Table 3.1.

Stabilisation


tant to know, strictly depend on the handlingand disposal methods adopted. Below, themost important parameters in relation to thedisposal/use operations listed in table 3.1are briefly discussed.

3.2.1. Agricultural UseDry MatterDry matter plays a role in the transport,application and spreading operations.Methods and systems of application to landalso depend to a great extent on rheologicalproperties.

Volatile Solids (Organic Matter)The reduction of volatile solids throughstabilisation is very important, mainly toavoid odour problems. Organic matter alsoexerts beneficial effects on land, but changesin the content of organic matter do notsignificantly modify sludge applicability.

Table 3.1 Disposal and use operationsparameters

Nutrients, Heavy Metals, Organic Micro-pollutants, Pathogens, pHApplication rates are affected by the contentof nutrients, heavy metals and organicmicropollutants in sludge and soil, whilehygienic risks are associated with the patho-gens presence. All of the above factors areinfluenced by pH.

3.2.2. CompostingTemperature, Dry Matter, Volatile SolidsThe process performance strictly depends ontemperature, dry matter and volatile solids interms of both biological evolution andhygienization (pathogens reduction). Inparticular, a concentration of solids of 40-60% and a temperature around 60C aregenerally required.

NutrientsThe C/N ratio is very important to ensure aproper process evolution and a good endproduct. Values of 25-30 should be main-tained.

Heavy Metals, Organic MicropollutantsHeavy metals and organic micropollutantscan either prove to be toxic to the process orreduce application rates of compost.

3.2.3. IncinerationTemperature, Dry Matter, Volatile Solids, CalorificValueThe economics of incineration depend to agreat extent on auxiliary fuel requirements

and, therefore, the above parameters are allimportant to ensure an autogeneous com-bustion.

Rheological PropertiesRheological properties are important as faras the feeding system is concerned.

Heavy Metals, Organic MicropollutantsThe toxicity of emissions (gaseous, liquid,solid) depend on the presence of heavymetals and organic micropollutants at originand/or when improper operating conditionsoccur.

3.2.4. LandfillingDry MatterIt is important to know whether the sludge isconsistent enough to be landfilled. Addition-ally, rheological properties are essential inrelation to the sludge bearing capacity.

Volatile SolidsThe amount of volatile solids has an impacton the development of malodours andprocess evolution, including biogas produc-tion.

Heavy MetalsHeavy metals can negatively affect theevolution of the biological process and thequality of the leachate.

From the above it follows that dry matter andvolatile solids are the most important param-eters in sludge characterisation involved inall the application/disposal methods. Theycan be modified through stabilisation andsolid-liquid separation processes, which areoperations almost always present in a wastewater treatment system.

3.3. Stabilisation

Stability can be defined as the quality ofbeing stable over the course of time; gener-ally, it is mainly related to biological andchemical aspects, and not physical (struc-tural) ones.

Many parameters are potentially available[2], but the concept of stability is usuallyassociated with that of odour, which dependson particular compounds difficult to analyti-cally detect and also to correlate with acertain degree of stability.

Quantitative odour measurement is possibleby asking each person of a panel of selectedpeople to sniff progressively diluted odorifer-

14

ous gases until the odour is no longer de-tected. These tests are cumbersome, expen-sive and cannot be conducted in the field.

The volatile/total solids ratio and/or thepercentage of volatile solids destroyed canbe used as stability index. Ratios below 0.60and percentages above 40% are, generally,an indication of achieved stabilisation.Oxygen uptake rates less than 2 mg/g-volatile-solids/h at 18 C are indicative of astable sludge.

The measurement of stability can alsoinvolve the evaluation of the organicsubstrate concentration, such as volatilesuspended solids, COD, BOD5 and organiccarbon, ATP and enzymatic activity [3].

3.4. Solid-Liquid Separation

Solid-liquid separation processes includethickening and dewatering.

Thickening is defined as the capacity of asludge to increase its concentration of solids(normally by 2-3 times) by filtration andgravitational/centrifugal acceleration. Abilityto thicken is commonly evaluated by allow-ing sludge to settle in a graduate cylinder.This measure is subjected to several errors(wall effects, bridging, liquid depth) whichcan be overcome by using proper columndiameter (100 mm) and height (500-1000mm) and incorporating a slowly rotatingstirrer [1]. Another technique is based onthe use of a low-speed stroboscopic centri-fuge: the test is fast and requires only a smallamount of sludge [5].

Dewatering is the further increase of concen-tration of solids; it can be accomplished byfiltration or centrifugation, usually precededby conditioning. Dewaterability can beevaluated by general parameters and specifictests, the latter referred to as a specifictechnique.

The classical parameter used to evaluatefilterability is the Specific Resistance toFiltration, which represents the resistanceoffered to filtration by a cake deposited onthe filter medium having a unit dry solidsweight. Values of the order of 10-12 m/kg orless are indicative of good industrial filter-ability (raw sludges generally exceed 10-14 m/kg ); specific resistance can be reduced byconditioning, most commonly performed bychemicals (organic or inorganic). By measur-ing resistance at different pressure, the

Compressibility coefficient is obtained,which provides information on the mostsuitable operating pressure level.

The CST (Capillary Suction Time) is asimple, useful and rapid way to evaluatefilterability, but only for comparative evalua-tions; the multi-probe CST apparatus enablesa direct, but less precise, estimate of thespecific resistance.

Specific tests to evaluate vacuum-filter andbeltpress performance are also available.The filter-leaf test reproduces the vacuum-filter cycle [9], while tests including drainingand filtration under vacuum or pressureallow a prediction of beltpress performance[4], [6].

Centrifugability is defined as the aptitude ofa sludge to be dewatered under the action ofcentrifugal force; moreover, sludge consist-ency must be such that it can be easily con-veyed by the screw. However, it is not possibleto reproduce on a laboratory scale all theconditions actually occurring in a full scalemachine, so centrifugability parameters(settleability, scrollability, floc strength) mustbe separately measured. The method pro-posed by Vesilind [8], allows both settlingand scrolling properties to be evaluated.Vesilind and Zhang [10], showed that sludgecan be characterised by correlating the finalsolids concentration to the number ofgravities and the time of centrifugation.Another method is based on the calculationof a Specific Resistance to Centrifugationindex based on the sludge floc strength,consisting of the measurement of CST afterdifferent periods of standard stirring [7].

3.5. Concluding Remarks

Many physical, chemical and biologicalparameters and tests are available for thecharacterisation of sewage sludge, thusallowing its behaviour when processed andimpact on the environment when disposed/used to be evaluated.

A problem relating to the tests discussedabove is that the methodologies adopted aregenerally conducted in individual laborato-ries under different conditions, thus produc-ing rarely comparable results. In order toovercome this problem, the EuropeanCommittee for Standardization (CEN) hasset up a Technical Committee (TC308) forthe standardisation of methods applied tocharacterise sludges.

15Sludge Characterisation


Furthermore, conventional parameters areoften specific to the method of treatmentand disposal/use adopted, so parametersand tests able to give more basic informationon the sludge in question would be needed.For this reason, many recent research

activities in the field of characterisationaddress the development of methods able toevaluate fundamental properties of sludge.These are briefly outlined in Appendix 14.3.

16

4. Transportation and StorageBy Ludovico Spinosa2

4.1. Introduction

The effective optimisation of a sewage sludgetreatment and disposal system requirescorrect planning of the operations linkingthe treatment steps to those of disposal/use,i.e. storage and transportation.

Storage systems integrated into the process-ing sequence allow the speed and capacity ofthe various waste water and sludge handlingaggregates to be adapted, while transporta-tion is a consequence of the fact that thedisposal sites are almost invariably far fromthose of production.

4.2. Storage

4.2.1. Operation DescriptionThe main function to be ensured by storageis the equalisation between a continuouslyentering sludge flow (production) and adiscontinuously exiting one (disposal) [8].

A more uniform composition for mixtures ofdifferent batches and the possibility ofisolating different batches could also beachieved, thus guaranteeing a more constantquality and facilitating analytical controls.

The elimination of differences betweeninput and output is particularly important inthe case of agricultural use as this practice issubjected to a great variability in time de-pending on cultivation type and weatherconditions. Consequently, sufficient sludgestorage capacity must be provided whenunfavourable conditions occur.

Although certain short-term storage possibili-ties are available within the waste watertreatment system (clarifiers, aeration tanks,etc.) and the sludge treatment system (thick-eners, digesters, etc.), sludge storage isgenerally accomplished by separate facilities:they can be internal to the waste water treat-ment plant itself or external when a central-ised sludge treatment platform serving se-veral plants is planned.

The selection of the most suitable method ofstorage basically depends on whether thesludge is in the liquid or dewatered state, i.e.it depends on its dry matter content. In

addition, knowledge of the sludges physicalconsistency, through its rheological charac-terisation, is of great help when definingwhat kind of installation to choose.

Moreover, during storage several phenom-ena can occur, including:

development of malodours, due toanaerobic conditions, which can bereduced through stabilisation;

variation in the fertilising value, includ-ing mineralization of organic matter withloss of ammonia;

hygienization, as a consequence ofpathogens reduction.

It follows that factors of primary importanceto be considered when designing sludgestorage facilities are:

solids concentration (which can beincreased through solid-liquid separationprocesses such as mechanicaldewatering),

rheological properties,

stability (increased through stabilisationprocesses, also able to perform somehygienization),

nutrient and organic matter contents,

pathogens content (reduced throughdisinfection processes).

4.2.2. EquipmentFor liquid sludge, the most suitable ways ofstoring are in [8]:

tanks/vessels having a square, rectangu-lar or circular cross section with verticalwalls;

excavated lagoons/ponds, normallylined at the bottom, with inclined walls.

Since open tanks can be the source ofunpleasant odours, they should be coveredand/or placed at a convenient distance fromurbanised areas. Often, installation ofaeration and mixing equipment is useful to

2 C.N.R. Water ResearchInstitute, Via F. De Blasio5, 70123 Bari, Italy

17


perform additional stabilisation and homog-enisation. Closed tanks should also be fittedwith special equipment to counteract the riskof dangerous gases escaping from the tanks.A lagoon normally occupies an area at leastdouble that of a classic vessel. Pumps arenormally used for filling/emptying opera-tions.

The most widely used systems of storage fordewatered sludge are dumps, basins andcontainers, depending on the obtainablesludge volume ratio in terms of m3-storage/m2-occupied-area (Figure 4.1): it ranges from1.50 to 0.80 for sludges behaving similarly tosolids, 0.80 to 0.40 for those showing a certaindegree of plasticity and below 0.40 for moreliquid sludges. The storage of dewateredsludge generally requires a drainage controlfor water and gases and, in areas with heavyrainfall, covering and sealing systems. Filling/emptying operations can be performed by awide range of equipment, including belt-,screw-, pneumatic- and positive displacementtype-conveyors, chutes and inclined planes,bucket elevators, grapple buckets, etc. Theequipment sometimes has to be fitted with aspecial apparatus to prevent it from blockingduring operation.

4.3. Transportation

4.3.1. Operation DescriptionSludge must almost invariably be transportedfrom the point of production to the dis-posal/use site, possibly via a centralisedprocessing platform.

Generally, the transport step is a large shareof costs for works serving large municipali-

Volume to area ratio forstoring same volumes ofdifferent sludges(solid > 0.80; plastic< 0.60; liquid < 0.30)

Figure 4.1.

(m)

1.8

1.5

1.2

0.9

0.6

0.3

0

(m/m)

(m)0 1 2 3 54

solid

liquid

plastic

ties, as they must transport great amounts ofsludge over a great distance to reach asuitable disposal site. However, higher costsas a consequence of great distances could becounterbalanced by savings due to largetransported volumes. The cost problem isgenerally less important for small communi-ties, and they can reduce investments indewatering equipment and plan centralisedsolutions.

Sludges can be transported by pipeline,barge, rail or truck.

Transport by pipeline is a practice suitablefor moving large quantities of sludge havinga quite low solids concentration, generallybelow 10%. The main disadvantages lie inhigh installation costs, relatively long con-struction time and, above all, in the fact thatthe pipeline has to be shut down duringmaintenance and repair works. The designof the pipeline can be carried out simply bymultiplying figures for clean water by empiri-cal coefficients, but the application of suchsimple criteria could be misleading, so morereliable procedures are suggested [6], [3].

Transport by rail and barge is more conven-ient for moving large quantities of sludgeover long distances. In this case the availabil-ity of suitable waterways and railway connec-tions is a further essential condition.

For the above reasons, transport by truck isthe most widespread method used. The mostsignificant advantages are relatively lowinvestment costs and a high degree of flexi-bility. Rerouting and alteration of collectionpoints are also easily arranged. Drawbacks arepossible leakage and odour/dust emission.

18

4.3.2. EquipmentIn pumping, sludges with solids concentra-tion of less than 2-4% behave like water,while above 10% it is necessary to use volu-metric pumps (single- or double-actingpiston or helicoidal rotor) or rotodynamicones with radial, axial or mixed flow. Highlyconcentrated sludges (with a concentrationabove 20%) can be transported by eccentrichelicoidal rotor pumps provided with ahopper and fed by a screw-drive [9]. Selec-tion criteria for pumps were defined by Frost[2], [4], while criteria enabling the mosteconomical pipe diameter and the pumpingsystem to be designed and optimised, forwhatever fluid may be pumped, are reportedin Castorani et al. [1].

All other transport systems require the mostsuitable container/tanker to be chosenaccording to the physical state of the sludge.For vehicles equipped with loading/unload-ing systems, the knowledge of sludgefluidodynamic and consistency characteris-tics, i.e. rheological properties, are of greatimportance as well.

Tanker trucks for transportation of liquidsludge are usually of uniform design. Largertankers are best suited to distances exceed-ing 20 km and annual transport volumes of4000 m3 at least. These vehicles are normallydesigned with a vacuum suction system forcollecting waste water and sludge, which isafterwards discharged by gravity.

Trucks for transportation of dewatered sludgeare equipped with unusually large watertighthatches for the loading/unloading opera-tions. In order to make optimal use of varioustypes of loading equipment, such tankers arefitted with large loading hatches and a seal toprevent odours from escaping. The interior ofthe tank is fitted with baffle plates to counter-act movement of the sludge during accelera-tion and braking. Rapid unloading is ensuredby tipping the tank to a maximum of about30 so that the sludge falls through the reartrap door, which can be swung open. Whenclosed, this door is rendered impermeable byseals and rotary locks. In tankers designed fortransporting dewatered sludge with a solidsconcentration exceeding 30%, the interiorbaffle plates can be extended and retracted inorder to adapt to the sludge viscosity. Slidingdoors on top of the vehicle can facilitate theloading operation (Spinosa and Sportelli,1986). All equipment and fittings of trucksmust be designed to exclude any possibility ofsludge leakage during the various handlingprocedures.

A very important factor in a sludge transportsystem is the loading and unloading equip-ment: it must be of simple and reliabledesign and permit rapid operations, espe-cially for short distances (to be covered inless than 1 h).

In any case, the rheological characterisationof sludge is very useful in the selection of thebest equipment to be used and optimalprocedure adopted.


Storage and transportation of sludge mustnot be regarded as isolated problems, but beconsidered as part of an integral system oftreatment and disposal: this will ensure thatthe overall system is used to its full capacityand, consequently, that overall costs arereduced.

In particular, storage allows the productioncapacity of sludge to be adapted to that ofdisposal/use, while proper transport systemsinvolve consistent reduction of costs andavoid possible environmental problems, dueto sludge leakage and malodour diffusion.

Several systems for sludge storage andtransportation are available. The selectionmainly depends on the dry matter contentand the consistency of sludge, so, in additionto the conventional evaluation of solidsconcentration, the rheological characterisa-tion appears to be a very useful tool in theprediction and estimate of the sludge behav-iour when submitted to the above-mentionedoperations and, therefore, in the equipmentselection.

A generally valid financial estimate forstorage and transport systems is not possiblebecause it is strongly affected by oil andlabour costs, which vary widely dependingon local conditions and economics. In somecases, national tariffs exist.

19Transportation and Storage


5.1. Conditions for Agricultural Use

The purpose of using sludge in agriculture ispartly to utilise nutrients such as phosphorusand nitrogen and partly to utilise organicsubstances for soil improvement.

5.1.1. Sludge Types and Sludge QualityIn principle, all types of sludge can bespread on farmland if they fulfil the qualityrequirements (heavy metals, pathogens, pre-treatment) laid down by the legislation ofthe relevant country (see Appendix 14.1).

Most often, the amounts of sludge allowed tobe spread are limited by the amount ofnutrients required by the plants and the totalamount of dry solids (see Appendix 14.2).

5.1.2. LegislationAll Western European countries and theUSA have acts or bills on the use of sludgeon farmland, but their legislation differs agreat deal. In Denmark, Sweden, Holland,Switzerland and the USA, the amount ofnutrients is restricted by law. In Norway,Finland, Germany, Ireland, Italy and Spain,the amount of dry solids is subject to restric-tions, whereas in France and Great Britain,the amount of heavy metals places restric-tions on sludge spreading [1]. In somecountries the regulations are so strict thatonly a minor part of the sludge can fulfil theconditions.

The following requirements are common tothese regulations:

Pre-treatment (reduction of the water-content in sludge, reduction of organicsubstances, reduction of pathogens)

Restriction on the amount of heavymetals contained in sludge

Restriction on the amount of dry solidsand heavy metals spread per unit of landand time

Restriction on the content of heavymetals in the soil on which sludge isspread, and requirements for the pH ofthe soil

5. Agricultural Use By Alice Saabye3

Restriction on the amount of nutrientsadded to the soil (nitrogen and phos-phorus)

Restriction on the choice of crops

Restricted access conditions to farmlandon which sludge is spread

Legislative compliance control

5.2. Description of Agricultural Use

In general, the sludge is stored in the treat-ment plant or it is taken to the farmer forimmediate storage.

Large amounts of sludge and limited storagecapacity often make it necessary to dewaterthe sludge to reduce its volume.

The sludge is normally spread on farmlandonce or twice a year in connection withploughing and seeding. Hence, the maxi-mum uptake of nutrients by the plants isobtained and thus a reduced washout of thenutrients to the ground and surface water.

In general, the farmers own equipment forspreading liquid and livestock manure is alsoused for spreading sludge. As the equipmentis not ideal for this purpose, it is oftendifficult to comply with the legislative re-quirements in relation to the amounts of drysolids and nutrients allowed to be added.

Consequently, it is important that theamount of dry solids required in the sludgematches the equipment available. If not, it isnecessary to invest in special spreadingequipment.

After spreading, the sludge is ploughed intothe soil. Legislation often requires that thismust be done very shortly after the spreadingto reduce odour nuisances.

Liquid sludge can be injected directly intothe soil.

3 I. Krger A/SGladsaxevej 3632860 SborgDenmark

20

5.3. Advantages/Disadvantages ofAgricultural Use

Spreading of sludge on farmland offers thefollowing advantages:

Utilisation of nutrients contained in thesludge, i.e. phosphorus and nitrogen

Utilisation of organic substances con-tained in the sludge for improvement ofthe humus layer of the soil (i.e. soilimprovement)

Known regulation on its application

Often the cheapest disposal route

Sludge spread on farmland may have thefollowing disadvantages:

Major investments in storage facilities assludge can only be spread on farmland afew times a year

Dependency on the individual farmersand considerable administration ofagreements

Lack of knowledge as to the content oforganic micro-pollutants and pathogenicorganisms in sludge and their impact onthe food chains

Thorough legislative compliance control

5.4. Financial Estimate

When making a financial estimate of sludgespread on farmland, the following costs mustbe taken into consideration:

Transport costs from treatment plant tostorage

Storage investments and operating costs

Transport costs from storage to farmer

Investments in spreading equipment(can often be omitted as the farmer useshis own equipment)

Expenses for spreading and ploughing(can often be omitted as the farmer useshis own equipment)

Expenses for analysis of sludge quality

Expenses for analysis of soil quality

Administrative expenses for e.g. declaration of sludge, conclusion of agreementswith farmers and control of application.

The price for agricultural use of sludge is inthe order of DEM 150-400/tonne of sludgewith 20% dry solids. It should be noted,however, that the price depends on localconditions and may differ considerably fromthe above.

21Agricultural Use


Sludge composting aims at biologicallystabilising sludges while controlling pollu-tion risks in order to develop agriculture orother end use outlets exploiting the nutrientor organic value of sludges. It can be appliedeither to non-digested sludge (e.g. Italy,France) or to digested sludge (e.g. theNetherlands). Composting involves aerobicdegradation of organic matter, as well as apotential decrease of the sludge watercontent, the efficiency of which depends onthe composting process.

Since organic substances and nutrients, suchas phosphorus and nitrogen, are in demandby farmers, compost, rich in organic nutri-ents, is considered a valuable soil improver.Sludges composted for agricultural end useare therefore of value to secondary markets.In the case of sludges treated by incinera-tion, waste sludges can be composted as apre-treatment to decrease the water content,thus increasing the efficiency of the incinera-tion process.

Nevertheless, it may be too expensive to firstcompost and then incinerate the sludge.Besides, some of the organic matter willdecompose, thus limiting the increase of thecalorific value linked to sludge drying.

6.1. The Influence of Variable Conditionsin Defining the Composting Process

Sludges can be composted if they havesufficient organic matter as well as a relevantwater content. In order to effectively com-post materials, certain nutrients are re-quired, including nitrogen and phosphorus,as well as proper moisture and aerationlevels that facilitate and enable, after mixingwith a co-product, the sustainability of micro-organisms.

Since liquid sludges have a high watercontent, they alone are not sustainablemediums for micro-organism growth. How-ever, they can be composted when used inrelevant quantities in a mixture with otherhighly fermentable organic wastes possessinga lower or a deficient water content.

Sludges with a water content greater than15% are easily co-compostable, particularly

6. CompostingBy Isabelle Coulomb4

when mixed with bark or other structuringmaterials making the compostable mixtureporous.

As a general reference, the water content ofa compostable mixture of organic wastesshould be around 55% while the organicmatter content should be greater than 70%,facilitating effective bio-degradation. A highmoisture content above 60%, reduces thetemperature, porosity and thus the oxygenconcentration while a low moisture content,below 50%, could limit the rate of compost-ing. At values of 10-15% the bacterial me-tabolism generally ceases to function. Bacte-rial activity is also influenced by pH, with theoptimal values being between 5.5 and 8.

A balance of the nitrogen and carboncontent is necessary for the proper growth ofmicro-organisms. The carbon to nitrogenratio (C/N) of the mixture is thereforecommonly used to define the optimumfunctional conditions. Although valuesranging between 25 and 30 are recom-mended, the types of molecules concernedmust be evaluated when establishing theideal ratio.

Structuring composting conditions andmixture ratios naturally depend on the typesof wastes to be treated as well as the qualityspecifications set for the resulting compost.

For example, the processing of organicmaterials would be different if the goal was toincinerate the end product, thus demandingthat the water content of sludges should bereduced, than if the goal of the compostingprocess was to produce a soil improver, asdefined by a particular agricultural outlet,which would therefore require that specificnutritional and structural properties shouldbe present in the end product.

Different ratios of wastes and differentprocessing policies would have to be imple-mented under the constraints of eachprogramme. Therefore the types of sludgesto be treated, the quantitative/qualitativeconstraints for incorporating them into acomposting process, and the efficiency of thecomposting biodegradation process itself,depend on the end product specifications asdefined in any given project.

3 SITA, 94 rue deProvence BP 693, 75425Paris Cedex 09, France

22

If, for instance, a compost end producttargeted for an agricultural outlet is consid-ered, many market and regulatory con-straints will have an impact on the definitionof the composting process itself. For exam-ple, although organic pollutants such as thepesticides, PCB, HAP, PCDD, PCDF do nothave regulatory limits at European levelbecause the concentration of these contami-nants has proven to be low, the contrary istrue for the heavy metal content in com-posts.

The heavy metal content of the final com-post product is critical. The end productstandards must meet the specificationsdefined by local and national legislation.Obviously, since the materials are compostedat a temperature level sufficient to killpathogens, the risk of pathogenic problemsis minimised, an advantage supporting theprocessing of organic materials by com-posting over other treatment methods. Inaddition, microbial competition duringcomposting also favours pathogen reduction.

Market conditions also have an impact onthe process design of a compost system. Inaddition to their inclusion in some nationalregulations, aesthetic contaminants such asplastic, glass, metals or stones are undesir-able because they may limit the marketingpotential of the final product.

In the case of agriculture end use composts,it must not only support local vegetationgrowth, it must also meet both the qualitativeand quantitative demands of the localmarket; the compost end product must benutritionally balanced to the local soildemand and produced in quantities whichdo not exceed the outlet application poten-tial.

For example, to utilise the final compost fora soil substrate application, the nutrient andsalt content must be low while the waterretention capacity must be sufficiently high,thus meeting the necessary water/aerationrequirements of specific types of vegetation.

The pH of substrate also becomes an impor-tant criteria in designating the type ofvegetation to be sustained and must beaccounted for in compost end quality specifi-cations. Whether designated as a soilimprover or a substrate application, decom-position of organic material must be suffi-ciently high. In addition, the process ofproducing a quality end-product mustensure germination and sustainability of

plant growth and ensure that no germina-tion of foreign seeds occurs as a result of thecompost.

The example of composting sludges as a pre-treatment method for incineration is techni-cally appealing because composting resultsin both mass reduction and decreased watercontent, thus providing a more efficientwaste stream for incinerators. A financial andmarket analysis must, however, be completedto validate the viability of this option: iscomposting in itself viable, and is the level ofbio-degradation necessary to justify theincineration of pre-treated, compostedsludges over the incineration of raw un-treated sludges?

6.2. General Description of the Process ofComposting and Plant ConceptionFactors

The process of composting is based onaerobic degradation of organic matter,under variable conditions as describedabove. In essence, the activity of micro-organisms causes both an increase in tem-perature, hence the pathogen destruction,and a release of energy, CO2, H2O, NH3 andother gases, while consuming oxygen.

The rate of the bio-degradation process canthus be controlled by regulating aeration,moisture content and mechanical processingof the material. For instance, the compostingprocess could be controlled and acceleratedby:

Forced aeration, which provides suffi-cient oxygen to meet demand, whileproviding an odour control mechanism;

Mechanical turning of compost, whichcontributes to the mixing and theincreased aeration of the material.

A particular market demand may also beaccommodated by mechanical screening to adefined particle size.

In developing an actual plant to processorganic wastes and sludges into compost, thefollowing system development and costs mustbe taken into account:

Waste reception and storage areas;

Mechanical mixing system to beapplied;

23Composting


Storage area for the compostablemixture, before aeration: Note, thisstep is not compulsory but can beinteresting for the flexibility of theoperation;

Fermentation/ bio-reactor area. Note:reactors, containers, lockers, tunnels,channels as well as their processretention times, ranging from 10 daysto five weeks, are defined in relationto the objectives of the programme;

Storage area for compost maturationand final stabilisation of the product;

Mechanical screening systems: the sizeof the screen depends on the localutilisation of the compost; the finefraction can be stored, commercial-ised or eliminated whereas the rawfraction can be recycled as a structuring material for composting;

Control of variable local facilitydemands such as: sewage water,odours, dust control and control ofother nuisances such as noise, flies,and the potential of foraging animals.

A wide range of composting systems existworld-wide, however they tend to be classi-fied into several general categories:

Open composting systems(see Figure 6.1) static pile :- natural ventilation- forced aeration with suction or

blowing in windrows or in silos turned pile :- natural ventilation- forced aeration in corridor silos, in

windrows or in tabular piles

Closed composting systems functioningcontinuously or periodically with forcedaeration (see Figure 6.2) rotating horizontal reactors (tube) static vertical reactor (tower) in-vessel systems (boxes)

The choice between the different systemsdepends on the technical and economicparameters. In general, when compostingsludge, process control, reduction of labour,and sensitivity to weather generally necessi-tate forced aeration technologies.

6.3. Advantages/Disadvantages ofComposting Sludges

AdvantagesComposting offers several advantages incomparison with other sludge treatmentmethods. In comparison with the direct

Sludge composting plantof Tortona (Italy):example of a turned pilecomposting system withforced aeration incorridor silos

Figure 6.1.

24

application of non-composted sludges inagriculture for example, composting allows:

A reduction in the volume of materials tobe transported for distribution in agricul-tural fields;

A facilitation, with respect to sludge, ofstorage and use in places and at times farfrom those of production;

A facilitation of the spreading processdue to the lower water content of theproduct;

Control of compost material specifica-tions, which results in a well-defined,stable end product composition with thepotential of improving the soil humuslayer and nutrient levels;

Control of nutrient content as defined byapplication and vegetation standards;

Product hygiene control before agricul-tural application;

Disadvantages The treatment cost is higher than simple

direct raw sludge application;

Aeration consumes energy;

There is a need for an outlet market forthe compost end product; competitivesoil improvers exist;

The need for mixing sludge with othermaterials in order to obtain an optimal C/Nratio, can be considered either as an advan-tage when bulking agent is another waste tobe treated (complementarity of treatment)or as a disadvantage when this bulking agenthas to be bought.

ConclusionAlthough costs are elevated when sludge iscomposted in contrast to, for example, directapplication of sludges in agricultural crops,the market and health aspects such aspathogen risk, odour, and nutrient controlin response to vegetation demand, oftenemphasise and necessitate the demand foralternative solutions, justifying composting asa viable and environmentally sound option.

6.4. Financial Estimates

Evaluating the costs of different treatmentoptions for sludges is necessary. For exam-ple, the following costs must be taken intoaccount when a sewage water treatmentplant decides to compost its sludges:

Sludge composting plantof La Roche sur Foron

(France): example of an in-vessel composting system.

Figure 6.2.

25Composting


The cost of transporting the sludge tothe composting plant;

Capital investment and engineering costsfor the composting system itself and theplant infrastructure (buildings, aeration,odour and air cleaning equipment,machinery for turning and mixing thecompost, sieves, conveyers if necessary,front loaders, ...);

Plant operating costs: personnel, energy(electricity, fuel), bulking agents (includ-ing the cost of transporting bulkingagent to the composting plant if neces-sary), maintenance, overhead expenses,taxes,...;

Quality control expenses: characterisa-tion of wastes and the compost endproduct as well as process evaluation;

Marketing costs, including marketstudies and marketing materials;

Cost of transporting the compost fromthe composting plant (when necessary).

An indication of price range for compostingis between DEM 275 - 525/tonne in France.See Table 12.1.


Although there are a lot of processes andpatents in the composting profession, manysystems are not viable under end productdemand and market demand constraints;they are either too simple and not adaptableto either waste and/or market demands or,they are too capital investment heavy and/orlabour intensive and the return on theinvestment is not justified. In developing anapproach for the treatment of waste sludgesby composting, all the intricate waste, endproduct, and market constraints must beevaluated in defining the process itself andstructuring a facility.

Consequently, the prescriptions mentionedabove concerning system and market plantanalysis and planning factors as well asadequate conditions for composting are thekey to success of a quality compost produc-tion. However, this quality improvement hasto be financially assessed.

26

7. DryingBy dne . Utvik5 with contributions by Albrecht R. Bresters6 and Martin Wrdemann7

7.1. Introduction

The purpose of this chapter is to describedifferent processing methods for drying ofsludge.

7.2. Conditions for Drying of Sludge -Amounts and Drying Rates

Basically, there are no specific pre-conditionsto be fulfilled in order to apply drying. Thismeans that all known sludges from munici-pal waste water treatment plants may beprocessed in a properly designed and oper-ated drying plant. However, as in all indus-trial processes, the total cost per tonneincreases when throughput decreases.

Presently, a small plant for municipal sludgeremoves less than 0.5 tonne of water/h,whereas a large plant evaporates nearly 30tonnes of water/h.

The main reasons for this great variation incapacity are:

1) Size of one or more WWTPs (inlet flow,m_/h)

2) Pollution, kg dry solid/m_ of effluentwater

3) Treatment process - chemical/biologicaletc.

4) Undigested or digested sludge

5) Dry solid content in dried sludge

Ad 4) When the sludge is digested, the drysolid content (DS) will be reduced by ap-proximately 20%, due to transformation oforganics into biogas.

Ad 5) The DS is normally raised to 40-50%as preparation for immediate subsequentincineration. To make a storable product formulti purpose utilisation, for instance asfertiliser, soil conditioner, fuel etc., the DS israised to 90-95% and granulated.

5 Stord International A/S,P.O.Box 9, N-5049 Sandsli- Bergen, Norway

6DRSH Zuivoringsslib BV,Hofplein 33, NL-3000 AZRotterdam, TheNetherlands

7Haskoning, P.O. Box151, 6500 Nijmegen, TheNetherlands

THIN SLUDGE

Mechanicaldewatering

Dewatered sludge

Air& vapour

Water toWWTP

Thermal dryingdirect/indirect

Dried sludge

Dustseparation

Granulationwhwn adequate

Emission to air

Heat recovery& odour removal

Water to WWPT

Coolingwhen adequate

PRODUCT

Generel flow sheet for adrying process

Figure 7.1.

27


7.2.1. Quality

The quality as fertiliser/soil conditionerand/or as fuel mainly depends upon:

1) Content of organic solids

2) Content of plant available fertilisingcomponents, mainly phosphorous,potassium and nitrogen

3) Ability to absorb and retain water

4) Content of heavy metals

5) Content of pathogenic bacteria andfungi viruses, helminth ova and toxins

6) Repopulation rate of pathogenic micro-organisms.

Whereas 2) and 4) depend upon the contentin the effluent water and the treatment, 1)and 3) are influenced by digestion, whichdegrades organic material.

The possible presence of pathogens is amajor concern and must be taken seriously.Proper drying, where the most relevantfactors are retention time/retention time

distribution and temperature, particle sizeand heat conductivity, is regarded the safestmethod in this respect. Under certain dryingconditions some N-NH3 may be lost. If this ispredominant and damaging, the loss may becounteracted by additives.

The actual flow sheet depends upon anumber of factors, of which type of dryerused (indirect/direct) and dry solids in endproduct (approx. 70% or 90%) are the mostdecisive ones. Heat recovery may be moreefficient with indirect dryers, whereas somedirect dryers may not require separategranulation.

7.3. Drying and Dryers

There are approx. 20 suppliers of dryingequipment (please refer to ISWA literatureand documentation, for instance the ISWAYearbook). The two distinctly differentdrying methods are indirect and directdrying. In direct dryers there is a directcontact between the material to be dried -the sludge - and the heated gas supplying therequired heat for evaporation and simultane-ously carrying the water vapour formed outof the system.

Example of a disc dryer

Figure 7.2.

28

7.3.1. Indirect DryersIn the other type, the indirect dryer, heat istransferred to the material to be driedindirectly by heat conduction through a heattransfer surface. Thus, the heating medium,for instance steam or thermal oil, is not incontact with the sludge. A small stream of airmay be used for transport of the watervapour formed, although an indirect dryermay well be operated without any air, therebykeeping the odour removal cost at a mini-mum and heat recovery at a maximum.

Among the indirect dryers the disc dryer iswidely used. An example of a disc dryer isshown in figure 7.2.

A disc dryer is a slow, rotating machine. Theconcept is simple, and the dryer is easy tooperate.

The stator consists of a nearly cylindrical,horizontal drum, enclosing the material tobe dried. Scraper bars, acting as mixers, areattached to the drum. On the top of thestator there is an open space serving both asdust separator and as a collector and duct forthe escaping vapours. The dryer is normallyoperated with a slight underpressure.

The rotor consists of parallel hollow discswelded to a horizontal shaft, and the steam -or other heating medium such as thermal oilor hot pressurised water - flows inside thediscs. Agitators are welded on the top of thediscs. A small flow of air is advantageousfrom an operational point of view, however,the drier may well be run without any air.Little or no air keeps dust separation andodour abatement costs at a minimum.

To ensure a permanently high heat transferrate, it is crucial to maintain the heat transfersurface - the discs - clean. The stickiness ofsludge is a challenge in this respect.

The ability of the disc dryer to handle stickyand easy-to-coat materials is generally dueto:

The self cleaning ability of all of the heattransfer surface (the discs) due to gentlefriction forces created when the discsturn in the material to be dried.

The action of the agitators and thescrapers causing adequate mixing andturbulence and maintaining a high heattransfer rate simultaneously with causingthe sludge to move through the dryer.

7.3.2. Direct DryersWith direct dryers, the heat for drying istransferred from the hot gases to the sludge.This requires an intensive contact betweengas and sludge material. The most importanttypes of dryers are the revolving drum dryerand the fluidised bed dryer.

With revolving drum dryers the sludge is fedinto one side of the dryer. Through therotation of the drum and the internals in thedrum, the sludge is transported to the otherend and simultaneously comes into a veryintensive contact with the hot gases. Theresult is a granulated sludge with a dry solidcontent of more than 90 %. In order toprevent clogging within the drum, the feedmaterial should have a dry solid content ofmore than 65%. Therefore, back-mixing ofdried sludge with dewatered sludge is nor-mally required before the material can befed into the dryer.

With fluidised bed dryers, the intensivecontact is realised by an upward flow of hotgases, carrying the sludge particles until theyare dried, resulting in a very turbulent gasflow. Depending on the sludge type, thedried sludge has a dry solid content of morethan 90% in the form of dust-free granules.The dust in the flue gas is transported by thegas stream, removed by cyclones and fedback into the dryer after mixing withdewatered sludge. Additionally, a heatexchanger can be included in the fluidisedbed, resulting in an intermediate dryingsystem between direct and indirect drying.

As a result of the intensive contact betweengas and sludge and the good heat transfer,the specific performance of the direct dryerscan be higher than that of the indirectdryers. Additionally, the mechanical designcan be simpler.

On the other hand, direct dryers have thefollowing disadvantages:

The used gases contain a large amountof pollution, especially strong odorouscomponents. This requires very substan-tial gas treatment.

This type of dryer is less suitable for usewith low temperature heat. This isespecially the case for the revolvingdrum dryer.

Explosions possible

It is, however, possible to recirculate part or

29Drying


all of the used gases and vapours, but thisleads to more complicated drying systems,through which the advantages of directsystems decrease substantially. The selectionof a direct or indirect drying system there-fore depends on sludge characteristics andother specific and local conditions. Bothsystems are subject to much practical experi-ence.

7.4. Advantages/Disadvantages of Drying

In terms of energy, water removal by evapora-tion/drying is generally more expensive thanmechanical methods such as pressing andcentrifugation/ decanterisation. Prior todrying, proper mechanical dewatering musttherefore be installed.

A drying plant, which in most cases includesgranulation, is more expensive to install thanmost other methods. On the other hand,drying results in a great volume reductionand a storable free flowing and hygienicproduct. Due to the great volume reduction,dried sludge implies reduced costs fortransportation, handling and storage.

The greatest advantage in having sludge in adry form as compared with various othermethods is the possibility of marketing theproduct for a number of applications and atthe most suitable time:

Fertiliser/soil conditioner in agricultureand forestry

Fuel in cement kilns, power plants andincinerators

Top soil, landscaping, landfilling anddisposal

For most applications it is of great impor-tance - and cost saving - that thermally driedsludge has been subject to safe hygienisationand considerable volume reduction.


As mentioned above, the disadvantage is thehigh investment cost relative to other meth-ods. No reliable and generally valid estimatesare known since local factors influenceheavily.

A few questions may clarify this point:

It may for instance be questionedwhether making biogas is favourable ornot. What is the price per kW/h ascompared with the market price forelectric power?

How much will digestion of sludgereduce the organic content and its valueas fuel and fertilising/soil conditioning?

The heat supplied to the dryer, forinstance through steam, is still present,but at a lower temperature. Is there aneed for this low temperature heat, andhow should it be priced?

Does the sludge owner have only onecustomer, except for deposit? How willthis one and only customer pay him inthe future, and what changes in thedeposit cost are expected?

An exampleA plant - without digestion - handling 2,400tonnes of DS/year in the form of 12,000tonnes of dewatered sludge (20% DS) a year,may have a cost of approx. NOK 1,300/t ofDS (approx. DEM 295, FFR 1,000, GBP 130)including capital (investment NOK 5-6 mill.)and operating cost (lime, polymer, transport,etc.).

If the cost of the building, storage, requiredheat and disposal of the dry material isincluded as well, the cost may typically be inthe order of DEM 600 to 700 per tonne ofDS.

30

Today considered the last method used inthe treatment of waste sludges, either aloneor in combination with other wastes, treat-ment by incineration represents 15% of thetotal mass of sludges treated in Europe.

The agricultural use of sludges, by directapplication, as well as landfilling of sludgesare subject to more and more regulatorycontrol. For this reason, incineration ofsludges is expected to increase, even thoughit can be a capital intensive investment and itis also subject to strict regulation pertainingto combustion criteria, management of theoff-gas treatment residues and treatment offly and bottom ashes.

Incineration of sludges can be performed indesignated incinerators or in municipal solidwaste incinerators under the particularconstraints for each type, where the processresults in combustion of the organic matterof the sludges.

After pre-drying, sludges can also be inciner-ated in cement kilns because they have a

8. Incineration (Vitrification, Co-incineration)By Isabelle Coulomb8 and A. Myrope9

high calorific value. Pollutants are stabilisedin the clinker which is an interesting way oftreating polluted sludges.

From an economic point of view thesemethods of treatment are mainly justified forsludges not allowed to be used in agricultureor incinerated in municipal solid wasteincinerators.

Another method of stabilisation is vitrifica-tion of the product. Japan has some experi-ence in this method, but the process remainstoo expensive. Therefore, these methods oftreatment will not be further detailed in thefollowing paragraphs.

8.1. Designated Sludge Incinerators - aGeneral Description

Specific sludge incineration facilities havebeen operating for many years. Rotary kilnfurnaces and the multiple hearth furnace ofthe classic or pyrolytic type are today moreand more frequently being replaced by

8 SITA, 94 rue deProvence, BP 693, 75425Paris Cedex 09, France

9Elyo, BP 4601, 235avenue GeorgeClemenceau, 92746Nanterre Cedex, France

Sludge incineration plantof Dordrecht

(Netherlands)

Figure 8.1.

31


fluidised bed systems, which tend to beeasier to operate (see Figure 8.1).

The fluidized bed system consists of a verticalcombustion chamber lined with a refractorymaterial at the base of which a bed of sand isbrought to high temperature and held insuspension due to a grate equipped withinjectors forcing a flow of hot air. Thesludges are introduced inside or above thebed of sand.

The gas must be heated to 850C to ensurecomplete burn-out of the flue gas. There-fore, except in the case of fresh primarysludges, the incineration of sludges comingfrom mechanical dehydration machines isnot able to be completed unless combustibleenergy is provided.

In order to limit the consumption of extracombustibles, a heat recovery system isinstalled for the hot gas leaving the combus-tion chamber. Three techniques are used torecover and reuse this heat: one heating thecombustion air in a smoke/air exchanger,another one pre-drying the sludge above thefurnace as a result of steam produced by a

recovery boiler, and the last one pre-dryingthe sludge in separate rotary disk dryersprior to incineration, the heat required forthis drying process being recovered by steamboilers behind the furnaces.

The second process is thermally moreproductive than the first one as it enables anachievement of autocombustibility of sludgeswith a dry matter content of 20-25% and witha rate of volatile material/dry material of 60-70% before pre-drying. Extra recovery of theevaporation steam from the dryer, and onthe condensates before they go back to theboiler tank, can also improve this result.

It should be noted, however, that this type ofheavy equipment and investment can usuallyonly be justified in the case of large installa-tions (capacity higher than 2.5 tonnes ofevaporated water per hour). The thirdprocess is applied in some modern plants inGermany, the Netherlands and the UK,which are autotherm with sludges at a drysolid content of approximately 23% (seeFigure 8.2).

Process scheme of thesludge incineration plantof Dordrecht (theNetherlands)Courtesy: Lurgi

Figure 8.2.

32

8.2. Co-incineration - a GeneralDescription

The use of municipal solid waste incineratorsto treat the sludges of urban stations isattractive, particularly if the incinerator isquite close to the water purification stationthat generates waste sludges, assuming thatthe capacity of the incinerator is not alreadysaturated by local municipal solid wastes.

Mixing municipal solid waste with wet sludgemust be carefully designed because a poten-tial decrease of the energy content of theproduction is not unlikely. However, a waymust be found to introduce the pre-dried oruntreated sludges into the incinerator.

One method of doing this consists of dryingthe sludges to a dry material content of 65%,which results in a calorific value similar tomunicipal solid waste (2000 th/t= 2000 kcal/kg). The sludges can then be injected intothe oven (hopper) taking into account thatthe quantity of municipal solid waste treatedmust be decreased proportionally to com-pensate for the weight of sludge.

This can be a drawback if the incineratorcapacity is already close to saturation, butmore so an advantage for periods of the yearwhen many medium size units have a lack ofwaste. The granulated material obtained

from sludges represents an important sourceof energy and can be easily stored withoutpresenting much of an odour problem.

Another solution consists of adoptingprocesses conceived to enable direct injec-tion of pasty sludges by means of a highpressure pump. The sludge is forcedthrough injectors which gauge and distributethe material above the grate (see Figure 8.3)

The sludge is injected at the entry of thefurnace by means of a pipe system crossingthe refractory bricks, between the waterpipes. The sludges fall by gravity and theincineration time and mixing of material arethe same as with ordinary municipal solidwaste (see Figure 8.4).

One of the most important points of thoseprocesses is the calibration of the systems. Ifthe sludge was injected in too large amounts,it would risk being incompletely incinerated;the elements are burnt on the surface andnot incinerated at the centre. The drymaterial content of the sludge requiredwhen pumping is 18 to 30 % DS. Differentsystems can treat a total sludge mass ofapproximately 20% in comparison withmunicipal solid waste. If the sludges are verywet, vaporisation occurs after injection sothat the total solid waste tonnage treateddoes not change very much.

Injectors of sludges usedin the incineration plant

of Monaco (France)

Figure 8.3.

33Incineration


Another method is also used, consisting ofmixing sludge with municipal solid waste inthe hauler. This may result in some problemsof volume and odours.

8.3. Advantages and Disadvantages

Advantages A significant reduction of the sludge

volume, after incineration

Energetic valorisation of sludges

Recycling of sludge treatment sub-products such as ashes and inert mate-rial, which can be used in filler materialfor asphalt and concrete production andin the fabrication of bricks

Low sensitivity to sludge composition

Reliable systems

Minimisation of odours, due to closedsystems and high temperature

Disadvantages Incinerators are capital intensive and

usually justified only in larger volumesituations; installations treating 2,000 to5,000 TDS (in France, for instance, thismeans stations from 200,000 to 800,000equivalent inhabitants, but in any case,

the sludge production depends on thewaste water treatment plant). In theNetherlands higher capacities arerequired: 10 000 to 40 000 TDS or more,whether the plant is combined with awaste water treatment plant (or inte-grated with another waste treatmentplant) or not due to the complex flue gascleaning and operating efficiency.

In case of co-incineration, the treatmentcapacity and treatment efficiency dependon the saturation of the incinerator byother solid waste streams and/or theratio of sludge mass to solid waste mass.


The following costs have to be considered inthe decision making process for treatment ofsludges by incineration:

Cost of storage systems necessary

Cost of furnace

Treatment of off-gas and other incinera-tion residues, i.e. bottom ash, fly ash,clinker

Other peripheral costs either for existingplants or in the case of new plants

Process scheme of thesludge incineration plantof Monaco

Figure 8.4.

34

Fixed, proportional operating costs:personnel, consumables (i.e. fuel,electricity and chemicals for flue gascleaning), maintenance, taxes, etc.

Cost of transporting the sludge to thetreatment site

Cost of quality control (raw sludge andsub-products)

Marketing costs generated by the recy-cling of some sub-products

On a first estimate basis, designated incinera-tion of sludge costs between 1,500 and 2,550FFR (440-750 DEM) per tonne of dry mate-rial, excluding taxes. This price includes theinvestment costs, the up-grading of existingsystems, the operating cost, and the finaldisposal of residues in a class I landfill. Thisis relevant to units capable of treating from2,000 to 5,000 tonnes of dry material peryear, which represents water treatmentstations from 200,000 to 800,000 equivalentinhabitants. The investment will be higherfor incinerators with pre-drying, but abenefit for operating costs can be accountedfor. In the Netherlands where incinerators,designated with larger capacities (i.e. 5tonnes of evaporated water per hour perincineration line in the Dordrecht plant),have pre-drying of the sludge and complexflue gas cleaning, costs are in the upperrange (550-750 DEM). The lower rangeapplies to countries where regulationsconcerning flue gas cleaning are less strin-gent.

Concerning co-incineration with adaptedinjectors (e.g. the incineration plant ofMonaco), the total cost, not including thetransport of sludges, but including provisionsfor depreciation, treatment of residues, etc.,is approximately 80 DEM/tonne of un-treated sludge, excluding taxes. This equalsapproximately 300 -425 DEM for a tonne ofdry solid, taking into account, of course, theinstallation size and the site conditions,making the process attractive as an effectivemethod.

Those prices correspond to an extra injec-tion of sludge without any decrease of thefull incineration capacity of municipal solidwaste. Investment and operating costs arethen limited to the injection systems. Whenthe municipal solid waste capacity decreasesbecause more than approximately 20% ofsludge in comparison with municipal solidwaste is injected, total costs of incineration

should be equally allocated to the sludge andthe other waste material. Prices will then behigher than previously mentioned (530 - 750DEM/tonne of dry solid).


Even though investment costs appear to bemore intensive than the cost of the othersludge treatment options, thermal treatmentof sludges by incineration is expected todevelop considerably in the years to come.Units of significant size can balance invest-ment costs, making it a technically andeconomically viable treatment process.

The combination of different waste streams,municipal solid waste and waste sludges, alsoenables optimisation of the incineratoroperations. One of the best examples isincineration of sludge granulate resultingfrom thermal drying. This material is easy tostore and can act as incinerator feed whenno other local treatment method is available.

Incineration units with either excess capacityor irregular flows (seaside or mountainlocations) are particularly interested in thepossibilities offered by sludge drying andincineration since they can store hygienicwaste products with a high calorific value.

Incineration results in a large volume reduc-tion of the waste. Depending on the possi-bilities of re-using ashes, the decrease of theamount of material to be landfilled will bemore or less important.

35Incineration


9. LandfillingBy Bla Dak10

10 Municipal SewageWorks, FovrosiCsatornzsi Mvek Rt.,Marcius 15-e tr 3, 1056Budapest, Hungary

9.1. Conditions for Application

A (wet) sludge production of 230 milliontonnes was estimated in the European Unionin 1993 (Korrespondenz Abwasser, 1993, 40,Jahrgang, Linder: Anforderungen an dieKlarschlammentsorgung in Europe). InGermany, 25% of the production was used inagriculture, 65% was landfilled, and 10% wasincinerated. The corresponding figures inSwitzerland were 50, 30 and 20, respectively,whereas the figures in France were 55, 25and 20, thus demonstrating large variationsfrom country to country. However, in thefuture the use of landfills will have the lowestpriority in the waste hierarchy and will onlybe

eea - sludge treatment and disposal

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