sludge treatment and disposal

Management Approaches and Experiences

Sludge Treatment and Disposal

By ISWA’s Working Group on Sewage & Waterworks SludgeAlbrecht R. Bresters, The Netherlands

Isabelle Coulomb, FranceBela Deak, Hungary

Bernhard Matter, SwitzerlandAlice Saabye, DenmarkLudivico Spinosa, Italy

Ådne Ø. 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

ISWALæderstræde 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

5

10. 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

7

Foreword

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 Ådne Ø. 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 Jiménez-BeltránChair 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, 1994-95“Oxford 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 60°C 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 sludge’s 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


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

(m³/m²)

(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. Krüger A/SGladsaxevej 3632860 SøborgDenmark

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 Ådne Ø. Utvik5 with contributions by Albrecht R. Bresters6 and Martin Würdemann7

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 one‘customer’, 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 850°C 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 Béla Déak10

10 Municipal SewageWorks, FovárosiCsatornázási Müvek Rt.,Marcius 15-e tér 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 chosen when no other ways to dispose ofthe sludge exist. The directions today aretowards agricultural use and incineration.

In the process of siting landfills it has alwaysbeen taken into account that even in case ofthe most careful setting and proper opera-tion, some degree of subsurface pollutionmight occur. This is the reason why geologi-cally vulnerable sites are avoided whenlocating landfill sites.

Vulnerable geological media, such as openkarstic areas and gravel terraces formingsubsurface aquifer layers, are strictly pro-tected for this reason.

Apart from the protection of the presentlyused and future aquifers another factor inthe selection of the site for landfills is thatthe landfill itself has to be on a ‘dry’ loca-tion, i.e. it should be located above theground water level. Minimization of envi-ronmental pollution can be achieved byproper selection of the site, organizeddisposal of the solid wastes, professionalsurface protection and continuousrecultivation of the disposal area, as theleaching of the pollutants by the groundwater and surface precipitation towards theaquifer layers is minimized.

It is also an important factor in the selectionof the location of the landfill site that thedistance between the bottom of the landfilland the unsaturated zone should be as greatas possible.

9.1.1. The Character and Treatment of Sludgebefore Final Disposal on LandfillThe waste water sludge at the waste watertreatment plants has different origins: it isproduced at the preliminary settling tanks(raw sludge), as excess sludge at the biologi-cal plant (activated sludge process), and itcan be the mixture of both of the abovesludge types. Waste water sludge can containall the pollutants contained in the raw(inflow) waste water, and the content oforganic material varies depending on theproportion of the industrial waste water, butusually it falls to the range of 60-70%.

Organic material is readily biodegradable(fat, proteins and carbohydrates),putrescible and causes odour problemsduring the storage period. The sludge is alsoconsidered to be an infectious material.

Sludges are classified as stabilized when theyhave undergone either aerobic or anaerobicstabilization processes or been chemicallytreated, which usually includes a liming step.The addition of the lime to the sludge forstabilization theoretically results in a betterdisinfection efficiency (pathogen removalrate) than for example anaerobic digestion.The disinfection effect of the aerobicstabilization (total oxidation process) is mostuncertain in that respect. Nowadays, thermalaerobic stabilization processes are also usedfor pathogen removal, and this system isconsidered to be much more efficient in thatrespect than the previously used systems.

In smaller plants, sludge drying beds are stillin use, but mechanical dewatering is becom-ing more and more widespread. As a resultof the mechanical dewatering, the originaldry material content (2-3%) of the liquidsludge is increased to 20-30%, which meansthat the sludge can be shoveled already atthis point. Dewatering machines requirechemical preconditioning/treatment of thesludge. Stabilized, dewatered sludge alwayscontains pathogenic microorganisms, whichhave to be taken into account. Lime treat-ment can increase the pH of the sludge upto pH 12, but the inactivation effect on thepathogens is only temporary.

36

37Landfilling

9.1.2. Regulations Governing the Construction ofSludge Disposal SitesIn recent decades landfilling of sludge was afavoured method, however, as explainedbelow, the relatively low cost of sludgedisposal is becoming more and more ques-tionable under the present conditions.

There are two alternatives for disposal ofsludge: mono-deposits where only sludge isdisposed of, and mixed deposits where thesludge from the municipal waste watertreatment plant is disposed of together withthe municipal solid waste. In the latter case,it is customary to utilize the gases from thedeposit (methane and CO2).

Conditions for sludge disposal (sanitarylandfill) are regulated by the ‘TechnicalDirectives’ in each country. Considering thepresent strict regulations and the limitednumber of the potentially suitable disposalsites only regional sludge deposits can beconsidered. It is also desired/required thatbelow the deposit a water tight (e.g. clay)layer should be established to preventinfiltration into the ground water.

Selection of the feasible site has to fulfillother criteria as well. Among these the mostimportant ones are the safety distance fromvarious establishments such as residentialareas, public roads, river dikes, etc. It is ofparamount importance that the selected siteshould not be covered even temporarily byinland waters, or be located on the area ofwater bases (aquifers). Another point is thatthe land should not be of high value from anagricultural point of view and the futuredevelopment plan is not to be intercepted bythe establishment of the sludge deposit.

Finally, economic points have to be consid-ered as well. Among these the distance oftransportation should be minimized, butoccasionally the presence of natural depres-sions, or pits of former mines are taken intoaccount. Traffic should be planned so as toavoid large trucks in residential areas.

Upon selection of the location of the sludgedeposit site the technical design follows. Themost important elements of the design arethe artificial water insulation and drainage.Percolation into the ground and surfacewater must be prevented at all costs. There-fore the landfill should have water tightlayers or lines on the sides (horizontal orsloping).

Establishment of the water insulation layer

starts with the formation of a protective fine(sand) layer below the plastic foil layer. Foilsare welded together on site. Above theselayers the drainage layer is establishedconsisting of a system of drainage pipes, withappropriate slope (for gravitational waterdrainage). Then a protective layer of sandfollows, protecting the underlying drainage-foil system from mechanical damage andconsequent leakage.

Actual setting of the deposit site can berealized in different ways. Depending on thecharacteristics of the area, fill-up deposits aredesigned in valleys or in other naturaldepressions in artificial pits or in plane areasby hill building methods. A combination ofthe above-mentioned methods is also possi-ble, i.e., upon filling up the depression a hillis established on top.

The remaining water content of thedewatered sludge usually does not reach thebottom of the deposit as a result of themicrobiological activity. Leachate water isgenerated from rain water and it is removedby the help of the built-in drainage system.Leachate water is highly polluted, thereforeits treatment is indispensable. This treatmentcan be carried out by traditional waste watertreatment methods, but occasionally theleachate is used for irrigation in forestry.Deposit sites has not only to be fenced, but aprotective forest range has to be establishedaround it as well.

Operation of the deposit will be discussedlater, but already here it is stressed that amonitoring well system has to be establishedaround the area for the detection of poten-tial pollutant migration. Sampled groundwater has to be analyzed in laboratories.

Finally, recultivation of the deposit has to betaken into account when designing such anestablishment. Usually, upon finishingdisposal operations shrubs and forest areplanted on the top of the area. As it appearsfrom the conditions described above, theestablishment of a sludge deposit site isextremely costly, requiring a useful life(filling up capacity) of at least 10 years toachieve cost effectiveness.

9.2. Practical Solutions

There are two alternatives for disposal ofsludge: mono-deposits where only sludge isdisposed of, and mixed deposits where thesludge from the municipal waste water

Sludge Treatment and Disposal38

treatment plant is disposed of together withthe municipal solid waste.

9.2.1. Mono-depositsThe largest constraint on the establishmentof a mono-deposit system in the temperateclimate is the provision of the appropriatewater content of the sludge. Usually, thiscannot be above the 65% percent value as ahigher water content impedes the mechani-cal means of transportation and handling.Suitable consistency of the sludge is checkedby the special shearing-force measuringapparatus. Dried sludge has - at least so far-not been disposed of on waste depositswhere the largest problem is the efficiency ofthe dewatering process. Among thedewatering processes described in detail inchapter 7, the chamber press machine and anew generation of centrifuges can theoreti-cally fulfill or exceed the criteria, after lime-iron salt conditioning.

As an example, the dry material content ofthe lime and iron salt conditioned sludge onchamber press is about 40-50% in the North-ern-Budapest WWTP. The sludge cakeproduced here is disposed of on an appro-priately designed deposit near the capital (20kms away) in the vicinity of the municipalityof Fótcsomád. This deposit is acknowledgedas the most advanced sludge deposit inCentral Europe.

There are strict rules for the operation ofmono-deposits. In theory, these rules alreadyapply when loading the trucks with sludge atthe waste water treatment plant. The mostimportant task is the organization of thedisposal on the deposit, transport vehiclesmovement, and the material movement onthe deposit. All of these processes have to bein harmony with each other. Strict regula-tions apply to the width of the depositedsludge layer (this depends on the consistencyof the sludge), the storage and distributionof the cover layer (soil) for the control ofodour and infection as well as the availabilityto animals (rodents). Collection of leachateand surface precipitation, their collectionmethod, treatment and disposal have to beregularly checked. Should the water samplein the monitoring wells indicate groundwater contamination, the technology has tobe modified. Apart from these rules a widerange of occupational health, hygienic andlabour safety rules have to be followed.

Although the physical conditions in a com-pactly sludge deposit are not really favorablefor gas formation, as in the mixed deposits to

be discussed later, some gas formation isexpected to occur in mono-deposits as well.The gas contains 50-60% methane, 40-50%carbon dioxide (in traces other gases also).The methane is considered a dangerous gas,therefore surface gas evaporation on thesurface of the deposit has to be controlledand checked. The most important task is toavoid gas accumulation in closed (covered)spaces (i.e. in rain water collection shafts).

9.2.2. Mixed DepositsIn forming the mixed deposit the predomi-nant ingredient is the municipal solid waste,whereas the sludge is considered as a second-ary additive as its proportion is usually only20-25% (see figure 9.1 and 9.2). As thecompanies responsible for the collection andtreatment of municipal solid waste are notidentical to the ones responsible for thesewerage and waste water treatment, the firstconstraint to be dealt with is the protest ofthe solid waste management companyagainst receiving and operating with sludge.The reason for this is that the sludge disposalmakes the waste disposal more complicated,especially when the water content of thesludge is high. In spite of this, the establish-ment of mixed deposits is economicallydesirable, and the requirement for thereduction of the water content of the depos-ited sludge is not as high as in the case ofmono-deposits. The water holding capacity(hygroscopy) of the solid waste is relativelyhigh. For the achievement of the propermicrobiological activity within the landfill, awater content in the range of 60-65% isrequired. The waste itself matures (biologi-cally available organic material content isdegraded by the microorganisms, andstabilization takes place) and the alreadymentioned gas formation is enhanced. Inthis deposit type the collection and utiliza-tion of the formed gas is a desirable (some-times compulsory) task (see below).

Regulatory principles for mixed depositswere compiled on the basis of the Germanregulations (LAGA-Deponie Merkblatt 4690,ATV-VKS, Regelwerk, Abwasser-Abfall,Arbeitsblatt A 301):

1. The most important rule of the mixeddeposits is that during processing thesuitable consistency has to be ensured,for the appropriate work of the mechani-cal machinery, to compress the solidwaste and the sludge in an optimal way.

2. Sludge can only be deposited afterdumping an at least 3 meters thick solid

Figure 9.1.

Disposal of sewagesludge and solid waste at

mixed deposit

39Landfilling


Sewage sludge landfilldisposal

Figure 9.2.

40

41Landfilling

waste ground layer. Continuously spreadsludge layers have to be avoided.

3. There are three built-in methods applieddepending on the way in which thesludge is mixed into the solid waste,• Pile deposit. In case of point (pile)disposal the maximum amount of sludgecan be 20-25 percent (w)• Mixed deposit.• Deposit in levels. In two or three levels(see Fig. 9.1). In case of this alternativethe mixing ratio is only 10% for thesludge. In the latter case the building inmethod at the deposit can be freelychosen.

4. Separated (i.e. point like) sludge dis-posal has to be closely followed in timeby the covering up of the sludge pilewith solid waste (see Fig. 9.2).

5. During disposal the formation of the off-gases is not a real hazard, as deposit gasesthough formed in deeper layers arereadily ventilated through a large surfacearea. Upon completing the disposalprocess by covering the surface layer withsoil, significant gas formation is expectedto occur. These gases are to be collectedand utilized. If utilization of the gasescannot be managed, collection and safeventilation of these gases have to beensured in another way (i.e. by ventila-tion chimneys followed by burning).

6. Leachate water percolating through thedeposited waste and sludge layers (it mayfluctuate between 0.01-0.1 l/sec quanti-ties) is considered to be highly pollutedand its chemical oxygen demand canreach values as high as 50-60,000 mg/l,and the ammonia content might reachseveral thousand milligrams per liter.

There are two possible ways of reducing sucha high pollution level. The first is the well-known waste water treatment, the second isthe recirculation of the leachate water to thedeposit (not allowed in some countries). Thelatter method provides twofold benefits: onthe one hand it provides the optimal watercontent necessary for the gas formation andon the other hand via the inner naturalbiodegradation processes (microbiologicaldegradation) the organic material content isgreatly reduced.

Many mixed deposits have been establishedall over the area of the FRG (regarding it asan example). For details consult MuA 62.

EU-Directives on waste

Number of Council Directive Title of rule

75/442/EEC Waste Framework

94/62/EC Packaging and packaging waste

91/689/EEC Hazardous waste

94/67EC Incineration of hazardous waste

COM (97) 105 final, 97/0085 (SYN) Landfill waste

Table 9.1.

Lfg. IX/81, Rettenberger and Tabasaran:Gemeinsame Ablagerung von Hausmüll undKlärschlamm.

9.3. Evaluation of Cost Efficiency

The specific price of establishing a sludgedeposit (including the cost of geological andhydrogeological investigations, area, liner(plastic or clay), leachate and gas collectionand treatment systems (drainage systems),fences, planting, soil for covering the sludge,monitoring wells, control and analysis ofleachate, weigh bridge, buildings, machines,pumps etc., but excluding the price of theland) is approx. HUF 10,000 or USD 67, orDEM 100 per square meter (1996 pricelevel). There are huge differences betweenthe Austrian and German specific prices withregard to the disposal cost of 1 tonne ofwaste. The disposal cost in Styria (Austria)varies between 800-3,500 ATS (USD 80-350).In Germany the price was in 1991 DEM 300 -600, showing a constant increase over thepast few years.

Table 9.1 shows the most important Direc-tives applying to waste management.


10. New Technologies(Gasification, Wet Oxidation)By Ådne Ø. Utvik11 and Bernhard Matter12

10.1. Gasification of Sludge

Gasification of waste water sludge is a rathernew method of sludge processing, anddetailed information on the method islimited. However, the method is brieflydescribed and discussed in order to makethis handbook complete.

10.1.1. The Gasification ProcessGasification is a thermal process whereby afeed stock containing combustible material isconverted with air (some times oxygen and/or steam) to an inflammable gas. BeforeWorld War II this technology was applied forthe supply of local power in industry andlater, due to a shortage of petrol in the war,also as fuel for cars, trucks etc.The most frequently used reactors forgasification are:

• The fixed bed reactor

• The fluid bed reactor

• The circulating bed reactor

Figure 10.1 shows the main steps in thegasification process.The first stage is toremove most of the remaining water bythermal drying. The dry solid content outputfrom the drier is 85-93% DS, depending onthe type of gasifier installed, see below.

Now the sludge is prepared for gasification,i.e. ‘incineration’ with sub-stoichiometricoxygen input. The various components inthe sludge are partly, and some of themcompletely, oxidised.

A large number of reactions take place in thereduction zone of the gasifier. However, theoverall process can be described by thefollowing three main gasification reactions:

• C + CO2 2CO

• C + H2O CO + H2

• CO + 3H2 CH4 + H2O

The input is mechanically dewatered sludge,digested or undigested.

Since the composition of the sludge varies

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

12 CDS, ColombiSchmutz Dorthe AG,Konsumstrasse 20, 3007Bern, Switzerland

➝

➝

➝

➝

➝

➝

greatly, the composition of the exit gas fromthe gasification process will also vary to agreat extent.

As mentioned, gasification of sludge is arather new method and presently at the stageof demonstration. Thus, there is no adequateand long time experience making a reliablecomparison between gasification and incin-eration possible.

10.2. Wet Oxidation

The organic content of sludge (approxi-mately 5% DS) is oxidated in specific reac-tors at temperatures of between 200_C and300_C and at pressure levels of between 30bar and 150 bar (low/high pressure sys-tems).

The necessary pressure may be reachedthrough high pressure pumps or throughspecially designed reactors: tubes, reaching1,200 meters deep into the underground, atthe bottom of which the fluid sludge passesthrough a zone of high pressure and tem-perature before rising up again to thesurface. In this application, dewatering is notnecessary for the input to the sludge treat-ment process.

The main output of the process is sludgecontaining more than 95% of mineralcomponents and less than 3% of low-molecu-lar organic substances. The sludge isdewatered (i.e. beltfilterpress) and thenrecycled or landfilled.

Since the filtration water is very rich inammonia, it needs to be treated locally (de-/nitrification) or led to a public waste watertreatment plant.

An upscale oxidation plant is running in theNetherlands. However, this plant seems tohave some operational difficulties.

10.3. Plasma Pyrolysis

Plasma arc heaters (torches) makes heatingof hydrocarbon waste materials (i.e. sludge)to high temperatures as possible. The hydro-carbon content of the waste is pyrolised at

42

2,000_C to 4,000_C into a gas mixture (80%CO2 and H2) which may be used in themanufacture of methanol or as fuel. Themolten slag-metal mixture solidifies into agas-like non-leachable solid.

The ratio between the energy output and theinput of electric energy is around 4.

Sludge gasification- flow sheet.

Courtesy of ScheldeMilieutechnologie B.V.

Figure 10.1.

The technology, widely applied in themetallurgic industry for decades, has beenadapted to waste management conditionsand requirements since the eighties. Upscaleplants treating municipal waste are underconstruction in the USA. In Europe, severalplants are operated mainly to achieve vitrifi-cation of slag and ashes as outputs of conven-tional waste incineration plants.

43New Technologies


11. Environmental ImpactAssessmentsBy Albrecht R. Bresters13, advice by M. Würdemann14

13 DRSH ZuivoringsslibBV, Hof plein 33, NL-3000AZ Rotterdam, TheNetherlands

14 Haskoning ConsultingEngineers , TheNetherlands

11.1. General

The environmental impact assessment study(EIA study) can play an important role in thedecision making process of large sludgetreatment projects. In the Netherlands, anEIA is obligatory if the sludge amountexceeds 5,000 tonnes of dry solids a year.

This EIA can be applied and executed in twodifferent manners:

• to support decision making at policylevel;

• to support the procedure for granting ofpermits.

The first type of EIA is probably the mosteffective one as it enables a comparison ofalternatives and decision making at a strate-gic level. This EIA may comprise subjectssuch as system and site selection.

For projects which may have an extensiveenvironmental impact, a project EIA isusually part of the procedure for granting ofpermits. The choice which has been made isfurther evaluated at a project level in termsof its environmental impacts and the possi-bilities of their mitigation.

In summary, the goal of a policy EIA is toevaluate systems (system selection, standalone solutions or solutions in combinationwith other waste or waste water treatmentplants or a power station) and to evaluatepossible sites.

The goal of the project EIA is to optimise theproject within stricter boundary conditions,such as the considered site, the local infra-structure and the existing facilities.

11.2. Set-up of the EIA

An EIA starts with a definition of the problemand the goal to be achieved. Within thecontext of this paper the EIA will deal withthe sludge problem in a certain country orregion. Considered on a broader basis sludgeis part of the waste water treatment chain andso it may in some cases be incorporated in theEIA on waste water treatment in a certain

region. But also in that case, the goal of theEIA is thus defined that in a certain region alarge amount of sludge has to be disposed ofin an environmentally safe manner. Thissludge may originate from municipal wastewater treatment plants, from industrial wastewater treatment plants or from both.

The goal of a policy EIA is to provide certainboundary conditions for new policies andenvironmental planning. The goal of theproject EIA, on the other hand, is morelimited, namely to define an optimal solu-tion within existing planning boundaryconditions. For instance, the questionwhether sewage sludge (or the organicresidual products of treated sewage sludge)is still allowed for agricultural purposes orlandfilling is quite important for the selec-tion of applicable technologies.

Once the legal and policy boundary condi-tions have been defined, it is very importantthat the actual systems and technologies tobe considered are evaluated at the same leveland in the same depth. This means that allinputs and outputs of the considered tech-niques (composting, thermal drying, incin-eration, vitrification, gasification, wet oxida-tion etc.) should be considered completelyin terms of their environmental impacts.Consequently, the effects of sludge transpor-tation as well as the possible environmentalimpacts of residual materials (for re-use assecondary raw material, fuel or for land-filling) should be included.

A problem may arise when the informationavailable on the various techniques is notreadily comparable. This can happen when‘state-of-the-art’ technologies are comparedwith ‘prototype’ solutions.

The description of the various techniques(‘alternatives’) should include reliable andcomparable information on the followingprincipal environmental criteria:

• air quality and emissions to air;

• surface water quality and resulting wastewater streams;

• soil and ground water conditions,residues etc.;

44

• noise effects;

• ecological impacts;

• safety;

• energy;

In the decision making process, however, alsoother criteria may play an important role:

• physical planning criteria, such as impacton landscape, required space and infra-structure site requirements;

• technical criteria, such as state-of-the-art,flexibility and technical complexity;

• last but not least, financial criteria, suchas investment and operating cost com-pared to the cost of non-action

The EIA study includes a comparison of thevarious alternatives, with an indication ofunknown factors which may influence thesystem selection. In a project EIA, once asystem has been selected, a number ofoptimisations within the system may beevaluated in terms of their relative environ-mental impacts, such as various ways ofsludge storage, reactor types or gas treatmentsystems.

The EIA can be finalised by indicating themethod of monitoring and evaluation of the(future) project.

11.3. Procedure

The basic procedure for an EIA in theNetherlands can be summarised as follows:

1. The policy decision (in case of a policyEIA) or the project (for a project EIA)are described by the organisation respon-sible or taking responsibility for theestablishment of policies or for theproject itself. The description includes abrief outline of the boundary conditionsand the subjects to be studied in the EIA.The comments of the public as well asthe parties involved are requested.

2. A national advisory commission presentsguidelines for the EIA. These guidelinesare finally established by the authorityresponsible for the policy planning and/or the granting of permits, taking intoaccount also comments from otherparticipants.

3. The actual EIA study is executed, andsubsequently the above-mentionedcommission advises the authority respon-sible.

4. The results of the EIA and the advice willfinally be used to define the policy and/or the permit conditions. Also duringthis phase of the procedure the possibi-lity of public participation exists.

The EIA procedure is governed by European(Directive 85/337/EEC) and by national law.

45Environmental Impact Assessments


12. How to Decide on SludgeDisposalBy Bernhard Matter15

12.1. Legal Boundaries

Legislation has the following impact onsludge disposal,

• Prescriptions as to the allowed disposalof sludge itself; utilisation in agricultureor forestry as sludge or as compostconditions for landfilling

• Prescriptions as to emission standards tobe met by sludge incinerators

• Prescriptions as to the quality of outletsfrom waste water treatment plants affectsthe amount and quality of sludge pro-duced

15 CDS, ColombiSchmutz Dorthe AG,Konsumstrasse 20, 3007Bern, Switzerland

Utilisation in agriculture/forestry 150 - 400

Composting 250 - 600

Drying 300 - 800

Incineration* 450 - 800

Landfilling 200 - 600

*Lower commercial prices possible in case of paying marginal prices for sludge incineration at waste incineration plants

Treatment costs in DEMper tonne of drysubstance

Table 12.1.

An overview of legislation on sewage sludgeis provided in appendix 14.1.

12.2. Cost

Costs are an important element in decidingon sludge treatment. They vary greatlydepending on local conditions and on thesize of the treatment facility. In order toachieve appropriate comparison of differentchoices of sludge treatment and disposal, it isimportant to take into account the overallannual costs, which are influenced by threemain components.

First, annual capital costs, depending on thesum to be invested, the method of financing,

the depreciation period and the interestrates on borrowed capital. Second, runningcosts due to energy, chemicals, maintenance,salaries of the personnel, taxes, insurance.Third, the costs of disposal or re-use of thefinal product or residue.

The above costs are all generated at thesludge treatment plant or further on in theprocess. If the sludge treatment is noteffected at the sewage treatment plant itself,usually transport costs and costs of storagefacilities due to transport methods are not tobe neglected. As an order of magnitude,specific treatment costs may vary as indicatedin table 12.1.

By making cost comparisons, that fact thatthe quality requirements of the input sludgeand thus the pre-treatment costs may bedifferent from one method to another alsohas to be considered. For instance, wetoxidation and in some cases also utilisationin agriculture do not require dewatering ofthe sludge. The above costs do not includetransportation and dewatering, nor anyother pre-treatment (aerobic or anaerobicstabilisation) of the sludge. The cost ofutilisation in agriculture includeshygienisation.

12.3. Decision Tree

A decision tree is presented in figure 12.1.The key words are commented on in table12.2.

The decision tree shows the different meth-ods for treatment and disposal of fluid,stabilised sludge. Stabilisation means aerobicthermophilic pre-treatment or anaerobicdigestion, both resulting in a sludge withlittle biological activity, thus minimising gasand odour emissions.

The conditions of deciding which way to goin the decision tree are numbered. Examplesof what may be a reason for choosing thisway rather than another are listed in table12.3. This table is neither exhaustive nor areall of the decisions unequivocal. It mainlyhas a illustrative purposes.

Non-stabilised sludge may be treated in a

46

similar way as polluted, stabilised sludge. Inthis case conditioning not only has to im-prove mechanical properties, but also ensurebiological stability. Before such stability isreached, it is important to treat the non-stabilised (fresh) sludge continuously asintermediate storage of fresh sludge maylead to odour and safety problems. In mostcases it is therefore reasonable to stabilisethe sludge as a first treatment step (e.g.anaerobic digestion).

Sludge TreatmentDecision Tree

Figure 12.1.

47How to Decide on Sludge Disposal


Table 12.3.

Comments on keywordsin figure 12.1Additional information isprovided in the otherchapters of this booklet.

Table 12.2.

Number Examples of conditions influencing the decisionin figure 12.1

1 Utilisation of thin and/or dewatered sludge in agriculture is possible

2 No use in agriculture or at the most as dry granular sludge

3 Mechanical properties have to be improved for intermediate storage, transportation orlandfilling

4 Landfilling is not desired

5 Incineration in a waste incinerator or similar furnace allowing input of dewateredsludge. Limits and variations of dry substance content after dewatering have to becontrolled

6 Granular spreading in agriculture, due to seasonal storage of dried sludge or to non-acceptance of other types of sludges by farmers

7 External valorisation of dried sludge as a fuel or disposal of surplus stock of driedsludge or mixing of dried sludge with dewatered sludge in order to reach input limitsof dry substance for the furnace

8 Application of thin (non-dewatered) sludge in agriculture

9 Green waste from gardens or other compostable waste is available for mixing withsludge. Utilisation of compost, e.g. as a soil conditioner, is possible

10 Mechanical properties have to be improved for intermediate storage, transportation orapplication in agriculture. Landfilling of surplus sludge.

11 External valorisation of dried sludge as a fuel or mixing of dried sludge withdewatered sludge in order to reach input limits of dry substance for the furnace.

Explanation of numberscontained in the decisiontree

Term in figure 12.1 Comment

Polluted A sludge is polluted if it does not fulfil the legal requirements for agricultural use.Pollution normally consists of an excessive heavy metal content.

Disinfection Disinfection is necessary for agricultural application of thin or dewatered sludge asanaerobic digestion alone does not guarantee sufficient elimination of germs. Usuallypasteurisation is chosen to produce a hygienic sludge.

Dewatering Thickening of thin sludge by mechanical methods (presses). The dry substance contentof dewatered sludge is approximately 15 to 35%

Conditioning Conditioning is mixing the dewatered sludge with an additive reducing the thyxotropicproperties and increasing the mechanical resistance in order to make the sludge easierto store, transport and deposit. Most additives are made on the basis of crushedlimestone.

Drying The dry substance content after thermal drying is usually 80 to 90%. The resultinggranular sludge is then biologically stable, easily storable and transportable and maybe used as fuel or as fertiliser. In the case of end disposal in a landfill or a wasteincineration plant, drying may be reasonable only up to a dry substance content of 40to 60%, which can also be reached by mixing of dried sludge with dewatered sludge.

Incineration The purpose of incineration is not only sludge elimination but also recovery of energy.This may be done at a special sludge incineration plant or with dried sludge as a fuel,at an external power plant or cement kiln. When incineration occurs at a nearby wasteincineration plant, pre-drying may be less important than for other uses.

New technologies Gasification, wet oxidation, plasma pyrolysis.

Landfill Landfilling of dewatered or even partially dried sludge is not or will not be allowed inall countries. Dewatered sludge often has to be conditioned before disposal. Theresidues (ashes) of incineration plants are usually disposed of at landfills.

Industrial use Dried sludge may be used in the fertiliser industry, in power plants, incineration ashesin the construction industry.

Agricultural use Sludge may be used in agriculture as thin, not dewatered sludge or as dewateredsludge, in the latter usually after conditioning. A third way is spreading of dry sludge asa commercial fertiliser, for instance if the sludge is dried and stored during the winterseason. Similar to the use in agriculture is the application in forestry, horticulture orviticulture.

48

13. Appendices

13.1. Legislation on Waste Water Sludgeas per 1 October 1996By Alice Saabye16

European waste policy is based on a hierar-chy of management priorities:

• Minimisation

• Recycling

• Incineration with energy recovery

• Landfilling

The following legislation affects treatmentand disposal of waste water sludge.

13.1.1. EU Countries

Urban Waste Water Treatment Directive(91/271/EEC)As the directive lays down stricter require-ments for waste water treatment, it leads to aconsiderable increase in the amount of wastewater sludge.

The directive requires that the EU membercountries should establish secondary treat-ment of waste water for waste water treat-ment plants exceeding 2000 PE by 31-12-2005. At discharge to fresh receiving waters,the waste water must be treated for nutrientsand organic substances (articles 4 and 5).

The directive prohibits discharge of sewagesludge into the sea and freshwater after31.12.1998 (article 14).

Directive on Protection of the Environment and inparticular of the Soil when Waste water sludge isused in Agriculture (86/278/EEC).This directive concerns the regulations thatmust be met if sludge is used on farmland.The following requirements are common tothese regulations:

• Pre-treatment

• Restriction on the content of heavymetals 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

• Restriction on the content of micro-pollutants

• Restriction on the amount of nutrientsadded to the soil (N and P)

• Restrictions on the choice of crops

• Restricted access conditions to farmlandon which sludge is spread

• Legislative compliance control

Draft Directive on Waste Disposal (95/09/15)Proposal for a Directive on the landfill ofwaste. The draft directive describes therequirements for design, establishment,operation and close-down of landfills.Landfills must be environmentally approvedby the authorities.

The directive classifies landfills in threecategories, which differ in relation to re-quirements for liners and control:

• Landfills for hazardous waste

• Landfills for non-hazardous waste

• Landfills for inert waste

The landfilling of waste water sludge is notforbidden, but restricted, as it is biodegrad-able waste. The target for restriction is tohave only 25% of the quantities of biode-gradable waste produced in 1993 landfilledin 2010 (Article 5).

Draft Directive on Incineration of Waste(94/08/20)The draft directive includes waste incinera-tion plants as well as sludge incinerationplants and lays down requirements for allemissions from these plants, e.g.:

• Air emissions

• Solid residues from incineration and fluegas cleaning (sludge ash)

16 I. Krüger I/S,Gladsaxevej 363,2860 Søborg,Denmark

49


• Waste water from flue gas cleaning(scrubber water)

• Leachate from ash deposit

Sludge incineration plants must be environ-mentally approved by the authorities.

13.1.2. USAFor many years, preparations have beenmade in the USA to work out sludge regula-tions covering all fields of application anddisposal. On 25 November 1992, the finaledition of part 503 ‘Standards for the Use orDisposal of Waste water sludge’ was ready.

In addition to requirements for agriculturaluse of sludge, the regulations include incin-eration and landfilling as well as detailedrequirements for pre-treatment.

The American regulations contain veryspecific directions for sludge treatmentbefore disposal including requirements forpathogens and vector attraction reduction.They also divide the sludge into two classes.If a number of further specified require-ments are complied with, sludge classified asClass A can be used without restrictions. Lessstrict requirements on sludge pre-treatmentapply to sludge classified as Class B, whichmust only be used under certain specific siterestrictions.

The directions applying to sludge in Class Aare split into six alternative methods andthose applying to sludge in Class B are splitinto three alternative methods including siterestrictions.

The pathogen reduction processes aredivided into two stages:

• Processes to significantly reduce pathogens (PSRP)

• Processes to further reduce pathogens(PFRP)

13.2. Plant Utilisation of Nutrients inSludgeBy Alice Saabye17

Sludge contains a number of nutrients. Ingeneral, the largest part of the nutrient valueis phosphorus, but a considerable amount ofnitrogen is also contained in the sludge.However, the content of potassium andmagnesium is low. The amount of nutrientscontained in the sludge varies a great deal,

which may be attributed to different treat-ment methods. It is therefore important thatanalyses of the current sludge should beavailable for inclusion in the fertiliser plansfor the fields onto which the sludge isspread.

13.2.1. NitrogenPlants cannot make immediate use of thenitrogen contained in the sludge as the mainpart is organic and needs to be convertedinto inorganic nitrogen by mineralisationbefore it can be absorbed by the plants.

Plants generally need addition of nitrogenearly in the season of growth, whereasmineralisation of nitrogen is not performeduntil after the spreading when the soilbacteria have been activated. Consequently,plants with a short season of growth, e.g.grain crops, cannot utilise the nitrogen verywell, whereas plants with a longer season ofgrowth, e.g. beets, corn and rye grass, areable to utilise the nitrogen to a greaterextent.

The type of nitrogen and its effect on plantsdepend on sludge type and treatment, cf.table 14.1.

As can be seen from table 14.1, the effect ofnitrogen from dewatered sludge on plants isless significant owing to removal of dissolvedinorganic nitrogen with the reject water.

13.2.2. PhosphorusSludge is generally a good phosphorusfertiliser, even if the phosphorus containedin sludge is much less readily soluble thanthe phosphorus contained in artificialfertiliser. On well-fertilised soils, the phos-phorus state is merely to be maintained. Thephosphorus accessibility in sludge dependson the treatment of the sludge. Table 14.2shows the phosphorus accessibility to theplants. The amount of citrate soluble phos-phorus shows how much of the phosphorusis accessible to the plants.

13.3. Basic CharacterisationBy Ludovico Spinosa and Vincenzo Lotito18

Many parameters and tests are available forthe characterisation of sewage sludge. Themain problem is that they are generallyspecific to the method of treatment anddisposal and not able to give fundamentalinformation on the sludge in question.

The most important parameters yielding

17 I. Krüger I/S,Gladsaxevej 363,2860 Søborg, Denmark


50

basic information are particle size distribu-tion, water retention and rheological proper-ties.

13.3.1. Particle Size DistributionThe main problem in particle size evaluationrelates to the measurement difficulty. Threeprincipal methods are known, i.e. directexamination, fractionation and counting,but each of them requires modifications ofthe sludge before and during measurements,which may result in an alteration by themeasurement techniques of the originaldistribution. A reproducibility problem isalso present due to the irregular shapes ofsludge flocs, so each step in a sizing proce-dure must be conducted carefully to pre-serve the original floc shape, size and struc-ture [2].

The main problem of direct examination isto fix sludge in an inert medium to allownon-disruptive examination. Thefractionation with a series of filters or screencould cause the retention of smaller particles

Sludge type 1st year uptake in plants Nitrogen type Possible effect for several years

LiquidPrimary sludge 30-35% of total nitrogen Mainly organic Yes

if spring spreading15-20% of total nitrogenif autumn spreading

Biological sludge 50% of total nitrogen Organic Yes

Anaerobic, 100% of NH4 Mainly inorganic Nodigested sludge + 15% of organic nitrogen (ammonia)

Dewatered:Primary sludge 15-20% of total nitrogen Organic Yes

Anaerobic, 15% of total nitrogendigested sludge Organic Yes

Table 14.1.

Utilisation of nitrogenfrom different types of

sludge

Nutrient 1st year uptake in plants Possible effect for several years

Phosphorus:Total phosphorus 50-80% Yes

Citrate soluble 100% Yes, if surplusphosphorus

Potassium 100% Yes, if surplus

Table 14.2.

Utilisation of phosphorusand potassium fromwaste water sludge

by layers where larger ones prevail. Finally,counting methods involve the use of differ-ent instruments, which measure signalsproportional to particle size: such instru-ments generally require mixing possiblyresulting in a change in the particle sizedistribution.

13.3.2. Water RetentionAlthough the names given to the variouswater fractions can be different, the follow-ing classification appears appropriate: freewater, capillary water, vicinal water andchemically bound water.

Classical analysis is carried out bythermogravimetry, which consists of submit-ting a small sample of material to drying atlow temperature under standard conditions:the results are presented as drying ratecurves consisting of a constant rate period, afirst falling rate period and a third one. Analternative method is based on the theorythat bound water does not freeze at tempera-tures below the freezing point temperature

51Appendices


of free water, so the free water can be deter-mined by noting the expansion caused byfreezing. A differential thermal analysisprocedure has also been used [4].

13.3.3. Rheological PropertiesA powerful technique in characterisingsludges is to classify them according to theirflow properties.

The fluidodynamic characterisation ofsludges has been widely applied in the pastto the study the flow of slurries, but due tothe existence of some correlation betweenthe rheological properties and the suitabilityof sludges to treatments, such asflocculation, thickening and dewatering, anddisposal, mainly landfilling has been evi-denced [1], [3].

The rheological behaviour of sewage sludgesis generally described as non-Newtonian(Ostwald-pseudo-plastic and Bingham-plasticmodels have been found to be the mostrecurrent for waste water sludge); activatedsludge has also been found to be thixotropic,which means that sludge rheology is alsodependent on the shear rate history.

A wide variety of viscometers is available.They fall into the two general categories oftube type and rotating type, whose applica-bility depends on the sludge physical state(liquid, plastic, solid).

The tube viscometers involve a faraginousprocedure and are not commercially avail-able; moreover, the tube diameter must belarge enough to prevent clogging.

Rotational viscometers are generally consid-ered to be more reliable. They may havecoaxial cylinders, rotating blades and cone-plate geometry. Drawbacks of coaxial cylin-der types are that cylinders must have a smallgap, so there is a risk of obstructions by solidmaterials, and slipping may occur at thecylinder/liquid interface. In the case ofviscometers with blades (or vane apparatus),the velocity gradient is less well defined, soonly a mean value based on the mechanicalenergy dissipated in the medium can beobtained. The cone-plate geometryrheometers can be excluded on the basis ofboth the large size of sludge particles relativeto the gap and the generally poor sludgeconsistency.

52

14. References

14.1. Sludge Characterisation

[1] APHA-AWWA-WPCF (1980). StandardMethods for the Examination of Water andWastewater, 15th Edn., Washington D.C., USA

[2] Bruce A.M. and Fisher W.J. (1984).Sludge stabilization - methods and measure-ment. In: sewage Sludge Stabilization andDisinfection. Bruce A.M. Ed., EllisHorwood, Chichester, U.K.

[3] Colin F. (1979). Methodes d’evaluationde la stabilité biologique des boues residu-aries, E.E.C.-COST 68 bis, Working Party 1Meeting, Institut de Recherches Hydro-logiques, Nancy, France, 25 Sept. 1979.

[4] Heide B.A., Kampf R. and Visser M.A.(1982). Manual for the selection and use ofpolyelectrolytes in dewatering sludge withbeltpresses, TNO Report 124E, Delft, TheNetherlands.

[5] Lockyear C.F. and White M.J.D.(1979). The WRC thickenability test usinga low speed centrifuge, WRC TechnicalReport TR 118, Stevenage, U.K.

[6] Spinosa L., Mininni G., Barile G. andLorè F. (1984). Study of belt-press operationfor sludge dewatering. In: Proc. of the E.E.C.Workshop Methods of Characterization ofSewage Sludge, Dublin, Eire, 6 July 1983, 16-29, D. Reidel, Dordrecht, The Netherlands.

[7] Spinosa L., Lotito V. and Mininni G.(1990). Evaluation of sewage sludge cen-trifugability. Proc. of V World FiltrationCongress, Vol. 2, 327-330, Nice, 5-8 June,1990.

[8] Vesilind P.A. (1971). Estimation ofsludge centrifuge performance, Journalof the Sanitary Eng. Division, ASCE, 97, SA2,234-238.

[9] Vesilind P.A. (1974). Treatment andDisposal of Wastewater Sludge. Ann ArborScience, Ann Arbor, U.S.A.

[10] Vesilind P.A. and Zhang G. (1983).Technique for estimating sludgecompatibil-ity in centifugal dewatering. Personal com-munication.

14.2. Storage and Transportation

[1] Castorani A., Spinosa L. and Trosi S.(1985). Preliminary criteria for the design ofsewage and sludge pumping systems. Phoe-nix International, 3/4, 22-29.

[2] Frost R.C. (1981). How to designsewage sludge pumping systems, W.R.C.Process Engineering, 85-S.

[3] Frost R.C. (1982a). Prediction offriction losses for the flow of sewagesludges along straight pipes. Water ResearchCentre Technical report TR175, WaterResearch Centre, Stevenage (UK).

[4] Frost R.C. (1982b). A method ofestimating viscosity and designing pumpingsystems for thickened heterogeneoussludges. 8th Int. Conf. Hydr. Trans. Solids inpipes, Johannesburg, SA, August 25-27.

[5] Spinosa L. and Sportelli S. (1986).Storage and transport of sewage sludge.Phoenix international, 3, 34-37.

[6] U.S.EPA (1979). Process designmanual for sludge treatment and disposal.EPA 625/1-79-011, Cincinnati (OH-USA).

[7] U.S.EPA (1984). Use and disposal ofmunicipal wastewater sludge. EPA 625/10-84-003, Washington DC (USA).

[8] Wiart J. (1993). Les differentsprocedes de stockage des boues d’épurationavant valorisation en agriculture. Etude del’Agence de l’Environment et de la Maitrisede l’Energie et al., Angers (F).

[9] Willis D.J. (1978). A literature surveyon sewage sludge pumping. B.H.R.A.TN 1500

14.3. Agricultural Use

[1] Handling, Treatment and Disposal ofSludge in Europe. Situation Report 1. AnOverview. ISWA Working Group on Sewageand Waterworks

53


14.4. New TechnologiesGasification

[1] D.E. Gough, I.F. Hammerton, T.R.Griale and C.K. Herdle: ‘Demonstration ofthe Oil-from-Sludge (OFS) Technology atthe Malabar Sewage Treatment Plant.’ 14thFederal Convention, Australian Water &Wastewater Ass., Perch, March 1991

[2] Anton Rottier / G.H. Huisman,Schelde Milieutechnologie B.V., personalcommunication

[3] Mr. Mayer, Noell-KRC, personalcommunication

14.5. Basic Characterisation

[1] Colin F. (1982). Characterization ofthe physical state of sludges. Cost 68a, Work-

ing Party N° 1, Institute de RecherchesHydrologiques, Nancy (F).

[2] Da-Hong L. and Ganczarczyk J.J.(1986). Physical characteristics of activatedsludge flocs. Dept. of Civil Eng., Univ. ofToronto. Personal communication to SludgeNetwork.

[3] Frost R.C. (1983). Inter-relationbetween sludge characteristics. In: Methodsof characterization of sewage sludge, T.J.Casey P. L’Hermite P.J. Newman Eds., D.Reidel Publ. Co., Dordrecht (NL), 106-122.

[4] Tsang K.R. (1989). Moisture distribu-tion in wastewater sludges. Dissertationat the Dept. Civil & Env. Eng. (P.A. Vesilind,Advisor), Duke Univ., Durham (NC), USA.

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sludge treatment and disposal

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