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European Commission

Disposal and recycling routes for sewage sludgePart 3 Scientific and technical report


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A great deal of additional information on the European Union is available on the Internet.It can be accessed through the Europa server (http://europa.eu.int).

Luxembourg: Office for Official Publications of the European Communities, 2001

ISBN 92-894-1800-1

European Communities, 2001Reproduction is authorised provided the source is acknowledged.


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IRU6HZDJH6OXGJHScientific and technical sub-component report

23 October, 2001

European Commission

DG Environment B/2


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3.2.1 Pre-treatment 20

3.2.2 Primary sludge 20

3.2.3 Secondary sludge 20

3.2.4 Mixed sludge 21

3.2.5 Tertiary sludge 21

3.2.6 Digested sludge 21

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3.4.1 Organic matter 23

3.4.2 Nitrogen and phosphorus content 24

3.4.3 Calcium enrichment 26

3.4.4 Other compounds of agricultural value 26

3.4.5 Heavy metals 26

3.4.6 Organic pollutants 27

3.4.7 Pathogens 28

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3.5.1 Pulp and paper industry 283.5.2 Tannery Sludge 29

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5.5.3 Gasification 65

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5.6.1 LCA for the ARA Region Bern, Switzerland 66

5.6.2 LCA for the City of Bremen, Germany 66

5.6.3 LCA for the French Water Agencies, France 67

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6.1.1 Behaviour in soil 71

6.1.2 Transfer to water 76

6.1.3 Uptake by plants 77

6.1.4 Uptake by livestock 82

6.1.5 Human exposure 84

6.1.6 Conclusion on heavy metals transfers in food chain 86

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6.2.2 Transfer to water 92

6.2.3 Uptake by plants 936.2.4 Uptake by livestock 93

6.2.5 Conclusion and human exposure 94

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6.3.1 Origin in sludge 95


6.3.3 Water contamination 98

6.3.4 Survival on plants 98

6.3.5 Pathogens transfers to animals and to natural ecosystems 99

6.3.6 Human exposure 99

6.3.7 Conclusion 100

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6.4.2 SO2 102

6.4.3 NOx 103

6.4.4 Particulate matter 103

6.4.5 Other Gases 104

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5HVXOWVDQGGLVFXVVLRQ 7.3.1 Results 113

7.3.2 Influence of sludge composition and transfer phenomena 119

7.3.3 Limits and evolution of the model 121

7.3.4 Sensitivity analysis 121

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This report aims to describe sewage sludge production, composition, treatment and disposal or

recycling, as well as to review scientific evidence regarding the migration and accumulation of

substances and elements contained in sludge into the environment and the food chain, and to

identify the associated risks. It focuses in particular on the recycling routes.

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Sludge is composed of by-products collected at different stages of the wastewater treatment

process. It contains both compounds of agricultural value (including organic matter, nitrogen,

phosphorus and potassium, and to a lesser extent, calcium, sulphur and magnesium), and pollutantswhich usually consist of heavy metals, organic pollutants and pathogens. The characteristics of

sludge depend on the original pollution load of the treated water, and also on the technical

characteristics of the waste water and sludge treatments carried out.

Sludge is usually treated before disposal or recycling in order to reduce its water content, its

fermentation propensity or the presence of pathogens. Several treatment processes exist, such as

thickening, dewatering, stabilisation and disinfection, and thermal drying. The sludge may undergo

one or several treatments.

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Once treated, sludge can be recycled or disposed of using three main routes: recycling to

agriculture (landspreading), incineration or landfilling. Other, less developped outlets exist, such as

silviculture, land reclamation, and other developing combustion technologies including wet

oxidation, pyrolysis and gasification. Each recycling or disposal route has specific inputs, outputs

and impacts.

Landspreading

Landspreading of sludge or sludge-derived material partially replaces the use of conventional

fertilisers, since it contains compounds of agricultural value. It also contains organic matter,

although under a form and at a level below that which would have a significant positive impact on

soil physical properties. Composted sludge however presents a more stable organic matter due to

the addition of a vegetal co-product during the process.

However, landspreading also involves the application of the pollutants contained in sludge to the

soil. These pollutants undergo different transformations or transfer processes. These processes

include leaching to groundwater, runoff, microbial transformation, plant uptake and volatilisation

and enable transfer of the compounds into the air and water, and their subsequent introduction into

the food chain.

Therefore outputs of sludge recycling consist of yield improvement, but also of emissions of

pollution into the soil, and indirect emissions into air and water. Other emissions into the air

include exhaust gases from transportation and application vehicles.


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Incineration

Incineration is a combustion reaction. Different techniques are currently performed, classified

between mono-incineration when sludge is incinerated in dedicated incineration plants, incineration

with other wastes, or co-incineration when sludge is used as fuel in energy or material production.

Other technologies are also being developed such as wet oxidation or pyrolysis.

Outputs are flue gases, ashes, and wastewater, as well as the production of energy. Therefore

incineration generates emissions into the air (particles, acid gases, greenhouse gases, heavy metals,

volatile organic compounds, etc.), soil (disposal of ashes and flue gas treatment residues to landfill,

atmospheric deposition of air emissions) and water (flue gas treatment wet processes). Emissions

into the air may be reduced thanks to flue gas treatment. Emissions depend on the process, but are

also influenced by the sludge type. Energy production generally counterbalances the energy needs

for sludge drying.

Operation of an incineration plant may also produce noise, dust, odour and visual pollution.

LandfillingThere are two possibilities in terms of sludge landfilling: mono-deposits, where only sludge is

disposed of, and mixed-deposits (most commonly observed), when the landfill is also used for

municipal wastes.

The inputs of landfilling are the waste and additional resources required for the operation of the

landfill site, such as fuel for vehicles, electricity, and additional materials when leachate is treated

on-site. Outputs consist of leachate, landfill gas and energy production when the gas is recovered.

Landfill operation therefore generates emissions into the air (mainly greenhouse gases like methane

and carbon dioxide, reduced when biogases are collected and burnt), and into the soil and water at

dumpsites (various compounds such as ions, heavy metals, organic compounds and micro-

organisms in leachate). The operation of a landfill also generates other impacts in terms of noise

and dust from the delivery vehicles, as well as odours, land use, disturbance of vegetation and thelandscape.

Other routes

Other sewage sludge recycling routes presently used in Europe include the use of sludge in forestry

and silviculture or in land reclamation.

)RUHVWU\DQGVLOYLFXOWXUH refer to different kinds of tree plantation and use. The term forestry is

mainly used when considering amenity forests, or mature forest exploitation. On the contrary,

silviculture is more specifically used when referring to intensive production. From the agricultural

and environmental point of view, differences exist in terms of the impact of landspreading as

compared to the use of sludge in forestry, relating to such factors as the plant species grown, thefauna and flora involved, and the soil types.

Agronomic benefits are increased tree growth and the provision of nutrients to the soil. However,

competition with weeds, especially in young plantations may be observed. Excessive rates of

sludge application may also lead to degradation of the upper layer of the soil and the humus, as

well as nitrogen leaching to groundwater. The use of sludge in a forest environment may cause an

alteration in the characteristics of the ecosystem and, in the case of a mature forest where there is

no need to have an additional input of nutrients, may disturb the natural biotopes. More research is

however needed on this issue.

When considering the risks to humans associated with the presence of heavy metals in sludge, it is

assumed that these are lower than those associated with spreading on agricultural land, as forest

products represent only a very small part of the human diet. However, some risks may still exist

due to the transfer of heavy metals to game or edible mushroom species, and in a general manner to

wild fauna and flora.


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After identifying gaps in knowledge, some recommendations are given in this report concerning

sludge application in forest or tree plantations.

Use of sewage sludge in ODQG UHFODPDWLRQ DQG UHYHJHWDWLRQ aims to restore derelict land or

protect soil from erosion through soil provision and increased vegetal covering. In the case of

industrial sites, topsoil may often be absent or if present, damaged by storage or handling. Soil or

soil forming materials on site may be deficient in nutrients and organic matter. Other problems may

exist, such as toxicity, or adverse pH levels. All these problems create a hostile environment for the

development of vegetation.

Possible solutions include the use of inorganic fertilisers or imported topsoil, which can be very

expensive depending on location and availability. An alternative solution is the use of organic

wastes such as sewage sludge, which is already performed in Sweden, Finland, Germany and the

United Kingdom.

Sludge application takes place using the same machinery as in recycling to agriculture. Some

specific machinery for sludge projection may be needed when applying sludge in areas where

access is difficult.

It was assumed that risks are lower than in the case of spreading on agricultural land, when its useis not related to food production. However, no data is available concerning the potential impacts on

wild fauna and flora. Moreover, the amount of sludge applied as well as the application of sludge to

sloping land to reduce erosion go against current regulatory prescriptions for the use of sludge in

agriculture, inducing risks in terms of pollutants application.

Developing technologies

Several technologies presenting an alternative to conventional combustion processes are currently

being developed or introduced onto the market. These technologies mainly include by the wet

oxidation process, pyrolysis, and the gasification process. Other technologies may be found, which

are most often combinations of these three main processes.These technologies present advantages in terms of flue gas and ash treatment. Moreover, they also

seem to have reduced impacts on the environment compared to conventional combustion processes.

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A review of current scientific knowledge concerning pollutants transfer mechanisms in the

different environment media and the food chain has been carried out in order to assess the possible

impacts on the environment and human health.

Each route has specific transfer processes, but transfers relating to landspreading covers most of the

significant transfers relating to the other routes, with the exception of air emissions.

Heavy metals

The presence of numerous metals in soil and sludge has been reported in the literature. Once

applied to the VRLOtheyare distributed between the different soil media. Scientific evidence shows

that they accumulate in the upper layers of the soil, due to binding to the different existing organic

or mineral particles. Their mobility and biovailability to plants and micro-organisms may be

influenced by several factors of which the pH level of the soil is the most important. Heavy metals

are naturally present in soil at varying levels, and may originate from several anthropogenic

sources such as fertilisers, animal manure, sludge, or atmospheric deposition. However, variety in

the metal levels in European soils may also be due to the diversity of the extracting methods used

rather than differences in the field. In order to ensure the quality of the comparisons, a

harmonisation of the sampling and measurement methods would be required.


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Micro-organisms species present in the soil are numerous. Some of them are important for soil

fertility and therefore for agricultural production. Concern has been expressed about the

consequences of metal provision to the soil on the micro-organisms population and biodiversity.

Available scientific literature shows contradictory results, depending on the species taken into

consideration, the local conditions of the experiments, and the confusion of short-term laboratory

experiments with long-term field trials. Some authors mentioned the ability of microbialpopulations to adapt to changing conditions, which may be considered a result of negative pressure

on the population. On the basis of long-term field trials, some studies concluded that soil micro-

organisms diversity and population could be negatively affected by sludge-borne metals in thelong-term, and by metal levels in soil which were in some cases below current regulatoryprescriptions. It must also be stressed that microbial activity indicators must not be used as the onlyindicators of microbial reaction to metal application, as they do not reflect changes in populationstructure.

/HDFKLQJWRJURXQGZDWHUappears to be a negligible phenomenon. On the contrary, UXQRII, whenit occurs, may play a significant role in metal transfer. Its importance depends greatly on the localsituation, and the fate of metals needs to be further documented.

3ODQWXSWDNHoccurs for all heavy metals and is described by transfer factors. Some metals (e.g.copper and zinc) are of biological importance for the plant.

It has been observed that heavy metals are concentrated in the roots and vegetative parts of plantsand are less present in the generative parts such as wheat grain.

Uptake will increase with increasing metal levels in soil, but only applies to the bioavailable part ofthe metals present in soil. However there may be no direct relation between total metalconcentration and bioavailable metals in soil. pH is the most important factor influencing metaluptake. In particular, a decrease in the pH value in soil in the range of pH 7 to pH 4 causes anincrease in the uptake of Cd, Ni and Zn. The same effect is observed for Cu, but is less marked.Lastly, when considering usual acidity levels in agricultural soils, a pH decrease had no observedeffect on Pb and Cr uptake. This information supports the setting of different limit values for Cd,Ni and Zn, and possibly for Cu, for soil with pH values of between 5 and 7 as well as for soil withpH values of higher than 7. Sludge spreading should also be avoided on soil with a pH value below5 and limit values should refer to the bioavailable part of metals in soil rather than to the totalconcentration, although it is not possible at the moment to define for all heavy metals what is thebioavailable fraction.

8SWDNH of metals by animals occurs through contaminated plant consumption or soil ingestion.However little information is available concerning metal quantities ingested and absorbed and theirsubsequent toxicity levels to animals. Metals do not seem to accumulate in meat. More focus isneeded concerning possible Pb and Cd transfer to offal, as in some cases this could lead to levelsnearing acceptable limits in foodstuffs. Transfer of Pb and Cd across the placenta and into the milkwas observed during indoor feeding trials, but there are likely to be few practical consequences for

finished animals. Concentration of Cu in the milk was not influenced by the ingestion of sludge-amended soil. A quantitative assessment of this contamination pathway is not available at thepresent time.

In a general manner, KXPDQH[SRVXUHWRKHDY\PHWDOV may be attributed to several sources anddepends on many factors such as diet, actual absorption, and food processing. Consumption ofcontaminated crops appears to be the main means of exposure to sludge-borne metals. It is assumedthat the specific contribution of sludge-borne metals to the human diet is very low, when takinginto account the observed level of metals present in soil, and considering the surface area overwhich sludge spreading takes place.

Organic pollutantsNumerous organic compounds are present in sludge. Once applied to the land, they are distributedthroughout all soil media and undergo several retention and transport processes. They are


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physically, chemically and biologically transformed in other intermediary compounds during their

mineralisation, for which no data is presently available. The degradation pathway of the organic

compounds and thus the duration before reaching negligible concentration in soils may greatly

depend on the aerobic or anaerobic degradation conditions.

/HDFKLQJRI RUJDQLF SROOXWDQWV to ground water appears to be insignificant but, unlike metals,

cannot be neglected in some cases. The importance of this mechanism depends on the properties of

the compounds and the soil. It appears on the one hand that many compounds present short half-life

values, reducing the risk of leaching to groundwater. On the other hand, persistent compounds such

as PCDD/Fs or PCBs show an affinity with soil particles and will therefore bind to soil rather than

leach to ground water. 5XQRII, when it occurs, may play an important role in the transfer of

organic compounds.

Even if definitive evidence is lacking, it appears that soil PLFURRUJDQLVPV are not affected by

sludge-borne organic pollutants in most cases and that they are able to adapt to changing

conditions.

Most organic pollutants are QRWWDNHQXSE\SODQWV. However, a risk of contamination of the food

chain exists when spreading sludge directly onto crops, especially on plants which are to beconsumed raw or semi-cooked.

Soil and sludge ingestion on land used for grazing is the main route for DQLPDO contamination.

Accumulation of bioaccumulative compounds such as PCDD/Fs, PCBs or PAHs may occur in meat

and milk. However, it is presently not possible to assess the quantities and fates of organic

compounds ingested by animals.

It appears that the consumption of animal products is the major source of KXPDQ exposure to

sludge-borne organic pollutants, due to the ingestion of soil by livestock. As in the case of heavy

metals, it is assumed that the specific contribution of sludge-borne organic pollutants to the human

diet is very low, when considering the reduced proportion of the utilised agricultural area onto

which sludge spreading takes place.

Lastly, it should be noted that at the present time no universally accepted and validated analyticalmethod exists for analysing most organic compounds. There is also a lack of data concerning levels

of organic pollutants in European sewage sludge as no regular survey has been performed in the

past.

Therefore, considering presently available knowledge on organic compounds, it appears at the

present time, that:

- transfer to water is low, micro-organisms adapt to changing conditions in soil, and numerousorganic compounds are rapidly degraded in soil. Attention should therefore mainly be given to

compounds with higher half-life time values,

- from the point of view of crop protection, no limit value seems to be necessary as transfers to

plant do not occur for most organic compounds,

- restrictions should focus on bio-accumulative compounds spread on grazing land such as PCBsand PCDD/Fs. In this case deep injection of sludge could reduce the risk of livestock

contamination by organic pollutants,

- a survey of organic pollutant levels in sludge should be performed by sludge producers,focusing on the specific organic pollutants identified within the waste water catchment area of

the WWTP.

Pathogens

There are five main types of pathogens observed in sludge: bacteria, viruses, fungi and yeast,parasitic worms, and protozoa. Humans and animals are sensitive to some of these organisms,

which may cause numerous pathologies ranging from simple digestion troubles to lethal infections.


16/72

Sludge-borne pathogens are mainly present on the VRLO surface or at shallow depths where sludge

has been ploughed into the soil. Pathogen penetration depends on the effective depth of the soil, its

texture (particularly its clay content), its organic matter content and also on possible cracks,

prolonged drought, faults or absence of vegetation.

Survival of pathogens in soil depends on numerous direct or indirect factors. Indirect factors are

climatic factors such as sunlight, temperature, desiccation or pH, characteristics of the soil (texture,

moisture etc.), quantity of sludge spread, the pathogen content of the sludge, its organic content and

the eventual presence of competing organisms. Direct factors are related to the biological

characteristics of the pathogen, and especially to the form under which it may survive. Parasiteseggs or cysts are the longest survivors one to two years in certain favourable circumstances.Depending on the conditions and the organisms themselves, survival periods may vary from a fewdays to several years. The pathogenic agent population decreases faster when the sludge is spreadon the soil surface rather than when it is ploughed into it.

Transfer to JURXQGZDWHU is only assumed to occur in some particular cases, while VXUIDFHZDWHUcontamination is more likely to occur when runoff water transports pathogens which are bound tosoil particles.

Survival on SODQWV is shorter than in soil, due to the effects of desiccation and sunlight.

Transmission to grazing GRPHVWLF DQGIDUP DQLPDOV takes place via ingestion of contaminatedfeed and soil.

+XPDQV can mainly be affected by consuming raw or semi-cooked contaminated vegetables ormeat.

Therefore the risks of sewage sludge application onto the land that may be addressed by goodpractices have to be taken into account as pathogens are present in sludge and may havesignificant impacts on humans and animals. In general, deep injection or ploughing down may berecommended during or after sludge application. Although those practices reduce the deleteriouseffect of weather on micro-organisms, contact with animals, wildlife and humans as well asdissemination into the environment will be reduced.

Sewage sludge may also contain plant pathogens, as well as weed seeds. They mainly originatefrom washing of vegetable and fruit, or from road or roof runoff after aerial deposition. Plantpathogens have in general low optimum growth temperature, so that disinfection will be achievedat a lower temperature than for mammalian pathogens.

3ROOXWDQWVWUDQVIHUPRGHOOLQJ

Based on the description of the transfer mechanisms of different sludge-borne pollutants in theenvironment, a PRGHO was developed in order to assess:

- the transfer of pollutants in soil (in particular due to runoff and leaching),- the transfer of pollutants to plants in order to make a comparison with limit values in

foodstuffs,

- the accumulation of pollutants in the soil,

- the time before reaching a given limit value of pollutants in soil.

2QO\KHDY\PHWDOVDUHWDNHQLQWRFRQVLGHUDWLRQ. Knowledge concerning organic pollutants doesnot enable accurate calculations to be made as very little is known about their behaviour anddegradation pathways in soils (moreover, it appears that organic pollutant transfer to plants isnegligible and that this particular route should not involve a significant human health risk). It is notrelevant to apply such calculations to pathogens.

7ZR VFHQDULRV DUH H[DPLQHG ZKLFK UHSUHVHQW WZR H[WUHPH VLWXDWLRQV RI ORZ DQG KLJKDFFXPXODWLRQ. Several assumptions were necessary in order to perform the calculation. 7KHUHIRUH


17/72

WKH UHVXOWV DUH LQGLFDWLYH YDOXHV DQG DUH QRW VXSSRVHG WR EH XVHG QHLWKHU LQGLYLGXDOO\ RU

ZLWKRXWLQGLFDWLQJWKHK\SRWKHVHVXVHG

The main results can be summarised as follows:

- on a one-year basis, it must be observed that pollutants brought to soil by sludge applicationrepresent a very low proportion of the amount of metals present in soil before sludge

application;

- plant uptake of sludge-borne metals may vary, but always represents a minor part of the amountof sludge-borne metals contained in soil; in the long-term, plant uptake will increase with

increasing soil concentration ;

- runoff is the main parameter in the model influencing the heavy metal accumulation in soil ;

- global plant uptake of metals present in soil always remains below the limit values forfoodstuffs. However, in the worst case, it may reach a significant proportion of these limit

values;

- on the contrary, uptake of metals originating only from sewage sludge application is very low,

and reaches, in the worse case of our modelling, 1 % of the limit value for foodstuffs ;- an equilibrium may be reached after several years between plant uptake and sludge application,

indicating that, in some cases, a limit value for metal levels in soil would never be reached ;

- the number of years required before a limit value is reached for metal accumulation in soilwould vary greatly between the two extreme cases considered herein: figures range from around

4,500 years to over 34,000 years in the case of low accumulation, and from 20 years to around

140 years in the high accumulation scenario.

*DSVLQNQRZOHGJH

Today, many uncertainties remain concerning the transfer of pollutants (especially organicpollutants) to the environmental media and the food chain. Several issues would need to be more

accurately documented. Amongst these issues, the following may be mentioned:

- The importance of the runoff process in the pollutants transfer should be assessed. Mechanismsneed to be understood, as well as quantities of pollutants concerned, and their fate.

- An issue of concern is the degradation pathway of the organic compounds in soil. Compoundsmay be degraded into intermediary chemicals before total mineralisation. The toxicity andleaching potential of these metabolites is not well known. Lysimeter and field studies should becarried out.

- Long-term impacts of heavy metals and organic pollutants, in particular on soil micro-organisms and fertility, are not well documented.

- More data is needed concerning the ingestion and absorption levels of organic compounds and,to some extent, heavy metals by animals.

- There is also a lack of knowledge concerning the specific contribution of sewage sludge topollutants transfers.

- A survey of the organic pollutants levels in sewage sludge should be performed in the MemberStates in order to gain an accurate appreciation of their occurrence. This may only be possible ifstandard analytical methods are set and broadly accepted.

- Available literature does not always enable a comparison between the different countries, as nocommon research protocol and no trans-national study has been carried out.

- More information is also needed concerning other routes for sludge recycling, such as landreclamation or use in forestry and silviculture. Research should be carried out to preciselyidentify the agricultural benefits of sewage sludge spreading and its environmental and sanitary


18/72

impacts (especially concerning organic pollutants for which no data is currently available).

Moreover, currently available information does not enable an assessment and comparison of the

benefits and risk as regard the diversity of European forests.

- Lastly, some interesting new technologies such as wet oxidation, pyrolysis or gasification havebeen developed. More information concerning their environmental impact and their application

is needed. Tests have not always been carried out on sludge, and this issue requires further

documentation.


19/72

2EMHFWLYHVRIWKHVFLHQWLILFDQGWHFKQLFDOSDUW

The objective of this intermediary report is to review scientific evidence on the migration andaccumulation of substances and elements contained in sludge into the environment and the food

chain, and identify the associated risks.

The present report aims to:

- present the main waste water treatment technologies and their impact on sludge composition;

- present the sewage sludge treatment processes and the different substances and elementspresent in sewage sludge;

- identify the main routes for sludge disposal and recycling and describe their technologicalcharacteristics;

- review current scientific knowledge on the main biophysical processes involved, flows ofdifferent elements of the sludge through the environmental media and the associated risks;

- develop a simple model describing the flows and accumulations of pollutants in theenvironment.

$V WKH WHFKQLFDO GHVFULSWLRQ RI LQFLQHUDWLRQ DQG ODQGILOOLQJ RI ZDVWHV DV ZHOO DV WKHLU

HQYLURQPHQWDO DQG VDQLWDU\ LPSDFWV KDYH DOUHDG\ EHHQ H[WHQVLYHO\ GRFXPHQWHG HOVHZKHUH

WKLVUHSRUWZLOOPDLQO\VXPPDULVHWKHDYDLODEOHFRQFOXVLRQVIRUWKRVHURXWHVDQGDGGUHVVWKH

VSHFLILFLWLHVIRUVOXGJH LQFLQHUDWLRQDQGODQGILOOLQJ LIDYDLODEOH 2QWKHFRQWUDU\ SDUWLFXODU

DWWHQWLRQ KDV EHHQ JLYHQ WR WKH GHVFULSWLRQ RI WKH EHQHILWV DQG LPSDFWV RI VOXGJH XVH LQ

DJULFXOWXUHDQGWKLVUHSRUWIRFXVHVHVSHFLDOO\RQWKHVHLVVXHV

The different steps covered in this report are presented below:

Wastewater

collection

Water

treatment

Sludge

treatment

Sludgerecycling

and disposal

Impacts


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21/72

:KDWLVVOXGJH"

:KDWLVVOXGJH"

Sludge is a by-product of the water clean up process. There are three main categories of sludge:

- sludge originating from the treatment of urban wastewater, consisting in domestic wastewater or in the mixture of domestic waste water with industrial waste water and/or run-off

rain water.

- sludge originating from the treatment of industrial wastewater, i.e. water used in industrialprocesses.

- sludge from drinking water treatment. Water has to be treated before its consumption. Theamount of sludge generated from drinking water treatment is significantly lower than that

generated from wastewater treatment.

The characteristics of sludge depend on the original pollution load of the treated water, and also on

the technical characteristics of the treatment carried out. Water treatment concentrates the pollution

present in water and therefore sludge contains a wide variety of matter, suspended or dissolved.

Some compounds may be usefully reused (organic matter, nitrogen, phosphorus, potassium,

calcium, etc.) whereas other compounds are pollutants (such as heavy metals, organic pollutants,

and pathogens).

6OXGJHW\SHV

Wastewatercollection

Watertreatment

Sludgetreatment

Sludgerecycling

and disposal

Impacts

Sludge from conventional wastewater treatment plants (WWTP) is derived from primary,

secondary and tertiary treatment processes. Most often, the sludge produced has a concentration of

a few grams per litre, and is highly biodegradable. Each process has a different impact on the water

pollution load. These are presented below.


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)LJXUH wastewater treatment and sludge generation

More information on the European wastewater treatment system is provided in box 1 at the end of

this chapter.

3.2.1 Pre-treatment

Pre-treatment consists of various physical and mechanical operations, such as screening, sieving,

blast cleaning, oil separation and fat extraction.

Pre-treatment allows the removal of voluminous items, sands and grease. The residues from pre-

treatments are not considered to be sludge. They are disposed of in landfills.

3.2.2 Primary sludge

Primary sludge is produced following primary treatment. This step consists of physical or chemical

treatments to remove matter in suspension (e.g. solids, grease and scum).

The most common physical treatment is sedimentation. Sedimentation is the removal of suspended

solids from liquids by gravitational settling. Sedimentation is usually considered first because it is a

simple and cost-effective method. Another physical treatment is flotation. Air is introduced into the

wastewater in the form of fine bubbles, which attach themselves to the particles to be removed. The

particles then rise to the surface and are removed by skimming.

In the mechanical stage, 50 to 70 % of the suspended solids and 25 to 40 % of the BOD5 can be

removed [Werther and Ogada 1999].

Chemical treatments are coagulation and flocculation. Coagulation and flocculation are used to

separate suspended solids when their normal sedimentation rates are too slow to provide effective

clarification.

Coagulation is the addition and rapid mixing of a coagulant to neutralise charges and collapse the

colloidal particles so they can agglomerate and settle. The flocculation is the agglomeration of the

colloidal particles that have been subjected to coagulation treatment.

3.2.3 Secondary sludge

Secondary sludge is generated from the use of specially provided decomposers to break down

remaining organic materials in wastewater after primary treatment. The active agents in these

systems are micro-organisms, mostly bacteria, which need the available organic matter to grow.There are various techniques such as lagooning, bacterial beds, activated sludge as well as filtration

or biofiltration processes.


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The lagooning technique uses the development of a bacterial population in a lagoon, which

converts organic matter into CO2 and biomass. Oxygen is fed into the system via the photosynthetic

activity of microphytes (unicellular algae) or macrophytes (plants), although an alternative

technique consists of artificial aeration of the lagoon. Practically water is passed through several

lagoons, each reaching a higher level of de-pollution. This technique is suitable for WWTPs with

large site areas.In bacterial beds, the effluent is in contact with bacteria, which are attached to a support.

In activated sludge, bacteria are kept in suspension in the vessel in aerobic conditions. At the end of

the process, the treated water has to be decanted off in order to separate the cleaner water from the

activated sludge. This treatment generates another type of sludge, called surplus activated sludge.

3.2.4 Mixed sludge

The primary and secondary sludge described above can be mixed together generating a type of

sludge referred to as mixed sludge.

3.2.5 Tertiary sludge

Tertiary sludge is generated when carrying out tertiary treatment. It is an additional process to

secondary treatment and is designed to remove remaining unwanted nutrients (mainly nitrogen and

phosphorus) through high performance bacterial or chemical processes.

These treatments are necessary when a high level of depollution is required, for example in

sensitive areas identified in the Member States.

Nitrogen consumes oxygen when a nitrification reaction takes place in the natural environment. It

is toxic under its ammoniac or nitrate phase, and is responsible of eutrophication. The removal of

nitrogen is a biological process leading to the production of N2.

Each step is carried out by specific bacteria, which need different conditions to grow.

The removal of phosphorus may be performed using chemical processes or biological treatments.

Chemical processes consist of chemical precipitation using additives followed by sedimentation.

Physical-chemical removal of phosphorus increases the quantity of sludge produced by an activated

sludge plant by about 30 %. Biological treatments employ specific micro-organisms, which are

able to store phosphorus. It accumulates within the bacteria enabling its removal with the rest of the

sludge.

3.2.6 Digested sludge

After water treatment, additional treatments need to be performed RQVOXGJH, in order to:-reduce its water content,

-stabilise its organic matter and reduce the generation of odours

-reduce its pathogen load,

-reduce its volume and global mass.

Several treatments can be applied to sludge to achieve this. These are described in a following part

of this report. One of those transforms the sludge in a way that it is considered as a new type of

sludge usually referred to as digested sludge. This so-called digestion process is described furtherin the following part.


24/72

,QIOXHQFHRIWKHZDWHUWUHDWPHQWRQWKHFRPSRVLWLRQRIWKHVOXGJH

Each kind of treatment has a specific impact on the composition of sewage sludge. We can define

four types of sludge:

- A : primary sludge, primary sludge with physical/chemical treatment or high pollution load1

- B1 : biological sludge (low load)- B2 : biological sludge from clarified water (low and middle load)- C : mixed sludge (mix of A and B2 types)- D : digested sludge

Composition of each kind of sludge is provided below:

$ % % & ''U PDWWHU'0 / 12 9 7 10 309RODWLOHPDWWHU90 '0 65 67 77 72 50S+ 6 7 7 6,5 7& 90 51,5 52,5 53 51 49

+ 90 7 6 6,7 7,4 7,72 90 35,5 33 33 33 351 90 4,5 7,5 6,3 7,1 6,26 90 1,5 1 1 1,5 2,1&1 11,4 7 8,7 7,2 7,93 '0 2 2 2 2 2&O '0 0,8 0,8 0,8 0,8 0,8. '0 0,3 0,3 0,3 0,3 0,3$O '0 0,2 0,2 0,2 0,2 0,2&D '0 10 10 10 10 10)H '0 2 2 2 2 20 '0 0,6 0,6 0,6 0,6 0,6)DW '0 18 8 10 14 103URWHLQ '0 24 36 34 30 18)LEUHV '0 16 7 10 13 10&DORULILFYDOXH N:KW'0 4 200 4 100 4 800 4 600 3 000

7DEOH impact of treatments on the sewage sludge compositionand properties [OTV 1997]

&RPSRVLWLRQ

Sewage sludge contains both compounds of agricultural value and pollutants. Compounds of

agricultural value include organic matter, nitrogen, phosphorus and potassium, and to a lesser

extent, calcium, sulphur and magnesium. Pollutants are usually divided between heavy metals,

organic pollutants and pathogens.

A table summarising the average composition of sewage sludge in the Member States is provided

in the appendix. However, this data has to be taken carefully, as years and time series are different.

The figures provided are mean values, and do not take into account differences between small and

larger WWTP. In addition, we are not always confident that it accurately represents the situation of

each country, especially in the Accession Countries and in some EU countries where no

comprehensive survey has been performed.

1 The load (Cm) is defined as the ratio between the daily mass of pollution to be removed and the mass of

bacteria used for depollution. Usually the following levels are defined:- high load: Cm>0,5 kg BOD5/kg sludge/day

- middle load: 0,2


25/72

In order to perform comparisons, typical composition of animal manure and slurry is also provided

in appendix.

3.4.1 Organic matter

Organic matter is mainly used for soil improvement. Known benefits of organic matter application

to soil are the improvement of the physical properties of soil such as structure or improvement of

the retention capacity of minerals and water. Other benefits of sludge application may be the

improvement of the soil bearing strength, or the reduction of the potential for surface runoff and

water erosion [ADAS 2000].

Degradation of the organic matter can also increase the soil content in compounds of agricultural

value (such as N, S, Mg etc.), which are slower released than in the case of mineral fertilisers and

therefore available for a longer period to crop [ADAS 2000]. Organic matter is lastly an energy

source for micro-organisms living in soil. Therefore sludge spreading may induce an increase of

the soil population and activity, and of its mineralisation capacity.

Sludge organic matter is mostly constituted of soluble matter, such as hydrocarbons, amino-acids,

small proteins or lipids. Its content in urban sewage sludge is high (usually more than 50 % of the

dry matter) but varies according to the treatment and conditioning2

carried out on sludge. Content

level may be reduced due to dilution after incorporation of lime or salts for instance. The table

below compares the content of organic matter of urban sewage sludge against other urban wastes

and animal manure.

2UJDQLF0DWWHUFRQWHQWRI'0

8UEDQVOXG HAerobic digestion 60 - 70Anaerobic digestion 40 - 50Thermal treatment < 40

Lime treatment < 40Composting 50 85

8UEDQFRPSRVW

*UHHQZDVWHVFRPSRVWLQJ

40 6030 60

$QLPDOPDQXUH 45 857DEOH content of organic matter in sludge after different treatments and in other urban waste

and animal manure [Lineres 2000]

However, a recent UKWIR funded literature review indicated that the minimum threshold level fordetectable effects of sludge additions on soil physical properties was c. 5 tonnes organic matter/ha,i.e. about 10 t dry solids/ha. Considering current quantity limitations for agricultural use of sludgeimplemented in the Member States, those benefits do not occur [Lineres 2000]. Improvement ofsoil physical properties may however be observed when using sludge for land reclamation(provided that limit values for pollutants applied are respected), as amounts of sludge used may bevery important.

Moreover sludge organic matter is mostly constituted of soluble matter, such as hydrocarbons,amino-acids, small proteins or lipids. There is only a small amount of lignin or cellulose entering inits composition. Therefore, sewage sludges organic matter mineralises fast, and its rapiddegradation could generate a peak in the nitrate and pollutant levels in soil

It may be observed that the specific case of composting induces the addition of stable organicmatter to the sludge, originating from the co-product. In this case, organic matter mineralises

slower, and nutrients are slower released, reducing the potential risk of nitrogen leaching to

2 for a description of the different types of treatments, see section 4


26/72

groundwater. ADEME [2001] observes that the rapidity of the compost mineralisation depends on

the type of compost, as well as from its maturity, and that compost decomposition may continue

several years following its application on soil. It is also assumed that composted sludge could have

a more significant impact on soil structure than non-composted sludge. However no information is

available in the literature concerning a lower threshold level for detectable effects of sludge

additions on soil physical properties.

3.4.2 Nitrogen and phosphorus content

The table provided in appendix shows the content of nitrogen and phosphorus in sludge in the

Member States. The ranges are between 20 and 80 000 mg/kg DM for Nitrogen and 10 and 90 000

mg/kg DM for Phosphorus.

The proportion of phosphorus and nitrogen in sewage sludge is comparable to the one of animal

manure.

7RWDO1(% of DM)

11+(% of N total)

3(% of DM)

8UEDQVOXGJH 0,9 5,2Liquid 1 7 2 - 70Semi-solid 2 5 < 10Solid 1 3,5 < 10Composted 1,5 3 10 20 0,2 1,5

8UEDQ&RPSRVW

&RPSRVWHGJUHHQZDVWH

0,961,0 2,4

0,390,04 0,44

/LWWHU

0DQXUH

2,2 4,4

4 7

10

50 70

0,61 1, 61

0,91 3,37DEOH content of nitrogen and phosphorus in sludge after different treatments and in other

urban waste and animal manure [Lineres 2000]

1LWURJHQ

Nitrogen is mostly found under organic form in sludge, and to a lesser extent under ammoniacform. Other mineral forms of nitrogen are only found as traces.

Treatments carried out on sludge can greatly influence their content of nitrogen and phosphorus, asshown in the table below. For instance, as most of the ammoniac is located in the liquid phase of

sludge, an important part of it will be removed during the thickening and dewatering steps. Thenitrogen content is also influenced by the operation of the WWTP, and the sludge storageconditions: in some cases a reduction of the nitrogen content in stored liquid sludge by 30 % after 4months has been reported [ADEME 1996].


27/72

7\SHRIWUHDWPHQW 7RWDO1(% of DM)

11+(% of total N)

/LTXLGVOXGJHaerobic digestion, gravity thickening

aerobic digestion, mechanical thickening

anaerobic digestion

lagooning

5 74 71 71 2

5 102 8

20 70N/A

6HPLVROLGVOXGJHaerobic digestion, mechanical dewateringanaerobic digestion, mechanical dewateringlime treatment

3 5.51.5 33.4 5

< 5< 5< 10

6ROLGVOXGJHaerobic digestion, lime treatment (press filter)compostedaerobic, dewatered on drying beds

anaerobic, dewatered on drying beds

2.51.5 32 3.5

1.5 2.5

< 1010 20

< 10

< 10'ULHGVOXGJH 3.5 6 10 15

7DEOH influence of treatment on the nitrogen content of some sewage sludge [ADEME 1996]

As plants can assimilate only mineral nitrogen, the agricultural value of the sludge is alsodetermined by the aptitude of its organic N to be mineralised. The nitrogen availability depends onthe type of sludge. It varies between 4 and 60 %, but within one type of sludge, great variationshave been reported (see table below). The nitrogen availability may be classified as follows:composted sludge < anaerobic digested sludge < aerobic digested sludge. The different treatmentscarried out on sludge may also greatly influence the availability of the nitrogen in sludge, without

knowing the influence of each one of them [ADEME 1996]. Other factors influencing theavailability of the nitrogen are extrinsic factors: temperature, humidity, pH and texture of the soil,and condition of landspreading. Loss of nitrogen can also occur if volatilisation of the ammoniactakes place, or if nitrates are leached. This may represent a possible risk of groundwater pollution.It may happen if the amount of sludge applied does not correspond to plant needs in nutrients orbecause of the fast degradation of sludge-borne organic matter which could give rise to a peak ofnutrient in soil.

6OXGJHW\SH $YDLODELOLW\

Aerobic digested sludge 24-61 %

Anaerobic digested sludge 4-48 %

Digested composted sludge 7 %

Composted raw sludge 4 %

Thermally dried sludge 7-34 %

7DEOH nitrogen availability according to laboratory results [ADEME 1996]

3KRVSKRUXV

Phosphorus is used by the plant for its growth, the rigidity of its cell walls, and for the developmentof its root system. Sludge-borne phosphorus is of particular interest as phosphorus is a limited

natural resource.Phosphorus in sludge is mostly present under mineral form: mineral phosphorus can representbetween 30 and 98 % of the total phosphorus, according to the type of sludge. As in the case of


28/72

nitrogen, the amount of phosphorus available in sludge depends on the treatments carried out, and

is not proportional to the amount of total phosphorus. Variations in composition are illustrated in

the table below. Of course, the amount of phosphorus in sludge is much higher when a specific

tertiary waste water treatment for phosphorus removal is carried out. It has also been observed that

composted sludge has a lower phosphorus content than non-composted sludge, due to the low

phosphorus content of the co-products used during the composting process. Contrary to nitrogen,phosphorus content in sewage sludge is not significantly reduced after storage.

7\SHRIWUHDWPHQW 3 2(% of DM)

3(% of DM)

Liquid sludge ;anaerobic digestion

Aerobic digestion

Primary sludge, lime treated

4.9 6.92.5 12.65

2.5 12

2.1 31.1 5.51.1 5.2

7DEOH influence of treatment on the phosphorus content of some sewage sludge [ADEME1996]

3.4.3 Calcium enrichment

Lime addition to sewage sludge is performed in order to stabilise the sludge (see chapter 4). Toreach a good level of stabilisation, it is recommended to add about 30 % of lime to the dry matter.Lime treatment of sludge therefore generates a product with a useful content in CaO that could beof interest on certain soils. However, as the calcium content may be highly variable in lime treatedsludge, it is generally necessary to analyse sludge before use.

Field studies have shown that lime treated sewage sludge has positive impacts on the pH, structureand permeability of the soil [Lineres 2000]. Calcium is also a useful element for the plant as itstrengthens its cell walls. Calcium supply to the soil may in some cases correct a deficiency of thiselement.

3.4.4 Other compounds of agricultural value

Other compounds present in sludge such as potassium, sulphur, magnesium, sodium and oligo-elements (e.g. boron, cobalt, selenium, iodine) may be of interest in crop production, each of thembeing useful for the plant development and growth. However, they may appear in sludge undervarious forms (for instance magnesium sulphate or magnesium oxide), and their efficiency willdepend on their availability.

It has been mentioned in the UK literature that, as atmospheric sulphur deposition continues to fall,sulphur fertiliser additions are increasingly generating yield responses. Biosolids applicationstypically supply between 140 and 200 kg/ha total SO3, making a valuable contribution to croprequirements [Chambers HWDO. 2000].

However, the agricultural value of those compounds related to their level in sludge is notextensively documented in literature.

3.4.5 Heavy metals

Numerous heavy metals are present in sludge. Heavy metals may affect plant health and growth,soil properties and micro-organisms, livestock and human health, and accumulate in theenvironment. Their impacts are more accurately described in chapter 6. On certain soils however,for example on copper deficient soils, the heavy metals content of sewage sludge can correct traceelement deficiency.


29/72

The average content of 7 heavy metals in the Member States is presented below. Data refers to the

information collected in the Member States for this study, which are summarised in appendix.

'LUHFWLYH

((&mg/kg DM

5DQJHLQWKH

0HPEHU6WDWHVmg/kg DM

&G 20 40 0.4 3.8

&U 1000 1750 16 - 275

&X 1000 1750 39 - 641

+J 16 25 0.3 - 3

1L 300 400 9 - 90

3E 750 1200 13 - 221

=Q 2500 4000 142 - 20007DEOH average content in sewage sludge of 7 heavy metals in the Member States

In all countries, the average values of composition are clearly under the limits of the 86/278/EECdirective. In most cases, the values given in this table are also below the national limit values set inthe regulations of each country.

For some compounds, such as cadmium and mercury, the values are quite homogeneous: between0.5 and 3.8 mg/kg DM. For other compounds however, there are great differences among Europeancountries. This may be due to the industrial context of each country.

Concerning the Accession Countries, we have collected data from Cyprus, the Czech Republic,Estonia, Latvia, Lithuania, Slovakia and Slovenia. The situation is heterogeneous. Compared withthe values in the Member States, Latvia has sludge of worst quality. The sludge of other countriespresents levels of contaminants, which are comparable or slightly higher than in the Member States(Cd, Pb, Cr in Estonia, for instance) or much higher (Hg, Ni). However, those values are all belowthe actual limits of the 86/278/EEC directive. In Slovakia, only one WWTP, representing 3,3% ofthe total sludge production, produces a sludge, which can not be applied on land (higher Cr contentdue to tannery effluent treatment).

There are three main origins for heavy metals in sewage sludge: domestic effluents, road runoff,and industry. For each metal, the proportion of each origin may be very different, and theimportance of heavy metals originating from the industry depends greatly from the industrialsituation of each country.

As a comparison, typical heavy metal levels in animal manure and slurry is provided in appendix.

3.4.6 Organic pollutants

A wide variety of organic chemicals with diverse physical and chemical properties may be found insludge. They also may affect soils, plant, animals and human health, and have impacts on theenvironment, which are described in chapter 5 and 6 of this report.

In this study we especially take into consideration the compounds to which it is most often referred,but many others are present as traces. The considered compounds are:

- PAH : Polynuclear aromatic hydrocarbons

- PCB : Polychlorinated biphenyls

- PCDD/F : Polychlorodibenzodioxins/furans


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- AOX : Sum of organohalogenous compounds

- LAS : Linear alkylbenzenesulphonates

- NPE : Nonylphenol and Nonylphenolethoxylates

- DEHP : Di(2-ethylexyl)phtalate

A description of each of them is provided in appendix.

So far, we have not received satisfactory data from the Member States concerning those

compounds. As they are often not mentioned in the national regulations, no survey has been

regularly performed describing the organic pollutant content in sewage sludge. To compensate the

missing information, the concentrations given have been collected from various documentary

sources. Therefore, those figures must be considered only as indicative values.

Concerning PCDD/F, data have been taken from the &RPSLODWLRQ RI (8 'LR[LQ H[SRVXUH DQGKHDOWK GDWD [AEA Technology 1999]. It shows that average concentrations of dioxins are quitesimilar among Member States, between 15 and 40 ng I-TEQ/kg DM. According to the study, this

would indicate that the sources of contamination in the Member States are similar. Industrial inputs

can also cause important contamination in sludge. In some cases, more than 1 000 I-TEQ/kg DM

have been reported.

Data available concerning other organic pollutant levels in sludge are not consistent and reliable

enough to draw any conclusion.

3.4.7 Pathogens

Sewage sludge contains various micro-organisms, especially when biological treatments are carried

out. Only some of them have health-related impacts. Sludge may also contain plant pathogens. As

for organic compounds, no satisfactory data could be found for the Member States and the

Accession Countries concerning their content in sludge.

Presence of pathogens in sludge is related to the sanitary level of the population, and the type ofindustry in the region. The types of pathogens usually considered are viruses, bacteria, protozoa,

and helminths. Their load in sludge varies along time. More information concerning treatments that

may be performed in order to reduce or destroy pathogens present in sludge is given in part 4.5, and

the fate of pathogens in the environment and their sanitary impact is summarised in part 6.3.

,QGXVWULDOVOXGJH

Data provided above refers mainly to urban sewage sludge. As a comparison, table 8 describes the

composition of some types of industrial sludge, namely pulp and paper, and tannery sludge. More

information is available in the WRc report, Survey of wastes spread on land [2001].

3.5.1 Pulp and paper industry

Composition of pulp and paper industry sludge depends on the paper production process. Usingvirgin wood fibre generates a liquid effluent mainly loaded with lignin and cellulose, thereforecontaining a higher level of stable organic matter. On the contrary, recycling of waste paperinduces additional steps such as de-inking and bleaching, and therefore generates a so-called de-inking sludge, containing colouring agents and chemicals. Reusing waste paper usually generates agreater amount of sludge than when using virgin wood fibres.

Pulp and paper sludge is therefore a mixture of cellulose fibres, ink and mineral components. Inksused to be produced by using heavy metals. Their usage has however been greatly reduced in the

last 20 years, therefore reducing their level in sludge. The higher content of cellulose fibres makesthe nitrogen availability lower than in the case of urban sludge. As a consequence, nitrogen isreleased more slowly into the soil after application, reducing the risk of leaching to groundwater.


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3.5.2 Tannery Sludge

Leather manufacturing generates liquid and solid wastes originating from the different steps in the

transformation of the mammalian skin into leather, performed by using several reactive products.

Liquid effluents contain collagen fixed to tanning agents and heavy metals originating from the

reactive products used during the tanning process. Sludge composition varies according to the

specific process performed on site.

As tannery wastewater is rich in proteins, nitrogen content in the sludge is higher than in the case

of urban sludge, and therefore of interest for landspreading. However, heavy metal (especially

chromium) content may prevent their use in agriculture.

3XOSDQGSDSHULQGXVWU\VOXGJH 7DQQHU\VOXGJH 'LUHFWLYH

(/(0(176 0LQ 0D[ 0HDQ 0LQ 0D[ 0HDQ

'U\VROLGV 1,7 65 31,60 4,10 13,21 7,38

&15DWLR 12,5 200 77,80

ZDWHUS+ 4,5 9,4 7,30 6,7 7,20 6,86

$JULFXOWXUDOYDOXH'0

2UJDQLFPDWWHU 19,1 90,4 63,90 47,61 68,87 54,15

17RWDO 0,4 4,9 1,31 3,59 5,60 5,05

11+ 0 0,3 0,02

&D2 0,52 19,9 12,50 13,35 21,41 16,02

0J2 0,02 6,5 0,86 0,30 0,51 0,38

3 2 0,19 8 0,68 0,40 0,88 0,62. 2 0,06 0,79 0,180 0,12 0,92 0,65

62 1,270

+HDY\PHWDOVSSPPJNJ'0

&DGPLXP&G 0,2 4,4 0,98 0,15 0,07 0,17 20 40

&KURPLXP&U < 1 44,5 34,10 92,00 162,50 127,60 -

&RSSHU&X 2 349 61,20 8,50 12,80 9,90 1000 1750

0HUFXU\+J < 0,01 1,4 0,240 0,03 0,04 0,03 16 25

1LFNHO1L < 1 32 12,40 1,10 2,07 1,53 300 400

/HDG3E < 1 83 13,10 2,25 5,15 3,67 750 1 200

=LQF=Q 1,3 330 135,10 20,40 30,60 26,80 2500 4000

$UVHQLF$V


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%R[ :DVWHZDWHU WUHDWPHQW V\VWHP LQ WKH (XURSHDQ 8QLRQ

DQGWKH$FFHVVLRQ&RXQWULHV

Tables below summarise the level of the wastewater treatment in the Member States and the

Accession Countries. In some countries however, such as Finland, access to sewerage may be

replaced by the use of septic tanks.

2&'('DWD (XURVWDW'DWD

$FFHVVWRSXEOLFVHZHUDJH1RDFFHVV

WR

VHZHUDJH

1R

WUHDWPHQW3 36 367 7RWDO

$FFHVVWR

VHZHUDJH&RXQWU\

% of population


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%R[6OXGJHTXDOLW\LPSURYHPHQW

During the last 20 years, sludge quality has considerably improved. Some examples are provided in

figures 2 and 3 and table 11 for heavy metals and organic pollutants. Other examples may be found

in WRc [1993] and ADEME [1999] reports. The data provided in figure 2 stems from the state of

Upper Austria and concerns 80 up to 140 rural, urban and industrial plants, of which capacities

vary between


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$2; mg/kg DM 250 350 140 280

3&% mg/kg DM < 0,1 0,01 0,043$+ mg/kg DM 0,25 0,75 0,1 0,6'(+3 mg/kg DM 50 - 130 20 - 601RQ\OSKHQRO mg/kg DM 60 120 -3&'') ng TE / kg DM < 50 15 - 45

7DEOH average contents of organic micro-pollutants in sewage sludge in 1988/89 comparedwith data from German publications till 1996 [Leschber 2000]

Those figures show that after a significant improvement, the level of pollutants is nearing a base

level. These improvements result from reducing the sources of the pollution, mostly point sources

such as industrial discharge.


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%R[ 6OXGJH SURGXFWLRQ LQ 0HPEHU 6WDWHV DQG $FFHVVLRQ

&RXQWULHV

6OXGJHSURGXFWLRQLQ0HPEHU6WDWHVThe amount of sludge produced in each Member State is presented in figure 4.

0

500 000

1 000 000

1 500 000

2 000 000

2 500 000

Germ

any

UK

F

rance

Italy

Spain

Po

rtugal

Swe

den

Netherland

s

Austria

Denm

ark

Finl

and

Be

lgium

G

reece

Irelan

d

Luxembu

rg

)LJXUH sludge production in Member States (t DM)3

The total amount of sludge produced in the 15 European Union countries is about 7 million tons of

dry matter (t DM). As shown in the figure 4, Germany is the first sludge producer, followed by the

United Kingdom, France, Italy and Spain, all producing more than 500 000 t DM in a year. These 5

countries generate altogether nearly 75 % of the European sewage sludge. All other countries

produce less than 250 000 t DM each. This situation roughly reflects the demography of each

country.

The amount of sludge is usually presented in tons of dry matter that should be multiplied tenfold4

to obtain the amount of raw sludge produced. The total amount of raw sludge produced in the EUshould be around 70 million tons. However, it is only a theoretical figure, as the original water

content of the sludge depends on its type and the treatment applied.

Figure 5 presents the sludge production in the European Union per inhabitant and per day.

According to this data, Greece produces the lowest amount of sludge per inhabitant (15,4 g

DM/inhabitant/day), whereas Denmark is the most important producer with 78 g.

0,00

10,00

20,0030,00

40,00

50,00

60,00

70,00

80,00

90,00

Denm

ark

Germ

any

Finlan

d

Swed

en

Portu

gal

Austria UK

Luxembu

rgSp

ain Italy

Fran

ce

Netherlan

ds

Irelan

d

Belgi

um

Gree

ce

)LJXUH sludge production in Member States (g DM/inh./day)

3 Source: report of the European Commission on the implementation of the European legislation concerningwastes for the period 1995-1997. Missing data completed from the study on the situation of the agricultural

use of sewage sludge in the European Union [ADEME, 1999].4 10 % corresponding to an average dry matter content of sewage sludge.


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These differences reflect the diversity of national wastewater treatment systems, including the

connection rate of each country. It is also directly linked to the quality, availability, and type of

sludge taken into consideration in the statistical data.

6OXGJHSURGXFWLRQLQ$FFHVVLRQ&RXQWULHV

In the Accession Countries, the production level is more difficult to assess because of

heterogeneous statistical systems, and the low reliability of the data. However, the production is

directly linked to the national equipment level. Figure 6 shows the amount of sludge produced in

the Accession Countries, when data is available. The values are between 400 tons in Malta and

330 000 tons in Poland (assuming a dry matter level of 10 %), which is the country with the largest

population.

0

50 000100 000

150 000200 000

250 000300 000

350 000

Polan

d

CzechRe

publi

c

Slovenia

Slovakia

Eston

ia

Lituania

Hung

ary

Latvi

a

Cyprus

Malta

)LJXUH sludge produced in the Accession Countries (t DM)5

The sludge production per inhabitant is presented in Figure 7, showing great differences between

countries. Indeed, there is no correlation between the amount of sludge produced and the

population. An explanation of such diversity is to be found in the differences in equipment level

and connection rate to water treatment, but also to the relatively low reliability of the statistical

data. For instance, there is presently no common classification of waste in the Accession Countries.

0,0020,00

40,00

60,00

80,00

100,00

120,00

140,00

160,00

Slovenia

Estonia

Cyprus

CzechRe

publi

c

Slovakia

Lituania

Latvi

a

Polan

d

Hungary

Malta

sludge production in the Accession Countries (g DM/inh./day)

5Data source: screening realised by the European Commission or data collected for this study. No data were

available from Bulgaria and Romania.


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%R[ &RPSDULVRQ RIVOXGJH VSUHDGLQJZLWKRWKHUIHUWLOLVDWLRQ

SURFHVVHV

Various fertilisation media in the Member States have been compared and are presented below.

The amount of nitrogen and phosphorus brought by the animal manure has been calculated using the

Eurostat figures for animal manure production in the Member States, and applying a coefficient toassess N and P content. This coefficient can differ among countries in the European Union, and a study

is currently being carried out by Eurostat to compare those figures. However, the results of this studywere not available when this report has been written. Therefore, we used coefficients from the French

CORPEN (Committee for the reduction of the water pollution by nitrates and phosphorus).

We have established an extreme scenario by assuming that all sludge produced in a country would beused in agriculture, although the disposal and recycling routes can greatly differ among the Member

States. It must also be reminded that the amount of mineral fertiliser used may sometimes be higher than

what it is needed. If only best practices would have been considered, this amount would certainly bereduced.

0%

20%

40%

60%

80%

100%

Finland

Greece

France

Sweden

Germany

Italy

Denmark

Spain

UK

Netherlands

Ireland

Portugal

Austria

Belgium&

N Animal manure

N Sludge

N Mineral Fertilizer

)LJXUH sources of nitrogen fertilisation in the Member States

In all Member States, sludge fertilisation is the least used media.

In the case of nitrogen fertilisation, the amount due to sludge use in agriculture is between 0,1 % inGreece and 2,4 % in Germany. However, considering the low figures, there is no strong differenceacross Member States.

Regarding phosphorus, the part of fertilisation from sewage sludge is between 0,2 % in Greece and 10%in Germany. Unlike nitrogen, there is an important heterogeneity between Member States, as theamount of phosphorus brought by mineral fertilisation is between 25 and 65 %. Phosphorus is a criticalfactor in fertilisation, and sludge utilisation in agriculture occurs on the basis of their phosphoruscontent in several countries.

0%

20%

40%

60%

80%

100%

Gree

ce

Finlan

dIta

ly

Fran

ceSp

ain

Portu

gal

Sweden UK

Irelan

d

Austria

Germ

any

Belgi

um&Luxembo

urg

Netherlan

ds

Denm

ark

P Animal Manure

P Sludge

P Mineral Fertilizer

)LJXUH sources of phosphorus fertilisation in the Member States


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39/72

6OXGJHWUHDWPHQWSURFHVVHV

Wastewater

collection

Water

treatment

Sludge

treatment

Sludge

recyclingand disposal Impacts

Sludge produced by wastewater treatment plants is usually processed to reduce the water content of

the sludge, its fermentation propensity and pathogens content. The different steps of the sludge

treatment are described in the table 12. The different treatments, which will be performed on sludge

will depend on its further disposal or recycling.

6WHSV 7\SHVRISURFHVVHV 2EMHFWLYHV

&RQGLWLRQLQJ Chemical conditioning

Thermal conditioning

- Sludge structure modification

- Improvement of further treatment

7KLFNHQLQJ Gravity thickening

Gravity belt thickener

Dissolved air flotation

- Obtain sufficient density, strength andsolids content to permit hauling for

further disposal process

- Reduce the water content of the sludge

'HZDWHULQJ Drying beds

Centrifuging

Filter belt

Filter press

- Reduce the water content of the sludge

6WDELOLVDWLRQ

DQGRUGLVLQIHFWLRQ

Biological processes:

$QDHURELFGLJHVWLRQ$HURELFGLJHVWLRQ/RQJWHUPOLTXLGVWRUDJH&RPSRVWLQJ

Chemical processes:

/LPHWUHDWPHQW1LWULWHWUHDWPHQW

Physical processes:

7KHUPDOGU\LQJ

3DVWHXULVDWLRQ

- Reduce the odour generation

- Reduce the pathogen content of thesludge

7KHUPDOGU\LQJ Direct

Indirect

- Highly reduce the water content

7DEOH the different steps of sludge treatment

A short description of existing treatment processes usually performed in Member States and

Accession Countries is provided below. For each of them, a classification is possible between batch

and continuous processes. Batch processes necessitate reaching a given quantity of sludge before

performing the treatment. On the contrary, continuous processes allow uninterrupted operation.


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&RQGLWLRQLQJ

A preliminary phase of chemical or thermal conditioning may be conducted to improve further

sludge thickening or dewatering.

Chemical conditioning is realised by using mineral agents such as salts or lime, or organic

compounds (polymers).

Thermal conditioning consists of heating sludge to 150-200 C for 30 to 60 minutes. Heat changesthe physical structure of the sludge, helping further dewatering. However, as part of the organicmatter may be hydrolysed during the process, it could trigger offensive smells and high pollutedfiltration or centrifugation water during the dewatering steps. It is possible to perform partialthermal conditioning by heating at a temperature of 40 to 50 C. This solution reduces thecontamination of centrifugation and filtration water. The advantages and disadvantages of each ofthose possibilities are summarised in the table below.

&RQGLWLRQLQJ $GYDQWDJHV 'LVDGYDQWDJHV

&KHPLFDOPLQHUDODJHQWV

- Improvement of the cohesionand the density of the sludge

- Increase in sludge amount- Reduction of the organic matter

content- Slow reaction

&KHPLFDO

RUJDQLFDJHQWV

- Reduction of the mass of sludge,- No modification of the

agricultural value- Lower quantities to be used- Easy to handle and transport

- Costs of the products

7KHUPDO - May be applied to all sludge- Efficient and stable process- Stabilisation and disinfection- Lower sludge amount

- Energy consumption- Odours- Increase in the pollution load of

the filtrate7DEOH comparison of the different conditioning processes

7KLFNHQLQJ

Thickening is a first step to reduce sludge water content. Sludge reaches 10 to 30 % dryness, andcan still be pumped. Various existing techniques are presented below.

*UDYLW\WKLFNHQLQJ

Gravity thickening is a widespread technique, performed in tanks usually fitted with a rotating

ploughing system. The gravitational forces bring the thickened sludge at the base of the tank fromwhere it is extracted. Water is collected at the top. The process is capable of thickening the sludgeby 2 to 8 times, bringing it from a few grams/litre to a few tens of grams/litre.

Performing costs are relatively low, as only an electricity supply is needed to operate the harrowand the pumps. The energy consumed is about 5 kWh/t DM.

*UDYLW\EHOWWKLFNHQLQJ

The gravity belt thickener consists of an endless filter belt on which thickening takes place in threephases: conditioning, gravity drainage and compression. Flocculated sludge is fed onto the belt and,as it moves along, water passes through the weave of the belt. At the discharge end of the machine,the sludge is further thickened by the compression caused by it being turned over onto itself. A

high-pressure wash station continuously washes the belt.

Sludge thickening with a gravity belt thickener is made possible by the addition of polyelectrolyteto the sludge.


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Gravity belt thickeners are used for all types of sewage sludge, although they are at their most

economical when handling sludge of less than 1 % DM feed and thickening to 6 % DM.

Primary sludge can be thickened to 10 % DM at which point it is difficult to process further

without expensive pumping systems. Activated sludge is normally thickened to 5 % DM.

Performing a gravity belt thickening requires about 50 kWh/t DM and water.

'LVVROYHGDLUIORWDWLRQ

The technique of air flotation can be used when the solid particles have a low rate of settlement,

and in sewage sludge treatment the process is used to thicken surplus activated sludge.

The specific gravity of fine suspended solids is lowered by the attachment of micro-bubbles and

brought to the surface, where the thickened sludge is removed by a scraper. Its application in

sewage sludge treatment involves dissolving air under pressure and subsequently releasing the

pressure in the flotation vessel. Adding a polymer is sometimes needed, when it is necessary to

reduce the matter in suspension.

The performance of this process is higher than the one of the gravity thickening. However, the

energy costs are also higher: 100 to 130 kWh/t DM.

&RPSDULVRQRIWKHGLIIHUHQWWKLFNHQLQJSURFHVVHV

The different thickening systems are compared below

$GYDQWDJHV 'LVDGYDQWDJHV

*UDYLW\WKLFNHQLQJ - Easy to perform- Low energy consumption- Low investment costs

- Needs important room- Low performance on

biological sludge

*UDYLW\EHOWWKLFNHQLQJ - Easy to perform- Compact - Work force need- Cleaning water consumption- Polymer use compulsory

'LVVROYHGDLUIORWDWLRQ - Easy to perform- Little room needed- Little H2S emission

- Not adapted to variableregimes

- High energy consumption7DEOH comparison of the different thickening processes

'HZDWHULQJ

Dewatering is the following step after thickening, and allows further reduction of the sludge water

content. Dewatered sludge has a dry matter content of up to 30 %.

'U\LQJEHGV

One of the simplest techniques for dewatering sewage sludge is the open air drying bed. This

technique is used mainly on small WWTPs whenever sufficient inexpensive land is available and

the local climate is favourable for year-round operation of the beds. This technique may be less

efficient in cold climates.

It consists of a sand and gravel area about 0.3 m thick on which sludge is spread. The water is

drained and sent to the head of the plant. The sludge is then atmospherically dried.

This process allows a DM content of 40 to 50 % to be reached in some countries, depending on the

duration of the drying. This level is reduced to 10% in Nordic countries. It offers the potential oflower operating costs and minimal maintenance requirements, which may offset the disadvantage

of high land requirements, weather dependency, and potential odours. However, this technique is

relatively labour intensive.


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&HQWULIXJLQJ

Centrifuging is a mechanical process that uses centrifugal forces to separate the thickened sludge

from the centrifugate. Centrifuges are used in dewatering applications because they are compact,

have high throughput capacity, and are simple to operate. Solid-bowl and basket centrifuges are the

most commonly types used.

It is possible to use centrifuging either as a thickening process or as a dewatering process.

The process can produce increases in the dry matter of up to 15 to 25 %. It is also possible to use a

high performance centrifuge, gaining an additional 5 %.

However, the energy needs of this process are significant: from 25 to 80 kWh/t DM, and it is also

necessary to add a polymer to the sludge.

)LOWHUEHOW

In the filter belt process, the sludge, mixed with a polymer, is dewatered on the same principle as

gravity belt thickening. It is then pressed between two belts. There are different kinds of machines

available, depending on the level of pressure applied to the sludge (low, middle or high pressure,

respectively about 4, 5 and 7 bars). The process may be combined with a gravity belt thickening.

It is possible to increase the level of dry matter by 10 to 20 %, depending on the type of sludge and

the pressure applied.

To perform this process, costs include polymer, water and energy (about 35 kWh/t DM).

)LOWHUSUHVV

It is possible by using this technique to reach a high dewatering level, between 30 to 45 %

generally. The investment costs however are quite high, especially for high capacities.

Plate and frame filter presses are commonly used to dewater sludge. Conventional filter presses

consist of rows of vertical plates between which sludge is injected under pressure. The filtrate iscollected before separating the plates. The sludge cakes then fall and are collected. In some cases,

membranes are placed between the plates, which can be filled with water in order to improve the

dewatering rate. In this case however operating costs are significantly higher.

A preliminary conditioning is usually required either with salts or lime. Electricity needs are about

30-40 kWh/t DM. Investment costs are reduced with increasing capacities.

&RPSDULVRQRIWKHGLII

sludge disposal3

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