Sewage sludge

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Pressed sewage sludge in the VEB Synthesewerk Schwarzheide (1990)

Sewage sludge is a waste from the completed treatment of wastewater in sewage treatment plants , which consists of water as well as organic and mineral substances , which in turn are in dissolved and solid form. Even if this waste has been dewatered or dried or treated in plant beds or in any other form, it remains sewage sludge.


A distinction is made between raw sludge and treated sewage sludge. Raw sludge occurs in sewage treatment plants as primary sludge in the mechanical cleaning stage or as excess sludge in the biological stage . Excess sludge consists mainly of microorganisms such as protists and bacteria . By aerobic or anaerobic treatment of the raw sludge obtained less odorous sludge ( treated sewage sludge or sewage sludge stabilized ). The anaerobic treatment takes place in larger sewage treatment plants in digestion towers ( digested sludge ) with the formation of sewage gas (methane content around 60%). In its initial state, sewage sludge is thin and dark in color. Solids contents of around two to five percent are achieved through gravity thickening.

Through the use of flocculants , the sludge is processed in such a way that it can be dewatered to a solids content (DM content) of up to 35%, for example by centrifuges , screw presses or belt filter systems. With the help of chamber filter presses or sewage sludge digestion systems, higher degrees of dewatering can also be achieved, with the organic matter additionally being biodegraded in the latter.

The sewage sludge is rich in nutrients , since microorganisms use the wastewater ingredients in the biological stage to build up the biomass and thus concentrate the nutrient salts contained in the wastewater . Nitrate , phosphate and other plant nutrients are of particular importance for agriculture .

The following is a table with the normal levels of nutrients in sewage sludge, which can vary greatly from time to time and from plant to plant.

nutrient in% TS Minimal mg / l Maximum mg / l Medium mg / l
Nitrogen (N) 1.5-5 0.5 1230 192
Phosphates (P 2 O 5 ) 1.5-5 1 1720 182
Potassium (K 2 O) 0.1-0.3 0.5 475 21st
Calcium (CaO) 4-6 0.5 3635 369
Magnesium (MgO) 0.6-2 0.5 610 49
Loss on ignition (organic components) 40-80

A large number of organic compounds with different properties and effects can be present in sewage sludge . These substances can be carcinogenic , mutagenic , toxic, etc. Heavy metal compounds are a particular problem . One example is chromium, which is nontoxic in elemental form, essential as chromium (III) and toxic and carcinogenic as chromium (VI). The German Sewage Sludge Ordinance and the Fertilizer Ordinance contain limit values ​​to minimize the dangers for people and the environment. Even if such substances are only present in low concentrations in wastewater, there is a risk that they will accumulate after agricultural application (" bioaccumulation ") and end up in the food chain. In addition to heavy metals, this also applies to the sum parameters AOX , PCB and PCDD .

Below is a table with information on these organic substance groups and chemical compounds and their concentration that were detectable in sewage sludge from the 1980s. Due to the legal bans on the use of some of the specified weed and pest control agents , which have since been issued, several of the specified compounds are currently no longer detectable in the sludge. Toxic organic compounds are currently still present in the sludge.

Substance group chem. connection min.-max. in µg / l usual concentration in µg / l
Polycyclic aromatic hydrocarbons Fluoranthene 0.10-43 -
Polycyclic aromatic hydrocarbons Benzo [ a ] fluoranthene 0.01-9 -
Polycyclic aromatic hydrocarbons Benzo [ a ] pyrene 0.01-40 -
Polycyclic aromatic hydrocarbons Benzo [ ghi ] perylene nn – 31 -
Polycyclic aromatic hydrocarbons Indeno [1,2,3- cd ] pyrene 0.01-23 -
Polycyclic aromatic hydrocarbons Pyrene 0.10-35 -
chlorinated hydrocarbons Hexachlorobenzene (HCB) nn-0.2 <1
chlorinated hydrocarbons p, p′-dichlorodiphenyldichloroethene (p, p′-DDE) nn-0.9 <0.2
chlorinated hydrocarbons DDT nn-0.2 <1
chlorinated hydrocarbons β-HCH nn-0.1 -
chlorinated hydrocarbons γ-HCH (lindane) nn-0.8 <0.05
chlorinated hydrocarbons Dieldrin nn-0.4 <1
Phthalates DEHP 70-100 -
Polychlorinated biphenyls PCB 101 = 2,2 ', 4,5,6'-pentachlorobiphenyl nn-0.9 -
Polychlorinated biphenyls PCB 138 = 2,2 ', 3,4,4', 5'-hexachlorobiphenyl nn – 5 -
Polychlorinated biphenyls PCB 153 = 2,2 ', 4,4', 5,5'-hexachlorobiphenyl 0.01-4 -
Polychlorinated biphenyls PCB 180 = 2,2 ', 3,4,4', 5,5'-heptachlorobiphenyl 0.01-1.2 -
Polychlorinated biphenyls Clophen A60 0.2-19 <1

A large number of trace substances are introduced into the wastewater from hospital wastewater and domestic wastewater , some of which can also be detected in sewage sludge. Wastewater treatment processes cannot completely eliminate trace substances. In an EU-funded project, sewage sludge from wastewater treatment was examined using different cleaning methods and concentrations of different drug groups were detected. The use of sewage sludge in agricultural fertilization can thus represent an entry path for trace substances in soil and water.

Sludge treatment

Sewage sludge is treated for further recycling. The following process steps can be used: thickening , conditioning , sanitizing , dewatering and drying . Which processes are used and in which order this is done depends on various boundary conditions (size of the sewage treatment plant , type of stabilization of the sewage sludge, local situation, spatial conditions, etc.).


Mechanical drainage

Mechanical dewatering devices (hydraulic presses, centrifuges , chamber filter presses, belt filter presses , screw presses ) often dewater the usually (aerobically or anaerobically) stabilized sewage sludge before post-treatment, recycling or disposal . In order to achieve extensive separation of the water contained in the sewage sludge, it is usually necessary to condition the sludge. This can be done by adding polymeric flocculants (rarely iron or lime milk ) to the liquid sludge. Adding lime amounts to around 20% to 35% CaO in the solids content and makes up a substantial part of the benefit when it is used as fertilizer. However, the original dry matter of the disposal volume is increased, which can lead to additional costs. Due to the mechanical dewatering, the solids content increases to over 30%, depending on the technology used, thus enabling the volume or mass of the sewage sludge to be disposed of to be reduced to a tenth of the original amount of wet sludge. In addition to the hydrostatic filters, there is also the vacuum belt filter. The drainage takes place by gravity with additional vacuum support. Depending on the application, either filter fleece or endless filter belts are used for vacuum belt filters, so that a high throughput can be achieved.

Biological drainage

Biological dewatering is the application of sewage sludge to compost beds. In these beds, which are mainly planted with reeds, the sewage sludge is first rapidly drained to a solids content of around 10%. The sewage sludge water seeping through the soil filter is collected with drainage systems and returned to the sewage treatment plant . The organic and mineral solids remain in the humification beds of the sewage sludge digestion plant. Through biological conversion processes over a longer period of time, the organic components are broken down and mineralized, which significantly reduces the mass. This creates high-quality sewage sludge soil from the sewage sludge residue , which, after clearing and subsequent storage, achieves dry matter contents of up to 60%. By breaking down 50% to 60% of the organic substances contained in sewage sludge (mass reduction ), sewage sludge digestion can effectively achieve lower residual quantities.


Sewage sludge with dry cracks

The drying process further reduces the weight and volume of the sludge. The water bound in the sludge is reduced with the help of evaporation or evaporation. Above all, the various drying processes are based on contact, convection or radiation processes to dissolve the bound water. The calorific value of the dried sludge depends on the type of drying, the residual moisture and the carbon content in the starting material used . The calorific value of dried raw sludge corresponds to that of dried lignite (up to 19 MJ / kg), dried digested sludge reaches approx. 11 MJ / kg.

Solar sewage sludge drying

For this purpose, the sewage sludge that arises in the wastewater treatment and is pre-dewatered is dried with the help of the sun's energy. Large areas of the sludge are brought into a drying hall. This hall resembles a greenhouse and has a transparent building covering made of foil, polycarbonate or glass.

The drying air in the hall is heated by direct and diffuse solar radiation ; this heats the air and the stored sewage sludge. In addition, the biological processes in the sewage sludge generate its temperature, which causes an increase in the inorganic carbon content. This heating increases the steam pressure in the sewage sludge compared to the air above it and the water evaporates from the sewage sludge. A built-in ventilation system in the hall (implemented e.g. by ventilation flaps, fans) ensures a controlled exchange of air. This removes or exchanges moist air from the drying hall.

The possible degree of drying of the sludge depends on the time, outside temperature and solar radiation. It indicates how much residual moisture there is in the sludge after drying. If you stay in the hall for a long enough time, you get a degree of dryness of around 90% in summer. In winter, the specific water withdrawal per m 2 of surface area decreases ; this means that evaporation (and thus also the degree of dryness of the sludge) is somewhat lower than in summer.

A granulate is produced by drying , which is used as a secondary fuel with a calorific value of 8-11 MJ / kg DM (corresponds to approx. 2-3 kWh / kg DM; conversion: 1 MJ = 0.2778 kWh) in coal-fired power and cement plants or can be used as fertilizer (see section Disposal ).

One of the largest solar sewage sludge drying plants with a drying area of 7200 m 2 is operated in Nicaragua using the “Wendewolf” method. The world's largest solar sewage sludge drying plant with a drying area of 20,000 m 2 has been on the Spanish Mediterranean island of Mallorca near Palma since 2008 . Approx. 30,000 tons of sewage sludge are dried per year in a batch process in 12 double halls. The sewage sludge drying is operated with the turning robot "electric pig". The size of the connected sewage treatment plants is 600,000 population equivalents.

Belt dryer

The drying of sewage sludge with a belt dryer is usually carried out in an air atmosphere. The air heats the sewage sludge strings on the belt that are generated by means of a perforated matrix . The escaping air is cleaned in a biofilter. The moisture may have to be condensed out. The energy for air heating is usually provided as waste heat from an industrial process. Belt drying is therefore often used as an example of low temperature drying .

Fluidized bed evaporation dryer (WVT)

For the fluidized bed evaporation dryer , sewage sludge particles are usually used, which are fluidized in a fluidized bed with superheated steam (atmospheric or with excess pressure). The particles are produced using an extruder or meat grinder . The water evaporated from the product is condensed in a heat exchanger and can be further used thermally ( saturated steam at 4 bar at approx. 140 ° C). The vapor (condensate) is then fed back to the sewage treatment plant. Saturated steam or thermal oil are usually used as the heating medium .

Disc dryer

With the disc dryer, the sewage sludge is heated on heated discs similar to a mixer. The escaping steam is discharged and condensed out. The resulting water-insoluble gases are cleaned. The condensate can be thermally reused by means of a heat exchanger (usually atmospheric at 70–90 ° C) and is then fed back to the sewage treatment plant. Saturated steam or thermal oil at high temperatures are usually used as the heating medium. This type of drying is therefore often used as an example for high-temperature drying .

Frying sewage sludge

In the deep-frying process, the sewage sludge, dehydrated to approx. 30% dry matter, is pelletized in a kind of meat grinder before the aqueous, moist intermediate product is fed to a continuous fryer. After approx. 8 minutes the dried pellets are available with a residual water content of 4% to 5%. The calorific value is about 22 MJ / kg DM due to the oil content.

The energy-rich pellets can be used to generate electricity. Depending on the energy level of a post-process, both the exhaust gases produced by the combustion and the heat in the vapor can be used thermally.

Recycling and disposal

Agricultural utilization

In the European Union, the use of sewage sludge as fertilizer in agriculture is subject to Directive 86/278 / EEC Use of sewage sludge in agriculture , which defines limit values ​​for concentrations for heavy metals .

In Germany , the Sewage  Sludge Ordinance (AbfKlärV) regulates the further use of sewage sludge from municipal sewage treatment plants. The use as fertilizer is only permitted on arable land - not on permanent grassland or fruit and vegetable growing areas. Sewage sludge that meets the requirements of the Sewage Sludge Ordinance with regard to the pollutant content and the requirements of the Fertilizer Ordinance with regard to the nutrient content are considered approved fertilizers in Germany. Sewage sludge of the highest quality that is precisely declared in accordance with the requirements of manure and which has advantages in terms of plant cultivation from the direct nutrient effect, the humus supply and the lime effect are called sewage fertilizers . According to the Federal Statistical Office , 45% of the treated sewage sludge from municipal sewage treatment plants in Germany in 2012 was used as fertilizer in agriculture and landscaping (material recycling), the rest was thermally disposed of. The recycling rates are very different from region to region, the federal state with the highest material recycling rate is Mecklenburg-Western Pomerania (2012: 96%). The still permissible pollutant content for agricultural sewage sludge was discussed for a long time. An amendment of the German Sewage Sludge Ordinance took place in 2017 and had a tightening of pollutant limits and the inclusion of additional criteria result. Due to the high proportion of pollutants in sewage sludge, its use as a fertilizer has already been discontinued in isolated cases or is increasingly criticized. Projections assume that the amount of microplastics that gets into the soil every year with sewage sludge is greater than the amount that ends up in the world's oceans . The federal government stepped up, however, the recovery of phosphorus from sewage sludge to use it for fertilizer. Scientists from the University of Giessen developed two possible processing methods. Waste management is already making use of the results from research and development. In the summer of 2019, the first recycling plant for the extraction of phosphorus from sewage sludge ash opened in Hamburg.

In the Austrian federal states of Tyrol and Salzburg , the spreading of sewage sludge is prohibited, as is the case in Vienna, while in the rest of Austria it is possible to a limited extent. The government planned a nationwide ban in 2013, but has not yet implemented it.

In Switzerland, on the other hand, the spreading of sewage sludge on forage and vegetable areas has been prohibited since January 1, 2003, and has been completely prohibited since October 1, 2006. The dried sewage sludge is thermally recycled in waste and sludge incineration plants as well as in cement works . Due to limited capacities, small quantities were also dumped and exported, mainly for co-incineration in lignite power plants in Germany. The reasons for the ban in Switzerland included the mercury content of sewage sludge and the content of endocrine disrupting substances. In 2013 a project to recover phosphorus in the form of urban mining from sewage sludge ash began in the canton of Zurich . The construction of a sewage sludge recycling plant on the area of ​​the city of Zurich was accepted by the voters in March 2013. The pilot project was carried out by the end of 2016. In June 2019, the building department of the Canton of Zurich announced that a preliminary project for a supraregional production facility in Zuchwil in Solothurn would be drawn up by the end of 2020. In this recovery plant, it should be possible to produce phosphoric acid under industrial conditions , probably from 2026 .


Due to the high content of organic substances (around 50%), which has a positive effect on the humus balance of the field when used as fertilizer, it has no longer been possible to dispose of sewage sludge by depositing it in landfills in Germany since June 1, 2005. According to the German Waste Deposit Ordinance (AbfAblV), only waste with a maximum of 5% organic dry matter may be dumped since this date.

Thermal recovery

Sewage sludge not used as fertilizer is used in thermal processes (incineration or gasification). Whether incineration of sewage sludge counts as recovery depends on the type of process. The calorific value is important for combustion, i.e. ultimately the carbon content . A sufficiently high calorific value can be achieved through previous drying, which, however, requires additional energy.

The following thermal processes are used for sewage sludge disposal:

In many thermal recycling processes, the plant nutrients contained in the sewage sludge are lost to the natural cycle of materials, since when incinerated together with other waste, the nutrients contained in the sludge are greatly diluted by the main ash. As a rule, these ashes cannot be used for nutrient recovery today. In the case of mono processes in which only sewage sludge is used, the phosphorus content in the ash is so high that recovery of the phosphorus could become economical in Germany, for example, due to the scarcity of resources.

In view of the amendment to the Sewage Sludge Ordinance in Germany, which took place in 2017, energy companies have been planning to replace fossil fuels with sewage sludge since around 2015, as increasing quantities are to be expected. The recovery of the phosphorus is also planned. In 2017, 70% of the sewage sludge was incinerated.

Sewage sludge digestion

Another recycling route for sewage sludge can be achieved with sewage sludge digestion . After dewatering in reed beds, the content of organic dry matter in the sewage sludge is largely reduced through microbial degradation, which also changes the material properties, the pore volume and other consequences. This creates hygienized and humus-like sewage sludge soil, which is suitable for the production of technical soils for use as a plant substrate in gardening and landscaping and for water balance layers in landfill cultivation, whereby valuable materials are returned to the material cycle and not destroyed as in incineration. Aspects of soil and groundwater protection must be taken into account for these types of use. Another possibility of feeding the nutrients back into the material cycle is adding them to composting and biogas plants .

Dangers to humans

According to § 3 BioStoffV ( Biological Agents Ordinance ), sewage sludge falls into the less severe 'risk group 2' (of 4 groups). Sewage sludge can contain microorganisms that cause infectious diseases in humans. A spread of such infectious diseases in the population through sewage sludge is unlikely. The absorption of sewage sludge (e.g. by swallowing, through the damaged skin (e.g. open wounds, eczema ) or by inhalation ( aerosols )) must be avoided.

In Germany, sewage sludge has not been allowed to be applied to permanent grassland or fruit and vegetable cultivation areas since 1992 in order to prevent the direct uptake of sewage sludge that may cling to plants (Section 4 AbfKlärV). In Switzerland at the beginning of 2003, the spreading of sewage sludge on pastures and on vegetable-growing areas was generally prohibited.

Handling precautions

  • To protect against swallowing, basic hygienic rules must be observed, such as hand washing before eating, drinking and smoking and before going to the toilet.
  • If direct contact cannot be avoided, suitable personal protective equipment must be worn. Protective gloves must be impermeable to microorganisms and are marked (symbol “impermeable to microorganisms”). This mark must be printed on the protective glove.
  • In the case of work that is very polluting, the body can be protected with a splash-proof disposable overall.
  • During work in which the possible formation of aerosols cannot be avoided by technical precautions, respirators of the highest filter level P3 must be worn to protect against inhalation . These are available as disposable masks for the mouth and nose or as a particle-filtering half mask with the option of changing the filter.


  • Gudrun Both, Harald Friedrich, Horst Fehrenbach, Hürgen Giegrich, Florian Knappe: New strategies for sewage sludge disposal in North Rhine-Westphalia. Proper and harmless recycling according to KrW- / AbfG and in accordance with soil protection . In: KA water management, sewage, waste . No. 48 (10) , 2001, pp. 1430-1442 .
  • Harro Bode: Sewage sludge treatment and disposal. How clear are the general conditions for the operators? In: KA water management, sewage, waste . No. 48 (12) , 2001, pp. 1758-1765 .
  • Andrea Bertsche, Susanne Klages, Christian Schaum, Ute Schultheiß, Helmut Döhler, Peter Cornel: Statistical evaluation of nutrient and pollutant levels as well as soil-improving ingredients in sewage sludge in Lower Saxony . In: KA water management, sewage, waste . No. 52 (5) , 2005, ISSN  1616-430X , p. 586-594 .
  • Reimar Leschber , Ulrich Loll: ATV manual . Sewage sludge. 4th edition. tape 4 . Ernst & Sohn, Berlin 1996, ISBN 3-433-00909-0 .
  • Wolfgang Bischof: wastewater technology . 10th revised and expanded edition. Teubner, Stuttgart 1993, ISBN 3-519-05247-4 .
  • Klaus Mudrack, Sabine Kunst: Biology of wastewater treatment . 5th revised and expanded edition. Spectrum, Heidelberg / Berlin 2003, ISBN 3-8274-1427-X .

Web links

Wiktionary: sewage sludge  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ Günter Fer: Sewage sludge: poison or fertilizer. In: gwf water · wastewater. Vol. 130 1989, No. 11, p. 599.
  2. Georg Schwedt: Pocket Atlas of Environmental Chemistry . John Wiley & Sons, 1996, ISBN 3-527-30872-5 , pp. 206 ( limited preview in Google Book search).
  3. ^ Günter Fer: Sewage sludge: poison or fertilizer. In: gwf water · wastewater. Vol. 130 1989, No. 11, p. 600.
  4. ^ Final report of the Interreg IV B project noPILLS , May 2015.
  5. Vacuum belt filter. April 17, 2019, accessed December 1, 2019 .
  6. ^ S. Nielsen, JD Larsen: Operational strategy, economic and environmental performance of sludge treatment reed bed systems - based on 28 years of experience . In: Water Science and Technology . tape 74 , no. 8 , 2016, p. 1793–1799 , doi : 10.2166 / wst.2016.295 .
  7. Wastewater lexicon: sludge drying, sewage sludge drying. In: water knowledge. Institute for Environmental Process Engineering, University of Bremen, accessed on April 14, 2011 .
  8. Use of secondary fuels (PDF; 1.0 MB).
  9. Examples of calorific values ​​(dry matter)
  10. Procedure info
  11. Solar sewage sludge drying in Managua ( Memento from June 26, 2013 in the Internet Archive ) (PDF; 126 kB).
  12. procedural information
  13. Solar sludge drying in the holiday paradise ( Memento from February 2, 2017 in the Internet Archive )
  14. ↑ Council Directive 86/278 / EEC of June 12, 1986 on the protection of the environment and in particular of the soil when sewage sludge is used in agriculture 86/278 / EEC (online,
  15. Tables of sewage sludge recycling type , Federal Statistical Office.
  16. No more fertilizer: sewage sludge in Osnabrück contaminated with microplastics In: , July 15, 2017, accessed on January 31, 2018.
  17. Soils in Swiss nature reserves contain considerable amounts of microplastic . In: Media release from the University of Bern . April 27, 2018 ( [accessed on May 2, 2018]). Soils in Swiss nature reserves contain considerable amounts of microplastics ( Memento from April 28, 2018 in the Internet Archive )
  18. Germany should produce phosphorus from sewage sludge - BMU press release. Retrieved May 28, 2020 .
  19. Fertilization: This is how the phosphorus comes from the sewage sludge. argarheute, accessed May 28, 2020 .
  20. Raw material recycling : Phosphorus supplier sewage sludge. Hazardous waste information, accessed on May 28, 2020 .
  21. ^ Supplement to the Tyrolean Field Protection Act of June 6, 2002, ( Memento of May 5, 2009 in the Internet Archive )
  22. Law on the ban on the application of sewage sludge LGBl 2000/08 (online,
  23. cf. z. B. Application of sewage sludge on soils , and other sewage sludge laws and sewage sludge regulations of the federal states.
  24. Government wants to stop spreading sewage sludge ( memento from May 10, 2015 in the Internet Archive ) , editorial, December 4, 2013
  25. A. Laube, A. Vonplon: Sewage sludge disposal in Switzerland - quantity and capacity survey . Federal Office for the Environment, Forests and Landscape, Bern 2004 ( Environmental Materials No. 181).
  26. ↑ A clear yes to sewage sludge . Tages-Anzeiger, March 3, 2013
  27. Phosphor Mining, Project Sheet No. 4, January 2016. Building Department of the Canton of Zurich; Office for Waste, Water, Energy and Air. Production of phosphoric acid from sewage sludge is within reach ( memento from September 16, 2016 in the Internet Archive ) .
  28. Media release from the building department of the Canton of Zurich: Converting sewage sludge into raw material: New process suitable for industrial production. June 3, 2019, accessed October 14, 2019 .
  29. Annex on sewage sludge gasification (PDF; 486 kB), on, accessed on January 18, 2017.
  30. Sewage sludge disposal using hydrothermal carbonation (PDF; 68 kB), on, accessed on January 22, 2017.
  31. Research report on phosphorus production from sewage sludge on behalf of the Federal Environment Agency
  32. Perspectives of sewage sludge co-incineration in coal-fired power plants of Vattenfall Europe Generation AG , 2014.
  33. 70% of municipal sewage sludge was incinerated in 2017. In: December 12, 2018, accessed June 4, 2019 .