Treatment options for sewage sludge

Published on November 13, 2011 by in Energy, Resources, Water


Leon Korving of AIFORO has more than 11 years of experience with treatment of sewage sludge as a member of the management team of SNB, one of the large sludge incinerators in Europe. In this position Leon also contributed to a large number of studies on treatment options for sewage sludge by STOWA, the Dutch institute for applied water research of the waterboards. This page gives a short introduction to treatment options for sewage sludge. If you need more specific information you can always contact AIFORO.


Sewage sludge is the semi solid residue left from the treatment of waste water. In a waste water treatment plant the sewage sludge helps to degrade the organic pollutants in the waste water through the microbiological activity in the sewage sludge. Therefore it is often described as “activated” sewage sludge. The sludge will also act as an adsorbent for a wide range of pollutants that are not readily degraded through microbiological processes. During the treatment of the waste water the amount of sludge in the plant grows and therefore part of the sludge has to be discharged. This waste material can be treated in many different ways. The different treatment options will be discussed below.


 Especially at larger water treatment plants the waste sludge is normally first digested in a sludge digester to produce biogas from the sludge. This biogas can then be used in a gasmotor to produce electricity and heat. Typically these digesters have a residence time of 20-30 days and operate at a temperature of 30-35 °C. In this stage the sludge still contains a lot of water and normally has a dry matter content between 4 to 8 %. Depending on the configuration of the water treatment plant the digester can supply 50-100% of all electricity required by the water treatment plant. In some configurations the water treatment plant can even produce a net surplus. In the last years several processes have been developed to improve the digestibility of the sludge by means of a pre-treatment of the sludge. The most popular and effective means seems to be a thermal hydrolysis of the sludge.


After digestion, the sludge is dewatered as far as possible by mechanical means. Typically with dewatering centrifuges or belt filter presses, but sometimes also with chamber filter presses or screw presses. In The Netherlands in this way a dry matter content of 20-25% is reached. In other countries, like for instance Germany, the sludge dewaters better and better dewatering results can be achieved. Generally the dewatering results are better with digested sludge. Apparently sludge from biological phosphorous removing plants dewaters worse than other sludges, but this still has to be proven with a serious survey.

Final treatment

The most popular disposal route of the sewage sludge has always been the application of the sludge as a fertilizer in agriculture. In a way this is logical as the sludge contains a lot of nitrogen and phosphorous and these are valuable nutrients. Also the organic matter in the sludge can improve the agricultural value of the soils to which it is applied. On the down side the sludge contains a lot of pollutants like pathogens, heavy metals and persistent organic contaminants. Therefore not always all stakeholders are pleased with this application. Also, if the sludge is brought to soils without any control of the nutrient balance this method can contribute to eutrophication of surface waters. Therefore agricultural application of sewage sludge is often controlled in many countries by a lot of legislation and is banned in other countries. Most probably agricultural application is best suitable for rural situations where the treated wastewater is not very polluted and where there is a real need for nutrients in agriculture.

If agricultural application is not possible or not wanted other disposal routes have to be considered. Landfilling of sewage sludge is not very often applied because the high water content requires special measures. In some special applications the sludge can be mixed with binding agents and ashes to produce a hardened product that can be used as a covering layer for landfills (Hydrostab).

 The most widely used disposal route for sewage sludge is incineration. Incineration reduces the volume of the sludge with a factor 10 and will destroy any organic pollutants in the sludge. If the sewage sludge is incinerated in a dedicated incinerator the sewage sludge ash can potentially be used as raw material for phosphate recovery. For mono-incineration of sludge typically bubbling fluidized beds are used that have proven to be very reliable. Pre-drying of the sludge to a dry matter content of 35-40% is required in order to be able to incinerate the sludge autothermally without any additional fuels. Waste heat from the incinerator can be used for the drying of the sludge. Modern incinerators should also be able to generate their own electricity so that the sludge can be treated in an energy-neutral manner. Treatment of the flue gas from the incinerator is necessary to remove pollutants like NOx, SO2, HCl and mercury from the flue gas. Normally no dioxins are produced if the incinerator is well designed and therefore the incinerator is a net-destructor of dioxins, since the treated sludge will normally already contain a certain concentration of dioxins. Special attention should also be paid to the formation of laughing gas, N2O, during the incineration. A good design of the incinerator and operating conditions can minimize the formation of this greenhouse gas.

An alternative to the mono-incineration of sewage sludge is co-incineration of the sludge in municipal solid waste incinerators (MSWI), coal fired power plants or cement kilns. The sludge can be co-incinerated in MSWI’s without any pre-drying of the sludge, but special precautions should be taken to ensure complete combustion of the sludge on the grate of the furnace. Coal fired power plants can also co-incinerate dewatered sludge. Especially brown coal power plants are suitable as these plants already use a low quality fuel source. In the Netherlands also composting of the sewage sludge is used as a measure to increase the heating value of the sludge to make co-incineration in a power plant easier. Co-incineration of sewage sludge in a coal fired power station will make a better use of the energy content of the sludge when compared to mono-incineration, but the down side is that the flue gas treatment of a power station is not designed to handle the flue gas from waste materials. For sewage sludge especially the emission of mercury from the sludge is an issue that should be considered. For co-incineration of sewage sludge in cement kilns it is necessary to completely dry the sludge in special sludge driers. Preferably the drying should take place with waste heat to ensure a positive energy balance. Several drying systems have been developed for sewage sludge over time and experiences have shown that specially designed systems are required to cope with the specific difficulties of sewage sludge. A mayor disadvantage of all co-incineration options is the fact that the phosphate from the sewage sludge is lost though dilution of the sludge with other waste materials. Visit this website for more information on the importance of phosphate recycling.

 New developments

In the last years the focus of sewage sludge treatment has been to find the best way to get rid of a waste material. Over time more attention has been given to energy optimization and this has resulted in the treatment options that have been described above. At this moment there is an increasing awareness that we should see waste materials like sewage sludge as a resource, while carefully controlling the pollutants in the sludge. With this focus new treatment methods are being developed. Sewage sludge can for instance be an important source of nutrients like nitrogen, phosphorous and sulfur. Especially the recovery of phosphorous is becoming more and more important and can be an important driver for the development of new technologies. Recent developments make it possible to recover the phosphorous from sewage sludge ash, thus favoring the mono-incineration of sewage sludge. New technologies like supercritical oxidation of gasification of sewage sludge could potentially lead to new methods to recover these nutrients from sewage sludge.

The organic content could also be a raw material for a biobased economy. Already important steps have been taken to produce bioplastics (polyhydroxyalkanoates, PHA’s) from the fatty acids in sewage sludge. Other options can be steps like the supercritical hydrolysis of sewage sludge to sugar molecules. These sugars could then also be fermented to produce lower alcohols like ethanol or butanol that can be important base-chemicals for green chemistry. These processes are currently being tested on lab or pilot scale and still need a lot of development. Nevertheless there seems to be a lot of potential for radical new methods of sludge treatment methods.

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