Sludge management on sewage treatment plants

Challenges and range of services

Our range of services in the field of sludge management at wastewater treatment plants takes various aspects into account. We are active in both the municipal and industrial sector and offer scientific advice and innovative solutions for new and old challenges:


  • Studies on sewage sludge fermentation for the determination of design parameters
  • Technical implementation up to pilot scale
  • Avoidance of bulking sludge and floating sludge
  • Process conversion from aerobic to anaerobic sludge stabilization
  • Design of digestions for an efficient operation of digestion


For more than 25 years, Fraunhofer IGB has been developing biotechnological processes for the treatment of waste and water - from the microbiological basics to pilot plant scale and industrial-scale plants. Basic and detail engineering on the basis of Fraunhofer patents are carried out by our industrial partners from the plant engineering sector.

Systematic analysis – basis for the optimization of sewage treatment plants

Sewage treatment plants in the municipal or industrial sector are usually standardized unique specimens that have been designed according to historically developed rules of technology. Sometimes not enough attention is paid to sludge treatment. Fraunhofer IGB offers a systematic assessment of sewage treatment plants through the specific evaluation of operational data books, design documents and installed technology. This translates into operational transparency, on the basis of which measures can be taken to optimize the sewage treatment plant.

Energy-efficient wastewater treatment plants: high-load digestion for sewage sludge

Schwerzen sewage plant.

Sludge digestion – alternative to sludge stabilization

Wastewater treatment plants remove organic substances from wastewater. If the sludge produced decays, biogas is produced as a product. However, only a good tenth of the more than 10,000 wastewater treatment plants in Germany have a digestion tower. Smaller operators in particular shy away from the costs involved in building a new digestion tower. Instead, they enrich the sewage sludge with oxygen in the already existing aeration tank and stabilize it. However, the aeration tanks require a lot of electricity and make the sewage treatment plants the largest municipal electricity consumers. At the same time, an enormous potential of energy is lost, since no biogas is produced during the aerobic sludge stabilization. Many larger sewage treatment plants, whose digestion towers are now obsolete, could also produce more biogas with modern technology, thus improving cost and energy efficiency.

High-load digestion of sewage sludge

Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB has developed an energy-effcient high-load process for the fermentation of sewage sludge. It was put into operation for the first time in 1994 at the sewage treatment plant in Leonberg. In the meantime, this process is being successfully applied by four other municipal sewage treatment plants. The outcome: The high-load digestion converts the sludge into biogas in a considerably smaller space and more cost-effectively than the conventional digestion towers.


Fundamental advantages of high-load digestion:

  • shorter retention time
  • smaller digestion space
  • enhanced degradation rate
  • higher biogas yield
  • no operational problems (foaming)
  • easier to dewater
  • lower operational and disposal costs


Limited disposal possibilities

The possibilities for disposal of sewage sludge from municipal wastewater treatment are being increasingly restricted. In future there will no longer be any demand for use in landscaping, utilization in agriculture is controversial, dumping of most sludges no longer possible. Incineration of sewage sludge will gain in importance, disposal is thus becoming more expensive. Aerobic sludge stabilization is expensive, frequently insuffcient and no alternative for sewage plants > 10,000 PE.

Wastewater treatment plant in Tauberbischofsheim with two-stage high-load digestion and microfiltration.

Intelligent utilization of sewage sludge as an energy carrier

The high-load digestion process developed at Fraunhofer IGB makes sewage sludge digestion a process that can, as a result of the effcient conversion of the sewage sludge contents into biogas, contribute substantially to the cost-effectiveness and energy effciency of sewage treatment plants. The process is therefore also suited for smaller treatment plants (10,000 PE) that so far stabilize the sludge aerobically with a high power consumption.

The sewage sludge is stabilized with net energy production by means of high-load digestion, can be dewatered to a higher TS level and the residual sludge disposed of by incineration at the lowest possible cost. The regenerative energy carrier biogas is derived as a product. The thermal energy requirements of the sewage treatment plant can be covered by the biogas obtained and further expenses can be saved by means of combined heat and power generation. The high-load digestion process therefore also represents an economically intelligent alternative and considerably improves the energy effciency of municipal sewage plants.