Sludge management on sewage treatment plants

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.

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.