Sewage sludge become a source of green energy at Braunschweig, Germany
Solution to which problem
The treatment of residual sludge from sewage treatment plants is a challenge that municipalities have to tackle. It is possible to achieve this by recovering the sludge to produce biogas by a process of thermal hydrolysis and anaerobic digestion.
The Steinhof WWTP has a capacity of 385 000 PE. With the help of 86 pumping stations it treats an average daily inflow rate of 60 000 m3/day. The wastewater is mechanically and biologically treated in a multi-stage process. During this process sewage sludge is produced, which is used as a fertilizer on agricultural areas.
The sewage sludge is used in two forms:
- irrigation: the clear water used for irrigation purposed is mixed with digested sewage sludge for the period in which the plants are able to absorb nutrients.
- the sewage sludge which is not used for irrigation is dehydrated at the treatment plant and is used on agricultural surfaces, also serving as fertilizer.
For the treatment of sewage sludge and energy recovery Steinhof WWTP chose Veolia solutions, including Exelys and Bio Thelys. One of these two global sludge reduction solutions works continuously and the other on demand; both combine thermal hydrolysis and anaerobic digestion. This solution optimize sludge treatment by producing:
- 25 to 35% less dry matter;
- 30 to 50% more biogas;
- no olfactory nuisance;
- a hygienic, stabilized digestate to control health risks and secure agricultural use.
The Braunschweig model was developed by Abwasserverband Braunschweig. Wastewater from the city and bioenergy from the countryside are combined to create a water nutrient energy cycle.
The wastewater from the city is mechanically and biologically purified in the waste water treatment plant in Steinhof. Then ½ of the water is fed to the infiltration system. The fields cover an area of 275 ha and their main purpose is the secondary cleansing of waste water. The remaining of the waste water is combined with sewage sludge and it is used to irrigate 2700 ha of agriculturally used land. Harvested products (maize and rye) are basis for the production of biogas. The biogas plant needs 44 000 tons of maize and rye annually in order to produce 19,3 million KWh of electrical energy. In the final stage methane gas is developed, then is transported via a gas dehumidification and compression facility through an underground gas pipe to Braunschweig where it is transformed into electricity and heating. The biogas is also used for own requirements and for the additional production of electricity at the biogas plant. The biogas production per day amounts to 24 000 m3, while the electricity amounts to 50 000 kWh/day.
Sewage sludge recovery solution makes sludge a resource and establishes local circular economy loops. Converting sewage sludge into biogas complies with environmental regulations and drastically reduces residual sludge volumes. It also creates energy that can be used by the plant itself or by selling it to the local grid.
Thanks to the sewage sludge the Braunschweig plant is 100% energy-efficient. Biological wastewater treatment, thermophilic sludge digestion and co-digestion with organic waste, cogeneration and recovery of biogas have resulted in the plant being energy self-sufficient.
Benefits of wastewater reuse:
- high wastewater treatment efficiency;
- solution for sludge disposal;
- less investments on WWTP,
- supply and distribution of irrigation water;
- resources conservation;
- promotion of groundwater recharge.