Waste: a Valuable Resource

Wastewater is rich in valuable content. While environmentally friendly cleaning used to be the primary focus, the recovery of energy and raw materials is becoming increasingly important. Therefore, in a growing number of places, we recover heat from the sewage upstream of the sewage treatment plants; the recovered heat can then be used to heat buildings.

During the sludge treatment in our six wastewater treatment plants, we use two different processes to produce electricity and heat.

In our wastewater treatment plant in Ruhleben, the sludge consists of up to 95 % water and is dewatered in centrifuges. The dewatered sludge is then called sludge cake. The water contained in the sludge cake, still around 75 %, evaporates on fluidised bed furnaces and the remaining solid material is incinerated there after. Depending on the amount of water left in the sludge, heating oil may have to be used in order to reach the specified combustion temperature of at least 850° C.

Energy and Fertilizer

The flue gases are cooled in heat recovery boilers, cleared of dust using electric filters and then cleaned in a flue gas purification plant. The harmful substances are removed so that their proportions in the flue gas are significantly below the admissible values at the chimney exit. The heat contained in the smoke is used to generate steam and electric power in the sewage treatment plant. Leftover ashes are used as landfill for mines that have been shut down. More than one third of all sewage sludge produced thermally is recycled in this way.

In the Münchehofe, Wansdorf and Schönerlinde sewage treatment plants, the sludge is treated in digesting tanks or digesting chambers. At a temperature of about 33° C, the whole process takes around 20 to 30 days. The organic substance is converted into biogas, which is then used to generate heat or electric power. Next, this "digested" and dewatered sludge is used to generate energy in power plants and cement works, as well as in the Ruhleben sewage treatment plant through co-incineration.

And we also extract magnesium ammonium phosphate from the sewage sludge, a high-quality mineral slow-release fertiliser which is commercially available under the name "Berliner Pflanze". Its use contributes to the conservation of the limited natural phosphorus resources.