Besides the clarified water, modern biological wastewater treatment plants produce sludge which mainly consists of the feces-consuming microorganisms.
In a wastewater treatment plant with a size of 500,000 pe that can be 1,000,000 m3 sludge per year, which even after its dewatering still weighs approx. 10,000 t.
However, the sewage sludge also contains undesired toxic components, such as heavy metals, halogenated hydrocarbons, and pathogens. This makes composting impossible and also the agricultural use as fertilizer is controversial
and will be made more difficult by planned legislative projects.