Sewage sludge is a mix of solids and water, which arises during the purification of sewage water in communal waste-water treatment plants. The solids, which are contained in the sewage sludge, consist of a mixture of solid contents, which have been separated from the waste water (primary sludge), and the bacterial sludge, generated by microbes (surplus sludge). The sewage sludge is separated from the waste water in the secondary clarifier. In most waste water treatment plants the sewage sludge undergoes anaerobic treatment in order to further reduce any organic substances that it still contains. This is followed by the thickening, drainage and in some cases drying of the sewage sludge.
As a rule, sewage sludge contains plant nutrients such as nitrogen, phosphorus, sulphur or lime. However, sewage sludge can also be tainted with substances such as hardly biodegradable organic compounds, heavy metals, nanomaterials, pathogenic micro-organisms or hormonally active substances.
Depending on legal regulations, communal sewage sludge can be applied to agricultural land, thermally treated by utilizing waste heat, composted or humified. (BAWP 2017)