Sewage sludge utilization: Technology with a future

The treatment of wastewater in mechanical-biological wastewater treatment plants produces large amounts of sewage sludge. In Germany, with more than 10,000 municipal sewage treatment plants, approx. 2 million t/a of sewage sludge dry solids (DS) must be disposed of. As a result of stricter wastewater regulations, treatment processes are expected to intensify further, meaning that increasing amounts of sewage sludge are to be expected in the short-to-medium term future.

Due to its harmful components, the land application of untreated sewage sludge is only possible under strict conditions. This has been prescribed by the German government in recent years by the amended Fertiliser Ordinances (DüV, DüMV) and Sewage Sludge Ordinance (AbfKlärV).

When municipal sewage sludge is subjected to a thermal treatment with complete carbon conversion, the resulting product is a decontaminated ash from which important nutrients such as phosphorus can be safely recovered. This requires a so-called mono-use of the sludge without mixing it with any other fuels. Only ash produced from pure sewage sludge is suitable for the recovery of the nutrients contained therein.

For many municipalities and wastewater treatment plants, this shift from land application of sewage sludge towards thermal treatment represents a considerable challenge. The new, more intensive sludge management chain should ideally not lead to increase in wastewater charges. Especially smaller plants among them often cannot find a suitable, financially viable solution with established technologies for thermal treatment.

Here we set new standards with our patented process. By means of gasification, in which the energy bound in the sewage sludge is converted into a more usable fuel gas form, high efficiencies in energy recovery from the sewage sludge are also possible in smaller plants. With the recovered energy, the operator’s energy costs can be reduced and the costs of the sewage sludge utilization plant can be recovered in a short period of time. The disposal or recycling of phosphorus-containing ash is unproblematic – the properties of the practically carbon-free ash are comparable with those from mono-combustion rather than those of carbon-rich residues from some alternative thermal treatment technologies.