Sewage sludge is generally perceived more as a problematic residue or waste rather than a valuable raw material. Previous methods of disposal have included agriculture and combustion in power stations or monocombustion plants, which use only sewage sludge as fuel. But a closer examination of the components of sewage sludge reveals it contains valuable raw materials. In addition to carbon, which is present in almost all objects in our everyday lives, sewage sludge also contains significant proportions of phosphorus – up to 5 percent by weight in dry state.
Phosphorus is essential for cell metabolism, and is therefore an essential primary material for all life. It finds its way into sewage sludge via the human food chain. Natural phosphorus resources are finite and increasingly contain harmful trace substances. The European Commission therefore considers phosphate or phosphate ore as a critical raw material, and much more recycling of phosphorus from sewage sludge will be required in the future. Germany’s Sewage Sludge Ordinance (Klärschlammverordnung), amended at the end of 2017, obliges German treatment plant operators to recycle at least 50 percent of the phosphorus in sewage sludge from 2029 or 2032 onwards, depending on the size of their facilities. To have the technical capability to put this ordinance into practice, they will require smart processes that go beyond the recycling of phosphorus to focus on efficiently using sewage sludge as a source of energy and carbon into the bargain.