Sewage sludge is generally regarded as a problematic residual material or waste, rather than a valuable raw material. Conventional disposal routes are agriculture and thermal utilisation in power plants or so-called mono-incinerators, which exclusively use sewage sludge as a fuel. However, if we take a closer look at the components of sewage sludge, it is clear that it contains valuable raw materials. In addition to carbon which can be found in almost all everyday objects, sewage sludge contains a significant amount of phosphorus – in its dry state, it can account for up to 5% of its weight.
As phosphorus is essential for cell metabolism, it is a raw material that all life depends on. It enters sewage sludge via the human food chain. Natural phosphorus resources are finite and increasingly contain harmful trace elements. As a result, the EU Commission regards phosphorus, or phosphate, as a critical raw material and the recycling of phosphorus from sewage sludge will be significantly expanded in future. Following the amendment of the Sewage Sludge Ordinance at the end of 2017, German operators of wastewater treatment plants will be required to recycle at least 50% of the phosphorus found in sewage sludge as of 2029 or 2032, depending on the size of the waste water treatment plant. The implementation of this ordinance requires smart processes that focus on the efficient utilisation of sewage sludge as a primary energy source and carbon carrier, in addition to recycling phosphorus as an added bonus.