Sewage sludge: A review of its risks and circular raw material potential

The increasing amount of sewage sludge, which is mainly related to the improvement of wastewater treatment technologies and the expansion of sewage systems, has become a serious problem for sewage sludge management. Sewage sludge is a valuable source of nutrients and organic matter. The use of sewage sludge for agricultural purposes presents a sustainable method of disposal in line with circular economy strategies, which are aimed at waste prevention, the effective use of resources and energy, and a nutrient closed-loop system. However, sewage sludge can contain heavy metals and toxic organic compounds which pose threats to the environment. Therefore, sewage sludge management is still a controversial topic. Taking into account disruptions in the fertiliser market caused by the pandemic and wars, it is necessary to consider local waste with fertilising potential as an alternative raw material to reduce fertiliser import dependency and ensure food security. This review collected data confirming the agronomic value of sewage sludge, including long-term field studies and data on the presence of pollutants along with possible ways to reduce them. Particular attention is given to the technologies available for fertiliser production from sewage sludge which are operating at full scale. This includes information about the processes, their efficiency, production scale, and the characterisation of fertilisers.