Proposing policy changes for sewage sludge applications based on zinc within a circular economy perspective

Sewage sludge production has been increased in the last years because the wastewater treatment is mandatory in urban areas with more than 2000 inhabitants since 2005. Most adequate disposal of sewage sludge should be agricultural soils to improve their fertility and fulfill bioeconomy and circular economy requirements. However, the higher level of heavy metals in sludge than in soils makes necessary to consider the inputs of heavy metals in soil as have been done by regulation 86/278/EEC and more recent documents (EU 3rd Draft Directive or the Environmental, economic and social impacts of the use of sewage sludge on land). Sewage sludge application is restricted by the mean amount of heavy metals applied with sewage sludge in soil in a 10-year period and the concentrations of heavy metals in the sludge and soils. The aim of this study was to evaluate the current and foreseeable limits of the application of different types of sewage sludge (anaerobic, pelletised and composted) considering the current and the possible future legislation of the most abundant heavy metal in the sewage sludge, which is Zinc (Zn) and taking into account the existing levels in acid soils in Galicia (NW Spain). Main results highlighted the adequacy of using composted sludge as amendment, while anaerobic and pelletised sludge as fertilisers to reduce future possible soil contamination. Sewage sludge regulations should aim to reach mean values of Zn to ensure soil microbial health and sustainability adequate levels of Zn in crops or pasture. Low quality sewage sludge application should be reduced in frequency or doses, allowing soils to get benefits of increasing soil organic matter without strong changes in Zn soil contents.