Municipal sludges as sources of energy or nutrients – What is the best?

Abstract

The decision on the optimal use of sludges from wastewater treatment plants is still very complex, even when applying the principles of sustainability and circular economy. Interest in the energy use of sludges is conditioned not only by their inclusion among biowaste but also by the availability of technologies that meet emission limits and newly by the possibility of extending anaerobic digestion processes to hydrogen production. The use of sludges in agriculture as a source of carbon and nutrients is limited by concerns about the occurrence of anthropogenic organic compounds and especially micropollutants that can migrate in individual environmental compartments. Based on information from the Globally Harmonised System of Classification and Labelling of Chemicals, 287 compounds of anthropogenic origin in sludge were divided into five groups: compounds causing environmental risk, health risk, acute toxicity, irritant, and no impact. The group of compounds causing environmental risk (highly toxic for the aquatic environment) includes 42 compounds. Phytotoxicity tests (Lepidium sativum) in relation to other parameters characterising sludge stability have shown that the key parameter for achieving a germination index >85% is the amount of organic matter expressed as chemical oxygen demand (COD) and conductivity.