Potentiality of sewage sludge-based organo-mineral fertilizer production in Poland considering nutrient value, heavy metal content and phytotoxicity for rapeseed crops.

Sewage sludge is a valuable source of nutrients and organic matter, which are beneficial for plant growth. The use of sewage sludge for agricultural purposes can be limited by the potential content of heavy metals and toxic organic compounds that pose a threat to the environment. The characterisation of sewage sludges from 11 Polish wastewater treatment plants (WWTPs) in terms of their fertilizing properties and heavy metal contents are presented in the study. Based on the chemical composition of sewage sludge from the WWTP in Żywiec, the compositions of organo-mineral fertilizers dedicated for rapeseed crop was developed. Alternative raw materials such as poultry litter ash and biomass ash were considered as components for fertilizer production. The production process of organo-mineral fertilizers included mixing sewage sludge with poultry litter ash/biomass ash and mineral fertilizers and granulation with mineral acids. The final products contained 24-34% of total NPK and fulfilled the requirements regarding heavy metal content according to Polish legislation. Using sewage sludge for fertilizer production is a promising solution for sewage sludge management, allowing the recycling of 82-140 tonnes of phosphorus (as P2O5) and 42-73 tonnes of nitrogen (N) annually in the WWTP in Żywiec.