Bio-technological conversion of sludge into organic fertilizers. Sulphate reduction and general sludge treatment

Abstract

NIVA has been in charge of laboratory tests for converting dissolved metal sulphates in sewage sludge into insoluble sulphides by biological sulphate reduction. We have also had an advisory role related to general engineering practices. The work has been a part of an EU-financed project on a multi-step bio-chemical detoxication process called "the Poltesanit method" to convert sewage sludge polluted by heavy metals into a material applicable as a fetilizer additive or as a soil conditioner. Our work has verified that it is possible to convert metal sulphates into sulphides by sulphate reducing bacteria (SRB) in a mixed anaerobic microbial community in sewage sludge at neutral pH. The concentrations of soluble metal sulphates in the sludge were significantly lowered throughout the seven-month test period. Ni, being the most soluble metal, showed the highest specific metal sulphate conversion rate (1.0 mg/d*g VS), followed by Fe and Zn (0.62 mg/d*g VS and 0.53 mg/d*g VS, respectively). Cu was barely observed in the aqueous phase. Although sulphate reduction was successfully obtained in the lab, full-scale implementation of the SRB process into the "Poltesanit" sludge treatment method is not recommended due to operational problems and safety issues. The project group decided to omit the initial SRB step of the "Poltesanit method", and instead feed elementary sulphur to the subsequent acid-producing sulphur oxidising process.