Survey on the quality of outlet biosolids and sludge heavy metals in the wastewater treatment plants of Mazandaran province

Background: Sewage sludge is rich in fecal coliforms, Salmonella, parasite eggs, and heavy metals that can cause disease in humans. The aim of this study was to evaluate the quality of outlet biosolids and sludge heavy metals in the wastewater treatment plants (WWTP) in Mazandaran province and to compare them with the Environmental Protection Agency (EPA) standards. Methods: This study was carried out on the sludge of the treatment plants of Sari, Joybar, Babol, Babolsar, Nowshahr, and Nowshahr-Chalus sludge compost plants. In this study, physical, chemical, and biological properties of sludge samples including retention time, pH, total solids (TS), color, heavy metals (copper, zinc, nickel, cadmium, and chromium) as well as the density of fecal coliform, Salmonella and parasite eggs were determined and reported. Results: Biosolids of Joybar, Babol, Babolsar, and Nowshahr-Chalus WWTP in terms of parasite eggs were in class A. Biosolids of Sari WWTP in summer after drying off naturally were located in Class A, and in summer and spring, were located in Class B based on the US EPA criteria. The average concentration of heavy metals (copper, zinc, nickel, cadmium, and chromium) was lower than the EPA standard. Conclusion: The results showed that mostly the treated sewage sludge in Mazandaran WWTPs were in Class B of the EPA guideline. The concentration of heavy metals in the biosolids of these WWTPs is lower than the EPA regulation and the use of these biosolids in agriculture is not limited for these metals.