Changes in chemical and biological parameters during co-composting of anaerobically digested sewage sludges with lignocellulosic material

This study reports a pilot experiment of composting of anaerobically digested sewage sludges deriving from the production of biogas in a wastewater treatment plant. Two composting piles (about 15 m3 each) were prepared by mixing 50% and 30% sludges with lignocellulosic material. The composting process was monitored through determination of the main physico-chemical properties. The maturity of the composts was assessed by determination of the respiration index and dehydrogenase activity. The data indicated that, at both sludges concentrations, the process produced a final compost suitable for agricultural applications. The germination and growth indexes of Lepidium sativum indicated the lack of phytotoxicity of the final materials. The suitability of the final product as a field improver and horticultural substrate has been attested through bioassays conducted on maize and chrysanthemum.