Comparison and toxicity assessment of co-composting process by pistachio wastes and date-palm straw combined with municipal sewage sludge

Abstract

This study aimed to compare and evaluate the toxicity of the co-composting process of pistachio dehulling waste combined with municipal sewage sludge (PW + SS) and date-palm straw combined with municipal sewage sludge (PS + SS). The process was evaluated over a 60-day period. During the process, physicochemical parameters (temperature, humidity, pH, electrical conductivity (EC), carbon to nitrogen ratio (C/N), ash, and volatile organic matter (VOM)), microbial indicators (Escherichia coli, Salmonella, and parasite eggs), heavy metals (copper, zinc, iron, manganese, lead, cadmium, nickel), and germination index were measured. In the PW + SS reactor, the maximum temperature reached 59 °C on the 32nd day and in the PS + SS reached 39 °C on 18th day. The pH level in PW + SS reached a minimum of 3.2 on the 14th day and then increased and finally reached 9.5. The pH level in PS + SS had an increasing trend and finally reached 7.1. Ash content had an increasing trend in PW + SS and finally reached 27.5% and reached 19.1% in PS + SS. EC, C/N, and VOM in both reactors had a significant decreasing trend. E. coli microbial indicator in both reactors decreased from ≥ 105 to zero at the end of the process. Toxicity assessment showed that the PS + SS compost was less toxic than the PW + SS compost.