Pretreated rice straw improves the biogas production and heavy metals passivation of pig manure containing copper and zinc

The high content of heavy metals in pig manure can inhibit biogas production and increase the risk of environmental pollution, while rice straw is an excellent co-digestion substrate and natural adsorbent. In this study, raw and pretreated rice straw were used to analyse the influences on biogas production and heavy metal passivation during anaerobic digestion. The results showed that more than 80% of copper occurred in oxidizable and residual fractions, while zinc transferred to the solid phase rather than passivated. Heavy metals were the first to be transformed into a solid phase through the adsorption and were passivated during the peak period of biogas production. Microwave and ultrasonic treatment could effectively increase the adsorption properties, whereas alkaline treatment tended to reduce this property but contributed more to anaerobic digestion. Anaerobic digestion was the primary factor of passivation. The contribution rate to copper and zinc reached 77% and 59%, respectively, under the addition of rice straw and increased to 96.4 and 85.8%, respectively, in the alkaline group. The addition of rice straw was beneficial to biogas production and heavy metals passivation, but the content of heavy metals in substrates and the cost of the pretreatment should be studied.