Increasing biogas and methane yield by adding sewage sludge and zero-valent iron nanoparticles during the single-stage anaerobic digestion with municipal solid waste

One of the most important problems of the anaerobic digestion (AD) process and the main obstacle to this complex structural conversion is lignocellulose and the difficulty of its biological conversion. One way to disrupt the lignocellulosic composition is using nanoparticles (NPs) with proper concentration. In this study, biomethane potential tests have been used to maximize the biomethane production from anaerobic co-digestion of municipal solid waste (MSW) and sewage sludge (SS) under different mixing ratios (MSW:SS: 100:0, 50:50, 60:40, 80:20 and 90:10) and different concentrations of iron (Fe) NPs (50, 70, 90 and 110 ppm) to speed up the AD process of the substrate, which increases biomethane production. Increasing the SS ratio up to 40% dramatically increased the biomethane yield, in a way that after 18 days of hydraulic retention time, 90% of methane (270 NmL/g VS) was obtained. The results showed that the highest biomethane yield (based on the optimal mixing ratio obtained in the first experiment, MSW:SS: 60:40) was obtained by adding 90 ppm Fe NPs using the statistical analysis (Duncan test) compared to the control (P < 0.05). Also, using iron NPs (Fe NPs) led to an increase in cellulose by 83%, a decrease in lignin by 62% and a decrease in hemicellulose by 32%, indicating an increase in the degradability due to the presence of NPs. Most of the biomethane was produced at 90 ppm Fe NPs concentration, showing an increase of 44% compared to control, and the maximum reduction in total solids and volatile solids were 36% and 28%, observed in this reactor, compared to the control sample.