Biogas yield using single and two stage anaerobic digestion: An experimental approach

Abstract

Untreated sewage sludge is one of the biggest problems in India and around the globe. In the current study, we try to figure out an efficient technique that uses sewage sludge (SS) food waste (FW) to generate biogas. In this work, FW and SS are co-digested in single-stage anaerobic digestions (SSAD) and two-stage anaerobic digestions (TSAD) at various processing parameters, such as temperature, carbon-to-nitrogen (C/N) ratio, volatile solid (VS) percentage content, retention time (RT), and pH value. The high-solid digestion of FW and SS in the SSAD process reduces the biogas production rate compared to the TSAD process with 35 days of retention time at mesophilic temperature due to the oily substance from FW accumulating in the scum floating layer in the SSAD process. By altering the mass ratio between FW and SS (based on VS%), the TSAD process could be more effective. This ratio of 60:40 between FW and SS would produce more biogas with maximum efficiency in TSAD compared to the SSAD process. The results highlight an increase in biogas production and VS removal efficiency of the TSAD process 41.90% and 9.8%, respectively, compare to the SSAD process. The cumulative biogas yield in the TSAD process was 1914.8 ml and in the SSAD process was 1349.4 ml when the ratio of FW and SS was taken to be 60:40. In the TSAD process, Methane (CH4) concentrations increased on average from 51% to 75%, due to better substrate hydrolysis and more readily accessible volatile fatty acids in the second stage. The analysis shows the co-digestion approach and TSAD process might work together to overcome the limitations of the conventional SSAD procedure for FW or SS when used separately.