Assessment of performance of Mesophilic Single-stage biogas digester for treating agricultural wastes

Abstract

Agricultural activities play a crucial role in global food production and economic growth, but they also generate substantial amounts of organic wastes and by-products. One of the leading causes of environmental pollution and health hazards has been the improper management of agricultural wastes. One promising and environmentally friendly solution for treating agricultural wastes is anaerobic digestion. Although studies have been done in Ghana and Central Region using anaerobic digestion to treat organic wastes, there is little or no knowledge available on the assessment of the performance of a mesophilic single-stage biogas digester for treating agricultural wastes. The primary objective of the research work is to develop such biogas digester to treat agricultural wastes. An 8 m3 pilot-scale single-stage digester with a manual stirrer operated at a mesophilic condition (30 oC) was used to treat agricultural wastes at three different hydraulic retention time (HRT): HRT 20, 23 and 26 days with a hydraulic flow rate of 300 L/d, 260 L/d and 230 L/d respectively. Cow dung was used as inoculum for the digester whiles pig manure, cabbage wastes, carrot leaves, jute leaves, amaranth plant and spinach leaves represented agricultural wastes. Selected physicochemical parameters (BOD, COD, pH, chloride, ammonia, total phosphorus, total solid, volatile solid, total nitrogen, and nitrate), pathogenic microorganisms (E. coli and Salmonella spp.) and heavy metals (lead, chromium, nickel, zinc and cadmium) were analyzed on the inoculum, influent and effluent. The results from the cow manure made it feasible and preferred inoculum for the anaerobic digestion of agricultural wastes. With regards to physicochemical parameters, the greatest elimination was seen in TS and VS at HRT 26, whereas TN, OC, COD NO3-, and TP were at HRT 23. The results of the pathogenic microbial treatment indicated an infinite reduction of salmonella spp. and a 2.02 log reduction in E. Coli all at HRT 26. Additionally, the data related to heavy metals indicated that all the initial values of these metals were higher in the influent than in the effluent, except for Zn and Pb at an HRT of 23 days, which saw an increment in their effluent concentrations. HRTs 23 and 26 days showed better treatment efficiency as compared to HRT 20. This research is a win-win solution for farmers and policy makers as it addresses waste management, energy, environmental and economic concern, whiles supporting sustainable agricultural and rural development. However, Pb and Zn showed higher effluent values which need additional treatment before using it for cultivation.