Experimental analysis on the effects of trace metals as micronutrients in enhancing biomethane production

Abstract Anaerobic digestion (AD) of microbial biomass has proven to be a significant breakthrough technique in producing biogas rich in methane. The quantity of biogas obtained by anaerobic digestion processes varies significantly based on the nature and characteristics of the substrates used. This research work focuses on the use of trace metals such as Fe, Cu, Zn, Mn, Mg, Ni in proper proportions to enhance the microbial consortium thus aiding in the production of biogas of desired quality. The substrate used for this study is Food Waste and Cow dung. Food waste from the college canteen was used as the substrate and cow dung was used as an inoculum for providing a catalytic effect in the anaerobic reactor. Food waste and cow dung in the ratio 75:25 was fed into the anaerobic digesters with varying concentrations of micronutrients supplemented to the reactors operating at a pH range maintained between 6.8 and 7.2 under room temperatures (22–27 ºC). The effect of these micronutrients on the anaerobic digestion process was observed by analysing the biogas yield, pH, alkalinity, total solids, and volatile solids of the samples. Sulphates of Fe, Cu, Mn, Ni and Chlorides of Zn and Mg was used in this study. Fe, Cu, Zn, Mn, Mg, Ni were fed to the anaerobic reactor at varying concentrations to arrive at the optimum dosage for the chosen substrates. The optimum dosage for the chosen substrate concentration was taken as that concentration which yielded maximum biogas yield with less retention time. Fe at concentrations varying from 1 mg/l–5 mg/l was fed to the anaerobic reactor and the optimum dosage for the chosen substrate concentration was noted at 1 mg/l. The reactor with an Fe concentration of 1 mg/l showed an increase in biogas production rate of about 68% compared to the sample without Fe supplementation as well as the ones with other dosages greater than 1 mg/l and less than 1 mg/l of Fe dosage. Each nutrient is subjected to an individual dosage analysis before arriving at the optimum dosage and then a mixture of the arrived optimum dosages will be analysed for further study. The process set-up will be conducted for a minimum retention period of 20 days and terminated when the results show a deep fall in the biogas production for consecutive days. Biogas produced for the nutrient supplementation of 1 mg/l of Fe, 0.5 mg/l of Cu, 1 mg/l of Zn, 0.5 mg/l of Mn, 1 mg/l of Mg and 0.5 mg/l of Ni yielded a biogas of 850 ml/g VS in 10 day retention time. Triplicate samples study were conducted and biogas yield measured daily to arrive at concordant results. The results showed an increase in the biomethane yields of the substrate by about 60% compared to the reactors which had no micronutrient supplementations. Furthermore, the study summarized that not all micronutrients are essential for a successful microbial metabolism to take place in an anaerobic digester as the micronutrient Manganese at varying dosages of 0.5 mg/l, 1 mg/l and 1.5 mg/l showed an antagonistic effect on the microbial activity in the reactor. The results obtained from this study showed a significant improvement in the quantity of biogas produced from the substrates supplemented with micronutrients at optimum dosages thus arriving at an efficient and effective method for treating waste in a sustainable way.