Evaluation of the Influence of Temperature and Co-digestion with Green Grocery Waste on Biogas Production from Cattle Manure

Abstract

In this study batch and continuous anaerobic digestion was carried out to evaluate the effect of co-digestion and temperature (controlled temperature and uncontrolled environmental temperature) on biogas production. Substrates used for co-digestion were cattle manure and green grocery wastes. In batch anaerobic digestion process anaerobic reactors containing different ratios of substrates were operated at controlled temperature (35°C) for almost 40 days to evaluate the full potential of substrates for methane production. Automatic Methane Potential Test System was used to record the methane yield. Results of batch anaerobic digestion process showed that co-digestion is more energy efficient process than single substrate digestion and methane production was enhanced to almost 10% by co-digestion of green grocery and cattle manure. To evaluate the effect of temperature on the continuous anaerobic digestion process,two independent 14 L anaerobic digesters with working volume of 10 L were selected. One digester kept at controlled temperature (37°C) and other placed at environmental temperature. For monodigestion cattle manure was fed to both reactors at organic loading rate of 1 g VS/L.day and retention time was 20 days. It was observed that biogas yield was affected by temperature and its production was decreased with decrease in temperature (seasonal variation). As expected biogas yield in the reactor incubated at 37°C was higher than the reactor placed at environmental temperature. Duringwinter the methane production from cattle manure at uncontrolled temperature was almost 10 times lower than the biogas produced at 37°C. Then the substrate for both of the continuous reactors was replaced with mixture of cattle manure and green grocery waste for evaluation of the effect of co-digestion on biogas production. Since in Pakistan presently almost all anaerobic digesters use cattle manure solely as substrate and are operated at uncontrolled temperature. There was a significant increase in biogas yield by applying codigestion. Biogas yield at controlled temperature with co-digestion (cattle manure and green grocery) was 20 times higher as compared to biogas yield in the reactor operated at environmental temperature and fed with cattle manure alone. This study shows that anaerobic digesters used in Pakistan, which are run at uncontrolled temperature and fed with cattle manure, can be significantly enhanced their production by applying co-digestion and controlled mesophilic temperature. The biogas can play a major role in the supply of energy source as alternate of natural gas in Pakistan, where natural gas has share of almost 50 % in the primary energy sources, which is expected to diminish in coming decades.