Abstract
M.Tech. (Chemical Engineering)
Biogas, as a renewable energy source, is regarded as a viable alternative to the burning of fossil fuels to meet energy demands. The production of biogas to meet energy needs not only has environmental benefits such as reduction in greenhouse gas emissions and responsible waste disposal but also socio-economic benefits, especially when applied to a rural setting, such as improvements in employment, professional qualification and overall food supply of the local population. The main objective of this study was to determine optimum biogas production from City of Johannesburg (CoJ) market waste, composed primarily of fruit and vegetable waste (FVW), under anaerobic condition.
The influence of operational factors such as temperature, pH, loading rate and retention time were evaluated. The co-digestion of FVW with cow dung (CD) at varying mixing ratios was also evaluated as a technique to optimize biogas production. Mono-digestion of FVW at thermophilic temperatures were optimum for both biogas and methane production. Changes in organic loading rate (OLR) indicated that with increases in OLR there were increases in both biogas and methane production. The highest biogas and methane production occurred for an OLR of FVW: 200g. The optimum retention time for biogas and methane production was determined to be 14 days and 10 days respectively. The explanation for the prolonged biogas and methane production has been accredited to the fact that there was a greater component of biodegradable material available for breakdown by methanogenic bacteria but even more critical than that was the alkaline pH of 7.18 of this substrate mixture at the commencement of digestion.
However, experimental results indicated that co-digestion is the preferred method to optimize biogas and methane production. According to the experimental results obtained, a mixing ratio of FVW: CD of 20:40g is the optimum substrate mixture for both biogas and methane production. The optimum environmental conditions for co-digestion was found to be at 37°C with a pH range between 7.12 to 7.18, while the optimal retention time was found to be 33 days. It was found that when the mixing ratios contained between 50% and less cow dung with corresponding increases in FVW, there were drastic reductions in the digester stability and overall production rates for those substrate mixing ratios. It was therefore concluded that in order to ensure sustained and good biogas and methane production via co-digestion, the CD content should remain above 50% of the total mixture.