Abstract
The Earth’s surface and its ocean temperatures are increasing annually. One of the causes is the burning of fossil fuels to produce energy. Renewable energy must substitute fossil fuels to reduce the effects of climate change. There are various renewable energy sources such as solar, hydropower, wind, geothermal, and biomass. Among the rest of the renewable sources, biomass is the most available. Biomass or biowaste can be broken down in an oxygen-free environment to release biogas. Biogas is a highly combustible gas containing 55-70% methane and 30 to 40% carbon dioxide with some hydrogen sulphide. Once it is generated and stored, this gas can be used for cooking and heating at the home scale. When purified and compressed, biogas can also be used to fuel motor vehicles. However, due to the technology's poor efficiency, people are reluctant to invest in the technology. Hence, this research investigates the performance of a medium-scale biodigester at a small farm in Devon, a town in the southeast of Johannesburg, South Africa.
Literature has shown that many variables affect biodigester's performance; however, many researchers list fermentation temperature and pH, feedstock characteristics, and the VFA:TA ratio as the most influential parameters in AD. Hence, the objectives of the investigation include developing a process monitoring system, which can monitor the factors that affect AD, i.e., temperature, pH, organic loading rate, VFA:TA, and ammonia concentration; and characterise the feedstock of the biodigester.
Four experimental programs were designed to fulfill this investigation's objectives, i.e., feedstock characterisation, biomethane potential test, design and building of data logger, and biodigester monitoring.
The experiments showed that the cow dung from the farm is a suitable feedstock for AD, with an average TS% of 13.77%, VS(VS% of TS) of 81.3%, and ash content 2.25%. Even though the feedstock on the farm is suitable for anaerobic digestion, the biodigester still performs poorly; because the results from the batch experiment illustrate that the cow dung can produce on average 37.76 NL of biogas per kg of cow dung. Hence, the floating biodigester should produce 3.78 Nm3 of biogas per 100 kg of cow dung. However, that is not the case since the biodigester operates under slightly acidic fermentation pH and low fermentation temperature.
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In conclusion, the study suggests solutions to optimise the biodigester performance; these proposed solutions include co-digesting cow dung with pig manure, adjusting the cow dung to water mixture ratio, recirculating the digestate, and pretreating the water. Other costly optimisation methods that were proposed are introducing an agitator and a heating system on the biodigester.