Co-digestion of Cassava Biomass with Winery Waste for Biogas Production in South Africa

Abstract

Renewable energy security for the future and better use of natural resources are key challenges that can be concurrently managed by a practical anaerobic co-digestion approach in the production of methane. For this study, co-digestion of cassava and winery waste was investigated for the production of biogas. Cassava biomass is a good substrate for biogas production due to its high carbohydrate yield per hectare (4.742 kg/carb) than most plants. Winery wastes constitute a lot of challenge in South Africa due to high amounts currently being dumped at landfills. Due to the chemical properties of the two substrates, it is envisaged that their co-digestion will produce more biogas than use of a single substrate. Biomethane potential (BMP) tests were carried out in a batch, mesophilic (37 °C±0.5) reactor using cassava and winery waste singly and in combination at a ratio of 1:1 and ran for 30 days. Biogas optimization was also evaluated. The optimal conditions for methane production from anaerobic co-digestion of cassava biomass and winery solid waste using response surface methodology (RSM). The effects of temperature, pH and co-substrate ratios on the methane yield were explored. A central composite design technique was used to set-up the anaerobic co-digestion experiment was determined. Once the optimized values were established, biogas production from co-digestion of cassava biomass with winery waste was investigated using a single-stage 5 L mesophilic batch digester and the microbial dynamics inside the digester during co-digestion of cassava and winery waste in the single-stage 5 L mesophilic batch digester. The samples were collected on days 1, 15 and 30 of the anaerobic digestion period and DNA extracted from them while 16sRNA bacterial sequencing was performed. The results for the BMP tests showed that cumulative methane yield for cassava, winery waste and in combination were 42, 21 and 38 mLCH4 respectively. It was concluded that biogas production from anaerobic digestion was dependent on many factors such as pH, substrate properties and the ratio of different feedstocks used during co-digestion. The results from the optimization study were pH 7, temperature of 35 °C±0.5 and co-digestion ratio of 70:30 cassava to winery waste. The maximum methane yield of 346.28 mLCH4/gVSadded was predicted by the quadratic model at the optimal temperature of 35 oC±0.5, pH of 7 and 70:30 ratio of cassava biomass to winery solid waste. Experimental results showed a close fit but higher methane yield (396 mLCH4/gVSadded) than predicted values as indicated by the coefficient of determination (R2) value of 0.9521. The response surface model proved successful in the optimization process of methane yield. The single-stage 5L mesophilic batch digester with a co-substrate ratio of 70:30 cassava to winery waste produced a total of 819.54 mL/gVS biogas with a 62 % methane content. The study of microbial community dynamics showed the presence of the bacteria that is responsible for each stage of anaerobic digestion. The study concluded that both winery waste and cassava substrates were favourable for biogas production and most underprivileged people in the rural areas with no access to electricity can produce & utilise it.