Anaerobic digestion of energy crop (cassava)

Abstract

Global energy demand is on the rise due to continuous increases in population, economic growth, and energy usage. Methane production through anaerobic digestion of organic materials provides a resourceful carrier of renewable energy, as methane can be used instead of fossil fuels for both heat and power generation and also as vehicle fuel, thus cutting down the emissions of greenhouse gases and hence contribution in the slowing down climate change. Several studies have been done on biogas, but in South Africa, these are biased towards industrial wastewater. Therefore, there is need to explore other alternatives for biogas generation. Furthermore, the sustainability of anaerobic digestion processes depends on the availability and the identification of the optimal substrate. The use of cassava in South Africa provides a great potential for the production of bioenergy especially biogas, due to its suitable chemical composition. Cassava codigested with other feedstock could be an alternative substrate for various communities for the production of biogas in South Africa. Since cassava is yet to be listed as a staple food crop in South Arica, its peels and other by-products from its processing can be suitable for renewable energy production for small medium enterprises (SMEs). This study’s overall objective was that of establishing the suitability of cassava tubers as an alternative source of biomass feedstock for biogas production in South Africa. The specific objectives of the study were: 1) Comparing the yield and rate of biogas production of cassava peels inoculated with cattle manure using a batch digester under anaerobic digestion conditions addressed in chapter four and five of the thesis; 2) Investigate the biogas yield and rate of different co-digestion ratios of cassava with vegetable and fruit waste using batch digestion under anaerobic digestion conditions presented in chapter six; 3) Optimize the production of biogas through process optimization by maintaining the optimum temperature during fermentation and compare inexperiments subjected to different treatment or treatment combinations and, 4) While chapter seven addresses the objective of using the experimental results to design an upscale system using baseline data information from experiment. Several feedstocks (i.e. cassava tuber, cassava peels, vegetable and fruit waste and cattle manure) were identified and analysed using the American Standard Methods for examination of Water and Wastewater (ASTM). Cassava was selected as it has several advantages compared to other crops, including the ability to grow on degraded land and where soil fertility is low. It also has the highest yield of carbohydrate per hectare (4.742 kg/carb) apart from sugarcane and sugar beet, which makes it suitable for bioenergy (biogas) generation. In the first instance, a batch experiment of were cassava peels were digested anaerobically with and without cattle manure to determine whether cassava peels (CP) in combination with cattle manure (CM) at different ratios shows better biogas yield. The following ratio combinations of mixture were used 100:0, 0:100, 80:20 and 20:80 (CM:CP). A theoretical methane production was conducted using elemental composition and the results were compared with the experimental ones. The test of biogas yield was conducted using an anaerobic digester of 600 ml at mesophilic (35 ± 1 °C) temperature. In the second experiment a 50 litres anaerobic digester was used to investigate the biogas yield of peeled cassava tuber compared to unpeeled cassava tuber that yield biogas of 635.23 L/kg VS and 460.41 L/kg VS respectively. This was based on the finding of the first experiments of biogas yield from cassava peels. The biogas yield with and without inoculum was measured and the biogas yield were modelled using two different models namely modified Gompertz and cone model. Finally, in parallel with the previous batch experiments another set of batch experiments were carried out under anaerobic conditions at mesophilic (35 ± 1 °C) temperature in a 600 ml digester, this experiments was conducted by co-digesting cassava (CB) with vegetable and fruit waste (CB:VF) at different ratios (100:0, 60:40, 40:60 and 50:50). The cumulative biogas yield were modelled for kinetics using modified Gompertz model. Based on the results obtained from the experimental study cassava co-digested with vegetable and fruits at a ratio of 40:60 which was found to produce the maximum yield, a mathematical design (upscale system) was designed. This designed biogas plant could be located in several communities especially those close to the landfills to reduce the cost of transportation from source.

The study’s results revealed that: • co-digestion influenced biogas production and methane yield. The final cumulative methane yields by the co-digestion of CM and CP at the CM:CP mixing ratios of 80:20 and 20:80 were 738.76 mL and 838.70 mL respectively. The corresponding average daily methane yields were 18.42 mL/day and 20.97 mL/day. This indicates that CP enhanced the production of methane in the co-digestion process with the 20:80 CM:CP ratio. • the feedstock of peeled cassava with inoculum, produced 28.75% more biogas yield when compared to peeled cassava without inoculum. This results highlights the important of inoculum in the anaerobic digester. • peeling the cassava tuber increase the biogas yield by 38% compared to the unpeeled tuber • cassava biomass co-digested with vegetable and fruit waste increased the methane yield compared to the mono-digestion with the highest methane production was achieved from the co-digestion of cassava biomass with vegetable & fruit waste at 40:60 ratio (CB: VF) Although several challenges hampering the smooth implementation of biogas generation in South Africa, this study concludes that cassava (peeled and unpeeled) co-digested with fruit and vegetables waste has potential to generate biogas thereby presenting a substantial opportunity to promote bioenergy production from cassava considering in many rural areas the needs for fuel and electricity are not satisfied fully. Finally, cassava anaerobic digestion facility at different scales could enhance additional benefits like the integration of nutrients and residual carbon into the land as fertilizer.