Browsing by Author "Sawyerr, Nathaniel Olugbenga."

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Anaerobic digestion of energy crop (cassava)
(2019) Sawyerr, Nathaniel Olugbenga.; Trois, Cristina.; Workneh, Tilahun Seyoum.; Okudoh, Vincent Ifeanyi.
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.
Denitrification of leachate using domestic waste at different levels of stability : simulations in batch test.
(2011) Sawyerr, Nathaniel Olugbenga.; Trois, Cristina.
Disposing of waste on land has been a method practiced by many countries because it is relatively inexpensive. This has led to the fast increase of landfilling option which is also due to increase of waste generation, resulting in the increase in the urgency of investigating cheap measures of treating wastewater (leachate) that is generated from landfills prior to its discharge to the environment. After the application of the process of nitrification using Sequencing Batch Reactor (SBR) such as is applied at Mariannhill landfill site, Durban, the treated leachate still contains high level of nitrate ranging from 500 – 2000 mg/ℓ, which greatly exceeds the discharge limit of 12 mg/ℓ. Ex-situ bio-denitrification has been used widely around the world in various technological applications (SBRs, anaerobic trickling filters, etc.) that generally employ expensive chemicals. Hence the need to investigate the removal of nitrates using in-situ biodenitrification processes using readily available carbon sources such as fresh commercial garden refuse (CGRraw) and composted commercial garden refuse (CGR10). Both carbon sources were mixed with waste that had been treated for 8 weeks (Cell 1) and 16 weeks (Cell 2). The aim of this study is to determine the viability of pre-treated general waste at different degrees of stability (carbon contents) as carbon sources for in-situ bio-denitrification in landfills. The focus was mainly on determining the suitability, the kinetics and the performance of the different substrate. The suitability of the substrates to perform denitrification was assessed based on the carbon content and carbon to nitrogen ratio in the substrate. On establishing suitability, the kinetic rate of denitrification was assessed for each substrate. The kinetics analysis was based on the time taken for full denitrification to occur and the concentration of the byproducts of the denitrification process such as Ammonia. Characterization tests were performed to determine the suitability of the substrates to be used as carbon sources for denitrification. In situ denitrification processes were simulated at smaller scale in the laboratory using anaerobic batch reactors, with biologically treated leachate and seeded Treated leachate from the Sequencing Batch Reactor. Batch tests were conducted at a nitrate concentration level of 500 mg/ℓ. The combination of 8 weeks treated waste with Fresh Commercial Garden Refuse (Cell 1 + CGRraw) and with Commercial Garden Refuse (Cell 1 + CGR10), respectively, provided the most suitable substrates for denitrification as they contained the highest carbon content as well as relatively high carbon to nitrogen ratio (C:N) . Although the 16 weeks treated waste together mixed with Commercial Garden Refuse (Cell 2 + CGR10) had the lowest C:N ratio, this could be due to a lack of homogeneity within the sample. The results of the batch tests confirms that 8 weeks treated waste (Cell 1) and 16 weeks treated waste (Cell 2) substrates were both too stable and contained too little carbon to attain full denitrification. In addition to the inability to attain full denitrification, Cell 2 leached out nitrate of approximately 500 mg/ℓ NO3-N back into the batch. The batch test results showed that the cells substrates augmented with CGRraw and CGR10 achieved positive results as full denitrification was achieved within a maximum of 7 days for Cell 1 and 14 days for Cell 2.