Investigating the upscaling of an anaerobic digester for biogas production from industrial wastewater

Abstract

South Africa faces challenges of water scarcity and carbon-based energy pollution, and is currently under pressure to explore and capitalize on alternative methods to produce green energy. In addressing these challenges, the application and optimization of anaerobic digestion (AD) technology presents a robust solution for the treatment of wastewater and bioenergy production if operated efficiently. Under the South African National Development Roadmap and the United Nations Sustainability Development Goals on clean water and sanitation (#6), climate action (#13), and clean and affordable energy (#7), this research addressed the water scarcity, energy, and environmental concerns in South Africa. The study was aimed at up-scaling and optimizing an anaerobic digester for biogas production from industrial wastewater. Sequentially, characterisation of the wastewater streams, evaluating and optimising the operational factors of the AD process as well as cost-benefit analysis were among the specific objectives carried out to achieve the overall aim of the study. The characterization results show that the sugar refinery and industrial sewage have the highest organic content with COD of 18770 mg/L and 4320 mg/L respectively. The sugar refinery stream had the highest concentration of volatile solids at 0.026 g/mL, as well as a high concentration of phosphates and nitrates, followed by industrial sewage, and the least concentration of volatile solids, phosphates and nitrates was observed from the oil refinery stream. The results of biochemical methane potential tests show the highest biogas production from the sugar refinery stream (148 mL/gTDS), followed by the industrial sewage (76 mL/gTDS), and the least production from the oil refinery (64 mL/gTDS) and municipal wastewater (45 mL/gTDS). Although the sugar refinery stream produces the highest biogas, it also shows difficulty in the removal of organic content with COD removal of 62.8%, indicating that a major fraction of organic content is not degraded and converted into biogas. Using the OFAT (one-factor-at-time) approach, the hydraulic retention time (HRT) and magnetite load were among the key operational factors evaluated. It was observed that, at the initial stage there was a poor response in biogas production and COD removal, followed by an exponential increase in biogas production between 9-18 days, a decline in biogas production between 19-22 days, and retardants or no production between 22-30 days. This suggested that there was a long lag phase due to poor microbial activity and an optimum HRT can be obtained between 18 – 22 days. The addition of magnetite reduced the lag phase significantly from 9 days to 3 days, indicating that the addition of magnetite improves the interspecies electron transfer and ultimately enhances biogas production. The addition of 0.4 – 0.6 g/L magnetite achieved high biogas production rates of 23 mL/d and 20 mL/d respectively between 9-12 days while the addition of 0.8 g/L resulted in catalyst overloading which inhibited the microbial activity and a rapid decline in biogas production to 2 mL/d after 9 days was observed. An increase in organic content from 4320 mg/L to 18770 mg/L resulted in a 30% increase in biogas production, however, a decline of over 40% in the removal of contaminants was noted due to a decrease in pH leading to the accumulation of long-chain fatty acids which are inhibitory to groups of microorganisms. At the upscale of 50 L AD, the operating conditions were optimised using the response surface methodology (RSM). The statistical analysis revealed that the quadratic models for the three responses studied (biogas yield, COD removal and colour removal) were significant with p-values below 0.05. The R2 values for the three responses were 0.99 with differences between predicted and adjusted R2 below 0.2, suggesting reasonable agreement with observational data points. The optimum operating conditions obtained were an HRT of 21 days, pH of 7.01 and magnetite load of 0.42 g/L with a desirability of 0.99. The optimum conditions were validated against two streams, industrial sewage (low organic content) and sugar refinery (high organic content) and achieved over 85% and over 60% in the removal of the contaminants, respectively. The addition of sensors to the up-scaled 50 L AD ensured accurate control and monitoring of operating conditions, thus enhancing methane content to 90%. Furthermore, a cost-benefit analysis of the optimized AD system with biogas production and its applicability as an alternative source of energy was conducted. The annual operating costs for the upscaled AD system exceeded the annual revenue, resulting in a net cash flow of -R8 506. The 50 L AD has a net present value of -R121 016, indicating that the system is not economically viable due to the high capital cost required for the design and commissioning of the system. Based on the cumulative cash flow, the 1 L, 5 L, 10 L, and 50 L systems will take 24.8, 21.9, 25.3, and 19.03 years to pay back the initial investment. Also, the benefit-cost ratio increases with an increase in AD size: 1 L (0.05), 5 L (0.12), 10 L (0.13), and 50 L (0.4), indicating that up-scaling has the potential to generate more revenue with a decrease in operating cost for the optimized and stabilized system. Therefore, upscaling of an anaerobic digester (AD) with the optimum conditions for treating industrial wastewater and biogas production was feasible. However, up-scaling beyond 50 L capacity is essential to increase biogas production, enhance revenue generation and improve the benefit-cost ratio to 1 and above. This would make the AD system economically viable for large-scale operations.