Assessment and feasibility of converting municipal solid waste to biogas: a South African case study.

Abstract

The present energy crisis in South Africa warrants the need for an alternative and sustainable energy supply. As a sustainable clean energy carrier, biogas has been demonstrated to be a promising renewable energy source for the generation of heat and electricity. The organic fraction of Municipal Solid Waste (MSW) has been reported as a promising feedstock for biogas production and characterisation of MSW is the basis towards successful waste to energy programs. The use of inappropriate equipment and technology choices based on insufficient data on waste volumes and composition have resulted in the failure of many interventions previously introduced in South African municipalities. Assessing the composition and quantity of available biomass for anaerobic digestion (AD), suitable pre-treatment technologies to enhance biogas production as well as optimization of AD parameters such as pH, temperature and substrate ratio were the core components of this research project. The digestate was evaluated and potential use as fertilizer and feedstock for pyrolysis assessed. A mesophilic bench-scale AD of Cow Dung (CD) and Fruit and Vegetable waste (F&V) obtained from a F&V market was conducted. F&V forms the greatest waste in the country, and this facility generates on average 2560 tonnes of waste per annum. This study concludes that utilisation of MSW for AD relies heavily on characterisation data, which is only possible through separation at source. The study recommends the development of a municipal organic waste facility and equally important, diversion of high-end food chains from entering the landfills. Other technologies such as Mechanical Biological Treatment (MBT) can be revisited for possible waste to energy programs at landfill leading to landfill space savings and reduced pollution. Waste pickers at landfills can be employed for this purpose as separation specialists. Bench-scale AD revealed that the benefits of substrate pre-treatment outweighed the effects of co-digestion ratio and pH. Reducing particle size from 1-2mm to <1mm, doubled the methane gas generation in a much shorter time and removed pH induced microbial inhibition in unbuffered reactors. Optimal pH was observed at 7.5-8.5. A co-digestion ratio of 80:20 (CD:F&V) produced higher methane yield for all pH variations in comparison to 60:20. The Digestate measured an average volatile solids loss of 46.4% with a C:N ratio of 12 and a heating value (HV) of 4.30 MJ/kg. Metal analysis of digestate showed presence of Nb, Sr, Si, N, P and K as constituents returned to the soil. The investigation of the digestate as a potential feedstock for pyrolysis yielded a carbon rich biochar with an HV value of 11.5 MJ/kg at 500 oC and a bio-oil rich in phenols, ketones and carboxylic acids which are important industry products.