Determining the waste composition profile to optimise biomethane production : a case study for the South African ice cream industry
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The management of waste is a global concern, especially dealing with issues such as limited landfill airspace and the negative impacts of waste on the environment. To minimise these impacts, the National Waste Management Strategy (NWMS) proposes a waste management hierarchy, where the disposal of waste must be regarded as the last resort. Additionally, the White Paper on Renewable Energy (2003) and the National Climate Change Response White Paper (2011) also highlight the fact that the waste sector, which includes biogas to energy projects, has the potential to mitigate greenhouse gases. Within this context, the research aimed to determine the optimal composition profile of waste from the ice cream industry to produce the most (quantity of) biomethane. A mixed-methods approach was used, where a literature review was combined with laboratory analysis of different waste composition variations. Waste composition variations consisted of ice cream waste only; ice cream waste mixed with ice cream sludge (waste variation 1); ice cream waste mixed with woodchips and ice cream sludge (waste variation 2); and ice cream mixed with wood chips and raw material (waste variation 3). The physico-chemical characteristics and the biomethane production output of the waste variations were determined. Based on a 28-day analysis, it was found that ice cream waste (only) had the potential to produce biomethane (1894.2±101.1 Nml CH4/kg VS), similar to results reported in literature for dairy-related wastes. The addition of ice cream sludge, however, slightly improved the biomethane production potential (1965±7.1 Nml CH4/kg VS). The waste composition variations where wood chips were added produced lower amounts of biomethane (1085.4±43.1 Nml CH4/kg VS and 1024.9±61.6 Nml CH4/kg VS, respectively). Considering the cumulative biomethane output over a 28-day period, ice cream only waste and waste variation 1 produced similar trends, with biomethane production stabilising on days 18 and 19, respectively. Waste variation 2 showed a rapid increase in biomethane production between days 4 and 10, with biomethane production stabilising on day 13, whereas waste variation 3 already stabilised on day 10. To explain the differences in biomethane production, the correlation coefficient between biomethane output and the physico-chemical characteristics, such as volatile solids (VS), total solids (TS), pH and moisture content, of the waste variations was determined. The only significant positive correlation was found between biomethane production and moisture content, where the waste variations with a higher moisture content produced a higher biomethane output. In conclusion, the results of the study indicated that ice cream waste does have the potential to produce biomethane and biomethane outputs were similar to other dairy waste-related studies. The results suggest that the addition of ice cream sludge, which also contains waste water from the ice cream manufacturing process, has the potential to improve biomethane production.