Browsing by Author "Porteus, Jacqueline"

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    Life cycle assessment of anaerobic digestion of cattle manure
    (Stellenbosch : Stellenbosch University, 2019-04) Porteus, Jacqueline; Louw, Tobias M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: The pressure on energy resources worldwide combined with the awareness of the major impact industrial processes have on the environment, triggers the development of alternative energy sources and methods to reduce waste. Anaerobic digestion of waste addresses both these criteria by simultaneously supplying energy and reducing waste that would otherwise have to be stored or burned. This study focuses on the anaerobic digestion of cattle manure and the processes associated with the products downstream of the digester that can potentially replace current sources of energy and nutrients. A pilot anaerobic digester at Stellenbosch University (SU) is used as the base for the mass balance but the process data used is obtained from literature. Six different sets of processes (scenarios) were evaluated based on the possible uses of the biogas and digestate outflows from the digester. Ecoinvent’s database together with GreenDelta as Life Cycle Assessment software provider was used to determine the life cycle assessment (LCA) of each scenario. The CML impact assessment method was used as it concentrates on the LCA categories as per the scope of this study. LCA is the methodology for determining relative environmental impacts of a process from cradle to grave. The CML environmental categories are acidification potential, climate change, ozone depletion potential, photochemical oxidation, terrestrial ecotoxicity, human toxicity, depletion of abiotic resources, aquatic toxicity and eutrophication. The results of each scenario are compared to a base case consisting of the normal operation of a milk cow stall, combined with offset processes for the six scenarios. In scenario 1 and 2 the biogas is used to heat SU’s indoor swimming pool while the digestate is either applied to fields as nutrient source or cleaned via pasteurisation for domestic use. Scenario 3 uses the digestate as nutrient supply while biogas is scrubbed and bottled for cooking. Scenario 4 converts the digestate solids into fertilizer pellets while a portion of the biogas is used for generating electricity. Scenario 5 and 6 both involve the cleaning and bottling of biogas for cooking. In scenario 5 the digestate solids are mixed with limestone for fertilizer production. The liquid phase is used for irrigation. Scenario 6 uses the liquid digestate as nutrient source in a photo bioreactor cultivating algae. The bio-oil produced is converted into biodiesel. The solid digestate is applied to agricultural fields as nutrient source. After normalizing the LCA results of the scenarios against the results of the base case, it was found that the application of digestate without phase separation has a lower environmental impact than digestate converted into fertilizer. Biogas used for heating and power generation has lower impacts on the environmental categories than biogas scrubbed and bottled for cooking. The impacts from the base case are higher than the impacts of an anaerobic digester combined with processes utilizing biogas and digestate in their raw states. Processes from the different scenarios were mixed to create an optimum scenario with even lower impacts, but scenario 4’s impacts remained the lowest overall. Operating an anaerobic digester fed with cattle manure will improve the environmental impacts of a cattle stall significantly. The application of biogas and digestate on the farm adds financial benefits for the farmer while the whole operation is more environmentally friendly.