Implementation of Large-Scale Anaerobic Digestion of Food Waste at the University of South Florida

Food waste represents a major sustainability issue in the United States. Food waste represents 18% of landfill space and is the largest contributor of landfill methane emissions. US EPA’s Food Recovery Hierarchy recommends alternatives to landfilling food waste, including source reduction, food donation and anaerobic digestion. The Student Green Energy Fund (SGEF) Food Recovery Project aims to follow suggestions outlined by the US EPA Food Recovery Hierarchy to encourage the University of South Florida (USF) to become a zero-waste campus. A multi-disciplinary approach aims to source reduce wasted food, look for opportunities for source reduction and anaerobically digest any remaining food waste from there. Currently, pilot-scale digesters are being operated to help demonstrate the feasibility of large-scale anaerobic digestion. This thesis explores different approaches for the anaerobic digestion of food waste to determine alternatives to the pilot-scale approach to help implement large-scale technology. A community partnership with a tea wholesaler was identified, providing opportunities for food waste co-digestion with both tea leaves and compostable sugarcane-based polylactic acid (PLA) plates. The objectives of this research were to: 1. Analyze the co-digestion of food waste with compostable plates and tea leaves, 2. Conduct an initial lifecycle assessment that compares incineration of food waste and anaerobic digestion of food waste for the entirety of USF’s food waste, as well as a larger-scale digester that can process USF and surrounding hospital waste. The results of the first phase of co-digestion showed that food waste digestion on its own will result in souring of the reactor and inhibition of methanogenesis. When food waste is co-digested with either tea leaves or compostable plates, the reactor remains healthy and produces methane, however both digesters saw lag periods of 21 and 30 days with tea leaves and with plates, respectively. The methane yield for tea leaves was 372 ml CH4 / g VS and for compostable plates was 445 ml CH4 / g VS. The digestion period was 92 days, at which point the tea leaves methane production stabilized to inoculum control levels, however the compostable plates were still producing more methane and did not reach their full methane potential. The second phase of tests investigated whether the methane yield of food waste on its own could be improved. This was done through the introduction of an alkalinity source using a combination of sodium bicarbonate and oyster shells, or through a separate digester with an F:M ratio of 0.5, as opposed to an F:M ratio of 1 applied in all other digestion sets. The third phase of tests investigated whether the lag period observed in Phase 1 for tea leaves and compostable plates could be reduced by introducing acclimated inoculum from Phase 1 into the digestion set. Phase 3 also mixed all three substrates together to see the effects of co-digestion of all three substrates. At the time of submission, Phases 2 and 3 were ongoing so only preliminary results were provided. An initial lifecycle assessment was conducted using the US EPA Waste Reduction Model (WARM). This model compares alternative disposal methods for solid waste disposal. The study compared USF’s current process of sending food waste to incineration at McKay Bay Waste to Energy Incineration facility to sending food waste to an on-campus anaerobic digester. Sending food to an anaerobic digester decreased GHG emissions by 8 mton of CO2 equivalents each year, and when scaled up to process both USF food waste and surrounding hospital waste, could decrease GHG emissions by 16 mton of CO2 equivalents each year. This decrease is negligible between both options. Therefore, it is recommended that a more comprehensive lifecycle assessment be carried out that can compare the full lifecycle impact of both plants and determine with better accuracy the impact that each alternative would have on the environment