Anaerobic co-digestion of food waste with wastewater biosolids is a sustainable option for managing increasing global quantities of food waste. During anaerobic biodegradation, different microbial groups act together: hydrolyzers convert polymeric food waste molecules to smaller molecules, fermentative bacteria transform the smaller molecules into volatile fatty acids (VFAs), and methanogens use VFAs and H2/CO2 to produce CH4. Conventional co-digesters are operated for maximum CH4 generation; however, VFAs have higher commercial value (e.g., $1900/ton for butyric acid compared to $160/ton for methane) and are widely used to produce pharmaceuticals, polymers, esters, fuels, and solvents. Accumulating VFAs requires shutting down microbial pathways for CH4 production. Common strategies to do this include: 1) changes in operational conditions (e.g., pH, overloading, temperature); 2) pretreatment of inoculum; 3) additions of chemicals (e.g., BES). However, when applied to continuous anaerobic digesters, these methods are complicated by issues such as microbial adaptation, higher cost, and chemical contamination. We explored methods to reduce and hinder methanogenic populations in anaerobic digesters for treating food waste mixtures composed of proteins and carbohydrates. We first examined the impact of inoculum (i.e., source of microorganisms). Three inocula were tested: 1) anaerobic sludge (AS); 2) thickened waste activated sludge (TWAS); and 3) AS+TWAS, along with a control set with no inoculum added. Second, we examined the effects of intermittent exposure to high temperatures (50, 65, or 80 °C) compared to a control at conditions optimal for mesophilic methanogens (37 °C). Our results show that the inoculum can have an impact on VFA and CH4 generation. The TWAS inoculum resulted in the highest VFA concentration (6.62 g/L) on day 11, while using AS led to the lowest VFA concentrations (4.26 g/L). Once CH4 production began in the reactors, we found that intermittent and sudden temperature changes could significantly inhibit methanogens and promote soluble chemical oxygen demand (sCOD) accumulation (a proxy for VFA production); however, the effect was temperature dependent. We are currently analyzing the microbial communities in these reactors to better understand the impacts on methanogenic populations. The findings from this research will help guide the selection of strategies to promote VFA production during the anaerobic co-digestion of food waste.

Exploring strategies to promote volatile fatty acid accumulation during anaerobic co-Digestion of food waste

ACS Spring Meeting 2021

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