Description
Executive summary
Re-engineering the anaerobic co-digestion (co-AD) of organic-rich, municipal solid waste (MSW) to yield high-value chemical products could improve the economics of organic waste management. Conventional co-AD, in which two distinct waste streams are digested together (e.g., food waste and wastewater solids), generates methane gas (CH4). However, the lower market value of renewable CH4 ($560/ton) translates into a long return on initial capital investment and limits revenue generation that could help offset maintenance and operating costs. Shutting down the biological pathways to CH4 production in co-AD (via inhibition of methanogenesis) can increase the production of volatile fatty acids (VFAs) without compromising solids destruction goals. Many of these VFAs have current market values in the range of $650 (acetic acid) to $3000 (caproic acid) per ton and are therefore significantly more valuable than CH4. The primary challenges facing implementation of this approach are the lack of effective methods to increase VFA production during co-AD operation and separate and recover VFAs from co-AD effluents.
The overall goal of the proposed research was to demonstrate the technical feasibility of enhancing VFA generation in co-AD and recovering VFAs from co-AD using electrochemical methods. To accomplish this overall goal, the following specific objectives were completed: (1) determine how co-AD operational strategies can be leveraged to improve VFA production, (2) measure VFA recovery efficiencies and rates using an electrochemical-based separation process, and (3) test a scalable electrochemical cell (e-cell) design to validate VFA recovery from co-AD effluent. To accomplish these objectives, we completed three tasks: (1) identify and quantify the VFAs generated in co-AD systems as a function of strategies to minimize CH4 production and improve VFA production, (2) determine which anion exchange membrane (AEM) properties enable efficient and rapid transport of individual VFAs and their mixtures in response to an electrical driving force, and (3) validate a proof-of-concept electrochemical cell that separates and recovers VFAs from real co-AD effluent.
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