Hydrogen production in two-phase food waste anaerobic digestion with acidogenic gas diversion: mechanism and control

Food wastes (FW) constitute about 34% of municipal solid waste in Hong Kong. Instead of landfill disposal, anaerobic digestion (AD) is a more ecologically accepted technology for converting FW into bioenergy. Theoretically, a higher AD efficiency can be achieved in two- phase AD system over single-phase, but merely 10% of commercially operated AD plants are two-phase AD due to higher investment cost and only slightly better methane generation. Our research group has developed an innovative approach of diverting acidogenic off-gas into methanogenic phase to stimulate hydrogenotrophic methanogenesis, thereby enhancing methane production by 31%. We also identified that H2-producing metabolism in the first phase is critical for improving the overall conversion efficiency of FW to methane in this scheme. However, the practical H2 yield is usually lower than the theoretical yield of 4 mol H2/mol glucose. Thus, improvement in H2 production during FW acidogenesis is the key issue to be overcome.

In general, acidogenic process can be divided into H2-producing pathway and non-H2- producing pathway based on the spectrum of fermentative intermediates. However, an increase in the production of reducing equivalent with H2 loss is the main factor for limited H2 formation during acidogenesis. In theory, promoting pyruvate decarboxylation to acetyl-CoA formation and accelerating the NADH pathway are the key strategies for enhancing H2 production. However, the accumulation of lactate and/or NADH may reduce the acetyl-CoA formation and increase the reduced form of byproducts such as lactate, ethanol and propionate, resulting in the diversion of electron equivalents from H2. In order to achieve an effective H2 production during FW acidogenesis, it is critical to understand the biochemical mechanism of FW acidogenesis that facilitates the conversion of NADH to NAD+ and the formation of butyrate.

Based on the above rationale, the project aims to gain a mechanistic understanding of H2 production in acidogenic phase through correlation of operating controls including OLR, pH and ORP, corresponding metabolic intermediates, microbial structure, and genetic H2 function. The linkage between H2-producing behavior and corresponding mechanisms could provide an insight to alleviate negative impact and manipulate H2 production during FW acidogenesis. Bio-augmented acidogenic system is also proposed to accelerate the conversion of acidogenic intermediates to consolidate electron transfer for H2 production. Overall, this study will enrich the principles of anaerobic acidogenesis of organic matter and develop an efficient H2- producing system with a better bioconversion of CO2/H2 to methane via hydrogenotrophic pathway.