A biochemical model describing volatile fatty acid metabolism in thermophilic aerobic digestion of wastewater sludge

Abstract

A biochemical model was developed to explain the mechanism of volatile fatty acid (VFA) metabolism in Thermophilic Aerobic Digestion (TAD). The effects of substrate addition on the metabolic behavioral patterns of TAD biomass, from a pilot scale system, were examined. The majority of the substrates examined under batch test conditions, with TAD process biomass, under microaerobic conditions, were oxidized to an acetate intermediate. The model predicts that, under microaerobic conditions (i.e. oxygen demand of the biomass is greater than the oxygen supply of the aeration equipment), the NADH (nicotinamide adenine dinucleotide) produced during oxidation of substrates can be reoxidized by operation of the respiratory chain. Therefore, the carbon flow can be uncoupled from the necessity to maintain redox balance via fermentative means. This uncoupling would allow the organisms in a TAD process, under microaerobic conditions, to simplify metabolism and maximize ATP (adenosine triphosphate) production by increasing the flux of substrates to acetate. The oxidation of compounds, which required the net reduction of NAD +, under anaerobic batch test conditions, could not proceed and remained in the unoxidized form. The oxidation of substrates, which required no net reduction of NAD +, could and did proceed under fermentative conditions. The model predicts that, under strict anaerobic conditions (i.e. fermentative), bacteria must achieve oxidation/reduction balance by diverting the catabolic flow of carbon to fermentative end products to consume NADH (e.g. propionate). The key factor in fermentation is the conversion of specific intermediates to fermentation products by pathways which recycle NADH to regenerate NAD +. The oxidation of substrates that require the net reduction of NAD + cannot proceed under fermentative conditions.