The efficacy of Volatile Fatty Acid (VFA) production in Thermophilic Aerobic Digestion (TAD) of primary sludge was investigated. This research program was carried out in a pilot scale, TAD process, located in the wastewater treatment pilot plant site, at the University of British Columbia. Preliminary results showed that the highest accumulation of VFA (950 mg/L as acetate) had occurred, under microaerobic conditions (air flow rate of between 0-0.17 V/V-h), in the first stage of the 150 L, 2-stage process. The two other aeration conditions examined (transition-air flow rate of 0.28 V/V-h and aerobic-air flow rate of 0.6 V/V-h) accumulated negligible amounts of VFA. Therefore, the subsequent research concentrated on the first stage of the TAD process, under microaerobic conditions. The two independent variables examined were air flow rates and solids retention times (SRT). The three SRTs tested were 3, 4.5 and 6 days. The four air flow rates examined were assigned the labels true anaerobic, low flow microaerobic, medium flow microaerobic and high flow microaerobic conditions. Net VFA production was found to be a function of both aeration and SRT. In general, as SRT and air flow rates decreased, net VFA production increased (specifically acetate and propionate). The measured concentration of any species of VFA, at any given time, was a function of both the relative rates of its synthesis and biodegradation. Decreasing or increasing the aeration rate and/or SRT resulted in a proportional change in VFA accumulation. The maximum measured acetate accumulation rate occurred under the 4.5 d SRT and the true anaerobic condition.

A biochemical model was developed in order to explain the process of VFA metabolism in TAD. In this process, under strict anaerobic conditions, bacteria must achieve oxidation/reduction balance by diverting the catabolic flow of carbon to fermentative end products (eg. propionate) that will consume NADH (Nicotinamide Adenine Dinucleotide). The key issue in fermentation is the recycling of NADH by the conversion of specific intermediates to different fermentation products which regenerate NAD⁺. The oxidation of intermediates that required the net reduction of NAD⁺ cannot proceed under fermentative conditions. Consequently, these catabolic intermediates added under batch test conditions, using TAD sludge, under anaerobic conditions, remained in their unoxidized form and persisted in the medium. The oxidation of intermediates which required no net reduction of NAD⁺ can and did proceed under fermentative conditions. Under strict anaerobic conditions, the VFA profiles in the pilot scale TAD process were similar to fermentation type processes (eg. an even distribution of VFA between acetate and propionate).

When the bioreactors were operated under microaerobic conditions (ie. oxygen demand is greater than oxygen supply), metabolism resulted in a characteristic VFA distribution profile with acetate as the predominant VFA produced (up to 80% of the total VFA). Propionate constituted the second largest fraction at 11%. Under this microaerobic condition, the NADH produced during oxidation of substrates could be reoxidized by operation of the respiratory chain. Therefore, the carbon flow could be uncoupled from the necessity to maintain redox balance via fermentative means. This separation would presumably allow the organisms in a TAD process to maximize ATP (adenosine triphosphate) production by increasing the flux of intermediates to acetate. The majority of the substrates examined under batch test conditions, with TAD process biomass, under microaerobic conditions, were oxidized to an acetate intermediate.