The influence of aeration and solids retention time on volatile fatty acid accumulation in thermophilic aerobic digestion of sludge

The efficacy of Volatile Fatty Acid (VFA) accumulation 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 published elsewhere showed that the highest accumulation of VFA (950 mg l(-1) as acetate) had occurred under microaerobic conditions (air flow rate of between 0-0.17 V V(-)1 h(-1)) in the first stage of the 150 1, 2-stage process [3]. The subsequent research reported in this study 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 now rates examined were described as true anaerobic, low flow microaerobic, medium flow microaerobic and high flow microaerobic conditions. Net VFA accumulation was found to be a function of both aeration and SRT. Decreasing or increasing the aeration rate and/or SRT resulted in a proportional change in VFA accumulation (specifically acetate). The maximum measured acetate accumulation rate occurred under the 4.5 d SRT/true anaerobic combination. Under strict anaerobic conditions, the VFA profiles in the pilot scale TAD process were similar to fermentation type processes which are characterized by an even distribution of VFA between acetate and propionate.