Pilot-scale operation of thermophilic aerobic digestion for volatile fatty acid production and distribution

Thermophilic aerobic sludge digestion is operated at elevated temperatures, to achieve sludge stabilization, volatile solids (VS) destruction, and pasteurization. The thermophilic aerobic digestion (TAD) of sludge, under a "microaerated" condition, whereby the system oxygen demand exceeds the supply, has been found to accumulate high concentrations of volatile fatty acids (VFAs), namely acetate and propionate. This VFA-rich TAD supernatant is a potential carbon alternative for phosphorus and nitrogen removal enhancement in biological nutrient removal (BNR) processes. This pilot-scale continuous TAD operation was designed to produce VFA-rich supernatant, without sacrificing sludge solids destruction efficiency. A molecular weight cut-off (MWCO) method was used to analyze the carbonaceous and nutrient distributions in the TAD supernatant. Results suggested that real-time aeration control is necessary to control the micro-aeration operation and optimize the VFA production. The oxidation-reduction potential (ORP) did not correlate well to the actual VFA concentration; however, the ORP monitoring did reflect the status of the micro-aeration condition and the existence of VFAs in the system. The average VFA concentration was found to be in proportion to the sludge concentration in the feed. Also, a certain portion of VFAs was found to associate with particulates and colloids in the TAD supernatant. Key words: digested sludge supernatant, molecular weight cut-off, microaerated, thermophilic aerobic digestion, volatile fatty acids, volatile solids destruction, ultrafiltration.