Abstract
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.
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