New concept for an aerobic sludge digestion process without aeration using sponge carriers

We propose a novel sludge treatment process involving aerobic digestion without aeration. A sponge carrier is used to trap sludge, followed by biological degradation of the sludge by predation and endogenous respiration. In the experiment, sludge was fed into the sponge carrier and sludge degradation was measured over 21 days at ambient temperature (average of 20°C). The suspended solids, volatile suspended solids (VSS), and total chemical oxygen demand of the initial sludge on day 0 were 403, 286, and 378 mg, respectively. Oxygen was naturally supplied from atmosphere or through recirculation of water with high dissolved oxygen. Sludge degradation occurred rapidly within the first 7 days, and after 21 days, VSS was reduced by approximately 40%. Molecular analysis showed changes in the microbial community structure and the presence of aerobic organisms during the 21-day experiment. The nitrogen stable isotope ratio (<i>δ</i>¹⁵N) increased by more than 3‰ from 0 to 7 days and became stable between 7 and 21 days. The increase in <i>δ</i>¹⁵N could be attributed to an increase in the average trophic level of the microbial community, which implies that predation within the microbial community effectively causes sludge degradation during the first 7 days. In summary, sludge degradation was effectively performed by predation within the first 7 days, followed by other functions such as endogenous respiration after 7 days under aerobic conditions. This proposed aerobic digestion process may be effective for sludge degradation without aeration.