The activated sludge process is the major treatment method for domestic and industrial wastewaters, in which excess sludge is produced daily. The excess sludge requires extensive treatment and disposal. In order to reduce excess sludge, an oxic-settling-anoxic (OSA) activated sludge process was developed. An anoxic tank is introduced to treat the settled sludge from the final clarifier. The treated sludge is then returned to the aeration tank where a high food loading is present....[ Read more ]

The activated sludge process is the major treatment method for domestic and industrial wastewaters, in which excess sludge is produced daily. The excess sludge requires extensive treatment and disposal. In order to reduce excess sludge, an oxic-settling-anoxic (OSA) activated sludge process was developed. An anoxic tank is introduced to treat the settled sludge from the final clarifier. The treated sludge is then returned to the aeration tank where a high food loading is present.

This study aims at examining the proposed "fasting/feasting" concept proposed by Chudoba et al. (1992) and Chen and Liu (1999) by investigating both sludge fasting and feasting phenomenon, and the effect of fasting and feasting treatments on sludge growth. Five activated sludge cultures, including an aerobic batch, an intermittently-aerated batch, an anoxic batch, an aerobic continuous and an OSA cultures were used.

The results showed that only the aerobic batch and the aerobic continuous cultures were fastable, in which both the biomass and polysaccharide storage were reduced during the 2-hr fasting treatment at an ORP level below 100 mV. It was found that activated sludge was only fasted when a no food and stressful environment was present. When the fasted cultures were treated in the feasting environment, the SOUR increased markedly and no significant accumulation of food storage was observed. Both the substrate utilization and biomass growth rates of the fasted cultures were accelerated and a "sludge feasting" was brought about after the fasting treatment. Due to the increase in energy requirement after the fasting treatment, an accumulation of excess energy was not observed in the feasting treatment, though a harvest of energy might have already occurred. It was found that about a 60% reduction of the ATP content was observed in the fasted cultures. The fasting/feasting treatment could not induce a significant reduction of Y_obs for both fastable and non-fastable activated sludge cultures. It can be concluded that sludge fasting and feasting phenomena does occur in fastable cultures but the fasting/feasting treatment cannot induce a reduction of sludge growth.

Keywords: Activated sludge culture; sludge fasting and feasting; ORP maintaining; observed growth yield; cell ATP content; polysaccharide storage; SOUR.