Demonstration of Intermitttent On/Off Aeration at a Municipal Wastewater Treatment Plant for Denitrification and Energy Savings

Final Report

Intermittent On/Off Aeration

A pilot test of effluent water-level control technology

In the study of intermittent on/off aeration, the wastewater containing high concentration of phosphorus during air-off period was continuously discharged as effluent. This situation might cause the high concentration of phosphorus in the effluent. In order to avoid the discharge of high-phosphorus effluent during air-off period, a laboratory-scale pilot test was also conducted to investigate the influence of effluent water-level control during air-off period on effluent phosphorus concentration.

Figure 3.10 represents the scheme of reactors used in this effluent water-level control study. The working-volumes of the reactor are 14 L at maximum water level and 8 L at minimum water level.

Figure 3.11 shows the operational concept of this study in one cycle, which is 24 hours period. There were three stages, anaerobic, aerobic and settling/withdrawing stage, consist of one cycle. The influent flowrate (Q_inf) was set as constant, 333 mL/hr, in the 24-hours cycle. During the anaerobic period, first 8 hours in one cycle, no effluent and recycled activated sludge was discharged and water level was increased from the lowest level to the highest level. During the aerobic period, the 15 hours followed by the anaerobic period, treated wastewater along with activated sludge was discharged because of the continuous influent and recycled activated sludge pumped into the reactor. The water level was kept at highest level. During the settling/withdrawing stage, the activated sludge was settled and supernatant was pumped out to the lowest water level.

After one-week operation of this reactor, phosphorus and nitrate concentration was measured in order to obtain the profile in the 24-hours cycle. Figure 3.12 shows the ortho-phosphorus and nitrate profile in the reactor during one cycle. The phenomena of ortho-phosphorus release during anaerobic period and uptake during aerobic period was observed in the reactor. However, the quantity of released phosphorus (about 7 mg-P/L) is small compared to the observations from traditional BPR plants (about 30 mg-P/L).

Figure 3.12 The phosphorus and nitrate profile in the reactor during one cycle

The result showed the possibility to stop discharging phosphorus during air-off period. Since this operation lasted only for one week due to the difficulty of obtaining large volume of wastewater, the evidence of success of this new operation is not strong. It is suggested that a long-term study on this new operation should be conducted to support the success of BPR at Ashland WWTP.

More information on this topic: Gerry Novotny