SIMULTANEOUS REMOVAL OF NITROGEN AND PHOSPHORUS IN INTERMITTENTLY AERATED 2-TANK ACTIVATED-SLUDGE PROCESS USING DO AND ORP-BENDING-POINT CONTROL

The 2-tank intermittent aeration method is an anaerobic-aerobic activated sludge process of time-sharing type in which 2 complete mixing reaction tanks are connected in series, and aeration and agitation are periodically repeated in each tank. We have developed a new control system for the process which can secure anaerobic, anoxic and aerobic conditions through a combination of DO and ORP-bending-point (corresponding to termination of denitrification) emergence time control. In the 1st tank, nitrification and phosphorus uptake occur in the aeration period, followed by denitrification and phosphorus release in the agitation. The 2nd tank performs nitrification and phosphorus uptake in the aeration and denitrification in the agitation. One cycle of aeration and agitation is approximately 2 hours. This control system was applied to the test plant (influent flow rate: 225 1/day) for two months under the conditions of HRT 16 hours and temperature 20 +/- 2-degrees-C. We achieved stable and high removal ratios: TOC 94.9 %, T-N 89.4 %, and T-P 95.5 %. We also investigated the mechanisms of nitrogen and phosphorus removal and their material balance.