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 activa ted sludge process of time-sharing type in which 2 complete mixing rea ction tanks are connected in series, and aeration and agitation are pe riodically repeated in each tank. We have developed a new control syst em for the process which can secure anaerobic, anoxic and aerobic cond itions through a combination of DO and ORP-bending-point (correspondin g to termination of denitrification) emergence time control. In the 1s t tank, nitrification and phosphorus uptake occur in the aeration peri od, followed by denitrification and phosphorus release in the agitatio n. The 2nd tank performs nitrification and phosphorus uptake in the ae ration and denitrification in the agitation. One cycle of aeration and agitation is approximately 2 hours. This control system was applied t o 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 phospho rus removal and their material balance.