ENHANCED NUTRIENT REMOVAL BY OXIDATION-REDUCTION POTENTIAL (ORP) CONTROLLED AERATION IN A LABORATORY-SCALE EXTENDED AERATION TREATMENT SYSTEM

Two laboratory scale extended aeration treatment systems (EATS) were built to study their nutrient removal performance before and after the introduction of oxidation-reduction potential (ORP) controlled aeration. Effects of various ORP set points (40, 70, 110 and 180 mV) and zoning of the aeration tank (AT) were examined. Each system had a 901 aeration tank and the operating HRT and SRT were 20 h and 20 days, respectively. Synthetic wastewater was used and the F/M ratios ranged from 0.13 to 0.15gCOD/gMLSS/d. All experimental runs including the control yielded a similar COD removal efficiency of 96.4 +/- 0.4% and an effluent COD of 14.4 +/- 1.3 mg. This indicates that ORP controlled aeration had no significant impact on the carbonaceous removal of the EATS. Very good nitrogen (N) and phosphorus (P) removal was obtained when there was a small anoxic selector (3.81) at the head of the AT and the tank zoned into a main ORP zone (721) followed by an aerobic zone (18.1). The selector was found very useful in the prevention and curing of bulking as well as in the removal of P. It effectively denitrified the nitrate from the return sludge stream and caused significant P release, that is important to subsequent P uptake in the ORP and aerobic zones. An optimum ORP set point of 110 mV was found. The aeration pattern was characterized by having an equal time length of aeration and non-aeration. The achievable total nitrogen (TN) removal was 93% with a residual TN of only 2.94 mg/l (N). Excellent P removal of 99.5 +/- 0.1% was attained under an ORP set point range of 70-180 mV. P was reduced to 0.05 +/- 0.01 mg/l (P) in the effluent.