Full Scale Test on Chemical P Removal during a Step Feed BNR Study at John E. Egan Water Reclamation Plant

A full scale nutrient removal study was conducted at the 30-MGD John E. Egan Water Reclamation Plant with a single-stage nitrification activated sludge process and sand filtration. One quarter of the plant's secondary treatment capacity was used in the study. Simultaneous nitrogen removal through a step feed BNR process and phosphorus (P) removal by chemical precipitation with ferric chloride (FeCl3) were tested to investigate the potential impact of adding FeCl3 to mixed liquor for P removal on the step feed BNR process and on the operation of other unit processes in the plant. During the P removal tests, which were conducted once in early spring and again in early fall of 2005, solutions containing approximately 33% FeCl3 by weight were injected at two different target rates into mixed liquor at the effluent end of the aeration tank. The average daily FeCl3 concentrations applied with respect to 7.5 MGD inflow to the aeration tank were 32.5 to 44.6 mg/L. The average soluble phosphorus (Sol-P) concentrations in 24-hour composite samples of the secondary effluent during the P removal tests ranged from 0.05 to 0.82 mg/L with an arithmetic mean of 0.27 mg/L. The total nitrogen (TN) removal by the step feed BNR process tested averaged 52% and 54% before and during the chemical P removal test in the spring and 52% and 58% in the fall. This indicates that adding ferric chloride to mixed liquor up to 44.6 mg/L for removing particulate FePO4 in the secondary clarifiers has no adverse impact on the step feed BNR process for biological nitrogen removal. The higher percentage TN removal during the P removal test in the fall was probably attributed to the higher MLVSS in the aeration tank resulting from higher solids content in the returned sludge after ferric chloride addition. The accumulation of ferric particles in WAS during the P removal tests appears to have caused the clogging of cloth belt used in the gravity belt thickeners (GBT) for sludge thickening. The sludge thickening performance of the GBT deteriorated during the chemical P removal tests. The performance was improved after the addition of ferric chloride to mixed liquor stopped.