Optimizing phosphorus removal in an A/O plant.

The City of Ann Arbor Wastewater Treatment Plant has retrofitted an Anaerobic/Oxic (A/O) process into its existing aeration tanks. In 1992, the A/O process was placed into full scale operation. It was started up and operated successfully. However, on several occasions, phosphorus removal was not achieved to the desired level. No exact cause had been identified. Speculations on what might have caused the low removal efficiency concerned the plant staff on the reliability of the A/O system control strategies. The need for chemical addition was, and still is necessary most of the time to comply with the plant's National Pollution Discharge Elimination System permit phosphorus limit of 0.6 mg/l as a 12 months moving average. In addition to the primary purpose of removing phosphorus, the A/O system was selected as an environmentally beneficial process by reducing chemical usage, thereby reducing sludge production and thus cost. The objectives of this research were to identify the possible causes of the A/O system disturbances and to determine the optimal operational strategy necessary to achieve required phosphorus removal. Bench tests were used to determine the polishing dose of ferric chloride required for optimal phosphorus removal and to identify points for optimal addition. A full scale experiment was conducted on one of the A/O treatment passes in the east portion of the plant. The effects of upstream processes on A/O and the impact of the A/O on the downstream processes were investigated. The major findings of the study were, besides leveling the flows, the need to closely monitor the sludge blanket levels in the settling tanks, the mixed liquor suspended solids concentration in the aeration basins and the dissolved oxygen concentration in the anaerobic and oxic zones. Application of the study findings improved the A/O operation and minimized the occurrence of upsets. Reliability was achieved, and total phosphorus concentrations of 0.6 mg/l and lower were obtained. The average monthly total phosphorus concentration in the final effluent in 1995 was 0.5 mg/l (0.3-0.9 mg/l) compared to 0.8 mg/l (0.4-1.0 mg/l) in 1992, 0.6 mg/l (0.3-1.2 mg/l) in 1993 and 0.6 mg/l(0.4-1.0 mg/l) in 1994. The average ferric chloride concentrations used were 3.0 mg/l in 1995 compared to 13.0 mg/l in 1993 and 11.0 mg/l in 1994. Sludge production in 1995 was 524.2 tons (382.7-807.1) compared to 584.8 tons (404.5-746.8) in 1993 and 586.9 tons (423.8-909.5) in 1994. The decrease was due to the reduction in chemical usage therefore achieving cost savings both in chemicals and sludge disposal. In summary, the findings of the study have let to a much improved operation of the A/O process.PhDEnvironmental Health SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105147/1/9635605.pdfDescription of 9635605.pdf : Restricted to UM users only