A bench-scale, continuous-flow, study was conducted to investigate the factors affecting phosphorus removal in a two-stage, intermittent aeration (IA) process. The two-stage process used consisted of an anaerobic zone and an intermittently aerated, completely mixed (IACM) tank, in which intermittent aeration was controlled by absolute ORP. The factors examined were the average ORP control level (used for aeration control), external acetate dosage, and intermittent aeration cycle. The degree of phosphorus removal in the two-stage IA process depended mainly on the presence of acetate in the anaerobic zone. The average ORP control level also affected the phosphorus removal at relatively low external acetate dosages. Small phosphorus release in the presence of nitrate and dissolved oxygen was observed during the air-off period. This suggests that a relatively long aeration cycle should be avoided in the two-stage IA process for preventing significant phosphorus release in the IACM tank.

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