A newly constructed biological nutrient removal treatment plant operating in the Phoredox mode was used to test the effects on plant efficiency of varying two important operating conditions - the point of introduction of return sludge flow, and the addition of primary sludge thickener supernatant to the bioreactor. Overall treatment results for Kelowna's weak sewage showed that the effluent was consistently below 10 mg/L BOD and 4 mg/L suspended solids, while total P was below 2 mg/L 94 percent of the time, and total N was below 6 mg/L 90 percent of the time. There was no significant difference in effluent nutrient quality between tests with all return sludge entering the anaerobic zone or with half entering the anaerobic zone and half entering the first anoxic zone. Primary sludge thickener supernatant added to the anaerobic zone typically resulted in an effluent P concentration of less than 0.1 mg ortho-P/L, while the absence of supernatant produced an effluent containing greater than 2 mg ortho-P/L.

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