A Step Feed Biological Nutrient Removal (BNR) system was operated under full design load conditions. The process proved capable of meeting ammonia toxicity limits and reducing total nitrogen to an effluent concentration of 5 mg/L. Through some simple modifications, the system also proved
capable of biological phosphorus removal. The most effective modification was to limit the nitrate returned to the first pass through the return activated sludge. In this case, limiting the effluent nitrate concentration to about 6 mg/L yielded good phosphorus removal. Operating changes to
the feed distribution and number of feed points made no discernable difference. Likewise, secondary anoxic zones made no discernable difference in nutrient removal. Prefermentation, however, improved both denitrification and phosphorus removal. Profiles through the step feed tanks showed significant
phosphorus release in all of the unaerated zones, not just the first pass, where nitrate was limited.
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