Economical modifications help attain better biological phosphorus removal

(Originally published in the June 2021 issue of Water Environment & Technology magazine. All rghts reserved.)

By Jon van Dommelen and Rob Smith

Many small water resource recovery facilities (WRRFs) are turning to biological nutrient removal (BNR) activated sludge upgrades to contribute cleaner water to their receiving lakes and streams and to meet tighter National Pollutant Discharge Elimination System (NPDES) limits on their effluents. Operators care about the detrimental effects of excess nitrogen and phosphorus loads and take pride in the idea of being a steward of the natural environment. However, the new systems often fall short of expectations and may even be in noncompliance with their NPDES permit.

A common problem for achieving biological phosphorus removal at small facilities is weak influent. Conventional BNR requires an ample source of readily degradable organic material in the wastewater to drive the biochemical reactions that convert nitrate to nitrogen gas and dissolved phosphorus to particulates. Unfortunately, many wastewaters do not have sufficient carbon for nutrient removal. Effective process control is one of the keys to overcoming this problem to make the proper adjustments to the wastewater treatment tanks to avoid noncompliance.

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BNR requires zone integrity by maintaining three separate environments: anaerobic, anoxic, and oxic. Without proper process control, the tank label may not match the environment. For example, an "anaerobic tank" may actually have an anoxic environment. The environments are defined in terms of dissolved oxygen (DO) and nitrate. However, orthophosphate and oxidation reduction potential are other parameters that provide insight into the environment. From a treatment perspective, the chemical environment is more relevant than how the tank is labelled.

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