While phosphorus and nitrogen are indispensable for the growth of aquatic flora, nutrient-rich wastewater results in eutrophication — an excessive growth of algae and plankton in bodies of water — which impairs freshwater and coastal marine ecosystems. To preserve these vital bodies of water and prevent eutrophication, wastewater treatment plants are mandated to include nutrient removal processes that target phosphorus and nitrogen.
The U.S. Environmental Protection Agency (EPA) has encouraged states to adopt numeric nutrient water quality criteria for Total Phosphorus (TP) and Total Nitrogen (TN). The Illinois EPA has taken it a step further with a regulatory process for developing watershed-specific limits by performing in-depth sampling and modeling studies known as Nutrient Action Reduction Plans (NARPs).
Publicly Owned Treatment Works (POTWs) are required to implement processes that reduce the effluent nutrient concentrations in wastewater to safe levels. Let's discuss different processes for nutrient removal in wastewater treatment and how municipalities can partner with wastewater engineers to optimize nutrient removal methods for each unique POTW.
Conventional wastewater treatment processes, though designed to meet secondary treatment effluent standards, typically do not remove phosphorus and nitrogen and certainly not to the extent required to protect receiving waters. Nutrient removal in wastewater treatment processes can be necessary to help POTWs comply with effluent limits specified within their discharge permits.
The revised EPA standards for TP limits apply to all major wastewater treatment plants (WWTPs) with an average design flow of 1 million gallons per day. The TP effluent limit of 0.5 mg/L is estimated to apply to all major NPDES-permitted POTWs beginning Jan. 1, 2030. This date and target varies depending on the watershed and proposed phosphorus reduction strategy.
The following table highlights the key phosphorus removal methods in wastewater treatment.
Total Nitrogen in wastewater is also being discussed in future permitting regulations and watershed-based studies.
The following table highlights the key nitrogen removal processes used during wastewater treatment.