An Alternative to Setting Total Phosphorus Effluent Limitations for Nutrient TMDLs

A nutrient TMDL methodology was developed to address impairments in a major river basin in New Jersey. Because WWTP point sources supply a relatively constant load even during dry-weather low-flow conditions, they exert a major impact on productivity. Given the fact that the ratio of orthophosphorus to total phosphorus in WWTP effluent cannot be assumed with any degree of certainty, and that it is the effluent orthophosphorus that drives productivity, it is necessary to set effluent limits for orthophosphorus directly in order to achieve water quality targets based on DO and pH. Since Biological Phosphorus Removal concentrates phosphorus in TSS, more advanced filtration technology is often necessary to satisfy stringent effluent TP limits. Expressing effluent phosphorus limits as orthophosphorus rather than total phosphorus therefore eliminates a significant regulatory disincentive for utilizing Biological Phosphorus Removal in a treatment system, and also directly controls the nutrient responsible for excessive productivity in streams.