The Devil is in the Details: How Reporting Methodologies Impact Compliance for Facilities with Stringent Phosphorus Limits

National Pollutant Discharge Elimination System (NPDES) permits for point-source discharges have been issued with increasingly low phosphorus limits in recent years. Federal regulations dictate that permit conditions for Publicly Owned Treatment Works should be stated as average weekly and average monthly discharge limitations unless impracticable (40 CFR 122.45(d)(2)). Some recent permits have included alternate calculation methodologies, such as seasonal or annual averages or medians, on the basis that nutrient impacts warrant different averaging periods or other site-specific considerations. Analysis of effluent phosphorus data from four Massachusetts facilities reveals considerable differences in reportable values using seven selected calculation methodologies applied to both concentration and load-based reporting. Results appear to also vary by the type of treatment process employed. Given these variations, permit writers and facility managers must give careful consideration to these seemingly minor differences in permit requirements to ensure that fair and appropriate methodologies are applied.