The Science of Phosphorus | Blog Series | 2 of 5

The Status of P Regulations

When a watershed does not meet attainment status for designated uses, there are three main mechanisms for regulation of phosphorus (P) in treated effluent from a water resource recovery facility (WRRF) discharged to surface water: technology-based standards, total maximum daily loads (TMDL), and water quality based emission limits (WQBEL). Technology-based standards are probably the most common form of regulation. However, it is the WQBEL which are currently the driving force for more widespread regulation. 

Technology-Based Limits

Technology-based limits (TBL) are an end-of-pipe approach that relies on available technologies to derive discharge limits. A TBL may be imposed in combination with other forms of regulation and are often imposed as interim limits to provide time for a utility to implement treatment solutions or explore alternatives to treatment for achieving more stringent requirements. The advantage of a TBL is that it is simple to define. It depends only on the present state of technology and therefore has widespread applicability.

One example of a TBL is the Bay View Water Reclamation facility which discharges treated effluent from Toledo’s lone WRRF into Maumee Bay, a tributary to Lake Erie. Like all major facilities in the Great Lakes watershed, the Bay View discharge permit includes a limit not to exceed a monthly average of 1.0 mg P/L or a weekly average of 1.5 mg P/L in treated effluent. 

The disadvantage of a TBL is not based on the specific properties of a particular watershed and therefore, may not be sufficient to bring a watershed into attainment status.   

Wastewater Plant Settling Tanks Aerial View

Total Maximum Daily Load

A TMDL is a water-quality based standard applied to impaired watersheds. Nutrient enrichment is one of the most common causes of impairment (WEF Nutrient Removal). A TMDL is hence a nutrient diet for watersheds. An important distinction of a TMDL is that it is based on load and not concentration. Each point source is assigned a portion of the total allowable load. The load restriction does not change as the facility expands such that the concentration of phosphorus in treated effluent must be reduced as flow increases. Thus, a facility must achieve greater levels of removal as flows increase. Another important distinction is that limits may be applied over a term longer than typical weekly or monthly limits, providing greater flexibility to offset periods of low performance with periods of high performance.

Water Quality-Based Effluent Limits

WQBELs have traditionally been applied to limit discharge of toxic substances. Phosphorus and nitrogen are not toxic in the traditional sense – a specific concentration will not lead to a predictable response, e.g. mortality.  Therefore, in order to facilitate standards development USEPA released the report National Strategy for the Development of Regional Nutrient Criteria in 1998.  Progress has been slow but nutrient water quality standards now exist in 23 states (view an interactive map, State Development of Numeric Criteria for Nitrogen and Phosphorus Pollution). Implementation of new standards varies by state but some very stringent limits for phosphorus are appearing in NPDES discharge permits. In Wisconsin, for example, treated effluent from WRRFs discharging to streams is limited to a maximum concentration of 0.075 mg TP/L. A compliance schedule of up to 9 years is provided to meet the new limit.



Additional Blog Posts of Interest

The Science of Phosphorus | Blog Series | 1 of 5

The Phosphorus Problem: Wastewater Treatment Options and Process Monitoring Solutions

6 Steps to Reduce Total Phosphorus in Water Resource Recovery Facilities

Monitoring Orthophosphate for Reduced Chemical Costs in Water Resource Recovery Facilities

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