Guidance for Interpreting Narrative Local
Limits Contained in MRWPCA Wastewater
Discharge Ordinance #2008-01
Developed by the MRWPCA Source Control Division
The local limits for discharges to the sanitary sewer are found in MRWPCA Wastewater
Discharge Ordinance #2008-01. Although the numeric limits contained in this Ordinance are
straight forward, the narrative limits need further definition. To assist the industrial or
commercial facility in understanding and implementing these narrative limits, the following
explanations and implementation guidelines were developed.
Each individual narrative limit for specific pollutants is divided into two parts:
1. The first part is a PROHIBITION that states "no person shall discharge any wastewater"
that contains the specific pollutants in such quantities as to cause or significantly
contribute to the stated undesirable results.
2. The second part states that dischargers of significant concentrations of the specific
pollutants will be required to install pretreatment equipment, incorporating the best
practicable technology (BPT) available to reduce these concentrations. The question then
arises: when is a pollutant concentration at or above the significant level? It became
apparent that some form of numeric indicator was necessary for both the facility and the
regulatory authority to make decisions and take appropriate action when necessary. For
this reason, the Source Control Division developed the following numeric indicators
using previous MRWPCA local limits, Treatment Plant removal efficiencies and EPA
pollutant limitations:
Pollutant: Total Toxic Organics (TTO)
TTO concentration < 1.0 mg/L: no action required.
TTO concentration > 1.0 mg/L but <2.5 mg/L: MRWPCA will increase monitoring or
require self-monitoring of the waste stream to determine if a trend exists. The industrial
or commercial facility will be required to implement source control or waste
minimization measures.
TTO concentration > 2.5 mg/L: is a significant discharge. The industrial or
commercial facility will be required to self-monitor the waste stream, install pretreatment
equipment using Best Practicable Technology (BPT), implement measures to reduce the
specific pollutant levels to below 2.5 mg/L, or cease the process that is causing the
excessive TTO concentration.
Pollutant: Oil and Grease:
Polar (animal or vegetable)
Oil/Grease concentration <300 mg/L: no action required.
3. Oil/Grease concentration >300 mg/L but <450 mg/L: MRWPCA will increase
monitoring or require self-monitoring of the waste stream to determine if a trend exists.
The industrial or commercial facility will be required to implement source control or
waste minimization measures.
Oil/Grease concentration >450 mg/L: is a significant discharge. The industrial or
commercial facility will be required to self-monitor the waste stream, install pretreatment
equipment using Best Practicable Technology (BPT), implement measures to reduce the
specific pollutant level to below 450 mg/L, or cease the process that is causing the
excessive animal/vegetable oil and grease concentration.
Pollutant: Oil and Grease:
Non-polar (petroleum or mineral)
Oil/Grease concentration <100 mg/L: no action required.
Oil/Grease concentration >100 mg/L but <150 mg/L: MRWPCA will increase
monitoring or require self-monitoring of the waste stream to determine if a trend exists.
The industrial or commercial facility will be required to implement source control or
waste minimization measures.
Oil/Grease concentration: >150 mg/L: is a significant discharge. The industrial or
commercial facility will be required to self-monitor the waste stream, install pretreatment
equipment using Best Practicable Technology (BPT), implement measures to reduce the
specific pollutant level below 150 mg/L, or cease the process that is causing the excessive
petroleum/mineral oil and grease concentration.
Pollutant: Toxic Inorganics, Ammonia, BOD, TSS
To date, no numeric guidelines have been developed for the narrative limits dealing with
significant dischargers of toxic inorganic pollutants, ammonia or BOD/TSS. In the
future, numeric guidelines may be developed to facilitate wastewater reclamation and
reuse, or to alleviate problems being caused by the specific pollutants.