All wastewater discharges to surface waters in the State of North Carolina must receive a permit to control water pollution. The Clean Water Act of 1972 initiated strict control of wastewater discharges with the responsibility of enforcement given to the Environmental Protection Agency (EPA). The EPA then created the National Pollutant Discharge Elimination System (NPDES) to track and control point sources of pollution. The primary method of control is by issuing permits to dischargers with limitations on wastewater flow and constituents. The EPA delegated permitting authority to the State of North Carolina in 1975.
The NPDES Unit is responsible for the issuance of wastewater discharge permits. This process includes determining the quality and quantity of treated wastewater that the receiving stream can assimilate, incorporating input from stream modeling, collaborating with Regional Office staff , and evaluation of the discharger’s location.
Where appropriate, the NPDES program establishes limits for flow (quantity discharged), conventional pollutants (BOD, pH, TSS, fecal coliform, oil & grease, etc.), toxicants (metals, volatile organics, etc.), and non-conventional pollutants such as ammonia and nutrients. Delegated states have the authority to establish state water quality standards which can be more stringent than federal standards established by EPA
In addition to administering the NPDES program in North Carolina, there is also the responsibility of enforcement of the discharge limitations. The penalty for discharging without a permit is a fine of up to $25,000 per day.
There are three main categories of pollutants that are regulated and/or monitored for in NPDES permits:
- Oxygen-consuming and conventional parameters
- Toxicants and non-conventional parameters
- Nutrients and parameters related to eutrophication
Aquatic organisms must have dissolved oxygen to survive. Biological and chemical effects exerted by treated wastewater may use oxygen in the stream. This "assimilation" of wastewater is predicted using mathematical modeling of the receiving waters and the wastestream in question. Protection of the water quality (instream) standard for dissolved oxygen is the goal of these types of models and the desired result of limiting oxygen-consuming wastes in an NPDES permit.
Typically, a Level B "desktop" model or a field-calibrated Level C model (such as QUAL2E) is used to determine acceptable levels of biochemical oxygen demand and ammonia. A permittee may also be required to maintain a minimum dissolved oxygen (DO) level in the effluent (i.e., at the end of the pipe). Limits for these pollutants are expressed in milligrams of pollutant per liter of solution (mg/L) as BOD5 (a five-day measurement of biochemical oxygen demand at 20 degrees C), ammonia nitrogen, and dissolved oxygen. For domestic sewage, a BOD5 maximum limit of 30 mg/L is given based on Title 40 of the Code of Federal Regulations (CFR) part 133.102, which defines minimum treatment. More stringent BOD5 limits are implemented where water quality mandates them.
Other "conventional" parameters that are frequently limited in an NPDES permit include total suspended solids (TSS), fecal coliform, temperature, pH, and oil and grease. Chemical Oxygen Demand (COD) is frequently limited in industrial permits. Modeling is not usually performed to determine acceptable limits for these parameters. Limits for fecal coliform, temperature, and pH reflect the water quality standards of the receiving stream. The same federal regulation that defines the maximum limit for BOD5 also establishes a maximum TSS concentration of 30 mg/L for domestic discharges. Oil and grease limits are usually established by best professional judgment (BPJ). Federal regulations often determine the limits for specific types of industrial discharges.
Toxicants are limited in NPDES permits via whole-effluent toxicity (WET) testing and chemical- specific limits. WET tests are given as limited, enforceable parameters. The receiving water and the dilution of the receiving stream determine the type of WET test given in a permit. For example, a small discharge into a large receiving water may require that an acute test, as opposed to, a chronic test be imposed. Also, a discharger into a saltwater system may opt to use a saltwater species instead of a freshwater species as the test organism.
Cadmium, chromium, copper, nickel, lead, zinc, cyanide, mercury, arsenic, silver, and selenium are toxicants, which are commonly limited or monitored for in NPDES permits. Volatile organic compounds and semi-volatile organic compounds are two other groups of toxicants that are found in NPDES permits. Limits may be given after determination that "reasonable potential" exists for a water quality standard violation to occur. The allowable concentration of a toxicant (i.e., potential limit) is determined by multiplying the receiving stream dilution by the water quality standard.
Eutrophication is a natural aging process of slow-moving bodies of water, such as ponds, lakes, reservoirs or estuaries. However, man-made eutrophication (sometimes referred to as cultural eutrophication) is an accelerated version of this natural process brought on by point source and non-point source additions of excessive nutrients. A system that has the potential to be affected by total phosphorus (TP) and/or total nitrogen (TN) may be classified as nutrient sensitive waters (NSW).
Limits for TP and TN are normally placed in permits after extensive water quality surveys are performed on the receiving waters. There are currently no standards for TP or TN in North Carolina. However, much the same as BOD5 and ammonia nitrogen are limited to protect the dissolved oxygen standard, nutrients are limited to prevent contraventions of the water quality standard for chlorophyll-a. It should be noted that the majority of NPDES permits require facilities to monitor for TP and TN only.