By Randy Hurst
In the first article in this 4-part series, we discussed "Technology-Based" Effluent Limits, or TBELs, for C-BOD, TSS, pH, fecal coliform bacteria, residual chlorine, and oil and grease. These limits are set by regulation and apply to all POTW discharges regardless of whether or not they are necessary to protect water quality. They are intended to move toward the Clean Water Act's national goal of "no discharge of pollutants" by requiring all treatment plants to implement readily available technology. For sewage treatment plants, TBELS are the Secondary Treatment standards; for industrial dischargers they are the Effluent Limit Guidelines (also called Categorical Standards).
The 1972 Clean Water Act also set a national "interim goal of water quality which provides for the protection and propagation of fish, shellfish, and wildlife and provides for recreation in and on the water [to] be achieved by July 1, 1983."  A related "national policy" is "that the discharge of toxic pollutants in toxic amounts be prohibited." These principles, sometimes called the "fishable/swimmable" goal, form the basis for protection of water quality by implementing "Water Quality-Based Effluent Limits," or WQBELs. The "interim" goals and WQBELs were intended to eventually be superseded by TBELs, which would incrementally implement the national goal of "zero discharge" as the technology to do so becomes available. However, over the last 40 years the emphasis has shifted away from the zero discharge concept and WQBELs have become a permanent part of the permitting system.
Unlike TBELs, which apply to all POTWs equally, WQBELs are site specific. They are based on a concept called "Water Quality Standards," which involves two sequential analyses to generate an "acceptable" level of pollutant discharge: a level that will not result in adverse effects to aquatic life or to people who use the water. The first step in setting water quality standards is to set the "Designated Use" of the water at issue: what sort of aquatic life is (or should be) present in the receiving water, and what sort of use will the water be put to by humans? Some states have created multiple classes of designated use. Maryland, for instance, defines four classes of waters (each with one subclass) and North Carolina, ten. Pennsylvania, however, classifies all waters into just three classes: warm water fish habitat, trout stocking, and cold water fish habitat; all "waters of the Commonwealth" are also designated for other uses such as a source for public water supply (after proper treatment), irrigation, and recreation.
Step two of setting Water Quality Standards is to establish Water Quality Criteria for individual pollutants to protect the Designated Uses. Criteria are based on various effects on aquatic life or on humans who consume the water. There are multiple considerations. In-stream dissolved oxygen levels are addressed by regulating oxygen demanding pollutants such as C-BOD and ammonia, as well as "nutrients" like nitrogen and phosphorus which contribute to algal blooms and oxygen depletion. Fecal coliform bacteria are limited based on human contact during swimming. Physical effects on aquatic life such as temperature and pH are regulated, primarily in cold water fish habitat. And "toxic" pollutants  such as residual chlorine, ammonia, aluminum, copper and toluene are regulated. Most toxic pollutants are evaluated for three kinds of effect: short-term ("acute") aquatic life effects (high concentrations occurring at the point of effluent discharge, but dissipating with dilution), long term ("chronic") effects (lower concentrations persisting in the stream after mixing of the effluent), and human health (assuming the pollutant will enter a public water supply after treatment). Given this complexity of analysis when setting limits to protect water quality, it's easy to see why Congress chose to establish the "floor" of Secondary Treatment to force installation of basic technology that would protect most waters from most pollutants most of the time.
Individual WQBELs are computed for each discharge using the water quality criteria applicable to the receiving water and considering the dilution that occurs when the effluent is discharged. Thus, each POTW's WQBELs are unique. Small plants discharging to large streams and rivers may have a lot of dilution and therefore no need for WQBELs at all because the actual maximum discharge concentrations resulting from the Secondary Treatment technology are substantially lower than the water quality-based effluent limits. Other plants may have to reduce particular pollutants to achieve some critical water quality criterion. A common example of a water quality-driven effluent limit is ammonia. Ammonia is both an oxygen demanding pollutant and toxic to aquatic life, so two analyses are necessary to set a limit; the lower of the two is used in the NPDES Permit.
This "lowest limit controls" concept also applies to certain Secondary Treatment pollutants. Although C-BOD is limited by the Secondary treatment standard of 25 mg/L as a monthly average, it is also a measure of oxygen demand, so in some cases C-BOD must be reduced to below the secondary treatment level to maintain in-stream dissolved oxygen at the criterion level, especially for waters classified for cold water fishes, which require high DO. Effluent C-BOD limits as low as 10 mg/L appear in some POTW permits for this reason. Similarly, since residual chlorine is toxic, we frequently see effluent TRC limits set much lower than Pennsylvania's Secondary Treatment limit of 0.5 mg/L to protect aquatic life in the receiving water; in fact, many POTWs have been required to abandon the use of chlorine disinfection altogether to protect water quality.
On the other hand, since there is no water quality criterion for TSS a WQBEL cannot be computed at all and no Reasonable Potential (discussed below) to violate a water quality criterion can exist, so there should be no POTW permits with effluent limits for TSS less than the Secondary Treatment TSS limits of 30 (monthly) and 45 (weekly) mg/L. [Author's Note: DEP objected to this paragraph and at its request it was omitted from the article published in the Keystone Water Quality Manager. Despite requests, DEP did not provide a reason for its objection.]
In the case of dischargers in the Chesapeake Bay watershed, the analysis is further complicated by the fact that Pennsylvania must honor Maryland's water quality standards. This results in "cap loads" for nutrients in waters crossing the state line, which DEP then doles out to various sources, including POTWs and non-point sources such as agricultural land use. Thus, although in many cases nutrient limits were not determined to be necessary to protect the receiving stream (i.e., the stream is not "impaired"), water quality-based effluent limits are still necessary to meet criteria set for the Chesapeake Bay many miles away.
By regulation, WQBELs are included in NPDES permits only when the regulatory agency determines that there is a "reasonable potential" for violating water quality criteria if the limits are not imposed. Thus, all WQBELs must be supported by an analysis showing that the discharge has a potential for causing water quality criteria to be exceeded unless an effluent limit is set. To make this determination, PaDEP uses computer programs that compute the maximum allowable effluent concentration using minimum stream flow, maximum (design) effluent flow, a rough estimate of in-stream mixing rate, and other factors (for instance, some metal criteria vary with water hardness and ammonia criteria are pH dependent, so effluent and stream water hardness and pH are factored into some calculations). If the reported maximum effluent concentration is greater than the calculated monthly average limit minus a "safety factor," then PaDEP assumes that there is a reasonable potential for a water quality impact and imposes the calculated effluent limit.
Since this "reasonable potential" decision is usually based on comparing highest one of three effluent samples obtained during the permit renewal process with the calculated maximum allowable monthly average concentration, it is statistically questionable. Some POTWs that are told that they will have effluent limits for these pollutants have obtained additional samples, either during the permit renewal testing or after seeing the draft permit, to generate statistically valid data for decision-making (and in some cases, "clean sampling" has helped to establish the true concentrations of the pollutant of concern). Some POTWs have avoided thousands of dollars in unnecessary testing costs over the life of their permits by generating data more representative of the monthly average discharge that show that they do not have the potential to cause a water quality criterion violation.
In some cases, a pollutant is present, but not quite at the threshold fraction of the calculated limit to justify an effluent limit. In these cases, PaDEP may include a "monitor" condition in the permit to generate additional effluent data to be used in a future permitting decision.
While most effluent limits are expressed as concentrations, not all limits are stated in numbers. In the next article in this series we will discuss the interesting topic of "narrative" effluent limits.
 This article was edited at the request of DEP for publication in the PWEA magazine. This version is the full article.
 As noted in the first article, the goal of "zero discharge of pollutants" was to be met by 1985.
 Imagine how much longer this article would be if we didn't have acronyms for National Pollutant Discharge Elimination System, water quality-based effluent limits, technology-based effluent limits, treatment standards for publicly owned treatment works, carbonaceous biochemical oxygen demand, total suspended solids, etc.
 Technically, only 126 specific pollutants are defined by the Clean Water Act as "toxic." For purposes of this discussion, however, all pollutants that have a toxic effect are called "toxic."
 The EPA Reasonable Potential standard is at 40 CFR § 122.44(d)(1).
Other articles in the NPDES Permit Basics - Understanding what's in your Permit: