By Randy Hurst
This is the third article in this 4-part series:  about NPDES permits for wastewater discharges from Publicly-Owned Treatment Works (POTWs). All POTWs are required by regulation to give a copy of the current NPDES Permit to all of their Certified Operators. Certified operators, in turn, are required by regulation to know what the Permit requirements are and report to the Owner when a violation is-—or is likely to-—occur. Hence, all Certified Operators should become familiar with the provisions of their permits. Because it is the most complex topic, we discuss effluent limits in the first three installments. The fourth article will explore reporting requirements.
There are two conceptually different bases for effluent limits: Technology-based and Water Quality-based. We will discuss Water Quality-based limits in the second article in this series.
The concept of Technology-based Effluent Limits (TBELs) was adopted by Congress in drafting the comprehensive Water Pollution Control Act amendments (now known as the Clean Water Act) in 1972. The idea behind TBELs is to force a reduction of pollutant discharges by requiring the use of available and practical technology. The concept is embodied in the very first section of the Act: "It is the national goal that the discharge of pollutants into the navigable waters be eliminated by 1985." (Hence the term "National Pollution Discharge Elimination System.") Complex analysis of the effects on the environment of each discharge is not needed; everyone must use available and effective technology, with the goal of eventually eliminating pollutant discharges altogether. In this way, all dischargers are treated alike and all pollutant discharges are reduced. This "level playing field" approach reduces a major source of conflict in permitting decisions-—deciding what level of pollutant discharge is acceptable. The law provides the answer: everyone must use the most effective and available pollutant reduction technology.
The Act defined two classes of TBELs: those for industrial dischargers and those for POTWs. Because of the wide variety of industrial processes that produce pollutants, the industrial limits (called, misleadingly, Effluent Limit Guidelines-—they are, in fact, regulations) are developed on an industry-specific basis. Currently, there are about 58 categories of Technology-based Industrial discharge limits. Those of you with Industrial Pretreatment Programs know these better as "Categorical" industrial discharge limits.
The TBELs developed for POTWs are called "Secondary Treatment Standards." Three pollutants are regulated by Federal Secondary Treatment Standards: BOD/C-BOD, TSS, and pH. The requirements for both BOD and TSS are defined by the allowable effluent concentrations, which are 30 mg/L as a 30-day average and 45 mg/L as a 7-day average; the rule includes slightly lower limits for C-BOD of 25 and 40 mg/l as a 30 and 7-day average respectively. There is also a pH limit of between 6 and 9 at all times. In addition to the discharge limits, POTWs must achieve 85% removal (on a 30-day average basis) of BOD/C-BOD and TSS, based on an influent/effluent comparison. All POTWs must meet these requirements, which are achievable using readily available technology.
In Pennsylvania, PaDEP has expanded the definition of Secondary treatment to be more comprehensive and more stringent than the Federal standards. PaDEP regulations adopt the Federal BOD/C-BOD, TSS and pH limits and removal rates, and add limits for three additional pollutants. The PaDEP regulations also redefine "Secondary Treatment" itself to be more restrictive than the EPA definition.
The definition of Secondary Treatment in PaDEP's NPDES regulations (Chapter 92a) adds the following two requirements: First, Secondary Treatment must "include significant biological treatment," which is further defined as the use of a "biological treatment process" (this term is undefined) that achieves at least 65% removal of BOD as a 30-day average. Additionally, the treatment plant must provide for the "disposal or beneficial use of sludge." All POTW and non-POTW sewage treatment plants in Pennsylvania must meet these operational standards.
The PaDEP regulations also add discharge limits for fecal coliform, total residual chlorine, and oil and grease to the Secondary Treatment standards. Keeping in mind that these limits are based on technology only, they are as follows. Fecal coliform: From May–September: 200 cfu/100 ml as a geometric mean and 1000 cfu/100 ml as a maximum at any time; from October–April the limits are a mean of 2,000 and a maximum of 10,000. Total Residual Chlorine: 0.5 mg/L as a 30-day average (except in Special Protection Waters, where the limit is zero unless special conditions are met). Oil and Grease: maximum of 30 mg/L in any (grab) sample, and 15 mg/L as a daily average (if only one sample is obtained in a day, then the lower limit applies); and "at no time cause a film or sheen upon or discoloration of the waters . . . or adjoining shoreline."
You might notice that the last limit mentioned differs from all the others mentioned so far in that there is no numerical measurement; it is a "narrative" limit that does not rely on scientific measurements. We will discuss narrative limits in the third article in this series.
Five of the pollutants discussed above-—BOD, TSS, pH, fecal coliform, and oil and grease-—are also called "conventional pollutants" since POTWs are capable of treating these pollutants using conventional treatment methods (as they existed in the 1970's when the standards were first defined). Thus, Secondary Treatment Standards are TBELs that apply to Conventional Pollutants (plus, in PA, TRC) by regulation and, with one exception, are numerical limits.
Because TBELS are set by regulation, no analysis of the effect on the receiving water is necessary to include them in permits. In fact, many streams are perfectly capable of absorbing C-BOD or TSS concentrations well in excess of the limits. That isn't the point. The goal of TBELS is to reduce the discharge of pollutants in the hopes of someday meeting the "zero discharge" goal set by Congress over 45 years ago. Over the last 20 years environmental protection groups have periodically urged EPA to "ratchet down" these standards by reducing the limits for BOD/C-BOD and TSS and by adding additional pollutants-—most notably phosphorus and nitrogen-—to the list of conventional pollutants, since readily available modern treatment processes are capable of routinely achieving low levels of these pollutants. To date, EPA has rejected these attempts, at least in part because it believes that attainment of water quality standards can achieve the same objective. We will discuss that concept in the next article.
A final twist on TBELs: In some cases, a permitting agency identifies a pollutant that is not regulated by TBELs, but which may require some limits in the permit. Usually, these pollutants are regulated as water quality-based limits (discussed in the next article), but in some cases the concept of imposing a technology-based "floor" is applicable. The Clean Water Act provides for case-by-case imposition of TBELs under limited circumstances. Section 402(a)(1)(B) states, somewhat densely, "the Administrator [of the EPA] may . . . issue a [NPDES] permit for the discharge of any pollutant . . . upon condition that such discharge will meet (A) all requirements [for TBELS in the Act] or (B) prior to the taking of necessary implementing actions relating to [TBELs], such conditions as the Administrator determines are necessary . . . ." This section is usually referred to as the "best professional judgment" rule. In essence, it allows a TBEL to be developed on a case by case basis when none has been formally adopted for a class of discharger, such as POTWs. However, it also restricts the use of best professional judgment to those parameters that do not already have promulgated TBELs (such as those regulated by secondary treatment standards). This restriction keeps the process "clean": once the EPA or DEP has gone through the process of determining what technology-based limits are appropriate and POTWs have built treatment plants to meet these standards, it would be counterproductive to have individual permit writers making up their own technology-based limits. This does not mean that more stringent effluent limits for C-BOD or TRC cannot be issued if necessary to protect water quality, however. This "lowest limit controls" principle is discussed in the second article in this series, Water Quality-based effluent limits.
 This version differs slightly from that published in the PWEA magazine Keystone Water Quality Manager. The final paragraph here does not appear in that article.
Other articles in the NPDES Permit Basics - Understanding what's in your Permit: