NPDES Wastewater Discharge Permits for Point Sources

The NPDES permits program is authorized by the CWA (CWA Section 402, 33 U.S. C. 1342). The initial permits issued in the 1970s and early 1980s focused on POTWs and industrial wastewater – typically “process” wastewater and cooling water where applicable. An NPDES Permit is required for a discharge of wastewater to waters of the United States (surface waters). This permit is issued by a state by delegated authority of the USEPA (1972). Since waters of the United States are also waters of the state, NPDES permits are actually combined NPDES/State Waste Discharge permits issued under dual authorities. NPDES permits are issued for a maximum of five years. Conventional pollutants contained in industrial, commercial, or domestic wastewaters are BOD, SS, fecal coliform, pH, temperature, free and residual chlorine, oil and grease, and DO. Toxic pollutants include synthetic organic chemicals and heavy metals. Nonconventional pollutants are any additional substances not in conventional or toxic categories, such as ammonia as nitrogen, nitrate–nitride, and total phosphorus.

NPDES permits for discharging wastewaters contain the following standard conditions common to all permits, site‐specific effluent limits, compliance monitoring and reporting requirements (see Section 4.4.4), and site‐specific conditions necessary for discharge control. Standard conditions describe the legal aspects of the permit and its revocability. They also describe the permittee’s duties and obligations; for example, the permittee is required to report changed conditions, to allow compliance inspections, and to reapply for a permit renewal. Finally, the permittee is informed regarding penalties that may be assessed for permit violations. Effluent limits imposed are based on technology‐based and water‐quality based standards (USEPA 1995b, 2010a; Hammer and Hammer 2012; Section D.3). The management of wastewater sludge is also controlled by conditions in a NPDES permit. The EPA is responsible for implementing and enforcing the NPDES program; however, individual states have the authority to perform these functions with oversight by the USEPA. An example of an industrial NPDES permit limits for criteria pollutants is shown in Table 4.6.

Table 4.6 NPDES permit limits for a pulp and paper mill’s wastewater treatment plant.

Source: From Das (1991).

Effluent parameter	NPDES permit daily limit		30‐Day average concentration	7‐Day average concentration
	Maximum	Average
BOD5	28 000 lb	20 000 lb
	40 mg/l	35 mg/l	30 mg/l	45 mg/l
TSS	46 000 lb	32 000 lb
	50 mg/l	45 mg/l	30 mg/l	45 mg/l
Flow			—	60 MGD
pH				6.0–9.0 all times
Rainbow trout	80% survival in 100% effluent
Bioassay	65% effluent for a 96‐hour period
Fecal Coliform/	20 000 (#/100/ml)	5 000 (#/100/ml)
% Klebsiella	N/A	98%

Note: Typical permit renewal in every five years. This table outlines only the quantitative limit. The entire permit is 26‐pages long.

BOD₅, five‐day biochemical oxygen demand; MGD, million gallons per day; N/A, not applicable; pH, negative log₁₀ of hydrogen ion concentration in gmol/l; TSS, total suspended solids.

The NPDES is a self‐monitoring program whereby the owner of the treatment plant is required to be responsible for compliance monitoring and reporting the performance of wastewater and sludge processing, including any violation of permit conditions. Effluent limitations and monitoring provisions specify the procedures to be used for sampling, flow measurement, laboratory testing, and record keeping. Operation and maintenance require reporting of treatment system failure, by‐passing of untreated wastewater, and process upset. Additional reporting requirements are any change in discharge, noncompliance notification, and compliance schedules. Enforcement for noncompliance is by administrative order to a monetary penalty. For serious violations, judicial action can be taken as either a civil or a criminal action (Hammer and Hammer 2012). Additional frequently asked questions and their answers pertinent to NPDES permitting are given in Section D.3.

EXAMPLE 4.9

An activated sludge extended aeration WWTP with a flow of 6.0 MGD has BOD of 320 mg/l, SS of 280 mg/l, and a total phosphorus of 7.35 mg/l, keeping the effluent the same as 19 mg/l of BOD, 20 mg/l of SS, and 1.25 mg/l of total phosphorous.

Calculate the following:

1. The BOD loading
2. BOD and SS removals by the plant
3. Pounds of BOD and SS removed per day
4. Total settled SS and phosphorous sludges in lb/day
5. If NPDES permit condition requires 85% removal of BOD, is the plant in compliance?

SOLUTION

1. BOD loading = 320  × 6.0 MGD × 8.34 = 16 012 
2. BOD removal =  × 100 = 94%SS removal =  × 100 = 93%
3.  BOD removal = (320 − 19)  × 6 MGD × 8.34  = 15 062  SS removal = (280 − 20)   × 6.0 MGD × 8.34  = 13 010 lb
4. = [(280 − 20) + (7.35 − 1.25)] × 6.0 MGD × 8.34 = 13 316 
5. Yes, BOD removal is 94%, therefore the plant well exceeded the 85% removal requirement by the NPDES permit condition.