10.3 Clean Water Act & NPDES Permit Compliance

6.2 Clean Water Act & NPDES Permit Compliance

Environmental compliance is the primary legal obligation of wastewater treatment operators. In South Carolina, point source discharges into receiving waters are strictly regulated to protect public health and preserve water resources.

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Regulatory Framework & SCDES Oversight

The Clean Water Act (CWA)

Passed by Congress in 1972, the Clean Water Act established the National Pollutant Discharge Elimination System (NPDES) program. The CWA makes it unlawful to discharge any pollutant from a point source into waters of the United States without an active NPDES permit.

State Regulatory Authority: SCDES

Federal EPA delegates NPDES authority to South Carolina, administered by the South Carolina Department of Environmental Services (SCDES) (formerly DES). SCDES issues and enforces state NPDES permits pursuant to SC Regulation 61-9 (Water Pollution Control Permits). Operating a facility without a permit or violating permit conditions carries severe state and federal civil fines and potential criminal liability.

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NPDES Permit Effluent Limits: TBELs vs WQBELs

NPDES permits establish dual-tiered discharge limits based on technology standards and receiving stream protection:

+-----------------------------------------------------------------------+
|                       NPDES PERMIT EFFLUENT LIMITS                    |
+------------------------------------+----------------------------------+
| Technology-Based Limits (TBELs)    | Water Quality-Based (WQBELs)     |
| National minimum treatment floor   | Site-specific stream protection  |
| (40 CFR Part 133 Secondary Stds)   | (TMDLs & Water Quality Stds)     |
+------------------------------------+----------------------------------+

1. Technology-Based Effluent Limits (TBELs)

TBELs establish national performance minimums for secondary treatment under 40 CFR Part 133:

• Biochemical Oxygen Demand (BOD₅):
  • 30-Day Average: 30 mg/L (with $\ge 85%$ minimum removal efficiency)
  • 7-Day Average: 45 mg/L
• Total Suspended Solids (TSS):
  • 30-Day Average: 30 mg/L (with $\ge 85%$ minimum removal efficiency)
  • 7-Day Average: 45 mg/L
• pH Standard Range: Strictly bounded between 6.0 and 9.0 Standard Units (S.U.) at all times.

2. Water Quality-Based Effluent Limits (WQBELs)

When TBELs are insufficient to preserve receiving stream water quality standards, SCDES imposes site-specific WQBELs derived from waste load allocations and Total Maximum Daily Loads (TMDLs):

• Carbonaceous BOD₅ (CBOD₅): Reduced to 5.0 to 15.0 mg/L for discharges into low-flow or sensitive streams.
• Dissolved Oxygen (DO): Minimum effluent concentration (e.g., $\ge 5.0\text{ mg/L}$ or $6.0\text{ mg/L}$) enforced via post-aeration cascades or blowers.
• Total Residual Chlorine (TRC): Low limits (e.g., $0.011\text{ to }0.019\text{ mg/L}$) to prevent aquatic toxicity, requiring chemical dechlorination using sodium bisulfite (\text{NaHSO}_3) or sulfur dioxide (\text{SO}_2).
• Microbiological Indicators:
  • Freshwater Discharges: Escherichia coli (E. coli) — Monthly geometric mean $\le 126\text{ CFU/100 mL}$; single sample maximum $\le 349\text{ CFU/100 mL}$.
  • Coastal / Shellfish Waters: Fecal Coliform — Monthly geometric mean $\le 200\text{ CFU/100 mL}$; 7-day maximum $\le 400\text{ CFU/100 mL}$.
• Nutrient Limits: Total Nitrogen (\text{TN} < 3.0 - 6.0\text{ mg/L}) and Total Phosphorus (\text{TP} < 0.5 - 1.0\text{ mg/L}) limits to mitigate eutrophication.

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Discharge Monitoring Reports (DMRs) & Math Formulas

Facility monitoring data must be compiled into monthly electronic Discharge Monitoring Reports (eDMRs) and submitted to SCDES by the 28th day of the month following the monitoring period.

Key Compliance Formulas

1. Daily Mass Loading Calculation

$\text{Loading (lbs/day)} = \text{Flow (MGD)} \times \text{Concentration (mg/L)} \times 8.34$

Worked Example: A WWTP discharges $2.5 \text{ MGD}$ with an effluent TSS concentration of $18 \text{ mg/L}$. Calculate daily loading: $\text{Mass Loading} = 2.5 \times 18 \times 8.34 = 375.3 \text{ lbs/day}$

2. Geometric Mean for Bacteria

Bacteria counts must be averaged using a geometric mean to prevent extreme outliers from distorting data: $\text{Geometric Mean} = 10^{\left( \frac{\sum \log_{10}(x_i)}{n} \right)}$

3. Percent Removal Efficiency

$\% \text{ Removal} = \left( \frac{\text{Influent Concentration} - \text{Effluent Concentration}}{\text{Influent Concentration}} \right) \times 100$

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Sampling Protocols: Grab vs. Composite & Holding Times

NPDES permits prescribe sample collection methods, preservation techniques, and maximum holding times:

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Non-Compliance, Bypass & SSO Reporting Protocols

SCDES Regulation 61-9 mandates strict emergency reporting protocols for permit non-compliance:

24-Hour Verbal Notification

The operator must contact the regional SCDES office by telephone within 24 hours of becoming aware of:

1. Any non-compliance endangering human health or the environment.
2. Any Sanitary Sewer Overflow (SSO) or unpermitted discharge.
3. Any unanticipated bypass of treatment process units.
4. Any daily maximum discharge limit violation for toxic pollutants.

5-Day Written Report

A formal written report must be submitted to SCDES within 5 calendar days, containing:

1. Exact description and cause of non-compliance/spill.
2. Exact period of non-compliance (dates and times).
3. Status of non-compliance (whether corrected or ongoing).
4. Steps taken or planned to reduce, eliminate, and prevent recurrence.

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SWPPP & Industrial Pretreatment Program (IPP)

Stormwater Pollution Prevention Plan (SWPPP)

WWTPs with design flows $\ge 1.0 \text{ MGD}$ must maintain an industrial stormwater permit and a facility SWPPP incorporating structural Best Management Practices (BMPs), spill containment, and routine site inspections.

Industrial Pretreatment Program (IPP)

Under 40 CFR Part 403, Publicly Owned Treatment Works (POTWs) receiving industrial discharges must establish an IPP to regulate Significant Industrial Users (SIUs) and Categorical Industrial Users (CIUs).

• Pass-Through: An industrial pollutant passes through the POTW into receiving waters untreated, causing an NPDES permit violation.
• Interference: An industrial discharge inhibits or disrupts POTW treatment processes, sludge use, or operational safety (e.g., toxic metal slugs killing nitrifying bacteria).