8.5 Sanitary Sewer Overflows (SSOs): Root Causes, Containment, Reporting & Mitigation

Sanitary Sewer Overflows (SSOs): Root Causes, Containment, Reporting & Mitigation

A Sanitary Sewer Overflow (SSO) is an unpermitted discharge of untreated or partially treated wastewater from a municipal collection network prior to reaching the wastewater treatment facility. SSOs represent acute environmental emergencies that threaten surface water quality, public drinking water supplies, and public health.

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1. Primary Root Causes & Public Health Hazards

SSOs are categorized into dry-weather events and wet-weather events based on their initiating root cause:

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|                        PRIMARY ROOT CAUSES OF SSOs                      |
+-------------------------------------------------------------------------+
| 1. Pipe Blockages (Leading Cause of Dry-Weather SSOs):                  |
|    - Accumulation of congealed Fats, Oils, and Grease (FOG).            |
|    - Severe tree root intrusions trapping rags and non-dispersible wipes|
|    - Vandalism or foreign object deposition (rocks, construction debris)|
|                                                                         |
| 2. Mechanical & Electrical Failures:                                    |
|    - Lift station total utility power blackout without standby generator|
|    - Pump mechanical failure, impeller jamming, or check valve seizure. |
|    - SCADA / float level control sensor failure.                        |
|                                                                         |
| 3. Wet-Weather Hydraulic Overloading:                                   |
|    - Extreme precipitation or rapid snowmelt driving massive I&I        |
|      surges exceeding the physical carrying capacity of the pipeline.   |
|                                                                         |
| 4. Structural Infrastructure Failures:                                  |
|    - Pipe crown collapse from biogenic sulfuric acid (H2SO4) corrosion. |
|    - External line strikes during third-party excavation.               |
|    - Washouts from riverbed erosion or geological slope failures.       |
+-------------------------------------------------------------------------+

Environmental and Public Health Hazards

• Pathogen Exposure: Raw wastewater carries dangerous concentrations of waterborne pathogenic organisms, including enteric bacteria (Escherichia coli, Salmonella enterica, Shigella), viruses (Norovirus, Hepatitis A, Enterovirus), and protozoan parasites (Giardia lamblia, Cryptosporidium parvum).
• Aquatic Eutrophication & Dissolved Oxygen Depletion: High concentrations of carbonaceous biochemical oxygen demand ($\text{BOD}_5$) and un-ionized ammonia ($\text{NH}_3$) rapidly strip dissolved oxygen from receiving streams, causing extensive fish kills and aquatic ecosystem devastation.
• Drinking Water Intake Contamination: Spills into rivers or alluvial aquifers threaten downstream municipal drinking water treatment plant intakes.
• Private Property Inundation: Surcharged collection lines force raw sewage backwards through building laterals, contaminating private basements and indoor living environments.

2. Emergency Response, Containment & Remediation Protocols

When an overflow occurs, certified collection operators must execute a systematic emergency response:

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|                   SSO EMERGENCY INCIDENT ACTION PLAN                    |
+-------------------------------------------------------------------------+
| Phase 1: Site Security & Safety                                         |
|   - Don mandatory PPE (Tyvek protective suits, nitrile/rubber gloves,   |
|     face shields, steel-toe boots).                                     |
|   - Establish perimeter control using barricades, warning tape, and     |
|     bilingual sanitary hazard warning signs to protect the public.      |
|                                                                         |
| Phase 2: Containment & Diversion                                        |
|   - Erect earthen berms, sandbag dikes, or absorbent containment booms  |
|     to halt overland flow.                                              |
|   - Seal storm drain catch basins with pneumatic plugs or magnetic mats |
|     to prevent raw sewage from entering the municipal storm network.    |
|                                                                         |
| Phase 3: Flow Elimination & Bypass Pumping                              |
|   - Deploy trailer-mounted, diesel-driven self-priming bypass pumps     |
|     between upstream and downstream intact manholes around blockage.    |
|   - Clear the mainline blockage via high-velocity hydro-jetting.        |
|                                                                         |
| Phase 4: Vacuum Recovery & Surface Decontamination                      |
|   - Recover all pooled liquid and sludge using combination Jet-Vac units|
|   - Collect gross solids, toilet paper, and plastics for landfill.      |
|   - Disinfect soil: Apply agricultural or hydrated lime (Ca(OH)2)       |
|     evenly over soil surfaces to raise pH >= 12.0 for pathogen kill.    |
|   - Hard Surfaces: Power-wash into vacuum pickup; apply EPA disinfectant|
+-------------------------------------------------------------------------+

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3. CDPHE Regulatory Reporting Requirements (Colorado Standards)

In Colorado, discharges of unpermitted wastewater are strictly regulated by the Colorado Department of Public Health and Environment (CDPHE) Water Quality Control Division (WQCD) under the Colorado Water Quality Control Act (CRS 25-8-601 et seq.) and Regulation 61:

The 24-Hour Verbal / Electronic Notification Mandate

• Mandatory Reporting Threshold: Any unauthorized spill, release, or overflow of untreated or partially treated wastewater that impacts or threatens to impact "State Waters" (including streams, rivers, dry gulches, irrigation ditches, stormwater conveyances, wetlands, or alluvial groundwater) must be reported to the CDPHE Environmental Spill Reporting Hotline within twenty-four (24) hours of the utility becoming aware of the event.
• Initial Information Required:
  1. Location of the discharge (street address and GPS coordinates).
  2. Estimated spill volume and duration.
  3. Receiving water body name or drainage pathway.
  4. Preliminary root cause determination.
  5. Emergency containment and mitigation actions taken.

The 5-Day Formal Written Spill Report

• A detailed, comprehensive written engineering report must be formally submitted to the CDPHE WQCD within five (5) calendar days following the incident.
• Mandatory Elements of the 5-Day Report:
  • Exact date, start time, and stop time (total spill duration in hours/minutes).
  • Defensible mathematical calculation methodology utilized to quantify total spill volume.
  • Map showing overflow location, overland flow path, and receiving water sampling points.
  • Surface water quality monitoring results (ambient dissolved oxygen, $E. coli$, and ammonia upstream and downstream of impact point).
  • Comprehensive root cause analysis (e.g., FOG accumulation, root blockage, power outage).
  • Description of all cleanup, containment, vacuum extraction, and lime disinfection measures.
  • Long-term Corrective Action Plan (CAP) detailing capital improvements, maintenance adjustments, or enforcement actions to guarantee the spill does not recur.

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4. Defensible Spill Volume Estimation Methodologies

Regulatory reporting requires operators to provide scientifically defensible mathematical volume estimates rather than unverified guesses:

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|                    SPILL VOLUME ESTIMATION METHODOLOGIES                |
+-------------------------------------------------------------------------+
| Method 1: Area and Average Ponding Depth                                |
|   Volume (gal) = Area (sq ft) * Avg Depth (ft) * 7.48 gal/cu ft         |
|                                                                         |
| Method 2: Overflow Rate Multiplied by Spill Duration                    |
|   Volume (gal) = Overflow Rate (gpm) * Total Duration (minutes)         |
|   *Flow rate estimated using empirical visual manhole bubbling charts   |
|                                                                         |
| Method 3: Lift Station Inflow Differential (SCADA Run-Time)             |
|   Spill Rate (gpm) = Basin Average Inflow Rate - Pump Pumping Rate      |
|   Volume (gal) = Spill Rate (gpm) * Power Outage Duration (minutes)     |
|                                                                         |
| Method 4: Vacuum Recovery Reconciliation                                 |
|   Total Spill Volume = Gallons Recovered by Vac Trucks + Residual Loss  |
+-------------------------------------------------------------------------+

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5. Capacity, Management, Operation, and Maintenance (CMOM)

The EPA and CDPHE Capacity, Management, Operation, and Maintenance (CMOM) program provides an overarching asset management framework designed to eliminate avoidable SSOs:

• Capacity: Utilize hydraulic computer modeling, continuous flow metering, and rainfall gauging to ensure sewer mains and lift stations possess adequate hydraulic carrying capacity for peak wet-weather design events.
• Management: Establish standard operating procedures (SOPs), GIS asset mapping, computerized maintenance management systems (CMMS), continuous operator safety training, and aggressive commercial FOG pretreatment ordinances.
• Operation: Execute scheduled hydro-jetting of high-risk sewer reaches, conduct weekly lift station mechanical inspections, and monitor root growth.
• Maintenance: Implement prioritized trenchless rehabilitation (Cured-in-Place Pipe / CIPP lining, pipe bursting, slip-lining), manhole epoxy rehabilitation, and maintain emergency bypass pumping equipment ready for instant deployment.

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6. Worked SSO Calculations

Worked Example 7.5.1: Area-Depth Volume Estimation for Overland Spill

A surcharged sewer manhole overflows onto a low-lying park lawn. Operators measure the resulting sewage pool as approximately 120 feet long by 45 feet wide. Depth measurements taken at grid intervals yield an average liquid ponding depth of 3.5 inches ($0.2917\text{ ft}$). Calculate the estimated total spill volume in gallons.

Step 1: Calculate surface area ($A$ in square feet): $A = 120\text{ ft} \times 45\text{ ft} = 5,400\text{ ft}^2$

Step 2: Convert average depth to feet: $\text{Depth} = \frac{3.5\text{ in}}{12\text{ in/ft}} = 0.2917\text{ ft}$

Step 3: Calculate volume in cubic feet: $\text{Volume} = 5,400\text{ ft}^2 \times 0.2917\text{ ft} = 1,575.18\text{ ft}^3$

Step 4: Convert cubic feet to gallons ($1\text{ ft}^3 = 7.48\text{ gal}$): $\text{Total Spill Volume} = 1,575.18\text{ ft}^3 \times 7.48\text{ gal/ft}^3 = 11,782\text{ gallons}$

Worked Example 7.5.2: Lift Station Power Outage Spill Estimation

A severe electrical storm causes a total utility blackout at a municipal lift station lacking an automatic standby generator. The lift station serves a basin with a continuous dry-weather diurnal inflow rate of 450 gpm. The wet well has a reserve emergency storage capacity of 3,000 gallons above the high alarm float before overflowing through the emergency vent.

If the utility power outage lasts for exactly 2.5 hours (150 minutes) before portable emergency generators restore pumping, calculate the total volume of raw sewage discharged during the SSO event.

Step 1: Calculate total wastewater volume entering the station during the 150-minute outage: $\text{Total Inflow} = 450\text{ gpm} \times 150\text{ minutes} = 67,500\text{ gallons}$

Step 2: Subtract the wet well internal reserve storage capacity: $\text{Overflow Volume} = \text{Total Inflow} - \text{Wet Well Reserve Storage}$ $\text{Overflow Volume} = 67,500\text{ gal} - 3,000\text{ gal} = 64,500\text{ gallons}$

Conclusion: The estimated total volume discharged during the SSO is 64,500 gallons. This spill must be reported to the CDPHE spill hotline within 24 hours followed by a 5-day written report.