Mitigating Storm Impacts in Sanitary Sewer Networks: WEF–EPA CMOM GuidanceBy: Dr. Hossein Ataei Far Introdution: In a separate sewer system, s...

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Mitigating Storm Impacts in Sanitary Sewer Networks: WEF–EPA CMOM Guidance
By: Dr. Hossein Ataei Far

Introdution:
In a separate sewer system, sanitary wastewater (from homes and businesses) is collected in one set of pipes and stormwater runoff is collected in an entirely different system and discharged separately, often directly to water bodies. This differs from a combined sewer, where both flows travel in the same pipe.
One of the most documented effects of flooding on separate sanitary sewer systems is increased inflow and infiltration.
• Inflow: stormwater entering the sanitary sewer through direct connections (e.g., roof downspouts or storm drains mistakenly tied into the sanitary system).
• Infiltration: groundwater entering through cracks, joint failures, or deteriorated pipes.
These become significant during heavy rainfall and can raise wastewater volumes above design capacities, effectively making the system behave like a combined sewer under stress.
Effects include:
1. Hydraulic overloading of pipes and pump stations.
2. Reduced efficiency of wastewater conveyance, because greater volumes slow flows and may cause surcharging and backups.
3. Reduced treatment efficacy at the wastewater treatment plant as I&I dilutes flows and changes pollutant loads.
During extreme rain or flooding, sanitary sewers can surcharge and lead to:
Basement backups and localized urban flooding.
• Sanitary Sewer Overflows (SSOs) — untreated sewage discharged into streets, storm drains, or surface water bodies before reaching a treatment plant.
These events pose public health risks (disease exposure) and environmental degradation (nutrients, pathogens entering waterways).
• Flooding alters the composition of influent wastewater:
Dilution can temporarily reduce pollutant concentrations, but large volumes can still surpass treatment plant hydraulics and biological capacity.
Rapid surges may reduce retention times, lowering nutrient removal efficiency and destabilizing biological treatment processes.

🌀 Flooding Impacts on Separate Wastewater Collection Systems
⚠️ Inflow and Infiltration (I&I) During Storm Events Flooding significantly impacts separate sanitary sewer systems by increasing inflow and infiltration (I&I):
• Inflow: Stormwater enters the sanitary sewer through direct connections, such as roof downspouts, yard drains, or storm drains that are mistakenly connected.
• Infiltration: Groundwater enters the sewer network through structural defects, including cracked pipes, defective joints, damaged laterals, or deteriorated manholes.
During heavy rainfall, I&I can rise rapidly, pushing flows above design capacity. Separate sewers may temporarily behave like combined sewers.
Common impacts include:
• Hydraulic overloading of gravity sewers, force mains, and pump stations
• Reduced conveyance efficiency, causing pipe surcharging, basement flooding, or surface backups
• Decreased treatment performance at wastewater treatment plants due to diluted influent flows and altered pollutant loads.
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📌 Overarching CMOM Recommendations (WEF – U.S. EPA Aligned Framework)
The CMOM (Capacity, Management, Operation & Maintenance) framework provides a proactive, risk-based approach to sewer system management. Supported by U.S. EPA and WEF, CMOM helps utilities reduce flooding risks, prevent sanitary sewer overflows (SSOs), and improve long-term performance.
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1️⃣ Shift from Reactive to Proactive Management
Core idea: Act before failures occur. Why it matters: Reactive repairs address symptoms, not root causes such as I&I, insufficient capacity, or poor monitoring. Practical actions:
• Maintain a sewer system risk register (SSOs, flood-prone locations, manhole surcharging)
• Perform regular condition and performance assessments
• Prepare a pre-storm readiness checklist
Best-case project: Portland, Oregon implemented pre-storm risk registers and proactive maintenance, significantly reducing basement backups.
Citation: U.S. EPA, Guide for Evaluating CMOM Programs, 2005
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2️⃣ Use Data and Technology for Informed Decisions
Core idea: Leverage operational data, modeling, and smart technologies to anticipate failures. Practical actions:
• Install flow meters and deploy SCADA
• Develop hydraulic models
• Maintain GIS-based asset inventories
Best-case project: Charlotte-Mecklenburg Utility used SCADA and hydraulic modeling to optimize wet-weather operations, reducing SSOs.
Recent support: Smart sewer programs now incorporate real-time sensors and analytics for proactive flow control (EPA Smart Sewers, 2025)
Citation: WEF, Wastewater Collection Systems Management, MOP 7
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3️⃣ Establish Clear Roles and Accountability
Core idea: Define CMOM responsibilities to prevent operational gaps.
Best-case project: Fairfax County, VA clarified CMOM roles across departments, improving emergency response during storms.
Citation: U.S. EPA Region 4 Management Guidance, 2006
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4️⃣ Develop and Maintain Written Procedures
Core idea: Standardized procedures ensure repeatable operations.
Best-case project: San Diego, CA documented wet-weather procedures, reducing response times to surcharging events.
Citation: WEF, CMOM Operation & Maintenance Reference Guide
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5️⃣ Asset Inventory and Critical Asset Identification
Core idea: Know the assets, their condition, and criticality.
Best-case project: Houston, TX, developed GIS-based inventories and tagged critical assets, prioritizing rehabilitation efficiently.
Citation: WEF, Technologies for CMOM Activities
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6️⃣ Assess and Reduce Inflow and Infiltration (I&I)
Core idea: Excessive I&I drives flooding and SSOs.
Best-case project: Milwaukee Metropolitan Sewer District conducted comprehensive I&I reduction programs, lowering wet-weather overflows by 30%.
Citation: U.S. EPA CMOM Self-Assessment Checklist
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7️⃣ Plan and Prioritize Based on Risk and Consequence
Core idea: Allocate resources based on impact severity.
Best-case project: Philadelphia Water Department applied risk-based prioritization for sewer rehabilitation, improving flood management.
Citation: WEF Collection Systems Committee, 2014
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8️⃣ Integrate Wet-Weather Performance into Routine O&M
Core idea: Make wet-weather readiness part of normal operations.
Best-case project: Seattle Public Utilities integrated storm triggers into daily O&M, enhancing hydraulic performance during heavy rainfall.
Recent support: Integrated stormwater-wastewater management strategies improve urban resilience (Grigg, 2024)
Citation: U.S. EPA CMOM Guide, 2005
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9️⃣ Preventive Maintenance and Cleaning
Core idea: Routine cleaning preserves hydraulic capacity.
Best-case project: Boston Water & Sewer Commission implemented risk-based cleaning and FOG control, reducing SSO frequency.
Citation: WEF, MOP 7
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🔟 Monitor, Review, and Learn from Events
Core idea: Learn from flooding or SSO events.
Best-case project: Tampa Bay Water conducts post-event reviews for all SSOs, updating procedures and models accordingly.
Citation: U.S. EPA Region 4 Guidance, 2006
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1️⃣ Link CMOM to Business and Strategic Plans
Core idea: Align CMOM with organizational goals.
Best-case project: San Francisco Public Utilities Commission integrated CMOM KPIs into corporate strategic plans, securing funding and leadership support.
Recent support: Investment priorities for sewer and stormwater systems are increasingly informed by the EPA CWNS 2024 survey.
EPA: Achieving Clean Water Act Goals Requires $630 Billion+ Over Next 20 Years, Jun 5, 2024 | Funding & Financing, Spotlight (EPA CWNS, 2024)
Citation: WEF, CMOM Operation & Maintenance Reference Guide
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2️⃣ Engage Staff Through Training and Organizational Culture
Core idea: Skilled and motivated staff ensure effective CMOM.
Best-case project: Denver Water implemented comprehensive staff training with wet-weather drills, improving proactive responses.
Citation: WEF, Technologies for CMOM Activities
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3️⃣ Prepare for Emergencies and Communicate Transparently
Core idea: Formal emergency plans and clear communication are essential.
Best-case project: New York City DEP established SSO and flood response communication protocols, enhancing public safety and awareness.
Citation: U.S. EPA CMOM Guide, 2005
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📚 References
• Ge, J., Li, J., Qiu, R., Shi, T., Huang, Z., Liu, Y., & Yuan, Z. (2024). Identifying periods impacted by sewer inflow and infiltration using time series anomaly detection. Water Research X, 25, 100278.
• Grigg, N. S. (2024). Stormwater Management: An Integrated Approach to Support Healthy, Livable, and Ecological Cities. Urban Science, 8(3), 89.
• Muttil, N., Nasrin, T., & Sharma, A. K. (2023). Impacts of extreme rainfalls on sewer overflows and WSUD-based mitigation strategies: a review. Water, 15(3), 429.
• U.S. EPA, 2025. Smart Sewers Program.
• U.S. EPA, 2024. Clean Watersheds Needs Survey (CWNS).
• U.S. EPA, WEF MOP 7, CMOM Guides

Figure referenced: Focused Field Investigations for Sewer Condition Assessment with EPA SSOAP Toolbox, NASTT 2014 No-Dig Show, Srini Vallabhaneni et al., 2014

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