Comprehensive Strategies To Reduce Inflow And Infiltration In Sanitary Sewer Systems

Reducing inflow and infiltration (I&I) requires a systematic approach beginning with source identification through flow monitoring and smoke testing, followed by targeted structural rehabilitation of defective pipe segments and manholes. Successful programs typically target the removal of 15% to 30% of peak wet-weather flow by addressing private-side defects and public-side infrastructure decay simultaneously.

Preliminary Assessment and Diagnostic Planning

Before initiating physical remediation, municipal engineers must establish a baseline for the hydraulic load. Inflow is defined as direct stormwater entry through illicit connections like roof leaders, cellar drains, and manhole covers, while infiltration involves groundwater seeping into the system through cracks, joints, and root-damaged pipe walls.

• Diagnostic Equipment and Materials:
  
  
  • Ultrasonic flow meters for accurate diurnal and peak flow measurement.
  • Digital CCTV inspection crawlers (NASSCO PACP-certified) for internal pipe assessment.
  • Smoke testing blowers and non-toxic smoke candles for detecting illicit cross-connections.
  • Dye-water testing equipment for tracing specific inflow pathways.
• Regulatory and Standards Requirements:
  
  
  • Adherence to NASSCO (National Association of Sewer Service Companies) PACP (Pipeline Assessment Certification Program) standards.
  • Compliance with EPA’s Capacity, Management, Operation, and Maintenance (CMOM) guidelines.
• Benchmarking and Duration:
  
  
  • Initial flow monitoring campaigns should span a minimum of 60 to 90 days to capture sufficient wet-weather event data.
  • Budgetary allocation often requires $5,000 to $15,000 per mile of sewer line for comprehensive assessment and professional data interpretation.

Systematic Execution for I&I Mitigation

Step 1: Flow Characterization and Sub-Basin Isolation

Utilize a network of flow meters strategically placed at manholes to segment the collection system into manageable sub-basins. By comparing the calculated dry-weather flow against real-time wet-weather spikes, technicians can isolate the specific sewer reaches contributing the highest volume of extraneous water.

Step 2: Source Identification via Smoke and CCTV

Deploy smoke testing crews during dry weather to force air into the system. Observe where smoke exits—such as yard drains, catch basins, or residential downspouts—to identify direct inflow sources. Simultaneously, perform CCTV inspections of identified reaches to locate structural defects like offset joints, circumferential cracking, or lateral connection failures that facilitate groundwater infiltration.

Step 3: Trenchless Rehabilitation of Public Mains

Rather than traditional open-cut excavation, employ Cured-in-Place Pipe (CIPP) lining. This process involves inserting a resin-impregnated felt liner into the existing pipe and curing it with steam or hot water. This eliminates pipe joints—the primary source of infiltration—and effectively seals cracks and voids without disrupting surface traffic.

Pro-Tip: Ensure lateral connections are reinstated using robotic cutters after the CIPP liner has fully cured to prevent long-term blockage and ensure seamless hydraulic flow.

Step 4: Manhole and Lateral Sealing

Manhole structures frequently leak at the frame-and-cover or the chimney section. Apply cementitious or epoxy-based structural liners to the interior chimney and utilize inflow dishes (inserts) beneath the manhole cover to prevent runoff from entering the system. Private-side lateral rehabilitation should involve cured-in-place spot repairs if the lateral is the source of significant groundwater intrusion.

Comparative Analysis of I&I Remediation Technologies

Common Field Failures and Remediation Protocols

• Failure: Incomplete Lateral Reinstatement
  
  
  • Root Cause: Improper robotic cutter calibration or insufficient curing of the CIPP liner leading to "dimpling" at the lateral connection.
  • Actionable Fix: Perform a post-reinstatement CCTV inspection immediately following the cut. If obstructions exist, utilize a secondary precision cutter to smooth the transition.
• Failure: Groundwater Migration Post-Lining
  
  
  • Root Cause: Failure to perform pre-lining grouting; groundwater pressure behind the liner creates a "blowout" or wrinkle during the curing process.
  • Actionable Fix: Always perform high-pressure water testing of joints and utilize pressure-grouting in segments with high hydrostatic pressure prior to liner installation.
• Failure: Inflow Re-entry via Replacement Material
  
  
  • Root Cause: Using low-quality gaskets in replacement manhole frame seals or pipe collars.
  • Actionable Fix: Mandate the use of ASTM C923-compliant watertight manhole flexible connectors to ensure a long-term seal against groundwater ingress.

Frequently Asked Questions

What is the difference between inflow and infiltration?

Inflow is stormwater that enters the system directly from sources like manhole covers and roof drains during rain events, causing immediate flow spikes. Infiltration is the slow, continuous leakage of groundwater through cracked pipes and loose joints into the sewer system.

Why is CIPP preferred over open-cut excavation?

CIPP is a trenchless technology that avoids the high costs and public disruption associated with digging up streets. It provides a structural, jointless pipeline that prevents future root intrusion and seals off infiltration points effectively.

How do I prioritize which pipes to fix first?

Prioritization is based on the data gathered from flow monitoring. Pipes located in sub-basins with the highest "I&I-per-inch-diameter-mile" metrics should be addressed first to achieve the greatest return on investment for peak flow reduction.

Can private property owners be held responsible for I&I?

Yes, many municipalities have ordinances requiring homeowners to disconnect illegal downspouts and sump pumps from the sanitary sewer. Enforcing these codes is a critical component of a comprehensive I&I reduction program.

Advancing Your Infrastructure Resiliency

Implement a data-driven sewer management program today to minimize treatment costs and prevent sanitary sewer overflows. Contact our engineering team for a professional audit and tailored rehabilitation strategy for your collection system.