Identifying and mitigating inflow and infiltration (I&I) in our aging wastewater systems is one of the most pressing challenges facing civil engineers, municipalities, and land developers today. With excess stormwater and groundwater finding its way into sanitary sewer systems, the consequences can range from increased treatment costs to environmental hazards and basement flooding.
In Part 1 of this series, we explored the fundamental aspects of I&I investigation methodology, highlighting the importance of detection and key approaches used in the early stages of an investigation.
This installment, Part 2, focuses on field investigation strategies designed to identify inflow and infiltration sources. We will walk through categorized inspections (Group 1, Group 2, and Group 3), highlight techniques like lot drainage inventory, fog testing, dye testing, and manhole inspections, and provide actionable insights for identifying I&I cases effectively.
Field Inspection Strategies to Identify Inflow and Infiltration Sources
Inflow and infiltration inspection starts with detailed fieldwork to identify where stormwater and groundwater are entering the sanitary sewer system. The strategies can be grouped into three main categories based on their scope and complexity:
Group 1 Inspections
Group 1 inspections are a cost-effective way to identify engineered stormwater features that are connected to the sanitary sewer. These inspections often reveal issues with low remediation costs while significantly reducing I&I.
1. Private Lot Drainage Inventory
The first step to tackle I&I is identifying private lot drainage systems that improperly discharge into the sanitary sewer system. Through detailed surveys, every lot in the study area is assessed for its visible drainage features like downspouts, cleanouts, and catch basins, along with their hydraulic discharge points. Most downspouts are confirmed to drain to the surface, but some may be found without visible surface discharge, requiring further investigation.
These data points provide a critical foundation for subsequent investigative steps, leveraging tools like Civica Infrastructure Inc.’s DataCurrent for geo-referenced data collection.
2. Fog Testing
When drainage features lack clear discharge points, fog testing becomes an essential part of the investigative process. This technique isolates a stretch of 200 meters of sanitary sewer using floating blockers, injecting a safe, visible fog into the system to trace any direct or indirect connections. Technicians can then identify sources such as foundation cracks, downspouts, and sumps that release fog, marking them as I&I contributors.
This accessible yet effective technique not only identifies direct inflows but also enhances public engagement, as stakeholders are informed well in advance of the testing timeline.
3. Dye Testing
To validate the hydraulic connectivity identified during fog testing, dye testing is conducted. Non-toxic, environmentally friendly dye is introduced into suspected drainage systems, confirming links to the sanitary sewer. The vivid yellow or green dye helps pinpoint otherwise hidden sources of I&I.
While fog testing can sometimes produce false positives due to trench migration, dye testing ensures accurate confirmation.
4. Manhole Inspections
Manhole inspections are an essential visual assessment to detect potential I&I sources. The process involves inspecting manholes for any signs of inflow through pickholes, cracks, or improper connections. Ponding depth above manhole covers is also measured to gauge I&I risks during wet weather, particularly in areas with high stormwater presence.
Manhole inspections provide critical insights and can address weaknesses in the sewer system that could otherwise go unnoticed during surface-level analyses.
Group 2 Inspections
Group 2 inspections focus on I&I detection techniques to identify additional inflow and infiltration in buried sewer systems during wet weather. These techniques target entry points such as service laterals, foundation drains, and groundwater infiltrating damaged pipes or manholes.
1. Subsurface Groundwater and Soil Assessments
Groundwater intrusion is a significant contributor to I&I challenges, particularly in areas with outdated infrastructure. Subsurface investigations assess groundwater levels and soil drainage properties, which can reveal how groundwater interacts with the sanitary sewer system. This step is particularly valuable during wet weather when infiltration is most prevalent.
2. On-Site Wet Weather Assessments
Conducting a wet weather inspection during rainfall is a powerful method for evaluating I&I under real-world conditions. Teams monitor surface runoff patterns, inspect manhole conditions, and assess system oversaturation during peak wet weather events. These on-site assessments provide valuable insights, helping engineers identify problem areas that static inspections might overlook.
Group 3 Inspections
After Group 2 inspections, further investigations are conducted to identify key I&I contributors. This leads to the start of Group 3 inspections to expand on the earlier findings.
1. Storm Sewer Exfiltration Testing
Another often-overlooked source of I&I comes from improper storm sewer connections to sanitary systems. Exfiltration testing can detect these cross-connections by tracing how stormwater leaks into unrelated sanitary sewer infrastructures.
2. Micro-Monitoring Along Targeted Areas
Localized sewer flow monitoring, known as micro-monitoring, is conducted to detect anomalies in flow patterns. For example, analyzing sewer lines along targeted roadways within study areas can help target locations with abnormal inflows during specific time periods, such as rainstorms.
3. Lot Grading Inspections
Surface grading plays a direct role in how stormwater interacts with private lots and public sewers. Lot grading inspections assess whether surface runoff is directed toward the sanitary system due to poor landscaping or grading practices. Correcting these issues can significantly reduce unnecessary inflows.
4. Overland Storm Drainage
Finally, overland storm drainage assessments focus on understanding broader stormwater flow patterns. Improper diversions or blockages in stormwater routing can lead to excess stormwater infiltrating sanitary sewer systems.
Contact Civica to Identify Inflow and Infiltration Sources
Successfully detecting inflow and infiltration requires an integrated approach that combines cutting-edge technologies like fog and dye testing with a deep understanding of local sewer dynamics. Civica specializes in identifying and mitigating I&I to ensure that your sanitary system operates efficiently—a crucial component in effective wastewater management.
Contact Civica to learn how our advanced methodologies, such as CCTV sewer inspection, sewer smoke testing, and comprehensive lot surveys, can transform your understanding of I&I challenges. And stay tuned for Part 3, where we will explore the quantification and mitigation strategies for inflow and infiltration. From capturing real-time flow data to implementing long-term fixes, the next part will deliver actionable insights to help you achieve sustainable wastewater management.
Learn More At –
Methodology of Sanitary Maintenance Hole Inflow and Infiltration
Methodology of Sanitary Maintenance Hole Inflow and Infiltration
Role of Flow Monitoring in Inflow and Infiltration (I&I) (Part 1)
Flow Monitoring and Its Role in Inflow and Infiltration (I&I) (Part 2)
Case Study: Inflow and Infiltration Reduction of Pilot Areas (Part 1)
Case Study: Inflow and Infiltration Reduction of Pilot Areas (Part 2)
Inflow and Infiltration and Wet Weather Flow Reduction of Pilot Areas (Part 3)
