Infiltration/Inflow Investigation
Your collection system experiences a significant increase in flow during wet weather. The treatment plant reports flows reaching 15 million gallons per day (MGD) during storms, compared to a dry weather flow of 8 MGD. The collection system has a permitted capacity of 12 MGD at the plant. You suspect infiltration and inflow (I/I). You have flow monitors at key locations. The system serves 20,000 customers. The area has a mix of older clay tile sewers and newer PVC. You need to develop a plan to identify and reduce I/I.
1. Outline a phased approach to identify and prioritize I/I sources.
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Phase 1: Flow monitoring and rainfall analysis. Use existing flow monitors to identify sub-basins with high wet-weather response. Compare flows to rainfall data to determine response time and magnitude. Phase 2: Smoke testing and CCTV inspection in high-priority areas. Smoke testing identifies illegal connections and broken pipes. CCTV identifies defects like cracks, root intrusion, and offset joints. Phase 3: Quantify I/I using flow isolation (e.g., weirs or temporary flow monitors) in subareas. Phase 4: Prioritize rehabilitation based on cost-effectiveness and severity. Consider public/private sources (e.g., roof drains, sump pumps).
2. What are some common sources of I/I in older clay tile sewers, and what rehabilitation methods would you consider?
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Common sources: deteriorated pipe joints, cracks, root intrusion, collapsed pipes, illegal connections (e.g., storm drains, foundation drains), and manhole defects. Rehabilitation methods: (a) Cured-in-place pipe (CIPP) lining for structural defects; (b) Grouting for joint sealing; (c) Pipe bursting or sliplining for larger defects; (d) Manhole rehabilitation (e.g., lining, sealing); (e) Public education and removal of illegal connections. Prioritize based on cost per gallon of I/I removed.