I/I Reduction Prioritization Under a Consent Order
A utility under a consent order must cut peak wet-weather flow by 25% within three years. Flow monitoring shows dry-weather flow of 0.35 m3/s and a peak of 1.40 m3/s during a 2-year storm. Smoke testing and CCTV identified: 12 km of vitrified clay pipe with joint offsets and root intrusion, 4 km of older concrete pipe with sags and cracks, 60 manholes with deteriorated frames and covers in a floodplain, and 800 private laterals of unknown condition. The budget allows rehabilitation of roughly 5 km of main and 30 manholes in year one. The utility has no private-property ordinance and limited right-of-entry authority.
1. Recommend how to allocate the year-one rehabilitation budget to maximize peak-flow reduction, with justification.
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Prioritize by cost per unit of peak-flow removed, not by pipe length or age alone.
- Manholes in the floodplain first: 60 deteriorated frames/covers are a direct, low-cost pathway for inflow during flooding. Rehabilitating 30 of the worst (highest flood exposure, visible defects) likely removes inflow at far lower cost per liter than mainline lining.
- Then target mainline segments with the highest measured infiltration/inflow, using flow isolation (night-flow and storm-response) to rank sub-basins. Lining 5 km indiscriminately wastes budget; concentrate on the segments where CCTV defects (sags, cracks, offset joints) coincide with high measured response.
- Root intrusion and joint offsets in the 12 km of clay are candidates for lining or joint sealing, but only where flow data show they contribute.
Private laterals are the wild card: 800 laterals of unknown condition could be a large share of the peak. Without an ordinance, the utility cannot compel repair, so year one should include lateral inspection in the highest-response sub-basins to size the problem and build the case for an ordinance in year two.
2. The 25% reduction target looks unlikely with mainline work alone. What program and regulatory strategy should management pursue?
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Mainline and manhole work alone rarely reaches a 25% peak reduction when private inflow is unquantified. Management should:
- Quantify the private-lateral contribution by inspecting a statistically valid sample in the highest-response basins; use the result to project basin-wide inflow.
- Pursue a private-property inflow ordinance and right-of-entry program, with grants or low-interest loans to reduce resident resistance. This is the highest-leverage action if laterals prove significant.
- Negotiate with the regulator: present the flow data, the quantified lateral contribution, and a phased schedule that front-loads high-yield work. Consent orders are commonly amended when the utility demonstrates a data-driven plan; asking for an extension without data invites denial.
- Continue flow monitoring to measure the effect of each project, so year-two spending follows demonstrated results.
Document everything: the regulator will judge the utility on whether it is measuring, prioritizing, and adapting, not only on whether the number is hit on the original schedule.