How Louisville Cut CSOs by 1 Billion Gallons per Year

Louisville, Kentucky serves a metropolitan area of over 630,000 people with a combined sewer system that, like many older cities, was overwhelmed during heavy rainfall. CSOs were sending untreated sewage into the Ohio River and local waterways — a violation that led to a federal consent decree.

Louisville's original Long-Term Control Plan (LTCP) budgeted $200 million for traditional infrastructure — tunnel storage, treatment plant expansion, and sewer separation. But when the city deployed system-wide real-time controls, they achieved the required overflow reductions for just $83 million, saving $117 million.

The Challenge

Louisville MSD (Metropolitan Sewer District) manages over 3,800 miles of sewer mains — one of the largest combined sewer systems in the country. During wet weather, the system routinely exceeded capacity at multiple points, resulting in billions of gallons of annual CSOs.

The consent decree required dramatic overflow reductions with specific volume targets and deadlines. The traditional engineering approach — the one every consultant recommended — called for massive deep tunnel storage systems similar to those built in Indianapolis, Milwaukee, and other large cities.

The Smart Approach

Louisville took a phased approach:

Phase 1: Comprehensive Monitoring

The first step was understanding the system's actual behavior. Louisville deployed sensors throughout the network to measure flow rates and water levels during storm events. This data revealed something critical: the system had significant unused capacity that wasn't apparent from engineering models alone.

Phase 2: Real-Time Control

Based on the monitoring data, Louisville implemented system-wide RTC — automated gates and control structures that dynamically redistribute flow during storms. The system maximizes use of available pipe capacity, temporarily stores water in underutilized parts of the network, and times flows to match treatment plant capacity.

Phase 3: Targeted Gray Infrastructure

Where RTC alone wasn't sufficient, Louisville added targeted gray infrastructure — but at a fraction of the originally planned scale. Smart monitoring data identified exactly where additional capacity was needed, eliminating waste on unnecessary construction.

The Results

• CSO reduction: 1 billion gallons per year of prevented overflows
• Cost savings: $117M saved vs. original $200M plan (58% reduction)
• Consent decree compliance: Meeting all required milestones on schedule
• Operational efficiency: Better visibility into system performance during all conditions
• Ongoing optimization: System continuously improves as more data is collected

Key Lessons

Scale Matters — But Not How You'd Expect

Louisville proved that system-wide RTC works at large scale. With 3,800 miles of sewer mains, this is one of the largest smart sewer deployments in the country. The complexity didn't prevent success — if anything, larger systems have more opportunities for optimization because they have more underutilized capacity.

Phased Deployment Reduces Risk

By starting with monitoring (lower cost, lower risk) before committing to RTC and infrastructure, Louisville validated their approach at each stage. If the data had shown RTC wouldn't work, they could have pivoted to traditional approaches without wasting resources.

Data Drives Better Infrastructure Decisions

When Louisville did invest in gray infrastructure, the investments were precisely targeted based on actual monitoring data — not engineering assumptions. Every dollar of capital spending was validated by real performance data.

See all tracked deployments on our City Tracker or read the South Bend case study.