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Analysis of Future Vulnerability

• Simulations of the recent past suggest that sewer overflow volumes are up to 15 percent higher than previously estimated.
• Future rainfall, population, and land-use changes could further increase overflow volumes.

Strategies to Reduce Future Overflow

• Expanding wastewater treatment plant capacity or cleaning existing deep interceptors could represent low-regret, near-term options.
• Large-scale investments in source reduction, or combining source reduction with treatment expansion and/or interceptor cleaning, could help reduce sewer overflow, but with a wide range of uncertainty regarding cost-effectiveness and relative strategy performance.
• None of these combined strategies, even including substantial source reduction investments, fully eliminates sewer overflows in current or plausible future conditions.

GSI Cost-Effectiveness

• GSI strategies, evaluated in isolation, yield poor cost-effectiveness for overflow reduction under commonly used rainfall, capital cost, and GSI performance assumptions.
• GSI and other source reduction strategies are more cost-effective in higher rainfall scenarios, and could provide hedging value against future climate change.