During heavy rains or other wet weather events, runoff combined with wastewater can overwhelm aging, outdated sewer systems. In North America, most of the combined systems are concentrated in older communities in the northeastern United States and around the Great Lakes and St. Lawrence River. Tetra Tech is helping to upgrade water infrastructure systems where the need is greatest.

Tetra Tech provides a complete range of services to address combined sewer overflow (CSO), including planning, engineering design, and construction-phase services. Tetra Tech understands that each community has a unique situation and works to craft the best solution to address specific consent decree requirements and other local needs.

In addition to traditional CSO control methods, Tetra Tech has implemented numerous real-time control systems, from the simplest form of local reactive control to system-wide predictive and centralized decision-support systems with online optimization.


  • Hydraulics and hydrology services
  • Watershed management and green infrastructure approaches
  • Modeling
  • Flow reduction (sewer separation and targeted elimination)
  • In-system storage (basins, tunnels)
  • Engineering design services
  • Combined sewer system upgrades
  • Sanitary sewer system upgrades
  • Instrumentation and controls
  • High-rate treatment and disinfection
  • Real-time control (RTC) systems
  • Long-term control plans

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