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CSO/SSO

During large storm events when combined sewer system capacities are stressed, combined sewer overflows (CSOs) can negatively impact the quality of receiving waters, such as rivers, lakes, and harbors.

Since the development of Combined Sewer Overflow control policies throughout North America in the 1990s, Mott MacDonald has assisted major urban clients in addressing combined sewer overflow issues. Our work has ranged from characterization studies, hydraulic modeling, to facility plans and designs to reduce the environmental impact of CSOs.

Mott MacDonald carefully balances the potential cost and triple bottom line criteria of CSO control alternatives with anticipated water quality improvements and regulatory compliance. We use holistic watershed-based approaches to integrate the potential grey and green abatement approaches into a sustainable basin solution. This approach drives the selection of control strategies for each unique CSO drainage area, which may include source control, stormwater best management practices, conveyance system real time controls, storage, collection, and/or treatment.

We have assisted clients in implementing source controls through public education programs, maintenance technologies, and stormwater controls to prevent contaminants from entering the sewer system.

Mott MacDonald has contributed to innovative water conservation programs that reduce flow in CSO areas. These programs encourage the use of low-flow plumbing fixtures, blue roofs, and rainwater capture and reuse. Our conservation approaches have encompassed community-enhancing techniques such as the creation of green corridors, street tree planting, and wetlands conservation.

Mott MacDonald assists clients in implementing the following CSO strategies:

  • Infiltration/inflow reduction
  • Treatment plant flow maximization and real-time control
  • Conveyance/trunk sewers
  • Trenchless technology implementation
  • Regulator screens
  • In-line and off-line storage tanks
  • High-rate primary treatment facilities (e.g. screening, vortex/swirl separation, chemical precipitation, and ballasted flocculation)
  • Storage and conveyance tunnels


CSO Storage, Pump Stations and Conveyance Tunnels

Mott MacDonald has designed and supervised CSO abatement systems throughout North America. We have helped our clients implement major construction programs with capital projects of more than $500 million.

From surface storage tanks and shallow connector tunnels to complex drop shafts, deep storage tunnels, and interceptors, Mott MacDonald has engineered hundreds of miles of large-diameter conveyance sewers and tunnels throughout North America and internationally. Our pumping stations are designed for draining storage tanks or tunnels and several exceed 100 MGD in capacity. To date, our projects have collectively reduced annual overflows by several billion gallons.

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