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Combined Sewage Storage Tunnel (CSST)

Operational information

The Combined Sewage Storage Tunnel (CSST) is connected to the most active, high-volume combined sewage overflow (CSO) locations within its service area, and functions as an underground, linear storage facility. It intercepts and stores surface runoff and wastewater (combined sewage) until it may be treated at the Robert O. Pickard Environmental Centre and returned safely to the Ottawa River.

A graphic map showing the general location of the two interconnected tunnels, as well as associated chambers, connections, and facilities.

Planning, design and construction of the CSST spanned more than a decade. Now in operation, the CSST will:

  • Significantly reduce the frequency of CSOs to the Ottawa River
  • Reduce the volume of CSOs to the Ottawa River by up to 43,000 m3 per event – or approximately 18 Olympic-sized swimming pools
  • Reduce the risk of basement flooding for approximately 7,000 residential properties in the north end of the Glebe and in Centretown
  • Twin a critical downtown sanitary sewer (the Interceptor Outfall Sewer, or IOS), allowing flow to be diverted for improved inspections and maintenance on this aging infrastructure

The CSST and related measures help ensure Ottawa’s compliance with provincial targets for CSO control. 

CSST planning, consultation and construction