A tunnel is being constructed that will carry treated water waste from the Ashbridges Bay treatment plant back into Lake Ontario.

The project consists of 50 vertical pipes, also known as in-lake risers, which extend from the tunnel to the lake bed.

The city of Toronto's waste water infrastructure is in a constant state of update and rebuild. Ashbridges Bay is the city’s largest and oldest water treatment plant. When complete, this project will replace the existing outfall built 70 years ago.

On shore, a 14-meter wide shaft, 85 meters deep, was dug so that a tunnel boring machine (TBM) could be lowered. The TBM will then dig under the lake bed in a southerly direction to where the risers are currently being constructed.

The work on the risers is being performed on a platform that can be seen from the Leslie Street spit and Woodbine beach.

"The risers are about one meter in diameter, stainless-steel pipes that will have ports or nozzles on the end, through which the effluent will be released into Lake Ontario.,” City of Toronto Chief Engineer Michael D'Andrea told CTV News Toronto.

The waste will flow from the plant to the shaft, through the tunnel and up through the risers.

"We've constructed 12 risers, they're installed and we are working on another eight risers," D'Andrea said, "Once that operation ceases this winter, we're basically on hold until the springtime in April, where we then begin the operation again."

Part of the design, D'Andrea explained, is to disperse the treated water waste from the risers over a broad area. He adds that when the project is complete, "you would have no knowledge that there were any infrastructure under the lake.”