Construction of the fifth and final phase of replacing the deep tunnel system that collects raw sewage and delivers it to Charleston Water System’s wastewater treatment plant will begin shortly.

The water system’s Board of Commissioners recently approved a $50.8 million contract with Southland Renda JV to build what is being called the new West Ashley Tunnel.

The work is not for street level storm water.

The project involves drilling a 1.6-mile-long, 120-foot-deep and 8-foot-diameter tunnel from Albemarle Road, under Wappoo Creek, to the Plum Island Wastewater Treatment Plant off Harborview Road, the water system said.

Along with a new pump station, piping and other improvements, the project is expected to take about 30 months.

The new tunnel replaces an existing tunnel, which dates to the 1960s, that carries wastewater from West Ashley, parts of Johns Island, and the towns of Hollywood, Ravenel and Meggett, the company said. The old line is both deteriorated and too small for current peak wastewater flows.

So far, three sections of the original tunnel system have been replaced covering the east side of the Charleston peninsula, another on the west side and a third under the harbor. A fourth section was added to collect wastewater from Daniel Island.

The West Ashley Tunnel is funded by a 2010 bond issue and supported by current sewer rates, the company said.