Cobb County is going deep underground again to solve anticipated wastewater treatment demands in the southwestern part of the county.

The county commissioners on Tuesday awarded $312 million in contracts to engineer and build a 5.5-mile tunnel that will drain about 40 percent of Cobb, an area generally south of Ga. 120 in West Cobb.

It will be the second tunnel for Cobb, which in 2004 opened a 9.5 mile tunnel in East Cobb.

The so-called South Cobb Tunnel will average 250 feet in depth as it runs south through bedrock from near the boundary with Douglas County and then east to a treatment plant on the Chattahoochee River near I-20.

The tunnel will allow the county to avoid replacing 16 miles of aging sewer lines and two pumping stations, said Steve McCullers, director of the Cobb County Water System, which provides wastewater treatment for Cobb and four of its six cities.

Work is expected to begin this summer and be completed in 2014.

"This is the largest project ever undertaken by the water system," McCullers said.