In 2010, Virginia’s Washington County Service Authority (WCSA), was unable to withdraw enough water from the Middle Fork of the nearby Holston River to meet its residents’ needs. The county’s initial withdraw permit was 4.6 mgd, but in order to increase it to 12 mgd, a new raw water intake had to be constructed at the confluence of the Middle and South forks of the river.
Located more than 3 miles away, the river posed some challenges. The location is home to historically sensitive ground and water with high turbidity. Designers from The Lane Group created a submersed intake structure served by a directionally drilled gravity line to a wet well outside the historically sensitive area on the South Fork, where the turbidity was lower.
The existing plant was enclosed by two rivers, a highway and steep topography, so construction of a new 12-mgd facility within this property was a feat. To combat the geography, designers added tube settlers and adsorption clarifiers, along with five new filters, bringing the plant’s total to eight.
Because the water was being pumped across an elevation much higher than the plant, there was an excess of water pressure from the new intake and pump station. The pressure had to be lowered before treatment. Instead of using a simple pressure reduction process, the design team proposed using Francis turbines in a new energy recovery station to generate electricity for the plant.
After pilot testing under cold weather, cold water and high turbidity conditions, the design was deemed a success.
Ultimately, the 12-mgd project with a $26 million budget could actually handle at least 15 mgd and was completed under budget. As of October 2012, the project was 80% complete and is expected to be fully operational by February 2013.
“We couldn’t be happier,” said Robbie Cornett, general manager of the WCSA.