A 25-million-gallon reservoir serves as an equalization basin between the Chattahoochee River and CCMWA's Quarles Treatment Plant, and it provides emergency storage capacity. Managers planned to use the asset to ensure service continued uninterrupted once they began installation work at the Wyckoff plant.
“Our treatment plants and reservoir were designed to provide not only adequate capacity, but also redundancy,” explains Process Engineer Thomas Ginn, PE. “If we need to take one of the plants off line, we can still meet the water needs of the community.”
Before beginning installation work at the Wyckoff plant, CCMWA engineers determined that greater reservoir capacity at the Quarles facility was needed to ensure they could meet water demand while portions of the Wyckoff plant were temporarily off line. Reservoir dredging had originally been scheduled as part of a 2013 – 2014 maintenance project, but priorities were changed due to the new EPA standards.
The reservoir had last been dredged in 1978 and was found to contain 68,000 cubic yards of sediment and treatment residuals. If left unaddressed, the material could have made its way into the plant, reducing efficiency and potentially compromising water quality.
The compressed time frame meant doubling the amount of equipment normally required for dredging and dewatering to four mobile belt presses and four recessed chamber presses. Mobile belt presses dewater sediment by moving it through a series of rollers that squeeze water out, leaving a “cake” of dewatered material. Recessed chamber presses use hydraulic plates to force out water. Combined, the presses could process approximately 8.5 tons of sediment per hour.
An auger dredging device on a floating platform conveyed the sediment from the bottom of the reservoir into mixing tanks. The sediment was then pumped into the presses where it was dewatered. The cake produced by the dewatering process contained alum, so the dewatered sediment had to be hauled 45 miles to the nearest Subtitle D landfill facility.
The treatment plant and reservoir are surrounded by homes, and there is just one plant entrance. To meet their deadline, managers had to schedule up to 50 trucks making four round-trips daily from the plant to the landfill.
“This project required extensive planning and coordination,” says Andrew Lovejoy, PE, of Civil Engineering Consultants. “It takes a lot of expertise to develop and execute a big project like this.”
The water authority had five bidders, and all of them listed the same two companies in their bids as subcontractors: Bio-Nomic Services and Metropolitan Environmental Services. In addition to dredging, the contract included maintenance on the banks and sluice gates.
“In four months we've had one complaint, and it was about dust being stirred up by the trucks,” Lovejoy added. “To have only one small complaint is very commendable considering the project's size.”
Ginn estimates the reservoir will save $400,000 on electricity this summer because plant operators can shift pumping from daytime peak to nighttime off-peak hours. He also plans to dredge every five years.
“This will decrease the potential for water quality problems or process issues at our treatment plants,” he says. “It's a very good, proactive preventive maintenance step.” Most gratifyingly, the authority will meet its Stage 2 deadlines ahead of time.
— Gibbs (firstname.lastname@example.org) is a freelance writer in Memphis, Tenn.