Cobb County-Marietta Water Authority’s Quarles Water Treatment Plant Raw Water Storage Reservoir has a volume of approximately 27.8 million gallons. Approximately 14 million gallons (69,000 CY or 50.4% of the reservoir) had filled with water treatment plant residuals from years of settling river sand from influent pumping and alum precipitant returned from recycle backwash operations at the water treatment plant.
The project included rehabilitating the reservoir by removing 69,000 CY of accumulated material and rehabilitating existing intake and outlet structures. Due to water plant operating constraints, the project schedule required that all material be removed in approximately five months. Two sister subcontractors teamed up to meet this schedule. The solids removal equipment included two dredges, four Plate and Frame Presses and four Belt Presses.
In-place solids content in the reservoir ranged between 12-20%. The Plate and Frame Presses were able to increase the cake solids to 65% with no polymer addition while the Belt Presses were able to achieve 45% with the addition of polymer. The dewatering equipment was able to reduce the in-place volume by a factor of almost four, proportionally reducing trucking costs and tipping fees.
CEC arranged the project to place the burden on the contractor to determine the most advantageous combination of dewatering and trucking. CEC did not specify solids content requirements; the only requirement was that material pass a paint filter test. CEC structured the method of payment for material removal based on restored capacity to the reservoir. CEC believed that it was in the Owner’s best interest to define the scope of the project in terms of restored capacity and not dry tonnage removed or some other arrangement.
Click here to read an article, published in March 2011 in Public Works magazine, that discusses CEC's Quarles Raw Water Reservoir Rehabilitation project.