Prior to November 2000, the Gwinnett County Department of Public Utilities (DPU) operated seven small- to mid-capacity wastewater treatment plants that were collectively treating more than 48 mgd. With population increases averaging about 18,000 per year, Gwinnett County is one of America's fastest-growing areas, and treatment capacity needs have been steadily increasing by two mgd per year since the early 1990s.
In 1996, a team of six engineering and construction firms consisting of Metcalf & Eddy, Piedmont, Olsen & Hensley, Moreland Altobelli, Jordan Jones & Golding, CH2M Hill and Precision Planning formed the Gwinnett Water Partnership of 2000 and launched plans for a new 20-mgd wastewater treatment plant that would have the potential for future capacity expansion. The design included two enclosed anaerobic egg-shaped digesters for solids degradation, making Gwinnett County the first in the southeast to employ this innovative new trend in wastewater treatment.
The egg digesters incorporate a circulation process in which the solids are removed from the tank, sent through a heater and then returned to the tank for further treatment. Red Valve manual and control pinch valves are used for throttling control and maintenance isolation. The pinch valve's abrasion-resistant elastomer sleeve makes it ideal for handling the thick sludge present in this stage of the process, and the enclosed design keeps the material contained. Several pinch valves also control flow into the granular-activated carbon adsorbers, an earlier stage of the process.
Egg digesters feature superior circulation benefits over a conventional mixing tank by reducing surface areas and eliminating dead spots. Mixing occurs throughout the entire tank, preventing settling and stratification.
Even before the facility went online in November 2000, the partnership was on site designing a 40-mgd expansion. Start-up for the expansion is planned for 2002.
Check Valves
The plant's secondary clarifiers were built on the same plane as three raw-water storage tanks, and the partnership was concerned that in the event of a tank overflow, the untreated waste would contaminate water in the clarifying tanks. To prevent the problem, a 48-inch Tideflex® InLine check valve from Red Valve was installed to prevent unwanted flow into the clarifiers. Pressure on the outside of the valve's patented Duckbill® seals the valve drop-tight, even around entrapped obstructions.
Outfall Diffusers
At the Crooked Creek outfall where the re-use water from the Gwinnett plant is currently being discharged, a multi-port diffuser was installed to better disperse the final effluent and protect water quality in the Chattahoochee River downstream. Designed by Montgomery Watson, the system includes 22 six-inch Red Valve Tideflex® diffuser valves. The variable-orifice design of the diffusers increases the jet velocity of the discharge to enhance mixing and dilution. The flattened-plume shape of the discharge stream created by the design also aids in dispersion.
Tideflex diffusers offer the added benefits of a check valve by preventing backflow into the outfall pipe. This stops the intrusion of sand, silt and mud that can greatly reduce efficiency and eventually clog the system.
The 40 mgd plant will discharge its effluent into a new outfall at Gwinnett County's Lake Lanier, where an even larger Tideflex® diffuser system is being designed. The new system, which incorporates a 96-foot multi-ported pipeline, will help maintain the optimum water quality that has already allowed Gwinnett to obtain a generous water permit. Gwinnett's current permit allows the county not only to discharge 40 mgd, but also to retrieve that same amount to treat and dispense as finished drinking water.
Booth 3741
