The City of Palmetto in south Fulton County, Georgia, supplies drinking water to over 4,000 residents. This water comes from two sources—into the “Palmetto pressure zone” from the City’s own water treatment plant, and into the “Coweta pressure zone” from neighboring Coweta County. To support growth, however, the City needed to expand the size of the Coweta zone, shifting demands out of the Palmetto zone. To complete this shift, the City’s 250,000-gallon elevated Carlton Road storage tank would need to be part of the Coweta zone.
One of the primary concerns with sending Coweta water through the Carlton Road storage tank was that it would increase water age. As water ages in a distribution system, total trihalomethanes (TTHMs)—byproducts of the chlorination process—increase information, which can trigger a violation if levels are too high.
Through modeling and historical data analysis, Gresham Smith estimated that the City would need to reduce chloroform, the most prominent species of TTHMs in the distribution system, by 60% to maintain confident compliance. To accomplish this, our team implemented an aeration solution in both of the City’s elevated storage tanks to volatilize the TTHMs. The results of performance testing exceeded project expectations, achieving over 68% reduction in one tank, and more than 89% in the other.
gallons of stored water treated
proven disinfection reduction
Competing on Common GroundTo ensure competitive equipment selection, Gresham Smith crafted a performance-based specification for the aeration systems, meaning that two very different types of equipment could compete on common ground.
The City received bids based on both systems and was able to choose the most cost-effective option. Both systems were specified for longevity and utilize NEMA 4X enclosed VFDs, dampening pads on the aeration blowers to reduce noise and vibration, and a stainless steel control panel.
Working in Tight CorridorsBoth specified systems were sized to fit through the 24-inch square hatches available on the domes of the elevated tanks. Because the selected equipment is free-floating on the water surface—with an inlet near the tank fill pipe to minimize short-circuiting—we were able to carry out the installation without draining the tanks.
A Customized DesignNumerous design criteria were taken into account, including tank volume, diameter and height; tank water levels; maximum estimated flow; maximum estimated fill rate; and historical TTHM levels by speciation. As a result, the City’s smaller 75,000-gallon tank employed a 10 HP, 240-V aerator, while the larger 250,000-gallon tank utilized a 15 HP, 480-V aerator.
The construction plan was also sequenced to allow the City to do in-tank maintenance concurrent with installation of the equipment to minimize interruption. Both aeration systems give the City peace of mind that as they grow, and as they encounter difficult testing conditions, they will fully comply with federal Disinfection Byproduct Reduction (DBP) regulations.
“Gresham Smith helped the City of Palmetto be proactive in meeting increasingly stringent requirements for drinking water. Their design allowed the City to safely augment our water system with the supply from a neighboring jurisdiction.”
–Mayor J. Clark Boddie, City of Palmetto