A plant team and consultants in Nashville keep two surface water treatment plants running during chemical feed system upgrades.
The Omohundro and K. R. Harrington conventional surface water treatment plants that provide drinking water to metropolitan Nashville recently received new chemical feed systems.
Both systems were installed without interrupting either plant’s operations and have enhanced energy efficiency and chemical feed accuracy, according to Gilbert Nave, assistant director for water operations with Metro Water Services in Nashville.
The feed systems at both plants were designed with a single brand of peristaltic feed pumps, helping to simplify maintenance, reduce parts inventories, and make it easy for staff to move between plants without requiring additional training on the chemical feed equipment.
The Omohundro facility along the Cumberland River is on the National Register of Historic Places. Its George Reyer Pump Station, which moves raw and finished water, was built in 1889, and the original chemical feed, sedimentation and filtration portions of the treatment plant were built in 1929. The pump station and treatment plant were expanded and upgraded several times over the years, and the plant is now rated at 90 mgd.
The Harrington facility lies east of Nashville, also on the Cumberland River, and draws water above the confluence with the Stones River. Built in 1974 with a 60 mgd capacity, it was expanded to a 90 mgd in 1992. A major flood that hit the Nashville area in May 2010 affected all portions of the plant.
The chemical feed systems had not been upgraded since 1992 at Harrington, and since 2001 at Omohundro. Because repair kits and replacement parts were becoming obsolete and expensive to obtain, Metro Water Services contracted with Gresham, Smith and Partners (GS&P) to design upgrades for both systems. The upgrades covered alum, carbon, fluoride, lime, polyphosphate, polymer and permanganate.
The engineering team worked with plant management, operations and maintenance staff members to select the key feed system components. “We held a series of workshops to get everybody’s buy-in for layouts and system sizing,” says Kristi Schnell, senior engineer with GS&P. “We sized the systems based on plant history, by analyzing eight years of monthly operating report data to determine trends in historical flows and chemical dosages. We also reviewed future plant flow projections.”
The team established goals for the project that included energy efficiency, reliability, maintainability and operational flexibility. Dale Mosley, GS&P principal, observes, “As with all changes in treatment plants, there were challenges in hitting the goals, but the biggest was keeping the existing chemical feed systems in operation while constructing and switching over to the new systems.
“Shutting down either plant during construction was just not an option. After the flooding, we found that operating with one plant shut down puts unacceptable stress on the water system.” An additional component of the project was to elevate both plants’ chemical feed systems above the 500-year plus 2 feet flood elevation for critical facilities, as recommended by the Federal Emergency Management Agency.
Early in the design process, computational fluid dynamic modeling demonstrated that some changes in the chemical dosing processes would enhance treatment and optimize chemical use. As replacements for the existing diaphragm-type chemical feed pumps, the team evaluated diaphragm, progressive cavity and peristaltic pumps. They chose peristaltic pumps, and Watson-Marlow Fluid Technology Group was the successful bidder.
The new systems include four sizes of peristaltic pumps, all with variable-frequency drives. “The pump outputs are easily changeable based on treatment plant flows,” says Mosley. “If the plant has an increase or decrease in flow, the peristaltic pumps slow down or speed up to deliver the chemical dosage required to meet the flow condition.”
In addition to pumps, the feed system valves and instruments were standardized to provide operational flexibility between plants.
“We desired a consistency between the two facilities for the benefit of the maintenance and operations staffs,” says Nave. “We can now train very easily between the facilities. If an operator has an opportunity to be promoted, or if a management staff member needs to be reassigned from one facility to the other, there is consistency in what they see. So the learning curve is small in how they operate the equipment.”
The process of installing and switching to the new chemical feed systems took a great deal of coordination and timing. “We used the existing bulk storage tanks and as much of the existing systems as possible,” says Mosley. “All the transfer and feed equipment is new. It was like assembling a complicated jigsaw puzzle to keep each chemical feed system operational while replacing equipment.
“At Harrington, one of the key objectives was to get all the chemical feed equipment at a higher elevation so they didn’t risk flooding again. To do that we had to move the equipment from the lower floor level to the ground floor level. We were able to phase construction to make it work.”
Another key aspect of the new chemical feed systems is a switch from dry powder chemicals to liquid chemicals to increase safety for the operators and limit materials handling. The only material now delivered as powder is carbon, which is mixed with water in a bulk tank to form a slurry.
To date, the chemical feed systems at both plants have been functioning as designed and have required only minimal maintenance. “We’re very satisfied,” says Nave. “The hoses for the peristaltic pumps are the weak link in the system and will require replacement on a regular basis. We’re in a learning curve on how long each hose lasts with each chemical. Once that is established, we will get that information set up in our computerized maintenance management system so we can be proactive in replacing the hoses.
“We now can fine-tune chemical dosage rates better. Previously, the operators manually changed the chemical feed pump rates whenever the plant flow rate changed. Now they are much more comfortable, to the point where they let the treatment flow pace the chemical feed pumps. Operators’ buy-in during design was key to this. They choose the feed settings and the system maintains them, right on the nose. The operators make the rounds to review each system but few if any adjustments are needed.”