Public works officials who manage the nation's wastewater treatment plants (WWTPs) would universally agree that a "weak" link in any plant's process chain can cause big-and often expensive-problems. Consider the case of the modest plant operated by the City of Tybee Island, GA. A six-man workforce maintains both the water and wastewater treatment plants at this barrier island community off the Savannah coast where summer tourists increase the permanent resident population of 4,000 to about 30,000.

Having reliable equipment can have a big influence on the combined operating budget for the two municipal services. Until recently, three 25-hp short-shaft progressive cavity pumps that recycled activated sludge from the primary clarifiers back to the aeration basin were absorbing scores of valuable maintenance man hours, according to George Reese, manager of the city's water and sewer utilities.

"It seemed as if we were replacing bearings and packings every week," he said. "It took two men four hours, at $25 per hour, to repack the units. If we had a more serious breakdown where we had to pull a pump, it could take four or five men and a crane truck. We were having trouble keeping up."

Tybee Island is the northernmost of Georgia's barrier islands and a popular getaway for southeastern mainlanders. The modest 5 mile by 2 mile footprint of land offers some historic streetscapes, a bird sanctuary and inviting sand beaches. The plant is on the northern side of the island and discharges treated effluent into the Atlantic Ocean at a point some 200-ft beyond the low-tide mark.

Because the monitored waters along the public beachfronts must meet high-quality standards, the flow through the WWTP demands uncompromising reliability. The .6-mgd flow during the off-season can reach 1.3-mgd during summer peaks and could have an economic and environmental impact from a failure in the process flow. Recurring problems with the return activated sludge pumps in the plant's process chain imposed disproportionate maintenance that presented more than just an annoyance.

"Sometimes I only got a year out of a pump," Reese recalls. "They cost $17,000 each and were so problem prone that I finally had to find more reliable replacements."

His favorable experience with submersible pumps in the island's 14 lift stations led him to investigate replacing the current pumps with a new-generation of submersible ones. In fact, a 30-day "try-and-buy" program enabled the utility to test the solids-handling of a new submersible pump on a side-by-side comparison with the existing progressive cavity units.

The 5-hp pump that replaced the first progressive cavity pump in a dry-pit structure quickly demonstrated exceptional reliability under the same (+/- 3 percent) solids encountered during operations. The original progressive cavity types were 25-hp units. The 5-hp submersible pump initially installed on a trial basis not only had a smaller and more energy efficient motor, but it also eliminated the noise of the progressive cavity units when operating in the dry pit beneath the operations office. The improved reliability quickly led to procuring the test pump and another unit.

Two of the submersibles are now in place with a third planned in the broader range of duties. The new pumps gain much of their efficiency from a patented impeller technology incorporated into the units. The design features a unique, semi-open impeller, combined with the relief groove in the volute that reduces the risk of clogging and maintains pumping efficiency, even when handling fluids with a high percentage of solids.

"I could easily buy two of the submersible pumps for the cost of replacing one progressive cavity type," Reese explained. "The first pump entered service on a trial basis early in 2007 and operated flawlessly, which led to a second pump being installed last September. The third will be procured in 2008.

"The only changes required were the downsizing of the 8-in discharge lines to 4-in lines. We equipped both with variable frequency drives that should help us get the most from their efficiency."

Reese joined the City of Tybee Island Water & Sewer Department in 1979 and has watched regulations change and water treatment technologies improve through the years. The one constant has been the predictable ebb and flow of mainlanders with the changing seasons who expect-as do state and federal regulators-the highest water quality at the beaches.

The activated sludge plant consists of two (50-ft in diameter) clarifiers, an aeration basin with four floating aerators, UV disinfection and a filter press with the two former clarifiers that now serve as sludge-holding tanks. The present infrastructure reflects a major upgrade in 2001. Dry solids are disposed at a mainland landfill. 
 
The municipal collection system consists of 3,000 connections along 40 miles of 6-in to 15-in lines and the network of 14 lift stations. Another upgrade that should increase capacity to 2-mgd is currently planned for the future.

Pumps & Systems, March 2008