Management of the wastewater treatment plant for the city of Great Bend, Kan., recently avoided another costly refurbishment of aging Archimedes screw pumps by replacing them with submersible pumps. The changeout saved more than $300,000 vs. the quote to rebuild two of the four, 54-inch Archimedes units that required major overhauls.
The operations at the wastewater treatment plant in Great Bend, a city of more than 15,000 in the middle of the state, were in potential jeopardy when two redundant screw pumps failed at each of two locations and a third pump began exhibiting audible bearing damage. The 40-year-old plant treatment plant is just inside the Arkansas River levee at the eastern boundary of the city. The wastewater collection system links 100 miles of lines and 18 lift stations with the facility. The facility uses crop fields at the municipal airport and private farm fields for the land application of most biosolids.
The failing, 54-inch Archimedes pump units stand side-by-side in wetwells at two locations in the treatment process of the plant’s wastewater flow. Under normal flow conditions, one pump operates at each point, with the accompanying unit used as a backup or for additional capacity during higher flow conditions. The first pair of pumps draws raw sewage from the collection system at the head of the plant and into the process chain. The other two pumps are located downline and recirculate wastewater from a wetwell and into a secondary process loop. Repeated mechanical failures of the massive screw pumps at these locations imposed an ongoing drain on the annual maintenance budget of the plant.
Past Their Prime“Each screw pump had been replaced several times in the past,” recalls Bob Schwartz, superintendent of the Water Pollution Control (WPC) Department. “The drive motor bearings or the gear reduction units seemed to cause us problems in one or more of the pumps every year. If the tube part and lower bearing unit needed to be replaced or refurbished, it would cost from $130,000 to $240,000. The replacement of an entire screw pump could cost from $225,000 to $250,000.”
The Wichita office of JCI Industries Inc., a company serving municipal and industrial fluid-handling markets, was engaged to assess the problem and to recommend a solution. Mike Younger, who worked more than 20 years in public works in Kansas before joining JCI, carefully studied the options before recommending that all four of the screw pumps be replaced with ITT Flygt Model 3201 submersible pumps equipped with variable frequency drives (VFD). The Flygt pumps are intended to deliver 3,200 gpm at the intake, vs. the two located at the secondary process loop, which are designed for 3,400 gpm. A fifth pump was purchased as a reserve unit.
The submersible pumps matched the project’s unique criteria:
- The submersible pumps are sealed units that can operate even in the often partially submerged conditions of the shallow wetwells, and are fitted with impellers that perform the mission without cavitating.
- The pumps fit into the existing wetwell configurations without having to undergo reconstruction.
- The four pumps can deliver 10 MGD, a significant safety factor for the plant flow range from 2.5 MGD up to 7.3 MGD during a rain event. The pumps run on alternating schedules to ensure even wear.
- Because of their smaller, 40-HP rating, the submersible pumps inherently require less energy than the former 75-HP screw pumps that always ran at full speed when activated. The VFDs on the submersible pumps also enable them to operate at 55 percent in normal flow conditions during day periods, and run at only 10 percent during the typically light flow rate experienced between 1 a.m. to 7 a.m.
- The submersible pumps are much smaller than the 11-ton screw pumps. The much smaller submersible pumps are easy to access, and the plant operators normally can self-perform their service requirements on location. Even this is minimal, with the manufacturer recommending an oil change once a year and a complete shop retrofit only every three years. However, many public works groups have been known to run the pumps several years longer between retrofits. Most importantly, the City could buy all five submersible pumps – installed – for much less than the screw pumps.
The installation took approximately two months for a three-man JCI crew to complete and the plant remained in operational compliance during the entire period.
A Series of UpgradesAs many wastewater treatment facilities operated elsewhere, the Great Bend plant embodies numerous responses to the increasingly stringent series of federal and state environmental regulations. In the late 1990s, the facility reacted quickly to modified permit requirements dictated by the Kansas Department of Health & Environment. The modifications went into the plant’s National Pollutant Discharge Elimination System. The permit now mandates additional wastewater treatment to reduce ammonia levels before discharge into the Arkansas River. This led to major changes in the infrastructure that supports not only the more advanced treatment process but also the mandates controlling land disposal of the plant biosolids.
Charlie Suchy, superintendent for WPC Collections, cites several major examples these changes have had on the plant infrastructure in recent years. These include a stripped clarifier, used now only as an emergency diversion basin, the sludge drying beds, the abandoned trickling filters and four RBCs that were important elements of the former treatment process. Instead, the plant now operates with the activated sludge treatment process and a more advanced biosolids processing method.
Nearly $6 million in upgrades occurred at the plant in recent years, including an EIMCO Carousel system and structural modifications. The walls of two clarifiers were heightened enough to nearly double their capacity to 450,000 gallons each. A 500,000-gallon post aeration basin also was built to support the more complex biological process and a 1.28-MG raceway structure for the UV disinfection system with 320 tubes. A new SCADA system provides remote alarm monitoring and has eliminated a third shift of on-site personnel.
A professionally staffed lab monitors the quality of both the land-applied biosolids and the effluent discharged into the Arkansas River. Starting in 1993, the issuance of 40 CFR Part 503 imposed major changes on the quality and disposal protocol of biosolids, according to David Hays, plant manager. To use surface application, the plant must achieve a volatile solids reduction of 38 percent.
Another change occurred at the gravity belt thickener used to process the sludge. The initial attempts to increase the solids content, and thereby reduce the number of daily loads hauled from the plant, proved more challenging than expected. The 5 percent sought would not flow efficiently due to the high pressure and the distance from the digesters. WPC resolved the operational problem by replacing two sludge pumps with progressive cavity pumps that can handle processed sludge with up to 6 percent solids.
WPC applies the sludge using a 3,000-gallon tank-truck fitted with a spreader attachment, or a “Big A” Model 3500 with a 2,000-gallon tank and 10-foot spray boom. Under normal conditions, the plant dispatches 15,000 gallons per day, five days a week. The improved sludge has cut the number of hauls from seven loads to three loads per day. WPC strives to maintain a 20-day capacity in the plant digester.
The addition of modern submersible pumps, in addition to other technologies, has advanced significantly in the field of wastewater treatment and management. Environmental rule changes and rising operating costs have driven most of the improvements. Great Bend is a small community of just 15,000 residents, but has a wastewater treatment facility that rivals the quality of those in much larger municipalities.