The scenic community of Waupaca, Wis., derived its name from the translation of a Native American term meaning “clear water.” It seems doubly ironic that community wastewater passes through a facility known as the Crystal River Lift Station, once notorious for its three pumps becoming clogged with filthy debris since their installation in October 2002.
The 360-gal-per-minute (gpm) lift station ranks among the largest of Waupaca’s 12 sewage pumping stations that serve more than 10,000 residents and 3,600 connections whose wastewater flows to a 1.5-million-gal-per-day (mgd) regional treatment plant operated by Waupaca’s utility. After undergoing treatment, the effluent, restored to standards, discharges into the Waupaca River.
It was not uncommon for the Crystal River Lift Station’s wet well to accumulate a thick mat of disposable cleaning heads, various types of wipes, towels, grease and even a band of underwear elastic. The array of crud originated from the county jail, nursing home, middle school, elementary school, hospital and other connections along the collection lines upline from the station.
“We experienced clogs once and sometimes as often as three times per day,” said Jeff Dyer, wastewater team leader. “The old pumps cavitated badly and weren’t reliable. They simply weren’t engineered to operate efficiently in that environment and they eventually faced replacement.”
Waupaca’s utility recognized a growing problem for the wastewater treatment industry. The increased incidence of ragging can be attributed largely to wet-impregnated and dry-electrostatic wipes used as protectant, cleaning, personal hygiene and household cleaning applicators. Unlike traditional woven material, these often porous sheets, and dusting and cleaning heads are manufactured from polymer fibers or film. Manufacturers market many as single-use “disposable” products that the general public can easily mistake as “flushable.”
When several state lawmakers began to propose a ban on the new-generation products, trade organizations began to react. A 2008 report by INDA—the global association of the nonwoven fabrics industry, based in Cary, N.C.— called attention to how wipes that pass through residential sewers should flow equally through the wastewater collection lines and be compatible with accepted wastewater treatment plant operations. INDA and EDANA, its European counterpart, have since drafted guidelines to establish what constitutes a flushable consumer product. NSF Intl. even sponsors an independent validation service to test and certify a consumer product’s flushability.
Although the convenient tissue-like wipes rarely block residential sewer lines, they can strangle traditional lift station pumps as the residual material entangles pump impellers and impedes or totally clogs the intakes. Federal legislation that reduced the flush volume for residential plumbing fixtures to 1.6 gal has only aggravated the problem. Many utility operators foresee the problem becoming as potentially serious as when disposable diapers first reached the market decades ago. The diapers, however, were large enough to block residential lines before they reached the street, which created work for many plumbers at homeowner expense. Consumer news reports and word of mouth eventually led the general public to recognize the difference between a disposable versus a flushable diaper.
As often is the case, the fibrous waste from these products, regularly flushed into the collection system to the Crystal River Lift Station, imposed recurring overloads that clogged the recessed impellers of the facility’s former 10-hp pumps. That, of course, forced teams from the four-man wastewater group of Waupaca’s Public Works Department to be dispatched on a 24/7 basis to deal with the incidents.
The pump blockages in the 26-ft.-deep wet well presented an inherently unhealthy environment complicated by occasional seasonal weather challenges. Dyer believes the station’s repeated blockages and call outs contributed to inevitable schedule intrusions and related expense in the wake of those dispatches. A two-man crew sent to clear the pumps doubled the rate of more than $31 per man-hour (when including benefits) as a result of these incidents, which typically demanded an hour or more to restore the pumps to operation. Also, if proactive monthly cleanouts exceeded the capability of the city’s Vactor truck, an outside contractor was called in with a more powerful unit that cost up to $1,500 per cleaning.
When the aging former pumps eventually needed either a major overhaul or replacement, the wastewater department favored the latter. Several types of pumps were under consideration when a territory manager for Xylem, the manufacturer of Flygt-brand pumps, responded to the utility’s inquiry. The discussion about the recurring clogging eventually focused on a Flygt N pump as a promising solution.
The pump was specifically designed to handle the growing challenges of today’s debris-laden wastewater flows. The pump’s improved efficiency derives largely from a self-cleaning, semi-open, back-swept impeller with a horizontally positioned vane that delivers superior hydraulic efficiency that also can contribute energy savings by eliminating the drag imposed on many earlier pumps with debris building up on their recessed impellers. The pump’s hydraulic design eases the passage of solids while self-cleaning the edges on the unit’s impeller with each revolution. By eliminating ragging, the pump prevents the steady buildup of fibrous material that can otherwise impose drag and compromise respective energy usage and operating efficiency.
The Water Environment Federation recognized the pump’s engineering features with the Collection System Innovative Technology Award for 2011.
A presentation was made to the utility and consultant representatives. To further demonstrate the reliability of the pump, Waupaca’s Public Works Department was extended a “try-and-buy” opportunity in 2008, which entailed a 10-hp pump installed and operated on a 60-day trial basis. If it ever clogged, the pump would be removed without debate at no cost.
The installation made use of the existing rail-type mounting system of the existing pumps. The swap quickly met expectations by performing flawlessly during the trial.
In fact, the trial pump was set to operate in permanent lead pump mode instead of the normal one-third operating sequence. Because the trial pump was always operating, the trial period essentially subjected the pump to a continuous 180-day operating test. During the test, the companion existing pumps continued to clog throughout the duration of the trial. This led the utility to procure a second N pump in 2009 and a third in 2012, Dyer said. The three 10-hp N-3127 pumps run in continuous sequence with run times averaging 1.4 hours each during 11 cycles on a typical day.
“The reliability of the replacement pumps has been excellent,” Dyer said. “The other concerns we had about repeated emergency callouts day and night during all types of weather are no longer a factor.”
Pump benefits Wisconsin’s Crystal River Lift Station