25 March 2014
The growing popularity of single-use towels, baby wipes and other non-woven ‘convenience’ products has become a maintenance nightmare for water-treatment operations. It should scare any operation that requires clean water.
The scenic community of Waupaca, WI, is spiritually connected to clean water.
Not only is its name a translation of a Native American term meaning “clear water,” its wastewater facility is known as the Crystal River Lift Station. So community leaders were well aware of the irony when this facility became notorious for its three pumps becoming clogged with debris after their installation in October 2002.
The 360-GPM Crystal River Lift Station is among the largest of Waupaca’s 12 sewage- pumping stations. It serves more than 10,000 residents and 3600 connections whose wastewater flows to a 1.5 MGD regional treatment plant operated by Waupaca’s utility. After undergoing treatment, the effluent discharges into the Waupaca River. It was not uncommon, however, 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 upline from the station.
“We experienced clogs as often as three times per day,” says 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.”
Waupaca’s utility faced a growing problem for the wastewater treatment industry: the increased incidence of ragging due largely to wet-impregnated and dry-electrostatic wipes popular for household cleaning and personal hygiene use. Unlike traditional woven material, these often porous sheets and cleaning heads are manufactured from polymer fibers or film. Manufacturers market many as single-use “disposable” products, which the general public often mistakes for “flushable.”
When state lawmakers considered a ban on this new generation of products, trade organizations reacted. A 2008 report by the U.S.-based Association of the Nonwoven Fabrics Industry, INDA, 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 its European counterpart ENDA have since drafted guidelines that define what constitutes a “flushable” consumer product. The National Sanitary Foundation sponsors an independent validation service to test and certify a consumer product’s flushability.
Although convenient tissue-like wipes rarely block residential sewer lines, they can strangle traditional lift-station pumps as their residual material entangles pump impellers and impedes or clogs intakes. Federal legislation that reduced the flush volume for residential plumbing fixtures to 1.6 gallons has only aggravated the problem for utilities. Many operators foresee the problem becoming as potentially serious as when disposable diapers first reached the market decades ago. 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 and flushable diaper.
Convenience products, inconvenient clogging
At the Crystal River Lift Station, the fibrous waste from these products caused recurring overloads that clogged the recessed impellers of the facility’s former 10-HP pumps. Each clog required that a team from the four-man Wastewater Group within Waupaca’s Public Works Department be dispatched to clear the pumps. Blockages in the 26-ft.-deep wet-wells presented an inherently unhealthy environment, further complicated by seasonal weather challenges. The station’s repeated blockages and call-outs created schedule intrusions and added expense. The cost for a two-man crew sent to clear the pumps, for example, doubled to more than $31 per man-hour, including benefits. Typically, more than one hour was required to restore the pumps to operation. And 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 $1500 per cleaning.
Preparing to install the N pump, workers attach an adaptor for use with an existing rail-type mounting system.
When the utility’s aging pumps eventually needed 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. (Editor’s Note: N-pump technology has since been incorporated into Flygt’s line of Experior products.)
To demonstrate the reliability of N-pump technology, Waupaca’s Public Works Department was offered a “try and buy” opportunity in 2008: A 10-HP unit would be installed and operated on a 60-day trial basis. If it ever clogged, the pump would be removed, without debate, at no cost. Installation was simplified with the use of the existing pump’s rail-type mounting system. The swap 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.
Since the pump was always operating, the trial period subjected it to a continuous 180-day operating test. During the test, the existing companion pumps continued to clog. This led the utility to procure a second N pump in 2009, and a third in 2012. The three 10-HP N-3127 pumps now operate 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,” reports Dyer, who adds that the facility’s “day-and-night emergency call-outs in all types of weather are no longer a factor.” MT&AP
Designed with Debris-Laden Flows in Mind
The Flygt N pump was specifically designed to handle the growing challenges of today’s higher debris-laden wastewater flows. Its improved efficiency derived largely from a self-cleaning, semi-open, backswept impeller with a horizontally positioned vane. The design contributed to energy savings by eliminating the drag imposed on earlier pumps from debris build-up on recessed impellers. The N pump’s hydraulic design eased the passage of solids while self-cleaning the impeller vane leading edges with each revolution. By eliminating impeller fouling, the pump prevents the steady build-up of fibrous material that can otherwise impose drag and compromise operating efficiency and energy use. The Water Environment Federation (WEF) recognized the pump’s engineering features with the Collection System Innovative Technology Award for 2011. N pump technology and its adaptive functionality have since been combined in what the manufacturer now designates as its Experior line.