Flushable wipes continue to inundate pump stations throughout North America, clogging pumps and pipes and often requiring operators to remove the offending debris by hand. This deragging process is time-intensive, hazardous and never-ending as new debris continuously flows into pump stations. Total shutdown, cleanout and rebuild of the pumps may be a weekly requirement depending on the volume of wipes present in the waste stream.
According to a 2013 report done by the Association of the Nonwoven Fabrics Industry, wipes usage will grow 16 percent year-over-year in North America alone (INDA; Cary, NC), meaning wipes will continue to pose significant problems in sewers, at water resource recovery facilities (WRRFs) and in pump stations.
Chopper pumps and dual-shafted grinders both provide economical, reliable solutions for preventing clogs and equipment failure caused by wipes and non-dispersible products. Pump station owners, however, should fully understand the differences between the design and function of these two equipment types before making a final design decision. Equipped with this information, pump station owners can minimize clogs and equipment failure caused by wipes and non-dispersibles.
Design & Operation
Dual-shafted grinders employ low-speed, high-torque grinding to break down troublesome solids. This configuration handles tougher items such as wood or rocks and is especially effective at shredding rags and consumer wipes when compared with single-shafted grinders or macerators.
As the name indicates, dual-shafted grinders comprise two stacks with rows of hardened, steel cutters that rotate toward one another. The cutter teeth actively grab solids and pull them through the cutter stack, shredding solids into smaller pieces. The shafts rotate at different speeds, so the cutters interact like a pair of scissors—slicing the solids rather than crushing them. This slicing action helps ensure a consistently small particle size; most pieces are 1/2-inch or smaller (see Figure 1, page 56). Dual-shafted grinders employ top and bottom shaft bearings and robust mechanical seals to prevent shaft deflection or seal failure when grinding high volumes of solids, such as wipes, rags and organics.
An in-line, dual-shafted grinder system comes in two parts, ensuring an efficient and streamlined installation. The flanged steel housing is bolted into the pipeline at both ends, and a removable cutter cartridge—including two shafts of cutters, the gear reducer and motor—slides down into the housing. This setup simplifies maintenance inspections and repairs by allowing the pipeline to be put back into service when the cutter cartridge is removed.
Chopper pumps are motor-driven, centrifugal pumps that macerate incoming solids prior to pumping them. This configuration protects the pump component from clogging and maintains flow through the system. The chopper component comprises a cartridge seal to handle slurries and reduce material wrapping, impeller vanes, auxiliary cutters and impeller blades. The auxiliary cutters move against the sharp impeller blades to chop solids before they enter the hydraulic parts of the pump. To accomplish this task, chopper pumps employ high speed and low torque to quickly chop and move solids.
Some chopper pumps, however, may not be as efficient at handling large volumes of tough solids. This condition is common in a pump or lift station near the end of a series of stations. Chopper pumps must operate at a relatively high speed to maintain the ability to pump fluids. This can reduce the maximum possible torque required to grind through solid debris such as rocks, wood, plastics and rag balls caused by wipes. The type and volume of waste chopper pumps can handle also may be limited by the equipment's design. In other words, if the chopper doesn't work, the unit can't pump effectively, resulting in clogs. A dual-shafted grinder, on the other hand, isn't required to maintain flow or move material through the entire system. The main function of a dual-shafted grinder is to grind any debris that contacts it, allowing the continual flow of waste through the pump station.
Selection Criteria & Positioning
Despite the limitations of chopper pumps, they can be effective if placed at the first of a series of lift or pump stations. If incorporated at an initial upstream point, the chopper pump is only handling the onset of waste from individual houses, buildings or apartment complexes from a localized area. Most of this initial debris is easily handled by a well-designed chopper pump.
As rags pass through collections systems in either long trunk lines or in a series of pump stations, they tend to form "ropes" or rag balls as they recombine with hair and grease (see Image 1). When chopper pumps are placed further downstream in a series of stations, especially last in a line of pump stations, the enormous volume of rag balls and wipes that have accumulated can create significant challenges.
Because of their construction and the torque of their cutting force, dual-shafted grinders can overcome this limitation, regardless of where one is placed within a line of pump stations.
The Otter Creek Water Reclamation District in South Elgin, Illinois, faced ongoing maintenance and cleanup costs at their Thornwood lift station due to a buildup of rags, wipes, trash, wrappers and other debris clogging the system and forcing pump shutdowns.
Thornwood is the largest of the distric's three lift stations, with an incoming flow of about 490 gallons per minute (gpm) or 110 cubic meters per hour (m3/h). The station's three 40 horsepower (HP) (30 kW) pumps must move sewage at about 600 gpm (136 m3/h) at 36 feet (11 meters) total dynamic head (TDH). Incoming debris would clog the pumps and force the district to take the system offline.
Operators would then have to disassemble the pumps to clear the buildup by hand, reassemble them and restart the system. The district engineer noted that the facility was seeing higher volumes of polyester-reinforced rags, which seemed to be a significant factor increasing clogging.
This buildup was so massive that, beyond regular cleanings by the operators, the station was forced to hire a Vactor truck four times each year at $4,900 a visit to remove rags and wipes from the wet well. The weight from the debris buildup would disconnect the cables leading to the submersible pumps, inadvertently shutting off and short-circuiting them. Altogether, the cost to deal with these maintenance and repair issues was costing the district nearly $20,000 a year in hard costs plus the additional maintenance time.
The district engineer suggested a dual-shafted grinder unit. Since the installation of the grinder in 2012, the lift station has experienced zero pump maintenance issues and has eliminated the previous costs associated with clogging problems.
As the Thornwood Lift Station example demonstrates, properly selected equipment can provide pump stations with exponential cost savings and reduce unscheduled maintenance. The initial investment in a dual-shafted grinder or chopper pump may deter some end users from incorporating a solution within their system. But when wastewater districts compare the up-front cost with the long-term benefits of solving the wipes problem for their pump or lift stations, the return on investment is difficult to ignore.
While end users cannot completely eliminate the impact of wipes and non-dispersibles on influent waste streams in their pump stations, they can dramatically improve the clogging and ragging situation with the right waste-reduction equipment.