Progressive cavity pumps, or screw pumps, have long been fixtures in wastewater treatment facilities. PC pumps are known for being extremely adept at transporting viscous and abrasive fluids, making them an excellent fit for thickened liquids such as waste activated sludge. 

PC pumps consist of a single-helix metal rotor that turns inside of a double-helix molded rubber stator. The rotor and stator construction forms a series of pockets that traverse the length of the pump, pushing fluid from the suction side to the discharge end in a smooth, pulseless flow that is proportional to the rotor speed. The tight tolerances between rotor and stator provide some superior operational benefits but also introduce weaknesses when dealing with wastewater sludge.

The sludge challenge
Wastewater sludge is a high-viscosity, solids-enriched fluid that often contains pump-clogging matter such as large solids, rags and fibers. It is inevitable that inorganic debris will slip through a plant’s headworks screens and become entrained in the process stream, which translates to trash in the plant sludge. This problem has become even worse in recent years with the proliferation of disposable wipes and personal hygiene products that are commonly found in today’s raw sewage. These materials settle within digesters and form larger masses that a PC pump cannot pass. Additionally some solid waste material like small rocks can cause damage to a PC pump’s rotor or stator requiring expensive repairs.

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For as long as PC pumps have been pumping wastewater sludge around treatment facilities, operators have been trying to protect the pumps from clogs and damage caused by unwanted debris. Initial attempts were made with single-shafted comminutors. High-speed macerators, which use a spinning blade working against a static orifice plate, have also been tried as a solution to safeguard PC pumps. Both technologies have had limited success and typically break down when dealing with tougher solids or fibrous materials like disposable wipes. The technology that most plant engineers have settled on is two-shafted sewage grinders.

Two-shafted grinders
A leader in the two-shafted sewage grinders is JWC Environmental and its pipeline mounted Muffin Monster suite of grinders. The Muffin Monster grinders utilize low speed with high torque to turn two shafts of hardened steel cutters. The tight tolerance cutters easily shred through any rags and trash in the sludge systems. Due to the high-torque design these grinders can also slice through tougher debris like wood and small rocks that could otherwise damage PC pump components. 

The in-line Muffin Monster grinders have been proven in plant sludge systems over the last 40 years to stop pump clogs. Plant operations personnel have benefited in that time from eliminating messy and costly pump repairs as well as stopping unnecessary employee exposure to biosolids. Treatment plant engineers have been so satisfied with Muffin Monsters over the years that the grinders have become a standard piece of equipment for protecting other sensitive plant systems like centrifuges and heat exchangers. 

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Keeping Monsters current
Over the last four decades the Muffin Monster grinders have also been evolving to keep up with changing requirements. For service and support JWC’s 30K Muffin Monster was updated to have a service-in-place cutter cartridge, that is supported by a cutter cartridge exchange program. This allows operators to order an exact replacement of the cutter cartridge from JWC and have it shipped to the treatment plant without ever taking the plant grinder out of service.  The maintenance team can then quickly swap out the cutter cartridge without ever breaking the pipeline flange bolts. The pre-built cutter cartridge comes with a new one-year warranty and the confidence of a factory-certified replacement.

More recently JWC Environmental has also worked to address the increase in disposable wipes in waste streams. The problem being encountered was that traditional two-shafted grinders tended to slice wipes into longer strips that were prone to reforming into pump clogging ragballs and ropes. To address the issue JWC developed Wipes Ready cutters for its Muffin Monster grinders. These new cutters are engineered to crosscut wipes and other fibrous material into smaller pieces so they remain pumpable.

Progressive cavity pumps will likely continue to be a standard piece of equipment in sewage treatment plants around the world. To keep the pumps operating efficiently and to keep operators away from messy and hazardous pump clean-outs, two-shafted grinders will be required. JWC Environmental and the Muffin Monster will be there to chew up debris and keep the sludge flowing.

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