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Comminution technology has been evolving quite rapidly in response to the increasing burden entrained solids have placed on treatment facilities. More advanced devices have been developed in rapid succession. The result has been an exciting and fluid race between the leading manufacturers to develop the best size reduction device. The latest grinder innovations to be introduced have coupled the power of twin shaft grinding with higher flow capabilities and screw screening systems. Here's a rundown on the past and present state of the art in wastewater solids reduction.
Wastewater solids such as tampons, sanitary napkins and plastic laden disposables like diapers and other solids are becoming commonplace in wastewater. Even entire shirts, blankets and uniforms are found in wastewater downstream of correctional and other institutions. Even where screening is employed some percentage of solids remain in the system and often weave into formidable bundles that can ensnare rotary equipment and nozzles. These solids must either be screened or ground to a size that can pass through the system without plugging of equipment
Since the 1950s, each decade has seen the development and installation of a new breed of solids reduction (comminution) devices. Each major new development contributed a set of improvements in performance. Over time many of these units have evolved and found specialized applications where they excel.
Of late, the pace of twin shaft comminutor development and innovation has accelerated markedly. These units use two counter-rotating banks of intermeshing cutters to cut and shred solids to fine particles. The shortfall of these units has been their banks of cutters provided little open area for fluids to pass through. The latest designs discussed below have solved this problem to form a compact solids screening system.
What is a Comminutor?
Comminutors are commonly employed where it is undesirable, impractical or uneconomic to remove solids due to the lack of disposal options. Even when screening equipment is employed, some solids inevitably slip through, so the use of a comminutor is useful as a backup. Alternatively, some plants find it desirable to put the solids through the plants' digestion processes and thus require size reduction equipment and not screening.
A Brief History of Comminution
An improved, higher powered, straight through comminutor has subsequently been introduced with a similar but improved design. It has a heavier solids reduction capability, rotary design as opposed to oscillating and a heavier screen. This unit, the DIMMINUTOR, easily replaces and upgrades older straight through Worthington style comminutors, which are no longer made. It features individually removable and replaceable cutters and a cantilivered design that eliminates the need for bottom seals at the gritty channel bottom.
Dual Shaft Comminutors (Grinders)
Twin shaft grinders use two slowly counter-rotating shafts with intermeshing cutter disks. As one cutter passes the opposing cutter at close clearance it shears solids trapped between the two and passes them downstream. This design provided good dry solids reduction and feeding capability. The design came originally from solid waste shredders with up to 300 hp motors for reduction of everything from steel drums and tires to construction debris. The drawbacks of these miniaturized shredders for liquid waste systems were poor ability to pass liquids. They were also excessively complicated assembly due to the use of multiple individual cutter and spacer disks. Another problem was the possibility of cutter stack collapse and disk cracking as every thin cutter was dependent on every other one for its position in the stack.
Cutter Cartridge Enhanced Twin Shaft Comminutors
Hi-Flow Twin Shaft Grinders with Diverters
In the later 1980s, the diverter concept was again introduced. Several units implemented flat diverter screens such as the "disc screen" which employed multiple parallel banks of rotating disks to convey solids to an adjacent grinder. Another design re-introduced the drum screen with an adjacent grinder.
While these designs increased the flow capability of their grinders, they also had a number of drawbacks. Flat screen or disc designs were subject to "plastering" or bridging of solids across the screen. Screen wear against bottom grit was problematic. Also, solids tend to get trapped within the screen center. Solids could also bypass the grinder altogether as these units have no way to assure that the solids don't escape in the gap between the screen and the grinder. In the drum type screen, only 1/4 of the screen can be active as the rest rotates in the wrong direction or doesn't feed the solids effectively. This results in increased headloss.
The applications for these shredders include institutional waste where high concentrations of solids are encountered. Often prisoners flush whole bed sheets, blankets and garments into the wastewater system in a deliberate effort to plug the system. The screenings discharged from a bar screen is another case where a heavy duty 10 or 15 hp shredder is suitable. While grinders excel at handling liquid-borne solids, heavier solid masses which are not suspended in a liquid carrier call for a heavy duty shredder type device.
Twin Shaft Grinders Using Two Cutter Diameters
An important criteria in choosing a grinder is hydraulic performance. Can the unit be placed in the flow without raising the head excessively to the point of backing up the system or cavitating the pump? For this, it is important to work closely with the manufacturer in sizing of the unit to the application. In general, a more open design will pass more flow and cause less backup than a unit with a higher percentage of closed area.
About the Author
William Galanty is a holder of 10 patents in the field of wastewater comminution and is the President of Franklin Miller Inc., a manufacturer of size reduction processors for municipal as well as industrial applications. He is experienced in the design and application of high powered industrial solid waste shredders, design and production of high pressure and complex industrial size reduction systems in addition to his experience in wastewater treatment comminution. Mr. Galanty has also testified as an expert witness on the design and application of wastewater grinders.
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