Comminutors
Comminutors are used to reduce the particle size of wastewater solids. The terms "Sewage Grinder" and "Comminutor" are two terms for a cutting device for sewage solids. The term "comminutor" originated with a device for chopping meat. It was later applied to equipment used for reducing pharmaceuticals and wastewater. The term grinder, as in meat grinder, is commonly used to imply a comminutor that reduces solids finely and often has multitudes of cutting edges. However, this is a loose terminology. There are units that finely reduce solids with only a few working elements. The original sewage grinders, installed as long as thirty years ago, were all called comminutors. Many of these machines are still in service to this day although some are sorely in need of hard to find spare parts. In response to operators' frustration with these units, manufacturers refrain from calling their current offerings - comminutors. But most persons skilled in this art agree that comminutor is the valid term for this type of device.
Applications
An important use for comminutors (grinders) is in the primary treatment of raw sewage solids in plant headworks and pump stations. Large, stringy solids can easily plug even supposedly "non-clog" pump impellers and necessitate the use of size reduction devices. In sludge, inline grinders are often installed ahead of pumps in recirculation lines and also to enhance the operation of dewatering equipment, digestors and nozzles which can easily become plugged. Belt filter presses and centrifuges are especially sensitive to oversized particles that can puncture filter membranes, reduce their efficiency or damage expensive centrifuge drums. As a result, grinders can easily pay for themselves in damage and downtime prevention.
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.
Evolution and Types
Drum Type Comminutors
Sewage comminutors date back to the early 1950s with the introduction of the bottom discharge Comminutor. This unit featured a rotating drum with attached cutter teeth. Solids would get caught on and rotate with the drum and get reduced as the teeth passed through a fixed comb. This design featured a large active screen area and heavy construction. The drawbacks of this unit's design included: a bottom discharge that required a special "L" shape channel construction; trouble handling certain solids, the rotating drum was subject to wear from bottom grit, and the teeth were extremely difficult to remove to service.
Straight-through Comminutors
Next to be introduced was the Worthington comminutor that fit into straight-thru channels. Its oscillating cutters wipe a semi-circular screen surface clear and sweep and cut the solids against vertically mounted stationary cutters. This design featured easier maintenance and a straight through configuration. The drawbacks were the screens were light and subject to puncturing. The unit was incapable of handling the increasingly heavy solids encountered due to low power and a lack of torque at the end of each stroke. The straight thru comminutor generally had a good reputation when maintained. However, if maintenance schedules were missed, unit performance would suffer.
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.
In-line Comminutors/Grinders
The first machine to address the problems of reducing solids directly inline was the Pipeline Delumper. Originally developed in the 1960's for the chemical process industry, this pressure rated unit quickly became invaluable to treatment plants. It could reduce heavy solids directly inline and improve flow properties of the system thus reducing maintenance problems for operators. The Delumper was the pioneer inline processor that helped treatment plants with a difficult and previously unsolved maintenance problem.
Dual Shaft Comminutors (Grinders)
After the US Navy in the early 1970s adapted the Pipeline Delumper as the wastewater processor of choice for its frigate fleet, a competing inline comminutor was introduced with a twin shaft design. This unit adapted and miniaturized a German twin shaft solid waste. shredder's mechanism. Others subsequently also started marketing this design for municipal applications.
Cutter Cartridge Enhanced Twin Shaft Comminutors
An improvement to the original twin shaft design was the introduction of cutter cartridge elements. These cartridges replace the multiple individual cutters and spacer disks with one piece solid cartridge elements. This increases cutter strength and eliminates multitudes of small gaps and associated stack re-tightening requirements. The cutter cartridge design is often used to retrofit twin shaft units with old individual cutter disks.
Design
The comminutor consists of a housing, inlet trough, right-angle gear-motor, drive shaft, double-sealed and shielded bearings, revolving sewage drum with four (4) rotating cutter bars, and one stationary cutter comb. The cutter bars and cutter comb are made of A2 chromecarbon tool steel.
The cutter bars are mounted at 90 degree intervals on the drum. The entire drum assembly revolves as a single integral unit. The teeth on the cutter bars pass through the stationary comb as the drum revolves, creating a cutting and shearing action between the ends of the comb and the top surface of the cutter bars which are recessed into the drum
The drive unit is a right-angle gear-motor integral type with reduction type worm gearing Class No. 3. This unit is designed and applied in complete accordance with applicable practices of the American Gear Manufacturers Association. The worm gear materials shall comply with all mechanical ratings allowed by the AGMA Rating Standard. The motor size is 1/2 hp for operation on all volt, phase and cycle service. All equipment is designed for outdoor service.
The flow-through of the comminutor is as follows: the sewage enters the comminutor through an open-top inlet trough, passes through the 1/4" slots in the revolving sewage drum, and discharges out the bottom of the comminutor. In the event of prolonged power outages and subsequent plugging of the drum, sewage can overflow the open-top inlet trough into the flow chamber, thereby preventing plugging of the inlet line
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