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Grinders and Comminutors - An
Guest article by William Galanty, President, Franklin Miller,
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Particle size reduction technology
has been evolving quite rapidly in response to the increasing
burden entrained solids have placed on wastewater treatment
facilities. The result has been an exciting race between the
leading manufacturers to develop the best size-reduction device.
The latest grinder innovations to be introduced have coupled the
powerful 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.
solids such as tampons, sanitary napkins, plastic-laden
disposables like diapers and other solids are becoming
commonplace. Even entire shirts, blankets and uniforms are found
in wastewater downstream of correctional and other institutions.
Where screening is employed, some percentage of solids remain in
the system and often weave into formidable bundles that can
ensnare rotary equipment and plug nozzles. These solids must
either be screened out or ground to a size so that they 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 or 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 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 in
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 (a.k.a. grinders,
macerators) 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 grinding pharmaceuticals and wastewater solids. 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.
Where are comminutors are used?
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
Early Comminution Equipment
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.
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.
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 water treatment plants. It could reduce heavy solids
directly inline and improve flow properties of the system thus
reducing maintenance problems for operators. The pipeline delumper
was the pioneer inline processor that helped treatment plants with
a difficult and previously unsolved maintenance problem.
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 was adapted from a
German twin-shaft solid waste shredder. Others subsequently also
started marketing this design for municipal applications.
Twin-shaft grinders use two slowly
counter-rotating shafts with intermeshing cutter disks. As one
cutter passes the other cutter, 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.
Enhanced, Twin-Shaft Comminutors
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,
This increased cutter strength and
eliminated multitudes of small gaps and associated re-tightening
requirements. The cutter cartridge design is often used to
retrofit older, twin-shaft units with individual cutter disks.
Comminutors with Diverters
To improve the flow capability of
twin-shaft units, various flow "diverters" have been
employed to move solids to the grinder while allowing the liquid
flow to bypass the grinder. The first of these grinders with a
"diverter" design was introduced the 1950's. This unit
employed a grinder unit in combination with a vertical rotating
drum screen to handle high flows.
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 Latest Developments
innovations in twin-shaft channel grinding, the FMI Super
Shredder, Taskmaster Rover and the Taskmaster Titan. These
units feature designs that handle higher flows than and avoid the
pitfalls of the earlier units.
Inline Shaftless Comminution
FMI Super Shredder employs a spherical rotor design with a
completely open "shaftless" center. It combines the
benefits of an open flow configuration for low headloss with a
high shear principle that produces a finely ground output. The
Super Shredder features a bi-directional cutting operation, heavy
solids handling, abrasion resistant hard-faced cutters and
stainless steel cutter construction.
The Super Shredder excels at
handling high flow rates with a minimum of headloss. As it only
has two cutting elements, it is simple to maintain. The design
uses the flow to help in feeding the unit, so it is best applied
to applications where the flow velocity is greater than 2 ft/sec.
Traveling Grinder Technology
Taskmaster Rover mates the power of twin-shaft grinders with
effective fine screening. In this unit, a grinder travels (roves)
back and forth across a semicircular sizing screen to intercept
and grind the solids to a size that must be fine enough to pass
through the screen slots.
Oversized solids are continuously
recycled back through the grinder for further reduction. The
unit's screen is positioned across the entire unit so there are no
gaps through which unprocessed solids can pass. As the screen is
positioned behind the grinder, it acts like a solids classifier to
recirculate oversized particles back to the grinder for further
processing. This design assures a high level of effectiveness and
a completely processed output.
The Taskmaster Rover uses a
stainless semi-circular screen. This design eliminates the
possibility of trapping of solids as occurs with screen drums.
Even in the event of a power outage, the unit may be automatically
restarted without the need for the operator to manually clear out
imbedded debris. In fact, The Rover's twin-shaft grinder has an
excellent screen sweeping capability and has the ability to
self-clear its screen, even if solids have accumulated during the
Twin-Shaft Grinders Using Two
recent improvement in high-flow grinding is the Taskmaster Titan.
This unit employs twin-shafts with intermeshing cutters but each
stack has a different sized cutter disks.
The benefit of this design is that
the smaller disks efficiently perform the cutting against an
apposing cutter disks while the larger disk allows much more fluid
to pass through. The result is a twin shaft grinder that requires
no more power than previous units but handles much higher flows
without the need for diverter screens and their potential
operational problems. It can also more effectively feed larger
solids into its cutters without repelling them.
Things to look for when
selecting a grinder:
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
- Will the design handle the
present and future flow capacity without excessive upstream
- Is the design easy to maintain
in the given installation?
- Is the unit built for long term
reliability and ease of maintenance?
- Is the unit built with materials
that are resistant to the corrosive or abrasive conditions of
- Is the unit capable of handling
the specific solids encountered in your system such as
industrial waste or institutional solids?
About our Author
William Galanty 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.
You can contact him at:
Mr. William Galanty
Franklin Miller, Inc.
60 Okner Parkway
Livingston, NJ 07039
Web site: http://www.franklinmiller.com/
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