am Reduction
of Bold Waste

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This report (SW -i 17),one of a series of staff papers
on current and developing technology
in the field of solid waste management,
was written by JAMES F. MANK

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Size Reduction
of Sold Waste

an overview
Many municipalities and solid waste
management organizations are express-
ing an increasing interest in size reduc-
tion of solid wastes. The information
that follows is provided to assist orga-
nizations and individuals in under-
standing the subject.

TYPES OF EQUIPMENT

  There are many types of size-
reduction machinery available. These
include crushers, cage disintegrators,
shears, cutters, chippers, rasp mills,
drum pulverizers, disk mills, wet
pulpers, and hammermills.  The type
most often used for processing muni-
cipal solid waste is the hammermill.
Basically, the hammermill consists of
a heavy rotor with hammers attached
to its outer circumference and en-
closed in a housing. As the rotor turns,

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the protruding hammers strike and
size-reduce the throughput material.
Most hammermills are the swing-
hammer type in which the hammers
are pivoted on the rotor to decrease
the possibility of the hammermill be-
ing damaged by extremely heavy
waste. There are also fixed-hammer
machines and grinder-type hammer-
mills. The grinder-type hammermills
(Eidal) have star wheels attached to
the outer circumference of the ham-
mermill rotor. Throughput material is
ground up between the rotating star
wheels and the hammermill housing.
Hammermills are available in hori-
zontal or vertical rotor designs. The
horizontal-shaft machine is usually fed
from the top, and the refuse is size-
reduced by the machine until particle
size is small enough to permit passage
through a grate at the bottom of the
machine. Vertical-shaft hammermills
are also fed from the top, and the
throughput material is gradually re-
duced in size as it flows down through
the machine.
The Office of Solid Waste Manage-
ment Programs (OSWMP) fiber recov-
ery demonstration project at Franklin,
Ohio, involves a wet pulper. The pul-
per is a type of disk mill that operates
under water and works essentially like
a large garbage disposer. Refuse is fed
in!o the machine and is reduced to a
pulp. The machine works best for
tMention of commercial products does not
constitute endorsement or recommendation for
use by the U.S. Government.
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pulpable or fibrous materials and is
used primarily for composting or fiber
recovery operations.
A rather unique portable shredder,
called a Cobey Roto-Shredder, was
the subject of study at the OSWMP’s
Crawford County, Ohio, sanitary land-
fill demonstration project. The actual
shredding is done in a rotating drum
that has fixed teeth attached to it. The
Roto-Shredder moves by its own power
along a windrow of refuse, and its ro-
tating drum shreds the refuse as the
machine moves down the windrow. In
actual practice, the Roto-Shredder
does not shred tires or other difficult-
to-shred items and only tears up paper
to a limited extent.
POSSIBLE APPLICATIONS
Examples of applications where size
reduction may fit into a solid waste
disposal system include: bulky or
municipal waste processing prior to
landfilling, a resource recovery effort,
or certain types of thermal reduction.
Size reduction of bulky waste before
landfiffing may result in landfill space
savings that more than offset the cost
of size reduction. Certain thermal re-
duction methods, such as a vortex in-
cinerator, fluidized bed reactors, or
some dual fuel boilers, may require
size reduction of the input refuse
before that method of processing can
be used. Most types of material recov-
ery systems, especially those that are
automated, cannot be implemented
without size reduction being included
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as part of the total system. The in-
come from the recovered materials
may help offset the waste processing
costs. In special cases where daily
cover would have to be hauled to a
sanitary landfill site, it may be accept-
able and economical to size-reduce
general municipal refuse and then land-
fill the material without a daily cover
requirement. (See OSWMP position
piper on milled waste fills.) This appli-
cation, combined with a metal- and
glass-recovery effort, is the reason for
most of the current interest in size-
reduction equipment. As explained in
the position paper, however, the
OSWMP has not given blanket approv-
al for this practice. The paper stresses
that each individual case should be
carefully studied and precautions
taken to prevent any deleterious effect
on the environment, especially from
leachate production.
In general, shredding refuse before
landfilling decreases the problem of
blowing paper, makes the refuse easier
to distribute and compact at the land-
fill, and makes possible a more uni-
form landfill density.
OPERATING COSTS
The main disadvantages of includ-
ing size reduction in any refuse dis-
posji system are the resultant in-
crea’ses in operating costs and the
possibility of introducing some unre-
liability into the disposal system
through unexpected downtime due to
breakdowns of the size-reduction unit.
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The following operating cost infor-
mation from various size-reduction
units has been obtained from several
OSWMP demonstration projects. In-
cluded in the total costs are amortiza-
tion, maintenance, labor, and utilities.
Because municipal solid waste is very
heterogeneous and contains many
difficult-to-shred items, the cost of its
size reduction is rather high. In gen-
eral, it is more costly to size-reduce
bulky, heavy waste than ordinary
municipal refuse. The cost of shred-
ding bulky waste at the OSWMP
Tacoma, Washington, project was
$6.34 per ton with a 40-ton-per-hour
800-horsepower Williams hammermill;
the cost of shredding’ bulky waste
with a similarly powered but slightly
lower capacity Williams hammermill
at the St. Louis, Missouri, bulky waste
size-reduction project was approxi-
mately $12.00 per ton. Convict labor
was used at St. Louis, and the project
director felt that the cost of size
reduction might be as much as 50 per-
cent lower if better labor conditions
existed and more care was taken to
keep unprocessable items out of the
machine. The Buffalo, New York,
bulky waste project used a Haze mag
Crusher, model no. SAP 5/M, which
operated at a capacity of about 11
tons per hour. The cost ran about
$9.37 per ton for noncombustible
bulk and $6.24 per ton for mixed
bulk. The reason for this cost differ-
ence is that bulky wood waste is
easier to process than noncombustible
bulky waste, such as refrigerators and
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water heaters. Jamming problems
were not as serious when mixed bulk
was processed.
The Madison, Wisconsin, project in-
volves size-reducing general municipal
refuse with a 1 5-ton-per-hour vertical-
shaft Heil Tollemache hammermill and
a 9-ton-per-hour horizontal-shaft
Gondard hammermill. The processed
waste is disposed 0 f in a sanitary land-
fill without daily cover. The cost pro-
jection for processing 280 tons of
solid waste per day with a two-shift-
per-day operation is approximately
$3.90 per ton. The cost of hauling the
shredded refuse to the disposal site is
about 5.40 per ton, and the cost of
spreading and compacting the waste
at the landfill is about 5.50 per ton.
Studies have shown that rats, flies,
blowing paper, and odors are no more
of a problem at the Madison landfill
than they are at landfills where a
daily cover is applied.
POTENTIAL PROBLEMS
Municipal solid waste is difficult to
process. Items such as crankshafts,
small 1-beams, and gears can get into
and cause severe damage to a size-
reduction unit. Most units have heavy-
metal rejection systems, but severe
damage can occur before an unprocess-
alle item is rejected. For this reason,
it is probably unwise for a solid waste
management organization to get into a
position where it would have no alter-
native waste disposal method if its
size-reduction unit were damaged
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severely and put out of operation for
an appreciable length of time.
Jamming is a major problem with
all size-reduction machines. Items
such as carpets, mattresses, bedsprings,
and wire rope will jam most machines.
Tires, especially truck tires, can also
be a problem if the horsepower of the
shredder is low or if the tires are not
distributed throughout the feed
material.
SELECTION OF EQUIPMENT
The OSWMP has a contract, com-
pleted May 31, 1973, that involved
development of a catalog of informa-
tion on available size-reduction equip-
ment suitable for processing refuse.
Also being developed as part of the
contract are an equipment evaluation
procedure that prospective purchasers
of size-reduction equipment can use
to objectively compare available
equipment, and a test procedure for
obtaining certain information and
performance data not available from
equipment manufacturers or existing
shredder installations. The final report
and equipment catalog also contain
information that wilt help prospective
purchasers of size-reduction equip-
ment to avoid common pitfalls such
as inadequate feed and discharge sys-
tems and inadequate drive motor
horsepower. The final report and
equipment catalog are currently under
technical review and probably will be
published in the near future through
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the National Technical Information
Service.
Including size reduction of refuse
in a solid waste disposal system will
not solve the solid waste disposal
problem. There will almost certainly
be some increased cost; and this ex-
pense must be carefully weighed
against the benefits, if any, that are
gained through possible increases
in the total disposal efficiency.
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