Page 238 - Materials Chemistry, Second Edition

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Municipal Solid Waste Processing; Materials Recovery Facilities 209
7 to 13 times higher than the OSHA standard. This finding dictates the use of facemasks while
working.
Plate counts at shredding operations have indicated that the total bacterial counts during shred-
der operation are as much as 20 times greater than the ambient, which contains about 880 organ-
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isms/m of air. In resource recovery facilities where shredders are used, coliform counts can
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increase from 0 to 135 per m (0 to 69 per ft ), and fecal streptococci from 0 to over 975 per m (0
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to over 500 per ft ) (Diaz et al., 1976; Vesilind et al., 2002). These data show the potential danger
of disease transmission by air during shredding of MSW.
The degree of dust collection is a function of the types and volumes of wastes handled, unit
operations, and local climate. The solution can vary from the installation of individual dust collec-
tion units at each operation along the line, to one or two centralized dust collection systems. Dust
collection systems include fans, ducts, cyclones, and baghouses. The plant worker may be required
to wear a simple dust mask during operations (Drobny et al., 1971; Vesilind et al., 2002).

7.7.2 ODORS AND THEIR CONTROL
Odors can be significant in MRFs, especially the “dirty MRFs” where raw MSW is brought to the
facility, stored, and subsequently processed. Odors can often be reduced or eliminated by minimiz-
ing storage time of raw materials or product followed by frequent wash-down of tipping floors.
Some of the most effective techniques involve applying negative pressure (suction) within an
enclosed MRF and treating the exhaust gases. These gases can be incinerated, passed through GAC
filters, or chemically (catalytically) oxidized. They can furthermore be scrubbed by passage through
a gravity spray tower, i.e., an enclosed chamber containing a fine mist of water or other solution.
There has been some work on biofiltering exhaust gases; i.e., forcing them through a mixture of soil
and gravel. Indigenous microorganisms will act upon and oxidize a wide range of these gases, using
them as substrates. In the interest of speed and cost, odors can simply be masked. A ‘disinfectant
spray’ can be released in the working area. Its purpose is primarily to coat the nostrils of workers,
thereby masking the malodorous MSW gases — it does not remove the noxious gases.
In situations with severe odors, multiple technologies may be required. Each technology may
be accompanied by problems (in addition to capital and operating costs) of its own.

7.7.3 NOISE SUPPRESSION

Much of the equipment used in MRFs generate noise: conveyor belts, crushers, pumps, front-end
loaders, and so on. The noise levels around a 3 ton/h hammermill range from 95 to 100 dBA with
much of the noise produced being low-frequency. The unit ‘dBA’ is a standard method of noise
measurement and stands for decibels on the A scale of the sound-level meter. This scale is an
attempt to duplicate the hearing efficiency of the human ear. In addition to a high constant noise
level, materials recovery facilities processing MSW produce considerable impact noise which is dif-
ficult to measure, and whose effect on human beings is poorly understood. The existing OSHA stan-
dard limits noise to 90 dBA over an 8 h working day. The corresponding limit set by the EPA is 85
dBA. Shredder operators must wear ear protection (Vesilind et al., 2002).
Engineering controls are difficult to establish in many situations. Sound muffling equipment
and soundproofing at specific work locations may be installed throughout the building or specific
pieces of equipment can be isolated, although this is often impractical.

7.7.4 AESTHETICS

Practical issues to be addressed in the design of MRFs include aesthetics and public health. In
order to be a good neighbor to the community, it is important for a MRF to be constructed
and landscaped in order to fit in well with an area zoned for commercial or industrial use. Berms
and attractive vegetation can be established at the perimeter of the property. Blowing litter