MUNICIPAL SOLID WASTE MANAGEMENT AND DISPOSAL - Environmentally Conscious Materials Handling - page 149

and automated sorting is the most appropriate to ensure high-quality materials
and minimize processing time. Recovery of recyclables at mixed-waste MRFs
ranges from 15 to 20 percent of the input waste stream. A clean MRF is simi-
lar to a dirty MRF in that it has a tipping floor, processing, and storage areas.
However, the recycled material is collected separately from the MSW, allowing
for higher-quality material. There is less separation needed for the recyclables,
compared to the dirty MRF. Most clean MRFs have two processing lines—one
for commingled containers such as glass, plastic, and metal, and the second for
fiber such as cardboard, newspaper, and high-grade paper. Similarly, clean MRFs
can be manual or highly automated.
In 2005, 545 MRFs were operating in the United States, with an estimated total
daily throughput of 86,000 tons per day (tpd). The distribution is 140 (24,351
tpd) in the Northeast, 156 (20,782 tpd) in the South, 132 (18,793 tpd) in the
Midwest, and 117 (22,042 tpd) in the West.

7 COMPOSTING

Composting is a biological process where the material is decomposed with the
help of microorganisms to a simpler organic matter that can be used as soil con-
ditioners. The compost can be sold or given away to landowners, landscaping
companies, and farmers for use in gardens, parks, and farms. Compost is environ-
mental friendly and can reduce or eliminate the need for chemical fertilizers. It
can also be used to remove solids, oil, grease, and heavy metals from stormwater
runoff and amend contaminated, compacted soils (U.S. EPA 2007).
Here are the three most common composting methods:
1. Aerated static pile composting . This involves introduction of air into the
stacked pile of mixed organic waste via perforated pipes and blowers.
Layers of loosely piled wood chips or newspaper can also be added to
facilitate the air passage from the bottom to the top of the pile. This
method produces compost in three to six months. However, it is weather
sensitive and thus may result in the loss of microorganisms responsible
for the composting process.
2. Aerated windrows composting . Organics are formed into long and narrow
piles that are turned with windrows turner equipment to reach required
temperature and oxygen requirements. The pile height is between 4 and 8
feet which allows for the generation of sufficient heat and oxygen to flow
to the center of the windrows. The pile width is usually between 14 and
16 feet. The disadvantage of this method is that it is used for only large
volume of material thus requiring abundant space. There could be odor
problems and leachate concerns with the application of this method.
3. In-vessel systems . Organic waste is fed into perforated barrels, silo drums,
or specially manufactured containers that are simple to use and require