Environmental Engineering Reference
In-Depth Information
into a smaller volume and higher density and then loaded to larger truck to a
disposal facility will help to reduce the costs for transportation. However, instal-
lation of these transfer stations will increase the cost accordingly. Separation of
waste components is important in the collection of recyclables. While separa-
tion helps recycling of the useful resources, it also increases the cost for waste
collection.
As discussed before, the collected recyclables or, in some cases, commingled
solid waste are transported to the MRF for further separation and processing.
The purpose of the MRF is to separate and recover materials and to improve the
quality of recovered materials for sale in the market. The processes employed
in an MRF are highly dependent on the types of waste or recyclables received
and subsequent uses of the end products. They typically includes sorting, size
reduction, screening, density separation, magnetic separation, compaction and
baling.
As shown in Table 5.2, food waste and yard waste together constitute 25 per-
cent of the MSW. Instead of going to landfills, this organic portion of the MSW
can go through a composting process to produce compost for use as a soil condi-
tioner for garden, farm, or landscape needs. Composting is a biological process
where the organic material is decomposed with the help of microorganisms to a
simpler organic mix that can be used as soil amendments. A critical step in the
composting process is the destruction of pathogens and the U.S. EPA provides
specific standards of pathogen control in composting systems.
Incineration has been used to reduce the residue of MSW in order to save the
landfill space. Most of the incineration plants in the United States are mass-burned
incinerator using MSW as collected for the fuel. Because the incineration process
releases air pollutant and produces ashes and is expensive compared with other
MSW disposals, it has been used only in areas where the landfill is not readily
available. Almost all the recently built incineration plants have energy recovery
facilities to offset the cost of operation and air pollution control. Because of
the recent success of recycling effort, the energy content of MSW has been
decreased; therefore, the demand of incineration with energy recovery has been
significantly reduced.
Landfill has been a necessary disposal component in a MSW management sys-
tem because there are significant residues produced in the recycling, composting,
or incineration processes. A landfill is a highly regulated and specially engineered
site for disposing of MSW on land. Because of the shortage of available land
space and the fact that residents usually do not want a landfill in their commu-
nities, locating a new landfill becomes a difficult task. Landfill is an important
component of the MSW management system because the waste diversion ratio
is still much less than 50 percent. The trend of the MSW management is to place
emphasis on the reuse and reduction of solid waste and to reduce the solid waste
sent to a landfill before it goes through recycling, composting, or incineration
process for resource recovery.
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