Civil Engineering Reference
In-Depth Information
Development of waste-management strategies at different levels depends on the
importance of waste cost in comparison with other production factors and on local
regulations, which may require following specific procedures, regardless of the
waste quantity involved.
As a general rule, waste management is performed in three main ways
(see also Fig.
4.5
): (1) waste reduction by using clean technologies,
(2) waste elimination, and (3) recycling of waste as raw material or
energy.
The clean-technologies approach is the most appealing, but the most expensive
and time consuming because it may require drastic transformations in all phases of
the process, from raw-material input to packaging and delivery. Technologists must
be involved in process change. Also important and not to be overlooked are the
effects of the packaging phase on urban and industrial waste, due to the plastic and
wood waste produced. In addition, the use of combustibles such as natural gas,
liquid, and solid fuels with low sulfur concentrations (less than 1 % in weight) can
be included in this area of waste management; it can be called clean technology in
facilities, because gaseous pollutants from combustion are reduced without abate-
ment plants at the end of the combustion process.
Elimination of solid and liquid waste by employing external agencies is the
simplest but generally the most expensive policy for an individual factory. Quite
often it is the only feasible one because the quantity of waste produced is too small
to justify an independent plant. Nevertheless, local regulations may require storage
and transportation in accordance with very strict rules, which may increase the final
cost of disposal. It must be pointed out that the difficulties in finding suitable sites
for this operation are growing dramatically, particularly in crowded areas such as
the most industrialized part of the world.
In the case of liquid waste, water discharge to external agencies costs according
to the pollutant content. Otherwise, liquid waste could be treated before discharge
by means of mechanical, chemical, and aerobic systems, which consume energy.
In a similar way, the treatment of gaseous streams at high temperature must be
performed in the factory, depending on local regulations (by burning the pollutant
streams and by keeping them for a given time at temperatures higher than 900-
1,000
C (1,650-1,832
F) and then cooling them to a lower temperature before
discharge from the stack). Notice that the cooling phase can be economically
performed by recovering heat from the exhaust through gas-to-gas exchangers,
thus reducing the energy consumed in treatment.
Whenever possible, recycling as raw material or energy is always the most
attractive approach.
The use of waste as raw material in the process itself remains the best form of
waste management, because in this case waste does not have to leave the factory
boundary. The use of waste coming from other factories and processes can be
