HAZARDOUS WASTE TREATMENT - Environmentally Conscious Materials Handling - page 180

3 BIOLOGICAL TREATMENT SYSTEMS

Biological treatment involves the application of microorganisms that utilize car-
bon and energy for growth to oxidize the organic matter present in the waste
stream through biochemical means under controlled conditions. The objective of
biological treatment is to promote and maintain microbial population (biomass)
that metabolizes (biodegrade) a target waste. The waste material that is adsorbed
by microorganisms is subject to different biochemical reactions, which take place
at temperatures ranging between 0 ◦ C and 40 ◦ C.
In order to carry out biochemical reactions at low temperatures, catalysts must
be present to lower the activation energy of these reactions. Catalysts are present
as enzymes, which are organic compounds produced by living organisms in
their life process. There are intracellular enzymes that carry out biochemical
reactions inside the cell and extracellular enzymes that are excreted from the cell
to carry out reactions outside the cell. In biological degradation, large organic
molecules are broken up by extra cellular enzymes through hydrolysis .Theyare
then subject to intracellular enzymatic reactions. Enzymes' role in metabolism is
to lower the energy required to activate a reaction and thereby speed up biological
activity.
Microorganisms use catabolic and anabolic metabolism. Catabolic metabolism
releases energy that is captured and transformed to support cell maintenance and
cell building activities. Energy release involves the transfer of electrons from
organic carbon. To complete this reaction, electron acceptors such as oxygen,
nitrate, and sulfate are needed. In catabolic metabolism, oxygen is consumed and
organic matter is removed. Anabolic metabolism is a cell-building metabolism.
The anabolic processes produce protoplasm as an end product, which is composed
of proteins, carbohydrates, DNA, and other components (LaGrega et al. 1994).
The biological degradation of an organic compound involves the transfer of
electrons from the waste to an electron acceptor. This process is called respi-
ration . There are two types of respiration: aerobic and anaerobic. In aerobic
respiration, bacteria use oxygen as a terminal electron acceptor. Here, oxygen is
reduced to water and organic carbon is oxidized to carbon dioxide. In anaerobic
respiration, terminal electron acceptors can be of inorganic compounds such as
nitrates being reduced to nitrogen gas; sulfates being reduced to hydrogen sulfide
gas; or carbon dioxide being reduced to methane.
During the biodegradation process, microorganisms must have the optimum
environmental conditions for cell growth. These are optimum temperature, pH,
moisture, and macro and micro nutrients.
Biodegradation of hazardous wastes can be accomplished using suspended-
growth or attached-growth biological systems. Suspended-growth treatment sys-
tems keep the microorganisms suspended freely in water. The microorganisms
are maintained in suspension within the liquid, and microorganisms convert
the organic matter or other constituents in the wastewater into gases and cell