Environmental Engineering Reference
In-Depth Information
reaction rate is limited by how much material diffuses through. Within the biofilm,
substrate is utilized for biological growth. Biofilm loses mass continuously due
to erosion of small pieces or sloughing of large pieces. Wastewater is dispersed
with rotating spray distributors that spin due to the jet action of sprays. They are
simple to operate, resistant to shock loadings, and require lower power. However,
the BOD removal rates are lower than activated sludge systems and they may
have higher suspended solids in the effluent. The two types of operating trickling
filters are low-rate and high-rate trickling filters.
3.6
Rotating Biological Contactors
A rotating biological contactor (RBC) is an attached-growth biological treatment
system that contains approximately 10- to 12-foot-diameter closely spaced plastic
numerous discs that are connected with flow across shaft located in a rectangular
basin. The rectangular basin is filled with the wastewater that needs to be treated.
Approximately less than half of the discs are immersed in wastewater. As the
discs rotate at a slow speed, they submerge into the wastewater and aerate as
they come out of the wastewater. Biofilm continuously grows on discs as the
rotation brings microorganisms in close contact with the wastewater. Rotation
also causes shearing action and causes sloughing of biofilm, keeping a constant
biofilm thickness. Aeration is provided by the oxygen transfer into the biofilm
while the discs are rotating outside of wastewater and by mixing turbulence
that entrains air into tank liquid. The discs are generally covered for temperature
control and to prevent photodegradation. The RBCs are used to treat low-strength
industrial wastewaters due to insufficient oxygenation provided by this system
(Woodside 1999).
3.7
Bioremediation
Bioremediation is the use of biological organisms to aid in the removal of
hazardous waste from a contaminated area. The goal of bioremediation is to stim-
ulate microorganisms with nutrients and other chemicals that will enable them
to destroy the contaminants. Bacterial bioremediation becomes an engineered
process when the natural bacterial feeding, growth, and reproduction cycles are
enhanced in some way as to increase the rate at which contaminants are removed
from a hazardous waste site. The method for enhancing these cycles is by opti-
mizing the environmental conditions for the bacteria, whether through increased
nutrient loading; stabilizing and/or maintaining ideal temperature, light, oxygen,
or moisture conditions; or other parameters that are beneficial for a bacteriological
community.
In hazardous waste management, some contaminants are more difficult to treat
than others. Some treatment processes, even though they work effectively, may
be prohibitively expensive. Bioremediation presents itself in many cases as a
less costly solution to these problems. It can break down some of the quite
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