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Odor Control
"Three odor control units
were recently installed at the plant. The units draw foul air in from
various treatment processes and through carbon canisters stored in the units.
The units trap the hydrogen sulfide gas that creates the odors. The
hydrogen sulfide is then washed off of the carbon canisters with water,
regenerating the carbon and allowing continual treatment of the odorous air."
Although
odor generation at wastewater treatment plants has always been a problem, odor
control at WWMA over the past few years has grown increasingly worse. The
number of complaints received from the residents in the WWMA service area have
increased over the last several years. With the recent approval by the EPA
of the Clean Air Act, communities around the state and country are evaluating
various methods of reducing the emission of odors from wastewater treatment
facilities.
There are several
process areas at WWMA that can be significant sources of odor. They
include the wet well area, the primary clarifiers, the sludge processing area,
the gravity thickeners, and the anaerobic digesters. Other areas, such as
the final clarifiers and aeration tanks, can also generate odors, but are not as
strong as those produced by the processes stated earlier. The main
odor-generating areas on which focus is given are: the wet well. the sludge
processing area, the primary clarifiers, and the gravity thickener.
Existing System Setup and Requirements
WWMA has historically used chemical masking as its method of odor control.
This system had not been very effective and did not provide for a permanent,
long-term solution. WWMA used a chemical masking agent that has a cherry
odor in order to cover up the odors emitted by areas that produce excessive
amounts of Hydrogen Sulfide. Additional "flavors" of
odor-masking agents had been tried with minimal success. The system
required that plant workers transport 55-gallon drums around the plant daily to
spray the odor-causing areas with the chemical masking agent. The chemical
was also sprayed on an as-needed basis when the odors became particularly
offensive and was added to the incoming wastewater flow as needed.
Reasons for Exploring Alternatives
The
main reasons for exploring alternative methods of odor control for WWMA were
high chemical costs and the ineffectuality demonstrated by these same
chemicals. In 1998, WWMA spent over $83,000 for chemical odor
control.
Alternative Odor Treating Technologies
Alternative I - Chemical Masking
The first alternative
examined was for WWMA to continue using a chemical masking agent to mask the
odors. This would require no change in procedure, except that more money
would probably be needed because odors produced at the plant have grown
increasingly worse. Another reviewed option would involve examining
alternate masking agents in hope of finding one that would be more efficient
and/or cost effective. This option would possibly increase cost or result
in wasted money and time if other chemical masking agents would be tried with
unsuccessful results. In addition to cost, safety must be taken into
consideration. Chemical masking is never recommended if the chemical
renders the odors from the plant undetectable.
Pros
and Cons
PRO:
System currently used by WWMA
PRO: Can be used when odors are
strong
CON: Somewhat to mostly
ineffective
CON: Extremely costly, especially
for being somewhat to mostly ineffective
CON: May require physically
moving the system around the plant when addressing odorous areas
CON: Requires chemical storage
CON: Clean Air Act amendments
require more than what can be accomplished by this method
Alternative II -
Biofilters
Biofilters
are natural odor removal media which involve passing the odorous gases through a
packed bed of nutrient-rich or biologically active microorganisms. The
odorous air is forced through the packing medium containing the microorganisms,
which then use the nutrients to chemically break down the hydrogen sulfide gas
in the air. The microorganisms live on the packing medium and feed on the
hydrogen sulfide, converting it to safer forms of sulfur, such as sulfides and
sulfates. There are different types of biological media available.
Some require organic carbon to effectively break down the hydrogen sulfide,
while other microorganisms do not. Problems with this method include large
land requirements for the biofilter area and acid production when converting the
hydrogen sulfide, which can kill the organisms needed to break down the
odors. Favorable points for the use of biofilters also exist: no chemicals
are needed (reducing safety and handling issues), no chemical costs, low capital
and maintenance costs, simple design, and long media life.
Alternative III - Wet Air
Scrubbing
Wet
air scrubbing is currently one of the more popular methods of odor
control. Odorous air containing hydrogen sulfide can be passed through a
tray, packed bed, or sprayed with a mist. All of these scrubbing methods
contain chemicals which can neutralize the hydrogen sulfide gas. The most
commonly used chemicals are sodium hydroxide (caustic soda) and sodium
hypochlorite (bleach). Other chemicals are being developed for use in the
wet scrubbers, but most are still in the research phase and have not been proven
for actual use in a wastewater treatment plant. Wet scrubbers have been
proven to be very efficient and require much less area than biofilters. On
the other hand, this method requires the use of chemicals and involves issues of
economics, safety, and chemical handling.
Although
wet scrubbers are very popular, the drawbacks previously mentioned have prompted
additional research into other non-chemical possibilities of treatment. At
the present time, these alternatives are very costly.
Alternative IV - Water
Regenerated Activated Carbon System
The
final option explored in all of the systems evaluated is a specialized activated
carbon system that uses water for regeneration and requires no manual
cleaning. This system consists of a series of chambers that houses
canisters filled with treated activated carbon, a new form of carbon that
converts hydrogen sulfide to sulfuric acid, which can easily and safely be
washed away. The system is set up so that while one of the chambers is
being regenerated with water, the other chambers remain online and continue
operating. This eliminates any need for regeneration with chemicals or
taking the system offline until the carbon can be regenerated. Only one
chamber is regenerated at a time so the system has enough capacity to work as
effectively as it had before regeneration began. Canisters house the
carbon and are easily removable and returnable, eliminating hazardous conditions
and the need for landfilling. This system requires very little maintenance
and can optionally be automated.
Pros
and Cons
PRO:
Chlorination system already in place
PRO:
Initial costs are low compared to UV
disinfection
PRO:
Operators familiar with chlorination
system; dechlorination system is similar
PRO: Method has been proven to
work for WWMA
CON: Chlorine and Sulfur Dioxide
are extremely toxic and can be lethal
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