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Odor Control

 

"Three odor control units were installed at the plant in 2001.  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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Last modified: May 3, 2018