There once were two cats from Kilkenny
Each thought there was one cat too many.
So they fought and they fit
And they scratched and they bit
And instead of two cats, there ain’t any!
Wastewater treatment plants and their neighbors sometimes can behave like the two cats from Kilkenny, fighting about odors and their impact on home values.
Odors are often elusive and sometimes unbearable. They can defy scientific documentation but continue to persist, like ghosts often seen but not heard. As neighbors continue to encroach on treatment facilities, odor management is increasingly important.
Dust can be an important component of odor, since it adsorbs odors and can be transported long distances. Fortunately, treatment operators have a variety of options for keeping odors under control and maintaining good relations with the community.
Multiple sources
Odors are generated at several locations in wastewater treatment plants:
• Influent pump stations (typical odorants are hydrogen sulfide, reduced sulfur compounds, ammonia).
• Headworks (screening and degritting).
• Primary sedimentation basins.
• Aeration basins (low hydrogen sulfide, volatile organics).
• Sludge storage tanks (hydrogen sulfide, reduced sulfur compounds, ammonia, amines, organic acids).
• Dewatering, drying, loading (polymer odors, organics, ammonia).
• Digesters (hydrogen sulfide, reduced sulfur compounds, ammonia, amines, organic acids).
The concentration of odorous contaminants depends on the rate and kind of ventilation used for the enclosed systems. For example, covered buildings can operate under negative air pressure and all odorous gases can be vented through an odor treatment system.
Odor evaluation methods can be classified in three broad categories. Analytical methods include gas chromatography, sensors for specific chemicals, colorimetric detector tubes (low cost), and electronic noses with proprietary sensors. Olfactory methods include field olfactometry using a standard odor intensity referencing scale (OIRS), dilution-to-threshold dilution factors, and the olfactory threshold of an individual, such as an odor inspector. Community odor surveys can also be used to gauge the degree and geographic extent of odors.
The general philosophy for controlling or treating odor emissions is to try simple methods first before implementing complex treatment strategies. This lowers the cost of an odor-management plan. Here is a look at the benefits and potential drawbacks of several basic methods for regulating odors and achieving the desired goal at the fence line.
Ambient air dilution
For contaminants not subject to regulatory requirements, such as VOC or HAPs, facilities can improve dispersion of odorous gases by increasing stack heights and optimizing the stack diameter to achieve an exit velocity of 3,000 feet per minute.
Source control
Source control involves covering the source to prevent the emission, or chemically treating the liquid-phase in the collection system to remove the odor-causing compounds and reduce sulfide corrosion.
Physical covers have limitations in that they concentrate contaminants inside the enclosed headspace increasing sulfide concentration and thereby accelerating corrosion inside the vessel. That involves capital expense for repair and makes access more difficult, as it may require confined-space entry.
Chemical treatment, if applied upstream in the collection system, can provide odor control and corrosion prevention at downstream points, such as manholes, air relief valves and re-pump stations. Treatments include addition of iron salts, nitrate solution, oxidizing agents like hydrogen peroxide, and chemicals that disrupt the sulfate-to-sulfide bioconversion process (such as the NanoCleanse enzyme solution or anthraquinone).
Chemical oxidants such as chlorine, sodium hypochlorite, potassium permanganate, ozone, hydrogen peroxide and ferric salts require on-site storage and handling of chemicals, raising safety, exposure and liability issues. Chemicals also require metering systems and proper mixing. Finally, chemicals can produce byproducts, such as precipitates.
Another option is air oxidation using venturis, blowers or compressors. These methods increase operating costs, limit gas-transfer rates and cause odorous compounds to be stripped out and released to the ambient air. Pure oxygen can be used, although it is five times more expensive than air, requires on-site storage and causes precipitation of iron oxide. Oxidizing compounds such as hydrogen peroxide are classified as hazardous and are consumed in large quantities since they oxidize everything in the wastewater — BOD, odorous compounds, and other constituents in the wastewater.
Use of chlorine or hypochlorite causes sulfides to oxidize to sulfate. The ratio of chlorine to sulfide is 8.9 under low pH and 2.2 at high pH. In field applications, the ratio used is 5 to 15, since chlorine also indiscriminately oxidizes other compounds. The major issue with chlorine and hypochlorite is the formation of halocarbons from the organics in the wastewater, and the release of these carcinogenic compounds into the ambient air.
Iron salts are specific to sulfides and do not react with other odorous compounds. Ferric salts, such as ferric chloride, react with sulfides to form sulfur while being reduced to ferrous iron. The ferrous iron reacts with dissolved sulfide to form ferrous sulfide, a light black precipitate. Ferrous sulfide also oxidizes to ferric sulfate in the aeration basin, wherein it can remove phosphorus. The major issue with iron salts is their hazardous nature, requiring double-walled tanks and piping.
Nitrate biochemically oxidizes the sulfide to sulfate in the bulk flow and in the surface layers of slime. It is also consumed biologically, since it serves as an alternate electron acceptor. Under anaerobic/facultative conditions, fairly high concentrations of nitrate and good mixing are needed to reduce sulfide concentrations in the water. Also, the sulfate formed can again reduce to sulfide under the proper conditions.
Enzyme solutions, such as NanoCleanse biochemically prevent formation of sulfides. This can be a cost-effective strategy to reduce the formation of hydrogen sulfide in anaerobic digesters.
Point-of-discharge control
Technologies used for point-of-discharge odor control are activated carbon adsorption, chemical sorbents, chemical scrubbers and biofiltration.
Activated carbon adsorption is simple and can be used to adsorb a wide variety of odorous compounds. While economical at low inlet contaminant concentrations, it is not effective for ammonia and other nitrogen-based compounds. Water-regenerable carbons can be reused only a couple of times, since their capacity decreases with each cycle. They use large amounts of water, and the metal oxides chemically convert mercaptans in the gas stream to disulfides, which sometimes makes the exhaust gas smell worse.
Chemical sorbents react with hydrogen sulfide and mercaptans to form solid products. Sorbents can use organic substrates such as wood chips or an inorganic support. Iron oxide sorbents react with hydrogen sulfide to form sulfides, which are oxidized by the air in the exhaust gas to form sulfur, and the oxide is regenerated. The sulfur deposited eventually clogs the system and requires change of the sorbent.
Recently, a new class of highly porous sorbent foam products have been developed that can be used for biogas and for odorous gases. The main advantage of these sorbents is that they do not react with carbon dioxide, an issue with chemical scrubbers using sodium hydroxide. Other sorbents used include potassium permanganate dispersed on porous supports to oxidize the odorous compounds.
Chemical scrubbers use a solution of sodium hydroxide and sodium hypochlorite to solubilize and oxidize the hydrogen sulfide and reduce sulfur compounds. In multi-stage systems, the consumption of sodium hypochlorite is reduced by first scrubbing with sodium hydroxide to solubilize the hydrogen sulfide, and then reacting with hypochlorite in the final stage.
Potential issues with chemical scrubbers are the cost of chemicals, on-site storage of hydroxide and hypochlorite, corrosion of liquid-handling systems, and release of chlorine gas, which is also odorous and can react with organics in the water (transferred from the gas phase) to form carcinogenic halocarbons.
Biofiltration uses either naturally bioactive media, such as compost, soil, or synthetic media, such as randomly packed synthetic material or monolithic media. Compost and soil media systems have a large footprint, eventually require media replacement, and can handle low inlet hydrogen sulfide concentrations.
Synthetic media systems are more robust, can handle higher inlet contaminant concentrations, have a smaller footprint, and offer higher treatment capacity. The most common media is polyurethane foam, either as randomly packed small cubes or as a monolith, formed by wrapping foam sheets into a cylinder inserted in the vessel as a one-piece cartridge. Biofilters biologically convert hydrogen sulfide to sulfate in the water, resulting in low pH within the media. A potential limitation of foam media is plugging due to biomass growth, resulting in higher gas-phase pressure drop over time.
Biofilters treating a mixture of organic and inorganic odors need to address biomass growth and media plugging. One such system uses a different synthetic media that can be periodically washed down while the system is still operating to prevent biomass accumulation. The water wash is operated automatically by the gas-phase pressure drop across the biomedia beds.
More on the horizon
Odor treatment is still an emerging area, and operators frequently try different technologies depending on the circumstances. For any facility contemplating odor control, an overall analysis up front can help in developing a comprehensive odor management plan. The choice of odor treatment technology is not a matter of one system over another but of developing a synergism of various methods to create an overall treatment system that is cost-effective and manageable.
ABOUT THE AUTHOR
Rakesh Govind, Ph.D., is a professor of Chemical & Materials Engineering at the University of Cincinnati and president of PRD Tech Inc., a provider of odor control and other clean environment technologies based in Cincinnati, Ohio. He can be reached at
rgovind837@aol.com.
