As cities grow, wastewater treatment plants and collection facilities become closer to the public eye…and nose. The treatment facilities and collections systems can produce significant nuisance odor issues, leading to complaints from the public. Odor control is therefore becoming an increasingly relevant topic in the wastewater treatment industry.
The primary odorant of concern in wastewater is hydrogen sulfide gas (H2S), though there are others. H2S is a colorless, potentially toxic gas with a rotten egg odor. There are many factors that contribute to the amount of odor that a wastewater collection system and treatment plant can produce. These include:
- Concentration of organic material and nutrients
- Sulfate concentration
- Temperature
- Turbulence
- pH
- Dissolved oxygen
- Detention time
Odor mitigation can be divided into two general areas: liquid phase treatment and vapor phase treatment. Liquid phase treatment consists of adding chemicals to the wastewater to maintain aerobic conditions, to shift the pH higher to keep gaseous compounds in the solution, and/or to bind the sulfides with the chemical added to prevent their release.
Vapor phase treatment consists of containing the foul air, transporting the foul air to a treatment unit, and removing the odorous compounds from the air stream before exhausting to atmosphere.
Vapor Phase Treatment
The first step in the vapor phase treatment method is to contain and transport the foul air to a treatment unit. There are a variety of engineered systems to accomplish this. In collection systems, the conveyance pipe itself serves as the containment and transport structure. Foul air is pulled by friction forces in a gravity sewer’s headspace.
Once the foul air is contained, it is withdrawn from the contained area. Once at a treatment unit, biological systems and adsorption engineered systems are routinely used to treat odorous air. Biotrickling filters are biological systems that remove odorous compounds using naturally occurring microorganisms that consume the compounds and produce non-odorous (or less odorous) compounds as a byproduct. Adsorption systems, such as carbon adsorbers, function by adsorbing compounds onto a dry media and allowing non-odorous air to exhaust.
Biological Treatment
Biotowers (or biotrickling filters or bioscrubbers – there are subtle differences, but we’ll discuss them together here) are biological treatment systems that usually consist of a tower filled with inorganic media, an irrigation system and a recirculation pump for pH control, and a leachate drain. Normally, reclaimed water is used for irrigation and to provide nutrients for microorganisms that have grown on the media, but potable water can be used with suitable adjustments to the system.
Biotowers typically perform best when airflows and odor concentrations do not fluctuate excessively. When H2S concentrations fluctuate rapidly, higher than normal concentrations will be exhausted from the biotrickling filter until the microbiology becomes acclimated to the higher concentrations. Biotrickling filters can effectively remove hydrogen sulfide to high efficiencies but vary in their removal of other reduced sulfur compounds. Biotowers are often the most cost effective choice for air streams with high H2S concentrations.
Adsorption Systems
Adsorption systems, such as carbon adsorbers, are a well-established odor control technology. Maintenance is generally limited to replacing the carbon or other dry media once the adsorptive capacity has been exhausted.
Because of the frequent carbon replacement needed at higher H2S loadings, carbon adsorbers are best suited for low concentrations of H2S (typically below 10 ppmv). Careful attention to specifying carbon properties can significantly extend the media life and efficiency. Dry media adsorbers are well suited for “polishing” the airstream after treatment by another odor control unit and for removing non-H2S compounds. In a carbon adsorber with fresh carbon, contaminant removal can exceed 99-percent. Proprietary adsorptive medias (other than, or in tandem with carbon) can be used to target specific odorants where special needs exist.
Chemical Scrubbers
Chemical scrubbers are widely used in the municipal wastewater treatment industry and are highly effective at removing hydrogen sulfide and other odorants. Chemical scrubbing technology is most economically used for higher airflows and lower H2S concentrations.
Chemical scrubbing systems have shorter residence times than biological systems and in some situations can have lower initial installation costs, but they typically have relatively high long-term operation and maintenance costs due to chemical purchase requirements, scaling within the scrubber systems requiring acid cleaning, and maintenance needed. Chemical scrubber systems are typically most effective at larger installations where space is limited, and a trained maintenance staff can be dedicated to operation and maintenance of the systems. They are also more appropriate for intermittent loads (for instance, for a dewatering building that isn’t operated 24 hours a day) than a biological system. Chemical systems may not be appropriate for lift stations and other remote facilities that are not continuously staffed.
Liquid Phase Treatment
Liquid phase odor control focuses on introduction of chemicals into the wastewater stream, either preventing the formation of odor compounds or preventing the release of compounds that have already formed. Inhibitors work by either inhibiting the bacteria that reduce sulfate to sulfide or by substituting for sulfate as an oxygen source. pH control works by raising the pH of wastewater to prevent the release of dissolved hydrogen sulfide into the air. When feasible, liquid phase odor control methods should be pilot tested to determine effectiveness and dosage before full scale installations are commissioned.
Air and Oxygen
Most odors in wastewater systems can be prevented by maintaining aerobic conditions. It is difficult to maintain aerobic conditions naturally, so supplemental oxygen or air can be added to maintain a dissolved oxygen (DO) concentration of least 0.5 to 1.0 mg/l. Adding air or oxygen will allow the oxygen to directly oxidize odor causing compounds and provide DO for aerobic metabolism by microorganisms.
Oxidizers
In addition to air and oxygen, other oxidizers can be used to chemically attack odor-causing compounds. Examples of oxidizers include chlorine, sodium hypochlorite, hydrogen peroxide, potassium permanganate, and ozone. Most oxidizers are highly reactive and present safety and environmental concerns that make them less desirable than other odor control alternatives.
Nitrates
When oxygen in wastewater is depleted, microorganisms will preferentially reduce nitrates before reducing sulfates. Since the microorganisms are not reducing sulfates, formation of many odorous compounds is prevented. Calcium and/or sodium nitrate can also be used to remove dissolved sulfide from wastewater.
Precipitants
Precipitants are chemicals added to wastewater that chemically bind to dissolved sulfide to create insoluble precipitates. Iron salts (e.g., ferrous chloride) are typically used. Precipitates typically don’t settle in the collection system but will readily settle with other solids in the wastewater treatment plant. Use of precipitants can significantly increase the solids output from a treatment plant.
pH Control
pH control works by raising the pH of wastewater to approximately 8.5 or higher. At a pH of 7.0, approximately one half of dissolved sulfide present consists of H2S. H2S can easily be stripped from the wastewater due to turbulence or to reach equilibrium with the atmosphere. By raising the pH, the percentage of H2S decreases. Chemicals such as magnesium hydroxide are typically used due to their non-hazardous nature.
Odor Treatment Key Takeaways
Odor control technologies are not a case where one size fits all. Each technology has a situation for which it is best suited depending on the wastewater characteristics, odor characteristics, site requirements, and operator preferences.
In general, chemical addition has relatively low capital costs and high life cycle costs. Biological systems tend to have higher capital costs but lower life cycle costs and do not present inherent chemical hazards. A good odor abatement strategy takes into account the specific needs of your project. Working with a team of water/wastewater engineers who are skilled in odor control can help you manage wastewater odors effectively.