Vapor phase or liquid phase, that is the question wastewater utilities ask when faced with finding a solution to noxious odors.
At some point, every wastewater treatment plant or water resource recovery facility has been subject to comments, concerns or questions about wastewater odor. Through the process of collecting and treating the wastewater from sewers and collection systems, nuisance odors are emitted and often become a point of concern for treatment or mitigation.
Watch this sponsored video interview with Vapex Environmental to learn more about odor control for wastewater systems.
What is wastewater odor?
Wastewater odor is predominantly expressed through the chemical compound of hydrogen sulfide or H2S, a colorless gas. H2S is a natural by-product of septic environments and/or anaerobic digestion processes. Anaerobic digestion is commonly used to treat liquid wastes at wastewater treatment plants as it creates an environment devoid of oxygen.
This type of environment can also be found within sewers and wastewater collection systems into which human wastes have been flushed. The waste flushed down a toilet can take several hours — or even up to a day in low-flow systems — to reach the wastewater treatment plant for treatment. During this time, septic conditions can occur, which explains why foul odors can be smelled when passing by a manhole or a sewer opening, particularly during the summer months because heat contributes greatly to these conditions.
What is wastewater odor control?
Wastewater odor control is the process by which the malodors in sewage are either prevented from being formed in the first place with liquid phase treatment — treatment within the wastewater — or by capturing and/or treating the vapor coming off the treatment process — vapor phase treatment.
Different types of odor control systems
Odor control technologies fall into one of two categories: liquid phase or vapor phase. The odor control solutions range from mechanical systems to chemical additives to naturally occurring oxidants.
Liquid phase technologies
In liquid phase odor control solutions, chemicals or treatment technologies are directly introduced into the wastewater liquid during the treatment process. Depending on the solution used, liquid phase odor control either aims to prevent the noxious odors in the first place or the solution will be applied to the wastewater stream at a point in the treatment process to have the greatest mitigation effect on foul odors.
Vapor phase technologies
For vapor phase technologies, the treatment is used to treat the air vapor resulting from the treatment processes. Hydroxyl radicals, wet scrubbing, activated carbon and biological systems are among the list of vapor phase technologies used by wastewater treatment facilities. The biological systems can be split into two categories — biofilters and bio-trickling filters — both of which use microorganisms, commonly referred to as bugs among wastewater professionals, to treat the odors.
Among the vapor phase technologies are oxidation treatment technologies, which are popular among wastewater professionals due to their natural occurrence and sustainability. An example of an oxidant is the hydroxyl radical (HO), which is a diatomic molecule consisting of one hydrogen atom and one oxygen atom.
Hydroxyl radicals are found naturally in the environment and aid in mitigating air pollution, namely greenhouse gases such as methane and ozone. The HO molecule is highly reactive because it has an unpaired electron, giving it a high oxidation potential. This potential makes HO a powerful tool for oxidizing H2S, ozone and other sulfides that are the common culprits of noxious odors in wastewater systems.
Hydroxyl radical technologies generally use a small footprint because they do not strip out the offensive odors, and when use in wet wells, they can provide some auxiliary effectiveness in mitigating fats, oils and greases (FOG). FOG build up can diminish flow through pipes, damage pumps and create treatment headaches at wastewater treatment plants, adding to a wastewater system’s maintenance needs.
Why is wastewater odor control needed?
The primary driver for wastewater odor control is to address or manage complaints from the community or nearby residences that recognize these offensive odors as a nuisance.
The smell of H2S is often equated to the smell of rotten eggs when found at low concentration levels in the air, but in heavy concentrations, the odor becomes intensely sweet smelling. Other offensive odors produced in wastewater treatment and collection systems include dimethyl disulfide, dimethyl sulfide, and mercaptans.
Due to the foul smell of these odors, customers often complain to the facility about them as they create a nuisance to the environment. Foul odors produced in this way are not governed by the Clean Air Act, which is enforced by the United States Environmental Protection Agency. The Clean Air Act was developed in 1990, and these odors are excluded from federal regulation because they are not deemed to be threats to public health. Instead, it was determined that states, cities and local governments would take up the regulations to address these odors within the proximity of where they are found.
While treatment plants generally are constructed away from residential areas, communities grow and change. Housing developments close in on the proximity to the treatment plants, which then results in more concerns and complaints about offensive odors.
Lastly, odor control can be useful for addressing public safety and the health of a wastewater collection system. For example, while the concentrations from sewers and wastewater plants are low, high concentrations of H2S can be fatal to humans.
Additionally, even low concentrations can be corrosive to metal surfaces and concrete, so while odor control often is driven by community concerns, it has additional benefits for utility asset management plans and operator safety.
Odor control and climate change
The overall mission of odor control is monitoring and positively impacting air quality. By reducing the chemicals expelled into the atmosphere, these technologies also contribute to minimizing the effects of climate change.
For years, wet scrubbing was the predominant odor control technology, but it required a lot of resources, particularly water, chemicals and electricity. Hydroxyl radical or oxidation systems also use those same resources, albeit with a naturally occurring molecule. Meanwhile, biological odor control systems are being considered because they use less water and electricity and do not require chemicals.
What’s the best wastewater odor control system?
Choosing the right odor control solution for a wastewater treatment plant requires a review of the practical engineering for the specified application. Utility owners and managers should consider liquid and vapor phase technologies, identify the footprint available to install the system, and the operational costs to maintain it over time.
Following the practical engineering discussions, utility managers should analyze the costs and benefits of the technologies available. In doing so they will gain the insights needed to make an informed decision on the capital and operational expenses associated with the technology they select.
