By Peter Chawaga
If you were somehow transported back to the 19th century unawares, the first thing to tip you off might be the smell. There you’d be, walking down a cobblestone street, possibly curious about the odd dress, the lack of electric light, maybe the inexplicable livestock milling about. But before you got your bearings, a whiff of nauseous air would announce itself and instantly you’d think, “I should not be here.”
A lot has changed over the last 200 years, but as far as daily existence is concerned, the smell of our surroundings today is probably the most noticeable. Formerly a chief contributor of offensive scents, the water treatment industry is now overcompensating for its sins. Today there is no shortage of odor control solutions — activated carbon, chemical wet scrubbing, tall stack dispersion, biofiltration, hydroxyl radicals, ionization — and the variety can make finding the best one for your plant a daunting task.
With this in mind, Richard J. Pope, PE, BCEE of Hazen and Sawyer, and Thomas J. Lauro, PE, Westchester County Department of Environmental Facilities, developed “Steps Leading to an Effective Retrofit Odor Control System,” a paper dedicated to narrowing the odor control field and identifying the right fit for a given existing plant.
For the readers’ convenience, Pope and Lauro divided their paper, which was presented at this year’s WEFTEC, into seven questions that will lead a plant to its most effective retrofit odor control system. This article will follow suit.
What are the regulatory requirements?
The case study that inspired Pope and Lauro’s work featured the New Rochelle Wastewater Treatment Plant (NRWWTP). The Department of Environmental Facilities that governed this plant set a general “good neighbor” goal regarding odors and with a residential neighborhood, park, and school nearby, this status wasn’t particularly easy to maintain. It’s necessary for all plants to fall in line with state and Department of Health regulations when addressing odor.
In the case of the NRWWTP, located in New York, the state has air quality standards that regulate hydrogen sulfide (at levels of 10 ppb) and the emission of volatile organic compounds (VOCs). These provided more tangible goals than a “good neighbor” guideline for the plant to follow if they hoped to get the permits necessary to install an odor control system. Staying informed of the various health and environmental departments that regulate your jurisdiction is a first step towards controlling plant odor.
What are the odors and how strong are they?
The NRWWTP staged an odor investigation. This was an evaluation of each plant process unit to determine the source and concentration of odors being emitted. A wastewater fate model was used to assess the emission of toxic VOCs from each and this established the baseline odors and an initial view of areas that needed control.
The paper includes the following list of locations that were sampled at the NRWWTP:
- Influent channel area in the influent pump station
- Primary settling tanks at the following locations: influent, mid-tank, effluent, effluent launder, and effluent channel
- Gravity thickener tanks exhaust
- Sludge storage tanks exhaust
- Sludge processing building exhaust
- Final settling tanks at the following locations: center feed well, mid-tank, and effluent launder
Methods of analysis included hydrogen sulfide measurement, testing of the dilution threshold and recognition threshold (values at which someone can detect or recognize an odor), reduced sulfur compound measurement, and carbon dioxide measurement.
How do odors impact off-site receptors?
An air dispersion model, run with the baseline data collected during the odor investigation, can determine the areas needing control, the impact of the monitored odors, and whether the plant has managed to comply with regulations. In the case of the NRWWTP, a locally approved model was used. It’s known as a Gaussian model, a mathematical simulation of the buildings, tanks, enclosures, and processes at the plant, and all of the identified odor sources. Odor receptors for the model were set up around the plant fence and in a rectangular grid at 100 and 500 meter intervals as they moved away from the fence line. It was also set up to evaluate plant emissions using five years of meteorological data.
The model found that beyond the solids storage, processing, and truck load areas of the plant, the headworks, influent channels, primary settling tanks, and primary effluent channels had to be controlled.
What steps and technologies are necessary to mitigate the plant odors?
Finally, after rigorous testing and analysis, you will be ready to determine how to best rid your plant of odor. Using the results of the odor investigation and air dispersion model, conduct a technology review of what’s available to cure your plant’s foul smelling woes.
The NRWWTP ended up going with a packed-tower wet scrubber to mitigate smells from its primary tanks, influent and effluent channels, solids storage tank, gravity thickening tank, solids processing building, and truck load-out areas. For them, it was the right solution because it was a proven technology, required limited space, and had high removal efficiency, quick fluctuation response, and low carbon dioxide levels.
To control odors at a new headworks building, NRWWTP staff chose a biofilter with the ability to remove 99 percent of hydrogen sulfide, over 90 percent removal of organic reduced sulfur compounds, and low operating and maintenance requirements.
What air permits are required?
Both of the solutions embraced by the NRWWTP, the packed-tower wet scrubber and biofilter, require air permits in New York. It’s likely the case that your area requires such certifications as well, so do your homework.
How are the controls installed while maintaining odor control and plant operations?
Pope and Lauro recommend assembling a “maintenance of plant operations plan” to transition between existing systems and new odor control units. If possible, this should include a temporary odor control unit in case anything goes wrong during the conversion.
To switch from the two existing mist scrubbers to the two packed-tower wet scrubbers at the NRWWTP, the team was able to switch the first wet scrubber for one mist scrubber while the other treated the total flow by itself. With that wet scrubber engaged and online, they had it handle the total flow while installing the second wet scrubber. The headworks building previously had no odor control, so installing the biofilter required no such acrobatics.
Will it work?
The most important question is saved for last. The NRWWTP case study was a success. To determine the answer at your plant, you should run a performance test. Review operating data to find out if regulatory requirements are being met. Are the odors from the identified plant sources being contained and treated? Are the discharges from the new technologies you installed exceeding expectations and compliance requirements? Check with your neighbors. Are they happy? Have you and your nose come back to the future?