Odor emissions have become a primary focus for operators across the country as decades of growth placed housing and commercial developments closer to wastewater collection and treatment systems, which are often a source of odors. In an era in which neighbors have increasingly become intolerant to odor and municipal budgets have shrunk, many operators seek effective odor control solutions that not only treat the full range of odor compounds, but also require minimal operation and maintenance costs.

Various odor control technologies, including carbon adsorption or masking agents, have helped facilities deal with the smells. Depending on factors such as scale and circumstance, such technologies have managed to be effective, though this market remains quite receptive to new ideas, and more powerful and less expensive methods.

One such new idea is actually a rather old one: seashells. Bord na Móna, Greensboro, N.C., has developed a patented, shell-based system that utilizes naturally occurring and renewable shells as its primary media. The system can control odors from wastewater pumping stations, wastewater treatment works, sludge handling, municipal solid waste and composting centers, as well as various industrial facilities requiring H₂S, odor or VOC removal. The technology, since its inception in the early 1990s, has been employed in more than 600 installations worldwide.

The sulfide-laden, odorous airstream is biologically treated in the liquid phase by recirculating water within the unit, allowing contact between selected microorganisms and odorous compounds. The bacteria reside on the shell media, which contain high levels of CaCO₃ and neutralize acid byproducts from sulfide oxidation. The physical, structural and chemical properties of the media allow smaller filters and higher efficiencies. The byproducts of the system are harmless, clean and free of odor.

The shell media have an adsorption capability and a chemical affinity for sulfur compounds. Their physical shape, size and rigidity render the media highly effective as a scrubber packing material, while their natural pH buffering and the slow-release nature of the media render the shells ideal for pH control. The technology is not new; the company has been installing this technology on a range of applications for more than 10 years, but many in the U.S. market are still hearing about it for the first time.

Development creates demand

The Mónashell odor filtration system above was installed and operated at the Cary, N.C. facility.

Some of the country's larger, more high-profile utilities exist in Florida, where there are frequent odor challenges due to high temperatures, humidity and septic conditions, and many collection systems are located adjacent to large housing developments, corporate offices and tourist destinations.

An odor problem arose at a master pump station adjacent to a new housing development within a central Florida utility's jurisdiction, where odor complaints from neighbors required action. The pump station, which contained a maximum of 3,000 cubic feet of odorous headspace at low water level, was originally built without odor control, most likely because there was no one around at that time to be affected by odor emissions. As in so many regions across the country, however, Florida's growth-based economy spurred substantial development in this once rural area. Where orange groves once stood, there are now gated communities complete with four-bedroom homes and swimming pools. Where bottomland forests covered the landscape, there are now shopping plazas and gas stations. The result: This once-remote pump station and associated fugitive odors are now nearly surrounded by inhabitants. Operators had to act quickly to maintain a good neighbor policy and avoid environmental regulatory fines due to sewage odor. The timing was right as the odor control company had already approached a prominent Florida utility to pilot its technology, and the county agreed to participate in a pilot study utilizing a 500-cfm unit.

The fully operational, modular unit was shipped to central Florida on a flatbed and installed in late September 2009. The county had prepared the site by establishing a water supply, electrics and drain, which accommodated the biofilter, fan and irrigation sumps. Installation consisted of a single day to place the biofilter and connect the water, electricity and drain. Grounds crew also ran a 15-foot duct from the wet well to the system's inlet.

Because only two small drives (the fan and pump) were required and no chemical or nutrient dosing systems were needed, installation was a relatively painless process. The supplier's staff commissioned the unit on Oct. 1 by inoculating the filter bed, setting airflow and completing a system check. Inoculation consisted of a one-time dry blend dose of microbial H₂S oxidizers.

Twenty-four hours after inoculation, the system was achieving more than 99-percent removal efficiency of average H₂S inlet levels of 41 ppm and maximum levels of 90 ppm. The system also achieved up to 95-percent reduction of difficult-to-treat organic sulfides (e.g., methyl mercaptans and dimethyl sulfide) just days after inoculation. As of Nov. 10, 2009, field tests indicated inlet H₂S levels of 40 ppm had been reduced to 0.009 ppm.

The pilot system continued to operate with minimal attention through July 2010 and H₂S monitoring and four rounds of odor panel testing were conducted. Data collected from May and June showed an average H₂S inlet concentration of 27.6 ppm, peaking at 124.5 ppm. The average H₂S at the outlet was reduced to 0.1 ppm, with a single spike to 0.4 ppm, so the removal efficiency remained at about 99.9 percent. Most gas detection alarms are set to go off at 5 ppm.

Cracking the odor code

Reduction of air contaminants is but one important aspect of air quality analysis at any emission source. Human perception of air quality is another. Odor threshold, the point at which an odor is no longer perceptible to a human test panel, can be quantitatively analyzed using a standardized protocol (ASTM E679-04).

This analysis was performed four times during the ongoing central Florida installation and odor was found to have been reduced by 98.9, 97.6, 99.6 and 99.9 percent, corresponding to outlet odor levels of 920, 620, 350, and 575 dilutions to threshold, which were excellent results for a single-stage odor control system treating high inlet H₂S levels. Several rounds of compound testing showed organic sulfides were consistently in the low-ppb range or below the threshold of detection.

The pilot program verified that the odor control system could provide effective H₂S and odor removal even with high inlet levels. The pilot program also allowed the collection system staff to gain familiarity with the operational requirements of the system. Because the system was able to effectively contain and treat odors from the lift station, the lease was extended to allow time to arrange for the permanent installation of an odor control unit at the site.

Other sites

In pilot testing conducted at a water reclamation facility in Greensboro, N.C. similar performance was obtained with much higher H₂S inlet levels. The average H₂S inlet level was 34.5 ppm, the average H₂S outlet concentration was 0.05 ppm. Inlet levels varied with the highest reading of 376.6 ppm, the outlet was 1.08 ppm, for a removal efficiency of 99.7 percent.

In March 2010, a full-scale odor control unit was installed by the Town of Cary, N.C. for treatment of odor emissions from a flow distribution box at its North Cary WWTP. The system treats 500 cfm of air containing up to 25 ppm H₂S. H₂S outlet levels have consistently been below 0.1 ppm, and often below the detection limit of 0.01 ppm. PE