The biology of odor control

In 1997, the Somerset (Mass.) Water Pollution Control Facility received five to 10 calls each week from residents complaining about odors. Composting was the main culprit, but the treatment plant’s headworks also were to blame.

That same year, as part of a $6 million project to modify the composting operation and control odors, the town installed biofilters; and the results have given Superintendent Frank Arnold a break from the telephone. “It’s worked unbelievably,” he says. “We still get an odor complaint – probably one a week – but it’s always the same person.”

The Somerset project is indicative of odor control efforts across the nation. Many plants that were built years ago in unpopulated areas now are surrounded by homes. In Somerset, for example, there are residences within hundreds of feet of the treatment facility.

“Most cities tried to locate treatment plants outside town, away from houses,” says Jim Clark, vice president in the Los Angeles office of Black & Veatch, a Kansas City, Mo.-based engineering firm. “In the last 20 to 25 years, as urban growth has continued in the country, all of a sudden the plants are surrounded by housing. At that point, people are complaining, and the plants have to do something about the odor.”

New plants often are designed with odor-control components, while existing plants are retrofitting, Clark adds. In both cases, biofilters are becoming a popular choice for the job.

A non-chemical reaction

Although biofiltration is proving effective in a growing number of odor control applications, it is not the only technology available. For example, activated sludge systems and biotrickling filters eliminate some foul air. But, while those technologies are designed primarily to treat wastewater, biofilters are designed exclusively to treat odors.

Put simply, biofiltration begins with a porous, organic medium (commonly, a combination of mulch, wood chips, soil, etc.) on which bacteria are cultivated. Odorous air is distributed slowly through the medium, where the bacteria react and oxidize the odor compounds.

The fact that the technology uses no chemicals is one of its biggest advantages, says John Willis, engineering and technical services manager in the Atlanta office of Brown and Caldwell, an engineering consulting firm based in Pleasant Hill, Calif. “Operationally, it costs about 5 to 10 percent of what a chemical system costs,” he says. “You don’t have chemicals, shipment, storage or pumps.”

Darrell Milligan, environmental specialist for the Hillsborough County (Fla.) Water Department, confirms the cost advantage. The county has 19 biofilters across its system, most of them at pumping stations. Milligan notes that, since the county began installing the filters, he has saved substantially on chemical, maintenance and personnel costs. “It looks like I’m only going to spend 50 percent of this year’s budget,” he says.

Serving the Tampa area, the Hillsborough County wastewater system includes 10 treatment plants and 500 pumping stations. In 1992, as part of a push toward total quality management, the county began experimenting with biofiltration. “We put one in the ground at a pumping station, and it proved its worth,” Milligan says.

In 1994, the county began installing biofilters at other pumping stations. “At wastewater pumping stations out in the communities, I really got beat up bad,” Milligan explains. “We had chemical scrubbers that were unreliable and trucks coming in and out with dangerous chemicals. And, if I had a mechanical malfunction, it destroyed the whole process. Now, [with the biofilters], I’ve gotten the chemicals out of the neighborhoods, and I’ve reduced the amount of maintenance to the point that I’ve been able to reduce staff.”

Based on the success of the “field” units, Hillsborough County installed a biofiltration system at the Sun City wastewater treatment plant in 1998. “There was a mist tower chemical scrubber that had been put there as an add-on after the plant was built,” Milligan explains. “It was falling way short of the goal.”

Situated in a retirement community, the plant handles 4.5 million gallons of flow per day. During a plant expansion, the county installed the biofilter, which treats 3,400 cfm of odorous air from the headworks, anaerobic selector basins and a pumping station that is part of the solids processing operation.

Prior to installing the biofilter, the plant received numerous complaints about odor. “Since it’s been installed, the complaints are nonexistent,” Milligan says. “It’s 99 percent efficient at removing hydrogen sulfide, which is the main compound we have to remove.”

Hillsborough County has gone on to install a smaller system (130 cfm) at its Van Dyke plant, which handles 500,000 gallons of flow per day. With biofiltration, the two plants have saved $118,000 that otherwise would have been used to purchase chemicals, Milligan says. He plans to continue installing biofilters at the county’s pumping stations.

Follow your nose

Placement of biofilters is not complicated. “In general, you would move air from odor sources, the main ones being the headworks, primary clarifiers and solids handling [stations],” Willis says.

That is the setup in Cobb County, Ga., where the South Cobb Water Reclamation Facility is installing its second biofilter. The first system is located near the plant’s headworks and is used to treat odors from scum screening and septage handling. The second biofilter, which is under construction, will treat odors from the thickened sludge storage and dewatering sites.

Unlike the plants in Somerset and Hillsborough County, the South Cobb facility is located in an industrial area and has generated few complaints. Nevertheless, plant odors did bother employees, and the county anticipated that they could potentially trouble some neighbors. “We’re not trying to rid the facility of odors entirely,” says Steve Geary, operations superintendent. “[But] we don’t want nuisance odors [affecting] the public.”

The plant, which handles 23 million gallons of flow per day, had never had a dedicated odor-control component. However, an expansion project presented an opportunity to add one.

The online biofilter, built in 1996, handles 4,000 cfm and cost $75,000 to design and build. According to Willis, whose firm designed the South Cobb systems, the second biofilter will treat 16,000 cfm and will cost the county $300,000.

Each filter (the small one measures 50 feet by 20 feet on the surface; the large one 120 feet by 35 feet) is housed in an outdoor bed with air-distribution pipes at the bottom. The pipes are covered with a layer of rock, a layer of filter media (a mixture of wood chips, dirt and compost) and 6 inches of pine bark to keep the media in place. Sprinkler systems hover over the units to keep the media moist.

Like Cobb County, the biofilter beds at Somerset use a mixture of wood chips and mulch. However, being enclosed, they do not require an extra layer of coverage.

The beds are situated side by side, and, combined, they cover 80 feet by 100 feet. Although, they are designed to handle 40,000 cfm of odorous air together, they are currently treating 20,000 cfm.

The plant has a daily flow of 3 million gallons of wastewater, and scrubbers are employed as part of the treatment. The biofilters were installed mainly as an addition to the plant’s composting process. “The composting operation has an active compost building, a screening building and a curing building, and all those buildings go to the biofilter,” Arnold explains.

Additionally, the biofilters serve as a backup to the chemical scrubber at the plant’s primary clarifiers. Odorous air can be diverted through the scrubber or through the biofilters, but, according to Arnold, “The biofilters worked out so well, it’s actually cheaper to do it without using the scrubber chemically.”

While Somerset can now bypass the scrubber at the front end of its operation, it cannot avoid scrubbing at the composting stage. Some air can be sent directly through the biofilters, but process air – moved through the compost piles and saturated with ammonia – must be scrubbed before it is diverted to the beds.

Aside from the ammonia scrubber, the facility’s odor-control process needs little attention, Arnold says. “There’s almost no monitoring,” he notes. “We check the pressure differential to make sure the [bio]filter isn’t binding; and every now and then, because the chips decompose in the media, we have to add more wood chips. That’s been done a couple of times in three years.”

Deceptively simple

Biofiltration case studies would seem to indicate that the positive effects of the technology are attained simply and guaranteed. Dig a hole, lay a few pipes, toss in a little mulch, breathe easy and count the money.

That is not necessarily the case, according to Clark. “There are people in the industry who think that biofilters are really cheap to build and operate,” he says. “But, if you really look at the numbers, that isn’t as obvious as some people think it is.”

First, there is the matter of land. “Biofilters take a lot more space than chemical systems,” explains Albert Gray, deputy executive director for the Water Environment Federation, Alexandria, Va. “You have to design for porosity and surface area. Porosity [ensures] that the odorous gas can pass through the medium, and the surface area is important because it provides a medium on which the bacteria can grow.”

Second, temperature and humidity must be maintained to ensure proper growth of the bacteria; and, finally, the medium must be changed periodically, Clark says. “Once it’s in place and you have the proper humidity control devices in place, you generally don’t have to do much with it for a long period of time,” he notes. “But, if you want the system to operate well with very little release of odorous compounds, you need to replace the media (every three to five years).

“You are saving on chemical costs, but, if you look at media replacement, the economics are really site-specific,” Clark adds. “Biofilters are not necessarily the low life-cycle alternative, even if land is available.”

Additionally, while biofilters can operate with more than 90 percent effectiveness in treating odors, that may not be enough for some plants. “Chemical and carbon treatment removes 100 percent of the odor,” Willis says. “If you have a limit you have to meet, biofiltration might be used as a secondary method. Or plants can try it out to be sure it can meet the limit.”

At the Hyperion Treatment Plant in Los Angeles, almost all the potential problems materialized in 1995 when the plant tried a small biofilter (1,000 cfm). “The main problem was one of design,” says Patty Jacobs, an environmental engineer for the plant. “There wasn’t adequate drainage, and we had an overactive sprinkler, so [the filter] was saturated all the time. We’d take core samples and find everything dead. Also, it was built out of wooden planks that weren’t sealed, so the foul air was coming out.”

Hyperion corrected the design flaws, but another problem surfaced: The biofilter was being used to treat odors from a digester gas desulfurization facility, and the bacteria in the biofilter were ineffective on the carbon dioxide and methane contained in the air stream. “It just never worked; it was a misapplication,” Jacobs says. “We gave it a good shot, but we replaced [the biofilter] with activated carbon in 1998.”

While it did not work with Hyperion’s digestor gases, biofiltration is being used successfully at Los Angeles’ Griffith Park Composting Facility. “It’s gone so well there, we hope to expand it,” Jacobs notes.

But another try at the treatment plant is not feasible. “Right now, the main reason we wouldn’t use it is [lack of] space,” Jacobs explains. The plant handles 380 million gallons of wastewater per day and ventilates 200,000 cfm of foul air. “To put a biofilter on that footprint, it would have to be huge,” she says. “It would have to be 200 feet by 200 feet. We physically don’t have the room for it.”

Additionally, she says, “In our experience, [biofilters] are temperamental and hard to control. You’re depending on bugs, and you have to get temperature and moisture right. [However], it’s not insurmountable; if it’s designed well, it’ll run well.”

Catching the bug

To take some of the guesswork out of biofiltration, several companies are manufacturing complete systems. “[In those cases, the biofilter] comes in a prepackaged unit,” Jacobs notes. “It’s pre-built, pre-designed and pretested, andit generally has the [necessary] controls and sensors; so it’s kind of a turnkey situation.”

According to Gray, there is no clear advantage between the prefabricated systems and those built on site. “Obviously, larger treatment systems, treating higher flows and having larger quantities of gas, are probably going to want to have systems built on site,” he says. “Smaller facilities with lower gas flows probably can handle it very nicely with prefabricated units that are assembled and installed on site. [However], it really is a very case-specific consideration.”

In Hillsborough County, Milligan started with “homemade” biofilters but soon found that he preferred the prefabricated systems. “They were easier to install,” he says of the units. “We’re not an engineering and construction service, and, [with prefabricated biofilters], I didn’t have to have the crew out there digging up real estate.” In the coming year, Milligan plans to replace two built-on-site filters with manufactured models.

While biofiltration may not be appropriate for every city or county, the number of success stories has grown over the years, leading Gray to predict that many more local governments will adopt the technology. “In the last 10 years, [biofilters] have gained momentum,” he says. “Experimentation with different media, like peat and humus and soil, have in-creased efficiencies. They’re getting better porosity; better surface area-to-volume ratios; better bacteria growth; and better temperature controls.

“They’re competing with chemical systems, and they’re proving to be cost-effective,” he notes. “They don’t require as much operator attention or hands-on control because they’re natural systems. I don’t think chemical treatment is going to continue to be a major player, except where odors are extremely difficult to handle. I think biological odor control is the wave of the future.”