Shell Game

A biofiltration system helps a Georgia wastewater utility eliminate hydrogen sulfide and mercaptan odors at a lift station
Shell Game
Seashells, a byproduct from the food industry, have high levels of calcium carbonate that neutralize acid byproducts of sulfide oxidation. The large surface area promotes vigorous biological activity without chemicals or carbon. (Photos courtesy of Anua)

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Two force mains with strong waste loads delivered 300,000 gpd to the Mars Hill lift station in Watkinsville, Ga. The station lies near a church and retirement home. Neighborhood residents complained to Chris Thomas, director of the Oconee County Utility Department, about the odor.

One 5-mile section of 6-inch force main serving many restaurants delivers hydrogen sulfide and mercaptan odors to the station. Waste from a bakery on a 3-mile section of 8-inch force main produces sweet alcohol. The odor was so complicated that Thomas called in experts to treat it.

“We tried everything they recommended, from hydrogen peroxide and chlorine to aerating and a misting system,” he says. “Nothing cured the problem. We realized we would have to step up to something more costly and complex.”

Thomas invited makers of odor-control units to conduct tests. “We wanted to avoid continued chemical costs and high maintenance.” Thomas visited pilot installations of the Mónashell biofiltration system from Anua. Maintenance involved adding a little water to the unit and replacing the media as required. The system eliminated odors.


Modular system

Each biofilter is custom-built based on the quantity of air and quality of influent. The Mars Hill unit is 8.5 feet square and weighs 20,000 pounds with the shells. Workers poured a concrete pad to prepare for its arrival. “We also needed piping and a water source,” says Thomas. “Everything else is pre-engineered.”

The skid-mounted modular system has a bolted fiberglass-reinforced paneled housing and shell media, control panel, two fiber-reinforced plastic irrigation sumps, two 0.4 hp water recirculation pumps, and a 1 hp V-belt fan with unit-to-fan ductwork and vertical exhaust stack. The blower moves air at 465 cfm.

When activated, the unit pulls in air from the lift station. An internal air deflector box directs the flow up to the distribution zone. The vacuum created by the fan in the outlet box draws the waste airstream through the shells, a byproduct from the seafood industry. High levels of calcium carbonate in the shells neutralize acid byproducts of sulfide oxidation, while the large surface area promotes vigorous biological activity without chemicals or carbon.

As air passes through the media, water captures odorous compounds. Then the microorganisms on the shells and in the water oxidize them. Clean air leaves through the stack on the fan outlet.

“We top off the sumps at the bottom of the unit with a continual trickle of water, about 5 gpd,” says Thomas. “It’s enough to replace evaporation losses and to refresh the recirculating water.”

Spray heads recirculate the water onto the media. As the water drains down, it makes contact with the incoming odorous air. The water collects in the base of the unit, then drains back into the sump for recirculation to the irrigation grid.


Breath of fresh air

After bringing the biofilter on-line, the staff noticed a sharp decrease in hydrogen sulfide odor the next day. Each morning brought more improvement until the area was completely odorless. Earlier testing by Black & Veatch verified that the biofiltration system attains a neutral pH, removing more than 99 percent hydrogen sulfide, 98 percent ammonia, and more than 95 percent sulfur compounds.

Workers check the lift stations every other day and clean a small screen on the biofilter weekly. They also read the airflow gauge and initially ran pH and simple lab tests to check the health of the system. “The frequency of the tests dropped as we established a baseline that determined how often we needed to do them,” says Thomas. “The first four months we watched for changes.”

The unit has an alarm for low flows or low water levels, but it is not tied to an autodialer or SCADA system. Thomas believed that was unnecessary, as the crew is always checking the stations.

“We did see a 5 to 8 percent increase in power consumption over last year’s figures,” he says. “However, the biofilter took away a major headache and stopped the complaint calls. It’s a sustainable, cost-effective solution to achieving odor-free air.”


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