A microbial treatment system designed to prevent odors from forming in collection systems and a diluted ozone formula that can eliminate atmospheric odors have saved the City of Tampa, Fla., about $175000 in annual odor control costs, according to Brad Baird, deputy director of the city's Department of Sanitary Sewers. Hydrogen sulfide emissions from a large pump station along the collection system have been measured at zero parts per million (ppm), says Dan Page, the city's wastewater technical supervisor.

The treatment started after naturally occurring microbes were injected into the 21-km (13-mi) central intercepting system and its tributaries, which handle about 30% of the city's 213853 m3/d (56.5 mgd) flow. The microbes use competitive exclusion to replace odor-forming bacteria in the existing biofilm, thereby reducing sulfides and other odor compounds in wastewater, says Rod Dickerson, president of Lafayette, La.-based Dickerson Consultants, which holds a patent on the method. The biofilm is comprised of the natural bacteria on the walls of the force mains and the floor of the gravity piping. To reduce odors further, ozone technology is applied to the vapor space and piping leading to the scrubber system at the last pump station before the flow reaches the Howard F. Curren Advanced Wastewater Treatment Plant. An electrical charge produces dilute quantities of ozone molecules, which mix with and oxidize odors while converting them to non-odorous compounds, according to Dickerson, whose company also has a patent on the ozone technology.

After treatment, the level of atmospheric hydrogen sulfide reaching the pump station scrubber inlet averages 200 ppm, according to Page. The technologies also are designed to make the collection system a more effective step in wastewater treatment, Dickerson says. The microbial treatment system, otherwise known as In-Pipe Technology, is more effective at preventing odors than the ozone formula, the Tri-O Air Treatment System, and leaves microbes in the system to destroy contaminants without producing odors. "We see to it that microbes are used in the right proportions so they efficiently eat up the contaminants," he says. Seeds from a library of microbes are selected, grown, and concentrated, Dickerson says. The microbes are concentrated up to 101³/ml, compared with 10⁸/ml for most microbes sold commercially on a dry weight basis, he says.

Hydrogen sulfide emissions from a large pump station along the collection system have been measured at zero parts per million (ppm).

Dan Page
Cityof Tampa, Fla.

Bacteria are divided into small vials, Dickerson says, and, through the use of microprocessor-controlled precision pumps, are uniformly distributed on a continuous basis into the farthest reaches of a collection system, mainly at pump stations.

In-Pipe Technology, Dickerson says, is the only such system that addresses the source of the odor: the biofilm. All the other chemical technologies curtail the hydrogen sulfide after it is formed or chemically affects the biofilm, he says.

Because every collection system has different requirements, a uniform pricing schedule cannot be used for the In-Pipe system, Dickerson says, adding that some cities have more pump stations than others. Tampa has used the In-Pipe and Tri-O technologies for about 2 years. Previously, the city paid $50000 per month to purchase Bioxide to control odors at three pumping stations along the central intercepting system, according to Page. The city now pays $30000 per month to Global Pollution Control Inc., a McKinney, Texas-based contractor that injects the microbes into the system and handles the Tri-O Air treatment System, he says. Another $14000 is used to pay for bleach and caustic chemicals for the scrubber system, but the contractor reimburses the city half that amount, leaving a total monthly payment of $37000, he says.

The microbes also can consume biochemical oxygen demand (BOD) and suspended solids in the collection system, eventually reducing energy aeration needs and biosolids production and disposal costs at a treatment plant, Dickerson says. Collection systems differ as to how much solids can be reduced; in some cases, a reduction of up to 70% may be achieved, he says. However, Tampa officials, who use the In-Pipe system for only 30% of the city's flow, say they have no record of reduced solids in the collection system leading to energy costs savings at the plant.

According to Dr. Ronald Linsky, executive director of the nonprofit National Water Research Institute (NWRI) in Fountain Valley, Calif., the In-Pipe Technology has "merit," and laboratory tests prove it can consume solids in the collection system. However, a larger pilot demonstration project using waste streams of 1893 to 3785 m3/d (500000 to 1 mgd) is needed to support the validity of the technology, including its ability to break down [BOD and suspended] solids, he says.

NWRI is working with Dickerson to submit the In-Pipe Technology for extensive pilot testing so researchers can analyze biofilm changes, Linsky says. At press time, Tampa was the only city to deploy the In-Pipe Technology.

Michael Richman, WE&T
Reprinted with the permission of
Water Environment & Technology
October 1997