Environmental Science & Engineering - www.esemag.com - November 2004
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Odour complaints stopped at large sewage transfer line

Long-running odour problem was solved by tapping a 30,000 cfm-capacity biofilter array into a 14 mgd average, 55 mgd maximum sewage transfer line.
Biofilter array has six 5000-cfm integrated, modular biofilter units, set up as two master units handling 15,000 cfm each, with separate exhaust fans.
Dampers on inlets and outlets to modular biofiltration units allow for isolation of each unit.
Management at Jefferson County Environmental Services in Alabama reports it has solved a long-running odour problem by tapping a 30,000 cfm-capacity biofilter array into a 14 mgd average, (US) 55 mgd maximum sewage transfer line. The 6-unit Basys™ array, which is also said to reduce corrosion, was manufactured by Biorem Technologies Inc. of Guelph, Ontario.

The array handles hydrogen sulphide at average concentration of 5 ppm, and maximum of 25 ppm, as well as other odour-causing compounds, such as methyl mercaptan, dimethyl sulphide (DMS), dimethyl disulphide (DMDS), and other Total Reduced Sulphur (TRS) compounds. The County has ordered five additional systems from the manufacturer for one of its wastewater treatment plants.

“Our main purpose was to stop the dozens of odour complaints we received over four or five years where the transfer line ran near a shopping mall and a residential area,” recalled Clyde C. Osborne, wastewater treatment plant superintendent. “Helping to reduce corrosion that was found during a routine inspection was a welcome by-product. When you go to that site now, you don’t smell anything. Once the system became fully operational, the complaints stopped.”

Osborne said a high-stack exhaust fan had apparently been solving the problem up- and downstream of the sensitive area for about 7 years, but complaints had persisted in the immediate area, stimulating a search for an alternative solution.

“We considered activated carbon towers, which we had used at a pretreatment facility upstream, but we didn’t like the expense of having to change media every three or four years, or the need to dispose of a hazardous material,” he said. “That led us to biological filtration, with presentations from three vendors. Biorem was chosen because their media was more uniform, and didn’t seem to present risks of filter clogging or attracting rodents.”

“Construction started early in 2002, with a local contractor able to handle the installation. Startup was in August, 2002, with media expected to last for 10 years. We’re expecting a low-maintenance, long-running solution to the problem. They provided sample testing three or four times the first year, and then we contracted for more, and haven’t seen any problems.”

The 60”, 35-mile concrete transfer line, built in 1987, receives effluent from about one-quarter of the system’s 164 pump stations, which range in capacity from 100,000- 5,000,000 (US) gpd. Corrosion had been enough of a problem to require activated sludge pretreatment, upstream from the odour complaint area, that was related to input from dairy product and soft drink plants. Consulting engineering services for the odour-control project were provided to the County by F.W. Daugherty Engineering & Associates, Inc. of Birmingham, Alabama.

The odour control system operates under negative pressure, with a main collection duct evacuating the headspace of a subterranean junction box that receives discharge from the transfer line. Air is also received from other sewer lines emptying into the box, thereby adding some corrosion control benefit. The collection duct branches above ground to the biofilter array. Flanges on the duct provide for pre-conditioning of the air via a pneumatic humidification manifold prior to its entry to the biofilters. Treated air leaving the biofilters is drawn out through the stacks of fans for discharge to the atmosphere. A shed enclosure houses the complete humidification system, including compressors, pumps, and all instrumentation and controls.

The biofilter array is set up as two units handling 15,000 cfm each, with separate exhaust fans. Normal deployment is for redundancy against operational interruption, with simultaneous operation available for maximum removal of hydrogen sulphide and other odour-causing TRS compounds. Continuous monitoring of hydrogen sulphide in the entering air guides the level of operation. Biorem provided six of its 5000-cfm Basys™ integrated modular biofilter units. Capacity could be added by connecting more filter modules.

Quarterly, preventive-maintenance media testing, within a 10-year contract program, has already begun. Testing consists of measurement of critical parameters such as pH, nutrient levels, and bacterial and fungal speciation and enumeration, as well as physiochemical properties such as moisture content and particle size. The company maintains that such testing has been a critical factor in achieving continuous, reliable performance for any biofilters, and says it has over 80 installations of its biofilters in North America.

The company adds that its Biosorbens™ biofiltration media, also suitable for VOC removal, was designed specifically to handle a wide range of odour-causing compounds, including TRS and others, as opposed to the traditional focus on hydrogen sulphide only, a focus which it says is represented by alternative technologies such as carbon adsorption, chemical and wet scrubbing, and bio-scrubbing.

“It’s important to remove all odourous compounds, because these additional ones are a significant contributor to odour complaints even when they are present in minute amounts,” noted Derek Webb, P.Eng., the company’s director of design and production engineering. “This is because they have very low odour detection thresholds, such as dimethyl sulphide @ 0.0001 ppm; crotyl mercaptan @ 0.000029 ppm; and ethyl mercaptan @ 0.00019 ppm.”

The media is said to maintain a balance between optimal growing conditions for the microbes, while offering ideal pressure drop through the media, high moisture retention, long operating life, and resistance to decomposition and compaction, while providing a surface on which the biomass film can flourish.

It is described as consisting of inert, uniformly-shaped hydrophilic cores that are produced with nutrient-rich organic and inorganic adsorbents. These characteristics are said to decrease residence time, and increase serviceable life to more than 10 years. Its high surface area-to-volume ratio is said to result in requiring only one-third the space needed by organic medias.

Contact Biorem Technologies Inc., info@bioremtech.com.

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