During the past decade, biological odor control has gained increasing acceptance in the U.S. municipal odor control marketplace. Biological odor control designs have evolved from basic pit and pile compost biofilters to large biotrickling filter (BTF) systems that provide a ten-fold decrease in footprint with higher odor treatment capacity, better control and predictability, and longer media life.
Biological odor control systems are designed to remove hydrogen sulfide (H2S) and odorous organic compounds from wastewater process air streams, preventing odors from being discharged into the environment, and thus complying with local odor control ordinances. BTF systems promote the growth of sulfur-oxidizing bacteria (principally Thiobacillus), which attach to an inorganic solid support medium. The support media may be mineral or synthetic, and are designed to resist compression and degradation from the acidic byproducts of the biological oxidation of sulfides.
As contaminated air is passed through the BTF, the contaminants are absorbed in the liquid film at the media surface, and the microorganisms oxidize the inorganic and organic sulfides to release energy used for cell growth. Water and nutrients are continuously trickled over the media to maintain the liquid film, sustain the bacteria population and remove waste byproducts.
BTFs are capable of removing more than 99% of the H2S, and more than 90% of the organic sulfides and mercaptans from a wastewater air stream. H2S is readily oxidized by the Thiobacillus bacteria. However, longer air contact time (residence time) is required to effectively oxidize mercaptans and organic sulfides.
PUF media
One BTF on the market is the Zabocs BTF system manufactured by Siemens Water Technologies. The system uses a rigid polyurethane foam (PUF) media, which has many properties that make it ideal for bacteria support. Bacteria adhere exceptionally well to the polyurethane surface, and the reticulated foam configuration provides a very high surface area per unit volume to improve the gas to liquid contact. The rigid foam resists compression, and is randomly dumped in the BTF tower to allow very low pressure drop.
BTFs must first be seeded with bacteria, then acclimated to the odor source. Seeding is generally accomplished by circulating return activated sludge (RAS) through the media. RAS contains millions of bacteria species, including many species of sulfur-oxidizing bacteria. The bacteria population naturally acclimates to the environment. Sulfur-oxidizing bacteria grow in population as they consume the sulfides from the air stream.
As they oxidize the sulfides, acidic sulfates are formed as byproducts, and are absorbed in the recirculating liquid, lowering the pH. The low pH (acidic) conditions discourage the growth of other bacteria and promote the selective growth of acidophilic bacteria. As the liquid pH drops, the odor removal improves. After about four weeks, the BTF will be removing 99% of the H2S, and will contain a dense, healthy colony of sulfur-oxidizing bacteria.
The BTF system may be operated in single-stage or two-stage mode. In single-stage mode, the irrigation liquid (containing water, nutrients, bacteria and acidic byproducts) is pumped over the media in a continuous recycle. This provides an abundance of bacteria and nutrients to continuously reseed the bed. In addition, the acidic byproducts maintain the media bed at a low pH to promote the selective growth of acidophilic, sulfur-oxidizing bacteria. A small amount of make-up water is added to maintain the liquid sump in the optimum pH range.
Two-stage mode
In two-stage mode, the BTF operates in two distinct pH zones. The first (upstream) stage is operated with continuous recycle at a low pH, identical to operation in the single-stage mode described previously. The second stage is irrigated intermittently by freshwater and nutrients, to maintain the media bed at a neutral pH. In this way, different types of bacteria are grown in the second stage, which improves the oxidation of organic sulfides and other volatile organic compounds.
The Zabocs BTF system consists of a BTF tower and the Nucirc Process Skid. The tower contains the PUF cube media, media supports and liquid distribution system.
The Nucirc Skid, adjacent to the biotower, is a stand-alone unit that houses the pumps and controls, as well as the nutrient and recirculation tanks. Mounting the recirculation pumps, nutrient dosing pump, and instrumentation and controls on a common skid greatly simplifies equipment installation and reduces commissioning time. A programmable logic controller and touchscreen display enable easy, user-friendly operation in either single-stage or two-stage operating modes.
Padre Dam application
The Padre Dam Municipal Water District (PDMWD) manages the water and wastewater supply for more than 125,000 residents in the city of Santee, Calif., and nearby towns in eastern San Diego County. The PDMWD has been operating a chemical scrubber at its wastewater treatment facility to control odors from the facility’s headworks. High chemical costs and frequent maintenance issues led district management to explore alternative odor control technologies.
After investigating several different options, including the BTF process, the district decided to run a pilot test of a biological system in parallel with its existing system. A Zabocs BTF pilot system was started up in October 2004 at the Padre Dam Influent Pump Station in Santee. The pilot BTF system was designed to treat up to 350 cfm of odorous air with H2S levels as high as 200 ppm.
Padre Dam engineers and specialists from Siemens Water Technologies monitored the results for performance and maintenance requirements. After a typical four-week acclimation period, the H2S removal efficiency stabilized at greater than 99%.
The system performed well over the next few months, and Padre Dam engineers concluded that the Zabocs BTF system was the most appropriate technology for their application. A full-scale system replaced the pilot system, and was started up in November 2005.
“The San Diego Air Pollution Control District (SDAPCD) air permit requires periodic monitoring for H2S, with a maximum allowable exhaust concentration of 0.1 ppm,” said Russell Yanigawa, a Siemens engineer. “The BTF system has performed well for the past six months, with inlet concentrations from 1 to 10 ppm, and no detectable H2S at the outlet. Maintenance has been limited to replacing the liquid nutrient once a month and daily inspection and recording of the H2S stack concentration as required by the SDAPCD air permit.”
“The system has performed as advertised, is easy to operate, has been reliable and is supported effectively by Siemens,” said Harold Bailey, director of operations and water quality at PDMWD.
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