New choices for vapor-phase odor control
For vapor-phase odor control at wastewater treatment plants, the most commonly applied technologies are packed-bed chemical scrubbers, activated carbon units and organic-media biofilters. In the past few years, enterprising equipment manufacturers and innovative design engineers have made significant advances in all three types of odor control technologies.
Packed-bed chemical scrubbers
In packed-bed chemical scrubbers, foul air passes through a bed of plastic media, where it contacts a scrubbing liquid flowing downward through the bed. The scrubbing liquid collects in a sump situated below the packing and is recirculated up to a nozzle or tray distributor located above the packing. Wet scrubbers utilize pH-controlled absorption and chemical oxidation to remove odorous compounds from an airstream. Low-pH scrubbing solutions are typically used to remove ammonia and amines, and high-pH solutions are used for hydrogen sulfide (H2S).
For wastewater applications, the most common scrubbing solution is sodium hydroxide (caustic) and sodium hypochlorite (bleach). Fresh chemical is automatically added through metering pumps controlled by pH and oxidation reduction potential (ORP) probes, which vary the chemical feed to meet fluctuations in the inlet concentrations. To prevent solids accumulation in the recirculated scrubbing liquid, a constant supply of makeup water is added to provide continuous overflow of the sump.
New low-profile package units
One of the innovations in packed-bed scrubber technology has been the introduction of a new type of low-profile package system. Standard packed-bed units are sized for face velocities of 450 to 600 feet per minute (fpm). In a vertical configuration, a 10,000 cubic feet per minute (cfm) unit would be 5 feet in diameter and more than 20 feet high. Vertical units often present problems for interior installations, particularly in existing structures. In the past, a low profile was achieved by essentially turning the unit on its side in a horizontal configuration; however, this approach reduced the effectiveness of treatment. In a vertical unit, air moves through the packing countercurrent to the liquid flow, while in a horizontal unit air travels through the packing perpendicular to the liquid flow. This arrangement allows a portion of the air flow to short circuit across the top of the media, which decreases the removal efficiency.
A new style of packed-bed unit has been developed that reduces the height of the unit while keeping a vertical configuration. This is accomplished by using multiple compartments set side by side.
The newest system developed by U.S. Filter/R. J. Environmental uses three compartments to provide two-stage treatment. The first compartment, which is also the first stage, has 5 feet of packing with its own sump and recirculation pump. An acid solution can be used in the first stage to remove ammonia. The last two compartments each have 5 feet of packing and a combined sump to provide second-stage treatment with caustic and bleach used to remove H2S and reduced sulfur compounds. When ammonia removal is not needed, the first stage can be employed as a pretreatment stage. In this mode of operation, final-stage blowdown liquid, which would otherwise be wasted, is routed to the first stage to achieve a 40-percent chemical reduction.
Our environmental consulting firm, Black & Veatch, has installed several of the new package systems in both new and retrofit applications and they have proven to be effective and reliable. A 10,000 cfm low-profile package has a height of less than 12 feet and will fit into most existing structures with ease. There are several high-quality system suppliers in the market, so competitive pricing can be obtained.
Innovative chlorine gas scrubber
In the past, the application of chlorine gas in wet scrubbers had been attempted, but was largely unsuccessful because of the inability to achieve effective chlorine strength in the scrubbing solution. Unlike sodium hypochlorite, which is a 12.5-percent solution stabilized with caustic at pH 9 to pH 10, chlorine gas provides a concentration of only 2 percent to 3 percent when injected into the makeup water. For high H2S concentrations, the large amount of carrier water required to meet the chlorine demand dilutes the caustic solution. At one existing installation, scrubber stack sampling showed that performance actually deteriorated when the chlorine gas feed commenced.
For our Scottsdale, Ariz., project, this problem was resolved by an innovative scrubber design that adds chlorine gas to the recirculated scrubbing solution instead of the makeup water. The chlorine gas injector is installed in a loop on the recirculation piping. The volume of flow through the loop is set by manually adjusting the valves, while chlorine gas flow is turned on and off at a solenoid valve controlled by the ORP probe. The ability to add chlorine without excessive dilution enables this system to maintain the high pH needed for effective treatment. The high pH also increases chlorine solubility and minimizes chlorine emissions, which have been problematic with these applications in the past. The two-stage system at Scottsdale is able to treat 30 ppm of H2S and achieve 99.9 percent odor removal. The main advantage of the chlorine gas system is a significant reduction in chemical costs for applications where treating high H2 S is requir
ed.
Activated carbon units
Another odor control technology commonly used at wastewater plants is carbon adsorption. Activated carbon removes pollutants by surface adhesion, and its highly porous structure supplies a large surface area. Deep bed systems typically consist of a stainless steel or fiberglass vessel containing a bed of granular activated carbon, through which the odorous air is discharged. Carbon units have face velocities of 50 fpm to 70 fpm and typically use a media depth of 3 feet. The lower face velocity of carbon units means that for the same airflow they have larger vessels than wet scrubbers. However, the space occupied by carbon vessels can be cut in half by using dual-bed units.
There are several types of carbon media available, and selection depends on the contaminants being treated. Virgin carbon media is the best choice for removing volatile organic compounds (VOCs).
Caustic-impregnated carbon is chemically treated with sodium hydroxide or potassium hydroxide to enhance its capacity to remove hydrogen sulfide. When the media becomes depleted, it can be regenerated in situ by soaking it in a caustic solution. However, many operators did not like handling the high-strength caustic chemical used for media regeneration. Due to this concern, we typically assume caustic-impregnated media will be replaced, rather than regenerated. Recently, a new type of carbon media was developed to resolve this concern.
New water regenerable media
Centaur® carbon is a new type of media produced by Calgon Carbon Corporation. This media can be regenerated using water, which is less of a safety concern than caustic regeneration. The regeneration process requires the unit be removed from service for one full day, followed by a second day of diminished capacity while the media dries. The initial wash water from the regeneration process is very acidic, at pH 1. No handling is required, but care must be taken with the discharge of the wash water. A low-cost neutralization unit can be installed, if sufficient dilution water is unavailable. The Centaur media and the caustic carbon media offer similar removal efficiency of over 99 percent odor reduction. The Centaur media has lower overall H2S capacity, so it has a shorter initial life. However, the ability to safely regenerate yields lowers long-term operating costs. We have applied Centaur media at two facilities where, after several regenerations, it continues to provide exc
e
llent treatment, and has demonstrated its value.
Compact carbon unit
Following the introduction of the Centaur media, Calgon introduced an innovative new package carbon unit called the PhoenixTM system. The Phoenix system uses a high-density type of Centaur media in replaceable canisters. The compact system incorporates automatic washing for ease of operation and maintenance. The vessel is fabricated from polypropylene for corrosion resistance and can be installed indoors or outdoors.
In the Phoenix unit, the carbon canisters are mounted on air distribution headers. Foul air flows from the outside to the inside of the canisters and is directed to a header standpipe where it enters the exhaust plenum. The flow rate through each canister is 200 cfm. The distribution headers are arranged in separate compartments, or banks, within the unit. The canisters within each compartment are automatically regenerated with water on a sequential basis, so only one bank is off-line at a time. The unit is sized with one bank off line, so treatment is maintained during regeneration. Regeneration produces a small amount of low pH discharge, which may require neutralization, if sufficient dilution water is unavailable.
After numerous regenerations, the spent canisters are replaced. The Phoenix unit has side portals that allow the easy removal and replacement of the canisters in just a few hours. The canisters are sealed so the operator does not come in contact with the carbon media. The exhausted canisters are shipped back to Calgon using the same shipping containers as the new canisters, eliminating the need for landfill disposal.
We have installed Phoenix systems at a pump station and to purify the off-gas from covered trickling filters. The units have just recently come on line, and we are awaiting operating data to assess the overall performance and reliability of these systems. A Phoenix system has been in operation for over a year at a Florida water plant treating high H2S from a gas stripper. The operators at that facility indicate that excellent performance was achieved with minimal maintenance problems, and they are now planning to purchase two additional systems.
Biofilters
Biofilters employ soil, compost or other media as a substrate for microbes that remove odorous contaminants from an air stream as it travels through the media. Sufficient residence time must be provided for microbes to accomplish effective contaminant treatment, so biofilters must use a low air velocity. For H2S removal, typical residence times range from 30 seconds to 60 seconds. Longer residence times may be necessary for airstreams containing higher concentrations. For a reasonable pressure loss, bed depth is limited from 3 feet to 5 feet with face velocities from 2 feet to 8 feet per minute, so biofilters have very large space requirements. The biofilter media must be replaced every two to five years.
The key elements in biofilter design are media, air distribution and moisture. Improper media can compact or become too soggy and lose its porosity quickly, whereas a properly formulated media will retain its structure over several years. Media porosity is also an important consideration to minimize head loss across the bed. Good air distribution through the biofilter media is an essential element of biofilter design. If air distribution is uneven, treatment will be inconsistent and channels can develop, allowing air to escape untreated. Many designs employ a low-cost network of perforated pipe embedded in gravel, but some of these systems have had poor distribution. Proper moisture control is essential to the process, because it prevents media drying and cracking, which allows the escape of untreated odors.
In response to the upsurge of interest in biofiltration technology, many manufactured biofilter systems have been developed. Two of these systems incorporate all the essential features needed for optimum treatment.
Modular biofilter
CVT America/U.S. Filter-Davis produces a modular biofilter that addresses the important issues of media, air distribution and moisture control. The system uses an engineered media developed and proven in Europe. The air distribution system consists of a bed of ceramic spheres, which provide even air distribution through the media. The biofilter modules are covered, so they are shielded from excess moisture due to rainfall and heavy snow. For moisture control, pressure compensated irrigation hoses are embedded in the media. Two independent hoses are used. A bottom hose maintains moisture in a humidification layer, while a top hose is used for media irrigation. Control equipment is installed to sense the moisture content of the media and automatically add water as needed.
The 1,500 cfm modules are transported to the site by truck, then unloaded into position. The installation is completed with a simple connection to the contaminated air inlet and utilities. In addition to simple installation, the modular concept promotes uniformity and ensures the same high-quality performance at all installations. We have not yet installed a CVT system, but we consider the company to be an acceptable supplier based on our assessment of existing installations. At a Michigan facility with 32 modules treating 48,000 cfm, operators attest to the ease of use, performance and reliability of the CVT system.
Baseplate and conditioner system
BioDigestor Technologies Inc. (BDT) provides a high quality biofilter that employs a baseplate air distribution system. This system requires the on-site construction of a concrete containment structure. The baseplate system is composed of prefabricated polyethylene units, which interlock to form a slotted floor on supporting legs with an underlying air plenum. This system provides even distribution with a minimum of pressure loss due to friction. The manufactured baseplate system is more costly than a simple perforated pipe system, but it provides performance that is more consistent.
BDT supplies an engineered media with different materials used for specific applications. Instead of wetting the media, moisture is introduced in a conditioner unit that humidifies the inlet air. Fine mist systems have had problems with nozzle scaling. The BDT system uses a coarse spray, which also allows for maintaining temperature with a water-heating device. Control equipment is installed to automatically adjust the amount of moisture supplied. The BDT system is uncovered, so the media is designed to be porous enough to allow excess moisture to pass through, with drainage provided at the base of the unit.
The BDT system has a higher initial capital cost than some of the "homemade" systems now being constructed, but it also has a proven track record and vendor guaranteed performance. We recently installed BDT biofilters for wastewater applications in Lawrence, Kan., and Jefferson City, Mo., and we will soon have data from those units. At the Mill Creek wastewater treatment plant in Cincinnati, a BDT system was designed to treat 10 ppm H2S with a peak of 50 ppm. The unit has performed flawlessly, even though the H2S concentration has ranged from 80 ppm to 250 ppm.
This article originally appeared in the 06/01/1999 issue of Environmental Protection.