Building Better Mousetraps—New Solutions for Odor Control

Municipal treatment professionals realize that removing wastewater odors most often means removing hydrogen sulfide (H2S). The three traditional "collect and treat" technologies for doing this are chemical wet scrubbers, biofilters and deep-bed activated carbon. All have been proven to be effective at removing odorous compounds, but each has inherent limitations. New innovations in media and design have been introduced that capitalize on the strengths of these technologies and give municipalities additional options.

Wet Scrubbers—Chemical Dependencies
Chemical wet scrubber systems spray a caustic mixture downward through a bed of packing countercurrent to the foul airflow. The intimate combination of air and liquid chemically neutralizes H2S. Although this process is highly effective, it requires intensive use of chemicals such as sodium hydroxide (NaOH) and sodium hypochlorite (NaOCl) which results in high operating costs as well as the potential for worker exposure. In addition, the electrical and mechanical devices required for operation require a significant amount of maintenance.

Biofilters—Space Eaters
Although common in Europe for many years, biofilters are only recently gaining popularity in the United States. A typical biofilter system uses perforated PVC pipe to direct foul airflow through a 3' to 4' bed of soil or compost. The naturally occurring microorganisms within the bed effectively reduce H2S and other odorous compounds.

Biofilters provide a relatively simple solution with minimal chemical usage and are viewed favorably from an environmental standpoint. Their biggest drawback is their large space requirement. While most chemical wet scrubbers operate at 300 to 400 fpm, a typical biofilter operates much closer to 5 fpm due to the increased contact time required for effective biological degradation. This translates to up to 45 times the space requirement of other comparable technologies. In addition, the bio-material itself often requires moisture and pH control which mandates complex control and monitoring devices. Because this material is often very compressible, uneven bed compression may occur, creating the potential for odors to bypass the system.

Finally, when the bed becomes acidic and needs to be replaced, the bio-material must be landfilled and may be considered hazardous. During this replacement, the biofilter is off-line and cannot continue to treat odors. With deep-bed activated carbon systems, foul airflow is directed through a vessel typically containing caustic impregnated carbon. The H2S reacts with the caustic, producing elemental sulfur which is then strongly adsorbed onto the carbon providing high H2S removal efficiency. When the carbon is exhausted, however, it must be chemically regenerated with a 50% caustic solution, and after several regenerations, the carbon must be landfilled. For H2S treatment levels greater than 10 ppm, deep-bed carbon systems are economically unattractive.

Catalytic Carbon—Making New Solutions Possible
A recent innovation, the development of catalytic activated carbon, eliminates some of the drawbacks associated with deep-bed activated carbon systems. Catalytic activated carbon is not chemically impregnated. It promotes an oxidation reaction that primarily converts H2S to H2SO4. Because the H2SO4 is very loosely adsorbed and is water soluble, the catalytic activated carbon can be water-regenerated when it is exhausted. Therefore, the landfill liability associated with the use of impregnated activated carbon is eliminated.

Retrofitting an existing deep-bed unit with catalytic activated carbon is a straightforward process that offers the benefits of chemical-free regeneration without incurring costs for capital improvements. It eliminates the cost and time associated with chemical regeneration or frequent bed exchanges of impregnated carbons.

Despite the improvements that catalytic activated carbon offers some inherent drawbacks of deep bed carbon technology remain. The water regeneration process requires the unit to go off-line, making extremely frequent regenerations impractical. After multiple regenerations the carbon eventually will lose its adsorption capacity for H2S and need to be replaced. Like impregnated media, carbon exchange is a labor intensive and dirty process.

The Phoenix System—A New, Proven Solution for Odor Control
Another approach to odor control has recently been introduced to take advantage of the features of catalytic activated carbon. The Phoenix™ system from Calgon Carbon Corporation features a shallow bed approach utilizing carbon canisters allowing radial airflow. The pre-loaded canisters are arranged in rows of vertical banks. The foul air enters the top of the Phoenix unit and is directed downward where it flows from outside to inside each individual canister. The treated air then flows upward through an internal distribution pipe and is released through the exit side of the plenum. Due to the canister design and an enhanced form of catalytic carbon, the Phoenix system allows for more effective water regeneration which greatly extends the life of the carbon. The use of separate banks allows canisters to be sequentially regenerated, keeping the system on-line during this process. When the carbon eventually needs to be replaced, side entry portals allow each canister to be easily and quickly exchanged. The canisters can be recycled, eliminating the need for landfill of spent material.

Clearly, there are a variety of odor control solutions that should be considered in any design analysis. Traditionally, these choices were limited to chemical wet scrubbers, biofilters and deep bed systems utilizing impregnated activated carbon, but the options have expanded to include catalytic carbon--either as part of an existing deep bed system or in the radial-flow, carbon-canister Phoenix unit. In making their choice, designers, contractors and municipal engineers should consider a range of application-specific factors including flow rate, contaminant concentration, space requirements, down time, maintenance, operational hazards and disposal of spent materials.

Daniel R. Brooks is a manager for <%=company%>