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.
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.
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.