Integrated Systems Approach Helps Combat Odor Problems

By Mary Malerbo And Leo Zappa

When it comes to treating odors emanating from municipal wastewater facilities, many designers spend a great deal of time, energy and cost searching for the elusive "magic bullet," the one system (or device) that effectively treats any and all odors produced by the wastewater process. Likewise, many suppliers of odor control systems promote their product(s) as "magic bullets" that are definitive solutions for all odor problems for a particular municipality.

As a leader in the odor control industry for more than 30 years, Calgon Carbon Corp. knows that there isn't any one system or technology that can solve every odor problem in every municipality. This doesn't mean, however, that some odor problems can't be solved. What's needed is a comprehensive approach, one that combines two or more systems or technologies to fully treat the problem.

Odor Types, Sources

The principle source of odor in wastewater operations can typically be traced to sewage collection system pipelines that carry septic wastewater to the wastewater treatment plant and/or certain processes within the wastewater plant.

Wastewater can contain hydrogen sulfide (H2S) and other odorous compounds. The origin of these odors include wastes discharged to the sewage collection system, unwashed grit, scum on primary settling tanks, organically overloaded biological treatment processes, solids thickening tanks, waste gas burning operations in which lower than optimum temperatures are used, chemical mixing operations, solids incinerators and digested solids in drying beds or solids holding basins.

Odors from these sources vary in degree of intensity depending on the compounds present in the waste stream and the amount of anaerobic decomposition present. Odors are liberated from wastewater when turbulence in the water causes odorous compounds to be volatilized from the liquid.

Hydrogen Sulfide

The most prevalent cause of odors is hydrogen sulfide. However, other compounds may accompany H2S, such as ammonia, organic sulfurs (e.g. mercaptans, dimethyl sulfide, dimethyl disulfide, and carbon disulfide), amines (e.g. indole and skatole), fatty acids and other volatile organic compounds (VOCs). This is especially true within wastewater treatment plants, as opposed to collection system facilities, where H2S is almost always the problem.

Accordingly, a typical odor control system in a WWTP is designed for H2S removal. This is because of H2S's low odor threshold, its predominance as an odorous agent, and the fact that H2S can be easily detected using relatively unsophisticated analysis. The other odorous compounds are commonly present at concentrations that are often at least an order of magnitude lower than the H2S. However, they are still at detectable levels.

Some odor control systems are only effective at removing hydrogen sulfide. These systems may not be terribly effective at removing non-H2S compounds due to mass transfer limitations or as a result of the chemistry that is employed. Depending upon the number and types of other compounds present in the air stream, and each compound's concentration, odor intensity, detectability, character, and hedonic tone, odors generated by additional, non-H2S compounds may be noticed at the outlet of the odor control system. The "absence" of the additional compounds is, in effect, an illusion. The odorous compounds are present, but the H2S masks the smell. Depending on a particular treatment plant's design and construction, that reality may not be readily apparent.

Up-Front Testing

Before spending thousands of dollars on odor control equipment, consider spending a fraction of that cost on comprehensive odor control testing to determine which compounds require treatment. Properly administered, this test is cost-effective and a wise investment. If it's practical to do so, bag a sample of the air to be treated to determine which compounds are present and which ones may require treatment (be sure the testing includes sulfides and mercaptans). This testing is obviously not feasible with design/build facilities.

Air testing would not be necessary if every odor control technology could treat all compounds. Since in reality this is not the case, educating yourself on the capabilities of each odor control technology is of utmost importance.

Vapor Phase Technologies

There are a variety of odor control technologies available that can be used separately or in combination with one another to solve virtually any odor problem. These are:

Adsorption
• Unimpregnated activated carbon (or other adsorptive media)
• Impregnated activated carbon (or other adsorptive media)
• Catalytic activated carbon (or other adsorptive media)

Absorption
• Wet caustic scrubbing
• Mist scrubbing

Biological
• Biofilter
– Constructed
– Engineered / Modular
• Bio-Trickling Filter
• Bio-Scrubber

Gas Phase
• Reactants
• Counteractants
• Ozone
• Masking Agents

Thermal Oxidation

All of these technologies work in the vapor phase. In addition to these technologies, there is the technique of adding various liquid chemicals to the wastewater stream. The objective of adding chemicals to the liquid wastewater is to reduce the amount of odorous compounds released into the air by curtailing their production in the wastewater.

Integrated Solutions

Certain odor control applications are difficult to address with a single stage system, regardless of the type of technology evaluated. No technology exists that will effectively treat every possible odor control problem. However, by combining two or more technologies, virtually any odor problem can be solved. The most common scenarios for combining technologies are discussed below.

High Hydrogen Sulfide Concentrations

Odor control applications that contain high levels (greater than 50 ppm) of hydrogen sulfide often must be addressed by two stage systems. Such systems could include a catalytic carbon system, chemical wet scrubber, or bioscrubber as a first stage, followed by a second stage, often times an activated carbon system or biofilter. In all scenarios, the first stage serves to reduce the concentration of hydrogen sulfide to a more manageable level and the second stage of the system polishes the stream.

Ammonia & Hydrogen Sulfide

Another common situation is the presence of both ammonia and hydrogen sulfide. Combined technologies can include a multi-stage wet scrubber, an acid scrubber followed by a catalytic activated carbon, or a bioscrubber followed by a standard activated carbon system for polishing.

Hydrogen Sulfide & Organics

Air streams with both significant levels of hydrogen sulfide and organics generally require two or more stages. Among the treatment combinations effective for this application are multi-stage wet scrubbers, catalytic carbon systems followed by standard activated carbon or biofilter systems; or biofilter systems followed by catalytic or standard activated carbon.

Gas phase treatment as well as liquid chemical addition into the waste stream itself can serve as useful complementary technologies to either mask residual odors which the other technologies could not entirely treat, or reduce the initial loading of odors in order to reduce the size and operating cost of the odor control technology employed. The size of the separate stages in the combined systems (i.e. removal efficiency) is driven by the economics for the project, both capital and operational.

Conclusion

Clearly, there are a variety of odor control solutions that should be considered in any design analysis. Historically, these choices have been limited to chemical wet scrubbers, biofilters and traditional activated carbon. Now, the options have expanded to include catalytic carbon — either as part of a traditional carbon adsorber system, or as a new generation system.

One of these "new generation" systems, Calgon Carbon's Phoenix Odor Control system, is part of an integrated treatment system that is being successfully used at the Massillon, OH, Wastewater Treatment Plant (WWTP) to remove ammonia and hydrogen sulfide.

Most importantly, there is a new mindset that recognizes that many challenging odor control problems can only be solved with an option that combines two or more of these technologies. When developing front end analysis and system design choices, designers, contractors and municipal engineers should consider a range of application-specific factors that include flow rate, contaminant concentration, space requirements, down time tolerance, maintenance, operational hazards and disposal of spent materials.

It's also prudent to conduct some low-cost, front-end testing to attain as comprehensive a picture as possible of the odorous compounds that may require treatment. Remember, there is no "magic bullet" but there are integrated, multiple-technology approaches that deliver real results.