Waste Water Odor Control in the 21st Century

We can all remember when wastewater treatment plants were located far away from everything -- out of site, out of mind. Odor was only a problem for those at the facility and it was something that you just "got used to," if you were around wastewater. However, now, with development booming and urban growth extending into even the most remote of areas, it has become an issue for everyone.

Smell, as you have probably noticed, is one of the most sensitive of our human senses. In fact, the typical nose can detect extremely minute levels of odorants, well below the one-part-per-million range. This means that people can readily detect odors in their surroundings -- both pleasant and offensive odors. Now that most wastewater treatment plants are being surrounded by neighborhoods, the potential for complaints, bad public relations and hard feelings is immense -- and growing.

Since the new neighbors moving nearer to the country's treatment facilities have finely tuned odor detectors in the middle of their faces, it is becoming more and more necessary for wastewater professionals to become odor-control experts -- or, at least, know someone who can advise them on how to solve their odor problems efficiently and effectively.
Most wastewater- related odor problems are generated via bacterial activity within the waste streams (typically referred to as liquid-phase odor).

Achieving reliable, cost-effective control of wastewater odor is a possible when you begin with a working understanding of its nature and sources. First, most wastewater- related odor problems are generated via bacterial activity within the waste streams (typically referred to as liquid-phase odor). Inherently odor-causing compounds are volatile and therefore are readily released to the air when given the opportunity (at which point they may be referred to as vapor-phase odor). Odor problems can therefore be effectively controlled in either phase, however, there are often practical or economic reasons to focus on a particular phase, or in an increasing number of cases, both.

Origin of Odor Compounds
Typically wastewater-related "problem" odor compounds are generated in the liquid phase -- during the process of conveying the wastewater from its inception to treatment area. While traveling through sewer lines, the wastewater can become anaerobic or septic (that is, the dissolved oxygen can become depleted) as a result of the metabolic processes of microbes commonly found in wastewater.

Under anaerobic conditions, specific microbes (sulfate-reducing bacteria) thrive. These microbes generate hydrogen sulfide (H2S) as a byproduct of their respiration. Unfortunately, H2S has a low solubility in wastewater and when it escapes from the wastewater and moves into the air (i.e., vapor phase) it is easily recognized by its characteristic offensive, rotten-egg odor. H2S can also be responsible for severe corrosion problems and toxic conditions within wastewater conveyance and treatment facilities.

In addition to creating odor along sewer lines and at lift stations, H2S is released into the air in areas such as wet wells, force main discharges, equalization basins, headworks, grit chambers and primary clarifiers. Although H2S is the most prevalent odor-causing agent in wastewater, other organic, odor-causing compounds generated by anaerobes, such as mercaptans and amines, are also typically present.

Early Odor Treatment While It's in the Liquid Phase
Controlling odor while it is in the liquid phase is often easier and more cost effective than treating large areas of atmospheric odor. Odor-causing wastewater is typically treated by altering the biological conditions in the wastewater or by adding chemicals to either control the formation of odor-causing compounds, or react with those compounds once they are formed. Liquid-phase treatment is most commonly applied in wastewater collection systems or during sludge storage/processing in the wastewater. However, liquid-phase treatment is often applied in a wastewater system for downstream control at the headworks of a treatment plant. Biosolids-handling facilities also have liquid-phase odor-control challenges that can be handled in a manner similar to the methods used for wastewater.

Among the liquid-phase treatments available, Bioxide® and Aquit® (patented processes, nitrate- and anthraquinone-based respectively, from USFilter) are two viable and proven options available that do not involve the handling of materials classified as hazardous. Used individually or in combination, these liquid-phase odor-control technologies promote the growth of favorable biology within the wastewater and have proven to be highly effective in eliminating vapor-phase odor. Other options commonly used for liquid-phase odor control include oxidizing agents such as hydrogen peroxide and chlorine-based products, sodium or calcium hydroxides and precipitants such as iron salts. While effective, the handling issues of these hazardous agents have to be taken into consideration in the selection equation.

Dealing with Odors in the Air
Once odors are released into the atmosphere and become vapor-phase odors, other control strategies have to be considered. Vapor-phase technologies ventilate/treat the point sources of odor such as wet wells, headworks, etc. Vapor-phase treatment options are primarily focused on the removal of hydrogen sulfide. Methods for vapor-phase odor control most typically include wet chemical scrubbers, biofilters/bioscrubbers, dry chemical scrubbers and activated carbon adsorbers.

Among these options, wet air scrubbing has proven to be one of the most flexible and reliable for vapor-phase wastewater odor-control technology and can, in fact, be used to treat virtually any water-soluble contaminant. In addition to treating H2S and organic odors, wet scrubbing has proven to be effective for ammonia removal. Wet scrubber design for these types or wastewater odors typically involve multi-stage, vertically orientated packed towers. The basic objective of the packed towers is to provide contact between the odorous air and scrubbing solution (water with pH-adjusting and/or oxidizing chemicals to dissolve and/or oxidize the odorous compounds. The commonly used chemicals are chlorine based (particularly sodium hypochlorite) and pH adjustment is typically accomplished via sodium hydroxide or sulfuric acid. Multi-stage, pre-engineered systems make for easy installation and reduce operation and maintenance (O&M) costs.

Although hydrogen sulfide is the most prevalent odor-causing agent in wastewater, other organic, odor-causing compounds generated by anaerobes, such as mercaptans and amines, are also typically present.

Biofiltration is used within the wastewater industry to treat a variety of biodegradable, water-soluble contaminants. In a biofilter or bioscrubber, the odor contaminants are solubilized from the vapor phase into an aqueous biofilm on the surface of an organic medium, such as compost, mulch or a longer-lasting, engineered, inorganic. Moisture content and temperature are important environmental conditions for proper microorganism growth within the biofilm. Bacteria then "consume" the odorous compounds via their natural metabolic processes. Biofilters are effective at removing sulfur-based odor compounds, such as H2S, organic sulfides and mercaptans, when operated under proper moisture, temperature and loading conditions. They are generally not effective at removing nitrogen-based compounds such as ammonia and amines. Biofilter and bioscrubber designs vary from standard preengineered systems to custom, built-on-site construction; but all require a significantly longer retention time of the odorous air than wet scrubbing. Therefore, footprint limitations often prevent consideration of this vapor-phase odor-control approach or result in higher capital costs (although operating costs are often lower due to no daily chemical requirements).