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Waste Water Odor Control in the 21st CenturyWe 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. 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 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 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 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). |