Nowhere to Hide

As communities grow more crowded, odor control has become a major concern in the design and operation of wastewater treatment systems

• By Vaughan Harshman, PE
• Jul 01, 2006

As populations and communities continue to grow and expand, the need to treat wastewater means more facilities near homes. So, out of sight, out of mind doesn't work anymore. Years ago, wastewater treatment plants were located far away from communities, and odor was only a problem for those at the facility. Now, with new home developments booming and urban growth extending into even the most remote areas, it has become an issue in all communities.

Smell is one of the most sensitive of our senses. This means that people can readily detect almost any odor in their surroundings. Now that many wastewater treatment plants are being surrounded by neighborhoods, the potential for complaints, bad community relations, and hard feelings is immense -- and growing.

Once neighbors become sensitized to odors, any odor event -- no matter how small or how short in duration -- will trigger complaints. Therefore, it is becoming more and more necessary for wastewater professionals to manage and control odors before the neighbors notice.

But odors aren't simply an issue of bad public relations. The same compounds that create these odors can also corrode and damage treatment plants and collection systems.

It is important to be proactive to address corrosion, to prevent negative images of the facility, and to avoid a crisis situation by allowing time to implement the appropriate solution. Most wastewater professionals are now faced with learning all they can about odor control and finding someone who can solve their odor control problems efficiently and effectively.

Wastewater Odors: Where They Start
Wastewater-related odors typically develop in the liquid phase very early in the treatment process during its collection and transport to the treatment plant. While traveling through sewer lines, wastewater can become anaerobic (the dissolved oxygen can become depleted) as a result of the bacteria commonly found in wastewater. Under anaerobic conditions, certain types of bacteria generate hydrogen sulfide (H₂S) as a byproduct. Unfortunately, H₂S, because of its composition, easily escapes from wastewater and moves into the air -- the vapor phase. It is recognized by its strong, offensive, rotten-egg odor.

In addition to creating odor along sewer lines and at lift stations, H₂S is released into the air in areas such as wet wells, equalization basins, headworks, grit chambers, primary clarifiers, and biosolids processing areas.

H₂S isn't just an odor issue. It can also cause severe corrosion problems and toxic conditions within wastewater conveyance and treatment facilities.

Other odorous compounds such as mercaptans and organo-sulfide compounds are typically found in wastewater systems where H₂S problems exist. Together, these compounds are usually considered together "sewer odor" compounds or "organics." Their effect on odor conditions is usually secondary to H₂S, but it must be considered.

Each waste collection and treatment system is unique, with its own complex and variable conditions, which can prove challenging when selecting the appropriate odor control solution. But understanding the makeup and sources of odor can help you better manage odors effectively.

Liquid-Phase Odor Control Techniques
Treating 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 adding compounds to the wastewater to either control the formation of odor causing compounds biologically and/or chemically, or to react with odor compounds once they are formed. Liquid-phase treatment is usually employed in wastewater collection systems or after treatment at the biosolids stage.

There are four major types of liquid-phase treatments available: nitrates, anthraquinone, precipitants, and oxidizers. When evaluating the correct technique for a particular application, utilities should consider costs, operational ease, and effectiveness.

An anthraquinone-based process interrupts the sulfate reduction process, controlling sulfides in the waste stream. Used in conjunction with a nitrate salt process, it is very efficient in removing odors.

Other options commonly used for odor control at the liquid-phase include: oxidizing agents or precipitants such as iron salts. Though effective, these hazardous agents must be handled with caution.

Iron salts are added to wastewater to precipitate dissolved sulfide. Iron salts are cost-effective but limited in terms of treatment level and range of compounds that can be controlled.

Oxidizing agents such as hydrogen peroxide can be added to wastewater. By reacting with dissolved sulfide, the agent converts sulfides to sulfate or sulfur and also oxidizes other odorous compounds. With the proper reaction time, oxidizers are very effective at controlling a wide range of odors.

Some processes use iron salts and hydrogen peroxide together to increase the effectiveness and cost efficiency of treatment beyond the capabilities of either technology alone. Well-suited for collection systems with multiple pumping stations and/or odor release points in series, this kind of process controls a wide range of odors very cost-effectively.

Another technique that is sometimes used is pH adjustment of the wastewater stream. This technique is effective at keeping odor compounds in solution at intermediate points, but moves the compounds (and the odor problem) downstream to a point of neutral pH. pH adjustment can be successfully employed if odor and/or corrosion are not concerns downstream at master pumping stations or treatment plants, but most utilities need to employ technology that removes the odor compounds from the system instead of simply relocating them.

Vapor-Phase Odor Control Technologies
Rather than treating an entire wastewater flow, it is often advantageous to contain and treat the odorous air that is released from a given process or area. Sources of odor such as wet wells, headworks, clarifiers, equalization basins, sludge tanks, digesters, dewatering areas, and other treatment plant processes are covered, and odorous air is removed from these areas by a system of ducts and fans. A treatment system then removes the odors from this air stream before it is discharged to the surrounding atmosphere.

Methods for vapor-phase odor control most typically include: wet air scrubbing, biofiltration, scavenger materials, and carbon adsorption.

• Wet air scrubbing: Wet air scrubbing is one of the most flexible and reliable technologies for vapor-phase wastewater odor control and can be used to treat virtually any water-soluble contaminant. Pre-engineered systems make for easy installation and reduce operations and maintenance costs.
• Biofiltration/Bioscrubbing: Biofiltration is used within the wastewater industry to treat a variety of biodegradable, water-soluble contaminants. Biofilters are effective at removing sulfur-based odor compounds such as H₂S, organic sulfides, and mercaptans. Recent advances in media, nutrient delivery systems, and process control have made biological treatment more reliable and cost effective than ever before.
• Scavenger technology: "Scavenger" technology typically uses media to remove sulfur compounds from gas streams, reacting with them and converting them to stable compounds. Since the media is consumed, scavenger-based approaches to odor control are generally limited to low levels of contaminants.
• Carbon adsorption: With carbon adsorption-based methods, the odorous air stream is exposed to adsorbent carbon material, which allows the odor-causing compounds to adhere to the porous surface of the activated carbon-based granules or pellets. Carbon adsorption systems are effective on H₂S and related sulfur-based compounds, as well as other volatile organic compounds.

Odor Control Services
For many communities, especially those that are experiencing incredible growth, keeping up with odor control can be unmanageable. By outsourcing the odor management function to full-service odor-control experts, utilities are able to control personnel and labor costs, meet the demands of a growing community to reduce customer complaints, and manage costs associated with odor control.

A service contract can involve a wide range of scenarios, but the most comprehensive service offerings have the vendor take responsibility for all odor-related problems. Under a single contract, a vendor would respond to all odor complaints, investigate the associated problem(s), and then suggest an appropriate solution to each problem. At times, this could involve operations or treatment process changes, specific pretreatment activities, or even the addition of equipment.

There are no capital costs to the utility as all products and services are provided on a consumption or time basis. The contract, based on performance, encourages implementation of the most efficient solutions.

Start Early, Engage Experts
Most wastewater treatment systems require a variety of technologies, so there are no easy answers to odor control. Many municipalities around the United States have employed the "full-service" model that has been used by Manatee County (see sidebar: A Sewer System Success Story). Faced with ever-expanding wastewater treatment networks, new neighbors, and ever-present budget constraints, utility managers are challenged to effectively manage wastewater odors. If not managed correctly, out of control odors could cause a negative image of the treatment facility, or worse yet, a public relations crisis with neighbors contacting elected officials and media about their odor plight. So it's important for utility managers to start early, do their homework and engage industry experts to effectively manage odors.

A Sewer System Success Story Manatee County, south of Tampa Bay, Fla. is popular with people of all ages, and the area is growing rapidly. Expansion of the county's water treatment infrastructure has become an almost continuous process -- creating challenges for county administrators. One such challenge is to effectively manage the county's odor and corrosion issues with limited resources. The county covers 747 square miles of flat land. Consequently, to move residential effluent to the county's three wastewater treatment plants requires the assistance of hundreds of pumping stations. Each pumping station is a potential site for odor, especially if conditions are right. A seasonal phenomenon that results in full utilization of the county's wastewater collection system during the winter (tourist) season and minimum utilization of the system during the hottest months of the year creates prime conditions for odor issues. Over the years, the county tried many different odor-control remedies, including chlorines, disinfectants, and peroxides. The county found them to be sporadically effective and sometimes dangerous. During this period, there were widespread odor complaints throughout the area served by the wastewater collection systems, and it was evident that the continuing exposure of the sewer system infrastructure to hydrogen sulfide (H2S) gas was causing significant corrosion. Traditionally, odor problems were attacked on a case-by-case basis. However, this process proved to be too cumbersome and time consuming, given the size of the Manatee County staff. To address the issue, the county looked for alternative and safer odor control technologies. It was, in fact, these safer odor control technologies that initiated a transition, in the industry, from dangerous oxidizers to advanced hydrogen sulfide (H2S) inhibitors. Then, it was decided that the county simply needed to outsource the entire problem. The only other option was to add considerably more personnel, and this was complicated by the fact that the odor problems were largely seasonal in nature. Plus, the county was continuing to expand at a rate of 5 to 10 percent per annum. At this juncture, the county contracted with an odor control solution provider to take responsibility for all odor-related problems, by responding to all odor complaints, and investigating the associated problem(s). The provider would then suggest an appropriate solution to each problem, which could involve operational changes or might require the addition of specific pretreatment activities. Or, where other approaches weren't viable, the addition of equipment was proposed. This full service approach dramatically reduced odor complaints in Manatee County and helped establish a new paradigm for odor management in the municipal sector of the wastewater industry. The Manatee sewer network conveys 22 million gallons per day and, currently, there are a dozen different technologies in active use at various sites in the Manatee County system, including both liquid- and vapor-phase technologies. Each technology has been specifically chosen for its unique effectiveness with the circumstances of individual sites within the Manatee County system.

This article originally appeared in the 07/01/2006 issue of Environmental Protection.