Exhaust dilution can be an effective option for eliminating odors from roof exhaust systems at wastewater treatment facilities and keeping the surrounding communities free from wastewater odors
Odoriferous exhaust gases generated by wastewater treatment facilities can create major problems. No one likes foul odors, especially people who are not responsible for them yet must live with them daily. In fact, many communities are becoming less tolerant of odor-generating facilities and respond with new laws and/or fines against them. Unfortunately, foul odors (in the form of odoriferous roof exhaust) are a fact of life and a necessary evil at many municipal and industrial wastewater treatment facilities. To complicate matters, different people perceive odors differently; many people who are most sensitive to an odor can be as much as ten thousand times more sensitive to that odor than those who are least sensitive to it, according to the American Industrial Hygiene Association (AIHA).
There are many ways to eliminate odors emanating from wastewater treatment facilities. One technology -- mainly used for exhaust odor management at chemical, pharmaceutical and university research laboratory workstations, and for hospital emergency diesel generator exhaust -- has been gaining increased interest as a solution for difficult wastewater treatment odor management problems. This technology, employed in mixed-flow impeller roof exhaust systems, can eliminate perceived odors from wastewater treatment facility process gases efficiently and economically.
Diluting Exhaust Dissipates Odors
Mixed-flow impeller technology works on the principle of mixing outside, ambient air with odoriferous exhaust gases and sending the exhaust stream in a powerful, vertical "jet plume" up to 120-feet high in a 10-mile-per-hour (mph) crosswind, based upon the outlet velocity, thus effectively diluting and dissipating odors and preventing them from permeating the neighborhood, entering adjacent buildings or re-entering the source building. Mixed-flow impeller fans/jet plume systems draw in nearly twice the amount of fresh air (through base-mounted bypass dampers) as exhaust air into the fans' exhaust; their vertical "jet plumes" exit exhaust nozzles at up to 7,000-feet-per-minute (ft/min) velocity. The jet velocity induces large amounts of outside air (up to 170 percent) to be drawn into the plume. This injection of fresh air causes immediate relief of odor perception by odor dilution. In most cases this system eliminates odor problems in the neighborhood; however, in extreme cases when it does not, mixed-flow systems are flexible enough to allow other available odor-control methods to be used in conjunction with the system.
The theory of mixed-flow impeller technology systems is simple: to eliminate odors by dilution, fresh air is mixed in with the wastewater process exhaust gases until a suitable concentration in parts per million (ppm) or milligrams per cubic meter (mg/m3) is reached and the odor is no longer perceptible or objectionable. Dilution is achieved two ways: directly by diluting the exhaust stream (plume) before it leaves the exhaust fan; and, indirectly, where the exhaust stream from the fan is diluted by the atmosphere before it reaches the property line, nearby air intakes, adjacent buildings or sidewalks.
A minimum outlet velocity of 3,000 ft/min is typically recommended to avoid stack downwash. The optimal system outlet velocity can vary from system to system. Wind studies or modeling and certain calculations can be performed to help obtain the optimal system. For comparable performance (at substantially higher costs) another system would require a centrifugal-type, belt-driven exhaust fan ducted to a dedicated steel stack that might be as tall as 100 feet in order to disperse the exhaust odor stream from the fan. Obviously, the cost and complexity of such a structure (and its many negative implications) work against it.
Exhaust emissions from wastewater treatment facilities may either be toxic or nontoxic, with toxic odors regulated by the U.S. Environmental Protection Agency (EPA), Occupational Safety and Health Administration (OSHA) and other government agencies. Nontoxic emissions generally are considered completely safe (or safe in the amounts that are likely to be generated); although even "safe" materials may have limits beyond which their emissions would be regulated.
Among the many methods to manage wastewater treatment odors are those technologies using chemical additives, such as potassium permanganate, sodium hypochlorite, ferris chloride, chlorine or other aggressive chemicals, as well as those employing precipitators, scrubbers, thermal oxidizers, charcoal filters or other expensive hardware to treat process exhaust prior to discharge. Of course, toxic odor generation may require more aggressive approaches in many cases (see sidebar, Additional Odor Control Considerations
Additional Odor Control Considerations). However, in applications where toxicity of the emissions is not an issue, use of mixed-flow impeller technology has been increasing in popularity over the past few decades.
There also are applications when a combination of one or more methods of odor control is required depending upon wastewater composition; and sometimes atmospheric conditions may also determine which approach or approaches might be best suited as well. However, for most applications, mixed-flow impeller technology prevents offensive odors from permeating neighboring buildings in particular and entire neighborhoods in general. It also prevents offensive odors (or toxic/noxious fumes) from being re-entrained into the wastewater treatment facility -- a critical issue. Because mixed-flow impeller fans are low profile (typically about 15-feet high), they also eliminate the negative implications with regard to neighbors' perceptions of tall exhaust stacks on the roof as "pollution generators" -- right or wrong. This is especially important at wastewater treatment facilities where employees or neighbors have previously complained of foul odor -- a tall stack can be a daily reminder of the presence of the odor.
Other Advantages of Mixed-Flow Technology Fans
Another major consideration with regard to mixed-flow impeller technology is the aesthetics associated with tall exhaust stacks on the roof. Mixed-flow fans are substantially shorter than tall stacks typically used with traditional centrifugal-type fans for similar applications. Elimination of tall, unsightly stacks, which are either prohibited by code or undesirable, is an added benefit. In addition, low-profile, mixed-flow impeller fans don't require structural reinforcements on the roof or complex, expensive mounting/stabilizing hardware, such as elbows, flex connectors, guy wires or spring vibration isolators, substantially reducing time and costs for installation. The reason for this is the lower vibration characteristics of mixed-flow impeller systems versus centrifugal-type fans. A mixed-flow impeller fan's radial vibration parallels the building's roofline, resulting in a substantially lower axial component of vibration forced vertically onto the roof. Conversely, in a centrifugal system, the high radial component of vibration is forced directly down into the roof (moving vertically up and down on the roof) and thus requiring costly structural reinforcements with deteriorating system performance and potentially unsafe working conditions. Mixed-flow impeller fans also typically consume about 25 percent less energy than conventional centrifugal fans, with resultant faster payback periods (see sidebar, Characteristics of Mixed-Flow Impeller Technology Systems).
Many of these gains can be attributed to the high blade and system efficiencies of mixed-flow impellers. The direct drive of the motor shaft to the impeller is a substantial benefit over belt-drive systems. Whenever there is an intermediary (transmission) between the driving source (motor) and driven equipment (impeller), there will always be a loss attributed to this exchange of work. A typical belt-driven fan can experience losses of up to 10 percent of its energy, even in an ideal installation. An ideal installation would include perfect alignment, belt tightness and many other variable factors.
Lower noise levels may also be (and usually are) advantageous in some locations. When noise is an issue, however, there are accessories available to deal with it, including acoustical fences and acoustical silencer nozzles. In addition to odor generation, noise pollution is also raising added concern in many communities; mixed-flow impeller systems offer a variety of cost-effective, highly efficient methods of reducing noise if it is a problem -- oftentimes without affecting fan height above the roofline.
Case Study: The Ocean County Utilities Authority
For a typical example of how this technology has been used on a practical basis, consider the experience of the Ocean County (New Jersey) Utilities Authority. The authority operates a number of treatment facilities as well as pumping stations, many of which are located in residential neighborhoods in resort communities. To assure that they remain "good neighbors," the Ocean County Utilities Authority always considered environmental odor control a high priority at its pumping stations. In fact, over the years there have been very few odor complaints from neighbors, mostly because of various odor control methods employed. When faced with the decision to replace rooftop ventilation fans at some of its pumping stations, the utilities authority selected mixed-flow impeller systems based on their effective odor dilution and plume dispersion capabilities.
Three mixed-flow impeller exhaust systems were initially installed at three different pumping stations, and more have been subsequently installed based on the systems' ability to "effectively generate higher static pressures at lower energy consumption levels," according to Brad Hazley, director of the authority's Southern Division. Additionally, Hazley said that "quick and easy installation were also key considerations influencing the authority's decision since only 'minimal alterations needed to be made.' Just a slight modification of the fan box was necessary when we connected the new systems to the roof. It did not entail a great deal of effort for the maintenance staff to remove the old centrifugal fans and replace them with the new, low-profile, mixed-flow impeller systems."
Hazley and his staff also said that the systems' performance for ventilating and controlling potential odor in the wet wells at their pumping stations was "more than satisfactory -- and we benefit from overall lower operating costs because of the energy savings when compared to previous and/or alternative systems."
Maintenance was another key consideration when Hazley and his staff were evaluating alternative solutions. He is pleased with the results, since the new systems are virtually maintenance-free (as opposed to typical belt-driven centrifugal fans). Maintenance-free operation provides Hazley with the confidence he needs to maintain the Utility Authority's "good neighbor" policy. "We certainly didn't want to get involved with a belt failure on a weekend or during off-hours, which could result in an odor complaint," Hazley said. He added that with the mixed-flow impeller systems' maintenance-free performance, "we don't have to take a chance on that occurring."
If you are dealing with a wastewater odor control problem -- whether real or perceived by workers and/or neighbors -- determining the best method for relief generally depends upon the compounds causing the odor and their concentrations, as well as factors such as exhaust flow rates, atmospheric conditions, and building location and/or configuration. Based on the increasing popularity of mixed-flow impeller technology for solving these problems, and the advantages this technology offers, it's time to give it equal consideration among the more traditional methods in making a prudent decision.
Characteristics of Mixed-Flow Impeller Technology Systems
Mixed-flow impeller systems operate on a unique principle of diluting contaminated exhaust air with unconditioned, outside ambient air via a bypass mixing plenum. The resultant diluted process air is accelerated through an optimized discharge nozzle/windband where nearly twice as much additional fresh air is entrained into the exhaust plume before leaving the fan assembly. Additional fresh air is entrained into the exhaust plume after it leaves the fan assembly through natural aspiration effect. The combination of added mass and high discharge velocity minimizes the risk of contaminated exhaust being re-entrained into building fresh-air intakes, doors, windows or other openings.
As an example, a mixed-flow fan moving 80,000 cubic feet per minute (cfm) of combined building and bypass air at an exit velocity of 6,300 feet per minute can send an exhaust air jet plume up to 120 feet high in a 10-mph crosswind. This extremely high velocity exceeds ANSI Z9.5 Standards by more than twice the minimum recommendation of 3,000 feet per minute (fpm). Because up to 170 percent of free outside air is induced into the exhaust airstream, a substantially greater airflow is possible for a given amount of exhaust -- providing excellent dilution capabilities and greater effective stack heights over conventional centrifugal fans without additional horsepower.
Mixed-flow impeller systems also reduce noise, use less energy and provide enhanced performance with faster payback over conventional laboratory fume-hood exhaust systems. A typical reduction of $0.44 per cfm at $0.10/kilowatt-hour provides an approximate two year R.O.I. (return on investment) Energy consumption for mixed-flow fans is about 25 percent lower than conventional centrifugal fans with substantially reduced noise levels, particularly in the lower octave bands. They also conform to all applicable laboratory ventilation standards of ANSI/AIHA Z9.5 as well as ASHRAE 110 and NFPA 45, and are listed with Underwriters Laboratory under UL 705.
Mixed-flow systems are designed to operate continuously with a minimum amount of required maintenance, providing years of trouble-free performance under normal operating conditions. Direct-drive motor bearings have lifetimes of L10 100,000 hours. This refers to a "sample" of 100 motors in which the bearings in 10 motors (10 percent) would fail within a 100,000-hour timeframe. It is a baseline for comparison of motor bearing lifetimes. Once a motor is integrated into a belt drive configuration, the bearing life can drop as low as L 10 40,000 hours. Non-stall characteristics of the system's mixed-flow wheel make it ideally suited for constant volume or variable air volume (VAV) applications, along with built-in redundancy and design flexibility. VAV capabilities are achieved via the bypass mixing plenum or by using variable frequency drives to provide optimum energy savings.
Virtually maintenance-free operation (there are no belts, elbows, flex connectors or spring vibration isolators to maintain) eliminates the need for expensive penthouses to protect maintenance personnel under adverse conditions. Consequently, additional savings of several hundreds of thousands of dollars may be realized in a typical installation.
Mixed-flow impeller systems are available with a variety of accessories that add value, reduce noise and/or lower energy costs substantially. For example, accessory heat exchanger glycol/water filled coils for use in 100 percent conditioned makeup air facilities add exhaust heat to intake ventilation air to save thousands (or hundreds of thousands) of dollars in energy costs.
Additional Odor Control Considerations
When evaluating dilution, either alone or combined with another odor control technologies, consider these basic guidelines:
- Odor-laden air must be pointed upward with rain protection that prevents downward flow (no rain caps, goosenecks or flapper dampers)
- Use as high a stack exit velocity as possible (at least 3,000 ft/min)
- Locate exhaust fans on the highest usable roof with regard to duct connections
- Use a combination of extra fresh air from the roof into the stack flow along with stack height to achieve desired odor-detection levels at the property line or supply air intakes
Keep in mind that dilution applies to the control of odor problems that are not subject to further regulatory requirements, such as standards for volatile organic compounds (VOCs) or hazardous air pollutants (HAPs). The cost for some types of control equipment depend on airflow rates (cfm). Thus, if additional controls are required, dilution could result in higher costs unless the other system is placed upstream of the dilution fan.
Other methods of odor control include:
- Prevention -- eliminating the source of the odor or substituting a non-odor-causing material
- Minimization -- reducing the amount of odor-causing material or causing it to evaporate at a slower rate
- Masking -- adding a pleasant odor to the air to hide or mask the objectionable odor
Masking is usually too costly to be used very often in municipal and industrial wastewater facilities. Some specific prevention and minimization strategies include eliminating the source of the pollution, changing raw materials and/or fuels, modifying process operation, recycling exhaust rather than venting it to the atmosphere, minimizing entrainment of pollutants into the gas stream, reducing the number of points in the system where materials can become airborne, recycling a portion of process gas and designing hoods to exhaust the minimum quantity of air necessary to ensure odoriferous pollutant capture.
Selecting an odor-control technology depends on the compounds causing the odors and their concentrations, as well as the air-stream flowrate, moisture content and variability.
This article originally appeared in the 03/01/2004 issue of Environmental Protection.