The sweet smell of success

• By Joseph Curreri
• Jul 01, 2000

Controlling odor emissions from wastewater treatment facilities is a challenge faced by plant managers nationwide. The problem is particularly - and understandably - acute at facilities located in urban areas. Such was the case at the treatment facility operated by the Arlington County Department of Environmental Services just outside Washington, D.C. Located in close proximity to light commercial and residential developments, the plant had a history of odor complaints. Local residents were so incensed by the plant's odor emissions that complaint calls were often made directly to the facility's board of directors and other county officials, as well as to the plant itself. Plant management, unable to resolve the problem with operational changes, hired an engineering consulting firm to investigate the odor problem. The firm concluded that the majority of complaints were caused by fugitive emissions from a new centrifuge facility, which dewaters the sludge. The existing scrubbers were unable to handle the fluctuations in odor created by changes in centrifuge operation and the routine steady-state loading. When one or more of the centrifuges were put on-line, large spikes in odor were generated. These spikes, combined with residual odors from the sludge storage, truck loading and wet well areas in the centrifuge facility were more than the scrubber system could handle. The problems were further compounded by an inadequate scrubber control and chemical feed system.

Problems impacting scrubbers' efficiency

As with many wastewater treatment facilities, the Arlington, Virginia plant uses packed bed scrubbers to remove hydrogen sulfide (H₂S). This type of scrubber works by coating the surface areas of internal packing with a thin film of solvent, such as sodium hypochlorite, or sodium hypochlorite and sodium hydroxide. The neutralization reactions for these chemistries is shown in Table 1 (Copies of Table 1 may be obtained by contacting [email protected]). The foul air is then passed across the packing where the H₂S enters the liquid film and is oxidized. Scrubber chemistry is conventionally regulated by monitoring the pH and oxidation reduction potential (ORP) of the recirculated liquid. ORP is a measurement that indicates the level of the chemical reaction process of converting a substance to another form by combination with oxygen. Unfortunately, there are two problems associated with regulating solvent addition in this manner. First, the oxidation reaction is extremely pH dependent. If H₂S is scrubbed at a pH below 7, elemental sulfur is formed as a reaction byproduct. Since sulfur is not very soluble in water, it frequently clogs the packing or nozzles of the scrubber. Conversely, if a scrubber is run at a high pH (above 9), it is using more solvent than necessary. Therefore, for optimum scrubbing efficiency the pH should be kept between 8.5 and 9. Secondly, organic sulfides and other chemical compounds can mask the oxidation potential provided by the solvent, rendering ORP useless as a means of regulating chemical addition. The scrubber may be starved for oxidant, but won't request the addition of sodium hypochlorite because the ORP of the recirculated liquid is within specifications. As a result, the H₂S in the air stream is only partially neutralized and foul odor emitted. This was the problem at the Arlington facility.

Controlling the chlorine levels

The solution to the problem of ORP masking lies in the oxidation reduction reaction itself (Table 1). By the very nature of this reaction, when chlorine (Cl₂) is present in the scrubber exhaust gas, H₂S is non-existent. Therefore, a control system that maintains a preset minimum concentration of Cl₂ in the scrubber exhaust should be an effective means of controlling H₂S emissions. The major obstacle in the development of such a control system has been the availability of a suitable technology for detecting and measuring chlorine in the harsh scrubber environment. Most sensors cannot handle the 100 percent condensing atmospheres or the complex and varied gas streams they are exposed to in wastewater applications without becoming contaminated or inoperative. Cross-sensitivity to other gases and high maintenance requirements are also a concern. PureAire Monitoring Systems Inc. and Bionics Instrument Co., in collaboration with Automated Environmental Controls, developed a proprietary gas galvanic, electrochemical chlorine sensor for this specific application that has proven highly effective and reliable at the Arlington treatment plant as at well as other facilities throughout the United States. Originally designed to detect and measure residual chlorine in liquid streams, the sensor operates efficiently in the 100 percent condensing relative humidity conditions typically present in wet scrubbers. The electrolyte in the sensor will operate properly for three months or more without recharging and has no known cross-sensitivity or adverse reaction to hydrogen sulfide.

Sensor operation

When used for automated control, the PureAire chlorine sensor is installed directly in the scrubber and wired to a companion transmitter (Figure 1). Any chlorine present in the scrubber air stream reacts with the sensor electrolyte and generates a microamp electrical signal proportional to the measured Cl₂ concentration. This signal is converted into a 4-20 mA signal and transmitted to a proportional-integral-derivative (PID) controller. The PID controller in turn, integrates the signal from the Cl₂ sensor with temperature, pH and flow sensor telemetry and then uses this data to regulate the rate of sodium hypochlorite addition (Figure 2). The system functions like a cruise control to ensure that the scrubber exhaust gas is free of H₂S at all times despite dynamic changes in the scrubber's operating environment. As the H₂S load increases and the residual Cl₂ concentration falls, chemical addition is automatically increased. Likewise, as the H₂S load decreases and residual Cl₂ levels begin to rise, the rate of sodium hypochlorite addition is decreased proportionately. Such a system served as the design basis for the automated control system installed on the centrifuge scrubber at the Arlington facility in December 1998. Once the system was tuned to meet the specific requirements for the site and placed online, odor complaints dropped dramatically. Plant officials were so pleased with the results that a second system was installed soon afterwards on the building scrubber. Automated control based on residual chlorine measurements has also proven effective on Calvert mist-type scrubber systems, in which attempts at automating the sodium hypochlorite injection rate using ORP and other types of sensors in the scrubber drain have been largely unsuccessful. Unlike packed bed scrubbers, mist scrubbers do not contain any packing. Rather, they work by creating very fine droplets of water with small quantities of sodium hypochlorite and sodium hydroxide which swirl around an otherwise empty scrubber cylinder. Conventionally, mist-type scrubbers use manual adjustments to control the sodium hypochlorite injection rate, resulting in hit or miss H₂S control. Therefore, in most applications, sodium hypochlorite injection is simply set for maximum load conditions.

Automated results

The effectiveness of an automated control system based on in-situ chlorine measurements was proven at the San Luis Rey wastewater treatment plant in Oceanside, Calif. In this installation, the plant has been able to maintain scrubber removal efficiency at optimum levels while significantly reducing chemical consumption. First year savings were more than $100,000. As demonstrated at these two wastewater treatment facilities, the use of a proprietary in-situ chlorine sensor and an automated controller running special software that integrates data from the Cl₂ sensors as well as other relevant analog and digital data is effective in mitigating foul odors. The system is adaptable to both packed bed and mist-type scrubbers and has been shown to reduce odor complaints, improve scrubber efficiency and decrease chemical consumption and costs. In addition, the system enables plant personnel to meet scrubber performance requirements without a significant increase in maintenance and upkeep.

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This article appeared in Environmental Protection magazine, July 2000, Vol. 11, No. 7, p. 46.

This article originally appeared in the 07/01/2000 issue of Environmental Protection.