Design Considerations for Odor Control Systems in Cold Weather Climates

Designing odor control systems for operation in cold weather climates requires additional considerations to ensure success. Overlooking these details can lead to decreased performance and even failure. Webster Environmental Associates (WEA) has been designing and testing odor control systems throughout the country for over 35 years and we've learned a number of lessons along the way. This presentation will provide a road map for evaluating the potential effects of cold weather on odor control systems and will include the following: -Seasonal effects on odor control loading -Odor control technology comparison for cold weather conditions -Estimating inlet air temperatures -Freeze protection options -Adjusting airflows for seasonal changes -Backup power supply requirements -Startup issues in cold weather -Provisions for re-acclimation of biological systems This presentation will include case studies and lessons learned. One case study that will be included in detail is a recent project located in Michigan that WEA designed with Tetra Tech. The design of this project was completed in 2018 and construction was completed in 2019. This project included the design and installation of a 9,000 cfm carbon adsorber and a 10,000 cfm bioscrubber. The carbon adsorber treats air from a series of rotating biological contactors (RBCs) and the bioscrubber treats air from primary clarifiers, a septage receiving facility, headworks, a grit facility, sludge storage tanks, and a series of channels. The first challenge of this project was that the client requested that the system be able to continually operate at ambient temperatures of -20 °F which was the historic low for the area. This requirement caused us to take a closer look at what the inlet temperatures to the odor control systems could be at this condition. We went back to the site during very cold weather and took temperature readings at the different sources. One of our surprise findings was that some of the channels didn't always have water in them, especially in the middle of the night when flows were low (and of course this would be the coldest time!). During that condition, the temperature of the air coming from the channels would be near the ambient temperature. We had a mechanical engineer on the team conduct heat loss calcs based on the data we collected and he determined that the inlet temperature at the bioscrubber could go below freezing when the ambient temperature is -20 °F. We evaluated multiple solutions, including an inline heater, capturing waste heat from another source, etc. In the end the most cost-effective solution was to provide an auxiliary heat source that would add heat into the primary clarifier (see Figure 1 below). This option was the least expensive as it utilized a standard makeup air unit (MAU). The MAU would be controlled by the bioscrubber control panel and only come on when needed. The below is an excerpt from the specified control logic: The MAU shall provide additional heat to the foul air system when the inlet air temperature goes below 36°F. Bioscrubber shall send signal to MAU to turn it on. The bioscrubber will turn off the MAU after 4 hours and re-evaluate the inlet temperature. If the inlet temperature again falls below 36°F, then the bioscrubber will initiate the MAU again for another 4-hour cycle. This cycle will continue until the inlet air temperature remains above 36°F without the MAU.
The next challenge that we faced was the significant seasonal fluctuation in hydrogen sulfide (H2S) levels. Prior to initiating design, WEA conducted an odor study which included two rounds of testing, one in the cool weather and one in the warm weather (see Figure 2 and 3). The results of the testing revealed that H2S averaged 34 ppm during the warm weather testing and 5 ppm during cool weather. However, we did some additional spot testing as design was proceeding during colder weather (in February) to verify loadings and found that the H2S actually drops below 1 ppm throughout facility during the winter. This created a challenge as the bioscrubber may struggle to stay acclimated over the winter, depending on how long the H2S stays depressed. In order to address this potential issue, the design required that the system have the ability to operate in a once-through or recirculation mode without any modification of piping or the addition of any equipment. If needed, the bioscrubber could easily be switched over to a recirculation mode temporarily in the spring to allow the bioscrubber to re-acclimate.
Several other hurdles were also encountered, including the need for the duct to be insulated and acclimation during the early spring. These will all be discussed in detail in the presentation. In addition to the Michigan project, we will also be presenting lessons learned from other projects in cold weather climates as time allows. Additionally, an overall comparison will be made between different odor control technologies operating in cold weather and provisions that need to be made to ensure efficient performance.