Odor Control Technology Isn't Enough Without Effective Capture

Odor Control Technology isn't Enough Without Effective Capture Effective odor control is not just about the technology, it is also about the capture and containment of the odor. Choosing and sizing a wet scrubber or any odor control technology is critical, however the best technologies are insignificant if nuisance odors continue to escape before they can be treated. Such issues with capture and containment of odor were present at Orange County Sanitation District (OC San) Plant No. 2 biosolids Truck Loading facility. OC San Plant No. 2 is located in Huntington Beach, California, along the Santa Ana River (which has a raised walking/bike path) and is located near residences, Huntington State Beach, a few parks, and some commercial properties. The Plant is currently treating an average wastewater flow of approximately 60 million gallons per day (MGD). The function of the Truck Loading facility is to store the dewatered biosolids and to load to the hauling trucks for offsite beneficial reuse. Newly installed centrifuges, which improved biosolids dewatering, increased odor generation at the storage and Truck Loading facilities and triggered complaints by plant personnel and community members. To address these nuisance odors, a scientific approach was implemented that consisted of four main parts: gathering data, targeted sampling, air movement monitoring, and designing a custom air capture system based on the information collected in the previous steps. This paper will provide an overview of the approach utilized and highlight key elements that aided in understanding and solving the problem. Gathering Data: A series of questions were asked to find where and when the problem was occurring. To understand and solve the problem, collaboration with the OC San Maintenance, Operations, and engineering staff was crucial. Through this collaboration, it was determined that the odor in the Truck Loading bays was caused by the centrifuge-dewatered biosolids in the truck loading operation. Odors were released when the biosolids dropped through the storage silo knife gate valves and filled the truck trailers. Odors continued to be released from the accumulation of biosolids on surfaces of the truck and Truck Loading bays from the loading process. Minor system contributors included fugitive odors where biosolids accumulate but were difficult to clean, including the floor grates associated with the truck scales, which trap biosolids from washdowns. This information determined that the central issue surrounding the Truck Loading facility was the lack of containment infrastructure. Continuous dataloggers were installed to identify the pattern of hydrogen sulfide and ammonia occurrence throughout normal operations for several weeks. Trucks generally drove into the loading bay from the north, where a door closed behind it, to prevent the wind from blowing the odor out through the facility. The original design of the building assumed that winds would dominate from the south side, and that thus air would be sufficiently contained by the door on the north while ventilation drew out the foul air. The presence of odors indicated that this design was less effective than anticipated in capturing the stronger lingering odor, and further analysis was needed to determine the best containment and ventilation approach. Targeted Sampling: Next targeted sampling was conducted to quantify the odor by pinpointing the constituents of the odor, and their concentrations so that effective capture and containment can be customized. This sampling and analysis measured a complete set of odor-causing constituents such as hydrogen sulfide and ammonia on a continuous basis as well as grab samples for other speciated reduced sulfur compounds/VOCs during the most concentrated periods of Truck Loading, at the higher areas of the loading bay, as well lower regions of the bays (Understanding that the mix of compounds present consisted of both heavier and lighter than air compounds). Air Movement Monitoring: Air movement in the Truck Loading bays was simulated using a series of smoke tests for both existing and considered future conditions. The smoke tests were conducted with smoke candles that released a large amount of smoke (up to 40,000 cubic feet) within a small period of time, to simulate the sudden release of odor while the trucks were being loaded. Each test simulated either existing or potential ventilation conditions. Potential conditions included simulating the presence of a door on the south side of the building, differing ventilation rates, and differing quantities of supply air provided. By observing the movement of the smoke during this array of conditions, conclusions were drawn about the movement of odor throughout the truck bay, including the locations of dead zones, the effectiveness of an additional door on the south side for containment, and the effectiveness of the existing ventilation system at clearing air through each region of the truck bay. The smoke tests made it clear that a second door was necessary to contain the smoke within each bay. Tests which simulated a door made clear other weaknesses with the existing ventilation system's layout, including dead zones near the ceiling. Design of a custom system: The above tests and observations indicated the specific problems that were causing issues with odor control at this particular facility. Alternatives evaluation and conceptual design of improvements to the system were entirely customized to the height and length of the building. First, a door was recommended on the south side of the Truck Loading facility, to contain the odors within the building. Positive air supply was recommended at low and high heights to move foul air with odorants heavier than air observed in step 2, and sweep dead zones found near the ceiling in step 3. A U-shaped exhaust duct layout at ceiling level was recommended to capture the most concentrated foul air on both sides, where the smoke tests indicated most of the smoke was accumulating. The above method of identification of the distinct contributors to the odor control issue at this facility was instrumental in creating a custom design to address the concerns thoroughly and efficiently. Without the observations and collaboration in the earlier steps, the conclusions that allowed for a custom solution would not have been possible.