A bird’s eye view of the facility, two times larger than any similar facility in North America.
The facility, which is twice as large as any other similar facility in North America, houses two units that Merrell Bros. refers to as “pods,” consisting of seven 216-x-42-foot interconnected greenhouses. Inside the pods, the biosolids are pre-dried and pasteurized through a newly patented process that uses solar energy to bring them from 80 percent moist to 60 percent dry and 40 percent moist. The greenhouse units are then covered with a clear LEXAN™ polycarbonate sheet, which allows a maximum amount of sunlight to enter the greenhouses.
In addition to providing sunlight, the pods are also responsible for keeping the air moving inside the greenhouses to encourage evaporation. Once the biosolids are finished pre-drying in the greenhouses, they are moved to belt dryers inside a separate processing building to fully dry and pasteurize. In order to comply with EPA regulations for pasteurization, the biosolids must be heated to 70˚C (158˚F) for a minimum of 30 minutes.
As Merrell Bros. was developing its process, it realized there were a number of issues to address. Specifically, the company had to figure out how to get each greenhouse and drying belt to filter massive volumes of air while still achieving an optimal humidity level and minimizing odor from the biosolids, which consist of primarily organic reduced sulfur compounds.
“We needed an air handling technology that would enhance the drying process, creating the best environment for the biosolids,” explained Ted Merrell, vice president and co-owner of Merrell Bros. “Figuring that out can be challenging, as you need something that can get the temperature just right. At the same time, we needed the best possible odor control since biosolids are chemically unstable.”
Typically, a conventional biofilter would be the optimal technology for this application. However, according to Jeff Jones, president of ECS Environmental Solutions, in the case of the Pasco County facility, implementing a traditional biofilter would likely be too cumbersome as the massive 460,000 cubic feet per minute (cfm) of airflow would require an extremely large footprint. Another complication is that no technology creates an absolute odor-free airstream, so a ground-level emission source would still adversely affect the fence-line odor threshold set forth in the facility design.
Instead, Merrell Bros. used 16 of ECS’s patented VX radial flow adsorber units to enhance the drying process and mitigate the odor. Each activated carbon VX unit in the facility takes in 28,500 cfm and is only 12 feet in diameter. The units disperse the air 40 feet high and at a rate of 3,000 feet per minute, reducing the facility’s fence-line impact. While the total odor removal rate of the activated carbon may be less than a biofilter, the combination of dispersion technology will help achieve the ultimate goal.
“We may have one set of parameters for odor control, but we also have to have moisture capacity,” said Merrell. “We needed to have 200,000 cfm of air exchanges in each of the greenhouse units to handle moisture processes. We put seven of those on each greenhouse pod — designed to draw 28,500 cfm of air each. We’re exchanging the air every five minutes, and an additional 60,000 cfm is being treated from the process building for a total airflow rate of approximately 460,000 cfm.”
To this end, special impaction filters were supplied by ECS to help remove moisture from the air before it enters the activated carbon.
