A four-month pilot study conducted at the Interstate Brands Wonderbread Bakery in Columbus, OH concluded that a two-stage biotrickling filter composed of environmentally-available bacteria immobilized on solid polypropylene fibers reduced ethanol emissions by 85 to 100 percent. As shown on the table on page 28, the system averaged 91 percent ethanol-removal efficiency during the study period, excluding filter downtime.

The project was funded by the EPA, USDA, the Lincoln Food Processing Center at the University of Nebraska and the American Bakers Association (ABA). The EPA supplied the trailer-mounted biofiltration unit, designed by PRD Tech, Inc. and incorporating proprietary PRD support media. PRD Tech worked with with the department of chemical engineering at the University of Cincinnati to conduct the study. Malcolm Pirnie, Inc. (Columbus, OH) designed and installed the duct work connecting the oven-stack to the biofilter.

The Food Manufacturing Coalition (FMC) for Innovation & Technology Transfer functioned as project manager via R. J. Phillips & Associates (Sterling, VA). Composed of eight food industry associations plus private firms, government entities and universities, the FMC is assessing opportunities for technology transfer to other food industry segments.

Results of the pilot study were presented by Dr. Rakesh Govind, professor of chemical engineering at The University of Cincinnati and director of PRD Tech, at the annual meeting of the American Institute of Chemical Engineers (AIChE) November 19 in Miami Beach. "Not only has this system proven to significantly reduce ethanol emissions, it can be significantly more cost efficient than currently-available technologies," Govind reported.

Biofilter drawbacks

Ethanol, a byproduct of fermentation, is a volatile organic compound (VOC). Through a complex interaction with sunlight and nitrogen oxides, ethanol and other VOCs contribute to the formation of ground-level ozone that can potentially have an adverse impact on human health and the environment. Roughly 98 percent of the VOCs produced in bakery ovens are ethanol.

The most common approach to reducing ethanol emissions is to treat them with biofilters, where bacteria degrade or transform the contaminants. Biofilters typically use peat or compost to sustain the microbial populations. But because the concentration of ethanol emissions varies with production schedule and with product (bread, pastry, cake etc.), bacteria growth can be uneven: overproduction clogs filters; underutilization starves the bacteria. Moreover, because bacteria consume the biofilter material, it must regularly be replaced. These systems can also be "extremely bulky," said Govind. Where space is at a premium, the system must often be located on rooftops, requiring structural modifications to support the load and thus adding capital cost.

Biotrickling benefits

Biotrickling, Govind explained, improves upon biofiltration. In biotrickling, a recycled water/nutrient stream distributes nutrients (such as phosphorous and nitrogen) to the bacteria and keeps the filter moist. Because the support housing the microbes is made of synthetic materials, and because the nutrients are trickled-in, the support media are not degraded and can last indefinitely. The system uses environmentally-available bacteria and no special strains are needed. The filter is self-cleaning, so bacteria cannot clog it with unlimited growth. Because the system is at the end of the baking operation, the microbes cannot contaminate the baked goods. While microbes thrive in the ambient exhaust temperatures, they cannot survive the high temperatures of upstream gases (more than 350° F).

Although ethanol concentrations vary with product, the biotrickling filter maintained removal efficiencies exceeding 80 percent for 99.6 percent of its operating time -- even when the ethanol concentration was abruptly increased. During shutdown periods, however, the microbial decay rate was slow, enabling the biofilter to regain more than 95 percent removal efficiency in just a few hours. Because there are no supplemental fuel requirements or catalyst replacements (as with catalytic oxidizers), biotrickling can save operating costs. Furthermore, the system operates at ambient temperatures and pressure, emits no nitrogen oxides and emits less CO₂. "There are no downsides to this technology," Govind observed.

The pilot study provides a plus- or-minus 30 percent estimate of capital and operating costs for a biotrickling filter applied to treating ethanol emissions from bakeries. More precise estimates can be calculated only after a system has been designed for a specific plant.

Biotrickling can potentially be adapted to other fermentation processes such as brewing, Govind added. The FMC licenses the technology.