ENGINEERING R&D
FEBRUARY 1999

A four month pilot study conducted at the the Interstate Brands Wonderbread Bakery in Columbus OH concludede that a two-stage biop-trickling filter composed of environmentally - availavble bacteria immobilized on solid polypropolene fibers reduced ethanol emmissions 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 proprietarty PRD support mediaPRD Tech worked with the department of chemical engineering at the University of Cincinnati to conduct the study. Malcom Pirnie, Inc. (columbus, OH) designed and installed the duct work connecting the oven stack to the bio filter.

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

In biotrickling, a recycled water/nutrient stream distributes nutrients (such as phosphorous and nitrogen) to bacteria immobilized on support media made of non-degradable polypropylene fibers. (Sources U. of Cincinnati; PRD Tech)

Results of the pilot study were presented by Mr. Rakesh Govind, professor of chemical engineering at the University of Cincinnati and the Director of PRD Tech. At the annual meeting of the American Insttitute of Chemical Engineers (AIChE) November 19 in Miami Beach. “Not only has this system proven to significantly reduce ethanol emmissions, it can be significantly more cost effective thean currently-available technologies.” Govind reported.

BIOFILTER DRAWBACKS
Ethanol a by product of fermentation, is a voliatile 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 potentialyy have an adverse impact on human health and the environment. Roughly 98 percent of the VOCs produced in bakery ovens are ethanol.

The most commom approach to reducing ethanol emmissions is to treat them with biofilters, where bacteria degrade or transform the contaminants. Biofilters typically use peat or compost to sustain the micro-bial Populations. But because the concentration of ethanol emissions varies with pro-duction schedule and with product (bread. pastry cake etc.). bacte-ria growth can be uneven: overpro-duction clogs fil-ters; 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 phosphorus and nitrogen) to the bacteria and keeps the filter moist. Because the support housing of the microbes is made of synthetic materials, and because the nutrients are not trickled in, the support media can last indefinitely. The system uses environmentally-available bacteria and no special strains are needed. The filter is self-cleaning, so bacteria can not 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. Then cannot survive the high temperatures of upstream gases (more than 350' F).

Although ethanol concentrations vary with product. the biotrickling filter main-tained removal efficiencies exceeding 80 percent for 99.6 percent of its operating time - even when the ethanol concentra-tion was abruptly increased during shut-down 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 require-ments or catalyst replacements (as with catalytic oxidizers), blotrickling 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 calcu-lated 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.

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