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Comment Processing & Handling

A rotating contactor improves quality of bacterial cellulose

By Chemical Engineering |

Although chemically identical to vegetation-derived cellulose, bacterial cellulose (BC) has a different tertiary structure, which imparts a number of unique — and commercially important — properties. For example, BC is nontoxic and biocompatible, and has a large water-holding capacity, which makes it suitable for dressing wounds in medicine. Other possible uses are cleaning environmental spills, oil recovery, water purification, and making food ingredients and paper products.

The major problem in producing large amounts of BC cheaply and efficiently is that it forms best when grown in a stationary culture. For example, Acetobacter xylinus — a member of the same genus responsible for the production of vinegar — produces cellulose at the interface of the growth medium and the air. In industrial stirred-batch reactors (SBRs), the high shear forces involved reduce the “doubling time” of A. xylinus by about 50%. These forces also tend to break up the cellulose fibers, which reduces the quality of the product, says Peter Gostomski, head of the Dept. of Chemical and Process Engineering at the University of Canterbury (Christchurch, New Zealand; edlinks.chemengonline.com/5828-535).

Gostomski’s research group, with cooperation from the Dept. of Chemical and Biological Engineering at Rensselaer Polytechnic Institute (Troy, N.Y.; edlinks.chemengonline.com/5828-536), is developing a rotating biological contactor (RBC) for growing a New Zealand strain of A. xylinus, which allows much better control of the reaction compared to static cultures.

Currently the Canterbury team’s RBC consists of a fermentation vessel with optimized geometry, and a shallower reservoir vessel. Within the fermentation vessel are two felt-covered, slowly rotating (5–15 rpm) cylinders, lying end-to-end, to which bacteria and cellulose adhere. Although the production rate for the NZ strain has not yet been optimized — the RBC produces 2–8 g/m2/d compared to 10–15 g/m2/d produced when operated statically — the quality of the cellulose is improved when produced in the RBC. For example, RBC cellulose has a water-holding capacity of 150–450 times its dry weight, which is up to double that for the static cellulose, he says.

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