Biofilm: A Hidden Threat

FROM THE AUGUST ISSUE: Biofilm on heat transfer surfaces in ethanol plant cooling water systems can cause corrosion, reducing efficiency and equipment lifespan.
By Andrew Ledlie | July 24, 2017

Biofilm in the cooling water system of a fuel ethanol plant can be problematic when it becomes insulative or contributes to corrosion. When plant managers miss the warning signs, plants become vulnerable to production bottlenecks, unplanned downtime for repairs and higher operating costs.

Biofilm is a slimy deposit formed by bacteria and can proliferate in as little as one week. On heat transfer surfaces, such as the plates in plate-and-frame heat exchangers, biofilm acts like insulation, reducing the ability of the cooling tower water to cool the process in mash and fermentation coolers and in ethanol condensers. Biofilm can be four times more insulating per micron of thickness than typical cooling water mineral scales. A layer of biofilm just 20 micrometers thick—thinner than a piece of transparent tape—can result in a significant loss of heat transfer.

When biofilm forms on cooling tower fill, it diminishes the evaporation rate. The resulting reduced cooling capacity and efficiency is particularly harmful during the summer when systems are already under higher cooling loads because of high temperatures. Consequently, many plants either scale back production or run chillers to compensate. Persistent biofilm can result in less-efficient overall production, as well as increased energy consumption and carbon intensity, as cooling tower fans and pumps are forced to run harder to deliver the same amount of cooling as a clean system would.

Beyond reduced efficiencies, biofilm causes microbiologically influenced corrosion (MIC). Metal surfaces become pitted as the acid produced by the organisms in the biofilm acts directly on the metal surface in concentrated, localized areas. This type of corrosion is all too familiar in ethanol plants, where leaks and failed heat exchangers can become a costly problem in as few as 10 years, despite the fact that the lifespan for this equipment should be 20 to 25 years.

Removal Challenge
Once biofilm takes hold, removing it is difficult. Unfortunately, the traditional chlorine approach does little to remediate the problem. Most ethanol plants are limited in the amount of chlorine that can be used to control bacteria for two reasons: They take cooling tower blowdown back into the front end of the plant, so the contents of the cooling water could end up in the dried distillers grains; or they directly discharge the blowdown into a ditch or creek, so they are limited by their National Pollutant Discharge Elimination System permits.

“Biofilm fouling is something I have been concerned about for years,” says Bob Jewell, energy systems chief at Chippewa Valley Ethanol Co. in Benson, Minnesota. “Unfortunately, we were limited in what we could do with biocide alternatives to chlorine because of our NPDES permit. I wanted to address biofilm before our new cooling tower was installed and it just so happened that our permit was due to be renewed at the same time.”

The CVEC plant installed a new cooling tower in the fall of 2016 and with it, employed the Solenis Biosperse XD3899 microbiocide to control biofilm, improving system cleanliness and reducing corrosion potential. Solenis also assisted CVEC in updating its NPDES permit.

Jewell also wanted to monitor the effectiveness of its chemical treatment program, an option offered with Solenis’ OnGuard 3B analyzer. The device provides a real-time, in-situ view of the cleanliness of a cooling water system and the effectiveness of the biocide program in controlling biofilm. The device employs an advanced ultrasonic probe that detects biofilm growth in cooling systems earlier and faster than any other commercially available technology. The 3B analyzer has enabled plants to reduce costs and downtime and to optimize chemical use by taking corrective action before biofilm can negatively affect process equipment.

Solenis calls the combination of its chemistry, automation and service ClearPoint Biofilm Monitoring and Control.

“From what I have seen so far, Solenis’ ClearPoint program is performing very well and meeting my high expectations,” Jewell says, one year after employing the system. “It’s definitely much better than our prior biofilm control program.” The program helps alleviate his worries about biofilm and corrosion, enabling him to focus on other areas of the ethanol-production process that require his attention, he says.

Author: Andrew Ledlie
Biorefining Marketing Manager