A 60,000 gallon (227,125 liter) volume district cooling system located in downtown Chicago had a constant problem with algae and biofilm growth in the cooling tower basin. The facility wanted to improve safety and control the algae and biofilm, without increasing the use of existing biocides, which already contributed between 50 to 70 percent of total treatment costs. Furthermore, pedestrian reactions to the delivery of chemicals labeled as hazardous in downtown Chicago was becoming a public relations concern.
MIOX Mixed Oxidant Solution (MOS) chemistry was evaluated by Fehr Solutions as a stand-alone biocide to replace a traditional dual-component (bleach/ isothiazolin) biocide program in a large HVAC cooling tower system. On-site generation of MOS from sodium chloride (salt) provides an attractive solution to concentrated biocides. MOS is generated at the site by passing a 3 to 4 percent solution of salt through an electrolytic cell. The brine solution is converted to a powerful oxidant consisting of sodium hypochlorite (bleach) and trace hydrogen peroxide, at a free available chlorine (FAC) concentration of 0.45 to 0.9 percent and a pH of 9.0.
MOS maintained lower water bacterial counts compared to the traditional program even at very low free available chlorine levels. No evidence of increased corrosion rates or breakdown of scale or corrosion inhibitors was observed.
Fehr Solutions conducted a three year study of MOS at the district cooling tower. A MIOX SAL-80 MOS on-site generator was installed and tested for performance. To determine effectiveness of MOS on microbial growth, MOS dosage rates were ramped up to a continuous feed of 8 pounds per day of MOS as free available chlorine (FAC). Within two weeks, the basin was cleared of algae and biofilm was substantially reduced. Within four weeks, all remaining detectable biofilm was removed. The oxidant concentration was routinely measured at 0.3 to 0.4 ppm as FAC. Even at the highest concentrations of MOS, no degradation of phosphonates, polymer or azole was measured. Further, no increase in corrosion rates occurred.
“This site has been using the MIOX systems since 2006 as the primary biocide for all of their open recirculating cooling systems. Contrary to popular belief these systems have performed as good, if not better than traditional bromine based oxidizing biocides at pHs in the 8.8 to 9.0 range and far better than the intermittent chlorine non-oxidizer system that was in place. All at a fraction of the operating cost,” adds Dr. Michael Fehr, technical manager at Fehr Solutions, LLC. “The MIOX systems are simple to operate and require minimal operator interaction. The generation of lower concentration oxidants makes feeding simpler with no plugged injectors or nozzles and no materials compatibility issues. The MIOX solution controls algae—even in direct sunlight—and bacterial counts are less than 100 cfu/ml on a routine basis. This is accomplished using a free chlorine residual of 0.3 to 0.5 ppm on a continuous basis.”
MOS eliminated algae and biofilm, maintained low bacterial counts, and decreased operating costs, without degrading the existing scale and corrosion inhibitors. The units are easy to operate and require minimal maintenance.
MOS is economical and cost-efficient and inherently safer than biocides, chlorine gas or bulk bleach. And it’s more effective at removing biofilm than other biocide programs. While conventional wisdom is that bromine products offer the best performance at high pH, Fehr Solutions’ studies show MOS is equally effective.
Enwave Chicago has since converted to MIOX disinfection for 3 additional cooling systems, with an average unit payback < 18 months. ◆
For more information about how MIOX can improve your cooling water treatment program, contact Aileen Culling at aileen.culling@miox.com or 505.224.1047.

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