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Case Study: District Cooling Water Treatment in Downtown Chicago

CASE STUDY

District Cooling Towers in Downtown Chicago

District cooling tower replaces traditional dual-biocide program with MIOX for cooling tower water treatment and achieves significant cost savings.

LOCATION

Thermal Chicago
60,000 gallon volume District Cooling
Downtown Chicago, IL

EQUIPMENT & CHEMISTRY

Dual SAL-80 (installed 2005)
Vault M45 (installed 2013)
MIOX’s Mixed Oxidant Solution (MOS)

PREVIOUS DISINFECTION

8 gal (30 L) of 12.5% hypochlorite/day, 2x/day slug dose
6 gal (23 L) of isothiazolin biocide/wk, 2x/wk slug dose

SYSTEM DESIGN PARAMETERS

RECIRCULATION RATE
• 33,000 gpm (12,500 Lpm)
• Delta T = 15°

SYSTEM VOLUME
• 60,000 gal (227,400 L)

MAKE-UP WATER
• 350,000 to 400,000 GPD (1325 to 1500 m3/day)

BLOWDOWN
• 100,000 to 150,000 GPD (380 to 570 m3/day)

SCALE INHIBITOR
• Phosphonate / polymer program

CORROSION INHIBITOR
• Azole yellow metal corrosion inhibitor
• N-nitrosodimethylamine (NDMA)

CUSTOMER’S CHALLENGE

A 60,000 gallon 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 50-70% 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’S SOLUTION

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.

MIOX’S MOS ADVANTAGE

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% 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 % and a pH of 9.0.

STUDY DESIGN & RESULTS

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 lbs/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 0.3-0.4 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.

CONCLUSION

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.
Thermal Chicago has since converted to MIOX disinfection for 3 additional cooling systems, with an average unit payback < 18 months.

Case Study: District Cooling Water Treatment in Downtown Chicago