District Energy - 2006 Third Quarter

tional microbiocide program. This six-month trial began in the spring and continued throughout the summer and fall cooling season. The goal was to achieve performance at least equivalent to that of conventional chemical treatment while reducing the number of chemicals applied to the cooling water and decreasing the corrosion rate.

This institution’s 20-year-old plant included a cooling system with three cross-flow galvanized cooling towers containing high-efficiency film fill with three 1,000-ton centrifugal chillers and a stainless steel (grade 304) plate-and-frame heat exchanger having a 400-ton capacity. The condenser tubes had been replaced the previous year with enhanced tubes.

Prior to the application of SONOXIDE treatment, the system was operated at five cycles of concentration and utilized an all-organic phosphonate corrosion and deposit control additive, an organic dispersant with supplemental azole and continuous brominating to 0.2 ppm to 0.4 ppm as free available chlorine. The oxidizing biocide was supplemented with a non-oxidizer, glutaraldehyde, at a dosage rate of 45 ppm slug feed once per week. Glutaraldehyde was included in the program to aid in biofilm control.

Upon trial startup, the water treatment program consisted only of low-power, high-frequency ultrasound for microbiological control and one advanced chemical cooling water treatment product with inherent biodegradable properties for deposit and corrosion control. No cooling water pH control was exercised. A sidestream coupon rack was installed and included carbon steel and copper corrosion coupons for the evaluation of corrosion rates, as well as a biofilm coupon for the determination of surface biological loading.

Results

Routine evaluation of total bacteria levels was performed using industry-standard microbiological assay slides. As shown in figure 1, ultrasonic treatment maintained consistent control of total bacteria in the acceptable range of 102 to 103 colony-forming units per milliliter (CFU/ml).

In addition to conducting visual
inspections, researchers assessed biofilm

Figure 1. Total Bacteria Counts Using Ultrasonic Treatment in University Trial.

Courtesy Ashland Specialty Chemical.

control through the use of a biofilm coupon. Total system biofilm control was achieved. The biofilm coupon revealed no slime or biological fouling present (see photo).

Surface biofilm control was further evaluated upon chiller inspection. A video probe inspection revealed clean, ‘like-new’ appearance of brushed, air-

Courtesy Ashland Specialty Chemical.

This biofilm coupon, removed from the sidestream coupon rack during a university trial of the ultrasound system, shows no evidence of slime or bio-fouling.

blown tubes in the first and second pass with no indication of pitting or under-deposit corrosion.

Analytical evaluation of corro-
sion coupons revealed excellent
corrosion rates with no evidence
of localized corrosion.

Analytical evaluation of corrosion coupons revealed excellent corrosion rates with no evidence of localized corrosion. The 65-day corrosion rate was 0.262 mpy (mils per year, where mil equals 0.001 inch) for mild steel and 0.004 mpy

for copper. If oxidative microbiocides had been present, it is anticipated that corrosion rates would have been higher and, therefore, the demand for corrosion inhibitor greater.

In this university trial, the ultrasonic treatment achieved the study’s objectives by reducing the total number of chemicals applied to the cooling system while achieving effective microbiological control. The ultrasonic device operated con-

Cooling system performance
was optimized by eliminating
insulating biofilm on heat
transfer surfaces.

tinuously without the need for routine maintenance or operator interaction. In addition, the treatment eliminated the need for halogen testing and maintenance of microbiocide feed and control systems.

Equally important, cooling system performance was optimized by eliminating insulating biofilm on heat transfer surfaces. Improvements in corrosion rates were noted and attributed to eliminating oxidant-induced corrosion commonly experienced when applying such biocides.