Poster Session Annual General Meeting of The American Society of Microbiology 100th
Los Angeles, California May 21-25, 2000
The Effect of Pulse-Power Technology on the Microbial Content and Biofilm Formation in Evaporative Cooling Towers Dennis J. Opheim, Ph.D. Department of Biomedical Sciences Quinnipiac University, Hamden CT
This study quantitatively compares the effectiveness of pulsed-power water treatment with traditional chemical water treatment on the microbial content and formation of biofilm in cooling tower systems. Six cooling towers operated for process cooling (75 to 175 tons of cooling each) by two major Connecticut manufacturers were treated with either the pulsed-power Dolphin HytronicTM System manufactured by Clearwater Systems, LLC, or by traditional chlorine based chemical treatment. The evaluations of effectiveness that were performed were: • • • • • •
examination of the towers for indications of biofilm growth and composition, quantitative determination of the microbial content of treated cooling tower water utilizing nationally recognized standard methods, formation of bacterial biofilms on glass coupons, destruction of artificially created biofilm containing bacteria and algae, determination of assimilable organic carbon levels in samples, effect of post-Dolphin treated water on bacterial viability.
All towers evaluated operated at 10 to 27°C depending on the season and the cooling tower's utilization pattern. The Dolphin treated towers had a pH range of 8.3 to 8.5, while the chemically treated towers had a pH range of 7.5 to 8.3. Visual examination of towers indicated the presence of significant biofilm and algal mats in those under chemical control, especially during summer months, whereas there was little or no biofilm or algal mat formation in the water of Dolphin treated towers. These conditions on the Dolphin treated towers existed even after two years of operation without physical cleaning of the tower systems. The Dolphin treated towers did not have any odors from microbial byproducts, while the chemically treated towers did have such odors in the summer months. Measurement of assimilable organic carbon (AOC) in the Dolphin treated tower indicated a level of approximately 700 µg AOC/L, an amount that has the ability to support considerable bacterial growth.
The bacterial plate counts were performed by spreading 0.1 ml of diluted sample onto Standard Methods agar (SMA) and incubating for 48 hours at 35째C. Also, microbial counts were taken on R2A medium after five days at 28째C of incubation. The results indicate that the bacterial populations were significantly lower in water from Dolphin treated cooling towers. Typical quantitative values are as follows: Standard Methods R2A medium Turbidity Free chlorine
Dolphin Treated Towers <5x103 CFU/ml 1.5 to 4.5x105CFU/ml 0.1 to 1.2 NTU <0.002 ppm
Chemically Treaded Towers 2x105 to 2x106 CFU/ml 2x105 to 3x107CFU/ml 9 to 40 NTU 0.1 to 10 ppm
Measurement of biofilm formation on glass coupons indicted that biofilm formation in three weeks was seven-fold greater in chemically treated towers than in Dolphin treated towers. The chemically-treated tower cell population peaked at a bacterial concentration of 1x107 bacteria/cm2. Artificially prepared biofilms, containing Anabaena and Chlorella (isolated from the walls of cooling towers) and a mixture of native bacterial species, were examined after 1 to 5 weeks of incubation in Dolphin treated cooling towers. At the end of one week, 98% of the Anabaena population was no longer attached to the coupon, and after three weeks the Chlorella population was reduced 90%. Control biofilms in aged chlorine-free water contained 80-87 % of the original number of photosynthetic organisms in the biofilm. When bacteria were incubated with water from Dolphin-treated towers and tested for cell viability after various periods of incubation, it was found that the Dolphin-treated water had significant antibacterial activity. The gram negative bacteria species tested included: Escherichia coli, Enterobacter cloacae, Pseudomonas flourescens and Klebsialla pneumoniae. The gram positive bacteria species tested included: Streptococcus salivaris, Bacillus subtilis, Staphylococcus saprophyticus and Staphylococcus epidermidis. The gram negative organisms, most of which were coliforms, were extremely sensitive to the agent in Dolphin treated water, but the gram positive organisms tested demonstrated only limited inactivation. Treating the samples with sodium thiosulfate or lowering the pH to 7.3 with hydrochloric acid blocked the bacteriocidal effect. The bacterial inactivating ability of the Dolphin-treated water lasted for a least one month after storage at 4째C. Conclusion Cooling towers under Dolphin control retained a high quality of water with a lower bacterial count, free of chlorine disinfectants, for a long period of time without special maintenance. Dolphin pulsed-power treatment appears to produce an antimicrobial effect responsible for a decrease in the levels of microorganisms present, a large decrease in biofilm formation and an improvement in the water quality attributes of clarity and odor.
Typical Test Results Total Biological Count - SMA (CFU/ml) in Dolphin-Treated Towers 20,000
15,000
10,000
5,000
0
7/13
7/27
8/10
8/24
9/7
9/21
10/5
10/19
11/2
11/16
Bio-Film Growth
Bio-Film in Cooling Towers 2.E+07 1.E+07 1.E+07 1.E+07 8.E+06 6.E+06 4.E+06 2.E+06 0.E+00 7/13
7/15
7/17
7/19
Dolphin Control
7/21
7/23
Chemical Control
Inactivation of Gram Negative Bacteria with Dolphin-Treated Water 10,000,000
CFU/ml
1,000,000 100,000 10,000 1,000 100 10 0
2
4
6
8
10
12
Hours 1 day storage
28 day storage
14
16