Biological Control in Cooling Towers Treated with Pulsed-Power Systems
| Dennis Opheim, Ph.D. Quinnipiac University |
John Lane, Director of Technology Clearwater Systems |
IWC-01-54
INTRODUCTION
Biological control is the foundation of cooling tower water treatment. Biofilm can directly cause corrosion problems (microbial induced corrosion, MIC), pathogen concerns (Legionella), increased pump pressure, heat-transfer problems, dermatological effects, and malodors. With the increasing restrictions being imposed at the federal and state level regarding chemical use in cooling towers (CAA, CWA, OSHA, FIFRA), biological control has become much more difficult. A safe and effective alternative to current treatment methods is needed.
Pulsed-Power Technology in the Food Industry
Pulsed-power technology is an FDA-approved electronic process for the pasteurization of pumpable fluids. A recent FDA report (ref. 1) describes in detail the successes with these systems as a low energy method of pasteurization. Although there are only two commercially available systems for pasteurization, PurePulse (a spin-off of Maxwell Laboratories) and Thomson-CSF, many different processes have been successfully tested. These processes all involve a pulsed electric field delivered to the fluid at a high power level. Four different types of pulses have been tested, and all show efficacy as a biocide. The four pulse shapes are:
1. Exponential Decay
2. Square Wave
3. Bipolar
4. Oscillatory (Sine Wave)
The typical setup exposes the fluid to 5 to 30 pulses of an electric field at a strength of 30 kV/cm. This exposure will result in several log kills of a variety of bacteria and yield longer shelf life than standard heat pasteurization using less energy and having no effect on taste. In these examples of pasteurization, the fluid is passed between two electrodes, and 20 to 40m sec electric field pulses are applied. There are a variety of configurations to produce this field. Once above a threshold value, the duration of the pulse, the strength of the pulse, and the number of pulses directly influence the effectiveness of the treatment. The original Maxwell patent (ref. 2) uses an induced electric field generated by a coil. For pasteurization this type of field is not optimal, since the electric field is not uniform over the cross-section of the pipe and actually goes to zero at the center of the pipe.
The mechanism of this effect is explained by viewing the cell membrane as a capacitor filled with a dielectric. The electric field pulse will either compress or expand this membrane depending on its polarization. The main effect of an electric field on a microbial cell is to increase membrane permeability due to its compression and expansion. If the electric field exceeds the critical electric field for the membrane, a pore will form in the membrane, which will expand and eventually rupture the membrane, resulting in the loss of cell viability (ref. 1).
Pulsed-Power Technology for Cooling Tower Water Treatment
An adaptation of this technique, The Dolphin System™, is commercially available from Clearwater Systems of Essex, CT, and is being used on the open loop of cooling towers in many parts of the country. This paper describes the effectiveness of pulsed-power system (PPS) technology regarding the control of biological parameters in three cooling towers equipped with a PPS.
The cooling tower device uses coils to induce pulsed electric fields in the recirculating water. There are several key differences between the PPS used for cooling towers and its cousin in the food industry.
1. The cooling tower device does not contact the fluid.
2. The pulse generated in the cooling tower device is an oscillatory pulse superimposed over an exponential decay pulse.
3. The energy demands of the cooling tower device are 1/10,000 of the energy used by the food devices and the field strength is similarly weaker.
4. In a cooling tower the bacteria are recirculated and exposed to 5,000 exponential decay pulses and 50,000 oscillatory pulses before exiting via blowdown.
5. The process goal in a cooling tower is not sterilization but control.
In cooling towers it appears that the bacteria are exhibiting a dose-response reaction (high repetition of low-level stimuli having a cumulative effect) to the application of low intensity electric fields and not the threshold response that the FDA states is required for sterilization (ref 1). Other theories currently under consideration for explaining this effect include that: (1) the combination of an exponential decay pulse with an oscillatory pulse creates a resonance effect which allows the field strength of the pulses to be additive as well as cumulative, and (2) the repeated exposure to low-level fields has a sub-lethal injury effect on the microorganisms that is effective in minimizing the microbes' ability to form biofilms.
With pulsed-power there is an additional method of biological control. It is well established in water treatment that coagulation and calcium carbonate precipitation will result in microbial reduction (ref. 3). Under pulsed-power treatment, precipitation occurs in the bulk solution as a powder. These growing particles incorporate microbes and limit their growth.
Method of Analysis for Biological Activity
There are a variety of techniques available for measuring microbial activity in cooling tower waters. For this investigation, heterotrophic plate counts (HPC) utilizing Standard Methods procedure 9215 (ref. 5) is used exclusively. There were a variety of reasons for this choice. The primary one is that this method is an EPA-recognized technique for monitoring viable bacteria in water samples and, as such, it is both well standardized throughout the country and the benchmark for evaluating biological activity. Other testing techniques such as dip strips or reaction vials are less expensive, yield results much more quickly than HPC, and are widely used. These techniques measure an attribute of the water that in chemically controlled towers have been shown to have a good correlation to HPC. However, no detailed studies have been completed on their accuracy with pulsed-power systems.
INSTALLATIONS
The evaluations reported in this paper were performed on two cooling towers at a large industrial plant and on one cooling tower at a skating rink; all towers are located in Connecticut. The towers at the plant are used to cool industrial processes and equipment; as such they are moderate sized towers (100 to 200 tons) and run 365 days per year. The skating rink has a 100-ton evaporative condenser tower for ammonia refrigeration purposes that is also run year round.
In 1998 a pulsed-power system (PPS) was tested as the exclusive water treatment for the open loop of one of the towers (Tower #1, Figure 1) at the industrial facility. Based on the success of that installation, all of the four other cooling towers at the facility were converted to PPS over the next year. These towers are located on roofs, within a few hundred yards of each other, and received the same care and maintenance after the PPS installations as before. One of the towers (Tower #2) was monitored extensively both before and after the PPS installation. The make-up supply for all of the towers is soft municipal water. A typical water analysis for the industrial applications is shown in Table 1.
The skating rink cooling tower (Tower #3) was converted to PPS control in late November of 2000. The make-up for the skating rink is moderately hard municipal water. A typical make-up water analysis for the skating rink application is also shown in Table 1.
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Figure 1 - Tower #1 at Industrial Facility Showing Typical PPS Installation
All three of the towers have a conductivity meter actuating a solenoid for blowdown. Each conductivity meter was set at 900 mSiemens/cm. Based on the average incoming water conductivity, this application will yield about 10 cycles of concentration for the industrial facility and 4 for the skating rink. Each of the industrial towers has a 20% side-stream bag filter using a 75-micron bag. There is no filtration on the skating rink tower.
One or two pulsed-power units were installed on each tower. One unit was placed in the recirculating line and on some installations a second was placed in the make-up line. The units were sized to match the full diameter of the existing pipe with nothing protruding into the water. Depending on size, each unit uses between 90 and 300 Watts.
METHOD OF ANALYSIS
All sampling was conducted in accordance with ASTM standard procedures (ref. 4) and all evaluations were completed by investigators from Quinnipiac University or a state-certified analytical laboratory. The sampler determined algal mat formation and obnoxious odors organoleptically. Total bacterial counts were determined by diluting samples in Standard Methods buffer (ref 5), spreading the cells onto plate count agar, and incubating plates for 48 hours at 35 oC (ref. 5). Turbidity was measured with a Hach portable Turbidimeter®, and free chlorine with Hach DPD Free Chlorine Reagent Powder Pillows.
Biofilm production was monitored by suspending glass coupons in the cooling tower basins for various periods of times - then air-drying, heat fixing, and staining with crystal violet. The cell density was determined by counting with light microscopy. Some biofilms were examined by epifluorescent microscopy after staining with acridine orange to estimate viable cells.
RESULTS
Figure 2 presents the bacterial density count for the two industrial towers over a 16-month period. Tower #1 was under PPS treatment throughout the duration of the study. Tower #2 was under chemical treatment until 3/22/00, when the PPS system was installed.
The details of the chemical treatment at the industrial facility are not available. Discussions with plant personnel indicate that the chemical supplier used a continuous feed sufficient to maintain free chlorine in the recirculating water of between 0.3 and 0.5-ppm with a periodic shock technique. This indication agreed with the collected data, measuring about 0.5-ppm free chlorine on most samples from the chemically controlled tower, with periodic high spikes of 5 to 10-ppm. The free chlorine level on the PPS system was typically < 0.01 ppm. Figure 3 presents the bacterial counts for the skating rink. There is no information on the specific chemical treatment used at the skating rink prior to PPS installation at the operation.
In Figure 2, each tower is plotted as a separate line. Tower #1 had been under PPS control for almost one year before the data were taken. The results show levels ranging between 102 and 104 counts, with 103 being a typical value. There is no discernible pattern in the values.
Tower #2 presents the bacterial counts while under chemical treatment (until 3/22/00) and then during PPS treatment The bacterial concentrations under chemical control are high and erratic ranging from 103 to 106 CFU/ml, with 105 being a typical value. The large drops in counts correspond to high levels of free chlorine in the system. The quick recovery of the biological counts is indicative of a viable biofilm in the system and a variable odor was present.
Field observations confirmed that under chemical control visible biofilms and algal mats were present in the tower. Tower #2 while under chemical treatment was by no means an example of a well-maintained tower; however, it does serve a useful function as a control comparison.
For the first two months after the PPS unit was installed, the biological data in Tower #2 were highly variable, ranging from 103 to 106 CFU/ml. The variable odor had cleared, and the visible biofilm layer disappeared in two to three weeks. After the initially high two months, the microbial concentration values decreased to levels similar to Tower #1,which had been treated with PPS-technology for one year.
Figure 3 shows similar patterns of change in microbial counts (CFU/ml) after the installation of the PPS at the skating rink. The plot presents data from the end of the period of chemical treatment through the transition to PPS treatment that occurred on 11/25/00. Approximately one month after installation the biological counts dropped to similar values as observed for the industrial facility. As with the industrial facility, the chemical control biological data are supplied only as an example of what could be expected with little to no treatment.
Results - Biofilm Control
Visual inspections on the PPS-controlled towers indicate that no observable biofilm is present. Pre-existing biofilm that was present under the chemical period appears to slough off a short time after the PPS is installed.
To quantify this observed effect, clean glass slides were inserted into the basins of both a chemically controlled (Tower #2) and PPS-controlled tower (Tower #1) at the industrial facility. Slides were removed weekly from each tower, heat fixed, and stained with crystal violet, and examined at 1000X magnification. These results, as presented in Figure 4, indicate that the microbial density in PPS-treated applications remained relatively constant. The levels of organisms attached to the slide in the chemically treated system increased with time.
Click on image for a larger view
Results - Turbidity
The change in turbidity is the most immediate and visibly obvious difference with the tower control under pulsed power as compared to chemical control. Figure 5 presents the turbidity data for the two industrial towers. The measured turbidity confirms what is visible to the eye; the water becomes much clearer soon after the PPS is installed.
Click on image for a larger view
Each of the industrial towers is equipped with a side-stream bag filter. The operators noticed that immediately after installation of the PPS, the filters needed cleaning more frequently (from once every 3 to 4 days to once every 1 to 2 days). This period of intensive cleaning lasted about one month. After this period, the bags required less cleaning than previously and are now cleaned only once every 6 to 8 days. It is believed that the intensive cleaning period corresponded to de-scaling and biofilm removal from the interior of the equipment. There was less biological activity and less material to be removed by the bags after this period. This pattern was repeated on each of the five cooling towers at the facility when they were converted to PPS.
SCALE PREVENTION
Under PPS treatment, precipitation will occur in the bulk solution as a powder rather than as a surface-nucleating scale. This bulk-solution powder generally exits via blowdown. PPS is broadly classified as a precipitation induction device (PID) (ref. 6). Operating by Faraday's law, a PPS generates pulsed electric fields in the fluid. As a PID, the characteristics of fluid velocity, conductivity, and turbulence will have little to no impact on the effectiveness of the device. PID devices force precipitation to occur on suspended particles, preferentially to surfaces of pipes or heat exchangers. Because of this tendency, systems can be run into the scaling regime without a scale forming on heat transfer or other surfaces.
The towers in this study are being operated only slightly above saturation, therefor little precipitate was routinely formed. The advantage of PPS as a scale preventer can be seen with the data in Table 2. The blowdown valve at the skating rink malfunctioned for a period of time, resulting in no blowdown of the system. The conductivity of the recirculating water climbed well over 2000 mS/cm and the water became highly scaling. Samples of the make-up water and the recirculating water were taken and evaluated. Those analyses are shown in Table 2. Looking at the ratios of the soluble salts (magnesium, sulfate, silicon, and chloride), the cycles of concentration for the system at the time of sampling was over 11.5. The calcium ratio at only 5.1 and alkalinity at only 2.9 show that a calcium carbonate precipitate was formed. By visual inspection, the heat transfer tube banks had absolutely no scale and a small quantity of white powder was present in the bottom of the cooling tower water tank. System pressure and temperature measurements showed no degradation of performance and biological counts were in control during this period. Even with a mechanical breakdown of the blowdown valve, the system operated without any loss of performance.
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CORROSION PROTECTION
All of these towers under pulsed power control operate at saturation with calcium carbonate, which is a cathodic corrosion inhibitor. In this type of water system, the expected corrosion rate on mild steel is 2 to 5 mils per year. The Cooling Technology Institute Guideline WTP-130 (ref. 7) lists corrosion rates in cooling towers on mild steel of 2 to 5 mpy as "good" and 0 to 2 mpy as "excellent". There are no corrosion coupon racks in the tower systems described in this paper, therefore no analytical data are available. Figure 6 contains mild steel corrosion data from a pulsed-power installation in Pittsburgh. This result is typical of the coupon studies we conducted. As can be seen, the general corrosion rate was 2 to 5 mpy with the longer-term coupons measuring less than 2 mpy.
Copper corrosion coupons were also performed on the Pittsburgh installation. All copper corrosion coupons showed less than 0.1 mpy, with the longer-term coupons showing less than 0.01 mpy.
Pulsed power technology offers significant advantage in corrosion control over chemical treatment, particularly with respect to copper. While copper is very resistant to most domestic waters, it is subject to pitting attack by either microbial growth or high levels of oxidizing biocides (chlorine, bromine). Typical chemical regimes use a triazole as a copper corrosion inhibitor to protect the copper from the oxidizing biocide. Triazoles form a protective film on the copper surface and protect the underlying metal. However, triazoles are attacked by oxidizing biocides and, if they are at too low a level, can actually acerbate localized galvanic attack by only partially covering the metal. Pulsed-power system eliminates these sources of corrosion, while maintaining its excellent biological control without oxidizing biocides.
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Figure 6 - Typical Mild steel corrosion with PPS
CONCLUSIONS
PPS controls bacterial concentration in cooling towers without any chemical treatment whatsoever. The formation of algal mats was significant in chemical-treated towers, especially during the summer months, whereas there was little to no algal mats in the PPS-treated towers. Biofilm formation on glass slides was 2 to 4 fold more rapid in a chemically treated tower than in one under PPS treatment. The turbidity of PPS-treated towers was typically less than 1, but was highly variable (5 to 30 NTU) in the chemically treated tower.
The full control of bacterial levels when switching to PPS may take several weeks. The erratic values of planktonic bacteria measured in Tower #2 after converting to PPS is believed due to the pre-existing biofilm shedding off and re-inoculating the water. After two months, most of this film was gone and the bacterial counts stabilized at a low level.
PPS is a commercially available alternative to chemical treatment of cooling towers. Using PPS instead of chemicals has the following advantages:
1. Elimination of Employee Health and Safety Issues relative to chemical storage and handling.
2. Elimination of environmental concerns from discharge of biocides by blowdown, drift, and air emissions.
3. Elimination of the corrosion issues caused by oxidizing biocides.
4. Excellent biological control with little employee oversight.
5. Elimination of scaling on pipes and heat transfer-surfaces by precipitation occurring in the bulk solution as a powder.
6. Excellent energy efficiency due to maintaining clean heat transfer surfaces even with temporary excursions.
7. Reduced maintenance from the elimination of chemical pumps and maintaining an overall clean system.
ABBREVIATIONS AND ACRONYMS
CAA - Clean Air Act
CFU/ml - Colony Forming Units per milliliter
CWA - Clean Water Act
EPA - Environmental Protection Agency
FDA - Food and Drug Administration
FIFRA - Federal Insecticide, Fungicide, and Rodenticide Act
HPC - Heterotrophic Plate Count
MIC - Microbial Induced Corrosion
NTU - Nephelometric Turbidity Units
OSHA - Occupational Safety and Health Administration
PID - Precipitation Induction Device
PPS - Pulsed-Power System
REFERENCES:
1. U.S. FDA, Center for Food Safety and Applied Nutrition, "Kinetics of Microbial Inactivation for Alternative Food Processing Technologies - Pulsed Electric Fields", June 2, 2000.
2. Hofmann, G.A. assigned to Maxwell Laboratories, Inc., US Patent 4,524,079, "Deactivation of Microorganisms by an oscillating Magnetic Field", 1985.
3. American Society of Civil Engineers & American Water Works Association, "Water Treatment Plant Design - second edition", McGraw-Hill, 1990, pp. 81 & 263.
4. ASTM standard D3370-76, "Standard Practices for Sampling Water".
5. Clesceri, L.S., Greenberg, A.E., and Eaton, A.D. (1998) "Standard Methods for the Examination of Water and Wastewater," (20th edition).
6. Cho, Y.I. and Liu, R., Control of fouling in a spirally-ribbed water chilled tube with electronic anti-fouling technology, Int. J. Heat Mass Transfer 42 (1999) 3037-3046.
7. CTI Bulletin No. WTP-130, "Guidelines for the Evaluation of Cooling Water Treatment Effectiveness", October 1981.
BIBLIOGRAPHY:
Beardwood and Therrie, Sept. 15, 2000, "Detection and Reduction of Biofilms in Industrial Cooling Waters", International Water Volume I, Issue 3 p 47-66.
Lane and Kutner, Cooling Tower Institute Paper No. TP00-03, "A Non-chemical Water Treatment Device", February 2000.
Lane and Peck, "Chemical-Free Treatment of Recirculating Water Using Pulsed-Power", internal communication Opheim and Rogers, " The Effect of Pulse-Power Technology on the Microbial Content and Biofilm Formation in Evaporative Cooling Towers", 100th Annual Meeting of the American Society of Microbiology, May 2000.
Dennis Opheim is Professor of Microbiology, Department of Biomedical Sciences, Quinnipiac University, Hamden, CT and John Lane is Director of Technology, Clearwater Systems, Essex, CT. / Edited by Loretta E. Corson - Clearwater Systems LLC.