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Controlling cooling tower water quality by hydrodynamic cavitation.

INTRODUCTION

In general, water in cooling towers needs to be treated to control microbial growth, scale formation, and metal corrosion. For any control process, the heat transfer performance of the cooling tower must also be maintained.

MICROBIAL GROWTH CONTROL

The conventional method for control of bacteria, algae, and fungi in cooling towers is by addition of antimicrobial pesticides (biocides). Various oxidizing and non-oxidizing chemical disinfectants are commercially available and have been successfully employed (Kim et al. 2002). These biocides are continually added to the recirculating bulk water to maintain a desired level of residual and make up for the loss through chemical reactions and blowdown. Methods of disinfectant addition include manual slug feed, automatic dosing based on the volume of makeup water, and timer-controlled automatic intermittent feed. Even with automatic addition methods, chemical treatment systems must be monitored and adjusted frequently to maintain the desired level of residual and verify control of microbial growth. In addition, some microorganisms may become resistant when subjected to continuous use of a single disinfectant, and periodic alternation of disinfectant type is often required to mitigate this effect.

In order to avoid use of chemical disinfectants, several nonchemical alternatives have been developed, including UV irradiation, ultrasonic cavitation, and hydrodynamic cavitation. A hydrodynamic cavitation device (HCD), the subject of this study, uses hydrodynamic cavitation in combination with scale filtration to provide a mechanical means of disinfection. As such, it eliminates the hazards and costs associated with chemical disinfectants and may prevent bacteria from becoming resistant to the disinfection technique.

Cavitation is the formation, growth, and implosion of vapor bubbles in a liquid. They can be created by sound waves (ultrasonic or acoustic cavitation), lasers, or by fluctuations in fluid pressure (hydrodynamic cavitation). Early cavitation research focused on the damaging effects of uncontrolled cavitation on pumps and rotating propeller blades. During the collapse of vapor bubbles, extremely high fluid velocities, pressures, and temperatures occur that can cause pitting and erosion of surfaces. More recent efforts have focused on controlling cavitation to produce beneficial effects.

Cavitation has been reported to kill bacteria through chemical reactions by free radicals formed from cavitation, cell disruption from pressure pulses, micro-jets, and heat associated with localized high temperatures. In aqueous liquids, cavitation leads to the formation of hydrogen and hydroxyl radicals and hydrogen peroxide (Kalumuck et al. 2003). These short-lived reactive radicals are capable of effecting secondary oxidation and reduction reactions (Suslick et al. 1997) including disinfection, but only in the immediate vicinity of the bubble.

During bubble collapse, a pressure pulse up to 1,450 psia (10 MPa) is produced in the immediate vicinity of the bubble. In addition to the pressure pulse, shear forces and shock waves may impinge on a bacterial cell wall within a few radii and cause cell lysis (Brennen 1995). Asymmetric bubble collapse can also lead to emission of micro-jets of fluid (seen as a downward conical structure in Figure 1). Lohse (2003) has postulated that jet formation may be responsible for cell-wall disruption based on electron micrographs of leukemia cells exposed to low-intensity ultrasound having conspicuous holes in their walls.

[FIGURE 1 OMITTED]

Thermal effects become very pronounced in the final stage of collapse when the bubble contents are highly compressed by the inertia of the inrushing liquid. Locally, temperatures can reach as high as 8,540[degrees]F (5,000[degrees]K) at the liquid/vapor interface. These regions of extreme temperature only exist for a fraction of a microsecond, e.g., 2 [micro]s (Brennen 1995). Nonetheless, the heat associated with the high temperatures could be sufficient to kill microbes located within a few bubble radii.

Cavitation appears to be effective against numerous strains of planktonic bacteria. Of particular note, Stout (2002) has demonstrated the efficacy of HCD against Legionella pneumophila serogroup 1 in a controlled laboratory setting under batch conditions (Figure 2). Along with the rapid kill of the bacteria, Stout also detected some oxidants expressed as free chlorine during the HCD experiment, which is consistent with the above discussion on the generation of free radicals and oxidants by hydrodynamic cavitation.

[FIGURE 2 OMITTED]

Scale Control

Scaling is the formation of hard deposits on piping, heat exchangers, and other process equipment surfaces. Calcium carbonate ([CaCO.sub.3]) scales are the principal cause of cooling-tower scaling problems and adversely and significantly affect heat-transfer efficiency. This scale formation is controlled by temperature, rate of heat transfer, pH, dissolved solids, and alkalinity. Cavitation has been reported to provide a favorable condition for [CaCO.sub.3] formation by (1) vacuum stripping [CO.sub.2] gas from water, thus increasing pH, and (2) locally increasing temperature. The latter mechanism likely predominates since [HCO.sub.3.sup.-] is the prevalent species at typical cooling tower pH (Hutchinson 1957).

High pH shifts the carbonate equilibrium to convert [HCO.sub.3.sup.-] to [CO.sub.3.sup.=], leading to increased precipitation of [CaCO.sub.3]. The formation of [CaCO.sub.3] is an endothermic reaction, and increased temperature promotes [CaCO.sub.3] precipitation (Snoeyink and Jenkins 1980).

A common indicator of the scaling or corrosive tendencies of water is the Langelier Saturation Index (LSI), which is based on aqueous [CaCO.sub.3] equilibria (Snoeyink and Jenkins 1980). The LSI represents the difference between the actual pH and the theoretical equilibrium pH that would occur in an equilibrated solution at measured concentrations of [Ca.sup.++] and carbonate species. A solution at equilibrium has an LSI of zero. If the LSI is negative, there is no potential to scale because [CaCO.sub.3] is dissolving, and it is generally interpreted that the water will be corrosive due to the loss of protective [CaCO.sub.3] coatings. As [CaCO.sub.3] dissolves, pH increases, pushing the LSI closer to 0. The opposite occurs when LSI is positive, where [CaCO.sub.3] forms and pH decreases. The farther the index is from zero, the greater the tendency to scale or corrode. Therefore, it is generally desirable to operate in a range where the LSI is positive but small.

Calcium carbonate can exist in three distinct mineral types: calcite, aragonite, and the less-common vaterite (Dictionary 1992; Merck 1996). Calcite crystallizes in a trigonal-hexagonal cell and typically forms scalenohedral (rhombohedrons) crystals, aragonite has an orthorhombic cell with dipyramidal (needles) shaped crystals, and vaterite has a hexagonal cell with dihexagonal dipyramidal (discs) shaped crystals. Though complete consensus has not been reached, calcite is usually assumed to form a hard, tenacious scale on surfaces, while aragonite and vaterite are generally thought to form softer precipitates that do not to adhere to surfaces as tenaciously as calcite (Coetzee et al. 1998). In pure solution under standard temperature and pressure, calcite formation would be favored given its lower solubility product constant of 3.36 x [10.sup.-9] over that of aragonite (6.0 x [10.sup.-9]) (Navrangpura 2005). The HCD manufacturer has claimed that hydrodynamic cavitation favors the formation of the aragonite form of [CaCO.sub.3], thus reducing fouling in cooling systems. Koestler et al. (2003) operated an HCD unit and found aragonite precipitates. It had been speculated that "magnesium-poisoning" could inhibit certain supersaturated solutions from precipitating calcite in favor of aragonite. More recent research indicates there may be a critical Mg:Ca ratio (Ahn et al. 2004) and that other cations such as zinc may play an important role in selecting the crystal form (Coetzee et al. 1998).

Corrosion Control

Corrosion is typically controlled by (1) maintaining water at alkaline pH, (2) removing corrosive dissolved gases (e.g., [O.sub.2] and [CO.sub.2]), (3) controlling bacterial activity, and (4) eliminating corrosive chemicals.

The effect of pH on iron corrosion generally falls into three distinct ranges: At pH <4, iron oxide films are dissolved; for 4<pH<10 the corrosion rate is limited by oxygen diffusion and is nearly independent of pH; for pH>10, iron is passivated and the corrosion rates decrease. Therefore, for the pH range typical of most cooling towers (7<pH<9), the dissolved oxygen concentration is the main controlling factor for steel corrosion (Betz Handbook 1980). To the extent that the cavitation process can reduce the dissolved oxygen concentration, the iron corrosion rate can be reduced. Similarly, zinc coating on galvanized steels is susceptible to oxygen corrosion. This corrosion may be reduced or eliminated via deaeration by mechanical, thermal, and/or chemical treatments (Nalco Guide 1993).

Microbiologically influenced corrosion (MIC) may be caused either actively or passively. Active attack is the direct chemical interaction of microorganism by-products (e.g., acids) with materials. In passive attack, a sessile biomass depletes oxygen at the metal surface, forming a concentration galvanic cell between the metal surface and the body of water. (Snoeyink and Jenkins, 1980). The establishment of the galvanic cell makes the area under the deposit anodic, which leads to the corrosion of metal in the area of the deposit. Because the HCD process only treats the flowing water and not the sessile microorganisms, the possibility of MIC must be considered in evaluating adoption of this process.

EXPERIMENTAL MATERIALS AND METHODS

The HCD test system described in this report was installed on a cooling tower system at an automotive test lab. The facility provides assembly, tooling, and testing analysis of products under development. A diagram of the cooling system is shown in Figure 3.

The cooling system removes heat from a closed-loop machine cooling system through two plate heat exchangers and rejects heat to the air through a two-cell cooling tower constructed of G-210 galvanized steel (1.9 mil thickness). It is of cross-flow, induced-draft design, having drift eliminators to limit drift losses to <0.005% of the recirculation flow rate. It employs a gravity flow distribution system with a covered deck, polypropylene nozzles, and a PVC honeycomb film fill. Each cell has a 20 hp (15 kW) fan that provides 103,000 cfm (49 kL/s) of airflow and 380 ton (1336 kW) of cooling. At rated capacity, the tower cools water from 105[degrees]F (41[degrees]C) to 85[degrees]F (29[degrees]C) at a 78[degrees]F (26[degrees]C) wet-bulb temperature and recirculation flow rate of 1400 gpm (88 L/s). The system contains ~7,000 gallons total capacity and is operated continuously.

The makeup water is treated drinking water from the city of Detroit (Table 1) fed to the cold well through a backflow preventer. Accumulation of dissolved minerals is limited by periodically releasing a portion of the system water (blowdown) to the sanitary drain and replacing it with a fresh supply of city water. Blowdown is regulated via a conductivity controller connected to a solenoid valve off strainers. The tower, its storage tank (cold well), and all ancillary piping had been drained, cleaned, and disinfected two months prior to the trial initiation.
Table 1. Makeup Water Analysis

Calcium (Ca) 26.8 mg/L
Magnesium (Mg) 8.80 mg/L
Chloride ([Cl.sup.-]) 7.5 mg/L
Sulfate ([SO.sub.4]) 23.9 mg/L
PH 7.29 S.U.
Silica ([SiO.sub.2]) 2.38 mg/L
Total alkalinity (as [CaCO.sub.3]) 72 mg/L
Phosphorous (P) 0.29 mg/L
Conductivity 214 micro Siemens


Chemical feeds to the cooling water system consist of an oxidizing disinfectant (12.5% sodium hypochlorite solution), a non-oxidizing disinfectant (1.5% isothiazolinone), and a proprietary scale inhibitor from a specialty chemical supplier. Each chemical feed is independently controlled by an automatic controller to deliver timed dosing of chemicals via chemical metering pumps in a side-stream loop. Sodium hypochlorite dosage averaged 0.09 gal/day (3.9 [micro]L/s) to maintain an average concentration of 0.32 mg/L free residual chlorine; isothiazolinone dosage averaged 0.12 gal/day (5.3 [micro]L/s) (concentration not measured); and corrosion inhibitor dosage averaged 0.14 gal/day (6.1 [micro]L/s) to maintain an average concentration of 0.69 mg/L. Conductivity, pH, temperature, product concentrations, and other system parameters are measured weekly to ensure consistent performance. Bacterial concentrations are manually monitored by adenosine-triphosphate (ATP) bioluminescence, and chemical dosing control timer setpoints are adjusted based on these readings.

Cold water is supplied from a 5,000 gal storage tank through strainers to two 896 [ft.sup.2] (83 [m.sup.2]) plate heat exchangers connected in parallel.

The HCD Hydrodynamic Cavitation Unit

The HCD continuously treated water at 60 gpm (3.8 L/s) via a 5 hp (3.7 kW) centrifugal pump. It was connected as a side stream treatment on the cold well tank. Water was pumped through the HCD and returned to the cold well. A separate bag filtration system was used to remove precipitated [CaCO.sub.3] and other suspended solids. It was operated off a side stream of the cold well using a 5 hp (3.7 kW), 140 gpm (8.8 L/s) submersible pump through a 200 [micro]m bag filter. Excluding the two pumps, there are no moving parts in the system.

The cavitation system consists of a pressure equalizing chamber and a cavitation chamber. Water is pumped into the pressure equalizing chamber at 94 psig (648 kPa) and then channeled into two pairs of conical nozzles positioned opposite each other in the cavitation chamber. As water is forced into the nozzles, it follows a spiral path around the axis of the cones before exiting through the nozzles. The rotation creates a vacuum of 13.5 psia (93 kPa), which leads to the formation of micro-sized gas bubbles in the water streams. Upon exiting each nozzle, the opposing streams collide at the midpoint of the cavitation chamber, causing a dramatic pressure increase that, in turn, leads to spontaneous bubble collapse.

Analytical Methods

Cooling tower water was analyzed for Legionella in accordance with ISO 11731 (ISO 1998; CDC 1992), while general bacterial enumeration was performed using heterotrophic plate counts (Method 9215D, Standard Methods 1998) and dip slides. Dip slides for bacterial enumeration were placed in an incubator after inoculation with tower water, held at 86[degrees]F [+ or -] 3.6[degrees]F (30[degrees]C [+ or -] 2[degrees]C) for 24[+ or -]2 hours, and compared against a calibration chart for preliminary bacterial enumeration. The slides were then placed back into the incubator for an additional 24 hours for final bacterial enumeration and reporting (mimicking the Standard Methods requirements for heterotrophic plate counts at 95[degrees]F [35[degrees]C]). Finally, the slides were returned to the incubator at 86[degrees]F [+ or -] 3.6[degrees]F (30[degrees]C [+ or -] 2[degrees]C) for an additional three days to obtain a five-day fungus, yeast, and mold count and were checked against a calibration chart for fungal enumeration.

Additional water parameters monitored included total suspended solids (TSS) (Method 160.2, US EPA 1983), volatile suspended solids (VSS) (Method 160.4, US EPA 1983), total dissolved solids (TDS) (Method 160.1, US EPA 1983), and alkalinity (total and free [carbonate]) (Method 310.1, US EPA 1983).

Temperature (Method 2550, Standard Methods 1998) and pH (Method 150.1, US EPA 1983) were measured using a portable pH meter having an accuracy of [+ or -]0.01 pH and [+ or -]0.1[degrees]C. Temperature measurements were taken by direct thermometer readings of the four heat exchanger ports (cooling tower in and out and process cooling loop in and out).

Conductivity was measured in accordance with Method 120.1 (US EPA 1983). Concentrations of chlorine residuals (total and free) were measured using N,N-diethyl-p-phenylenediamine (DPD) agents. Ca and Mg were measured by ICP/MS (EPA Method 200.8, US EPA 1994). Hardness was calculated from Ca and Mg concentrations.

Particulate samples were analyzed using X-ray fluorescence (XRF) and X-ray diffraction (XRD). For both analyses, specimens were prepared by wet grinding in ethyl alcohol and drying. XRF specimens were first heated to 1200[degrees]F (650[degrees]C) for two hours in air, then cooled and spread as a loose powder in a thin window analysis cup. XRF intensities were measured using a wavelength dispersive spectrometer. XRD specimens were prepared by dusting the surface of an off-axis cut quartz single crystal prepared with a thin layer of petroleum jelly. Data were collected in focusing (Bragg-Brentano) geometry using Cu-K radiation on a diffractometer in steps of 0.03[degrees] from 5[degrees] to 90[degrees]. The observed diffraction profiles were compared to peak positions and relative intensities found in the Powder Diffraction File (ICCD 2003) to determine the crystalline phase assemblage.

Corrosion was monitored using coupons made of mild steel, galvanized steel, copper, and stainless steel in a "coupon rack" or zigzag layout of piping external to the main circulating loop. The coupons were placed in the rack following their galvanic series: galvanized steel (the most active) followed by mild steel, stainless steel, and finally copper in direction of flow. This prevented the more noble metal from cathodically depositing on downstream coupons and creating a galvanic cell. Each coupon was oriented so that water flowed from the back of the coupon, where it was attached to a nonconducting coupon holder. Individual coupons were orientated with their broad face in a vertical position to minimize the possibility of debris accumulating on the faces. The coupons were exposed to the system water for 65 days and then given metallographic analysis.

Calculation Methods

The LSI was calculated as follows (Snoeyink and Jenkins 1980; Benefield et al. 1982):

LSI = p[H.sub.actual] - p[H.sub.s]

where

[pH.sub.actual] = actual, measured pH

[pH.sub.s] = pH of water if in equilibrium with [CaCO.sub.3] at the solution concentrations of [HCO.3.sup.-] and [Ca.sub.2+], = [pK.sub.2] - [pK.sub.so] + p{[Ca.sup.2+]} + p{[HCO.sub.3.sup.-]}

[K.sub.2] = {[H.sup.+]} {[CO.sub.3.sup.=]}/{[HCO.sub.3.sup.-]}

[K.sub.so] = the solubility product constant of [CaCO.sub.3]

The procedures described by Snoeyink and Jenkins (1980) and Benefield et al. (1982) were used to correct the above equilibrium constants and activities for ionic strength and temperature.

The overall heat-transfer coefficient [U.sub.o] for each heat exchanger was calculated as follows:

[U.sub.0] = Q/([A.sub.0][DELTA][T.sub.lm])

where

[U.sub.o] = overall heat-transfer coefficient (Btu/h.[ft.sup.2].[degrees]F)

Q = heat-transfer rate (Btu/h)

[A.sub.o] = heat-transfer area ([ft.sup.2])

[DELTA][T.sub.lm] = log mean temperature difference ([degrees]F) = ([DELTA][T.sub.2] - [DELTA][T.sub.1]) / ln ([DELTA][T.sub.2] / [DELTA][T.sub.1])

[DELTA][T.sub.2] = cold-fluid temperature difference, [T.sub.2in] - [T.sub.2out], ([degrees]F)

[T.sub.2in] = heat exchanger process water inlet temperature ([degrees]F)

[T.sub.2out] = heat exchanger tower water outlet temperature ([degrees]F)

[DELTA][T.sub.1] = hot-fluid temperature difference, [T.sub.lin] - [T.sub.1out], ([degrees]F)

[T.sub.lin] = heat exchanger process water outlet temperature ([degrees]F)

[T.sub.1out] = heat exchanger tower water inlet temperature ([degrees]F)

Changes in the overall heat transfer efficiency during operation of the HCD unit were determined from the fouling factor ([R.sub.f]), which is a measure of the resistance to heat transfer due to fouling. The higher the fouling factor, the poorer the heat transfer. The overall heat transfer coefficient data were used to calculate the change in fouling factor using by the following formula:

[R.sub.f] = 1/[U.sub.0,initial] - 1/[U.sub.0,final]

The concentration of dissolved substances in the system water is a balance between their concentrations in the makeup water and the loss of system water through blowdown, drift losses, and evaporation losses. The cycles of concentration (CoC) were calculated by two methods: (1) by taking the ratio of the concentration of a conservative substance (in this case calcium) in the system water to its concentration in the makeup water (Metcalf and Eddy 1991),

CoC = (Concentration of Ca in recirculating water)/(Concentration of Ca in makeup water),

and (2) by taking the ratio of the rate of the total water loss to the estimated rate of loss due to evaporation and drift as shown below (Mortensen 2003):

CoC = (E + D + B)/(D + B)

where

E = evaporation loss (gal/min) = recirculation flow rate (gal/ min) x temperature drop ([degrees]F) x 0.0008

D = drift loss (gal/min) = recirculation flow rate (gal/min) x 0.00005

B = blowdown (gal/min)

Because the evaporative and drift losses are difficult to measure directly, they were estimated as shown above.

Operation of the HCD Unit during the Study

Baseline data were obtained by daily sampling of system performance for ten days prior to the HCD being activated. During this period, several changes in operating conditions were made in preparation for the HCD testing--isothiazolinone and corrosion inhibitor feeds were turned off; the conductivity blowdown setpoint was reduced from 1100 to 450 [micro]S/cm to purge chemicals from the system; and the coldwell tank was drained by one-third of its volume. At the start of testing, the HCD and the bag filtration units were activated, the sodium hypochlorite feed was terminated, the blowdown valve was closed, and the conductivity setpoint was increased to 1100 [micro]S/cm. Free residual chlorine was below the detection limit of <0.1 mg/L two days after hypochlorite feed was terminated. After over one month of steady-state operation, the conductivity blowdown setpoint was increased to 1250 [micro]S/cm to see if the cycle of concentration (CoC) could be further increased without adversely affecting scaling and corrosion performance.

RESULTS AND DISCUSSION

The HCD field test data were analyzed to evaluate the performance of the HCD unit with respect to its impact on bacterial concentration, scaling, heat transfer, and capital and operational costs.

Bacterial Concentration

The heterotrophic plate counts and dip slide results from samples taken from the cooling tower bulk water during the study are shown in Figure 4. Before starting the HCD study, the two-week baseline bacterial concentration was [10.sup.3] to [10.sup.4] cfu/mL or lower when the cooling water was being chemically controlled with chlorine and isothiazolinone. Values dropped to ~[10.sup.0] cfu/mL during the city water purging. Soon after the HCD unit was started, the bacterial concentration increased to about [10.sup.6] to [10.sup.7] cfu/mL before beginning a slow decline to a stable level of ~[10.sup.4] cfu/mL, which was maintained consistently over >2 months in the absence of added disinfectant. The manufacturer indicated that similar initial increases had been observed in other pilot tests. Mason et al. (2003) also observed a similar phenomenon when using sonication on suspensions of Bacillus subtilis and attributed it to declumping, in which bacterial clumps are broken into a greater number of individual bacteria in a suspension before appreciable disinfection has occurred.

[FIGURE 4 OMITTED]

The HCD unit was initially operated with a 200 [micro]m bag filter to continuously remove suspended solids. When the bag filter was changed to one with 400 [micro]m openings for a week, there was no solids accumulation so it was replaced with a 200 [micro]m filter bag that was used throughout the remainder of the study. The amount of solids removed was not measured. No filter had been used when the cooling system was chemically managed. Therefore, it is not clear whether or not this additional treatment step (i.e., bag filtration) had any effect on the removal and control of microorganisms in the cooling tower system.

There was a marked difference between plate count and dip slides bacterial counts at low concentrations. The data in Figure 4 suggest that dip slides cannot accurately measure bacterial concentration when the concentration is relatively low (< [10.sup.4]-[10.sup.5] cfu/mL) and did not provide a useful comparison to plate counting after the initial "declumping" period. Legionella was found to be below the method detection limit (1 cfu/mL) throughout the study based on biweekly sampling as it had been before this study during the previous three years of semiannual sampling.

Visual tower inspections performed weekly did not find any sessile bacteria growth on the distribution deck, nozzles, fill, basin, or drift eliminators either before or during the trial. Therefore, no effort was made to measure sessile bacteria populations.

Scaling

CoC is a key metric to gauge the efficiency of water usage of the cooling system. By operating at higher CoC and conductivity, the amount of blowdown released, thus, the consumption of makeup water, is reduced. Calculated CoC values plotted as a function of makeup water flow rate indicated that there is diminishing benefit of increasing CoC beyond ~5 to reduce makeup water usage.

Therefore, the cooling tower system was operated to achieve a CoC of ~5. Changes in CoC and makeup water flow rate before and during the study period are shown in Table 2 and Figure 5. CoC was initially calculated using both calcium and conductivity ratios, as well as on a volumetric basis. Chlorides were not measured during the study since sodium hypochlorite had been the primary disinfectant under the chemical program and chlorides had not been historically analyzed. The volumetric basis produced widely erratic results (probably due to poor estimates of the evaporation rate). Conductivity and calcium CoCs tracked very closely, and only calcium CoCs are presented here. Following system purging, the conductivity averaged 1,043 [micro]S/cm during the trial. During the first half of the study period, the average flow rate of makeup water decreased from approximately 4900 gal/ day (0.21 L/s) to 3600 gal/day (0.16 L/s). The flow rate decreased further to approximately 2000 gal/day (0.09 L/s) after the blowdown conductivity setpoint was increased to 1250 [micro]S/cm. The average CoC of 4.9 during the trial was comparable to the 150 day pre-trial CoC of 4.7 and did not adversely affect pH, LSI (Figure 6), or the system's ability to control corrosion (discussed below). Interestingly, while a well-managed chemical program is capable of sustaining 5 CoCs and greater, the three-year timeframe prior to the trial averaged only 3.5 CoC due to two unexplained periods when the tower operated at 2 CoCs and less.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]
Table 2. Cooling Water Cycles of Concentration Before and During the
Study Period

 Previous Three Years 150-Day Pretrial Trial

Average 3.5 4.7 4.9
Standard Deviation 1.4 0.4 0.3


Throughout the chemical program and during the study, the pH remained stable at 8.7-8.8 and the LSI remained positive (Figure 6), decreasing from 1.99 under the chemical program to 1.54 after switching to the HCD. The changes in the LSI suggest a decreased tendency to form scale during the study period, but that [CaCO.sub.3] coatings (if present) would remain and provide some continuing corrosion protection. Since it is generally desirable to operate in a range where the LSI is positive but small, the HCD program appeared to provide a more favorable chemical environment for scale control.

XRF analysis of the scale particles collected in the filter bag show that it is dominated by calcium compounds but with significant amounts of silicon, magnesium, aluminum, zinc, phosphorous, and iron (Table 3). For the purposes of estimating the amount of carbon and oxygen contributing to the matrix corrections, all alkali and alkaline earth metals were assumed to be in the form of (relatively) insoluble carbonates. All other elements were assumed to be common oxide forms other than the halogens. Based on pure element sensitivities, the sum of the concentrations of all compounds is ~94%, indicating that the estimated amounts of carbon and oxygen are consistent with the absorption required to account for the measured intensities. The presence of zinc in the scale is noteworthy, since it is not a significant contaminant of city water nor likely to be blown into the cooling tower (see further discussion in "Corrosion Control" below).
Table 3. Composition of Scale Collected from the Filter
Bag on Different Dates as Determined by XRF

 Composition (wt %) Sample

Parameter 1 2 3

[CaCo.sub.3] 71.10 76.21 74.44
[Sio.sub.2] 9.91 7.45 8.71
[MgCO.sub.3] 6.11 4.94 5.39
[Al.sub.2][O.sub.3] 2.68 2.12 2.22
[ZnCO.sub.3] 2.55 2.66 2.58
[P.sub.2][O.sub.5] 2.30 2.32 2.35
[Fe.sub.2][O.sub.3] 2.05 1.94 2.03
[SO.sub.3] 1.25 1.12 1.07
[Na.sub.2][CO.sub.3] 0.592 - -
[K.sub.2][CO.sub.3] 0.461 0.356 0.413
Cl- 0.412 0.306 0.249
[TiO.sub.2] 0.231 0.205 0.199
[SrCo.sub.3] 0.166 0.125 0.116
[BaCO.sub.3] 0.082 0.094 0.084
CuO 0.072 0.065 0.060
[Mn.sub.3][O.sub.4] 0.057 0.051 0.057
[ZrO.sub.2] 0.009 0.006 0.007
PbO 0.006 0.007 0.006
Br- 0.006 0.005 0.003
NiO 0.005 0.006 0.005
Sum of concentrations 94.0% 91.0% 97.1%
Loss on ignition at 650[degrees]C for 2 h 24.0% 21.4% 15.5%
650[degrees]C for 2 h


XRD patterns on the three scale samples collected from the bag filters during the mid and latter part of the study period for all three samples were quite similar. Aragonite was not observed in any of the three samples, contrary to the manufacturer's claim that the HCD unit would tend to produce aragonite instead of calcite, which is reportedly less scaleforming than calcite. It is interesting to note, however, that the tower basin and cold well, which were free of visible deposits at the beginning of the trial, developed sediment that appeared similar to the fine, off-white silt observed on the filter bags. This tendency toward calcite may be explained by the presence of Mg and Zn, which have smaller ionic radii (0.72 and 0.74 vs. 1.00 A [Shannon 1976]) and can substitute for Ca in the calcite lattice. Substitution of smaller cations is known to favor formation of calcite over aragonite (Deer et al. 1992). A small shift of the measured calcite peaks toward higher angles (smaller lattice spacings) is consistent with this interpretation. The patterns of the samples matched the pattern of calcite, not that of aragonite. In all cases, the samples showed mostly calcite and calcite monohydrate in addition to variable amounts of unidentified materials. None of the samples contained observable aragonite or other [CaCO.sub.3] phases. It was also verified that the sample preparation procedure did not influence the observed phase assemblage.

Corrosion Control

Corrosion is typically controlled by maintaining water at alkaline pH. Except during the purging period prior to the HCD trial, pH did not change appreciably during the course of this study. The 8.8 pH of the (alkaline sodium hypochlorite) chemical program decreased to about 8 during purging but increased to 8.7 within five days of HCD start-up and remained 8.7 [+ or -] 0.1 pH units for the duration of the trial. If significant [CO.sub.2] stripping were occurring during operation, then equilibrium might be expected to shift away from carbonic acid and the system pH would increase. Since dissolved gases were not monitored in the bulk solution, insufficient data exist to provide a definitive explanation for this effect at this time.

The results of nonpassivated corrosion-coupon tests are shown in Table 4 as well as the data from two prior tests obtained during the chemical program. The cleaning procedures are described in GCC (2003). Other than the galvanized steel coupon, no appreciable corrosion was observed, and the other three coupons appeared brand-new after cleaning. The observed corrosion rates of copper and mild steel were either equivalent or better than those obtained during the chemical program. Corrosion rates observed under both the chemical and HCD programs fall into the "Negligible or Excellent" category set by the National Association of Corrosion Engineers guidelines shown in Table 5 (Boffardi 2000) and are also within acceptable limits established at the beginning of the trial, as shown in Table 4. With regard to galvanized steel, the observed corrosion rate (4.3 mil/year) during the study period was much higher than the acceptable rate set at the trial beginning (1 mil/year). However, it was discovered that an unpassivated galvanized steel coupon had been used and, hence, would not have properly modeled actual tower conditions if the tower remained properly passivated. Zinc concentration in the recirculating water remained relatively stable during the trial and was 0.2 [+ or -] 0.1 mg/L. Since no data exist for either the bulk water zinc concentration or the corrosion rate of galvanized steel prior to the start of the HCD study, it is unknown whether or not the corrosion was worse during the HCD program. No "white rust" was observed on galvanized tower surfaces, and the cause for the elevated zinc in the precipitate sample remains indeterminate at this time.
Table 4. Measured Corrosion Rates (mil/year) of Test Coupons

Date Days Exposed 316L SS Copper

Historic 23 <0.1
Pretrial 61 <0.1
HCD 65 <0.1 <0.1

Date Galvanized Steel Untreated Mild Treated Mild
 Steel Steel

Historic 1.3
Pretrial 0.5
HCD 4.3 0.3

Table 5. Guidelines for Corrosion Rates (mil/year)

Rating Copper Mild Carbon Steel

Excellent [less than or equal to]0.1 [less than or equal to]

Very good 0.1-0.25 1-3

Good 0.25-0.35 3-5

Moderate 0.35-0.5 5-8

Poor 0.5-1 8-10

Very poor to >1 >10
severe


Heat Transfer Efficiency

Calculated values of the overall heat-transfer coefficient for both heat exchangers and their regression lines (Figure 7) appeared to show a marginal improvement during the study period. However, the number of readings collected was not sufficient for a meaningful statistical analysis.

[FIGURE 7 OMITTED]

The testing operations, by their inherent nature, vary greatly in heat load due to the fluctuating number of tests being conducted and the duration of the tests. Therefore, the variation of the heat transfer coefficient was plotted against observed heat load as shown in Figure 8. Heat load was calculated using the measured flow rate of the tower water supply pumps and the temperatures entering and exiting the cooling tower. As is evident from correlation in Figure 8, some of the variability in the heat transfer coefficient might be due to the variation in heat load, and it is, therefore, difficult to quantify the extent to which the use of the HCD unit impacted the heat transfer efficiency.

[FIGURE 8 OMITTED]

Other Benefits

Using a side-stream hydrodynamic cavitation disinfection system may offer several intangible benefits: (1) reduced health and safety risks by avoiding the use and handling of chemical disinfectants (which also reduces record keeping and employee training requirements); (2) "de-skilled" labor force by negating the need to have a state licensed, certified pesticide applicator when FIFRA restricted-use chemicals (such as isothiazolinone) are being applied.; (3) elimination of potential bacterial acclimation to disinfectants; (4) operational simplicity through elimination of chemical storage tanks, chemical metering pumps, and process controller; and (5) reduced regulatory reporting requirements (e.g., USEPA Toxics Release Inventory reporting) and improved corporate citizenship by supporting "green" and sustainable manufacturing.

In addition, the HCD and filtration system has a 50 [ft.sup.2] (4.6 [m.sup.2]) footprint, as compared to the > 150 [ft.sup.2] (14 [m.sup.2]) conventional chemical disinfection system, and does not require secondary containment.

Limitations

The HCD system is a side-stream treatment system and therefore has all the limitations associated with such a system. All side-stream treatment systems treat only the water circulating through the systems and, thus, only the planktonic fraction of the total microbial population being circulated with the water. The main drawback of these treatment systems is their inability to treat sessile microorganisms (i.e., biofilm) that are attached to the surface of a cooling tower system and are not circulated through the side-treatment systems. This drawback could become magnified if there are "dead volumes," portions of the system that are temporarily isolated from recirculation by design or for operational reasons. In these isolated areas, microbial growth can proliferate unnoticed. When an isolated branch of the system is returned to circulation, that dead volume is deposited directly into the cooling tower flow and may inoculate the system. While the same scenario can occur in a chemical disinfection system, the impact may be mitigated due to the presence of disinfectant residuals, which would react with the released microorganisms throughout the circulation and also reach sessile microorganisms. Therefore, cooling systems containing more dead volumes are less suitable for disinfection with side-stream, nonchemical methods.

Corrosion control for protection of galvanized surfaces may require continued application of corrosion inhibitors. The results in this study indicated significant corrosion of a galvanized steel coupon, and zinc, which is a likely corrosion byproduct, was found in the filtered scale. No data are available on zinc corrosion rates prior to the start of the HCD study, and the use of an unpassivated coupon may not have properly modeled the cooling tower if it remained adequately passified.

Long-term effectiveness and system robustness, defined as the resistance of a system's response to changes of its inputs, were not characterized in this study. Although the bacterial count stabilized at a fairly constant level that was similar to the historical performance of the cooling system observed under the chemical program, ~2 weeks was required for the bacterial count to stabilize following the "declumping" event, and the two-month study period was too short to make an assessment of long-term performance.

SUMMARY AND CONCLUSIONS

A field study was conducted at a manufacturing facility to evaluate the performance of a hydrodynamic cavitation device with respect to disinfection, scaling, corrosion, and heat transfer efficiency.

During the study period, the HCD unit performed as well as the chemical program that it replaced in terms of total heterotrophic plate count without adding any chemicals (including disinfectants). After an initial, brief increase in the count possibly due to declumping of bacterial colonies, the count stabilized to and maintained at ~[10.sup.4] cfu/mL over ~2 months. No Legionella was detected during either the chemical program or the HCD field test. While a short-term test (six hours) on the effectiveness of the HCD unit against Legionella has been shown by Stout (2002) in a laboratory setting, the field effectiveness of the system for Legionella still needs to be more formally verified. Similarly, the short duration of this trial precluded an evaluation of the long-term effectiveness of the technology, and a lengthier study needs to be conducted in the future.

The HCD unit performed as well as the chemical program that it replaced in terms of CoC. When using HCD, the system operated at a CoC of 4.9, which is comparable to the average CoC of 4.7 during the 150-day pre-trial period under the chemical program, without adversely affecting pH, LSI, or corrosion. Loose scale particles were filtered from the system with a 200 [mu]m bag filter as part of the HCD system.

A 65-day corrosion test using metal coupons revealed no appreciable corrosion except for galvanized steel. The observed corrosion rates of copper and mild steel were either equivalent or better than those obtained during the chemical program.

Heat transfer efficiency may be affected by operation of the HCD unit. The high variance in the data and moderately strong correlation between the observed heat transfer coefficient and the overall heat load suggest that further study is required to verify this conclusion.

Since the technique does not produce persistent, mobile disinfection throughout the system, there will be specific situations where it is not appropriate as the sole method for disinfection.

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W.A. Gaines

B.R. Kim

A.R. Drews

C. Bailey

T. Loch

S. Frenette

W.A. Gaines is principal engineer in the Environmental Quality Office, B.R. Kim is technical leader and A.R. Drews is technical expert in the Research and Advanced Engineering Department, and T. Loch is senior technical specialist in the Manufacturing Executive Office, Ford Motor Company, Dearborn, MI. C. Bailey is environmental engineer in Powertrain Operations, Ford Motor Company, Livonia, MI. S. Frenette is director, Engineering and Development, Sali Group, Ann Arbor, MI.
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Author:Gaines, W.A.; Kim, B.R.; Drews, A.R.; Bailey, C.; Loch, T.; Frenette, S.
Publication:ASHRAE Transactions
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Date:Jul 1, 2007
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