Environ. Sci. Technol. 2008, 42, 9363–9369
Efficiency of Artemia Cysts Removal as a Model Invasive Spore Using a Continuous Microwave System with Heat Recovery SUNDAR BALASUBRAMANIAN,† JEFFREY ORTEGO,† KELLY A. RUSCH,‡ A N D D O R I N B O L D O R * ,† Department of Biological and Agricultural Engineering, Louisiana State University Agricultural Center, Baton Rouge, Louisiana 70803, and Department of Civil and Environmental Engineering, Louisiana State University, Baton Rouge, Louisiana 70803
Received July 29, 2008. Revised manuscript received October 1, 2008. Accepted October 15, 2008.
A continuous microwave system to treat ballast water inoculated with Artemia salina cysts as a model invasive spore was tested for its efficacy in inactivating the cysts present. The system was tested at two different flow rates (1 and 2 L · min-1) and two different power levels (2.5 and 4.5 kW). Temperature profiles indicate that the system could deliver heating loads in excess of 100 °C in a uniform and nearinstantaneous manner when using a heat recovery system. Except for a power and flow rate combination of 2.5 kW and 2 L · min-1, complete inactivation of the cysts was observed at all combinations at holding times below 100 s. The microwave treatment was better or equal to the control treatment in inactivating the cysts. Use of heat exchangers increased the power conversion efficiency and the overall efficiency of the treatment system. Cost economics analysis indicates that in the present form of development microwave treatment costs are higher than the existing ballast water treatment methods. Overall, tests results indicated that microwave treatment of ballast water is a promising method that can be used in conjunction with other methods to form an efficient treatment system that can prevent introduction of potentially invasive spore forming species in non-native waters.
Introduction The frequent intake of seawater as ballast from one part of the world and its introduction into another part during maritime shipping operations presents high risks of introducing nonindigenous marine species in the environment (1, 2). The International Maritime Organization (IMO) estimates that each year about 10 billion tons of ballast water are transported and exchanged around the world during maritime shipping (3). As a result of this exchange of seawater, an estimated 3000 to 10 000 different species of organisms are transported through ballast water every day (4), including plankton, toxic algae, water fleas, jelly fish, mussels, clams, sea slugs, and larval fish (5). Once introduced into the new * Corresponding author phone: 225-578-7762; fax: 225-5783492; e-mail:
[email protected]. † Department of Biological and Agricultural Engineering. ‡ Department of Civil and Environmental Engineering. 10.1021/es8021107 CCC: $40.75
Published on Web 11/14/2008
2008 American Chemical Society
environment, some marine species could become invasive as they lack natural predators and upset the ecological balance of that region. In the U.S. alone, bioinvasion through different pathways in which ballast water discharge is an active contributor has been estimated to cost the country $123 billion a year (4). In addition to the displacement of native flora and fauna, the transported organisms could affect human health directly or indirectly through consumption of contaminated marinebased food products (6). Some organisms contained in ballast water, such as dinoflagellates once consumed by marine species like shellfish are known to induce toxins in shellfish (5, 6). Consumption of these toxins with the shellfish will result in adverse health hazards to the consuming marine species and even in humans. Hence, prevention of ecological and human risk factors through improper ballast water disposal needs to be addressed. Different treatment methods have been proposed for safe disposal of ballast water: ballast water exchange (7, 8), chemical methods (9-11), heat treatment (12), use of ultraviolet (UV) radiation (13), and filtration (14, 15). Novel techniques such as ultrasonic, magnetic methods, and use of electrical pulses (16) have also been investigated for their treatment efficacy. Each of the methods mentioned above has its own advantages and disadvantages. For example, chemical treatments (using ozone, pesticides, glutaraldehyde, etc.) are considered to be effective, but they also cause corrosion of the tanks, introduce toxic chemicals, and sometimes generate undesirable byproducts. Heat treatments are considered a good option as there is no formation of byproduct and the coolant water can be easily discharged into the ocean if not used. However, cold weather conditions reduce the effectiveness of heat treatment, and in most cases, it is difficult to achieve uniform heating rates (12). This is particularly a problem in inactivation of heat resistant organisms like spores and cysts. We have recently described a microwave-based method for ballast water treatment as a suitable alternative to the available treatment methods for a number of nonspore organisms (17). This technology has high heating rates (due to its short heating and exposure span) when compared to conventional heating and requires fewer accessories to install (pipes, other conduits, boilers, etc.). Incident electromagnetic waves (in the microwave region) heat dielectric material (ballast water in this case) through two major mechanisms. The first one is based on vibration/ rotation of the polar molecules (mostly water) in the liquid under the influence of the oscillating electric field component. The vibrations and rotations happen at frequencies on the order of MHz/GHz and generate intermolecular and intramolecular friction. This polar rotation, coupled with movement of a very large number of charged ions at the same frequencies (the second mechanism) results in instantaneous heating throughout the liquid (18). The extent of temperature rise produced depends on the dielectric properties of the liquid (19-21). This study was undertaken as part of a larger research effort to understand the effectiveness of microwaves in preventing introduction of invasive species during the deballasting process. For this purpose, a continuous microwave heating system was used to process synthetic ballast water containing Artemia cysts, and the microwave process effectiveness was ascertained by its ability to inactivate the cysts in conjunction with a heat recovery system. Detailed investigations on the system microwave power requirements to effectively inactivate the cysts at the studied process conditions with and VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. (A) Schematics of the continuous microwave heating system; (B) AutoCAD rendering of microwave system (generator, circulator and water load are not shown). without the use of heat exchangers was also undertaken to enhance the understanding of microwave system’s overall feasibility for treating ballast water.
Materials and Methods Preparation of the Synthetic Ballast Media and Test Cultures. Sterilized deionized water was mixed with autoclaved synthetic sea salt (Crystal Sea MarineMix, Marine Enterprises International, Baltimore, MD) to form salt solutions at a concentration of 30 parts per thousand (ppt) closely representing the saline water found in the ocean. The water salinity was checked using a refractometer (Aquatic Eco-Systems, Apopka, FL) and allowed to achieve a chemical equilibrium for 48 h prior to the start of experiments. Preparation of the Test Culture. The pH of the prepared synthetic ballast media was adjusted to between 8.5 and 9.0 using sodium bicarbonate followed by inoculation of approximately 80 L with Artemia cysts (lot no. 03325, Artemia International LLC, Fairview, TX) at a concentration of 120 cysts/L of the media. A constant supply of air was carefully maintained using a small air pump to ensure that the cysts were suspended in the ballast water and did not stick to the sides of the barrel. After the microwave experiments, the surviving Artemia cysts were enumerated. First, the mixture was illuminated with sunlight until the Artemia germinated (24-48 h) and later the barrel was moved to lower intensity lighting conditions. Once the germination process started, the amount of Artemia germinated was monitored periodically. For enumeration, 2-3 mL of the Artemia solution was sampled using a pipet and placed on a filter paper so that it was evenly distributed. After draining, the filter paper was placed on a slide and the number of Artemia nauplii germinated was carefully counted. Continuous Microwave System. The experiments were conducted using a 5 kW, 915 MHz continuous flow microwave unit (Industrial Microwave Systems, LLC, Morrisville, NC). This unit is comprised of a power generation unit, wave guides, circulator, water load, power coupler, tuning coupler, an elliptical focusing cavity holding the applicator tube through which the process fluid was pumped and a cooling system to remove the excess heat generated by the magnetron. The entire processing system consisted of the feedtank, microwave processing unit, insulated holding tubes, cooling water circulation systems, a tube-in-tube heat exchanger unit (0.019 m nominal inner diameter, 0.038 m 9364
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nominal outer dia, and 1 m long) for heat recovery, and positive displacement pump to circulate the fluid through the system (Figure 1). Supporting Information Figure S1 shows the microwave system with the holding tube and tubular heat exchanger setup. More detailed information on the microwave system setup and operation is given by Boldor et al. (2008) (17). In Figure 1 the inlet and outlet temperatures of the product entering and leaving the microwave unit are denoted by Tin and Tout, respectively. Four sampling locations (ports) on the holding tube (denoted by h1, h2, h3, and h4) were placed at equidistance of 1.5 m from each other, such that the residence times of the ballast water inside the holding tubes reached a predetermined value at a particular flow-rate. For the current experimental protocol two flow-rates for the ballast water treatment were selected: 1 and 2 L · min-1. The residence times at each sampling location for the selected flow rates of the ballast water are given in Supporting Information Table S1. The initial tuning of the system was performed using synthetic seawater at 30 ppt salinity with no organisms present. Tuning stubs height (stub 1 ) 93 mm, stub 2 ) 42 mm, stub 3 ) 44 mm; stub 3 being closest to generator) in the tuning coupler were adjusted at the start of the study using a network analyzer (HP8753C, Hewlett-Packard, Palo Alto, CA) to maximize the power absorbed in the focusing cavity. Experimental Procedure for Cyst Inactivation Studies. The experiments were aimed at determining the effectiveness of the system in inactivating the heat resistant cysts present in ballast water. In addition to the two flow rates, two power levels (2.5 and 4.5 kW) were selected as the test parameters to form a 2 × 2 experimental design. The temperature responses during each experiment were recorded. Samples were collected at the various sampling sites after treatment (corresponding to different retention times) and the survival percentage of the cysts was determined. The control experiments were carried out in a thermal water-bath with temperature control. The temperature of the control bath was selected depending on the processing temperature reached during the microwave treatments for each flow rate and power combination. The inlet and outlet temperatures were measured using 10 T-type thermocouples (no. OSK2K1540/PP3-60-T-116U1-SMPW-M, Omega Engineering, Inc., CT) as previously
FIGURE 2. Sample temperature profiles during continuous microwave treatment of Artemia cysts in ballast water at various flow-rate and power combinations; (A) 1 L · min-1/ 2.5 kW (B) 2 L · min-1/ 4.5 kW. described (17). The arrangement of the thermocouples at the outlet of the microwave application chamber is shown in Supporting Information Figure S2. A data logger (model no. TC-08, PICO Technologies, Cambridgeshire, UK) connected to a PC-compatible computer with Windows XP (Microsoft Corp., Redmond, WA) operating system was used to record the responses from the ten thermocouples. The captured data was processed using MS Excel (Microsoft Corporation, Seattle, WA) and Sigma Plot (Version 9.01, Systat Software, Inc., San Jose, CA). All the experiments were performed in triplicate. Experimental Procedure for Electrical Power Requirement Studies. To investigate electrical efficiency of the system under a variety of conditions, the study included experiments with and without heat exchangers (tubular) using fresh water and 30 ppt synthetic salt water with no organisms. One of the heat exchangers (Supporting Information Figure S3) was custom built at the Louisiana State University Agricultural Center’s Department of Biological and Agricultural Engineering, and consisted of 76.2 cm length, 0.953 cm O.D. TP 304/TP 304 L stainless steel tubing which was passed through a larger (3.81 cm O.D) section of 304 stainless steel. The unprocessed water was pumped from a reservoir through the outer tube of the heat exchanger and after passing through the microwave system was returned to the heat exchanger through the smaller diameter pipe in a counter-current flow. The second heat exchanger (Supporting Information Figure S4) used in the study was commercially available, and consisted of a length of 0.9525 cm diameter titanium tubing in helicoidal shape passed through a 10.16 cm diameter section of PVC piping. The inner titanium piping side was used as the hot side, and the PVC piping was used as the cold side. The continuous microwave system was operated in TE10 mode, and impedance matching was performed using tuning stubs located along the waveguide of the system. The system was tuned separately for salt water and fresh water prior to conducting the experiments. The fluids pumped through the heat exchangers (if used) and the microwave cavity was heated at various power levels (1-5 kW in increments of 1 kW). The temperatures of the fluids entering and leaving the microwave were sampled using T-type thermocouples and the data was recorded using the PC-based data logger. All data collected from the network analyzer was imported into Microsoft Excel and SigmaPlot for processing and plotting. Energy utilization and efficiency were determined using the specific heat of the target material
as well as the mass flow rate and temperature difference. These values were then used to calculate overall efficiency at each power level. Power levels for fresh water experiments were measured using power diodes connected to the directional coupler. For saltwater (clean and inoculated) the power diodes malfunctioned and the absorbed power was calculated using the following formula (22): Q)m ˙ Cp∆T
(1)
Where Q ) absorbed power, Watts m ˙ ) mass flow rate, kg/s, Cp ) specific heat capacity of salt water, J/kg-K; assumed 4000 J/kg- K, and ∆T ) change in temperature, K.
Results and Discussion Observed Temperature Distributions for Artemia cysts during Microwave Treatment. Boldor et al. (21) have shown that there is a significant decrease in dielectric loss of the synthetic ballast water with increase in microwave frequency and decrease in temperature. Investigation on the temperature profiles observed for organisms in different operating conditions (i.e., temperature) will help in designing an effective microwave treatment protocol for the ballast water wherein the desired heating load is delivered in an energy efficient manner. Sample temperature histories recorded at the outlet, inlet and the feed tank for the inoculated ballast water are shown in Figure 2 and Supporting Information Figure S5, and it can be observed that the outlet temperatures reach equilibrium at a rapid rate (30 s at 2 L · min-1). The holding tubes 1 and 2 (h1 and h2, respectively) have a much slower response time until their temperatures reach values close to the outlet temperatures (Figure 2B). The tubular heat exchanger used in the preheating stage resulted in the microwave inlet temperatures above 30 °C (Supporting Information Figure S6). The heat exchanger thus helped to utilize the heat generated during microwave heating, reducing the total energy input into the system. The system’s design produced uniform temperatures at the outlet (as indicated by the low standard deviations) and high temperature changes from the inlet to the outlet, delivering the required heating loads during processing (Table 1). A combination of low flow rate and high power (1 L · min-1/ 4.5 kW) resulted in a temperature increase of more than 73 °C with the maximum absolute temperatures above 100 °C VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Temperature Increase Observed during Microwave Treatment (At a 30 S Time Interval) of the Ballast Water Containing the Artemia Cysts temperature, °C
operating parameters flow rate (L · min-1)
power (kW)
feed tank (maximum), Tinitial
microwave inlet (maximum), Tin
microwave outlet (maximum), Tout
difference ∆Ttotal
β (curve fitting)
R2 (adjusted)
2.0 2.0 1.0 1.0
2.5 4.5 2.5 4.5
27.34 ( 0.01 25.88 ( 0.00 27.19 ( 0.00 27.08 ( 0.01
30.96 ( 0.01 30.94 ( 0.02 39.85 ( 0.06 40.40 ( 0.16
49.20 ( 0.75 63.20 ( 0.12 72.62 ( 0.12 >100a
21.86 ( 0.74 37.31 ( 0.12 45.43 ( 0.12 >73
0.1433 0.1162 0.0550 0.0751
0.9660 0.9907 0.9871 0.9859
a The experiments were stopped since the temperatures reached were above 100 °C which is beyond the survival temperatures for Artemia cysts.
FIGURE 3. Percent (%) Artemia cysts germination in microwave and control experiments for the 2 L · min-1 study: (A) at 50 °C for a power input of 2.5 kW; (B) at 64 °C for a power input of 4.5 kW. (Supporting Information Figure S5B). This was expected as low flow rates increase the residence times inside the applicator tube, thus resulting in more power being absorbed. Analyzing the output temperature profiles by curve fitting, it was observed that the heating patterns follow a first order heating curve (23): y ) yo + A(1 - e-βt)
(2)
Where y ) outlet temperature, yo ) initial sample temperature, A ) temperature difference between the initial and target temperature, ∆T, β ) time constant dependent on the rate of temperature increase, and t ) time. The values of time constants β for ballast water containing Artemia cysts at the various flow rate and power combinations are shown in Table 1. These values indicate that the rates of heating at 2 L · min-1 were nearly similar irrespective of the power level; and were nearly twice the values at 1 L · min-1. This was expected considering that first order systems are only dependent on time in this case; the input power affecting only the absolute value, not the response time of the system. This information can be used in the design and optimization of microwave-based ballast water treatment systems. Inactivation Results Obtained for Artemia salina Cysts. Artemia are considered to be resilient to treatments like ozone (24), and survive at temperatures between 15 and 55 °C (25). The cysts however are even more resistant and can survive in harsher conditions; temperature extremes, severe desiccation, alternate desiccation and rehydration conditions, and prolonged exposure to high intensity UV and other types of radiation (26, 27). This ability of Artemia cysts to survive a variety of environmental conditions and readily adapt to them 9366
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make it an ideal candidate for studying the efficacy of microwave treatment. Figure 3 shows the percent germination of Artemia salina cysts after the microwave (at 2 L · min-1) and control treatments at temperatures of 50 and 64 °C, respectively, for power input levels of 2.5 and 4.5 kW. The figure shows that the microwave treatment was better at controlling the cysts than the control treatment, with a significant advantage at the lower temperature. The results obtained in the microwave and control treatments at 72 °C are shown in Supporting Information Figure S7. At 64 and 72 °C, none of the cysts survived for either treatment after 100 s. This shows that the microwave treatment is as at least as effective as heat treatment at high temperatures and it is better at controlling the cysts present in the ballast water at lower temperatures. The results obtained from the experiments show that complete inactivation of the cysts can be obtained at times less than 100 s and temperatures below 65 °C. Microwave heating produces faster heating rates since the target temperatures are realized within a matter of seconds as illustrated in Figure 2, assisting in faster cyst inactivation. The same conclusion was reached by other researchers that investigated the effect of rapid heating (100 °C/min) vs slow heating (4 °C/min) rates, and whom demonstrated a dramatic reduction in the cyst population at faster heating rates (28, 29). The inactivation results obtained by continuous microwave heating is in contrast to the results obtained on studies conducted using combination of sonication and chemical oxidants like ozone and hydrogen peroxide (30). Using sonication at 1.4 kHz and the two chemical oxidants (at 100 ppm) only 83% mortality
FIGURE 4. Temperature difference between the fresh water (left) and synthetic seawater (right) during the microwave treatment at different power levels with and without the use of heat recovery systems at a flow rate of 1 L · min-1. Ps denotes the power savings encountered while using the heat exchanger unit. was obtained for the cysts when exposed for 5 min. The is used (see Ssupporting Information page S13). Though the system in this stage of development is expensive compared mortality increased to 92% on exposure for 20 min. to other treatment methods, it can be used in conjunction With the exception of 1.0 L.min-1/2.5 kW combinations of parameters, the microwave heating was effective at killing with other methods and the environmental advantages of all cysts before the first sampling point in the holding tube this technology cannot be overlooked. (100 and 200 s for 2.0 and 1.0 L · min-1, respectively). The Electric Power Utilization Efficiency Studies. As diskilling rate for this exception was modeled using a first order cussed earlier, implementation of the described system in equation with the holding time as parameter, and the kill ships with no heat regeneration increases costs beyond rate coefficients for the control and the microwave experiunacceptable limits. To make this technology competitive ments are shown in Figure 3. The cumulative killing effect with the existing methods, it is necessary to either use of the microwave is presented in Supporting Information alternate methods with synergistic effects or couple the Figure S8, where the efficacy of a particular treatment is microwave heating with exchangers for heat regeneration to calculated by integrating the area under the heating and reduce operational costs. To this purpose, the heating holding portion of the process. The percent germination of characteristics in different media (fresh water and 30 ppt salt cysts at the end of the microwave treatment at the various water) at different power levels (1-5 kW) at a flow rate of 1 power-flow rate combinations is shown in Supporting L · min-1 were investigated for two heat exchangers. Results indicated a substantial improvement in power Information Table S2. transfer when using either heat exchanger. The dielectric Overall, both microwave and regular heating seem to loss of salt water is significantly higher than that of fresh provide total destruction of Artemia cysts at these high water, which results in more rapid heating due to the increase temperatures. In the case where survival was noticed, the in available and mobile ions within the solution. At a given microwaves were observed to have a definite advantage over power level, higher temperatures were achieved with the conventional heating in terms of killing rates (Figure 3). high dielectric loss salt water solution (Figure 4). The Power Absorbed During Cyst Inactivation Studies. The experiments with salt water had to be stopped at a maximum power absorbed by the medium was calculated using power level of 4 kW as more power resulted in boiling. It can equation 1 and substituting the ∆T values obtained for ¨ T of 45 °C at 1 L · min-1, only be observed that for a total A different power-flow rate combinations. One assumption 3.5 kW of power is required with a heat exchanger compared made was that the specific heat of salt water was 4000 J/Kg · K; to 5 kW without. For example, using these heat recovery the value obtained from literature for seawater (31). The systems the temperature of the salt water at the microwave calculated power values absorbed by the medium containing entrance is about 75% higher at 3 kW (Figure 5) compared the Artemia cysts and the input power as read on the to no heat exchanger (35 °C vs 20 °C). microwave control panel are shown in Supporting InformaOverall conversion efficiency was determined by comtion Table S3. As the dial was an analog instrument small paring the calculated power absorbed (from eq 1) and the variations from the assumed values were possible. The power input power reading from the control panel of the microwave efficiencies ranged from 90 to 100% in the microwave system. system. As expected, power efficiency increased dramatically The overall coefficient of performance calculated, including with the use of either heat exchanger (Table 2), with the the effect of the heat exchanger, ranged from 1.1 to 1.4. More most substantial increase observed for the PVC/titanium heat detailed studies on the thermal and dielectric properties of exchanger. During heating of the synthetic ballast water, in the medium containing the cysts will be helpful in undersome cases the power conversion efficiency exceeded 100% standing the heating phenomena observed. From an earlier due to the use of heat exchangers for heat recovery. preliminary economic analysis using the same setup laboraIn general, the efficacy of the system increases with tory system with no heat exchanger, the implementation of increasing output power. This effect can be explained by the this system for inactivating Artemia nauplii and adult Artemia increasing mobility of available ions as temperature increases, at a temperature of 55 °C will cost about $2.55/m3 and an estimated $1.09/m3 for a system having an efficient heat leading to increased power absorption and therefore inexchanger (17). However, for complete inactivation of cysts creased temperature. Higher conversion efficiency obtained temperatures of about 65 °C are required, demanding either is also dependent upon the characteristics of the heating more efficient heat exchangers than those used in this study medium and the presence of any heat recovery system. The or an increase in operating cost to approximately $3.02/m3 processed medium characteristics also influence the perfor an initial temperature of 25 °C when no heat exchanger formance as shown in Supporting Information Table S4. VOL. 42, NO. 24, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Sample temperature profile histories encountered during microwave treatment of fresh water (left) and synthetic seawater (right) at 1 L · min-1 while using the tubular SS heat exchanger.
TABLE 2. Calculated Microwave Power Conversion Efficiencies Obtained Using the Different Media in the Presence and Absence of Heat Recovery Systems (No HE, No Heat Exchanger, SS HE, Stainless Steel Heat Exchanger, PVC/Titanium HE, Poly Vinyl Chloride/Titanium Heat Exchanger) microwave power conversion efficiency (%) for fresh water
microwave power conversion efficiency (%) for salt water
input power, kW
no HE
SS HE
PVC HE
no HE
1 2 3 4 5
57.27 ( 23.1 53.23 ( 4.3 54.55 ( 1.9 58.58 ( 2.1 61.56 ( 2.8
52.90 ( 4.8 55.47 ( 1.4 62.53 ( 6.3 69.83 ( 4.8 75.65 ( 1.9
72.64 ( 7.3 82.52 ( 3.7 86.55 ( 4.1 90.61 ( 11.6 99.07 ( 0.5
87.02 ( 26.7 73.00 ( 8.3 75.69 ( 2.8 81.64 ( 2.3 98.21 ( 13.0
When fresh water was used as the medium, the coefficient of performance of the microwave system with the heat exchanger unit (denoted by COP1) for almost all power levels was below 1.0. However, when salt water was used as the medium, the COP1 values were above 1.0 for all cases except one (stainless steel exchanger at 1 kW power input). Overall, by using the heat exchanger units, the COP2 values (calculated total power absorbed/calculated microwave power absorbed) was greater than unity at all power levels. This shows that the system performance is significantly enhanced by the use of heat exchange units. These results indicate a probable methodology for the microwave treatment of ballast water, and they can be used to design a variety of systems capable of processing large amounts of water. On-board systems could be installed on large ships by placing the generators separate from the tank discharge. Waveguides would then transport energy with virtually no losses to focusing cavities located at the ballast discharge outlets, where the hot treated water would be cycled through a heat exchanger to improve efficiency and reduce exit temperatures to ecologically safe levels. A land based system would have microwave generators located within the port facility, and the use of waveguides, ship-to-land discharge tubes, and heat exchangers would ensure proper regulation of heat treatment. A third option would be to house the system on a barge, which would then take on the ballast while the ship is at berth (or waiting for a berth), resulting in no loss in transit time. Overall, the continuous microwave system could be used to deliver uniform heating loads and shows promise of being an effective tool for ballast water treatment, especially if used in conjunction with other methods. Future studies will be focused on its results in treating real ballast water, on the scale-up of the system and on optimizing the system for maximum power utilization. 9368
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SS HE 74.60 ( 21.3 90.00 ( 7.6 100.90 ( 2.0 114.53 ( 14.7 NA
PVC HE 94.67 ( 24.0 99.76 ( 5.4 106.40 ( 11.5 107.67 ( 1.4 NA
Acknowledgments We thank Industrial Microwave Systems LLC (Morrisville, NC), Laitram LLC (Harahan, LA) for their logistical support and for the loan of the microwave system. We also thank Department of Commerce and National Oceanic and Atmospheric Administration for their financial support of this research project (award no. NA05OAR4171072). Published with the approval of the Director of the Louisiana Agricultural Experiment Station as publication no. 2008-232-1905.
Supporting Information Available Figure S1 shows the continuous microwave system setup with the holding tubes and the tubular plate heat exchanger. Figure S2 shows the arrangement of the thermocouples at the outlet of the microwave applicator tube. Figures S3 and S4 show the schematics of the two types of tubular heat exchangers used in the study. The temperature profiles during microwave treatment of ballast water containing the Artemia salina cysts at different flow rate and input power combinations are shown in Figures S5 and S6. The percent germination of Artemia cysts after the microwave and control treatments are shown in Figure S7. The time-temperature histories experienced by the cysts during the treatment is shown in Figure S8. Table S1 shows the residence times at the different flow rates and sampling locations on the microwave process line. Table S2 shows the survival percentage of Artemia cysts after the microwave treatment. Table S3 shows the calculated absorbed power values during the microwave treatment of the ballast water containing the cysts. Table S4 shows the calculated power absorbed values and coefficient of performance values at various power input levels for the studied microwave system with different heat exchangers and using different media. A sample calculation of the treatment cost economics is presented on page S13. This material is available free of charge via the Internet at http://pubs.acs.org.
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