Final Technical Report. Pilot-Scale Evaluation of the Silver Bullet Device in Controlling Biological Growth in Cooling Tower Systems

Transcription

1 Final Technical Report Pilot-Scale Evaluation of the Silver Bullet Device in Controlling Biological Growth in Cooling Tower Systems Xiao Ma 1, Scott Duda 1, Janet E. Stout 1,2, Radisav D. Vidic 2 1 Special Pathogens Laboratory, Pittsburgh, PA 2 Department of Civil and Environmental Engineering, University of Pittsburgh, Pittsburgh, PA November 1, 2012

2 Executive Summary Pilot scale testing of the Silver Bullet cooling tower water system (known as Silver Bullet System) started on August 30 th 2012, and ended on October 15 th This report is prepared as the final technical report for evaluation of the Silver Bullet System s efficacy in cooling tower water biological control. A model cooling tower system that simulates realistic field conditions (including heat load, evaporative cooling, blow down system, and water makeup) has been used for this test. The model cooling tower system consists of two cooling towers: Tower 1 (T1) is set as the control tower with no treatment and is denoted as control tower in this report; Tower 2 (T2) is set as the device tower with Silver Bullet System installed and is denoted as device tower in this report. This model cooling tower system was previously used in ASHRAE RP project that evaluated six non-chemical devices for their ability to control biological growth in cooling towers using identical procedures as those applied in this study. Routine cooling tower performance and water quality records show normal operation during the study period with the control tower (T1) at an average of 4.91 cycles of concentration based on conductivity and the device tower at an average of 4.46 cycles of concentration. The heterotrophic plate counts (HPC) and Legionella testing results show that the installed system can control bacterial growth in cooling tower bulk water as both HPC and Legionella bulk water concentrations in the device tower (T2) were significantly lower than in the control tower (T1). The average HPC bacteria concentration in the pilot-scale cooling tower treated with the Silver Bullet Device (T2) was reduced by 98.8% and Legionella concentration was reduced by 78.1%. HPC bacteria concentration in the device tower biofilm was also significantly lower (approximately 85.9% reduction) than in the control tower. Due to the low dose of disinfectant generated in-situ by the small scale device tested in this study, Legionella concentration in the biofilm was not significantly impacted. 1

3 1. Materials and Methods 1.1. Pilot-Scale Cooling Tower and Sampling Procedure A pilot-scale cooling tower system was used for this investigation. The system consists of two identical pilot-scale cooling towers that are supplied by the same make-up water source (dechlorinated City of Pittsburgh water) and operated simultaneously under similar operating conditions. The pilot-scale cooling towers were designed to closely simulate realistic field conditions, including heat load, evaporating cooling process, blowdown system, and water makeup. A schematic process diagram of a pilot-scale cooling tower is shown in Figure 1. During the investigation, one tower (T1) was operated as a control tower with no treatment and is denoted as control tower in this report; the second tower (T2) was operated with Silver Bullet System installed and is denoted as device tower in this report. Under identical operating conditions, operation of the two pilot-scale cooling towers will encourage similar formation of both planktonic growth and surface occurring biofilm (sessile biomass). The effectiveness of the Silver Bullet System to control biological growth is evaluated based on direct comparison of biological growth in the device tower and the control tower. Figure 1. Schematic of Pilot-Scale Cooling Tower 2

4 Sterilized sampling bottles were used for sampling during the whole investigation. Bulk water samples for biological and water quality analysis were collected from cooling tower sumps; biofilm samples were collected from autoclaved stainless steel biofilm sampling coupons that were installed in both towers as soon as the they reached 5 cycles of concentration. Detailed biofilm sampling protocol is described in Appendix A. All biological samples were transported to the reference laboratory within 2 hours after sampling and all samples were chilled during transport. The experiment lasted for 6 weeks from August 30 th 2012 to October 15 th Silver Bullet System was operated immediately following the start-up of the cooling tower system and was functioning during the entire experiment. Bulk water samplings were conducted every Monday and Thursday each week and these samples were analyzed for heterotrophic plate counts (HPC), Legionella, chloride, total suspended solids (TSS), and total dissolved solids (TDS). Biofilm sampling was conducted every Monday and these samples were analyzed for HPC and Legionella. ph, conductivity, calcium hardness, total alkalinity, and water temperature were monitored daily during this experiment Testing Methods Table 1 shows the list of physicochemical and biological parameters that were monitored during this experiment, frequency of measurement, and methods of analyses. These parameters were used to evaluate the ability of Silver Bullet System to control biological growth in the pilotscale tower by comparison to the results from the untreated control tower. Table 1: Physical, chemical, and biological parameters monitored Parameter ph Make-Up Water Frequency of Measurement T1 and T2 Frequency of Measurement Daily Conductivity Daily Daily Total Alkalinity Daily Daily Calcium Hardness Temperature Daily Daily Daily Chloride Weekly Weekly Total Suspended Solids (TSS) Total Dissolved Solids (TDS) Weekly Weekly Weekly Weekly Method of Analysis Field Testing Field Testing Field Testing Field Testing Field Testing Laboratory Analysis Laboratory Analysis Laboratory Analysis Device/Standard Method Fisher Scientific ph Meter Fisher Scientific Accumet Conductivity Meter LaMotte Test Kits LaMotte Test Kits Fisher Scientific Chloride Meter ISO ISO

5 Bulk Water HPC Concentration Twice Per Week Twice Per Week Laboratory Analysis Standard Methods 9215 Biofilm HPC Concentration Bulk Water Legionella Concentration Biofilm Legionella Concentration Weekly Twice Per Week Weekly Weekly Twice Per Week Weekly Laboratory Analysis Laboratory Analysis Laboratory Analysis Standard Methods 9215 ISO 11731:1998 ISO 11731:2004 ISO 11731:1998 ISO 11731: Results and Discussion 2.1. Bacteriological Analysis Heterotrophic Plate Counts (HPC) HPC concentrations in make-up water and in the bulk water of device and control towers are shown in Table 2 and Figure 2 while Table 3 compares the HPC concentrations in the make-up water used in this study to those observed in other potential make-up water sources (ASHRAE, 2010). Table 2: HPC concentrations in bulk water Sampling Make-Up Water T1 (Control) HPC T2 (Device) HPC HPC (CFU/mL) (CFU/mL) (CFU/mL) 9/3/ /6/ /10/ /13/ /17/ /20/ /24/ /27/ /1/ /4/ /8/ /11/

6 Table 3: Concentrations of heterotrophic plate count (HPC) bacteria in various water sources Water Source HPC Planktonic Bacteria Count Reference(s) (log CFU/mL) Drinking water from household taps (<1 mi. from treatment plant), NJ Range = LeChevallier et al., 1987 Tuscon, AZ household tap Average = 3.5 Pepper et al., 2004 Tuscon, AZ household tap Range = Average = 4.0 Chaidez & Gerba, 2004 Warm tap water, hospital building Average = 4.8 Sheffer et al., 2005 Rainwater harvesting system, Seoul, Korea Range = Amin & Han, 2009 Hot water systems, Copenhagen, Ovesen et al., 1994; Bagh Average = 4.0+ Denmark et al., 2002 Hot water tank, apartment building, Copenhagen, Denmark Range = Bagh et al., 2003 Michigan freshwater lakes (bulk water) Average = 3.5 Jones et al., community taps and 5 industrial Jousimies-Somer et al., Range = process water basins 1993 Cooling tower water (basins) Range = Bentham, R.H., 1993 Drinking water distribution systems, Durham and Raleigh, NC Range = < Zhang & DiGiano, 2002 Drinking water distribution systems, Irvine and Garden Grove, CA Range = < Ridgway & Olson, 1982 ASHRAE Project 1361-RP Range = Average = 4.4 ASHRAE, 2010 This Study Range = Average = 4.6 As shown in Table 2 and Figure 2, HPC concentrations in the control tower ranged from CFU/ml to CFU/ml and ranged from 660 CFU/ml to CFU/ml in the device tower. HPC in make-up water ranged from 6100 CFU/ml to CFU/ml. T-test assuming equal variances (Table 4(a)) indicates that HPC concentrations in the device tower bulk water were significantly lower than in the control tower bulk water at 95% confidence level (p = < 0.05). On the other hand, the same t-test (Table 4(b)) indicates that HPC concentrations in make-up water and device tower bulk water were not statistically different at 95% confidence level (p = 0.16 > 0.05). These results clearly indicate that the Silver Bullet system was able to suppress any growth of heterotrophic bacteria in the planktonic phase. 5

7 HPC concentration in bulk water (CFU/ml) 1.E+08 1.E+06 Make-up Water Bulk Water Device Tower Bulk Water 1.E+04 1.E+02 1.E+00 Figure 2. HPC concentrations in bulk water Table 4(a): Two sample t-test assuming equal variances: HPC concentrations in bulk water of the control tower vs. device tower T1 (Control) HPC T2 (Device) HPC Mean 1.96E+6 2.3E+4 Variance 9.4E E+8 Observations Pooled Variance 4.7E+11 Hypothesized Mean Difference 0 df 22 t Stat P(T<=t) two-tail E-07 t Critical two-tail Table 4(b): Two sample t-test assuming equal variances: HPC concentrations in make-up water vs. device tower bulk water Make-Up Water HPC T2 (Device) HPC Mean E+4 Variance 1.28E+9 8.2E+8 Observations Pooled Variance 1.0E+9 Hypothesized Mean Difference 0 df 22 t Stat P(T<=t) two-tail t Critical two-tail

8 HPC Concentrations in bulk water (CFU/ ml) As shown in Table 5 and Figure 3, the average HPC concentration in the bulk water of the tower treated with Silver Bullet Device was 98.8% lower than the average HPC concentration in the control tower bulk water. This is a much better performance in comparison with nonchemical devices tested in the ASHRAE 1361-RP study (ASHRAE, 2010) where none of the devices showed reduction in HPC concentrations in the bulk water above 60%. Table 5: Average HPC concentrations in bulk water during the Silver Bullet Device study in comparison to studies with non-chemical devices evaluated in the ASHRAE 1361-RP study Make-Up Water HPC (CFU/mL) T1 (Control) HPC (CFU/mL) T2 (Device) HPC (CFU/mL) Percent Reduction Device Silver Bullet Device Magnetic Device Pulsed Electric Field Device (4-5 cycles of concentration) Pulsed Electric Field Device (6-7 cycles of concentration) NA Electro Static Device NA Ultrasonic Device Hydrodynamic Cavitation Device NA 1.00E E+06 Make-Up Water Control Bulk Water Device Bulk Water 1.00E E E+00 Device Figure 3. Average HPC concentrations in bulk water from the Silver Bullet Device in this Study and Devices from ASHRAE 1361-RP Similar with bulk water, the Silver Bullet System also inactivated HPC growth in biofilm. As shown in Table 6 and Figure 4, HPC concentrations in the biofilm collected from the control 7

9 HPC concentration in biofilm (CFU/cm 2 ) tower ranged from CFU/cm 2 to CFU/cm 2, while HPC concentrations in the biofilm collected from the device tower ranged from CFU/cm 2 to CFU/cm 2. T-test result shown in Table 7 indicated that HPC concentrations in the biofilm of the device tower were statistically lower than in the control tower at 95% confidence level (p = 0.03 < 0.05). Table 6: HPC concentrations in biofilm Sampling T1 (Control) HPC (CFU/cm 2 ) T2 (Device) HPC (CFU/cm 2 ) 9/10/ /17/ /24/ /1/ /8/ E+07 1.E+06 1.E+05 Control tower Device tower 1.E+04 1.E+03 1.E+02 1.E+01 1.E+00 Figure 4. HPC concentrations in biofilm 8

10 Table 7: Two sample t-test assuming equal variances: HPC concentrations in biofilm of control tower vs. device tower T1 (Control) HPC T2 (Device) HPC Mean Variance 2.7E E+10 Observations 5 5 Pooled Variance Hypothesized Mean Difference 0 df 8 t Stat P(T<=t) two-tail t Critical two-tail Thus, the biological analysis results suggested that Silver Bullet System significantly inhibited the growth of HPC bacteria in both bulk water (98.8% reduction) and biofilm (85.9% reduction). Legionella As shown in Table 8 and Figure 5, Legionella were detected in only a few make-up water samples (4 out of 12) but were detected in all water samples (12 out of 12) from both control and device tower. Therefore, Legionella can easily grow in warm and aerated water that characterizes cooling tower operation. Legionella concentrations in bulk water of the control tower ranged from 140 CFU/ml to > 6000 CFU/ml while the concentrations detected in the recirculating water of the device tower ranged from 40 CFU/ml to 2980 CFU/ml. Sampling Table 8: Legionella concentrations and serogroups in bulk water Make-Up Water T1 (Control) T2 (Device) Concentrati Sero- Concentration Sero- Concentration Seroon group (CFU/mL) group (CFU/mL) group (CFU/mL) 9/3/ NLp 140 Lp6 40 Lp5+6 9/6/ >6000 Lp Lp1+6 9/10/ Lp Lp Lp1+6 9/13/ Lp Lp1+6 9/17/ Lp Lp1+5 9/20/ Lp Lp Lp1 9/24/ Lp Lp1+6 9/27/ Lp Lp1+6 10/1/ Lp Lp1+6 10/4/ Lp5 >6000 Lp Lp1+6 10/8/ Lp1 450 Lp1 10/11/ Lp1 240 Lp1 *NLp: Non-Pneumophia species; LpX: Legionella pneumophila serogroup X 9

11 Legionella pneumophila concentration (CFU/ml) 1.00E E+03 Make-up Water Bulk Water Device Tower Bulk Water 1.00E E E+00 Figure 5. Legionella concentrations in bulk water T-test results shown in Table 9 indicate that Legionella concentrations in the recirculating water of the device tower were statistically lower than in the control tower (p = < 0.05) by 78.1%. Table 9: Two sample t-test assuming equal variances: Legionella concentrations in bulk water of the control tower and device tower T1 (Control) Lp T2 (Device) Lp Mean Variance Observations Pooled Variance Hypothesized Mean Difference 0 df 22 t Stat P(T<=t) two-tail t Critical two-tail Table 10 and Figure 6 compare the results of Legionella concentrations enumerated from biofilm samples collected during this study. Legionella concentrations in control tower biofilm ranged from 2210 CFU/cm 2 to >10695 CFU/cm 2, and Legionella concentrations in device tower biofilm ranged from 2888 CFU/cm 2 to >10695 CFU/cm 2. Table 11 provides the results of two 10

12 Legionella pneumophila concentration (CFU/cm 2 ) sample t-test assuming equal variances among these two data sets. Based on the finding that no statistically significant difference was observed between the results obtained for the device tower and control tower (p = 0.93 > 0.05, Table 9), it can be concluded that Silver Bullet System did not affect the concentration of Legionella in the biofilm during this study. Table 10: Legionella concentrations in biofilm T1 (Control) T2 (Device) Sampling Concentration Serogroup Concentration Serogroup (CFU/cm2) (CFU/cm2) 9/10/ Lp Lp /17/2012 >10695 Lp1+6 >10695 Lp1+6 9/24/2012 >10695 Lp1+6 >10695 Lp1+6 10/1/2012 >10695 Lp Lp1+6 10/8/ Lp Lp1 1.E+05 1.E+04 1.E+03 1.E+02 Device Tower 1.E+01 1.E+00 Figure 6. Legionella concentrations in biofilm 11

13 Table 11: Two sample t-test assuming equal variances: Legionella concentrations in biofilm of the control tower vs. device tower T1 (Control) Lp T2 (Device) Lp Mean Variance Observations 5 5 Pooled Variance Hypothesized Mean Difference 0 df 8 t Stat P(T<=t) two-tail t Critical two-tail Water Quality Analysis Water quality of the make-up water as well as the recirculating water in device and control towers was monitored during the entire experiment. As shown in Table 12, no significant differences in recirculating water quality between the control tower and the device tower were observed throughout the study. Mean Value of Water Quality Parameter Table 12. Mean value of tower bulk water quality and t-test results Make-up Water Device Tower Two sample t-test of T1 and T2, p value Conductivity (ms) 0.4± ± ± ph NS 8.6± ± Alkalinity (mg/l) 49.6± ± ± Hardness (mg/l) 108.3± ± ± Chloride (mg/l) 62.7± ± ± TSS (mg/l) 4.9± ± ± TDS (mg/l) 197.1± ± ± *NS: Not sampled The cycles of concentration of each tower calculated based on the conductivity were between 4 and 5 during the test (Figure 7). The device tower reached 4 cycles of concentration on the fourth day of experiment, and control tower reached 4 cycles of concentration on the sixth day of operation. The mean conductivity of recirculating water in the control tower was slightly higher than in the device tower, but the difference is not obvious (2.1 ms vs. 1.9 ms). Figure 8 shows the results of ph monitoring during the study while the other water quality 12

14 ph Conductivity (ms) parameters and cooling tower performance data are shown in Appendix B. In addition to routine water quality monitoring, water samples collected on October 3, 2012 were sent to Silver Bullet Water Treatment LLC for analysis and those results are compared to water quality analysis obtained as part of this study in Table 13. The results indicate that the conductivity analysis of make-up water performed by Silver Bullet was much higher than the results obtained as part of daily water quality monitoring in this study. As a result, there is a large difference in the cycles of concentration based on conductivity analysis by these two teams. Figure 7 indicates that there is some fluctuation in conductivity measurement of the recirculating water, which can be explained by the differences in blowdown and make-up water schedule in comparison to the sampling schedule. However, analysis of all the data collected in this test study indicate that the cycles of concentration were maintained in this study at desired levels Make-up Water Device Tower 0 Figure 7. Conductivity of make-up water and recirculating water in control and device towers Device Tower Figure 8. ph of tower bulk water 13

15 Table 13. Water quality parameters determined by Silver Bullet Water Treatment laboratory and in this study on 10/3/2012 Water Sample Conductivity (ms) Total Hardness (mg/l) Calcium Hardness (mg/l) ph H 2 O 2 (mg/l) Device Tower (1.941) (660) 8.78 (8.67) (1.832) (680) 8.63 (8.67) Make-up Water (0.452) (120) 7.85 Control CoC 6.65 (4.1) (5.7) Device CoC 7.11 (4.3) (5.5) Values in parentheses were obtained as part of regular daily water quality monitoring 3. Summary and Conclusions Long-term (6 weeks) pilot scale study indicated that the installation of the Silver Bullet system achieved HPC concentrations in the recirculating water of the treated (device) tower that were statistically identical to concentrations in the make-up water. The industrial biofouling control recommendation for sessile bacteria (below 10 5 CFU/cm 2 ) had not been achieved by the end of test as the sessile heterotrophic bacteria varied between 10 5 CFU/cm 2 and 10 6 CFU/cm 2. However, the Silver Bullet Device significantly reduced concentrations of planktonic (98.8%) and sessile (85.9%) heterotrophic bacteria in the treated cooling tower as compared to the values obtained from the untreated cooling tower. The widely accepted industrial biofouling control recommendation for planktonic bacteria (below 10 4 CFU/mL) was achieved with the Silver Bullet system within two weeks of operation. The Silver Bullet system did not affect the concentration of Legionella grown in the biofilm, i.e., Legionella concentration in the device tower biofilm and the control tower biofilm were not statistically different at 95% confidence level (p = 0.93 > 0.05). These results indicate that the disinfectant generated in-situ by the Silver Bullet system was at low concentrations and could not penetrate the highly resistant Legionella biofilm as effectively as in the planktonic phase. However, Legionella concentrations in the recirculating water of the cooling tower treated with the Silver Bullet System were significantly lower (78%) than in the control tower. In comparison to six non-chemical devices evaluated in the ASHRAE 1361-RP study, the Silver Bullet Device performed significantly better in terms of the ability to control planktonic and sessile growth of HPC in a pilot-scale cooling tower. 14

16 4. References ASHRAE, 2010 Biological control in cooling water systems using non-chemical treatment devices Final report for ASHRAE 1361-RP. Amin, Muhammad Tahir, and Mooyoung Han Roof-harvested rainwater for potable purposes: application of solar disinfection (SODIS) and limitations. Water Science & Technology. 60.2, Bagh, Lene Karen, H-J Albrechtsen, and E. Arvin Biofilm formation in a hot water system. Water Science & Technology. 46(9), Bagh, Lene Karen, Hans-Jørgen Albrechtsen, Erik Arvin, and Kaj Ovesen Distribution of bacteria in a domestic hot water system in a Danish apartment building. Water Research. 38(1), Bentham, R. H. (1993). "Environmental factors affecting the colonization of cooling towers by Legionella spp in South Australia." International Biodeterioration and Biodegration 31: Chaidez, Cristobal, and Charles P. Gerba Comparison of the microbiologic quality of point-of-use (POU)-treated water and tap water. International Journal of Environmental Health Research. 14(4), Jones, Garth W., Liza Baines, and Fred J. Genthner Heterotrophic bacteria of the freshwater neuston and their ability to act as plasmid recipients under nutrient deprived conditions. Microbial Ecology. Vol. 22, No. 1, Jousimies-Somer, Hannele R., Sirkku Waarala, and Marja-Liisa Väisänen Recovery of Legionella spp. from water samples by four different methods. Legionella: Current Status and Emerging Perspectives. Eds. James M Barbaree, Robert F. Breiman, and Alfred P. Dufour. Washington, D.C.: American Society for Microbiology, LeChevallier, Mark W., Timothy M. Babcock, and Ramon G. Lee Examination and Characterization of Distribution System Biofilms. App. And Env. Microbio. 53(12), Pp Ovesen, K., F. Schmidt-Jørgensen, and L. K. Bagh Bacterial growth in hot water systems. Danish Building Research Institute. Report No Hørsholm, Denmark; 1994 [In Danish, summary in English]. Pepper, I. L., P. Rusin, D. R. Quintanar, C. Haney, K.L. Josephson, and C.P. Gerba Tracking the concentrations of heterotrophic plate count bacteria from the source to the consumer s tap. International Journal of Food Microbiology. 92, Ridgway, H.F., and B.H. Olson Chlorine resistance patterns of bacteria from two drinking water distribution systems. Appl. & Eng. Microbio. October 1982, p Sheffer, P. J., J. E. Stout, et al. (2005). "Efficacy of new point-of-use water filters for preventing exposure to Legionella and waterborne bacteria." Amer J Infect Cont 33(5): S20-S25. Zhang, Weidong, and Francis A. DiGiano Comparison of bacterial regrowth in distribution systems using free chlorine and chloramines: a statistical study of causative factors. Water Research. 36:

17 Appendix A. Biofilm Coupon Sampling Protocol **Close valves to bypass coupon rack** 1. Put on gloves 2. Unscrew coupon holder from rack and remove 3. Swab bottom of coupon before unscrewing 4. Hold coupon over sterile conical tube and unscrew nut 5. Release coupon into 5 ml conical tube bottom side down 6. Swab top of coupon and add 10 ml of sterile water to tube 7. Place swab into tube and agitate vigorously to remove attached material from swab. Cut or break swab and leave in the tube 8. Transport sample to lab as soon as possible 9. Vortex for 30 sec before testing the sample liquid. Process for HPC using appropriate dilution scheme 10. Total CFU recovered is calculated by multiplying CFU x 10 ml sample volume 11. Divide total CFU by surface area of the coupon (5.61 cm 2 ). Report coupon results as CFU/cm Save coupon and sterilize for reuse 16

18 Figure A1. Biofilm Sampling Coupon Holder Cross-Section 17

19 Alkality (mg/l) Conductivity (ms) Appendix B. Water quality parameters Make-up Water Device Tower 0 Figure B1. Conductivity of make-up water and cooling tower bulk water Make-up Water Device Tower 50 0 Figure B2. Alkalinity of make-up water and cooling tower bulk water 18

20 Chloride(mg/L) Hardness (mg/l) Make-up Water Device Tower Figure B3. Hardness of make-up water and cooling tower bulk water Make-up Water Device Tower 50 0 Figure B4. Chloride concentration in of make-up water and cooling tower bulk water 19

21 Total Dissolved Solids (mg/l) Total Suspended Solids (mg/l) Make-up Water Device Tower 5 0 Figure B5. Total suspended solids of make-up water and cooling tower bulk water Make-up Water Device Tower Figure B6. Total dissolved solids of make-up water and cooling tower bulk water 20

22 ph Temperature Difference (F) Device Tower Temperature Difference Temperature Difference 0 Figure B7. Temperature drop across the cooling tower Device Tower Figure B8. ph of make-up water and cooling tower bulk water 21