MIXED-OXIDANT APPLICATION IN COOLING TOWER MAINTENANCE

Transcription

1 MIXED-OXIDANT APPLICATION IN COOLING TOWER MAINTENANCE Paul Petersen, President, Trident Technologies, Inc., San Diego, CA and Wesley L. Bradford, Ph.D., Manager, Environmental Programs, Los Alamos Technical Associates, Inc., Los Alamos, NM and Chief Scientist, Product Development for MIOX Corporation, Albuquerque, NM January 14, 2000 ABSTRACT Tests using the MIOX-generated mixed-oxidant solution instead of oxidizing biocides (chlorine or bromine) for cooling tower maintenance, and eliminating all other biocides, in first an established cooling tower and subsequently in two new cooling towers found: (1) aerobic bacteria counts consistently less than 1000/mL in the cooling water (no colonies on the standard Easicult TTC 1 dip slide test), a result rarely achieved using chlorine alone or chlorine with other biocides; (2) ease in maintaining Free Available Chlorine (FAC) concentrations at mg/l using a standard ORP controller; and (3) removal of biofilms from small areas of cooling surfaces where they had accumulated in the previous maintenance program, and a consequent reduction or complete elimination of microbiologically-induced corrosion. Using organic corrosion inhibitors, metal coupon tests showed higher rates of pitting-type corrosion on steel using the mixed-oxidant solution than using oxidizing biocides or chlorine. When the corrosion inhibition program was shifted toward stabilized phosphate with the ph controlled at , pitting-type corrosion on steel was eliminated, and levels of general corrosion were within industry norms and equal to those using oxidizing biocides or chlorine. Azoles used for corrosion control on copper heat exchange surfaces were unaffected by the mixed-oxidant solution and copper corrosion rates were normal. The benefits of the mixed-oxidant solution observed in these tests and general application for cooling tower maintenance include: aggressive disinfection -- elimination of biofilms, inactivation of pathogenic microorganisms including Legionella species, and nil or low aerobic bacteria counts, all without additional biocides; safe operations because only salt, water and power are used to generate the non-hazardous, mixed-oxidant solution, liability exposure and associated management costs are reduced (no Risk Management Plan (RMP) or Process Safety Management (PSM) plans required), staff and community safety are improved, and costs for safe transportation, handling, and storage of chlorine gas or hypochlorite are eliminated. low maintenance -- automated systems require only minimal operator attention. cost competitive competitive with chlorine gas and cheaper than sodium or calcium hypochlorite; low operating costs and no safety costs usually result in a lower lifecycle costs. 1 Registered Trademark of Orion Diagnostica. Available in the USA from LifeSign L.L.C., Somerset, NJ.

2 2 BACKGROUND Cooling Tower Maintenance The major challenges in cooling tower maintenance include: 1. Controlling deposition on cooling surfaces (CaCO 3, CaSO 4 and silica (SiO 2 ) deposits); 2. Providing corrosion protection for copper, copper/nickel tubing, admiralty (copper alloy), and carbon and stainless steels; 3. Controlling microbiological growth including biofilms on cooling surfaces and bacterial counts in the cooling tower basin water; and 4. Controlling airborne impurities contaminants and particulates washed out of the air and other contaminants that enter external to the water source. These challenges can be addressed and at least partially controlled by additions of various antiscalants, corrosion inhibitors, and biocides, plus use of a disinfectant continuously supplied to provide a disinfection residual. Chlorine is currently the preferred disinfectant because of relatively low cost, ease of use and control, and the ability to maintain a disinfection residual. However, the use of biocides is being viewed with increasing concern by the regulatory agencies from the standpoints of safe transportation, storage, and use, and residues of the biocides in discharges of cooling tower blowdown. The State of California now requires registration for and imposes a mil tax on use of all biocides, and increasing regulation in the future is anticipated. Thus, clearly there is need in the cooling tower maintenance industry for a disinfection system that will provide enhanced cooling water disinfection, allowing elimination of biocide use, while maintaining simplicity, reliability, and ease of operation and causing minimal corrosion and scaling impact on the exchange and cooling surfaces of the cooling tower system. The MIOX System and the Mixed-Oxidant Solution The MIOX Corporation began manufacturing and marketing systems in 1994 following a period of intense development and testing. The systems are designed primarily for treatment of potable and waste water but have found applications in swimming pools and cooling towers as well. MIOX systems use an electrolytic process in a brine of common salt (NaCl) to generate mixed oxidants in solution on-site and on demand, so there is no need to transport or handle hazardous materials. In addition, the mixed-oxidant solution produced is dilute and is not a hazardous material, presenting no safety hazard to workers. The only potential hazard is generation of hydrogen gas, which is a necessary product of electrolysis. Hydrogen gas is forcevented to the atmosphere prior to the oxidant tank so risks are minimized by system design. The mixed-oxidant solution consists primarily of chlorine (as HOCl and OCl - depending on ph), as well as other chlor-oxygen species which are short-lived when presented with an oxidant-demanding substance, ie. they react rapidly and are no longer detectable in the water, leaving the chlorine component as a measurable residual. This feature of the other-oxidant component of the mixed-oxidant solution was demonstrated first by Dowd (1994) and continues

3 3 to be seen in virtually all MIOX system installations. The relative disinfecting capabilities of the mixed-oxidant solution, which are significantly greater than those of chlorine alone, are believed to be caused by synergism of the oxidants working together; synergy between oxidants has been demonstrated by other researchers (Kouame and Haas, 1991) and is now being actively investigated in the water treatment research community. The mixed-oxidant solution has been shown to provide the disinfection needed to meet or exceed all bacteria/residual standards in potable water and to exceed all other current chlorine-based technologies in kill effectiveness of viruses while also reducing disinfection by-product concentrations compared to chlorine. For example, the mixed-oxidant solution can inactivate Giardia lambla cysts and Cryptosporidium parvum oocysts by more than 99.9% at practical doses (as FAC) of 5 mg/l (Venczel et.al., 1997; Casteel et.al., 1999); in the case of C. parvum oocysts, no inactivation occurs by equal doses of chlorine alone. Disinfection ability of the mixed-oxidant solution in contrast to chlorine has also been studied in simulated cooling water (Barton, 1996) with three bacteria species relevant to the cooling industry: Bacillus stearothermophilus, Pseudomonas aeruginosa, and Legionella pneumophila. The results of those tests at ph 8 after 10 minutes exposure are shown in Table 1. Mixed oxidants were significantly more effective than chlorine at achieving total inactivation of all bacteria, even at doses as low as 2 mg/l (as FAC). Table 1: Bacteria present (CFU/ml): Type of Microorganism: Bacillus stearothermophilus Pseudomonas aeruginosa Legionella pneumophila Initial Microorganism Concentration: ph = 8; Exposure = 10 minutes Chlorine Equivalent Concentrations (mg/l): Mixed Oxidants 2 mg/l Dose 4 mg/l Dose NaOCl Mixed Oxidants NaOCl 2 x CFU/ml 1400 CFU/ml 0 CFU/ml 12 CFU/ml 1 x CFU/ml 1200 CFU/ml 0 CFU/ml 110 CFU/ml 1 x CFU/ml > 2 CFU/ml 0 CFU/ml > 2 CFU/ml

4 4 Due to the mixture of oxidants, the mixed-oxidant solution presents a variety of advantages over chlorine alone beyond disinfection. Additional beneficial features of the mixedoxidant solution potentially relevant to cooling tower maintenance have been observed and documented in laboratory and field. Oxidation of Iron and Manganese: Oxidizes both Fe 2+ and Mn 2+, leading to removal in subsequent coagulation/flocculation or by filtration or settling. Greenfield Municipal Utilities, IA, uses the mixed-oxidant solution to remove Mn 2+ under conditions at which it could not be removed by either chlorine or potassium permanganate (Herrington and Bradford, 1997). Biofilm Removal: Sloughs distribution system biofilm and prevents regrowth. A KOA campground in Great Falls, MT, that had previously used hypochlorite reported sloughing of biofilms within 2 weeks of startup of a MIOX system, as well as an entire summer of operation with no total coliform violations, no clogging regrowth, no biofilms in the showers, no slime growths in the water park or swimming pool (for the first time ever, the operator did not need to use biocides at the waterpark), and an excellent taste to the water, contrary to the prior experience using hypochlorite (Crayton et.al., 1997). Ammonia Oxidation at Sub-breakpoint Doses: Causes ammonia oxidation in ammonia- and chloramine-containing waters at doses well below those required using classical breakpoint chlorination (Bradford and Cisneros, 1995). Cash Water Supply, TX, effectively replaced three chlorine booster stations producing approximately 17 lbs. per day for their chloraminated water with one small MIOX unit producing only 1.5 lbs per day (Daniel, 1995). Residual Maintenance: Creates a longer-lasting chlorine residual than traditional chlorination, often at a lower dosage. This is the feature most commonly reported by users of MIOX systems. The experience at Santa Fe, NM (Herrington et.al. 1999) and at Lamar County Water Supply, TX are examples. Lamar County maintains a free chlorine residual > 1 mg/l over 25 miles from the disinfection station, as compared to < 0.2 ppm using gas chlorine. This feature of the mixed-oxidant solution in cooling waters is illustrated further in Tables 1 and 2 below. Testing in simulated cooling water conducted earlier (Bradford et.al, 1997) had shown that the ability of the mixed-oxidant solution to maintain both a FAC and a Total Chlorine (TC) residual in warm waters with and without added ammonia (NH 3 ) was superior to that of chlorine as sodium hypochlorite (NaOCl). The results of those tests over a period of 60 minutes after dosing the test solutions at phosphate-buffered ph 6.5 are shown in Table 1, and at phosphatebuffered ph 9.0 are shown in Table 2. The implication of these results is that smaller (compared to chlorine) doses of FAC as mixed-oxidant solution are required to maintain a target FAC or TC residual in the cooling water.

5 5 Table 1: FAC and TC in Test Solution: ph = 6.5; 2 mg/l NH 3 ; Temperature = 50 o C FAC 5 minutes 10 minutes 15 minutes 30 minutes 60 minutes Dose FAC TC FAC TC FAC TC FAC TC FAC TC mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l FAC Dose as Mixed-Oxidant Solution FAC Dose as NaOCl Table 2: FAC and TC in Test Solution: ph = 9.0; 2 mg/l NH 3 ; Temperature = 50 o C FAC 5 minutes 10 minutes 15 minutes 30 minutes 60 minutes Dose FAC TC FAC TC FAC TC FAC TC FAC TC mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l FAC Dose as Mixed-Oxidant Solution FAC Dose as NaOCl EXPERIMENTAL APPROACH AND RESULTS Trident Technologies conventional chemical water treatment programs for cooling and boiler water systems provide exceptional protection against corrosion, deposit formation and microbiological growths. Nevertheless, always looking for new technologies and approaches to providing improved program performance, Trident decided to assess the effectiveness of mixedoxidant solution as a replacement for oxidizing and non-oxidizing biocides on a cooling tower.

6 6 Initial Testing at an Established Cooling Tower The cooling tower used in the testing was at the time of the testing about 8 years old, is a galvanized, induced draft design with polyfilm fill, and capacity of 700 Refridgeration Tons (RT). The cooling tower owner agreed to the test and the tower and interior surfaces were inspected and videotaped prior to conducting the test. The performance of the cooling tower treatment program was excellent prior to replacing hydantoin with mixed-oxidant solution. The program was under control using a well-established dual biocide and proprietary all organic corrosion/scale inhibitor program. The ph was controlled in the range using H 2 SO 4, and gluteraldehyde and hydantoin biocides were used routinely, establishing a disinfection residual of mg/l free halogen. Under this control program: (1) aerobic bacteria counts as determined by the Easicult TTC dip slide test were normally 10 3 /ml in cooling water with occasional excursions to higher concentrations; (2) some algae growth occasionally occurred in localized areas of the cooling water basin; (3) the water was somewhat cloudy as is normally the case using biocides which have surfactant characteristics and tend to foam; (4) the fill surfaces were in good condition; and (5) corrosion of steel surfaces was of the general type (non-invasive, non-pitting) at a rate of < 2 mils/yr, and corrosion of copper surfaces was at a rate of < 1 mil/yr, both within industry norms. In January, 1997 Trident initiated the evaluation. All biocides (both gluteraldehyde and hydantoin) were taken off-line and a MIOX system was substituted for microbiological control for a period of 6 months. All other maintenance operations remained unchanged, including ph control using H 2 SO 4. The ORP controller set-point was initially set at mv (versus Ag/AgCl with saturated KCl) until it was determined that ORP control was established using the system. The set point was subsequently lowered to 350 mv.

7 7 Over the 6-month test period, Trident personnel observed the following; Weekly aerobic bacteria counts taken in the cooling water basin using the Easicult TTC dip slide test indicated complete sterility (< 10 3 /ml); The FAC residual was easily maintained stable at 0.5 mg/l; No algae growth occurred within the cooling tower basin; no terbutylazine shocking was needed; The water in the basin was crystal clear this had never been observed; enhanced clarity was due in part to the absence of foaming caused normally by the added biocides; No biofilm growth was observed on the cooling surfaces; rather there was evidence for the removal of biofilm supporting scale that had formed in prior operation; and Midway through the 6-month test, corrosion rates on steel and copper coupons were within industry norms (< 2 mils/yr general corrosion on steel and < 1 mil/yr on copper). Midway through the 6-month test, the ph control was removed. All other maintenance conditions remained the same. Over the subsequent 3-month period, the ph of the cooling water climbed slowly but steadily; otherwise, except for corrosion rates discussed below, the observations of the performance of the cooling tower remained identical to those noted above, including that no scale deposition occurred despite increased ph. However, pitting-type corrosion began to appear on steel coupons. With the hypothesis that the mixed-oxidant solution, being a stronger oxidant than the oxidizing biocide used previously, was degrading the all-organic corrosion inhibitors, a new inhibitor package using stabilized inorganic phosphates was substituted for the all-organic program and ph control was reestablished in the ph range. With this change, pitting-type corrosion was eliminated and general corrosion rates on steel coupons were within industry norms (< 2 mils/yr). The azoles used for corrosion control on copper appeared to be unaffected; the copper heat exchange surfaces were in good condition at the start of the test and the mixed-oxidant solution caused no observable change in the surfaces.

8 8 Operation of Two New Cooling Towers In April, 1997, with sufficient experience available from the test of use of a MIOX system in the established cooling tower as described above, the owner agreed to install a similar system instead of conventional oxidizing and non-oxidizing biocides and to use the maintenance protocol and inhibitor program developed by Trident in a test on two new cooling towers. Both towers are stainless and galvanized with polyfilm fill; induced-draft design with capacity 1800 RT. Mixed-oxidant solution, the Trident corrosion inhibitor program and the operating protocol established in the test have been used from the day of startup with findings as of August, 1999, after two years of operation, as follows: FAC concentrations were easily maintained at mg/l using a Lakewood/Osmonics ORP monitor/controller; Despite using no other biocides, weekly Easicult TTC dip slide tests in the cooling water consistently indicate sterile conditions (< 10 3 /ml); No biofilm or scale buildup has been observed on the cooling surfaces and no algae growths have occurred in the cooling water basin; and With the Trident stabilized phosphate inhibitor program in place for corrosion control of steel and use of azoles for corrosion control of copper, corrosion rates on steel and copper surfaces are under control and within industry norms. No pitting-type corrosion has been observed on steel surfaces.

9 9 CONCLUSIONS Operational Benefits Testing of mixed-oxidant solution replacing oxidizing (chlorine or bromine) and nonoxidizing biocides for maintenance of a disinfection residual, first at one established cooling tower and subsequently in the new cooling towers, showed that the mixed-oxidant solution: 1. Eliminated the need for additional biocides; 2. Maintained aerobic bacteria counts in the cooling water at below detection on a standard Easicult TTC dip slide; the ability to maintain low bacteria counts presents advantages in maintaining cooling surfaces free of biofilm; 3. Was easy to use with existing ORP monitor/control instrumentation and maintained a stable FAC residual; 4. Eliminated biofilm buildup on cooling surfaces and algae growths in the cooling water basin; eliminating biofilms also eliminates under-film corrosion by iron- and sulfate-reducing bacteria and eliminates points of collection of scale-forming deposits; and 5. When used in conjunction with Trident s stabilized phosphate program for steel protection and azoles for copper corrosion protection, mixed-oxidant solution did not adversely affect corrosion rates within industry norms on steel and copper. 6. Is cost-competitive with chlorine gas and lower than the cost of sodium or calcium hypochlorite; no hazardous chemicals also means no RMP as required by USEPA and no PSM plan as required by OSHA, reduced safety equipment and training costs, and reduced liability exposure.

10 10 REFERENCES Barton, L., 1996, Disinfection of Simulated Cooling Tower Water, University of New Mexico, Albuquerque, NM, March, Bradford, W.L. and R.I. Cisneros, 1995, "Denitrification of Aqueous Ammonia Solutions Using the MIOX Mixed-Oxidant Solution at Chlorine Concentrations Near Stoichiometric Equivalency with Ammonia Nitrogen", Los Alamos Technical Associates, Inc., Los Alamos, NM, LATA/MX-95/0015 Bradford, W.L., F.A. Baker, and R.I. Cisneros, 1997, Results of Tests Comparing the Disinfection Effectiveness of Mixed-Oxidant Solution and Sodium Hypochlorite on Simulated Cooling Water, LATA/MX-97/0027, Los Alamos Technical Associates, Los Alamos, NM, and MIOX Corporation, Albuquerque, NM, January, Casteel, M.J. M.D. Sobsey, and M.J. Arrowood, 1999, Inactivation of Cryptosporidium parvum Oocysts in Water and Wastewater by Electrochemically Generated Mixed Oxidants, presented at the International Conference on Minimizing the Risk from Cryptosporidium and Other Waterbourne Particles, Paris, France, April 19-23, 1999, International Association on Water Quality, International Water Services Association, and the International Ozone Association. Crayton, C., A. Camper, and B. Warwood, 1997, Evaluation of Mixed-Oxidants for the Disinfection and Removal of Biofilms from Distribution Systems, presented at the Water Quality Technology Conference, Am. Wat. Wks Assoc. Meeting, November 9-12, 1997, Denver, CO. Daniel, E., 1995, Pilot Study/Engineering Report for MIOX On-Sight [sic] Mixed Oxidant Generator for Cash Water Supply Corporation, Cash Water Supply Corporation, Greenville, TX, July, Dowd, M.T., 1994, Assessment of THM Formation with MIOX, Master s Thesis, University of North Carolina, Department of Environmental Sciences and Engineering, School of Public Health, Chapel Hill, NC. Herrington, R.E. and W.L. Bradford, 1997, Pilot Study Report Mixed-Oxidant Disinfection System at Greenfield, Iowa, poster session, Water Technology Conference, American Water Works Association, Denver, Co., November 9-13, Herrington, R.E., W.L. Bradford, and M. Hamman, 1999, Performance of a Conventional Surface Water Plant Using Mixed Oxidants for Microflocculation and Final Disinfection, presented at the American Water Works Association (AWWA) 1999 Annual Conference, Chicago, IL, June 20-24, Kouame, Y. and C.N. Haas, 1991, Inactivation of E. coli by Combined Action of Free Chlorine and Monochloramine, Wat. Res., 25(9):

11 11 Nalepa, C.J., R.M. Moore, G.L. Golson, T.W. Wolfe, and P.R. Pukorius, 1999, A Comparison of Bromine-Based Biocides in a Medium-Size Cooling Tower, CTI Journal, 20(1): Venczel, L.V., M. Arrowood, M. Hurd, and M.D. Sobsey, 1997, Inactivation of Cryptosporidium parvum Oocysts and Clostridium perfringens Spores by a Mixed-Oxidant Disinfectant and by Free Chlorine, Appl. Environ. Microbiol., 63(4):