Purocide 325
2-chloro-4,6-dialkylamino-s-triazine
Manufacturer Name: Bellacide® 325
Manufacturer: FMC Corporation

Impact of Cooling Water Treatment Trends on Algae Control

Jeffrey F. Kramer, FMC Corporation, Princeton, New Jersey

IWC-95-47

Keywords: cooling water, algae, biocide, terbuthylazine, quaternary ammonium compounds

Summary: Several biocides were evaluated in the laboratory for their efficacy against algae under the high pH, alkalinity, and hardness conditions increasingly found in industrial cooling water systems. The algicidal activity of terbuthylazine was not affected by high pH or water hardness levels. Combinations of terbuthylazine and halogen-based biocides proved to be synergistic against algae.

INTRODUCTION

Over the last ten years there have been many changes in the way industrial cooling water systems are treated. Most of these changes have been the result of stricter health and safety standards and environmental regulations. For example, chromate-based treatment programs have almost completely disappeared due to a hazardous chemical listing and restrictions in chromate discharge imposed by the U.S. Environmental Protection Agency (EPA).¹

Since chromate had been under environmental scrutiny for many years, there was a gradual conversion to alternative treatment programs such as phosphate-based programs in the late 1970's. Phosphate-based programs operate between pH 7.0-8.0 and thus require acid feed to maintain pH in the proper range. The early phosphate-based programs were not tolerant of pH upsets and thus required constant monitoring for optimal performance Furthermore, the storage of concentrated acid is regulated under the Emergency Planning and Community Right-to-Know Act (Title III) of the Superfund Amendments and Reauthorization Act of 1986 (SARA).² This law requires facilities to report on hazardous and toxic chemicals present in the community and prepare emergency plans in the event of a spill.

With many companies undergoing re-engineering in the 1990's, the number of people involved in water treatment has declined which makes phosphate-based programs less attractive due to their labor intensive nature. In addition, the reporting requirements under SARA Title III associated with using acid for pH control are an added cost to phosphate-based programs. The need for a low maintenance program which eliminates the need to feed acid has led to the development of the so called all-organic programs.

The most significant result of the introduction of all-organic programs has been the operation of systems at increased cycles of concentration resulting in cooling water with higher pH, alkalinity, and hardness levels than in the past. The resulting alkaline cooling water conditions have created an environment that is more conducive to microbiological growth. This is of concern since research indicates that microbiological fouling may be a precursor to corrosion, scaling, and deposition processes in industrial cooling water systems.^3,4 Algae also thrive under alkaline cooling water conditions. Algae not only contribute to microbiological fouling but they can supply nutrients to bacteria and act as a demand for biocides.

It is important to understand bow these conditions impact the effectiveness of traditional cooling water biocides such as chlorine, bromine, and quaternary ammonium compounds when used for algae control. It is also desirable to know how algae specific inhibitors like terbuthylazine function under these conditions and if they complement the traditional cooling water biocides in any way. The results of these studies are discussed in this paper.

EXPERIMENTAL

ALGAE CULTURES
The following algae were used in this study: Chlorella vulgaris UTEX 26 and Phormidium inundatum UTEX LB 2517. Stock cultures were maintained in a synthetic algal nutrient medium.⁵ The medium was dispensed in 100-ml aliquots into 250-ml shake flasks. The flasks were capped and sterilized at 121°C (250°F) for 20 minutes. Once the medium had cooled to room temperature, it was inoculated with one milliliter of an algae culture. Stock cultures were incubated at 24°C (75°F) with 16 hours of cool white fluorescent light (4306 lux) per day for two to three weeks. Flasks were continuously shaken at approximately 100 oscillation per minute. When good growth was evident the flasks were removed from the shaker and used as inoculum or moved to low light conditions for storage.

MINIMUM INHIBITORY CONCENTRATION (MIC) TESTING
MIC tests were conducted in the same synthetic algal nutrient medium used for stock culture maintenance. The medium was used as is for tests conducted at pH 7.5. The medium was supplemented with sodium bicarbonate at 0.5 grams per liter for tests conducted at pH 8.5. The total hardness level of the medium was adjusted by the addition of appropriate amounts of calcium chloride and magnesium sulfate stock solutions (37,850 ppm calcium or magnesium as calcium carbonate) in a 2 to 1 ratio.

Each flask was inoculated with one milliliter each of a culture of Chlorella vulgaris and Phormidium inundatum. Biocide was added to the flask in an amount calculated to give the desired biocide concentration. Halogen levels, expressed as free residual chlorine, were verified by the DPD method using a Hach DR/3000 spectrophotometer.⁶ Flasks were incubated for three weeks under the same conditions used for stock culture maintenance. Halogen-based biocides were redosed on a weekly basis during the test. Stock solutions of halogen-based biocides were prepared fresh the day of use. Flasks were inspected at the end of the test for algal growth. A rating scale of 0-3 with "0" being no visible growth, and "3" being heavy growth was used to determine the level of inhibition by the biocide. The minimum inhibitory concentration is reported as the lowest concentration of biocide which results in a clear flask (no visible algal growth).

FOAM TESTING
Biocides were evaluated for their tendency to produce foam. Biocides were added to deionized water in an amount calculated to give the desired biocide concentration. Fifty milliliters of the mixture was added to a 100-ml stoppered graduated cylinder. The graduated cylinder was vigorously shaken for 30 seconds. Immediately after the shaking was stopped the foam height above the liquid was measured.

COMPATIBILITY TESTING
The compatibility of biocide with common scale and deposit control additives used in all organic programs was determined using a dynamic tube-blocking apparatus.⁷ In the tube-blocking apparatus a synthetic cooling water, generated by mixing separate cation and anion solutions, is pumped at a flow rate of 1 meter per second through a coiled 1.1 mm ID 316 stainless steel capillary tube immersed in a water bath at 50°C (122°F). Any deposition reduces the bore of the tube and causes an increase in pumping pressure. The pressure change in the capillary tube is monitored by a pressure transducer and plotted against time on a chart recorder.

The synthetic cooling water had the following composition: calcium, 300 ppm as calcium; magnesium, 90 ppm as magnesium; carbonate, 102 ppm as carbonate; bicarbonate, 538 ppm as bicarbonate; pH 8.4. The synthetic cooling water had a LSI of 2.8. Biocide was added to the cation solution and additive was added to the anion solution prior to mixing.

RESULTS AND DISCUSSION

With the trend toward operating industrial cooling water systems at higher cycles of concentration, the effect of pH on the algicidal activity of various halogen-based biocide programs was initially determined. Sodium hypochlorite, liquid bromine, and bromo-chloro-dimethylhydantoin (BCDMH) were evaluated for their ability to inhibit algal growth at both pH 7.5 and 8.5 when dosed once per week. 'Me algae specific inhibitor terbuthylazine (TBZ) was also tested but was dosed only once at the start of the test. The results of this test are shown in Table 1.

BCDMH was the most effective halogen-based biocide at both pH 7.5 and 8.5. At a free residual of 0.3 ppm, BCDMH significantly inhibited algal growth at pH 7.5 and gave complete inhibition at pH 8.5. Chlorine was less effective requiring a free residual of 0.6 ppm to completely inhibit algal growth. The activity of chlorine was not affected by pH. This result is surprising since as pH increases the concentration of hypochlorous acid, the more active form of chlorine, decreases.⁸ Bromine was ineffective at the levels tested. This has significant implications for algae control in large industrial cooling water systems which operate above pH 7.5, since bromine is typically the halogen-based biocide of choice in these systems.⁹ Low levels of TBZ showed excellent algae inhibitory properties in this test. TBZ was more effective at pH 8.5 that at pH 7.5.

Since TBZ has only algicidal activity, it is always used in conjunction with an oxidizing or non-oxidizing biocide as part of a total microbiological control program. Over the years TBZ has been used in combination with a wide variety of oxidizing and non-oxidizing biocides. These TBZ/biocide combinations have shown exceptional ability to remove and control the regrowth of algae.¹⁰ Based on these reports we were curious to see if TBZ would improve the performance of the halogen-based biocides, especially bromine, against algae. In these tests, flasks were treated with a single dose of TBZ in combination with weekly doses of the halogen-based biocide. The results of this test are presented in Table 2.

The halogen-based biocide/TBZ combinations showed improved algae inhibitory properties compared to the use of each biocide alone (compare the results in Table 1 and Table 2). In all cases the level of halogen-based biocide required to inhibit algal growth was reduced by the addition of as little as 0.1 ppm active TBZ. The synergistic effect of the halogen-based biocide/TBZ combinations against algae was most pronounced with chlorine and bromine (compare the amount of algae growth in Figure 1 and Figure 2). The level of chlorine necessary to inhibit algae growth was reduced from 0.6 ppm to 0.1 ppm at pH 7.5 and 0.2 ppm at pH 8.5. TBZ significantly improved the activity of bromine against algae reducing the amount necessary for control from greater than 0.6 ppm to 0.2 ppm at pH 7.5 and 0.3 ppm at pH 8.5. The results indicate that the addition of TBZ can enhance the effectiveness of halogen-based biocide programs against algae.

Quaternary ammonium compounds are used extensively for the control of algae in industrial cooling water systems. For this reason, they were also evaluated for their ability to inhibit algae growth versus pH. Two different quaternary ammonium compounds were evaluated; one based on alkyl (C₁₂ 40%, C₁₄ 50%, C₁₆ 10%) dimethylbenzylammonium chloride (ADBAC) and one based on poly[oxyethylene(dimethyhmino)-ethylene-(dimethylimino)-ethylene] dichloride (polyquat). The results of this test are presented in Table 3.

Both ADBAC and polyquat showed excellent algae inhibitory properties. Algae growth was completely inhibited by as little as 1 ppm active (the lowest concentration tested). The inhibitory properties of both ADBAC and polyquat were not affected by pH.

In addition to the increased pH and alkalinity levels, increased hardness levels would also result from the operation of industrial cooling water systems at higher cycles. It is well known that the biocidal activity of quaternary ammonium compounds is adversely affected by had water.¹¹ Therefore, the effect of water hardness on the biocidal activity of the quaternary ammonium compounds was determined. Since the synthetic algal nutrient medium used in the previous test has essentially zero hardness, it was supplemented with hardness ions to make up media of varying hardnesses for this test. TBZ was also evaluated in this test. The results of this test are shown in Table 4.

The MIC of TBZ against algae was not affected by hard water. TBZ was slightly inhibitory at 0.3 ppm active and gave complete inhibition at 0.5 ppm active even in extremely hard water. ADBAC and polyquat differed greatly in their level of hard water tolerance. While ADBAC appeared to be very tolerant of hard water conditions, the activity of polyquat decreased rapidly as hardness levels increased. This may be due to the higher charge density of the polyquat resulting in a decreased electrostatic association between the polyquat and the algal cell in the presence of hardness ions.¹²

Although the algae inhibitory properties of ADBAC were not adversely affected by pH or water hardness, quaternary ammonium compounds of this type are known to cause foaming problems in cooling towers which can limit their usefulness.¹³ For this reason, the foaming tendency of ADBAC, polyquat, and TBZ was compared. The results of this test are shown in Table 5.

Neither TBZ or polyquat produced any foam in this test. By contrast, ADBAC produced a significant amount of foam. In addition, the foam produced was persistent, taking several minutes to completely disappear. The results indicate that TBZ has several advantages over the use of quaternary ammonium compounds for algae control.

Anionic scale and deposit control additives are widely used in industrial cooling water systems. Quaternary ammonium compounds, due to their cationic nature, can potentially react with these additives resulting in a loss of additive performance. For industrial cooling water systems operating at high cycles, such an interaction can mean disastrous results. For this reason, the affect of polyquat on the calcium carbonate deposition control properties of a common polyacrylate-based scale inhibitor was determined. The testing was conducted in a dynamic tube-blocking apparatus as depicted in Figure 3. TBZ was also evaluated in this test. The results of this test are shown in Figure 4.

Polyquat adversely affected the performance of the polyacrylate-based scale inhibitor in the dynamic tubeblocking test presumably due to electrostatic interaction. The calcium carbonate deposition control properties of the polyacrylate-based scale inhibitor was reduced by 50% by the addition of 5 ppm active polyquat. TBZ bad no affect on the performance of the polyacrylate-based scale inhibitor. This result is not surprising since TBZ is non-ionic.

CONCLUSIONS

The results indicate that terbuthylazine is well suited for algae control in today's cooling water environment. It has advantages over other cooling water biocides commonly used for the control of algae such as effectiveness at high pH and hardness levels, non-foaming properties, and compatibility with anionic scale and deposit control additives. Terbuthylazine is synergistic with chlorine, bromine, and BCDMH which reduces the overall amount of biocide necessary to control algae.

REFERENCES

1. Henley, M., Industrial Water Treatment, 27 14-7 (1995)
2. 40 CFR Parts 355, and 370-374.
3. Characklis, W.G., Biotechnology and Bioengineering, 22 1923-8 (1981).
4. Obuekwe, C.O., C.W.S. Westlake, F.D. Cook, and J.W. Costerton, Appl. Environ. Microbiol., 41 766-71 (1981).
5. Schneider, B.A., "Pesticide Assessment Guidelines Subdivision G: Product Performance", p.127, U.S. Environmental Protection Agency, Washington, D.C. (1982)
6. American Public Health Association, "Standard Methods for the Examination of Water and Wastewater", p. 292, American Public Health Association, Washington, D.C. (1980)
7. Wilson, D., Norwegian Society of Chartered Engineers, Oilfield Chemicals Symposium, Geilo, Norway (1991)
8. Kemmer, F.N., "The NALCO Water Handbook", p.22.8, McGraw-Hill Book Company, New York, New York (1988)
9. Giusto, M., Industrial Water Treatment, 23 33-9 (1991)
10. Smith, A.L., R.A. Muia and M.O. Clancy, Cooling Tower Institute, TP 88-15 (1988)
11. Block, S.S., "Disinfection, Sterilization, and Preservation", p. 237, Lea & Febiger, Philadelphia, Pennsylvania (1991)
12. Ibid, p. 231
13. Sweeny, P.G. and F.J. Himpler, Corrosion 94, Paper No. 450 (1994)

Figure 1 - Not reproduced due to the poor quality of our copy

Figure 2 - Not reproduced due to the poor quality of our copy

Figure 3 - Dynamic Tube-Blocking Apparatus

Figure 4 - Effect of Biocide on the Calcium Carbonate Deposition Control of a Polyacrylate-Based Scale Inhibitor

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