Classifications

• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/50—Treatment of water, waste water, or sewage by addition or application of a germicide or by oligodynamic treatment
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/72—Treatment of water, waste water, or sewage by oxidation
  • C02F1/76—Treatment of water, waste water, or sewage by oxidation with halogens or compounds of halogens
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F1/00—Treatment of water, waste water, or sewage
  • C02F1/72—Treatment of water, waste water, or sewage by oxidation
  • C02F1/76—Treatment of water, waste water, or sewage by oxidation with halogens or compounds of halogens
  • C02F1/766—Treatment of water, waste water, or sewage by oxidation with halogens or compounds of halogens by means of halogens other than chlorine or of halogenated compounds containing halogen other than chlorine
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2209/00—Controlling or monitoring parameters in water treatment
  • C02F2209/06—Controlling or monitoring parameters in water treatment pH
• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F2303/00—Specific treatment goals
  • C02F2303/20—Prevention of biofouling

Definitions

• the present invention relates, generally, to a method of controlling microbial fouling in an aqueous system. More particularly, the present invention relates to a method of eliminating microorganisms in an aqueous system, which is based on a detachment of slime formed in the aqueous system using a chlorine biocide, a sulfamate ion source and a water-soluble bromide ion source.
• Water systems such as cooling towers installed in factories or buildings, have the most optimal environment for the growth and proliferation of microorganisms.
• pathogens such as Legionella bacteria may be rapidly spread, and all people coming in and out the building are thus exposed to, especially, the potentially lethal Legionella.
• microbial contamination often leads to the formation of slime in cooling towers.
• slime gives rise to significant problems, as follows: cooling water velocity drops and heat transfer efficiency is reduced resulting in energy loss; oxygen is depleted under slime layers, and anaerobic bacteria cause pitting corrosion on heat exchanger surfaces which leads to heat exchanger replacement .
• microbicides are used in aqueous systems that are liable to contamination with microorganisms, such as cooling towers.
• oxidizing chlorine biocides are widely used for economic benefits.
• chlorine biocides have several problems such as reduced effects on control of the growth of microorganisms in the water contaminated by ammonia, air pollution and offensive odor problem by high volatility, increased corrosivity with increased concentration in water, etc.
• expensive non- oxidizing biocids such as isothiazolone can be used, which have low corrosiveness and excellent effects on control of microorganisms when it is used above MIC (Minimum Inhibition Concentration).
• Disinfection efficiency of microbiocides applied to aqueous systems is typically analyzed by adding a microbiocide to an aqueous system, collecting water from the aqueous system, performing microorganism culturing and counting the cell number.
• microorganisms are detected at low levels, slime often forms in aqueous systems. This phenomenon is induced by mainly sessile bacteria.
• microorganisms are, according to their habitats, classified into planktonic bacteria freely floating in water and sessile bacteria attached to surfaces.
• the conventional method cannot easily detect sessile bacteria directly participating in the production of slime, while detecting only planktonic bacteria.
• slime is responsible for many problems associated with microbial contamination. For example, when a microbicide is applied to cooling towers to eliminate Legionella bacteria most lethal in public buildings, only suspending bacteria are killed, while bacteria present in slime remain viable. Therefore, diverse bacteria inhabiting in slime proliferates again with time.
• several problems such as unpleasant odor and damaged beauties originate mostly from slime. To date, such slime was removed by using high-pressure water after periodically stopping operations of aqueous systems.
• such a physical method is very cumbersome and costly, especially for large chemical factories because of the reduced operation time.
• An alternative method for removing slime without stopping operations of aqueous systems is to add excessive chlorine to aqueous systems.
• high concentrations of chlorine causes significant problems such as increase of corrosion rates and generation of unpleasant odor, and thus use of chlorine is actually impossible.
• removal of slime may be achieved by continuous use of non-oxidizing biocides such as isothiazolone of over MIC, but this method is uneconomical.
• sulfamic acid a white crystal that is not volatile, not moisture-absorptive and odorless, is used for metal cleaning, descaling, etc. due to its property of being a strong acid when dissolved in water.
• Sulfamate salts are used for preparation of flame retardants or weed-controlling agents, and also utilized as a stabilizer for chlorine.
• sulfamic acid is used in the form of being dissolved in water, that is, an acidic aqueous solution.
• the sulfamic acid solution has a remarkably lower corrosiveness to metals than hydrochloric acid and sulfuric acid solutions. This relatively weak corrosiveness of the sulfamic acid solution results from that sulfamic acid reacts with most metals to produce sulfamate salts with a low corrosiveness to metals.
• sulfamic acid is an amphoteric compound that can act as either an acid or a base, and, thus, its nature changes according to pH. Further, sulfamic acid easily reacts with other compounds. However, sulfamic acid's nature was not completely identified. When sulfamic acid is used as a stabilizer for chlorine, volatility of chlorine is greatly reduced, whereas its rapid-acting disinfection efficacy is reduced. Such reduced disinfection efficacy of chlorine upon use of sulfamic acid is described by Stuart et al., "Swimming Pool Chlorine Stabilizers" Soap and Chemical Specialties, Aug. 1964.
• hypochlorite free residual chlorine level of 0.6 ppm
• sulfamic acid is used as a stabilizer in low concentrations (0.5 to 1.0 ppm)
• disinfection efficacy of hypochlorite is rarely reduced, whereas hypochlorite almost loses its disinfection efficacy when the concentration of sulfamic acid is increased over the range (25 to 50 ppm).
• Efforts to use chlorinated sulfamic acid as a microbicide were made, as described in U.S. Pat. No. 3,328,294 granted to Self et al., U.S. Pat. No.
• hypobromite is produced by, (1) in case of directly using chlorine gas, reacting chlorine gas and water to generate hypochlorite and adding a bromide salt solution to the hypochlorite solution; or by, (2) in case of using hypochlorite, adding a bromide salt solution to a hypochlorite solution.
• Use of such combination of chlorine and bromide for controlling the microbial growth is also described in U.S. Pat. No. 3,795,271 granted to Saunier et al. In particular, as indicated by Trulear et al., "Recent Advances in Halogen Based Biocontrol.” Corrosion, 1988, Vol.
• hypobromite is produced immediately before being applied to the cooling towers by injecting a sodium bromide solution into a supply line of a hypochlorite solution in a prescribed molar ratio.
• This application method of hypobromite is generally used to improve the production yield of hypobromite. If there is no mention that a bromide ion solution should be directly added to the cooling towers, hypobromite is produced immediately before being applied to cooling towers by reacting hypochlorite with a sodium bromide solution.
• the '852 patent indicates that sulfamic acid should be used in higher amounts than a bromide ion.
• sulfamic acid is used as a stabilzer, as described in patents granted to Dallmier et al., including U.S. Pat. Nos. 5,683,654, 5,795,487, 5,942,126, 6,136,205, etc., in which stabilized bromine is produced by primarily reacting a chlorine oxidant, that is, hypochlorite with a water-soluble bromide ion to generate hypobromite and reacting the hypobromite with sulfamic acid.
• the stabilized bromine that is, bromosulfamate is limited in use economically although having higher disinfection efficacy than chlorosulfamate.
• biocides comprising a bromide ion and stabilized chlorine instead of the stabilized bromine are disclosed in U.S. Pat. No. 6,037,318 granted to Na et al., U.S. Pat. No. 6,478,972 Bl (hereinafter, referred to as simply '972 patent) granted to Shim et al., and International Pat. Application No. KR03/00423.
• hypobromite is produced by primarily reacting hypochlorite with sulfamic acid and reacting the generated stabilized chlorine with a bromide ion in an aqueous system.
• U.S. Pat. No. 6,110,387 (hereinafter, referred to as simply '387 patent) describes a method for stabilizing bromine biocides in water, comprising adding a sufficient amount of a sulfamate source (0.25 to 2 millimoles per liter) and a sufficient amount of a bromide ion (0.34 to 2 millimoles per liter) to a swimming pool and then periodically introducing a chlorine oxidant to maintain an available bromine concentration in the range of 2 to 6 ppm in the swimming pool.
• a sufficient sulfamate ion and a sufficient bromide ion is supplied to form stabilized bromine in water.
• the '387 patent does not mention the nature of the produced biocidal bromine species.
• hypobromite is used as a biocide
• decomposition of phosphonates such as 1,1-hydroxyethylidene diphosphonic acid (HEDP) is increased to an about two-fold degree, compared to the case of using hypochlorite as a biocide.
• HEDP 1,1-hydroxyethylidene diphosphonic acid
• sulfamic acid as a stabilizer for hypobromite can reduce the decomposition of the phosphonates to the level of the case of using hypochlorite as a biocide.
• Biocidal activity of chlorosulfamate produced by reaction of hypochlorite with sulfamic acid is described in detail by Delaney et al., "Bactericidal Properties of Chlorosulfamates", Proceeding of the American Society of Civil Engineers,
• chlorosulfamates penetrate into biofilms in the degree comparable to chloride ions, and can easily penetrate biofilms unlike general oxidizing biocides due to their low reactivity to biofilms, indicating their use as biocides for biofilm control. Microbial contamination of aqueous systems can be also prevented, as described in U.S. Pat. No.
• Fig. 1 is a graph showing dissociation of hypochlorite according to pH.
• a method of controlling microbial fouling in an aqueous system comprising adding a chlorine oxidant, a sulfamate ion source and a water-soluble bromide ion source to the aqueous system having a pH of 5 to 10 in an amount maintaining a total residual chlorine concentration of 1 to 9 ppm, in an amount maintaining a sulfamate ion concentration of 0.01 to 0.2 mmole/L (millimole per liter) and in an amount maintaining a water-soluble bromide ion concentration of 0.005 to 0.125 mmole/L, respectively, wherein the chlorine oxidant and the sulfamate ion source are used in a molar ratio of 1 :20 or less.
• a method of killing microorganisms in an aqueous system comprising the steps of adding a chlorine oxidant, a sulfamate ion source and a water-soluble bromide ion source to the aqueous system having a pH of 5 to 10; killing planktonic bacteria in the aqueous system by hypochlorite produced by the chlorine oxidant added to the aqueous system and/or hypobromite produced by reaction of the produced hypochlorite with the water-soluble bromide ion source added; detaching slime formed on a surface of an equipment or an apparatus in the aqueous system from the surface and dispering the slime by chlorosulfamate produced by reaction of the hypochlorite produced in the aqueous system with the sulfamate ion source added and/or bromosulfamate produced by reaction of the chlorosulfamate with the water-soluble bromide ion source added
• Chlorosulfamate is produced by reaction of hypochlorite with sulfamate ion according to the following reaction formula. NH 2 SO 3 " ⁇ NHClSO 3 - + H 2 O NHClSOs " ⁇ NCl 2 SO 3 - + H 2 O
• hypochlorite and sulfamic acid were, in the present invention, reacted at various ratios and then added to an aqueous system, or individually added directly to the aqueous system and reacted therein.
• chlorosulfamate showed decreased microbicidal activity due to the higher production of monochlorosulfamate than dichlorosulfamate, whereas it retained the effects of penetrating into slime formed in the aqueous system and inhibiting microorganisms to adhere a submerged surface.
• bromide ion is reacted With hypochlorite to produce hypobromite before being added to an aqueous system, the bromide ion is not detected in the aqueous system.
• bromide ion when the bromide ion is directly added to an aqueous system, the bromide ion still exists in the aqueous system even upon being added in smaller amounts than hypochlorite.
• the residual bromide ion penetrates into slime and reacts with chlorosulfamate therein, leading to detachment of the slime by forming bromosulfamate.
• a chlorine oxidant, a sulfamate ion source and a water-soluble bromide ion source are introduced into an aqueous system having a pH of 5 to 10, preferably 6.5 to 9.5, several oxidizing biocides having microbicidal activity are produced, including hypobromite, chlorosulfamate and bromosulfamate produced by reaction of chlorine with a bromide ion, reaction of hypochlorite with sulfamate and reaction of hypobromite with sulfamate, respectively.
• the method of the present invention is applied to an aqueous system having a pH of 5 to 10, and preferably, 6.5 to 9.5.
• the pH of an aqueous system to which the present method is applicable that is, the degree of dissociation of hypochlorite affects production of hypobromite, chlorosulfamate and bromosulfamate in the aqueous system.
• hypochlorite in a pH of below 5.0, hypochlorite is present as HOC1 and Cl 2 , while being just as OC1 " in a pH of over 10. Therefore, in an aqueous system containing hypochlorite, sulfamate ion and bromide ion, predominant biocides vary according to pH of the aqueous system, which may have different properties. In detail, in a pH of below 5, the reaction of the hypochlorite with the bromide ion is stimulated, and hypobromite production is thus increased, leading to increase of bromosulfamate production.
• the produced biocide has a a high oxidizing power, but is poor in penetrating into slime.
• a pH of over 10 the reaction of the hypochlorite with the water-soluble bromide ion is inhibited, and, thus, chlorosulfamate with low oxidizing power is mainly produced.
• the produced biocide has an excellent effect on penetration into slime, but have a poor microbicidal effect. Therefore, a pH range suitable for the method of the present invention is 5 to 10, and preferably, 6.5 to 9.5.
• amounts of the chlorine oxidant, sulfamate ion source and water-soluble bromide ion source, used in the method of the present invention are determined taking into consideration their effect on inhibition of slime attachment to a submerged surface and microbicidal effect in an aqueous system to be treated.
• the chlorine oxidant is preferably used in an amount maintaining a total residual chlorine level of 1 to 9 ppm. In case that the total residual chlorine level is below 1 ppm, free residual halgen with a high oxidizing power is produced in relatively low amounts even in the presence of the bromide ion source, and, thus, biocides with low biocidal activity are mainly produced.
• sulfamate when sulfamate is added in very high amounts or the content of the sulfamate becomes higher than that of chlorine with time due to the consumption of chlorine by reaction of chlorosulfamate produced in an aqueous system with organic materials, biocides with low biocidal activity are mainly produced, where they retain an inhibitory activity against slime attachment to a submerged surface of an equipment or apparatus in the aqueous system. Therefore, it is sufficient that sulfamate is added in an amount of 0.2 mmole/L or less in an aqueous system.
• the bromide ion source is added in an amount maintaining a water-soluble bromide ion concentration of 0.005 to 0.125 mmole/L. In case that the bromide ion concentration is below 0.005 mmole/L, free residual halogen with a high oxidizing power is formed at low levels, resulting in insufficient disinfection.
• hypobromite production is increased in an aqueous system, and, thus, production of bromosulfamate with a high oxidizing power is elevated, resulting in increase of biocide consumption as well as discharging of a large quantity of unreacted bromide ions to blow-down water. Therefore, this case is uneconomical and contrary to the present object to inhibit microorganisms to adhere a submerged surface and detach slime formed on the surface.
• the chlorine oxidant and sulfamate ion source are introduced into an aqueous system at a molar ratio of 1:20 or less.
• the sulfamate ion source When the sulfamate ion source is used above the range, that is, over 20 in the molar ratio, the resulting biocides have reduced biocidal activity.
• biocidal properties of chlorosulfamate produced by reaction of hypochlorite with sulfamic acid are described by Delaney et al., "Bactericidal Properties of Chlorosulfamates" Proceeding of the American Society of Civil Engineers, Journal of the Sanitary Engineering Division Feb. 1972 (pp23). According to the report, the chlorosulfamate concentration capable of killing 99% of E. coli in a pH ranging from 7 to 8 is 1,000 mg/L (1,000 ppm).
• the present invention limits available concentrations of total residual chlorine to 1 to 9 ppm through several experiments due to the following reasons: when the chlorine concentration is less than 1 ppm, free residual halogen with a high oxidizing power is formed at low levels even under the presence of a bromide ion source, resulting in insufficient disinfection; and, when the level of total residual chlorine is over 9 ppm, free residual halogen with a high oxidizing power is produced at high levels under the presence of a bromide ion source, resulting in improved disinfection.
• the concentration of the bromide ion source suitable for the object of the present invention limiting the concentration of the total residual chlorine to 1 to 9 ppm, ranges from 0.005 to 0.125 mmole/L, which is equal to 0.4 to 10 ppm when expressed in weight.
• the present invention has an advantage, as follows: a sulfamate ion solution is prepared as by neutralization of sulfamic acid, and a water-soluble bromide ion solution is prepared using sodium bromide (NaBr), where each solution may be used in combination with components required for the prevention of corrosion and scale formation, while hypochlrorite is directly applicable to an aqueous system to be treated according to conventional methods without modification, if its amount to be added is determined.
• NaBr sodium bromide
• the chlorine oxidants may be selected from among chlorine, sodium hypochlorite, potassium hypochlorite, lithium hypochlorite, magnesium hypochlorite or calcium hypochlorite, trichloroisocyanuric acid, sodium dichlorocyanuric acid, dichlorohydantoin, and mixtures thereof, and sodium hypochlorite and chlorine are preferred.
• the sulfamate ion source useful in the present invention may be selected from the group consisting of sulfamic acid or salts thereof, such as sodium sulfamate, calcium sulfamate, ammonium sulfamate, and mixtures thereof, and sulfamic acid is preferred.
• the sulfamate salts are prepared by neutralization sulfamic acid with a base.
• the bromide ion source useful in the present invention may include sodium bromide, calcium bromide, lithium bromide, chlorine bromide and bromine, and sodium bromide is preferred.
• the corrosion inhibitors may include an anodic corrosion inhibitor, such as chromate, nitride, orthophosphate, silicate or molibdate, a cathodic corrosion inhibitor such as zinc, polyphosphate or phosphonate, and a copper corrosion inhibitor, such as mercaptobenzothiazole, benzothiazole, or tolyltriazole.
• organophosphates and acryl polymers as the scale inhibitor.
• the organophosphates are exemplified by triethanolamine phosphate (TEAP), aminotrimethylene phosphonic acid (AMP), l-hydroxyethylidene-1,1- diphosphonic acid (HEDP), 2-phosphonobutane-l,2,4-tricarboxylic acid (PBTC), etc.
• TEAP triethanolamine phosphate
• AMP aminotrimethylene phosphonic acid
• HEDP l-hydroxyethylidene-1,1- diphosphonic acid
• PBTC 2-phosphonobutane-l,2,4-tricarboxylic acid
• acryl polymers may include acrylic homopolymers, acrylic copolymers, and acryl terpolymers.
• the method of the present invention can be applied to any aqueous system using water.
• the aqueous system to which the method of the present invention can be applied includes, but are not limited to, cooling towers of buildings or factories, industrial water systems such as water systems used in paper-making processes, wastewater recycling systems or gas washer systems, freshwater systems using reverse osmosis membranes, gas scrubber systems, ponds and water slides.
• industrial water systems such as water systems used in paper-making processes, wastewater recycling systems or gas washer systems, freshwater systems using reverse osmosis membranes, gas scrubber systems, ponds and water slides.
• Sodium hypochlorite (NaOCl) solution was used as a biocide in the present Examples, which were added to an aqueous system, and an effective amount of chlorine was measured as 12 % by the DPD-F AS method.
• a 0.1% NaOCl solution was prepared by dilution. After an oxidizing halogen biocide was added to the aqueous system, the change of free residual halogen and total residual halogen concentrations with the passage of time was measured by the DPD-F AS method.
• the free residual halogen indicates HOC1, OC1 " , HOBr or OBr " , and the level of combined residual halogen reacted with ammonia-containing organic materials was calculated by subtracting the free residual halogen level from the total residual halogen level.
• microorganism populations were counted using 3M Petrifilm (aerobic count plate) and percentage viability (viability %) of the microorganisms was calculated by an equation of (initial number-viable number)/initial number.
• sample No. 2 treating with the biocidal composition including a sulfamate salt, free residual chlorine was not detected, while only combined residual chlorine was detected. In sample No. 3, trace amounts of free residual halogen were detected. Compared to sample No. 1 treated with only hypochlorite, sample No. 2 treated additionally with the sulfamate salt showed remarkably increased microbial viability, whereas retaining a total residual chlorine concentration of 1.3 ppm even after one day. In the sample 3 treated with the biocial composition including the bromide ion, total residual chlorine was maintained to about 1.1 ppm even after one day, as well as biocidal ability was improved in comparison with sample No. 2. In sample No.
• EXAMPLE 1 Microbial viability and inhibition of slime attachment to a submerged surface according to various concentrations of sulfamate ion
• Beaker tests were performed to investigate microbial viability and inhibition of slime formation when water is treated with hypochlorite and bromide ion along with sulfamate ion of various concentrations.
• an organophosphate, 2-phosphonobutane-l,2,4-tricarboxylic acid (PBTC), and a polymer were added to water in amounts of 6 and 10 ppm, respectively.
• the water was adjusted to pH 8+0.2, and maintained at 30 ⁇ 2°C.
• River water bacterial density: 1,500,000 CFU (colony forming unit)/ml
• was used in this test which contained 40 ppm of calcium hardness (based on calcium carbonate) and 22 ppm of M-alkalinity (based on calcium carbonate).
• the river water was agitated at 30 rpm during this test, and covered with a vinyl product to prevent evaporation of water while being not completely closed to allow for air ventilation.
• a carbon steel coupon of 5x2x0.2 cm was dipped into the water in each beaker while hanged by a thread, and washed with acetone and dried before use.
• the degree of slime attachment to the carbon steel coupon was evaluated by visual inspection, and expressed as five grades (1 : no detection of attached slime; 5: very high slime attachment).
• Total residual halogen concentrations and the degree of the slime formation on the carbon steel; and microbial viability according to various concentrations of the sulfamate ion are given in the following Tables 2 and 3, respectively.
• chlorosulfamate which serves as an oxidizing biocide, is reduced in its oxidizing power, that is, its biocial activity against microorganisms because of a decrease in active chlorine concentrations, caused by the reaction of the chlorosulfamate with microorganisms or other organic materials.
• EXAMPLE 2 Microbial viability and inhibition of slime attachment to a submerged surface according to various concentrations of the bromide ion
• EXAMPLE 3 Microbial viability and inhibition of slime attachment to a submerged surface according to various concentrations of hypochlorite
• water was treated with various concentrations of hypochlorite along with a sulfamate ion and bromide ion according to the same method as in the Example 1, and microbial viability and the degree of slime attachment to the carbon steel coupons were measured.
• the results are given in Tables 6 and 7, below.
• EXAMPLE 4 Microbial viability and inhibition of slime attachment to a submerged surface according to pH
• pilot cooling tower tests were conducted.
• a pilot cooling tower was prepared, which has a water capacity of 120kg and a water flow rate of 1,600 kg/hr.
• the pH of water was controlled within +0.2 from the target pH value.
• the temperature difference through the cooling tower was 5 ° C .
• the cycle of concentration was maintained at 6 by controlling below-down amount of water to 2.8 kg/hr.
• PBTC and a polymer were continuously added to the water to a level of 6 and 10 ppm, respectively, to prevent corrosion and scaling of the carbon steel slices.
• the pilot cooling tower was maintained at 35+2°C.
• river water was used, which had 40 ppm of calcium hardness (based on calcium carbonate) and 22 ppm of M-alkalinity (based on calcium carbonate) and contained microorganisms of 1.8xl0 b CFU/ml.
• the pH of the river water was adjusted to the test range with diluted sulphuric acid and caustic soda solution.
• the turbidity of the water was measured using the Hach's DR- 2010, and expressed as FAU (Formazin Attenuation Units).
• Sulfamate ions and bromide ions were initially added in prescribed amounts to the water, and then continuously added to the water individually in combination with PBTC and the polymer in order to maintain desired concentrations in the water, taking into consideration the amounts of their loss. Hypochlorite was continuously injected into the water by a pump, based on the water flow rate. In order to investigate slime removal from the pilot cooling tower, first, a corrosion inhibitor and a scale inhibitor were added to the water for four days to allow slime formation in the pilot cooling tower. Typically, when a non-ionic dispersing agent or an excessive biocide is introduced into an aqueous system with high microbial contamination, slime is removed while the turbidity of the aqueous system is increased.
• biocial compositions were added to the pilot cooling tower, and the water was evaluated for change in turbidity, change in residual halogen concentrations and microbial viability.
• slime removal from the pilot cooling tower was determined by directly investigating the degree of microbial contamination in the pilot cooling tower.
• the results are given in Table 10, below, in which the results are expressed as three grades: 1: no detection; 2: a little; and 3: plenty.
• the grade 1 means the case in that no slime is detected in the pilot cooling tower by visual examination and by hands.
• the grade 2 means the case in that no slime is visually detected while slime is sensed by hands.
• the grade 3 means the case in that slime is visually detected. TABLE 10 Water turbidity, the number of viable microorganisms, the degree of slime removal and total residual halogen levels in the pilot cooling tower
• the method of the present invention is effective in inhibiting attachment to a submerged surface of microbial slime formed by mainly sessile bacteria in an aqueous system, as well as in killing bacteria contained in the slime and planktonic bacteria.

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