Factors Influencing the Effectiveness of Swimming Pool Bactericides

Techniques for culturing, harvesting, and testing bacteria to evaluate bactericidal chemicals for swimming pools are described. Concentrations of 25, 50, and 100 mg of the chlorine stabilizer cyanuric acid per liter increased the time required for a 99% kill of Streptococcus faecalis by 0.5 mg of chlorine per liter at pH 7.4 and 20 C from less than 0.25 min without cyanuric acid to 4, 6, and 12 min, respectively. The effect of concentrations of ammonia nitrogen in the range found in swimming pools on the rate of kill of 0.5 mg of chlorine per liter and of chlorine plus cyanuric acid was tested. At concentrations of ammonia nitrogen greater than 0.05 mg per liter, faster rates of kill of S. faecalis were obtained with 100 mg of cyanuric acid per liter plus 0.5 mg of chlorine per liter than with 0.5 mg of chlorine per liter alone. When water samples from four swimming pools with low ammonia levels were used as test media, 0.5 mg of added chlorine per liter killed 99.9% of the added S. faecalis in less than 2 min, but water from a pool with a large number of children required 60 to 180 min of treatment.     

Influence of Stabilizer Concentration on Effectiveness of Chlorine as an Algicide

Cyanuric acid, used as chlorine stabilizer in swimming pool waters, has a relatively minor effect on the algicidal efficiency of free chlorine. The toxicity of free chlorine to three swimming pool algae was reduced slightly by 25 mg of cyanuric acid per liter if inhibiting, but less than algicidal, concentrations of chlorine were employed. Higher stabilizer concentrations (50, 100, and 200 mg/liter) generally resulted in no further reduction in the algicidal efficiency of free chlorine beyond that observed at 25 mg/liter.     

Determinants of the microbiological characteristics of South Australian swimming pools.

A survey of 100 swimming pools has been conducted to assess the effectiveness of disinfection practices against various microorganisms and to check compliance with recommended chlorine levels and pH. Microbiological quality was assessed from densities of total coliforms, Escherichia coli, and Pseudomonas aeruginosa, total colony counts, and the presence or absence of amoebae, including the pathogen Naegleria fowleri. Although a free chlorine residual of 1.0 mg/liter and a pH range of 7.0 to 7.6 are recommended by local health authorities, 41 pools had a lower free chlorine residual and 37 had a pH outside this range. Multiple logistic regression analysis was used to test the association of field measurements with the microbiological data. The analysis demonstrated a strong positive association of free chlorine with bacteriological quality and the absence of Naegleria spp. No other field measurement was predictive in this regard, although the absence of all amoebae was associated with a relatively low water temperature and pH. Using a mathematical model derived from this analysis, it was estimated that 99% of pools would have acceptable bacteriological quality and 94% would be free of Naegleria spp. at a free chlorine residual of 1.0 mg/liter. However, at the mean water temperature (23 degrees C) and pH (7.5) seen in this study, other amoebae would still be detectable in 500-ml samples taken from 40% of pools at this chlorine level.     

Determinants of the microbiological characteristics of South Australian swimming pools

A survey of 100 swimming pools has been conducted to assess the effectiveness of disinfection practices against various microorganisms and to check compliance with recommended chlorine levels and pH. Microbiological quality was assessed from densities of total coliforms, Escherichia coli, and Pseudomonas aeruginosa, total colony counts, and the presence or absence of amoebae, including the pathogen Naegleria fowleri. Although a free chlorine residual of 1.0 mg/liter and a pH range of 7.0 to 7.6 are recommended by local health authorities, 41 pools had a lower free chlorine residual and 37 had a pH outside this range. Multiple logistic regression analysis was used to test the association of field measurements with the microbiological data. The analysis demonstrated a strong positive association of free chlorine with bacteriological quality and the absence of Naegleria spp. No other field measurement was predictive in this regard, although the absence of all amoebae was associated with a relatively low water temperature and pH. Using a mathematical model derived from this analysis, it was estimated that 99% of pools would have acceptable bacteriological quality and 94% would be free of Naegleria spp. at a free chlorine residual of 1.0 mg/liter. However, at the mean water temperature (23 degrees C) and pH (7.5) seen in this study, other amoebae would still be detectable in 500-ml samples taken from 40% of pools at this chlorine level.     

Enhanced inactivation of salmonella and pseudomonas biofilms on stainless steel by use of T-128, a fresh-produce washing aid, in chlorinated wash solutions

The effect of the washing aid T-128 (generally recognized as safe [GRAS] formulation, composed mainly of phosphoric acid and propylene glycol) on inactivation of Salmonella and Pseudomonas populations in biofilms on stainless steel was evaluated under conditions of increasing organic matter loads in chlorinated wash solutions dominated by hypochlorous acid. Biofilms were formed statically on stainless steel coupons suspended in 2% lettuce extract after inoculation with Salmonella enterica serovar Thompson or Newport or with Pseudomonas fluorescens. Coupons with biofilms were washed in chlorine solutions (0, 0.5, 1, 2, 5, 10, or 20 mg/liter at pH 6.5, 5.0 and 2.9), with or without T-128, and with increasing loads of organic matter (0, 0.25, 0.5, 0.75, or 1.0% lettuce extract). Cell populations on coupons were dispersed using intermittent, pulsed ultrasonication and vortexing and enumerated by colony counts on XLT-4 or Pseudomonas agars. Cell responses to fluorescent viability staining of biofilm treatment washing solutions were examined using confocal laser scanning microscopy. Results showed that 0.1% T-128 (without chlorine) reduced P. fluorescens biofilm populations by 2.5 log(10) units but did not reduce Salmonella populations. For both Salmonella and Pseudomonas, the sanitizing effect of free chlorine (1.0 to 5.0 mg/liter) was enhanced (P < 0.05) when it was combined with T-128. Application of T-128 decreased the free chlorine depletion rate caused by increasing organic matter in wash waters and significantly (P < 0.05) augmented inactivation of bacteria in biofilms compared to treatments without T-128. Image analysis of surfaces stained with SYTO and propidium iodide corroborate the cultural assay results showing that T-128 can aid in reducing pathogen viability in biofilms and thus can aid in sanitizing stainless steel contact surfaces during processing of fresh-cut produce.     

Limax Amoebae in Public Swimming Pools of Albany, Schenectady, and Rensselaer Counties, New York: Their Concentration, Correlations, and Significance

A survey was conducted on 30 halogenated public swimming pools, located in Albany, Schenectady, and Rensselaer counties, to determine their open-water limax amoeba densities. Six were outdoor pools. Other variables measured were the standard plate count, total seston, free residual chlorine or bromine, total alkalinity, total hardness, orthophosphate, total soluble phosphorus, specific conductance, pH, temperature, and several engineering parameters including the rate and type of filtration as well as a saturation index. Amoebae were isolated on agar plates at 37°C using heat-killed bacterial suspensions of Enterobacter cloacae or Escherichia coli. Most probable number estimates of amoebic densities ranged from not detectable (<0.01) to 110 amoebae per liter. The median concentration of amoebae was 0.9/liter. Eighty percent of the pools examined had less than 5 amoebae per liter. Significant correlations (P < 0.05) were found between amoebic densities and the log₁₀ of the standard plate count, orthophosphate, and total soluble phosphorus. No significant difference was found between amoebic densities in outdoor and indoor pools. Preliminary tests for the presence of the human pathogen Naegleria fowleri were inconclusive.     

Simultaneous degradation of atrazine and phenol by Pseudomonas sp. strain ADP: effects of toxicity and adaptation

The strain Pseudomonas sp. strain ADP is able to degrade atrazine as a sole nitrogen source and therefore needs a single source for both carbon and energy for growth. In addition to the typical C source for Pseudomonas, Na(2)-succinate, the strain can also grow with phenol as a carbon source. Phenol is oxidized to catechol by a multicomponent phenol hydroxylase. Catechol is degraded via the ortho pathway using catechol 1,2-dioxygenase. It was possible to stimulate the strain in order to degrade very high concentrations of phenol (1,000 mg/liter) and atrazine (150 mg/liter) simultaneously. With cyanuric acid, the major intermediate of atrazine degradation, as an N source, both the growth rate and the phenol degradation rate were similar to those measured with ammonia as an N source. With atrazine as an N source, the growth rate and the phenol degradation rate were reduced to approximately 35% of those obtained for cyanuric acid. This presents clear evidence that although the first three enzymes of the atrazine degradation pathway are constitutively present, either these enzymes or the uptake of atrazine is the bottleneck that diminishes the growth rate of Pseudomonas sp. strain ADP with atrazine as an N source. Whereas atrazine and cyanuric acid showed no significant toxic effect on the cells, phenol reduces growth and activates or induces typical membrane-adaptive responses known for the genus Pseudomonas. Therefore Pseudomonas sp. strain ADP is an ideal bacterium for the investigation of the regulatory interactions among several catabolic genes and stress response mechanisms during the simultaneous degradation of toxic phenolic compounds and a xenobiotic N source such as atrazine.     

Soil amoebas as biological markers to estimate the quality of swimming pool waters

The legal biological survey of swimming pool waters is based on both the level of bacteriological contamination and the amount of material of fecal origin. The great number of soil amoebas and the occasional epidemiological risk involved led us to consider using these organisms as possible biological markers to estimate the quality of pool water and the extent of disinfection. During a 1-year survey of 54 public swimming pools, 765 superficial pool and tap water samples were collected. One portion (50 ml) drawn from 1-liter samples was filtered and cultured for amoebas. In specimens considered contaminated we detected at least 20 amoebas per liter, whereas uncontaminated samples contained fewer than 20 amoebas per liter. By keeping the threshold value voluntarily low, we were able to compare tap water with pool water and to monitor the quality of various disinfection procedures (i.e., chlorine, bromine, and Cu-Ag). The data suggest that the filters were not always protective against a high concentration of amoebas. Furthermore, these disinfection procedures were not equally efficient according to estimates based on biological criteria. In addition, the quality of swimming pool water also depends on the quality of its source tap water. Thus, the numeration of soil amoebas can be used as an additional biological marker to estimate the quality of swimming pool water.     

Soil amoebas as biological markers to estimate the quality of swimming pool waters.

The legal biological survey of swimming pool waters is based on both the level of bacteriological contamination and the amount of material of fecal origin. The great number of soil amoebas and the occasional epidemiological risk involved led us to consider using these organisms as possible biological markers to estimate the quality of pool water and the extent of disinfection. During a 1-year survey of 54 public swimming pools, 765 superficial pool and tap water samples were collected. One portion (50 ml) drawn from 1-liter samples was filtered and cultured for amoebas. In specimens considered contaminated we detected at least 20 amoebas per liter, whereas uncontaminated samples contained fewer than 20 amoebas per liter. By keeping the threshold value voluntarily low, we were able to compare tap water with pool water and to monitor the quality of various disinfection procedures (i.e., chlorine, bromine, and Cu-Ag). The data suggest that the filters were not always protective against a high concentration of amoebas. Furthermore, these disinfection procedures were not equally efficient according to estimates based on biological criteria. In addition, the quality of swimming pool water also depends on the quality of its source tap water. Thus, the numeration of soil amoebas can be used as an additional biological marker to estimate the quality of swimming pool water.     

Effect of pH, application technique, and chlorine-to-nitrogen ratio on disinfectant activity of inorganic chloramines with pure culture bacteria.

The influence of pH, application technique, and chlorine-to-nitrogen weight ratio on the bactericidal activity of inorganic chloramine compounds was determined with stock and environmental strains of Escherichia coli, Salmonella spp., Pseudomonas aeruginosa, Klebsiella pneumoniae, and Enterobacter cloacae. The rate of inactivation increased from 1.5 to 2 times as the chlorine-to-nitrogen weight ratio was adjusted from 2:1 to 5:1, 5 to 6 times as the pH was decreased from 8 to 6, and 5 to 6 times as the concentration was increased from 1 to 5 mg/liter. Separate additions of free chlorine and ammonia (concurrent addition and preammoniation) into seeded water at or below pH 7.5 resulted in killing comparable to that observed with free chlorine (99% inactivation in less than 20 s). At pH 8, inactivation by separate additions was considerably slower and was comparable to that by prereacted chloramine compounds (99% inactivation in 25 to 26 min). Determination of the effectiveness of inorganic chloramine compounds as primary disinfectants for drinking water must consider the method of application, pH and concentrations of chlorine and ammonia.     

Inactivation of particle-associated coliforms by chlorine and monochloramine.

Sieves and nylon screens were used to separate primary sewage effluent solids into particle fractions of less than 7- or greater than 7-micron size. The efficiency of separation was determined by using a particle counter. Indigenous coliforms associated with the particle fractions were tested for their resistance to chlorine and monochloramine. Coliforms associated with the less than 7-microns fraction were inactivated more rapidly by 0.5 mg of chlorine per liter at 5 degrees C and pH 7 than coliforms associated with the greater than 7-microns fraction. Homogenization of the greater than 7-microns fraction not only resulted in an increase in the number of less than 7-microns particles, but also increased the rate of inactivation to a rate similar to that of the less than 7-microns fraction. With 1 mg of monochloramine per liter at 5 degrees C and pH 7, particle size had no appreciable effect on the rate of inactivation. At pH 8, however, the less than 7-micron fraction was inactivated more rapidly than the greater than 7-micron fraction. The time required for 99% inactivation of the particle fractions with monochloramine at pH 7 or 8 was 20- to 50-fold greater than the time required for the same amount of inactivation with chlorine at pH 7. The results indicate that coliforms associated with sewage effluent particles are inactivated more rapidly with 0.5 mg of chlorine per liter than with 1.0 mg of monochloramine per liter. However, greater than 7-micron particles can have a protective effect against the disinfecting action of chlorine.     

Inactivation of particle-associated coliforms by chlorine and monochloramine

Sieves and nylon screens were used to separate primary sewage effluent solids into particle fractions of less than 7- or greater than 7-micron size. The efficiency of separation was determined by using a particle counter. Indigenous coliforms associated with the particle fractions were tested for their resistance to chlorine and monochloramine. Coliforms associated with the less than 7-microns fraction were inactivated more rapidly by 0.5 mg of chlorine per liter at 5 degrees C and pH 7 than coliforms associated with the greater than 7-microns fraction. Homogenization of the greater than 7-microns fraction not only resulted in an increase in the number of less than 7-microns particles, but also increased the rate of inactivation to a rate similar to that of the less than 7-microns fraction. With 1 mg of monochloramine per liter at 5 degrees C and pH 7, particle size had no appreciable effect on the rate of inactivation. At pH 8, however, the less than 7-micron fraction was inactivated more rapidly than the greater than 7-micron fraction. The time required for 99% inactivation of the particle fractions with monochloramine at pH 7 or 8 was 20- to 50-fold greater than the time required for the same amount of inactivation with chlorine at pH 7. The results indicate that coliforms associated with sewage effluent particles are inactivated more rapidly with 0.5 mg of chlorine per liter than with 1.0 mg of monochloramine per liter. However, greater than 7-micron particles can have a protective effect against the disinfecting action of chlorine.     

Removal of mercury from chloralkali electrolysis wastewater by a mercury-resistant Pseudomonas putida strain

A mercury-resistant bacterial strain which is able to reduce ionic mercury to metallic mercury was used to remediate in laboratory columns mercury-containing wastewater produced during electrolytic production of chlorine. Factory effluents from several chloralkali plants in Europe were analyzed, and these effluents contained total mercury concentrations between 1.6 and 7.6 mg/liter and high chloride concentrations (up to 25 g/liter) and had pH values which were either acidic (pH 2.4) or alkaline (pH 13.0). A mercury-resistant bacterial strain, Pseudomonas putida Spi3, was isolated from polluted river sediments. Biofilms of P. putida Spi3 were grown on porous carrier material in laboratory column bioreactors. The bioreactors were continuously fed with sterile synthetic model wastewater or nonsterile, neutralized, aerated chloralkali wastewater. We found that sodium chloride concentrations up to 24 g/liter did not inhibit microbial mercury retention and that mercury concentrations up to 7 mg/liter could be treated with the bacterial biofilm with no loss of activity. When wastewater samples from three different chloralkali plants in Europe were used, levels of mercury retention efficiency between 90 and 98% were obtained. Thus, microbial mercury removal is a potential biological treatment for chloralkali electrolysis wastewater.     

Removal of Mercury from Chloralkali Electrolysis Wastewater by a Mercury-Resistant Pseudomonas putida Strain

A mercury-resistant bacterial strain which is able to reduce ionic mercury to metallic mercury was used to remediate in laboratory columns mercury-containing wastewater produced during electrolytic production of chlorine. Factory effluents from several chloralkali plants in Europe were analyzed, and these effluents contained total mercury concentrations between 1.6 and 7.6 mg/liter and high chloride concentrations (up to 25 g/liter) and had pH values which were either acidic (pH 2.4) or alkaline (pH 13.0). A mercury-resistant bacterial strain, Pseudomonas putida Spi3, was isolated from polluted river sediments. Biofilms of P. putida Spi3 were grown on porous carrier material in laboratory column bioreactors. The bioreactors were continuously fed with sterile synthetic model wastewater or nonsterile, neutralized, aerated chloralkali wastewater. We found that sodium chloride concentrations up to 24 g/liter did not inhibit microbial mercury retention and that mercury concentrations up to 7 mg/liter could be treated with the bacterial biofilm with no loss of activity. When wastewater samples from three different chloralkali plants in Europe were used, levels of mercury retention efficiency between 90 and 98% were obtained. Thus, microbial mercury removal is a potential biological treatment for chloralkali electrolysis wastewater.     

Comparative assessment of different biocides in swimming pool water

For disinfection of swimming pool water chlorine of chlorine-based products are normally used. In practice, these products have proven their worth regarding killing of pathogenic micro-organisms. Detailed values of their biocidal activity in swimming pool water were not found in literature. In the given study the efficacy of sodium hypochlorite (NaOCl) versus five micro-organisms was investigated.It is known that chlorination of swimming pool water may lead to formation of specific unwanted products like haloform. Nowadays, the concentration of those by-products in swimming pool water is limited and specific measures exist to minimize their formation. Nevertheless, there is increasing interest in alternative methods without by-product formation like e.g. hydrogen peroxide (H₂O₂) treatment.In the given study the antimicrobial activity of sodium hypochlorite was compared with that of different hydrogen peroxide-based products. The test procedure used was specifically designed to simulate practical conditions in a swimming pool but at the same time to lead to adequate reproducibility. Five test organisms were selected being relevant for the swimming pool area: Pseudomonas aeruginosa, Escherichia coli, Legionella pneumophila, Staphylococcus aureus and Candida albicans.The swimming pool water for the test was artificially prepared. Water hardness, temperature and pH value were adjusted to a defined level. Regarding simulation of organic load it was found that a mixture of urea, creatinine and several amino acids was most appropriate.Addition of the test organisms was done in three portions: one big in the beginning and two smaller after 10 and 20 min to simulate recontamination by bathers. Total test period was 30 min. The number of surviving cells was determined after 30 s as well as after 10, 20 and 30 min.Sodium hypochlorite was tested at a concentration of 1 ppm active chlorine. Compared to that three products based on hydrogen peroxide were investigated: pure hydrogen peroxide, hydrogen peroxide + silver nitrate and a trade product based on hydrogen peroxide.Sodium hypochlorite resulted in total kill of the inoculated organisms after 10, 20 and 30 min corresponding to a log 4 reduction. In contrast to that the biocidal effect achieved by the hydrogen peroxide-based products was significantly lower than one log cycle notwithstanding a very high concentration of up to 150 ppm.The test results confirm the very good killing activity of sodium hypochlorite versus micro-organisms relevant for the swimming pool area. Products based on hydrogen peroxide, with or without silver ions, are from a microbiological point of view no real alternative to chlorine disinfection in swimming pools.     

铜绿假单胞菌PA1550基因对泳动及蹭动的影响

[目的]:研究与铜绿假单胞菌运动能力相关的基因.[方法]:以一株临床分离的铜绿假单胞菌PA68做受体菌,应用人工Mu转座技术建立了库容为2000的突变子文库,从中筛选出泳动能力和蹭动能力丧失或减弱的突变子,通过基因克隆、测序,GenBankBLAST比对测序结果,互补基因表达确定与铜绿假单胞菌运动能力相关的基因.[结果]:突变子Y46在丧失了泳动运动能力的同时,蹭动能力也发生了减弱.在Y46突变子中,Mu转座子插入到功能完全未知的基因PA1550中.对极性效应及PA1550所在操纵子的分析表明,Mu转座子对插入点下游的基因的转录并不造成影响.[结论]:PA1550与铜绿假单胞菌的泳动及蹭动能力有关.     

Ancient Evolution and Recent Evolution Converge for the Biodegradation of Cyanuric Acid and Related Triazines

Cyanuric acid was likely present on prebiotic Earth, may have been a component of early genetic materials, and is synthesized industrially today on a scale of more than one hundred million pounds per year in the United States. In light of this, it is not surprising that some bacteria and fungi have a metabolic pathway that sequentially hydrolyzes cyanuric acid and its metabolites to release the nitrogen atoms as ammonia to support growth. The initial reaction that opens the s-triazine ring is catalyzed by the unusual enzyme cyanuric acid hydrolase. This enzyme is in a rare protein family that consists of only cyanuric acid hydrolase (CAH) and barbiturase, with barbiturase participating in pyrimidine catabolism by some actinobacterial species. The X-ray structures of two cyanuric acid hydrolase proteins show that this family has a unique protein fold. Phylogenetic, bioinformatic, enzymological, and genetic studies are consistent with the idea that CAH has an ancient protein fold that was rare in microbial populations but is currently becoming more widespread in microbial populations in the wake of anthropogenic synthesis of cyanuric acid and other s-triazine compounds that are metabolized via a cyanuric acid intermediate. The need for the removal of cyanuric acid from swimming pools and spas, where it is used as a disinfectant stabilizer, can potentially be met using an enzyme filtration system. A stable thermophilic cyanuric acid hydrolase from Moorella thermoacetica is being tested for this purpose.     

The carbon monoxide releasing molecule CORM-2 attenuates Pseudomonas aeruginosa biofilm formation

Chronic infections resulting from biofilm formation are difficult to eradicate with current antimicrobial agents and consequently new therapies are needed. This work demonstrates that the carbon monoxide-releasing molecule CORM-2, previously shown to kill planktonic bacteria, also attenuates surface-associated growth of the gram-negative pathogen Pseudomonas aeruginosa by both preventing biofilm maturation and killing bacteria within the established biofilm. CORM-2 treatment has an additive effect when combined with tobramycin, a drug commonly used to treat P. aeruginosa lung infections. CORM-2 inhibited biofilm formation and planktonic growth of the majority of clinical P. aeruginosa isolates tested, for both mucoid and non-mucoid strains. While CORM-2 treatment increased the production of reactive oxygen species by P. aeruginosa biofilms, this increase did not correlate with bacterial death. These data demonstrate that CO-RMs possess potential novel therapeutic properties against a subset of P. aeruginosa biofilm related infections.     

Effect of chlorine treatment on infectivity of hepatitis A virus

This study examined the effect of chlorine treatment on the infectivity of hepatitis A virus (HAV). Prodromal chimpanzee feces, shown to induce hepatitis in marmosets (Saguinus sp.), was clarified, and the virus was precipitated with 7% polyethylene glycol 6000, harvested, and resuspended. The suspension was layered onto 5 to 30% linear sucrose gradients and centrifuged; the fractions containing HAV were dialyzed, and a 1:500,000 dilution of this preparation induced hepatitis and seroconversion in 2 of 4 marmosets. A 1:50 dilution of this preparation served as inoculum. Untreated inoculum induced overt hepatitis and seroconversion in 100% (5 of 5) of marmosets inoculated intramuscularly. Inoculum treated for various periods (15, 30, or 60 min) with 0.5, 1.0, or 1.5 mg of free residual chlorine per liter induced hepatitis in 14% (2 of 14), 8% (1 of 12), and 10% (1 of 10) of marmosets, respectively, and induced seroconversion in 29, 33, and 10% of the animals. Inoculum treated with 2.0 or 2.5 mg of free residual chlorine per liter was not infectious in marmosets as determined by absence of hepatitis and seroconversion in the 13 animals tested. Thus, treatment levels of 0.5 to 1.5 mg of free residual chlorine per liter inactivated most but not all HAV in the preparation, whereas concentrations of 2.0 and 2.5 mg of free residual chlorine per liter destroyed the infectivity completely. These results suggest that HAV is somewhat more resistant to chlorine than are other enteroviruses.     

Inactivation of Naegleria gruberi cysts by chlorinated cyanurates.

The resistance of Naegleria gruberi cysts to chlorine in the presence of cyanuric acid was compared at pH 5 and 7. An amperometric membrane electrode was used to measure HOCl concentrations independently of the chlorinated cyanurate species, thus permitting an analysis of the role of free chlorine versus chlorinated cyanurates in cyst inactivation. In the presence of cyanuric acid, the products of the HOCl residual and the contact time required for 99% cyst inactivation were 8.5 mg . min/liter and 13.9 mg . min/liter at pH 5 and 7, respectively. The Watson's Law coefficients of dilution (n) were 1.3 and 1.6 at pH 5 and 7, respectively. The results strongly suggest that HOCl is the predominant cysticide with no measurable cysticidal effect of the chlorinated cyanurate species.