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emerging pathogens including legionella. Chlorine doses of 1.0 to 4.0mg / L were used.
Three different temperatures( 5, 15 and 25 degrees C, or 41, 59 and 77 degrees F, respectively) and three different pH( 6.0, 7.0 and 8.0) values were examined. The observed CT values for 2-log( 99-percent) reduction of L. pneumophila at pH 6 ranged from 40 to 500min-mg / L, depending on the temperature. Observed CT values at pH 7 and pH 8 ranged from 50 to > 320min-mg / L and 25 to > 1 000minmg / L, respectively.
These CT values were at least an order of magnitude higher than those reported by Kuchta et al.( 1983) below. The wide range of CT values reported in the literature could be due to different water quality conditions and test protocols used for inactivating legionella.
• Kuchta et al.( 1983) studied the effects of various chlorine concentrations, temperatures and pH levels on legionella in tap water. The chlorine residuals used( 0.1 and 0.5mg / L) were consistent with residual levels that would be expected in PWSs. The observed CT value for 2-log( 99-percent) reduction of L. pneumophila at pH 6 was 0.5min-mg / L at a temperature of 21 degrees C( 69.8 degrees F).
Observed CT values at pH 7 and pH 7.6 ranged from 1 to 6min-mg / L and < 3 to 9min-mg / L, respectively. The authors noted that contact times for the clinical and other environmental sources of legionella were as long as, or longer, than those required for river samples, although long contact times were needed regardless of serogroup or origin.
The authors concluded that low chlorine concentrations( 0.1mg / L) allowed legionella to survive for relatively long periods of time. Increasing the total chlorine concentration predictably enhanced the bactericidal effect, resulting in a 99-percent( 2-log) kill within the first 5 minutes at a concentration of 0.5mg / L.
The following pilot studies evaluated the efficacy of chlorine disinfection for inactivating legionella without co-occurring microbial organisms. Both studies were completed using warm water conditions.
• Saby et al.( 2005) tested the efficiency of several disinfectants in a hot water system pilot unit. The pilot unit was supplied by tap water pre-heated to 30 degrees C( 86 degrees F). Legionella-contaminated water was mixed with the tap water before heating. Colonization of the biofilm by legionella was found after seven weeks. After colonization of pipes in the pilot unit, various treatments were tested. Shock hyperchlorination at 50mg / L of free chlorine residual for 12 hours was found to be very effective in reducing legionella in the water; however, the pipe networks were recolonized in three to four weeks. The authors stated this could be explained by the inefficiency of shock hyperchlorination treatment on bacteria in biofilms. Continuous chlorine at a dose of 3mg / L for two periods of four weeks was also examined. The results showed that treatment with chlorine was effective at maintaining low levels of viable bacteria, including legionella. However, a malfunction of the chlorination system resulted in a positive result for legionella within 28 hours. The authors concluded that continuous chlorination allows only for containment of legionella and that technical problems with treatment could result in rapid recolonization. Temperature control at 40 degrees C( 104 degrees F) and 55 degrees C( 131 degrees F) was also evaluated as part of this study. While temperature control at 55 degrees C was the best technical and economic solution to legionella control, continuous chlorination was also a good solution.
• Muraca et al.( 1987) compared chlorine, heat, ozone and UV for inactivating L. pneumophila in a model premise plumbing system. A suspension of L. pneumophila was added to the system and allowed to circulate. Chlorine disinfection consisted of maintaining a residual concentration between 4 and 6mg / L through multiple additions of chlorine.
Chlorine experiments were conducted at 25 and 43 degrees C( 77 and 109.4 degrees F, respectively). Continuous chlorination at a dose of 4 to 6mg / L resulted in a 5- to 6-log( 99.999- to 99.9999-percent) decrease of L. pneumophila in six hours. Chlorine disinfection at 43 degrees C( 109.4 degrees F) inactivated L. pneumophila more reliably and completely than disinfection at 25 degrees C( 77 degrees F).
Due to thermal decomposition of chlorine residual, more chlorine was needed to maintain a residual of 4 – 6mg / L at 43 degrees C( 109.4 degrees F) than at 25 degrees C( 77 degrees F)( a total of 40mL of Clorox bleach( 5.25 percent chlorine) as opposed to 18mL). The authors noted that in addition to the higher doses required to overcome residual decomposition, a drop in chlorine levels or failure of chlorination equipment could allow legionella to survive. As a result, the authors concluded that chlorination of hot water systems is more difficult to regulate than that of cold water systems.
Chlorine can be applied by facilities for routine treatment of both hot and cold domestic water. PA
HEALTH AND SANITATION
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www. plumbingafrica. co. za November 2017 Volume 23 I Number 9