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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
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www . plumbingafrica . co . za November 2017 Volume 23 I Number 9