HEALTH AND SANITATION
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For primary disinfection in the municipal water system,
this combination of concentration and reaction time is
expressed as C (mg/L) × T (min) or CT. For continued
protection against potentially harmful organisms in
distribution systems or premise plumbing systems,
some level of chlorine needs to be maintained after the
initial application. The remaining chlorine is known as
residual chlorine.
The addition of chlorine to water creates two chemical
species that together make up ‘free chlorine’. These
species, hypochlorous acid (HOCl, electrically neutral) and
hypochlorite ion (OCl-, electrically negative), behave very
differently. Hypochlorous acid is more reactive than the
hypochlorite ion and is also the stronger disinfectant and
oxidant. The ratio of hypochlorous acid to hypochlorite ion in
water is determined by pH. At low pH (6–7), hypochlorous
acid dominates, while at high pH (>8.5), the hypochlorite
ion dominates. Thus, the pH of the incoming water may
be a factor when deciding upon the use of chlorine as a
disinfectant or in the engineering design when addressing
issues such as CT for the target organism(s).
Chlorine was first used in the US as a primary disinfectant
of drinking water in Jersey City, New Jersey, in 1908
(USEPA, 1999b). Chlorine is widely credited with virtually
eliminating outbreaks of waterborne disease in the United
States and other developed countries. Among PWSs that
disinfect, chlorine is the most commonly used disinfectant
(AWWA Disinfection Systems Committee, 2008).
CHARACTERISATION OF EFFECTIVENESS
AGAINST LEGIONELLA
Both laboratory and full-scale studies have been
conducted to assess the effectiveness of chlorine against
legionella. These studies included a range of physical
and chemical water conditions such as chlorine dose
and residual levels, temperature and pH. Kim et al.
(2002) reviewed available literature on the efficacy of
various disinfectants against legionella; findings related
to chlorine disinfection include the following:
• Relatively high doses of chlorine (2–6mg/L) were
needed for continuous control of legionella in water
systems (Lin et al., 1998a).
www.plumbingafrica.co.za
•
•
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Muraca et al. (1987) reported that chlorine was
more effective at a higher temperature (43 degrees
C (109.4 degrees F) compared to 25 degrees C
(77 degrees F)), but it decayed faster at the higher
temperature.
The association of L. pneumophila with protozoa
including amoebae required much higher doses
of chlorine for inactivation (Kilvington and Price,
1990). Kim et al. (2002) noted that this association
with protozoa may explain why chlorine can
suppress legionella in water systems but cannot
usually prevent its regrowth.
The laboratory studies described below examined the
effectiveness of chlorine in inactivating legionella under
a range of pH, temperature and chlorine residual levels,
although the temperatures tested in some studies were
lower than temperatures likely to occur in a building’s
hot water system.
Results showed a wide range of CT values needed for
all inactivation levels. While experiments performed
to compare efficacy of disinfectants can be useful to
demonstrate relative efficacy under the conditions of the
experiment, it should not be implied that these values could
be used in the field for premise plumbing water systems.
•
•
Gião et al. (2009) found that L. pneumophila (strain
NCTC 12821) could not be detected using cell
culture after exposure to 0.7mg/L of chlorine in
the laboratory for 30 minutes at room temperature
(20 degrees C, or 68 degrees F). With a chlorine
concentration of 1.2mg/L, cultivability was lost after
10 minutes. Viability of these cells was only slightly
affected when measured using the rapid SYTO
9/propidium iodide fluorochrome uptake assay.
When cells that had been exposed to 1.2mg/L
of chlorine for 30 minutes were co-cultured with
Acanthamoeba polyphaga, they recovered their
cultivability after 72 hours.
Jacangelo et al. (2002) conducted laboratory studies
to examine the efficacy of current disinfection
practices (e.g. chlorine dioxide, free chlorine and
monochloramine) for inactivation of waterborne
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November 2017 Volume 23 I Number 9