MIL-HDBK-1191
located in attics, ceiling spaces, equipment rooms, crawlspaces, or other
unconditioned spaces, or by being located adjacent to hot water or steam
piping. All of these risk factors should be taken into consideration
when designing the domestic water system to help determine what special
measures, if any, are called for to help control legionella.
Scale, Sediment, and Biofilm. Scale, sediment, and biofilm
9.2.5.1
are contaminants that support Legionella bacteria colonization. The
extent to which these support colonization is a function of factors
including potable water service quality (including the presence of living
aquatic amoebae such as are found in biofilms), system operating
temperatures, and pipe material. Standard control technologies for scale
and sediment deposits normally minimize the contribution of these factors
to legionella colonization. Biofilms are resistant to some treatments.
For addition and alteration projects, designers must be aware of the
potential presence of established biofilms, sediments, and scale.
Exercise caution in reusing existing piping system components without an
analysis of the existing conditions. It may be contributing to
Legionella colonization and subsequently, may lead to contamination of
new service.
New designs should consider
9.2.5.2
Control/Treatment Technologies.
incorporation into the water supply system design one of the treatment
methods or technologies recognized as efficacious in legionella control,
some of which include the following:
(a) Chlorine Injection. This is a long established water
treatment method and continues to be used. Many hospitals will maintain
a 1-2 mg/L free residual chlorine at the tap. Although this is in
accordance with CDC recommended treatments (reference 9d), there are
several concerns with using chlorine long term:
- legionella bacteria are more tolerant of chlorine than
other bacteria.
- accelerated corrosion of plumbing systems by the continuous
injection of chlorine.
- carcinogenic trihalomethane production above the
recommended maximum levels if free residual chlorine increases beyond the
recommended 1-2 mg/L at the tap (future restrictions are being considered
by the Environmental Protection Agency),
- reduced efficacy at elevated water pH levels.
- potential handling and storage hazards.
(b) Monochloramine Treatment. Monochloromine, which is a
combination of (free) chlorine with other elements or compounds with,
such as ammonia, eliminates some of the disadvantages associated with
chlorine, e.g. it lowers the concentrations of possible carcinogenic
disinfectant by-products, and may be more efficacious against legionella.
Monochloramine demonstrates prolonged disinfectant properties, and the
treatment method has been in use for many years. Recent research data
from one state authority indicated that hospitals using monochloramines
were culture-negative with no nosocomial legionellosis cases, whereas
hospitals using free chlorine for disinfection had legionella bacteria in
their water systems.
(c) Chlorine Dioxide Treatment. This technology has
advantageous characteristics in that it is not corrosive, does not
produce carcinogenic by-products, and maintains a relatively good
efficacy level over a broad pH range. However chlorine dioxide is
9-6
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