MIL-HDBK-1191
required to be generated on site and may be explosive, thereby raising -
handling and storage concerns.
(d) Copper-Silver Ionization. Some successes have been
reported in reducing Legionella concentrations in buildings treated by
this method, but there remains a question with the long-term performance.
The equipment costs, as well as installation and maintenance costs, are
relatively low. Ionization process by-products are not toxic and the
process provides a residual effect. The optimum efficacy level varies
with each individual potable water distribution system, and copper-silver
concentration limits set by water regulations may limit ionization
efficacy. Elevated pH levels will adversely effect efficacy, and
undesirable copper and silver deposits may form at the bottom of hot
water generator and storage tanks.
(e) Alternative Technologies. Alternative technologies
include ultraviolet light and ozone. Additional data regarding the
efficacy of these treatments are required. Consideration for
incorporating these or other technologies shall be coordinated with the
User's Design Agent.
Reducing Legionella Potential in Existing Facilities. High
9.2.5.3
velocity water flushing may be to some degree effective to purge
excessive scale and sediment from existing piping or equipment.
Additional measures for reducing legionella colonization potential
include cleaning or replacement of hot-water storage tanks, hot water
generators, faucets, and showerheads. Piping disinfection may be
accomplished via hyperchlorination at levels of up to 10 mg/L of free
residual chlorine, or by thermal shock (hot water flushing) using water
heated to a temperature of 65 degrees C (150 degrees F) or more for a
duration of at least 5 minutes.
Design Considerations. Treatment systems for legionella
9.2.5.4
control shall be sized on the total potable water demand both cold and
hot. Treatment shall be applied to the water service main. Designs
should incorporate the following practices to minimize the potential for
legionella colonization:
(a) Whenever practicable, specify pipe, equipment, and
fixture components having direct water contact to be of materials which
inhibit bacteria colonization, such as copper. For inpatient MTF's,
avoid materials that support colonization, such as natural rubbers and
some plastics, whenever practical.
(b) Design distribution mains to maintain balanced flow
throughout. Piping runs should be as short as practicable. Avoid
creating piping "zones" which experience infrequent use, and therefore
stagnation. As practicable, route mains close to fixtures to minimize
the lengths of branch piping runs. Avoid creating stagnant piping
sections, e.g. dead legs. Do not locate cold water piping near hot water
piping or other heat sources.
(c) Locate pressure balancing and thermostatic mixing anti-
scald valves as close as practicable to the fixture discharge. The
maximum pipe run should be 2 meters (6 feet).
(d) Piping between fixture shutoff and discharge should be
self-draining where possible.
9-7
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