HEALTH AND SANITATION
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Disinfectant type and residual – Some
reactor components may not be tolerant
of certain disinfectants or high doses, so
UV equipment manufacturers should be
consulted about exposure of UV reactors
to chemical disinfectants.
UVT – Components in the water can
absorb UV light and reduce the dose
delivered to the microorganisms from the
UV reactor. UVT (which can be calculated
from UV absorbance) is a key parameter
in making sure that the UV reactor is
properly sized for the facility.
Iron and manganese – These
constituents can foul quartz sleeves,
leading to decreased UV output. Iron
concentrations >0.1 mg/L may cause
operational issues.
The operation of a small UV reactor is
typically governed by two key parameters:
the flow through the reactor and UV sensor
reading(s). Over time, UV sensors will drift out
of calibration. For this reason, the readings
from a UV duty sensor installed in the reactor
should be compared against a reference
sensor temporarily inserted in the reactor.
Public Water Systems (PWSs) typically make
these sensor checks on a monthly basis. If
the calibration ratio between the duty and
reference sensor readings is >1.2, then
follow-up actions such as recalibration or
replacement of the UV sensor should be
taken. For installations that use an online
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UVT monitor to control UV output, weekly
comparisons between online and benchtop
UVT measurements are recommended.
INSTALLATION CONSIDERATIONS
There are several sources of design guidance
for the application of UV disinfection to
potable water supplies. These references
cover a range of applications, from those
producing only a few gallons per day to
millions of gallons per day. The following
checklist is tailored to institutional settings
for legionella control:
• Hydraulics should allow for even flow
through the reactor. Control valves and
reducers should be avoided within five
pipe diameters upstream of the UV
reactor. Pipe expansions should also be
avoided for at least 10 pipe diameters
upstream of the reactor to avoid jetting
and swirling flow through the UV reactor.
• Redundancy or other measures should
be built in to allow a UV reactor to be
taken out of service for cleaning, lamp
replacement, and other maintenance.
• Valves to isolate UV reactors may be
necessary. In some cases, such as
when UV reactors are flooded with
cleaning chemicals, special valve
arrangements may be beneficial on the
outlet and inlet piping.
• Power quality analysis includes review
of sub-second power interruptions and
voltage sags at the location of
Continued on page 31 >>
February 2018 Volume 23 I Number 12