On-Site Wastewater Treatment and Disposal Systems - Forced ...

6.5.3.5 Construction Features
Commercially available UV units sold primarily for water supply disin-
fection are applicable for onsite wastewater disinfection. Most of
these units are self-contained and employ high-intensity UV irradiation
over a thin film of water for short contact times.
The self-contained unit should be installed following the last treatment
process in the treatment sequence, and should be protected from dust,
excessive heat, and freezing. It should be accessible for maintenance
and control. As described in the previous section, the unit should be
equipped with a cleaning device (manual or automatic) and an intensity
meter that is properly calibrated. Flow to the unit should be main-
tained relatively constant. This is often achieved by means of a pres-
sure compensated flow control valve.
Some larger UV modules are available that consist of a series of lamps
encased within quartz glass enclosures. The module may be placed within
the flow stream such that all water passes through the module. UV lamps
positioned over discharge weirs, and therefore out of the water, are
also available. These systems are not as efficient as flow through
units since only a fraction of the lamp arc intercepts the water. Con-
trol of the water film over the weir plate (V-notch or sharp crested) is
difficult to maintain unless upstream flows are carefully regulated.
Cleaning and metering devices are required for both of these systems.
Depending on upstream processes and the UV unit employed, the UV system
may be operated on a continuous flow or intermittent basis. For small
flows, self-contained tubular units and intermittent flow would be
employed. Influent to the unit could be pumped to the UV system from a
holding tank. In order to obtain full-life expectancy of the UV lamps,
they should be operated continuously regardless of flow arrangement.
Where UV modules are employed, continuous flow through the contact
chamber may be more practical.
6.5.3.6 Operation and Maintenance
Routine operational requirements include quartz glass enclosure clean-
ing, lamp replacement, and UV intensity meter reading. Since UV disin-
fection does not produce a residual, the only monitoring required would
be periodic bacterial' analyses by skilled technicians. Periodic mainte-
nance of pumping equipment and controls, and cleaning of quartz jackets
during lamp replacement, would also be required.
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