Current through Register Vol. 41, No. 3, September 23, 2024
A. Disinfection can be achieved through
exposure of microorganisms to a sufficient level of UV at the germicidal
wavelength for an adequate period of time.
B. Design parameters. The following
parameters are important to UV disinfection design:
1. The absorbance coefficient is a measure of
the UV absorbing characteristics of the irradiated fluid as measured by a
single beam spectrophotometer at 253.7 nanometers, using both filtered and
unfiltered fluid samples. The units of this parameter are absorption units per
unit distance from the UV source.
2. The contact period is the period of time
that a microorganism is exposed to a given intensity and is a function of the
residence time distribution (RTD) of flow moving past an arrangement of UV
lamps which can be determined from tracer tests.
3. The UV dose is a function of the product
resulting from multiplying the average UV intensity, by the contact period (T)
and is expressed as (microwatts)(seconds)/square centimeter
(UW/SQ.CM/SEC).
4. The dose
response is a measure of the inhibition of cell replication, and is indicated
by the ratio of the monitored log counts of an indicator organism prior to and
following exposure to a given UV dose.
5. The dispersion coefficient (E) is a
measure of turbulent mixing (square centimeters per second) within the fluid
passing through an arrangement of UV lamps. The value of E established by the
RTD variance should be correlated with the contact time necessary to provide
the required dose response.
6. The
intensity is an expression of the rate (units of microwatts per square
centimeter) at which energy is delivered from the source into the surrounding
liquid. UV intensity will dissipate by dilution and will be absorbed by the
medium as the distance from the source increases. The UV intensity provided for
disinfection purposes should be approximated on the basis of the physical
properties of the UV lamps, the physical arrangement of lamps within a flowing
liquid stream, and the properties of the wastewater effluent (Kab).
7. Lamp assemblies are defined as the
arrangement or grouping of UV lamps occupying the cross-section of a channel or
reactor.
8. Photoreactivation is a
process whereby certain organisms regain the ability to reproduce upon exposure
to secondary light.
C.
Design dose. This disinfection process shall only be considered as conventional
when designed to treat effluent with BOD5 and suspended
solids concentrations of no more than 30 mg/l and that consistently maintains a
filtered KAB(Base e) of no more than 0.4/centimeter. The minimum average design
intensity and dosage provided by each lamp assembly shall be specified.
Conventionally designed lamp assemblies shall not receive a maximum flow in
excess of three mgd unless sufficient operating data is submitted to verify
disinfection performance for similar wastewater flows in excess of three mgd.
1. Conventional UV process design shall
provide a minimum average dose of 50,000 microwatt-seconds per square
centimeter after the UV lamps have been in operation for 7,500 hours or more
unless sufficient information is provided to demonstrate that the required
level of disinfection can be obtained at a lower dose level.
2. UV designs based on dose-response models
shall be verified by acceptable bioassay test results, and the expected
influent level of indicator microorganisms shall be determined to verify the
design.
3. Photoreactivation
effects should be accounted for by the UV design.
D. Features. The current configurations
acceptable for UV disinfection equipment include contact systems with submerged
UV lamps enclosed in quartz tubes and noncontact systems with UV lamps situated
adjacent to the flow surface or adjacent to teflon-lined tubular channels
carrying treated effluent. Conventional UV disinfection system design shall
include, as a minimum, two separate lamp assemblies with each assembly capable
of providing the level of disinfection necessary to meet the disinfection
standard at average daily flow. If no more than two lamp assemblies are
provided for treatment works discharging to critical waters, then each assembly
shall be capable of disinfecting the maximum daily flow. Upstream screens
should be provided for unfiltered effluent to prevent breakage of quartz tubes
by debris. In addition, these systems should be protected against "shock"
hydraulic loads from pump station flows.
1. As
quartz effectively passes the germicidal portion of light emitted by UV lamps,
a quartz tube should be used to enclose UV lamps that are submerged in the
treated effluent. The quartz tubes shall be watertight and not subject to
breakage under normal usage. As teflon also passes the germicidal portion of
light emitted by UV lamps, teflon lined channels may also be used to separate
UV lamps from direct contact with treated effluent. Lamp alignment should
provide for maximum contact periods and for reduced opportunity for blockage by
debris around the submerged lamps. The downstream fluid head should maintain
full flow within teflon lined channels. The strength needed to prevent channel
deformation in relation to wall thickness should be established by the designer
for these channels. The teflon tubes should normally be supported to prevent
sagging during operation. Provisions should be made for air bleeding of this
system by the operator when necessary.
2. Lamp spacing in channels or reactors
should be sufficient to use the light in the solution rather than absorb it on
adjacent lamps and walls. The lamp spacing should provide for the absorbance of
the fluid disinfected. For good quality secondary effluent (absorbance (Base e)
0.3/cm or less) the spacing between lamps should be no more than eight cm with
good mixing provided along intensity gradients. The arrangement and numbers of
lamps included in each assembly shall be designed to facilitate proper
maintenance. All electrical connections to submerged lamps shall be watertight
and designed so as to remain dry during maintenance operations.
3. UV lamp specifications should include as
minimum the following or demonstrated equivalent:
a. Availability (at least two
manufacturers).
b. 90% or more
emitted light output at 253.7 nanometers.
c. A minimum arc length that exceeds lamp
length.
d. A rated output of 120
UW/SQ.CM. or more at 1.0 meter from the source.
e. A rated operating life in excess of 7500
hours during which time the UV output exceeds one-half of the rated
output.
f. The lamps should not
produce significant ozone or hydrogen peroxide.
g. Temperature control should provide for
maintaining 105°F to 120°F surface temperature.
4. A single ballast should be utilized to
provide power to no more than two UV lamps. Ballasts may be mounted side by
side in a control box and shall be specified or labeled to indicate their
corresponding UV lamps. A set of lights should indicate the on-off status of
each lamp and should be visible without opening the control box. The ballasts
generate a significant amount of heat, and forced-air ventilation or positive
cooling of control boxes shall be provided. The set of ballasts serving each
assembly of UV lamps shall be mounted in separate (physically separated)
arrangements or control boxes. Control boxes shall be designed and installed in
such a manner that replacement of individual ballasts will not result in
discharge of undisinfected effluent.
5. The system of electrical connections shall
be designed so as to minimize maintenance problems associated with breakage and
moisture damage. The electrical system shall be designed so that routine
maintenance can be achieved without loss of disinfection efficiency.
6. UV lamp assemblies shall be so located as
to provide convenient access for lamp maintenance and removal. Provisions shall
be made so that lamp assemblies may be observed and the channel surface
physically inspected. Flow channels should be entirely accessible for cleaning
to remove film deposits of material interfering with UV disinfection.
7. At least one UV intensity meter within
each assembly of lamps shall be provided to indicate operating conditions. The
intensity reading should be indicated on the control panel for each lamp
assembly. For treatment works with a design average daily flow of one mgd or
higher, flow metering shall be provided and appropriate spectrophotometric
equipment shall be provided to measure the UV absorbance of the wastewater. An
elapsed time meter shall be provided to indicate the total operating time of
the UV lamps.
E. Dose
control. For treatment works with a design average daily flow of one mgd or
more, UV system design should include a control system to turn appropriate
lamps on or off in order to conserve energy. The reliability of proposed
automated control systems connected to flow sensors shall be demonstrated
through submission of acceptable supporting information. Manual control should
be based on diurnal flow variations.
1. A
spare UV lamp (and quartz tube, if appropriate) shall be provided as a minimum
at all UV installations. The number of additional spare lamps (and quartz tubes
if appropriate) provided shall equal the nearest whole number equivalent to 10%
of the number of lamps required to disinfect the maximum flow rate. Spare
ballasts shall also be provided at all UV installations in numbers sufficient
to operate the spare lamps.
2. UV
equipment design shall provide for routine chemical cleaning with a proper
acid/detergent cleanser. A chemical mix tank, circulation pump and
upstream/downstream connections should be provided. A weak acid such as citric
acid may be utilized for chemical cleaning of quartz tubes, but a stronger acid
is recommended for more effective and more economical maintenance. Acid levels
with flows returned to the treatment process should be monitored and controlled
through pH measurements. A high pressure wash of the quartz tubes or
teflon-lined channels should be utilized as a follow-up to chemical cleaning.
The system design shall provide for direct scrubbing of surfaces and lamp
removal for testing of UV output. As UV transmissibility of quartz and teflon
will diminish with time, the design should provide for periodic measurements of
these values. As continuous methods of cleaning UV lamp and channel surfaces
have not been established as reliable means of maintenance, these methods,
including mechanical wipers and ultrasonics, shall not be accepted as sole
maintenance methods, i.e., they may be used together with conventional
maintenance methods as previously described in this section.
F. Hydraulics. The distances
across light intensity gradients for flow past UV lamps should be short
compared to the length of the chambers in the direction of flow, and measures
should be taken to assure mixing across these gradients, with minimal
longitudinal mixing, as measured by the dispersion coefficient. UV system
design should provide an estimated E value of no more than 100 square
centimeters per second.
1. For lamp assemblies
with a dispersion coefficient equal to or more than 50 square centimeters per
second, the minimum contact period shall be 10 seconds, assuming that the flow
path length is equivalent to the linear distance that the design dosage is
provided. The contact period of the UV system flow pattern shall be of
sufficient duration to provide the design dose response in relation to the
established E value.
2. All UV
systems shall be furnished with a means for dewatering as necessary for
cleaning. The depth of irradiated flow shall be controlled as necessary to meet
the disinfection standard at all flow rates.
G. Safety. UV lamps should not be viewed in
the ambient air without proper eye protection as required by VOSH and other
applicable regulations. A minimum of one pair of UV protective eye glasses
shall be provided. The system design should prevent exposure of bare skin to UV
lamp emissions for durations exceeding several minutes. Electrical interlocks
should be provided to shut off high voltage systems in accordance with VOSH
requirements and as requested by other local and state standards when such
energized connections are exposed and could come into contact with
operators.
H. Monitoring.
Facilities shall be included for collecting a sample following the contact
period prior to discharge, to determine the effectiveness of the disinfection
method.
1. As most UV disinfection equipment
represents new technology and limited performance data is available for these
systems, an initial period of increased sampling frequency and testing
requirements for pathogenic bacterial indicators, such as fecal coliform, may
be required. The required initial testing program should take place over a
period of one year or more under reasonable operating conditions with a minimum
sampling frequency of at least once per week.
2. Disinfection of secondary effluent by UV
irradiation should consistently maintain a fecal coliform level below 200
organisms per 100 milliliters of sample or the level established by the permit
or certificate issued.
3. Indicator
organism test results should be correlated with other measurements at the time
of sampling, including flow rate, effluent suspended solids, UV absorbance
coefficient, and lamp operating conditions such as total operating time, the
number in operation, and voltage and intensity.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
