Section 770. Ultraviolet light irradiation (UV)

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.

Historical Notes

Former 12VAC5-581-830 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-770, Virginia Register Volume 20, Issue 09, eff. February 12, 2004.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.