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UV Disinfection Controls
The UV system operation is automatically controlled to maintain the required dose for disinfection by varying the output setting of the lamps based on plant effluent flow as plant flow changes.

Calculations for maintaining the required dose are shown below.
 
The operator will determine the number of banks of lamps to have operating based on the range of plant effluent flows occurring. Each bank of lamps has adequate disinfection capacity for 5.5 mgd. Based on the projected plant flows, two banks in service will provide adequate capacity for the peak design flow.

A MANUAL-OFF-AUTO selector switch is located on the front of the System Control Enclosure (SCE) to select the operating mode. Typically, the UV system will be operated in an automatic mode with the SCE automatically controlling system operation.
Mode DESCRIPTION
Auto The UV lamps' output is controlled automatically by the UV system SCE based on plant effluent flow to maintain the UV dose.
When first put in Auto, all lamps in the in-service banks will be operated at 100% output for approximately 60 minutes.
Manual Turn banks on or off using the On-Off override switch on the EBE enclosures.
Maintenance

With the system in the Auto mode, the Maintenance mode can be selected at the SCE touchscreen.
In the Maintenance mode, the Operator has access to the Maintenance touchscreen and can turn banks of lamps on or off and can set the lamp output from 50% to 100%.

Control Variables
UV Dose
UV dose can be changed by varying the intensity of UV lamps in operation for a given flow. Since the UV lamps are grouped by banks (i.e. one bank per channel) and are flow-paced, the UV dose is adjusted by changing the UV bank flow set points. This flow set point is measured by the downstream Parshall flume with the flow set point per channel calculated by dividing the total flow by the number of on-line channels. The minimum UV dose is calculated based on actual average intensity. Normally, the UV intensity is controlled automatically through the Wedeco system (See the Wedeco O&M Manual for more details).

UV Lamp Cleaning Frequency
When the quartz sleeves enclosing the UV lamps become coated with scale or other buildup, the effective output of the lamps, and the UV dose delivered to the effluent is decreased. The lamps are normally cleaned automatically with a wiper system. The UV lamp cleaning frequency may vary seasonally because of differences in final effluent quality (see the manufacturer's O&M manual for more details).

UV Lamp Age
The output of UV lamps, and the UV dose delivered to the effluent, will decrease over time because of normal aging of the lamps. After 1 year of use, or 8,760 hours, lamp output is roughly 65 percent that of new lamps. Lamps that are nearing or have exceeded their rated lifetime will require more frequent cleaning to maintain the same UV dose as new lamps would provide.


Non-controllable Variables
Influent Flow Rate and Duration
The flow rate and duration that enters the UV channel depends on the preliminary and secondary treatment operations, which depend on the treatment facility influent flow rate, and therefore cannot be controlled.

Secondary Effluent Characteristics
The secondary effluent TSS concentration, turbidity and presence of UV blockers, or low UV transmittance, cannot be controlled and will influence how the controllable variables (e.g. UV dose) are manipulated. As effluent quality changes, the UV bank dose set points can be changed to compensate.


Calculations
UV Dose Calculation
The intensity of the UV radiation and the contact time determine the UV dose received by bacteria in the effluent, and hence the effectiveness of the process. UV Dose is the standard indicator of the UV effectiveness and is expressed as follows:

UV Dose (mJ/cm²) = UV Intensity (mW/cm²) * Retention Time (sec)

The presence of suspended solids in the UV influent water absorbs UV energy, thus the worst-case intensity (at farthest point from UV source) is used in the calculation.

Channel Volume Calculation
The UV Channel volume actually refers to the irradiation volume of the UV reactor, i.e. the volume in which the bacteria are exposed to UV radiation. This is a fixed value calculated in the following manner:

UV Channel Volume = (UV Channel width * top water level * lamp arc length) - volume of quartz sleeves

Retention Time Calculation
The retention time is the amount of time that the bacteria are in contact with the UV radiation. Head loss and the velocity calculations ensure that optimal hydraulic conditions exist in the channel. The retention time is calculated by dividing the UV channel volume by the flow rate within the UV Channel.

Retention Time (sec) = Channel Volume (m³) / Flow Rate (m³/sec)


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Last Updated: 9/16/2013 9:40:43 AM
Version 4.0.1