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UV Disinfection Overview
Purpose
Disinfection is the process of inactivating pathogens (disease causing) and other microorganisms. The plant effluent is disinfected with ultraviolet (UV) light prior to discharge to the Columbia River.
Theory of Operation
UV light kills bacteria and viruses by destroying their genetic material and preventing reproduction. UV light can kill microorganisms without significantly altering the physical and chemical properties of the effluent, unlike chlorine, which leaves a residual.

An automatic UV dosage control system maintains an adequate UV dosage while minimizing power consumption. The UV lamp power is automatically adjusted in response to inputs from the plant effluent flow meter and a UV intensity monitoring system.  The flow and effluent transmittance inputs are continually monitored in the UV system control center and the necessary lamp power setting is increased or decreased to maintain a fixed UV dose regardless of changes in effluent flow or quality. 
Description
The UV disinfection system includes three parallel channels, each with capacity to treat 5.5 million gallons of plant effluent at the system design conditions (Figure 1). Each channel has a single bank of lamps, with 5 modules per bank and 16 lamps per module, for a total of 240 lamps in the three channels (Figure 2). Two of the UV channels are required for effluent disinfection at the plant peak design flow of 11 mgd. The third UV channel serves as a backup.

The UV banks contain low pressure, high intensity ultraviolet lamps arranged parallel to the direction of flow in the channel. The lamps expose the water to UV light with a wavelength of approximately 254 nm. The lamps are provided with a pneumatically operated automatic wiping system for lamp cleaning. The plant air system provides the air for system operation. Each channel has a low level probe to detect if the water level in the channel becomes too low. A UV intensity monitoring system is provided for each channel. The intensity reading is used to automatically adjust the lamp output as required to meet the dosage requirements for disinfection.

Each bank of lamps is powered from an electronic ballast enclosure (EBE), located along the east wall of the UV room. The UV system is controlled from a single system control enclosure (SCE) located adjacent to the electronic ballast enclosures.

The UV System Control Enclosure (SCE) has a programmable controller that provides the dose pacing control, monitoring, and alarms for the UV system. A touch screen is provided at the SCE for operator interface.
Each UV channel is provided with upstream and downstream slide gates to isolate a channel for maintenance. A constant upstream level gate (hydraulically balanced and requiring power to operate) is provided downstream of each UV channel to maintain the level relatively constant over the range of operating flows.

A portable cleaning tank has been provided in the event the lamps periodically need additional cleaning with a dilute acid solution. The automatic wiping system can be operated in the portable cleaning tank.
Item Design Criteria
UV System Configuration
Number of Lamps
Number of Channels
Number of Banks per Channel
Number of Modules per Bank
Number of Lamps per Module
240
3
1
5
16
UV Design Transmittance (minimum) 50%
UV Design Dose >40,000 microwatt seconds/cm2
Effluent Fecal Coliform Standard
30 Day Geometric Mean
7 Day Geometric Mean

200 mpn
400 mpn
Channel Dimensions
Channel Width
Water Depth
Total Depth
Approximate Length

46.88 inches
37.4 inches
69 inches
24 feet

Equipment
The disinfection system includes the following equipment:
  • Channel Isolation Slide Gates
  • Disinfection Channels
  • System Control Center
  • Power Distribution Enclosure
  • Lamp Modules
  • Lamp Cleaning System
  • UV Intensity Sensor
  • Automatic Level Control Gates
Channel Isolation Slide Gates6
  Each UV channel is provided with upstream and downstream slide gates to isolate a channel for maintenance.
Disinfection Channels
  The UV disinfection system includes three parallel channels, each with capacity to treat 5.5 million gallons of plant effluent at the system design conditions (Figure 1). Each channel has a single bank of lamps, with 5 modules per bank and 16 lamps per module, for a total of 240 lamps in the three channels (Figure 2). Two of the UV channels are required for effluent disinfection at the plant peak design flow of 11 mgd. The third UV channel serves as a backup.
System Control Center6
The UV System Control Enclosure (SCE) has a programmable controller that provides the dose pacing control, monitoring, and alarms for the UV system. A touch screen is provided at the SCE for operator interface.
Power Distribution Enclosure6
The power distribution center provides electric power to each bank of lamp modules.

The distribution center is housed in a stainless steel enclosure mounted across the channel and consists of a service entrance and a power distribution system.

Power is distributed to individual UV lamp modules through stainless steel receptacles. All UV lamp modules are individually ground-fault and overload protected for safety.
Lamp Modules6
The UV banks contain low pressure, high intensity ultraviolet lamps arranged parallel to the direction of flow in the channel. The lamps expose the water to UV light with a wavelength of approximately 254 nm. The lamps are provided with a pneumatically operated automatic wiping system for lamp cleaning.
Automatic Level Control Gate6
A constant upstream level gate (hydraulically balanced and requiring power to operate) is provided downstream of each UV channel to maintain the level relatively constant over the range of operating flows.

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Last Updated: 9/16/2013 9:38:55 AM
Version 4.0.1