Section 35 - UV Disinfection

35.01 General Overview

The purpose of disinfection is to destroy disease-causing organisms in the final effluent prior to ultimate discharge into the receiving water. The means of disinfection is ultraviolet (UV) radiation. Secondary effluent flows from the secondary clarifiers into a flow splitting box at the influent to the UV channels. Flow split is accomplished based on headloss over a rectangular weir. The flow in each channel passes through a manually-cleaned bar screen which intercepts materials that could foul or damage the UV lamps. A counter-weighted flap gate at the discharge end of each channel maintains a constant water surface elevation in each channel. A motorized slide gate at the entrance to each channel provides isolation when a channel is shutdown for flow pacing or maintenance.

After leaving the UV channels, effluent flows through a small withdrawal well where non-potable plant water and reuse water (wetlands enhancement and/or irrigation) can be removed from the flow stream. The remaining effluent flows through the parshall flume before discharge to the outfall structure in the Columbia River.

The UV dose is adjusted based on effluent flow measurement. Banks of UV modules are energized/de-energized and UV channels put into and out of service as plant flows vary. One bank of modules in each channel will remain energized whenever the channel is in service to base load the channel. Service hours are equalized by rotating the lead bank to the bank with the least run time every time the lead channel rotates. The flow setpoints are as follows:

Channel No.3 has the capability to operate isolated from the secondary effluent flow. Slide gates control the isolation of this channel. When wetlands treatment is not required, this channel is available to operate in parallel with the remaining channels. In addition to isolation slide gates in the influent and effluent channels, a motorized slide gate controls the channel influent flow in the same manner as the other two channels and a sluice gate is used to isolate the discharge pipe to the wetlands.

Upon expansion of the effluent treatment complex, the northern most UV channel will remain the "wetlands treatment channel" and piping to the existing "wetlands" channel No.3 will be re-routed.

The UV control system for the wetlands treatment channel is operated manually while disinfecting the wetlands flow stream and in flow pacing mode while disinfecting the plant effluent flow stream. The wetlands treatment system is not required for the plant effluent flow treatment capacity and can serve as a backup plant effluent disinfection system.

Regular cleaning and inspection of the UV lamps is required to maintain effluent disinfection quality. Cleaning involves removing the modules from the channels. To accommodate this maintenance activity a UV cleaning system is provided. A bridge crane is used to remove UV modules or an entire UV bank from the channels and transport them to the cleaning area. A curbed hose down area is provided to remove loose material from the modules. The modules or banks are then lowered into a basin where an acidic cleaning solution removes scale build-up. A diffused air system agitates the cleaning basin to further enhance material removal. A dilute concentration of citric or phosphoric acid is most commonly used as a cleaning solution.

Three UV channels will be built as part of the Phase I work. Each channel is wide enough for 23 UV modules in each bank. To meet specified treatment criteria, only 19 modules per bank are supplied. Space equal to the size of 2 modules are installed on each side of each bank for the addition of future modules. Each bank is constructed to accommodate the future number of modules. Using future modules will not increase the flow capacity of each channel, but will increase the level of disinfection achieved.

35.03 Computer Assisted Control Strategies

35.03.a. General Overview

At the MPWRF, the UV disinfection system is controlled through the System Control Center (SCC) located in the Effluent Disinfection Building. The operator uses a panelview system at the SCC to monitor alarms and change system variables. The operator can also use the SCS to monitor alarms and equipment status of the UV system.

35.03.b. Control Strategies

Operation:

1. The influent gates are controlled through the SCC when the LOR handswitch is placed in REMOTE. The SCC opens and closes the gates based on the flow and selected channel orientation (see Table 1 section 35.01). The operator can monitor the position of each gate using the SCS located in the Operator Laboratory and in the Administration Building.
   
   By placing the LOR handswitch in LOCAL, the operator can manually open and close the gates using the OSC handswitch.
   
   
2. If the gate fails to open or close properly, MAJOR ALARM QA-1600X will sound. This alarm is displayed on the SCS located in the Operator Laboratory and in the Administration Building. To reset the PLC/SCS the gate must be placed in its proper position (either open or closed). A COMMON UV DISINFECTION SYSTEM ALARM (QA-16000) will also occur. This alarm is displayed on the SCS, on LCP-12 and activates the Alarm Autodialer. Resetting the MAJOR ALARM will also reset the COMMON UV ALARM.

35.03.b.2. UV Disinfection Channel

Operation:

1. The operator uses the Allen-Bradley 2711-TCI PanelView Touch Screen to control and monitor the Trojan UV3000 system. The SCS computers located in the Operator Laboratory and the Administration Building can be used to monitor system operation only. The operator views the various equipment and control displays in the system by pressing the appropriate soft key on the PanelView screen. The following is a brief overview of each menu and screen available on the PanelView. For further information please reference the Trojan Technologies Operation and Maintenance Manual on the UV Disinfection System.
   • Control Menu - used to access all control screens
   • Status Menu - used to access all status screens (status screens are identical to control screens, the only difference being that no control capabilities are provided, only status of the system)
   • Flow Control Screen - used to indicate the current flow through the system. Flowpacing setpoints and deadband are set through this screen
   • Time Setup Screen - provides a time and date display. Changes to the time and date are entered through this screen
   • Mod/Lamp Status Screen - used to monitor module and lamp status. The operator uses this screen to monitor, and if desired change the Bank Mode Control
   • Channel/Bank Setup Screen - used to set up a variety of system variables. These include:
     • Lead channel rotation
     • Bank warm up delay
     • Gate response
     • Bank time off delay
     • Channel drain
     • Multiple lamp failure
     • Alarm delay
     • Lead/lag channel selection
   • UV Intensity Screen - used to monitor UV intensity for each bank
   • Last 20 Module and Lamp Faults Record Screen - used to view the last 20 module/lamp faults. The operator can scroll up or down the screen to view the entire fault record
   • Alarm Status Screen - used to monitor the status of each individual alarm. The operator can scroll up or down through the screen either by page or individually
   • Alarm History Screen - used to review the history of alarms that have occurred, listing the alarm, the time and date it occured and the time and date that it was acknowledged
   • System Overview Screen - used as an overall view of the UV system. Most of the status points and other indicators and variables are displayed on this screen.

35.03.b.3. UV Channel Drain Valves

Operation:

1. The channel drain valves are completely controlled through the PLC/SCS system. The operator must use the SCS CRT to manually open or close the valves or to monitor the open/close position of the valve. The valve either opens or closes depending on the status of the UV channel. When a channel is put on line by the SCC, the PLC tells the valve to close, when the channel goes off-line, the the valve opens.
   
   The valve will remain in the open position until the channel goes back on line.

35.03.b.4. UV Cleaning Blower

Operation:

1. There is no SCS control available for turning the UV Cleaning Blower on and off. The operator can monitor blower on status through the SCS, however, blower control is only possible at the Effluent Disinfection Building.

35.04 Operating Strategies

35.04.a. General Operating Strategy

The influent channel gates are opened and closed manually by placing the LOR in LOCAL and using the OSC handswitch. The gates are controlled by the System Control Center (SCC) when the handswitch is placed in REMOTE.

The UV banks and influent channel gates in each disinfection channel are controlled by the System Control Center (SCC, LP-16000). The operator can monitor alarms and change system variables through a programmable user interface consisting of a CRT and keyboard mounted on the SCC. The SCC receives a 4 - 20 mAdc plant flow signal from the plant PLC located in LCP-16. The SCC uses the flow signal to turn the lamp banks on and off and to open and close the channel gates. For a detailed description of operations and user adjustable parameters see the UV manufacturer's control description.

The flow pacing signal is a 4 to 20 mA analog signal produced by the plant PLC in LCP-16. When the SCC is in flow pacing mode, it receives the 4 to 20 mA signal from the plant PLC and automatically adjusts the number UV lamps that are energized and number of channels in service in proportion to the signal.

Whenever a UV channel is taken out of service (its influent gate closed), the channel drain valve at the end of the channel is opened to drain the channel. The drain valve remains open until the channel is placed back into service to prevent water from collecting in the channel.

35.04.b. Normal Operating Conditions

Blank.

35.04.d. Startup/Shutdown Conditions

• UV Disinfection Channel
• UV Cleaning Basin Blower

35.04.e. Key Control Data and 4 Level Alarm Table

35.04.f. Contingency Plans

A contingency Plan is written to handle an unpredictable occurrence that has a reasonable chance of happening. Most Contingency Plans deal with situations where having a written procedure can help prevent loss of a unit process, handle a process upset or respond to a hazardous situation. The following is a list of Contingency Plans developed for the UV Disinfection process. The plans are kept in the Contingency Plan Notebook located in the Operator Laboratory and the Shift Supervisor Office.

• Loss of Use of UV Disinfection System

35.04.g. Standard Operating Procedures

A Standard Operating Procedure (SOP) is written to handle operator activities performed on a regular but not necessarily daily basis. Most SOPs deal with potentially hazardous activities making it important that a written, step-by-step procedure be available. The following is a list of SOPs developed for the UV Disinfection process. The procedures are kept in the SOP Notebook located in the Operator Laboratory and the Shift Supervisor Office.

• Entering the UV System Influent Channel
• Entering the UV Module Channels
• Entering the UV System Effluent Channel

35.05. Troubleshooting

35.07.a. Sampling Schedule