Section 15 - Screenings Removal

15.01 General Overview

The purpose of providing screenings removal is to protect downstream mechanical devices from damage due to the presence of large objects or stringy material. Additionally, the screens will remove debris that collects in the treatment process and sludge facilities and is not amenable to treatment. This material will be removed by bar screens installed in the influent channels. The bar screens will be cleaned mechanically under normal conditions and manually during emergency operation. The mechanical screens will discharge the collected screenings via a chute into a shaftless screw conveyor. The screenings will then be conveyed to a screenings press where they will be compacted. The liquid from the presses will be collected and discharged downstream of the screens.

There are a total of five channels, four channels 4'-0" wide and one channel 6'-0" wide. All the channels were constructed during Phase I. With completion of Phase II, three mechanically cleaned bar screens have been installed in three of the four 4'-0" channels and a manually cleaned bar screen is installed in the 6'-0" channel. An additional mechanically cleaned bar screen will be installed in the remaining 4'-0" channel during Phase III. Each bar screen channel includes a Parshall flume installed downstream of the bar screen to control the water level and flow velocity through the screens. The Parshall flume is also used for flow measurement purposes. The Parshall flumes in the mechanical screen channels is sized at 1'-6" to provide proper upstream velocities and allow accurate flow measurement of flows up to 21 MGD. The Parshall flume in the manual screen channel is sized at 3'-0" to provide proper upstream velocities and allow accurate flow measurement of flows up to 43 MGD.

Specific design features of the screenings facility include:

• Detectors to monitor for potentially dangerous concentrations of explosive gases.
• Ventilation of odorous air to the air treatment system.
• Mud valves installed in the screen channels to facilitate channel de-watering and grit removal.

15.02 Design Criteria

The design criteria for the Screenings Removal Building Phase II are presented below.

15.03 Computer Assisted Control Strategies

15.03.a. General Overview

At the MPWRF, the influent bar screens are mechanically cleaned and the screenings conveyed to a screening press. The operator controls the recycle timer settings and Influent Channel level setpoint using the SCS located in the Marine Park Administration Building. The Parshall flumes located after the bar screens are used to measure influent flow. The PLC calculates the total influent flow and displays the result on the SCS. The operator uses the SCS to monitor alarms and equipment status. The PLC/SCS system also monitors the Screenings Building for combustible gas level and sounds an alarm if an elevated level is detected.

15.03.b. Control Strategies

Operation:

1. After leaving the influent pump station, the raw influent enters the screenings area through the influent channel. An ultrasonic level sensor (LE-02000) monitors the influent channel level. Level transmitter LIT-02000 transmits the level reading as an analog input to the PLC and the SCS located in the Operator Laboratory and in the Administration Building. At LCP-01 the influent channel level is displayed as an LED bar graph.
   
   
2. If the channel level increases above elevation 42.75', high level float LSH-02001 sends alarm LAH-02001 to the PLC. This alarm is displayed at LCP-01 and on the Annunciator page of the SCS Display. The influent channel level must fall below 42.75' to reset the PLC/SCS system.

15.03.b.2. Mechanically Cleaned Bar Screens

Operation:

Operation:

1. When selected to REMOTE, the conveyor is started after a set time delay following initiation of a barscreen cleaning cycle. It continues to operate after the barscreen stops for a set time delay. The setpoints of both the on delay and off delay times can be viewed on the Bar Screen page of the SCS CRTs. The conveyor cycle can be diasbled to allow operation of the barscreens without activating the conveyor.
   
   
2. When the HOR switch is selected to the HAND position the conveyor may be started or stopped locally. The conveyor is equipped with an emergency stop pull cord.
   
   
3. In either the REMOTE or HAND position the conveyor will stop and send an alarm to the SCS if there is a high torque event, or a broken screw as detected by the no motion switch. The no motion switch has a time delay relay to allow the conveyor to start.

15.03.b.4. Screenings Presses

Operation:

Operation:

Operation:

Operation:

15.04.a. General Operating Strategy

When selected to REMOTE, a screenings press cycle is started after a set time delay following start of the screenings conveyor. The setpoint and progress of the on delay timer is viewable through the SCS CRTs. If both presses are selected to REMOTE, the first press selected to REMOTE will function as LEAD with the other press assigned as backup. If the LEAD press fails, the backup press is started if it is in REMOTE.

When in HAND mode, the bar screen forward cleaning cycle is started by momentarily placing the F-R handswitch in the FORWARD position. The bar screen is run in reverse as long as the F-R handswitch is held in the REVERSE position. The intent of the reverse switch is to provide momentary reverse action to remove jammed material from the screen. If a bar screen failure is detected, it is indicated by the fail light.

When selected to REMOTE, the conveyor is started after a set time delay following initiation of a bar screen cleaning cycle. It continues to operate after the bar screen stops for a set time delay. The setpoints and progress of both the on delay and off delay timers can be viewed through the SCS CRTs.

When the bar screen, screenings conveyor, and screenings presses are all selected to REMOTE mode, they function together in an automatic sequence. This automatic sequence can be disabled at the Bar Screen page of the SCS. The bar screen cleaning cycle is started by either a high influent channel level ahead of the bar screen, or by a recycle timer. Both the high channel level setpoint and the re-cycle timer preset can be set through the SCS CRTs. The channel high level float switch activates an alarm to signal that the influent channel is backed up and water is spilling over the weir gate into the manual bar screen channel.

A gas sensor in the screenings area continuously monitors for the presence of combustible gases. When the level of combustible gases reaches the alarm level as set on the alarm module located on LCP-01, the alarm beacons and alarm horns are activated. The alarm horns are silenced by pressing the horn silence buttons. The alarm condition is also monitored by the PLC/SCS system. The alarm condition is acknowledged and reset by the operator at the alarm module. When the level of combustible gas reaches the warning level as set on the alarm module, an input to the PLC is activated to notify the operator of the conditions through the SCS. The warning alarm is automatically cleared when the LEL level drops below the warning trip level.

15.04.b. Normal Operating Conditions

Note: Normal plant operations will use the PLC and the SCS CRT to enter setpoints, select equipment operating modes and monitor equipment status. Local control of equipment will be covered under section 10.15.c. Alternate Operating Conditions.

15.04.c. Alternate Operating Conditions

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15.04.d. Startup/Shutdown Conditions

• Influent Channels
• Mechanically Cleaned Bar Screens
• Screenings Conveyor
• Screenings Presses
• Parshall Flumes
• Raw Sewage Sampler
• Screenings Effluent Channel

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

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15.04.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 Activated Sludge process. The plans are kept in the Contingency Plan Notebook located in the Operator Laboratory and the Shift Supervisor Office.

• High Hazardous Gas Conditions in the Influent Channel
• Response to High Influent Channel Level
• Response to High/Low pH in Raw Sewage
• Response to Raw Sewage Sampler Failure
• Response to Barscreen(s) Failure

15.04.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 precedure be available. The following is a list of SOPs developed for the Influent Channel and the Mechanically Cleaned Bar Screens. The procedures are kept in the SOP Notebook located in the Operator Laboratory and the Shift Supervisor Office.

• Entering the Influent Channel
• Working on the Mechanical Bar Screens
• Working on the Screenings Press(es)
• Entering the Screening Effluent Channel

15.05. Troubleshooting

15.06. Safety

15.06.a. Confined Spaces

15.06.a.1. Influent Channel

The operator is responsible for reviewing the SOP on Entering the Influent Channel. A confined space entry form must be filled out and properly signed before entry into the channel.

15.06.a.2. Screenings Effluent Channel

The operator is responsible for reviewing the SOP on Entering the Screenings Effluent Channel. A confined space entry form must be filled out and properly signed before entry into the channel.

15.06.b. Mechanically Cleaned Barscreens

Whenever maintenance is being performed on the barscreens they should be locked out electrically, using the standard procedures according to the Lockout/Tagout Policy. Plant flow should be secured to the barscreen using channel gates, if work is to be done in the channel itself. Work in the barscreen channel is considered a confined space, and the appropriate permit should be filled out prior to entry.

Due to the automatic function of this equipment care should be taken whenever working around the mechanical barscreens. Care should also be taken when manually removing debris off the barscreens and ancillary equipment to avoid being cut by sharp objects.

15.06.c. Screenings Conveyor

Whenever maintenance is being performed on the Screenings Conveyor it should be locked out electrically, using the standard procedures according to the Lockout/Tagout Policy.

Due to the automatic function of this equipment care should be taken whenever working around the screenings conveyor. Avoid wearing of loose clothing and jewelry while working on the screenings conveyor.

An emergency pull stop cord is provided for the screenings conveyor in both the Screenings Room and the Press Room.

15.06.d. Screenings Presses

Whenever maintenance is being performed on the screenings presses they should be locked out electrically, using the standard procedures according to the Lockout/Tagout Policy.

Due to the automatic function of this equipment care should be taken whenever working around the screenings press.

15.07.a. Sampling Schedule