Section 25 - Primary Clarification

25.01 General Overview

The purpose for providing primary clarification is to remove the settleable portion of the suspended solids in the wastewater. The primary clarifiers will be constructed in three phases. During each phase, a group of three clarifiers will be constructed. Each clarifier will be 150 feet long, 20 feet wide and 11 feet deep. The clarifiers operate in a plug flow manner with flow entering one end through influent baffles and exiting at the opposite end over launder weirs. As the wastewater passes through the quiescent zone, the settleable solids will fall out and collect on the bottom of the clarifier.

Each clarifier has a sludge conveyor and a cross collector. The sludge conveyor travels the entire length of the clarifier and scrapes settled sludge from the bottom of the basin to a sludge hopper located at the influent end of the clarifier. The cross collector scrapes settled sludge in the sludge hopper to the sludge pump suction. The sludge is then pumped by a progressive cavity pump to the Westside Treatment Plant via the Southside Interceptor.

The sludge conveyor serves a dual purpose by moving scum from the surface of the basin to a scum trough which spans the basins and serves all the clarifiers. The scum is lifted into the trough by helical scum skimmers. Once in the trough, the scum is flushed into a scum sump with primary effluent supplied by a submersible pump located in the effluent channel. The scum and scum flush water is pumped by a progressive cavity pump to the Westside Treatment Plant via the Southside Interceptor.

Specific design features of the primary clarifiers include:

• Structure over the scum trough.
• Chlorine solution applied to the effluent launders.
• Primary influent and effluent sampling.
• Flow splitting achieved with manually adjustable vertical slots at the entrance to each clarifier.

Initially, three basins will be constructed. With one of the three basins out of service, the annual average overflow rate will equal 1190 GPD/FT2 and the peak hour flow overflow rate will equal 3465 GPD/FT2. At the design year with 8 of 9 units in service, the annual average overflow rate will equal 900 GPD/FT2 and the peak hour flow overflow rate will equal 2618 GPD/FT2.

The design criteria for Primary Clarification is presented below.

25.03 Computer Assisted Control Strategies

25.03.a. General Overview

At the MPWRF, primary clarification removes a percentage of the suspended solids. The operator selects how many clarifiers are to be in operation. Each clarifier has a sludge pump and a scum collection trough connected to a scum pump. The sludge pumps will operate continually and are monitored by the PLC and SCS in the event of failure. High torque conditions of the sludge collector are also monitored by the PLC, and will alert the operator if alarm conditions exist. The operator uses the SCS to monitor alarms and equipment status associated with primary clarification. The PLC/SCS also monitors the Primary Clarifiers for combustible gas level and sounds the alarm if an elevated level is detected.

25.03.b. Control Strategies

This section tells the operator how to interact with the PLC and the SCS, describes each control loop and describes how and where alarm and equipment status is transmitted.

25.03.b.1. Combustible Gas Detection

Operation:

1. A gas sensor in the primary influent channel 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 beacon and alarm horn are activated. The alarm horn is silenced by pressing the horn silence button. 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.

25.03.b.2. Clarifiers

Operation:

1. All primary clarifiers operate continuously. A collector can be momentarily stopped by pressing the STOP pushbutton located at the collector. A collector is shutdown if the drive mechanism over torques as sensed by the overload switch. This interlock is reset by pressing the overload reset button.

25.03.b.3. Primary Sludge Pumps

Operation:

1. All primary sludge pumps will operate continuously. A pump can be momentarily stopped by pressing the stop pushbutton located at the pump. A pump will shut down if there is high pressure in the discharge line as sensed by the high pressure switch. This interlock is reset by turning the ON/OFF switch to OFF.

25.03.b.4. Scum Skimmers

Operation:

1. When in REMOTE, the skimmers are controlled by a recycle timer with adjustable ON/OFF setpoints.

25.03.b.5. Scum Flush Water Pumps

Operation:

1. When in REMOTE, a pump is started when the scum skimmers are started and continues to run for a set time delay after the skimmers stop. Only one pump runs at a time and they alternate each time the skimmers are started. A local pump failure is caused by either a high motor temperature or motor casing leak as sensed by a moisture detector in the motor casing

25.03.b.6. Primary Scum Pumps

Operation:

1. When in REMOTE, a pump is started at EL 35.25 feet and stopped at EL 31.5 feet in the scum pit. Only one pump runs at a time and they alternate each time a pump is commanded to start. The pump will shut down if there is a low level in the wet well as sensed by the low level float switch. This interlock is automatically reset when the level in the wet well rises to the pump start level.

25.03.b.7. Primary Effluent Sampler

Operation:

1. The sampler contains an integral timer to initate a sample cycle based on a time interval when in the TIMER mode, or initiates a sample cycle when in the REMOTE mode.

25.03.b.8. Primary Effluent Channel

Operation:

1. The continuous on-line pH meter monitors primary effluent pH and provides a HIGH or LOW pH alarm through the SCS.

25.04 Operating Strategies

25.04.a. General Operating Strategy

All primary clarifiers operate continuously. A sludge collector can be momentarily stopped by pressing the STOP push-button located at each collector. If a drive mechanism overtorque condition is sensed by the overload switch, the PLC will shut down that collector. Pressing the overload reset button will reset the interlock.

All primary sludge pumps will operate continuously. A pump can be momentarily stopped by pressing the STOP push-button located at each pump. If a high pressure condition is sensed by the high pressure switch, the PLC will shut down that pump. Turning the ON/OFF switch to OFF will reset the interlock.

When set in REMOTE, the PLC starts a scum pump at elevation 35.25 FT and stops it at elevation 31.5 FT in the scum wet well Only one pump will run at a time and the pumps will alternate each time a pump starts. The PLC will shut down a pump if a wet well low level is sensed by the low level float switch. This interlock is automatically reset when the level in the wet well rises to the pump start level.

A gas sensor in the primary influent channel 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 beacon and alarm horn are activated. The alarm horn is silenced by pressing the horn silence button. 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 condition through the SCS. The warning alarm is automatically cleared when the LEL level drops below the warning level.

25.04.b. Normal Operating Conditions

Note: Normal plant operations will use the PLC and the SCS CRT to monitor the operation of the primary clarifiers. The operators will monitor the primary clarifiers, the primary sludge pumps, the scum skimmers, the scum flush water pump, the primary scum pumps, the primary effluent sampler, and the combustible gas detector.

25.04.b.1. Primary Clarifiers

The desired number of primary clarifiers are placed in operation using the ON/OFF handswitch for the drive mechanism located at MCC-03, and the drive arms are visually checked to verify rotation before placing on line. The PLC/SCS will monitor each drive mechanism and will notify the operator in the event of an OVERLOAD condition.

25.04.b.2. Primary Sludge Pumps

The primary sludge pumps are operated continuously whenever the clarifier is on line. The PLC/SCS monitors each pump and will notify the operator in the event of a high discharge pressure on any pump.

25.04.b.3. Scum Skimmers

The Helical Scum Skimmers move the scum collected by the clarifier sweep arms into a trough. The helical scum skimmers are placed in REMOTE position, and operate based on an adjusable timer. The skimmer operation is monitored through the PLC/SCS, and will notify the operator in the event of a failure.

25.04.b.4. Scum Flush Water Pumps

The Scum Flush Water Pumps are operated in the REMOTE position, and start whenever the Helical Scum Skimmers run. The flush water pumps are monitored by the PLC/SCS, and will notify the operator in the event of a high temperature of the motor (MOTOR), or if moisture if evident in the motor (MOISTURE).

25.04.b.5. Primary Scum Pumps

Progressing gravity scum pumps automatically pump primary scum and scum flush water to the Westside Facility based on the level of the primary scum tank. The first pump placed in REMOTE will become the lead pump, with the second pump acting as a backup in the event of failure of thelead pump. The operation of these pumps is monitored by the PLC/SCS and will notify the operator in the event of either a failure to run (RUN), or a high discharge pressure (PRESS).

25.04.b.6. Primary Effluent Sampler

The Primary Effluent Sampler is placed in the REMOTE position, and with the sampler operated in the TIMER mode. The PLC/SCS will monitor the operation of the sampler and notify the operator in the event of malfunction. The operator may initiate a manual sample at any time by placing the handswitch to HAND.

25.04.b.7. Primary Influent Channel

A combustible gas detector is located in the primary influent channel, and continuously monitors the atmosphere for LEL of combustible gas. The PLC/SCS will notify the operator in the event hazardous gas levels exceed a preset level. Local alarm beacons and horns will also notify the operator. A local alarm silence pushbutton may be pushed to silence the horn, however, the alarm will not reset until the hazardous gas level falls below the preset alarm level.

25.04.b.8. Primary Effluent Channel

A pH meter located in the primary effluent channel continuously monitors the pH level of the primary effluent. The PLC/SCS monitors the pH level and will notify the operator in the event pH levels of the primary effluent exceed preset levels.

25.04.c. Alternate Operating Conditions

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25.04.d. Startup/Shutdown Conditions

STARTUP

Primary Influent Channel

1. Inspect primary influent channel for foreign debris that may cause problems with impeding flow, etc.
2. Verify that the Hazdardous Gas Detector is operational.
3. Check the Influent Channel for correct gate placement.

Primary Clarifiers

1. Verify all flights are operational, and check for obstructions of same. Check motor drive, gear-reducing box, chains, etc. Place the clarifier drive mechanism in the ON position.
2. Check and confirm that the torque overload alarm light is not on.
3. After first checking other ancilliary equipment associated with primary clarification, open the influent gate to the primary clarifier.

Primary Sludge Pumps

1. Check all suction and discharge valving to the sludge pump(s) and verify that they are all in the open position.
2. Do not start the sludge pumps until there is sufficient water or sludge in the clarifier.
3. After there is sufficient liquid in the clarifiers to operate the pumps without damaging them, start the pumps and verify that high discharge pressure doesn't exist. If high pressure exists the pump will shut down on "HIGH PRESS" alarm.
4. Under normal operation the sludge pumps will operate continuously.

Helical Scum Skimmers

1. Check operation of the helical scum skimmer. Assure skimmer rotates smoothly, and is free of obstructions. Check the level of the scum tank.
2. Verify that the skimmer is in REMOTE position.
3. Adjust the skimmer timer to the desired time interval. The normal time intervals for the skimmer will be determined through practice and experience.

Primary Scum Flush Water Pump

1. Check that Primary Scum Flush Water Pumps are operational, and in REMOTE position at local panels, LP-08041 and LP-08042.
2. Verify that the MOTOR and MOIST alarms are not activated on either pump.
3. Check that each pump operates whenever skimmer starts, and runs for a preset period of time after the skimmer completes its skimming cycle.

Primary Scum Pumps

1. Check the level of the primary scum tank.
2. Verify that both suction and discharge valves to both pumps are in the open position.
3. Verify that both pumps are in the REMOTE position.
4. Check that there are no alarm conditions at the scum tank (ie. "High Level, etc.).

Primary Effluent Sampler

1. Check that the Primary Effluent Sampler is in the REMOTE position.
2. Verify that there are no alarm conditions on the sampler.
3. Initiate a manual sample by pushing the MANUAL sample button, and verify that the proper amount of sample is taken.

Primary Effluent Channel

1. Verify that the pH meter is in the proper location, and that there are no alarm conditions for HIGH or LOW pH.

SHUTDOWN

Primary Influent Channel

1. Shut off associated channel air diffuser to the influent channel. Check the effect on the channel air blower after shutting off the air.
2. Close the channel gates to the influent channel.
3. Open the channel drains after closing the primary clarifier influent gate.
4. Hose down residual sludge in the channel to avoid odors.

Primary Clarifiers

1. Close the influent gate to the clarifier.
2. Continue to run the clarifier drive until all remaining sludge and scum are removed.
3. Open the clarifier drain line.
4. Hose down the clarifier as it empties, as much as possible.

Primary Sludge Pumps

1. Continue to run the primary sludge pumps until the primary clarifier is almost empty. The sludge pumps are the only means of completely emptying the clarifier, however, they should never be operated without liquid as damage to the stator may occur. Close supervision of the pump operation is necessary to avoid pump damage from running dry.
2. Shut off the pumps when the clarifier is empty.

Helical Scum Skimmers

1. After all scum is removed from the clarifier surface, or the water level is otherwise lower than the scum trough, secure the helical scum skimmer by turning to the OFF position.
2. Hose any remaining debris from the scum trough into the scum pit.

Scum Flush Water Pumps

1. The operation of the Scum Flush Water Pumps is automatic, and no special shutdown procedures are necessary for these pumps.

Primary Scum Pumps

1. The operation of the Primary Scum Pumps is automatic, and no special shutdown procedures are necessary for these pumps. It is doubtful that this process will ever be shutdown as at least one primary clarifier will always be in service.

Primary Effuent Sampler

1. Under normal operating conditions the Primary Effluent Sampler will be in the REMOTE position, and the sampling cycles will be controlled by an external flow signal. An alternate control mode will be the TIMER mode which initiates a sampling cycle based on a preset timer. To shut the sampler down in either mode place the switch in the OFF position.
2. If the sampler is to remain off for an indeterminate amount of time it should be thoroughly cleaned.

Primary Effluent Channel

1. The Primary Effluent Channel is equipped with a continuous on-line pH meter. Since its doubtful the effluent channel will ever be totally out of service there are no special procedures in shutting it down. However, the pH meter may be removed from service from time to time, and the meter's operating manual should be followed for this procedure.

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

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25.04.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 Influent Pumping process. The plans are kept in the Contingency Plan Notebook located in the Operator Laboratory and the Shift Supervisor Office.

• Response to Primary Effluent Sampler Failure

25.04.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 Influent Pumping process. The procedures are kept in the SOP Notebook located in the Operator Laboratory and the Shift Supervisor Office.

• Entering Primary Influent Channel
• Entering Primary Clarifier
• Entering Primary Effluent Channel

25.05. Troubleshooting

25.06. Safety

25.06.a. Confined Spaces

25.06.a.1. Primary Influent Channel

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

25.06.a.2. Primary Clarifiers

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

25.06.a.3. Primary Effluent Channel

25.07.a. Sampling Schedule