Section 30 - Activated Sludge

30.01 General Overview

30.01.a. General Overview

The activated sludge process oxidizes and removes soluble and finely divided suspended material that is not removed at the headworks and primary treatment. This is accomplished in the aeration tanks by aerobic organisms during the time the primary treated sewage is detained in the aeration tank. Soluble or finely divided suspended matter is stabilized in the aeration tanks by bacterial oxidation to carbon dioxide, water, and new cellular material. This increased volume of cellular material is removed as a portion of the return activated sludge settled in the secondary clarifiers.

The aeration basins will be constructed in three phases. A group of three basins will be constructed under each phase. Each basin is 160 feet long, 29 feet wide, and 17.7 feet deep. The basins are capable of operating under four different modes: plug-flow, plug-flow anoxic, step-feed and contact stabilization. The preferred mode of operation will be plug-flow anoxic.

All modes introduce return activated sludge (RAS) at the head of the aeration basins; mixed liquor will exit at the opposite end of the aeration basin over the effluent weirs in all modes.

In the plug-flow and plug-flow anoxic process modes, primary effluent will enter over the influent weir gates and exit over the effluent weirs into an effluent channel. In the step-feed and contact stabilization mode, primary effluent will enter through a sluice gate to a 24-inch pipe with adjustable discharge ports to the aeration basin. Air is supplied to all aeration zones within each basin.

Each basin is divided into five aeration zones. The first zone has a baffle wall and mixer for plug-flow anoxic operation. The other four aeration zones contain fine bubble membrane diffusers. The aeration basins are provided with process air by the process blowers. The rate of air supply will be based on dissolved oxygen (DO) in the basin.

30.01.b. Secondary Clarification

The secondary sedimentation process settles the biological floc from the activated sludge mixed liquor allowing the clarified secondary effluent to be withdrawn from the overflow weir. Two clarifiers are provided for the first phase of construction. Four additional clarifiers will be provided for the ultimate buildout plan. A sodium hypochlorite feed point is provided at the weir ring in each clarifier. The purpose of this disinfection point is to allow a periodic shock application to eliminate any biological growth on the weirs.

30.01.c. Secondary Scum

The design criteria for low pressure Channel Aeration and the Activated Sludge System are presented below.

30.03 Computer Assisted Control Strategies

30.03.a. General Overview

At the MPWRF, the process blowers supplying air for the activated sludge are controlled through the PLC. The operator selects which control mode the PLC uses to run the blowers using the SCS located in the Marine Park Administration Building. The operator also uses the SCS to monitor alarms and equipment status.

30.03.b. Control Strategies

Operation:

1. Four constant speed multistage centrifugal blowers are used to supply air to the aeration basins and the aeration basin effluent channel. The inlet flow control valves (ICV-1101X, X = 1 - 4) throttles the inlet to each blower. The inlet throttling valve manual loading station (IHK-1102X) controls the position of the inlet valve. Control devices for each blower and the inlet throttling valve manual loading station are located at the local panel located in the blower building. Each local panel contains an HOR handswitch for blower control and an AUTO/MANUAL selector for the inlet throttling valve loading station.
   
   With the HOR handswitch in REMOTE and the inlet throttling valve set on AUTO the operator can select one of three blower operating modes through the SCS CRT.
   
   In MANUAL mode, the operator can start and stop each blower and adjust the inlet valve position through the SCS CRT.
   
   In AUTO mode, the air flow PID controller modulates the inlet valve for each blower to maintain an airflow (SCFM) setpoint for all the blowers.
   
   In CASCADE mode, the D.O. PID controller sets the air flow PID controller setpoint. The D.O. PID controller adjusts the airflow setpoint to maintain a set D.O. level in the aeration basins. The operator uses the SCS CRT to select which D.O. reading to use for this loop. Options include, basin 1 D.O., basin 2 D.O., basin 3 D.O. or an average of the basin D.O.'s.
   
   
2. If a blower fails while running in REMOTE, a blower fail signal is sent from the local panel to the PLC. The PLC generates a RUN FAIL alarm (QA-1102X) which is displayed on the SCS located in the Operator Laboratory and in the Administration Building. To reset the PLC/SCS set the HOR handswitch to OFF. If one of the local sensors indicates a high bearing temperature, excessive vibration, low oil level or a surge condition, the blower is turned off and the PLC generates a LOCAL FAIL alarm. This alarm is displayed on the SCS and on LCP-12. To reset the PLC/SCS set the HOR handswitch to OFF. If all the blowers fail or are turned off, the PLC will generate an ALL BLOWERS OFF alarm (QA11020) which is displayed on the SCS and on LCP-12. To reset the PLC/SCS the operator must turn on a blower.

30.03.b.2. D.O. Control

Operation:

1. The operator is responsible for setting the high and low D.O. setpoints using the SCS CRT. If the aeration basin D.O. as measured by AIT-09041, AIT-09042, or AIT-09043 exceeds the high D.O. setpoint then the PLC generates alarm AAH-0904X. This alarm is displayed on the SCS located in the Operator Laboratory and in the Administration Building. To reset the PLC/SCS, the D.O. must fall below the high setpoint. If the D.O. falls below the low D.O. setpoint then the PLC generates alarm AAL-0904X. This alarm is also displayed on the SCS located in the Operator Laboratory and in the Administration Building. To reset the PLC/SCS, the D.O. must rise below the low setpoint.
   
   
2. The PLC also monitors the D.O. probe readings to verify that none of the probes is giving a incorrect or false reading. If any of the probes is found to be giving a bad reading, the PLC will generate alarm QA-0904X. This alarm is displayed on the SCS located in the Operator Laboratory and in the Administration Building. To reset the PLC/SCS press the reset button on the SCS CRT.

30.03.b.3. Secondary Clarifiers

Operation:

1. To start and stop the secondary clarifier drive mechanisms, the operator uses the ON/OFF handswitches located on MCC-09 and MCC-12. The operator cannot start or stop the secondary clarifier drive mechanism from the SCS CRT.
   
   
2. If high torque warning switch NSH-1404X senses a high torque condition (torque load reaches 40 percent of the ultimate torque) it sends a signal to the PLC which generates high torque alarm NAH-1404X. This alarm is displayed on the SCS located in the Operator Laboratory and in the Administration Building. The PLC/SCS resets when the torque condition clears. If the high-high torque switch NSHH-1404X senses a high-high torque condition (torque load reaches 85 percent of the ultimate torque) it sends a signal to the PLC which generates a high-high torque alarm NAHH-1404X. This alarm is also displayed on the SCS. To reset the PLC/SCS press the torque RESET button on MCC-09.

30.04 Operating Strategies

30.04.a. General Operating Strategy

The output of the blowers is controlled manually by placing the manual loading station into MANUAL mode and adjusting the position of the inlet throttling valve.

Both the aeration basin air flow and bypass valve are controlled locally by placing the manual loading station into MANUAL mode and adjusting the position of the valve. Under current design conditions, the air flow valve is normally selected to local MANUAL mode and opened 100 percent. The bypass valve is normally placed in local AUTO mode for remote control through the PLC/SCS.

The aeration basin air flow valve is provided for future control of air flow to the three basins currently installed. When aeration basins are added in the future, the control scheme will be modified so that a constant air header pressure is maintained and flow to each set of basins is controlled by the air flow control valves.

The first blower placed into REMOTE mode will function as the LEAD and start immediately. The second and third blower placed in REMOTE mode, can be started through the SCS CRT by the operator. All the blowers placed in REMOTE can also be stopped throught the SCS CRT. The LEAD blower may be changed by operations through the SCS CRT. If a blower is not started by the operator and it is in REMOTE, it functions as a standby and will come on should another blower fail. Upon power failure, only the channel blower that is designated as the LEAD will re-start.

In the REMOTE mode, the blowers function in a LEAD, 1st LAG, 2nd LAG, and STANDBY sequence as defined below. The output of the blowers is controlled, automatically by placing the inlet valve manual loading station into AUTO mode. The air flow split between the basins is controlled by manual valves on the individual basin air headers.

When the blowers are in REMOTE mode and the inlet valve manual loading stations are in AUTO mode, there are three modes of control available through the PLC/SCS, MANUAL, AUTO, and CASCADE. The mode selected is used to control all four blowers. There is a PID loop faceplate for each blower at the SCS CRT allowing operating personal to monitor air flow and blower performance and to make control changes as defined below. The PLC/SCS is programmed for bumpless transfer between modes. This means that the air flow to the basins will not make an instantaneous change when the mode of the controllers in change between MANUAL, AUTO and CASCADE.

When in MANUAL mode, each blower can be individually started and stopped and have its inlet valve position adjusted through the SCS CRT.

When in AUTO mode, the inlet valve for each blower is modulated by a PID flow controller to maintain an air flow setpoint to the three basins. The sum of the air flows to each basin is used as the process variable for the PID controller.

When in CASCADE mode, the air flow PID controller setpoint is "cascaded" from or set by the output of a D.O. PID controller. The D.O. PID controller adjusts the air flow setpoint to maintain a D.O. setpoint. The process variable for this loop is selected through the SCS CRT. Options include, basin 1, basin 2, basin 3, or average basin D.O. If average D.O. is selected, an alarm is generated if one of the probe readings is significantly different from the other two. That probe reading is removed from the average calculations until the alarm is reset at the SCS CRT.

The blowers are sequenced automatically when AUTO or CASCADE mode is selected. Any time a blower is started automatically, its inlet valve is initially at its minimum open position. The blower selected as LEAD operates continuously. The 1st LAG blower is started when the LEAD blower reaches its maximum capacity and the system demand continues to increase. This is determined when the output of flow controller is greater than 99 percent and the flow controller setpoint is greater than the capacity of one blower for a preset time. The output of the flow controller is then initialized to a value where two blowers deliver the same capacity as a single blower. The LEAD blower and 1st LAG blower then operate in parallel. The 2nd LAG blower is started when both LEAD and 1st LAG blowers reach their maximum output and the system demand continues to increase. This is determined when the output of flow controller is greater than 99 percent and the flow controller setpoint is greater than the capacity of two blowers for a preset time. The output of the flow controller is then initialized to a value where three blowers deliver the same capacity as two blowers. All three blowers will then operate in parallel.

The blowers are sequenced off in reverse order as the system demand decreases. The 2nd LAG blower is stopped when the flow controller setpoint falls within the maximum capacity of two blowers. The value of the flow controller setpoint at which this happens is lower than the value of the 2nd LAG blower start setpoint to prevent cycling. As the system demand continues to decrease the 1st LAG blower is stopped when the flow controller setpoint falls within the maximum capacity of a single blower. No initialization of the flow controller output occurs when stopping blowers. The inlet valves of the blowers are set to their initial minimum open position when the blowers are stopped.

The LEAD blower is selected by a handswitch on the SCS CRT. The following are the blower sequences based on which blower is operating as LEAD:

LEAD	1st LAG	2nd LAG	STANDBY
1	2	3	4
2	3	4	1
3	4	1	2
4	1	2	3

In order to be included in the automatic sequence, a blower must be selected to REMOTE and the local panel manual loading station for the inlet valve must be selected to AUTO. If a blower is called to run and the switches are not in these positions, the next blower in the LEAD/LAG sequence will be started if it is selected to REMOTE and its inlet valve control manual loading station is in AUTO. In REMOTE mode, upon failure of the LEAD or a LAG blower, the STANDBY blower is started.

Split range control is programmed to prevent blower surge when air flow demands are between the range of one and two, or two and three blowers. When operating in AUTO or CASCADE mode, the air flow controller adjusts the blower inlet valve(s) in an attempt to maintain the air flow controller setpoint. If the controller closes the inlet valves to their minimum open position, the PLC/SCS will switch to split range control. In split range control, the bypass valve is modulated to increase air flow demand so that the blower(s) move away from surge conditions. The output from the air flow controller is reversed and is used to modulate the bypass valve from its minimum open position to its full open position. Split range control is stopped if the air flow controller setpoint decreases so that a blower can be stopped or if the setpoint increases so that the controller output increases above the minimum open position of the inlet valves.

Surge conditions are identified by the blower current reading(s) approaching the current value at which the blowers experience surge. The blower surge current values will be correlated with blower inlet valve position. For example, an inlet valve position of 15 percent may move the blower(s) and blower current into surge conditions. This then would be the inlet valve minimum open position.

The bypass valve is also used to control the air flow to the aeration basin effluent channel. A minimum air flow to this channel is required to prevent settling of solids and to prevent plugging of the diffusers. The minimum open valve position for the bypass valve is determined when a single blower is delivering its minimum air flow rate and the air requirements of the effluent channel are still being met.

A clarifier is shutdown if the drive mechanism over torques as sensed by the high-high torque switch. This interlock is reset by pressing the high torque reset button. The clarifier manufacturer will provide control panel for the scum withdrawal system. This will allow the operators to adjust the amount of secondary scum generated by selecting the degree to which the scum trough is rotated into the water surface to collect scum.

Equipment protection interlocks -- alarms will be activated when the torque load reaches 40 percent of the ultimate torque. If the torque continues to increase, up to 85 percent of the ultimate torque, the clarifier drive motor will be de-energized.

30.04.b. Normal Operating Conditions

For AUTO operation use the following:

For MANUAL operation use the following:

30.04.b.2. Aeration Basins

There is no SCS CRT control or monitoring available on the butterfly valves directing air flow to the individual aeration basin air headers or for the butterfly valves used to balance flow to each aeration zone. The operator is responsible for adjusting these valves to equalize air flow split between aeration basins and to taper air flow along the length of each basin. The operator is also responsible for putting the D.O. probes in the proper zone within each aeration basin.

30.04.b.4. Ducking Scum Skimmers

The ducking scum skimmers are neither operated nor monitored through the PLC/SCS system. Rather, the operation is totally controlled through local panels LP-14041 and LP-14042 located on each secondary clarifier. To operate the skimmers the operator must set the scum trough rotation delay timer, the scum trough open time and the scum collection arm pass counter. Once adjusted these setting should not be changed unless there is a change in scum removal efficiency. Equipment Status Table 30.04.b.4. summarizes the proper equipment configuration for normal plant operation.

30.04.c. Alternate Operating Conditions

Blank.

30.04.d. Startup/Shutdown Conditions

• Process Blowers
• Secondary Clarifiers

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

30.04.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.

• Response to Influent Toxic Dump

30.04.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 Activated Sludge process. The procedures are kept in the SOP Notebook located in the Operator Laboratory and the Shift Supervisor Office.

• Entering the Aeration Basin Influent Channel
• Entering and Cleaning the Aeration Basins
• Entering the Aeration Basins Effluent Channel
• Entering and Cleaning the Secondary Clarifiers

30.05. Troubleshooting

30.06. Safety

30.06.a. Confined Spaces

30.06.a.1. Aeration Basin Influent Channel

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

30.06.a.2. Aeration Basins

The operator is responsible for reviewing the SOP on Entering and Cleaning the Aeration Basins. A confined space entry form must be filled out and properly signed before entry into any basin.

30.06.a.3. Aeration Basin Effluent Channel

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

30.06.a.4. Secondary Clarifiers

30.07.a. Sampling Schedule