Section 40 - Air Treatment System

40.01 General Overview

The Air Treatment System for the Marine Park Water Reclamation Facility treats contaminated air from the various facilities. Sources of odor which are contained and treated include:

• Influent Pump Station Wet Well
• Screenings Building
• Grit Chambers
• Screenings/Grit Handling Building
• Primary Clarifiers

Odorous air is treated using two liquid scrubber packed towers.

The packed tower liquid scrubbers consist of a cylindrical tower made of corrosion-resistant material containing a bed of plastic packing material. Scrubber liquor (Sodium Hydroxide and Sodium Hypochlorite) is sprayed over the top of the packing material and flows down through the bed, coating the packing with a thin layer of liquid. After flowing through the tower, the liquid is collected in a sump and is recycled back to the top of the tower. Makeup chemicals and water are added to the recycled flow to maintain removal efficiencies, and a small amount of spent liquid (blowdown) is removed and discharged to the primary clarifiers. The odorous gas stream enters the side of the tower and flows up through the coated packing material.

The liquid scrubber towers operate 24 hours per day since odors are continuously emitted from the various sources. There are two scrubber towers. Each can handle 50 percent of the design odorous air flow. When one or both towers are out of service, Sodium Hypochlorite can be added at the pump station wet well and grit chamber effluent for supplemental odor control.

The design criteria for the Screenings/Headworks Buildings Air Treatment System is presented below.

The design criteria for the odor control system chemical use and storage are presented below for Phase I:

40.03 Computer Assisted Control Strategies

40.03.a. General Overview

At the MPWRF, the Scrubber Recirculation Pumps, Blow Down Valve, ORP and pH levels are automatically controlled through a PLC. Odor Scrubber Tower Blower operation and makeup water addition are manually controlled through control panels located in the ATS control area. Setpoints for the control loops are entered into the PLC through the SCS located in the Marine Park Administration Building. The operator also uses the SCS to monitor alarms and control status. The operator has the ability to start and stop chemical feed pumps P-17025, P-17026, P-17201 and P-17202 (all located in the Chemical Storage Building) through the SCS CRT.

40.03.b. Control Strategies

Operation:

1. The scrubber liquor recirculation pumps (P-05011 and P-05012) are controlled at MCC-03 and MCC-04. There is a HOR handswitch for each pump. In REMOTE mode, the PLC controls pump operation.
   
   The recirculation pumps are designed to run continuously. Occasionally, the operator will need to turn off a pump manually, for maintenance or other reasons. In HAND mode, the operator can manually start and stop the pumps.
   
   
2. If either pump fails while in the REMOTE mode, the PLC will send a RUN FAIL alarm to the SCS. The alarm is displayed on the SCS located in the Operator Laboratory and in the Administration Building. Place the HOR handswitch in OFF to reset the PLC/SCS. If both recirculation pumps fail or are turned off, the PLC will generate a PUMPS FAIL alarm (QA-05010). This alarm is displayed on the SCS and on LCP-01. To reset the PLC/SCS, one of the pumps must be turned on.

40.03.b.2. Odor Control Scrubber Sump Level

Operation:

1. The Odor Control Scrubber Sump Level transmitter (LT-06011, LT-06012) measures and transmits to the PLC the Scrubber Sump level.
   
   
2. The operator is responsible for setting the high and low Scrubber Sump levels using the SCS CRT.
   
   
3. The PLC uses the high and low setpoints to activate LAH-06011, LAL-06011, LAH-06012 and LAL-06012. The PLC sends these alarms to the SCS. These alarms are displayed on the SCS located in the Operator Laboratory and in the Administration Building. To reset the PLC/SCS, the Scrubber Sump level must increase above the low level setpoint (for a low level alarm) or decrease below the high level setpoint (for a high level alarm).

40.03.b.3. Odor Scrubber Blowdown

Operation:

1. In REMOTE mode, the PLC operates FCV-05090 according to the Scubber Sump level.
   
   At the high Scrubber Sump setpoint, the PLC opens FCV-05090 to drain excess Odor Scrubber Liquor from the Scrubber Sump. At the low Scrubber Sump setpoint, the PLC closes FCV-05090.
   
   
2. The operator may choose to operate FCV-05090 manually by using the OPEN and CLOSE positions on HS-05090.

40.03.b.4. Odor Exhaust Fans

Operation:

1. EF-05911 and EF-05912 are controlled at MCC-03 and MCC-04. In REMOTE mode, the PLC monitors the fan operation.
   
   
2. Both fans are designed to run continuously. If the PLC receives a fan motor stop indication, it sends a fan OFF alarm to the SCS. The alarm is displayed on the SCS located in the Operator Laboratory and in the Administration Building. Place the HOR handswitch to OFF to reset the PLC/SCS.

40.03.b.5. Chemical Addition

Note: Chemical feed pumps and controls are all located in the Chemical Building. Local panel LP-17200 controls the Sodium Hydroxide feed pumps (P-17201 and P-17202). Local panel LP-17020 controls the Sodium Hypochlorite feed pumps (P-17025 and P-17026).

Operation:

1. Pumps P-17025, P-17026, P-17201 and P-17202 are all controlled through LP-17200 and LP-17020 located in the Chemical Building. In REMOTE mode, there are two methods for controlling the chemical feed pump sequencing and stroke; AUTO and MANUAL. The mode can be selected for each pair of pumps independently.
   
   In MANUAL mode the operator starts, stops and sets pump stroke through the SCS CRT.
   
   In AUTO mode, the PLC controls pump operation. The chemical feed PID controller sets the pump stroke to maintain the ORP or pH at the desired setpoint. The controller setpoints are entered through the SCS CRT.
   
   
2. If both pumps of each pair are selected to REMOTE, the first pump placed in REMOTE becomes the LEAD pump. The operator can change the LEAD pump by using an SCS CRT handswitch.
   
   
3. In the event of a LEAD pump failure or if the LEAD pump cannot maintain the ORP or pH setpoints the LAG pump starts. If the PLC senses a chemical feed pump failure, it will send a pump FAIL alarm to the SCS. Place the HOR handswitch to OFF to reset the PLC/SCS.

40.04 Operating Strategies

40.04.a. General Operating Strategy

The scrubber towers and make-up water are manually operated at the odor control area. The scrubber recirculation pumps, blow down valve, Sodium Hydroxide and Sodium Hypochlorite dosages are automatically controlled.

Variations in loading to the air treatment system are expected to change on a seasonal basis. It is not anticipated that rapid control changes will be required.

The quantity of makeup water added to the scrubber sumps or recirculation pump suction will vary as a function of the chemicals used for treatment. When Sodium Hydroxide and Sodium Hypochlorite are added, makeup water must be added to sweep precipitates from the sump and replace any water lost to evaporation. When Sodium Hydroxide alone is added, makeup water must be added to displace hydrogen sulfide saturated water from the sump. Makeup water addition is controlled manually with a setpoint adjustment to achieve the desire flow ranges. A drain line at the bottom of the sump can be manually opened to allow any precipitate to be removed.

The blow down valve is controlled automatically by placing the OCR handswitch into the REMOTE position. The blow down valve removes excess recirculation water from the tower. The blow down valve automatically opens on high level and closes on low level in one of the scrubbing towers.

In REMOTE mode, there are two modes for controlling the chemical feed pumps sequencing and stroke; AUTO and MANUAL. The mode can be selected for each pair of pumps independently. The PLC provides two chemical feed PID controllers and faceplates, AIK-05020 for ORP control and adjustment of Sodium Hypochlorite addition and AIK-05030 for pH control and adjustment of Sodium Hydroxide addition.

When a pair of pumps is select to MANUAL mode, each pump can be started or stopped, and the pump stroke is individually adjusted through the SCS CRT by the operator. For Sodium Hydroxide addition, if the pH reaches the high pH alarm point and either pump has been started in MANUAL, that pump is stopped. The pump is re-started at the low pH alarm.

When a pair of pumps is selected to AUTO mode, the stroke of the pumps is adjusted by the chemical feed PID controller to maintain the associated ORP or pH reading at the desired setpoint. The controller setpoints can be changed through the SCS CRT. The pH setpoint should be set near 8.5 and 9.0 because Sodium Hypochlorite is strongly disassociated at these pH values. The disassociated hypochlorite ion is the oxidizing agent used to treat the odorous air. The ORP setpoint should be a positive value indicating a small hypochlorite ion excess.

In AUTO mode, each pair of pumps is sequenced automatically. If both pumps in a pair are selected to REMOTE, the first pump placed in REMOTE is defined as the LEAD. The LEAD pump may be changed by operations using a SCS CRT handswitch. The LEAD pump is started or stopped based on the closeness of the pH or ORP reading to setpoint and the requested pump output. When the pH or ORP readings deviate from the setpoint value by more than 5 percent for a preset time, the LEAD pump for the associated pair is started. Once started, the pump stroke is automatically adjusted by the controller to maintain setpoint. When the controller output is less than 10 percent and the pH or ORP reading is within 5 percent of setpoint, the LEAD pump is stopped. The LAG pump for each pair is started when the output of the associated controller is greater than 99 percent and the pH or ORP readings deviate from the setpoint by more than 5 percent for a preset time. The output of the controller is initialized to 50 percent and sent to both pumps. When both pumps in a pair are on, they operate in parallel. When the output of the controller drops to less than 30 percent the LAG pump is stopped and the output of the controller is initialize to 60 percent and is sent to the LEAD pump.

40.04.b. Normal Operating Conditions

40.04.c. Alternate Operating Conditions

Blank.

40.04.d. Startup/Shutdown Conditions

• Scrubber Liquor Recirculation Pumps
• Odor Exhaust Fans
• Chemical Feed Pumps

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

40.04.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 Air Treatment System process. The plans are kept in the Contingency Plan Notebook located in the Operator Laboratory and the Shift Supervisor Office.

• [None defined at this time.]

40.04.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 Air Treatment System process. The procedures are kept in the SOP Notebook located in the Operator Laboratory and the Shift Supervisor Office.

• Entering and Cleaning the ATS Scrubbers

40.05. Troubleshooting

40.06. Safety

40.06.a. Confined Spaces

40.06.a.1. ATS Scrubbers

40.07.a. Sampling Schedule