Toronto Water Ashbridges Bay Treatment Plant Aeration Tank 2 - Process Control Narrative

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3 Toronto Water Ashbridges Bay Treatment Plant Aeration Tank 2 - Process Control Narrative Issued For Construction Rev Issued For Construction % Submission For Programmers Rev Date Description

4 Ashbridges Bay Treatment Plant Aeration Tank No. 2 - Process Control Narrative 1 PROCESS DESCRIPTION PROCESS DEFINITION OBJECTIVES AND MEASURES Objectives Measures Train Information/Calculations INFLUENT SOURCES EFFLUENT DESTINATION PROCESS CONTROL STRATEGIES Aeration Distribution System... 5 Description... 5 Aeration Distribution Conduits Secondary Bypass Sluice Gates Secondary Bypass Sampler Aeration System Description Process Air Blower Group Secondary Treatment Trains 1, 3-11 Control Strategy Setpoints and Alarms Secondary Treatment Train 2 Control Strategy Setpoints and Alarms Process Air Blower Group for Secondary Treatment Train 2 Normal Operation Equipment Summary Process Air Distribution System Aeration Tank List of Tables Table 1: Secondary Treatment Performance Metrics for Secondary Treatment Train Table 2: Secondary Treatment Performance Metrics for Secondary Treatment Train Table 3: Performance Metrics... 4 Table 4: Information/Calculations... 4 Appendices Appendix II Setpoints Table Appendix III Secondary Alarms Table Table of Contents i

5 1 Process Description 1.1 PROCESS DEFINITION This Process Control Narrative has been developed for the fine bubble aeration system upgrades for Aeration Tank No. 2. The Aeration Facility includes Aeration Tanks where the microorganisms assimilate organics and convert them into a settleable floc. Process Air is supplied to the Aeration Tanks to provide the oxygen required by the microorganisms and ensure the Aeration Tank remains adequately mixed. The Aeration Tank contents, generally referred to as mixed liquor (ML), flows from the Aeration Tanks to Secondary Clarification. Odorous Process Air produced as a result of the biological activity and excess Process Air introduced into the Aeration Tanks is treated in a Secondary Odour Control Facility and exhausted to atmosphere. There are eleven (11) Aeration Tanks and eleven (11) Secondary Clarifiers in the secondary treatment system. One Aeration Tank and one Secondary Clarifier normally operated as one set or train. Therefore, there are eleven (11) Secondary Treatment Trains. The aeration tanks 1 to 11 supply process air to the mixed liquour using coarse bubble aeration. The upgrades will convert Aeration Tank No. 2 to fine bubble aeration with anoxic swing zones. These 11 Secondary Treatment Trains will achieve two different levels of effluent quality, including: Secondary Treatment Train 1, 3-11: achieve BOD removal; Secondary Treatment Train 2: the Operator needs to select the desired effluent quality prior to operating Aeration Tank 2, including CBOD removal (CBOD Removal Mode) and nitrification (Nitrification Mode). Process Description 1

6 1.2 OBJECTIVES AND MEASURES Objectives Remove solids and organics from the secondary treatment influent to meet or exceed effluent targets Identify the treatment capacity for nitrification of one set of aeration tank and secondary clarifier (Secondary Treatment Train 2 only) Minimize bypass volume and frequency Pump excess activated sludge to solids processing Minimize secondary solids inventory Minimize cost of operation Minimize odours Measures Train 2 Table 1: Secondary Treatment Performance Metrics for Secondary Treatment Train 2 EFFICIENCY Measures EFFECTIVENESS Measures QUALITY Measures ammonia removal % of influent ammonia removal Influent and effluent TKN/ammonia s Nitrate removal % of nitrate denitrified Influent and effluent nitrate s Total nitrogen removal % of influent total nitrogen removed Influent and effluent total nitrogen s CBOD removal % of CBOD removed Effluent CBOD TSS removal % of TSS removed Effluent SS Average flow capacity % of Certificate of Approval design flow Measured flow (ML channel flow RAS flow) Peak flow capacity % of Certificate of Approval peak flow Measured flow (ML channel flow RAS flow) through secondary treatment Aeration costs blower horsepower consumption Blower motor amperage, KWh use Alkalinity consumption % alkalinity consumed Influent and effluent alkalinity Information/Calculations Table 2: Secondary Treatment Performance Metrics for Secondary Treatment Train 2 Parameter Sampling Location Sample Type Analysis Location Frequency Aeration Tank 2 Flowrate Aeration Influent CBOD 5 Aeration Influent TKN Aeration Influent ammonia Aeration Influent nitrate / nitrite Aeration Influent alkalinity Calculated from on-line ML and RAS Flow rates North and South settled sewage channels North and South settled sewage channels North and South settled sewage channels North and South settled sewage channels North and south settled sewage channels On-line SCADA Continuous instruments 24 hr composite laboratory daily 24 hr composite laboratory daily 24 hr composite laboratory daily 24 hr composite laboratory daily 24 hr composite Laboratory daily Process Description 2

7 Parameter Sampling Location Sample Type Analysis Location Frequency Aeration Influent TSS Aeration Influent ph Aeration Influent temperature North and South settled sewage channels North and South settled sewage channels North and South settled sewage channels Aeration Tank DO Aeration Tank #2 (12 locations with three per pass) Aeration Tank DO(Portable Instrument Verification) Portable TSS Probe (ML TSS Verification) On-line probe SCADA Continuous On-line probe SCADA Continuous On-line probe SCADA Continuous On-line probe SCADA Continuous All cells manual probe Each online DO weekly probe location End of Pass D manual probe Operations Weekly Aeration Tank Mixed Liquor End of Pass D On-line probe SCADA Continuous (ML) Aeration Tank MLSS End of Pass D grab laboratory weekly ML volatile suspended solids End of Pass D grab laboratory weekly (MLVSS) Aeration Tank 30 minute settling ML Channel grab operations daily test SVI ML Channel calculated operations Calculated Air flow rate for drop legs Air flow rate to Aeration Tank #2 Air pressure within air header B (below ground air header) 12 meters on 12 groups of drop legs 12 meters on 12 groups of drop legs Pressure transmitter on below ground air header Hydraulic Retention Time (HRT) Aeration Tank#2 Flow On-line instruments Secondary Effluent BOD 5 Secondary Effluent TSS Secondary Effluent TKN Secondary Effluent ammonia Secondary Effluent nitrate Secondary effluent TSS Secondary Effluent ph Secondary Clarifier #2 effluent channel Secondary Clarifier #2 effluent channel Secondary Clarifier #2 effluent channel Secondary Clarifier #2 effluent channel Secondary Clarifier #2 effluent channel Secondary Clarifier #2 effluent channel Secondary Clarifier #2 effluent channel Secondary Effluent temperature Secondary Clarifier #2 effluent channel Secondary Effluent ammonia Secondary Effluent nitrite / nitrate Secondary Effluent TKN Secondary clarifier alkalinity Secondary Clarifier #2 effluent channel Secondary Clarifier #2 effluent channel Secondary Clarifier #2 effluent channel Secondary Clarifier #2 effluent channel On-line instrument SCADA Continuous On-line instrument SCADA Continuous and calculated On-line instrument SCADA Continuous SCADA 24 hr composite laboratory daily 24 hr composite laboratory daily 24 hr composite laboratory daily 24 hr composite laboratory daily 24 hr composite laboratory daily Calculated On-line probe SCADA Continuous On-line probe SCADA Continuous On-line probe SCADA Continuous On-line probe SCADA Continuous On-line probe SCADA Continuous On-line probe SCADA Continuous 24 hr composite Laboratory daily Process Description 3

8 Parameter Sampling Location Sample Type Analysis Location Frequency Return Activated Sludge Flow RAS flow meter in On-line instrument SCADA Continuous discharge header Waste Activated Sludge Flow WAS flow meter in On-line instrument SCADA Continuous discharge header Daily Waste Volume WAS flow meter in On-line instrument SCADA Calculated discharge header Solids Retention Time (SRT) Secondary Train #2 TSS & Flow Online SCADA Calculated instruments Sludge Blanket Levels Secondary clarifier #2 On-line instrument SCADA Continuous Clarifier Surface Overflow Rate Secondary clarifier #2 Flow On-line calculated daily instruments Clarifier solids loading rate Secondary clarifier #2 ML Flow calculated daily Table 3: Performance Metrics EFFICIENCY Measures EFFECTIVENESS Measures QUALITY Measures MOE Requirement Sludge settleability Effluent TSS s Online TSS monitor Secondary Treatment Effluent Quality Level of Nitrification (Secondary Clarifier 2 only) Online TKN/Ammonia Analyzer Effluent Quality Effluent BOD 5 Concentrations 24-hr Composite sample testing Secondary Treatment Table 4: Information/Calculations Parameter Monitoring Location Sample Type Frequency Secondary Effluent TSS combined Secondary Effluent 24 hr composite daily Sludge Blanket Levels each Secondary Clarifier - on-line Surface overflow rate Secondary clarifier 2 Calculated Daily Solids loading rate Secondary clarifier 2 Calculated daily 1.3 INFLUENT SOURCES 1. Primary Effluent 2. Centrate, Subnatant from Waste Activated Sludge Thickening and Digester Overflow 1.4 EFFLUENT DESTINATION 1. Secondary Effluent to Secondary Effluent Distribution System 2. Return Activated Sludge to Pass one on Aeration Tank 3. Secondary Scum to Digestion 4. Waste Activated Sludge to WAS Thickening and/or Primary Clarifier for co-thickening 5. Emission Air to Secondary Odour Control Components 1. Aeration Distribution System 2. Aeration System Process Description 4

9 3. Submersible Anoxic Zone Mixers 4. Secondary Clarifiers 5. Secondary Effluent Distribution System 6. Return Activated Sludge Pumping System 7. WAS Pumping System 8. Secondary Scum Pumping System 9. Secondary Odour Control System 1.5 PROCESS CONTROL STRATEGIES Aeration Distribution System Description Aeration Distribution System conveys and distributes Primary Effluent, Digester Supernatant and WAS Thickening Subnatant among the Aeration Tanks. During high flow conditions, the Aeration Distribution System diverts excess flow to Secondary Treatment Bypass (see schematic below). Aeration Distribution System Disinfection Secondary Bypass Sampler and Flow Measurement Secondary Treatment Bypass Lake Ontario Secondary Bypass Gates Primary Effluent Process Air Aeration Distribution Conduit Aeration Influent Aeration System Digester Supernatant Solids Treatment Aeration Distribution Conduits (Existing programming not to be modified under this contract) Aeration Tank 2 is in service at all times except when taken off line for maintenance or repairs. The flow distribution to the Aeration Tank is governed by the Secondary Clarifier on-line capacity and is adjusted by the operator. Each Aeration Tank and associated Secondary Clarifier operate as a set or treatment train. There are four (4) Aeration Tank Inlet Sluice Gates per Aeration Tank, one at the head of each pass of the Aeration Tanks. The open status, closed status and intermediate position status of all Aeration Tanks Inlet Sluice Gates are SCADA monitored remotely based on the Aeration Tank Inlet Sluice Gates limit switches and position indication. There is a Mixed Liquor (ML) Flowmeter, located in the Mixed Liquor Conduit from pass four of each Aeration Tank to the Aerated Mixed Liquor Distribution Channel of the associated Secondary Clarifier. The ML flowrate is SCADA monitored at all times. Process Description 5

10 Equipment Summary Four (4) Aeration Tanks Inlet Sluice Gates, TAB-STR-G-0201, TAB-STR-G-0202, TAB- STR-G-0203 and TAB-STR-G-0204 One (1) Mixed Liquor Flowmeters, TAB-STR-FIT Secondary Influent Flowmeters: TAB-STR-FE-0002 Two TSS/pH/Temp meters: TAB-STR-AIT-0281, TAB-STR-AIT Aeration Distribution Conduits - Control Strategy The normal mode of operation of the Aeration Tanks is Step Feed Mode. The Aeration Tank Inlet Sluice Gates to all four passes open automatically to an initial operator selected position. For Aeration Tank 2, the positions of the Inlet Sluice Gates will be adjusted to achieve a flow split of approximately 10%/30%/30%/30% between passes 1, 2, 3, and 4 respectively. The overall influent to Aeration Tank 2 is between 60,000 m 3 /d and 95,000 m 3 /d. The Aeration Tanks can also be operated in Plug Flow Mode. The Aeration Tank Inlet Sluice Gate(s) associated with pass one of each Aeration Tank opens fully in REMOTE COMPUTER AUTOMATIC mode. The gate(s) may be adjusted to an intermediate position by the operator to maintain an equal distribution of Primary Effluent to pass one of each Aeration Tank as required. The operator selects Step Feed Mode or Plug Flow Mode for Aeration Tanks from SCADA. The operator selects online/offline mode for each individual Aeration Tank. In Plug Flow Mode the south Aerated Distribution Conduit conveys Primary Effluent to the first pass of all Aeration Tanks in operation. In Step Feed Mode the north Aerated Distribution Conduits convey Primary Effluent to passes two and four, while the south Aerated Distribution Conduit conveys Primary Effluent to passes one and three of each Aeration Tanks in operation. The initial position of the Aeration Tanks Inlet Sluice Gates is adjusted automatically when the operation mode is selected. In Plug Flow Mode the Aeration Tanks Inlet Sluice Gate of pass number one will be opened for each selected on-line Aeration Tank (setpoint: X1 %). In Step Feed Mode the Aeration Tanks Inlet Sluice Gates of pass number one, two, three and four will be opened for each selected on-line Aeration Tank (setpoint: Y1, Y2, Y3, Y4 % respectively). In any of the selected modes, operator adjusts position of Inlet Sluice Gates from REMOTE COMPUTER AUTOMATIC mode, to meet the required mixed liquor flow Setpoints and Alarms Refer to Appendix II for a list of adjustable setpoints through the SCADA operator interface and Appendix III for a list of alarms. Process Description 6

11 Aeration Distribution Conduits Normal Operation Calculation Flow rate through each Aeration Tank is calculated based on the number of online tanks and the total influent flow: Secondary Influent Totalized Flow: Q INF = Q 1 + Q 2 + Q 5 + Q 6 where, Q1 = TAB-STR-FE-0001 Q2 = TAB-STR-FE Q5 = TAB-STR-FE0005 Q6 = TAB-STR-FE-0006 Flow rate per online Aeration Tank: Q AT = Q INF /# where, # = number of online tanks Starting The following conditions must be met prior to starting Aeration Tanks Operator selects Aeration Tanks online Operator selects Aeration Tanks operation mode: Plug Flow Mode or Step Feed Mode Aeration Tanks Inlet Sluice Gates in REMOTE COMPUTER AUTOMATIC MODE Running When Aeration Tank selected as on-line: OPEN Aeration Tank Inlet Sluice Gate(s) (setpoint: X%) Operator adjusts position of Aeration Tanks Inlet Sluice Gates to meet required flow through the Aeration Tank based on calculated flow rate and Mixed Liquor Flowmeter reading Operator selects performance mode for Aeration Tank 2 Only: BOD Removal Mode or Nitrification Mode Stopping Operator selects Aeration Tank off-line CLOSE Aeration Tanks Inlet Sluice Gate(s) Aeration Distribution Conduits Fault Response Condition 1: Aeration Tanks Inlet Sluice Gate fails to OPEN/CLOSE SCADA issues an alarm Condition 2: Aeration Tanks Inlet Sluice Gate fails to reach position setpoint SCADA issues an alarm Condition 3: Mixed Liquor Flowmeter failed Process Description 7

12 SCADA issues an alarm Programmed Interlocks There are no programmed interlocks associated with the Aeration Distribution Conduits except for the standard software interlocks provided in the device software modules Secondary Bypass Sluice Gates Programming of these sluice gates is not required as part of this contract. Secondary Influent flow is monitored by Secondary Influent Flowmeters and effluent quality is monitored by the Secondary Effluent Turbidity Meters. The operator opens one or both Secondary Bypass Sluice Gates in to control the Secondary Bypass flow as required based on feedback from the Secondary Bypass Flow Meters. Once the Secondary Influent flow can be handled by the on-line capacity of Secondary Treatment, the SCADA system prompts the operator to terminate the Secondary Bypass. The operator manually terminates the Secondary Bypass closing the Influent Secondary Bypass Sluice Gates and shutting down the bypass Chlorinators in use. If the Secondary Bypass Sluice Gates failed to close after a pre-set time, an alarm will be initiated. Level in the Secondary Bypass is monitored by Secondary Bypass Level Switches. A Flood alarm is initiated when the secondary bypass level switch high detects high level in secondary bypass Equipment Summary The following equipment is included in the Secondary Bypass Control: Secondary Bypass Flowmeters: TAB-STR-FE-0003, TAB-STR-FE-0004 Secondary Effluent Flowmeters: TAB-STR-FE-0008, TAB-STR-FE-0009 Secondary Bypass Sluice Gates: TAB-STR- G-0001, TAB-STR- G-0002 Secondary Bypass Flood Level Switch: TAB-STR-LSH-0001, TAB-STR-LSH-0002 Secondary Effluent Turbidity Meters: TAB-STR-AIT-0031, TAB-STR-AIT Secondary Bypass - Control Strategy If the Secondary Influent (Primary Effluent) flow rate exceeds the maximum flow (setpoint: QINFmax, m 3 /d) and/or Secondary Effluent turbidity exceeds the maximum turbidity (setpoint: Nmax, NTU) in either conduit and/or the Secondary Sludge Blanket level exceeds the maximum level (setpoint: Lmax, m) for any of the on-line Aeration Tanks an advisement is issued to the operator. The operator starts standby Chlorinator(s) and initiates the opening of one or both of the Influent Secondary Bypass Sluice Gates (TAB-STR-G-0001, TAB-STR-G-0002) under REMOTE COMPUTER MANUAL control. The operator will decide the flow rate through the bypass conduit(s) based on the Secondary Influent flow rate. The Secondary Influent flow rate is adjusted by the operator based on the online Secondary Treatment capacity, i.e. number of Aeration Tanks online and operating conditions. Once the Secondary Bypass has been initiated, the position of the Secondary Bypass Sluice Gates is adjusted by the operator to maintain Secondary Influent flow. Once the Secondary Influent flow matches the flow rate setpoint an advisement is issued to prompt the operator to terminate the Secondary Bypass. The operator manually terminates the Secondary Bypass closing the Secondary Bypass Sluice Gates in REMOTE COMPUTER MANUAL and stopping the by-pass Chlorinator(s) in LOCAL MANUAL mode. The status of the Secondary Bypass Sluice Gates is monitored in SCADA Setpoints and Alarms Refer to Appendix II for a list of adjustable setpoints through the SCADA operator interface and Appendix III for a list of alarms Secondary Bypass Normal Operation Process Description 8

13 Under normal operating conditions, Secondary Bypass Sluice Gates are CLOSED and all Secondary influent flow is directed to the Aeration Tanks Calculation The following parameters are calculated by SCADA: Secondary Influent Totalized Flow: Q INF = Q 1 + Q 2 + Q 5 + Q 6 where, Q1 = TAB-STR-FE-0001 Q2 = TAB-STR-FE Q5 = TAB-STR-FE0005 Q6 = TAB-STR-FE-0006 Secondary Influent Bypass flow: Q bpss = Q 3 + Q 4 where, Q3 = TAB-STR-FE-003 Q4 = TAB-STR-FE-0004 Sum of Secondary Influent flow and Secondary Bypass Flow: Q calculated total = Q INF + Q bpss Starting Secondary Influent Bypass Sluice Gates in REMOTE COMPUTER MANUAL mode Running SCADA issues annunciation if: secondary influent flow QINF > Maximum flow setpoint QINFmax and/or secondary effluent turbidity > setpoint Ymax and/or secondary sludge blanket level >setpoint Lmax Operator starts bypass evaporator and chlorinator in LOCAL MANUAL mode Operator opens one or both Secondary By-pass Sluice Gates as required in REMOTE COMPUTER MANUAL mode Operator positions Secondary Bypass Sluice Gate(s) to achieve required flow based on Secondary Bypass Flowmeters Stopping SCADA constantly compares Q (sum of Secondary Influent flow and Secondary Bypass Flow) with maximum flow setpoint QINFmax Advisement will be issued by SCADA to terminate Influent Secondary bypass if: secondary influent flow QINF = or < maximum flow setpoint QINFmax and/or secondary effluent average turbidity = or < setpoint Nmax and/or secondary sludge blanket level = or < setpoint Lmax Process Description 9

14 Operator closes Influent Secondary Bypass Sluice Gates from SCADA in REMOTE COMPUTER MANUAL mode Programmed Interlocks There are no programmed interlocks associated with the Secondary Bypass Sluice Gates except for the standard software interlocks provided in the device software modules Secondary Bypass Sampler Local composite samplers collect samples during a Secondary Bypass event. The samplers located on each Secondary Bypass Conduit are automatically initiated by the Secondary Bypass event as indicated by the Secondary Bypass flow meters. Sampling of the Secondary Bypass event is flow proportional. Samplers are monitored for running status and general alarm conditions Equipment Summary Secondary Bypass samplers: TAB-STR-SP-3001, TAB-STR-SP Secondary Bypass Samplers - Control Strategy Sampling is automatically initiated by SCADA once Secondary Bypass has been initiated and secondary influent flow is detected by Secondary Bypass flowmeter. Sampling of the Secondary Bypass is flow proportional to flow measured by related Secondary Bypass Flowmeter Setpoints and Alarms Refer to Appendix II for a list of adjustable setpoints through the SCADA operator interface and Appendix III for a list of alarms Secondary Bypass Samplers Normal Operation Running START sampler(s) based on flow detected at appropriate Secondary Bypass flowmeter Stopping STOP sampler(s) based on no flow at appropriate Secondary Bypass flowmeter Programmed Interlocks There are no programmed interlocks associated with the Secondary Bypass Samplers except for the standard software interlocks provided in the device software modules Aeration System Description The Aeration System is operated to provide an aerobic environment for the removal of organic pollutants in a controlled biological process. The organic material is consumed by aerobic bacteria to produce new cells, carbon dioxide, and water. Each Aeration Tank consists of four parallel passes and can be operated in Plug Flow or Step-Feed Mode. Step-Feed is currently the preferred mode of operation at the Ashbridges Bay Treatment Plant. Process Description 10

15 Settled Sewage, from each of the three Primary Treatment Facilities, flows by gravity to the North and South Aeration Distribution Channels. The Aeration Distribution Channels are continuously aerated to prevent settling of suspended solids within. The Settled Sewage is distributed approximately evenly between the four parallel Passes in each aeration tank. Return Activated Sludge (RAS) is pumped from each set of Secondary Clarifier Sludge Hoppers to the inlet of Pass 1 of the Aeration Tank, where it contacts the Settled Sewage. The RAS and Settled Sewage mixture is called Mixed Liquor (ML). ML from each Aeration Tank overflows a weir at the discharge end of Pass 4, and flows through an underground box conduit, by gravity, to the Mixed Liquor Channel upstream of the Aeration Tank s corresponding Secondary Clarifier. All Secondary Clarifiers are online at all times, except when the corresponding Aeration Tank is out of service. The Process Air Supply System consists of ten (10) Process Air Blowers, two (2) Main Process Air Supply Headers, and twenty-two (22) Secondary Process Air Supply Headers (i.e. two (2) per Aeration Tank). Process Air is applied to the Aeration Tank contents to provide dissolved oxygen for microbial growth, and to mix the Aeration Tank contents. The Process Air Supply System includes the following two sub-systems: 1. Aeration System for Secondary Treatment Trains 1, 3-11: it includes eight (8) Process Air Blowers 3-11, one (1) above grade Main Process Air Header (Main Air Header A), and twenty (20) Secondary Process Air Supply Headers (i.e., two (2) per Aeration Tank). 2. Aeration System for Secondary Treatment Train 2: it includes two (2) Process Air Blowers 1-2, one below grade Main Process Air Header (Main Air Header B), and two (2) Secondary Process Air Supply Headers (i.e., one (1) per two passes). Aeration System Ferrou Chlorid ss e RAS Primary Effluent Air Process Air Blower Group Blowers 1-2 Process Air Distribution System Aeration Tank Secondary Clarifier 2 Air Process Air Blower Group Blowers 3-11 Process Air Distribution System Proces s Air Aeration Tanks Secondary Clarifiers 1, 3-11 Ferrou Chloride s RAS Primary Effluent Process Air Blower Group There are two Air Blower Groups- one group serving secondary treatment trains 1, 3-11 and another group serving secondary treatment train 2. General operating descriptions of the two blower groups follow. Process Description 11

16 Secondary Treatment Trains 1, 3-11 Control Strategy not required to be programmed under this contract There are eight (8) Centrifugal Blowers available for service. Three (3) of the Blowers (TAB-STR- BL-4003 to TAB-STR-BL-4005) are each rated at 51,500 m 3 /h, three (3) (TAB-STR-BL-4007 to TAB-STR-BL-4009) are each rated at 110,500 m 3 /h and two (2) (TAB-STR-BL-4010, TAB-STR- BL-4011) are each rated at 115,500 m 3 /h. The Blowers are operated to supply Process Air to inservice Aeration Tanks to oxygenate and mix the Aeration Tank contents. The Blowers also supply air to the North Settled Sewage Conduit to prevent deposition of solids in the conduit. The Operator manually starts or stops Blowers, as required, to match biochemical oxygen demand (BOD5) loading patterns, maintain Mixed Liquor Dissolved Oxygen (DO) targets and minimize energy cost. Blower duty sequence is selected by the operator based on experience and Blower run times. Each Blower is equipped with motor protection devices that monitor essential data, i.e. bearing temperature monitors, vibration monitors, suction and discharge temperature monitors, discharge pressure monitors, and lubricating oil system status monitors. If a critical condition is detected in any of the Blower Motors an alarm is issued to the operator and that Blower is stopped. PROCESS AIR BLOWER INLET GUIDE VANE OPERATION: The Process Air Blowers discharge into the above ground main Process Air Header (Main Header A) supplying air to Aeration Tanks 1, The blowers are operated manually to start / stop. Each of the Blowers has a motorized modulating inlet guide valve (IGV). Modulation of the IGVs will control the flow of air through the Blower. Four different operating modes are provided for the Inlet Guide Vanes of the Process Air Blowers: A) LOCAL Mode: When LOCAL/OFF/REMOTE selector switch, located on each vane actuator, is in LOCAL, the vane actuator can be locally controlled using actuator-mounted OPEN/STOP/CLOSE pushbuttons. When LOCAL/OFF/REMOTE selector switch is in REMOTE, the vane actuator can be remotely controlled from Area Control Panel CP-0105 or SCADA. B) REMOTE-ACP MODE: When COMPUTER/CP selector switch, located on the Control Panel CP-4001A (and CP-4002A) by each blower, is in CP, the vane actuator can be remotely controlled using OPEN/CLOSE cam-switch located on the Area Control Panel CP This is default control mode during blower start-up. C) SCADA-MANUAL Mode: When COMPUTER/CP selector switch, located on the Control Panel CP-4001A (and CP-4002A) by each blower, is in COMP, and SCADA-MANUAL is selected at the HMI, the vane actuator can be manually controlled by SCADA by entering a position setpoint in the range of % at the HMI. D) SCADA-AUTO Mode: When COMPUTER/CP selector switch, located on the Control Panel CP-4001A (and CP-4002A) by each blower, is in COMP, and SCADA-AUTO is selected at the HMI, the valve actuator can be automatically controlled by SCADA using two different options: IGV Operation (SCADA-AUTO mode) Fixed Pressure Setpoint Option Variable Pressure Setpoint Option Process Air Inlet Valves Options (SCADA-AUTO mode) DO Control Option Fixed Air Flow Control Option Air Flow Control Option Fixed Pressure Setpoint Option Process Description 12

17 When in Fixed Pressure Setpoint Option, the PLC modulates the running Process Air Blower IGV to maintain an operator adjustable pressure setpoint on the Main Air Header A. Main Air Header A pressure will tend to decrease with increasing air demand, as Process Air Inlet Valves on the Aeration Tanks are opened to increase air flow (automatically by SCADA). In response, the PLC will open the modulating Process Air Blower IGVs to bring the pressure back up to the setpoint. Conversely, Main Air Header A pressure will tend to increase with decreasing air demand, as Process Air Inlet Valves on Aeration Tanks are closed to decrease air flow (automatically by SCADA). In response, the PLC will close the modulating Process Air Blower IGV to bring the pressure back down to the setpoint. Adjustment of the Process Air Inlet Valves directly impacts the pressure in the Main Process Air Supply Header. The Blower Inlet Guide Vanes are automatically adjusted to maintain the Main Header pressure within an Operator selectable Pressure Operating Band (setpoint: Pmin to Pmax, kpa). Control of the Blower Inlet Guide Vanes is based on the signals from the Pressure Meter (TAB-STR-PIT-6000A) which is mounted on the Main Header A. Process Description 13

18 Variable Pressure Setpoint Option When in Variable Pressure Setpoint Option, the pressure setpoint of Main Air Header A is modified using Most Open Valve Potion Control (MOVP). MOVP incorporates an adjustment blower operating setpoint, thereby minimizing blower operating pressure and power draw. In MOVP control, the process setpoint (e.g., DO levels for individual cells of Aeration Tanks) is fixed but the blower pressure setpoint is variable. When more than a preset number of control valves are at their full open position (for example greater than 80% open), the header pressure remote setpoint will be incrementally increased (SCADA to allow for adjustment increments, from 0.1 to 0.25 kpa). In order to meet the pressure setpoint, PLC will decrease the opening position of the IGV. Further action should be locked out for an adjustable time period (e.g. from 10 to 120 minutes). If the full open position persists for more than two valves, SCADA incrementally increase the header pressure again by decreasing the opening position of the IGV, and the system is locked out for another adjustable time period. When the system senses that no valves are in their full open position, the header pressure remote setpoint will be incrementally decreased. PLC will increase the opening position of the IGV to reduce the pressure reading of Main Air Header B to meet the remote pressure setpoint. MOVP maintains a minimum headloss condition in Main Air Header A via a remote setpoint from SCADA. This minimum headloss condition will be defined as the point at which at least one of the Process Air Inlet Valves modulating flows to the Aeration Tanks is in its full open position Setpoints and Alarms Refer to Appendix II for a list of adjustable setpoints through the SCADA operator interface and Appendix III for a list of alarms Secondary Treatment Train 2 Control Strategy There are two (2) Centrifugal Blowers (TAB-STR-BL-4001 and TAB-STL-BL-4002) available for service for Secondary Treatment Train 2. Each of these two blowers is rated at 51,500 m 3 /h. At any given time, only one of the two blowers is sufficient to supply Process Air to Aeration Tank 2 (continuously) and Passes 3/4 of Aeration Tank 1 (periodically in winter only) to oxygenate and mix the Aeration Tank contents. The Blowers also supply air to the South/East Settled Sewage Conduits and the ML channel to prevent deposition of solids in the conduits/channels. The Operator manually starts or stops Blowers, as required, to match biochemical oxygen demand (BOD5) loading and nitrification patterns, maintain Mixed Liquor Dissolved Oxygen (DO) targets and minimize energy cost. Blower duty sequence is selected by the operator. If it is required to run multiple blowers, select one (1) blower in SCADA-AUTO mode to be the LEAD blower. The other blower in SCADA-AUTO mode will be SUPPORT Blowers. When in the SUPPORT position the IGV will be in a fixed position which is manually set by the Operator (this may be fully open or the most efficient operating point determined by the operator). For the LEAD blower the IGV actuator is automatically controlled by SCADA using the following control modes. Each Blower is equipped with motor protection devices that monitor essential data, i.e. motor over-current and over-load relays, bearing temperature monitors, vibration monitors, suction and discharge temperature monitors, discharge pressure monitors, and lubricating oil system status monitors. If a critical condition is detected in any of the Blower Motors an alarm is issued to the operator and that Blower is stopped. PROCESS AIR BLOWER INLET GUIDE VANE OPERATION The two Process Air Blowers (TAB-STR-BL-4001 and TAB-STL-BL-4002), operated as one duty one standby, discharge into the blow ground main Process Air Header (Main Header B) supplying air to Aeration Tank 2. Header B will be physically isolated from Header A and the remaining Process Description 14

19 blowers.(continuously These two blowers are operated manually to start / stop. Each of the Blowers has a motorized modulating inlet guide valve (IGV). Modulation of the IGVs will modulate the flow of air through the Blower to control the header pressure. Four different operating modes are provided for the Inlet Guide Vanes of Process Air Blowers 1 and 2: E) LOCAL Mode: When LOCAL/OFF/REMOTE selector switch, located on each vane actuator, is in LOCAL, the vane actuator can be locally controlled using actuator-mounted OPEN/STOP/CLOSE pushbuttons. When LOCAL/OFF/REMOTE selector switch is in REMOTE, the vane actuator can be remotely controlled from Area Control Panel CP-0105 or SCADA. F) REMOTE-ACP MODE: When COMPUTER/CP selector switch, located on the Control Panel CP-4001A (and CP-4002A) by each blower, is in CP, the vane actuator can be remotely controlled using OPEN/CLOSE cam-switch located on the Area Control Panel CP This is default control mode during blower start-up. G) SCADA-MANUAL Mode: When COMPUTER/CP selector switch, located on the Control Panel CP-4001A (and CP-4002A) by each blower, is in COMP, and SCADA-MANUAL is selected at the HMI, the vane actuator can be manually controlled by SCADA by entering a position setpoint in the range of % at the HMI. H) SCADA-AUTO Mode: When COMPUTER/CP selector switch, located on the Control Panel CP-4001A (and CP-4002A) by each blower, is in COMP, and SCADA-AUTO is selected at the HMI, the vane actuator of the LEAD Blower can be automatically controlled by SCADA using two different options: IGV Operation (SCADA-AUTO mode) Fixed Pressure Setpoint Option Variable Pressure Setpoint Option Process Description 15

20 Fixed Pressure Setpoint Control Mode When in Fixed Pressure Setpoint Option, the PLC modulates the lead Process Air Blower IGV to maintain an operator adjustable pressure setpoint on the Main Air Header B (i.e. a direct-action PID controller with dead-band). Main Air Header B pressure will tend to decrease with increasing air demand, as Process Air Inlet Valves on the Aeration Tank 2 are opened to increase air flow (automatically by SCADA). In response, the PLC will open the modulating Process Air Blower IGVs to bring the pressure back up to the setpoint. Conversely, Main Air Header B pressure will tend to increase with decreasing air demand, as Process Air Inlet Valves on Aeration Tank 2 are closed to decrease air flow (automatically by SCADA). In response, the PLC will close the lead Process Air Blower IGV to bring the pressure back down to the setpoint. Adjustment of the Process Air Inlet Valves directly impacts the pressure in the Main Process Air Supply Header. The Lead Blower Inlet Guide Vane is automatically adjusted to maintain the Main Header pressure within an Operator selectable Pressure Operating Band (setpoint: Pmin to Pmax, kpa). Operator adjustable setpoints will be available to set the minimum and maximum open position of each IGV. The main header pressure setpoint will have an associated deadband which will minimize fluctuations in the IGV position. The deadband gap will be operator adjustable on the setpoint page. Control of the Blower Inlet Guide Vanes is based on the signals from the Pressure Meters (TAB-STR-PIT-6000B) which are mounted on the Main Header B. Variable Pressure Setpoint Control Mode When in Variable Pressure Setpoint Option, the pressure setpoint of Main Air Header B is modified using Most Open Valve Position Control (MOVP) as a direct-action step controller that cascades the pressure setpoint to the Fixed Pressure Setpoint Controller. MOVP incorporates an adjustment to the blower operating setpoint, thereby attempting to minimize blower operating pressure and power draw. In MOVP control, the process setpoint (e.g., DO levels for individual cells of Aeration Tank 2) is fixed but the blower pressure setpoint is variable. When the system sense that several valves are in their most open position, the blower is called to provide more air to the tanks by increasing the pressure set point. This will improve air distribution to the grid, with higher power draw. If the average MOVP exceeds the MOVP setpoint (initial value 80% open), the header pressure remote setpoint will be incrementally increased (SCADA to allow for adjustment increments, from 0.1 to 0.25 kpa). In order to meet the increased pressure setpoint, PLC will increase the opening position of the lead blower IGV. Further action should be locked out for an adjustable time period (e.g. from 10 to 120 minutes). If the maximum IGV position is reached, the controller should be locked out to prevent wind-up of the pressure setpoint beyond the ability of the blower to meet the pressure demand. When the system senses that no valves are in their most open position, the blower will reduce air supplied to the tanks by incrementally decreasing the header pressure remote setpoint. This will reduce the power draw of the blowers and cause air valves in the system to open further. PLC will decrease the opening position of the IGV to reduce the pressure reading of Main Air Header B to meet the remote pressure setpoint, and the system is locked out for another adjustable time period. If the minimum IGV position is reached, the controller should be locked out to prevent wind-up of the pressure setpoint beyond the turn down range of the blowers. MOVP maintains a minimum headloss condition in Main Air Header B via a remote setpoint from SCADA. This minimum headloss condition will be defined as the point at which at least one of the Process Air Inlet Valves modulating flows to the specific zones of Aeration Tank 2 is in its full open position (80%). An operator adjustable table shall be provided to allow the operators to select the valves to be included in the most open valve averaging. An example of this table is noted below. Process Description 16

21 Valve Tag No. V-0246 V-0256 V-0266 V-0276 V-0247 V-0257 V-0267 V-0277 Included in MOV Averaging Control Yes No The MOV logic is shown in the following flow on the next page. Over Current Protection Mode: The PLC is to be equipped with over-ride control for Over Current Protection. In any SCADA- AUTO mode (Fixed or Variable Pressure), Blower Amperage feedback from the motor protection relay will hold or "freeze" the IGV position if within 5% of the Full Load Current set point for the blower. A selectable Over Current Protection status indicator on the set point screen will be used to enable or disable the over-ride control. A HI and HI-HI alarm will be issued if the Blower Amperage exceeds the trigger values. Control Mode Failure If Variable Pressure Mode is selected and the mode fails as a result of the failure of the air valves included in the MOV Average control, the control mode shall default to Fixed Pressure Mode. If either Fixed Pressure or Variable Pressure Mode is selected and the mode fails as a result of a failure of the pressure probe, the control mode of the blower IGVs shall default to COMP- MANUAL. The HMI shall display the current control mode. Bumpless Transfer When the controller is in manual, the PLC shall force the blower's header pressure set point to track the process variable. Once the controller is in automatic, the header pressure set point remains at the new value. This will prevent a bump in the process during transfer of control from COMP-MANUAL to COMP-AUTO. MOV Control Logic Flow Chart Process Description 17

22 Setpoints and Alarms Refer to Appendix II for a list of adjustable setpoints through the SCADA operator interface and Appendix III for a list of alarms Process Air Blower Group for Secondary Treatment Train 2 Normal Operation Calculation None required Starting The following conditions must be met prior to starting Air Blowers: Air Header routing valves are in appropriate positions Process Description 18

23 Operator selects Blower control mode: either Fixed Pressure Setpoint Control Mode or Variable Pressure Setpoint Control Mode Duty blower is selected and IGV is in COMP-AUTO Running Operator starts duty Process Air Blower under LOCAL MANUAL CONTROL Modulate inlet guide vanes to meet target Header Pressure (setpoint: Pmin to Pmax, kpa) under REMOTE CONTROL PANEL mode. Once desired pressure setpoint is reached, inlet guide vanes are modulated under REMOTE COMPUTER AUTOMATIC mode to maintain Header Pressure per operator selected blower control mode whether Fixed Setpoint Pressure Option or Variable Setpoint Pressure Option (setpoint: Pmin to Pmax, kpa) If guide vane for operating blowers at maximum position and pressure in the header drops below operating range for an adjustable duration, SCADA issues an advisement to start additional blower(s) to meet capacity If guide vane for operating blowers at minimum position and pressure in the header increases to above the operating range for an adjustable duration, SCADA issues an advisement to stop blower(s) to meet capacity Stopping Operator stops Process Air Blowers under LOCAL MANUAL CONTROL SCADA issues alarms Equipment Summary Two Secondary Process Air Blowers (Aeration Tank 2): TAB-STR-BL-4001 and TAB-STR- BL-4002 Air Blower discharge pressure transmitter (Aeration Tank 2): TAB-STR-PIT-4001 and TAB- STR-PIT-4002, Main Header Pressure Transmitter: TAB-STR-PIT-6000B Main Process Air Supply Header Routing Valves: TAB-STR-V-4015, TAB-STR-V-4012, TAB- STR-V-4013 and TAB-STR-V Process Air Distribution System Aeration Tank 2 Main Process Air Supply Header The below ground Main Process Air Supply Header (Main Header B) which conduct Process Air from the Blower Building to the Secondary Process Air Supply Headers for Aeration Tank 2. The Operator manually opens / closes the appropriate Routing Valves, including: TAB-STR-V-4015, TAB-STR-V-4012, TAB-STR-V-4013, TAB-STR-V The status of these valves is SCADA monitored via Limit Switches. Secondary Process Air Supply Headers Process Description 19

24 There are two (2) Secondary Process Air Supply Headers for Aeration Tank 2, one supplies air to Passes A and B, the other supplies air to Passes C and D. Each Secondary Process Air Supply Header is connected to both Main Process Air Supply Headers. Operator manually opens / closes the appropriate motorized air valves on the Secondary Process Air Supply Headers to supply process air into Aeration Tank 2 from the Main Air Header B only, including TAB-STR-V- 6002C, TAB-STR-V-6002D. Air flow rates in the Secondary Process Air Supply Headers are SCADA monitored continuously via Flow Meters (TAB-STR-FIT-6002C and TAB-STR-FIT- 6002D). Each of the Secondary Process Air Header is equipped with six (6) air flowmeters and modulating Process Air Inlet Valves, including: Secondary Process Air Header Air Flow Meter Process Air Inlet Valves Passes A/B Passes C/D TAB-STR-FIT-0245A TAB-STR-FIT-0246A TAB-STR-FIT-0247A TAB-STR-FIT-0255A TAB-STR-FIT-0256A TAB-STR-FIT-0257A TAB-STR-FIT-0265A TAB-STR-FIT-0266A TAB-STR-FIT-0267A TAB-STR-FIT-0275A TAB-STR-FIT-0276A TAB-STR-FIT-0277A TAB-STR-V-0245 TAB-STR-V-0246 TAB-STR-V-0247 TAB-STR-V-0255 TAB-STR-V-0256 TAB-STR-V-0257 TAB-STR-V-0265 TAB-STR-V-0266 TAB-STR-V-0267 TAB-STR-V-0275 TAB-STR-V-0276 TAB-STR-V-0277 These Process Air Inlet Valves are operated under three different modes: A) LOCAL Mode: When LOCAL/OFF/REMOTE selector switch, located on each actuator, is in LOCAL, the valve actuator can be locally controlled using actuator-mounted OPEN/STOP/CLOSE pushbuttons. When LOCAL/OFF/REMOTE selector switch is in REMOTE, the valve actuators are controlled by SCADA, either SCADA-MANUAL or SCADA- AUTO. B) SCADA-MANUAL Mode: When SCADA-MANUAL Mode is selected at the HMI, the valve actuator can be manually controlled by SCADA, by entering a position setpoint in the range of 0 100% at the HMI. C) SCADA-AUTO Mode: When SCADA-AUTO Mode is selected at the HMI, the valve actuator can be automatically controlled by SCADA; each valve is modulated to maintain an operator adjustable dissolved oxygen (Aeration Tank 2 DO Setpoints) or the air flow setpoints in the associated zones, depending on the operator selection. The operator can select one of three possible control options: DO Control Option, DO-Air Flow Control Option, and Fixed Airflow Option for each combination of DO probe, air flow meter, and modulating air valve. The operator can also select a set-point for each of the options. The detailed description of each option is summarized below. Duty DO Sensor Selection Two of the control modes rely on the use of DO sensors. There are 12 DO sensors, one for each air flow control valve. In order to simplify operations, duty DO probe assignments will be built into the control program. Process Description 20

25 The swing zone air control valves will operate based on the swing zone DO probe and there will not be any change in duty selection. The DO probes and related swing zone air control valves are noted in the following Table. These valves and DO probes will always be mapped together for each pass. Zone Control Valve Tag No. Swing Zone Pass A V-0245 AE-0248 Swing Zone Pass B V-0255 AE-0258 Swing Zone Pass C V-0265 AE-0268 Swing Zone Pass D V-0275 AE-0278 DO Probe Tag No. For the Oxic Zones in all four passes, initially a duty probe will be selected for each pass, then the duty pass will be selected. Duty Probe Selection In Each Pass The duty probe in any pass can be one probe or both probes. If one probe is selected as the duty, then both the control valves in that Pass will operate based on control from the duty DO probe. If both DO probes are chosen as duty, then each control valve operates based on its respective DO probe. The Duty Probe selection shall allow of the operators using a table format to select the duty probe or probes in in each pass. The ACP needs the functionality to allow one or both probes to be selected as Duty. Pass Oxic Zone 1 Oxic Zone 2 Tag No. Duty (Y or N) Tag No Duty (Y or N) A AE -0249A AE-0249B B AE-0259A AE-0259B C AE-0269A AE-0269B D AE-0279A AE-0279B One the duty DO Probe is selected in each pass, then the lead Pass shall be selected. Based upon air flow requirements, if Pass A is selected as a lead pass, then the Pass B air flow control valves will modulate based on output from the Pass A DO probes. Similarly, if Pass C is selected as a lead pass, then the control valves in Pass D shall modulated based on the output from the Pass C DO probes. The combinations of lead and following passes are as noted in the following Table. The lead Pass DO probes will control the control valves in the lead and following Passes. Option Pass A Pass B Pass C Pass D 1 lead lead lead lead 2 Lead Follows A Lead Follows C 3 Lead Follows A Follows A Follows A Process Description 21

26 4 Follows D Follows D Follows D Lead Summary Option 1 all Passes are selected as lead and the control valves operate independently Option 2 Pass A and Pass C are lead and the control valves in Passes B and D modulate based on the DO probe duty selection Passes A and C. Option 3 Pass A is lead and the control valves in Passes B, C and D modulate based on the DO probe duty selection for Pass A. Option 4 Pass D is lead and the control valves in Passes A, B and C modulate based on the DO probe duty selection for Pass D. The duty controlling DO probes shall be highlighted on the HMI. A flow chart showing the duty probe/control valve selection is as follows. Process Description 22

27 DO Control Option When in DO Control Option, the system attempts to maintain the DO setpoint using a directacting step controller with deadband. The control loop modulates the air control valves to maintain the DO level at the chosen set point. The PLC will open the valve to increase the DO and close the valve to decrease the DO. The PLC will periodically calculate a valve position setpoint based on an error calculation as follows: DO Error = Dissolved Oxygen Setpoint Measured Dissolved Oxygen X 100% Analyzer Range ( ppm) Valve Position = Last Valve Position + (DO Error X Step Size) Due to the dynamics of oxygen utilization and oxygen transfer, there may be significant lag between the time of adjustment of the air control valves and the resultant increase or decrease in dissolved oxygen measured at the associated DO probe. To accommodate this lag, the PLC will be programmed to make an adjustment, and then wait an appropriate amount of time (loop update time) before calculating a new valve position. The loop update time, and the Step Size used in the above equation, will reside in the PLC, and be adjustable by the Operator (30 seconds to 300 seconds). Furthermore, to prevent unnecessary valve adjustment, a deadband (0.1 to 0.5 mg/l) will be programmed into the PLC, such that no change in valve position will be made as long as the measure dissolved oxygen is within the deadband of the setpoint. Outside the deadband, the step controller will work as usual. If the minimum or maximum air valve position is reached, the controller should be locked out to prevent wind-up of the valve position setpoint beyond the range of the actuator. The DO Error calculation is also limited within the PLC to prevent the logic from calculating a large stepped change. The limit is currently set at +/-3% and is only adjustable in the PLC. DO- Air Flow Control Option In DO-Air Flow Control mode, the system evaluates the DO error and another step adjustment loop compares the targeted air flow against the Air Flow Rate at that current injection point and modulates the corresponding Process Air Inlet Valve, using a direct-acting step DO controller with deadband that cascades the airflow setpoint to the air control valve using a direct-acting PID controller. The target airflow will be calculated as follows: DO Error = Dissolved Oxygen Setpoint Measured Dissolved Oxygen X 100% Analyzer Range ( ppm) Target Air Flow = Last Air Flow Reading + DO Error x Step Size The air control valve will modulate to meet the new target air flow rate. If the actual air flow is more than the setpoint, the PLC decreases the valve position. If the actual air flow is less than the setpoint, the PLC increases the valve position. 1 Note that instrument span may not be as noted. Instrument span based upon instrument calibration and will match span as entered into the PLC from the HMI popup. 2 Note that instrument span may not be as noted. Instrument span based upon instrument calibration and will match span as entered into the PLC from the HMI popup. Process Description 23

28 When in DO-Air Flow Control mode, the air valve modulates to maintain an air flow setpoint on an adjustable cycle delay (5 minutes to 10 minutes), with an adjustable step size. The step size shall be from 1 to 10. If within the adjustable dead-band for Airflow, the controller takes no action. The deadband is used to minimize wear on the valve and actuator. The target air flow calculation based on DO error will occur at less frequent intervals (defined by loop update time) due to the process lag between airflow and resulting change of DO. If the minimum or maximum air valve position is reached, the controller should be locked out to prevent wind-up of the target airflow setpoint beyond the range of the valve. Diurnal Air Flow Option This option can be applied when the DO probes are out of service for maintenance, or during start-up of a tank. In the fixed air flow option, the air flow setpoint for each group of drop legs is copied from a diurnal air flow setpoint table. The diurnal air flow setpoint table is adjustable by Operators. This control mode allows operators to input a typical diurnal air flow pattern for each group of drop legs within Aeration Tank 2. A master Diurnal Air Flow Table will be developed based on 12 timed set points, with the air flow being able to be changed every 2 hours starting at 0:00 hrs to 24:00 hours. Each pass will have a separate specific air flow, which will be based on an operator adjustable percentage of the total air to each pass. The initial value will be 25% to each pass. The values in the four pass specific diurnal air flow tables will add up to the air flow in the master Diurnal Air Flow table. A sample of the Master Diurnal air flow table is shown below Control Mode Failure If the DO-Airflow mode is selected and the mode fails as a result of a failed airflow meter, the control mode shall default to DO Control mode. Process Description 24