Advanced Control in Water and Wastewater Systems

Transcription

1 Advanced Control in Water and Wastewater Systems Mandy Poole, P.E. Jason Neighbors Dave Green June 6, 2014 AGENDA What is a control loop? Control Theory Pump Control Levels of Control Case study on Wastewater Treatment Plant Aeration Systems Measuring the Savings Wheaton Sanitary District Results Discussion 1

2 CONTROL LOOPS: Fundamentals A controller tries to maintain the measured process variable (PV) at a given set point (SP) in spite of disturbances (D). Process Disturbance (changes) System Set Point PLC Controller Output Some Process Process Variable Measured Variable Sensor Control Objective: The state we want the system to be in Process Variable: Our measured result Sensor: the thing that measures the variable we are controlling Measured Process Variable (PV) : the signal as-transmitted from sensor Set Point (SP): the desired value we want the system to achieve Controller Output (CO): signal to something that can be changed Disturbances (D): things outside of our control that change in the process that affect the process variable CONTROL LOOPS: Fundamentals an Example Objective: Maintain the temperature of a room despite the falling outside temperature. Process Disturbance (outside temperature) System Set Point 70 o Thermostat Controller Output (open gas valve) Room Temperature Actual Room Temp Measured Variable Temp: 68 0 Sensor (Thermistor) Process Variable: Room temperature Sensor: Thermistor in a wall-mounted thermostat Measured Process Variable (PV) : Voltage transmitted from the thermistor Set Point (SP): Desired room temperature Controller Output (CO): Signal to the gas valve / furnace Manipulated Variable: Possible gas flow rate to furnace Disturbances (D): Outside temperature, insulation/heat loss, sun, snow 2

3 CONTROL LOOPS: Fundamentals Water Example Objective: Maintain the temperature of a room despite the falling outside temperature. Process Disturbance Chlorine Residual 1 mg/l PLC Controller Output (chlorinator) WTP Water Chlorine Residual Measured Residual Free Cl: 1.5 mg/l Chlorine Residual Analyzer Process Variable: Chlorine Residual Sensor: Chlorine Residual Analyzer Measured Process Variable (PV) : 4-20 ma signal coming from analyzer Set Point (SP): Desired residual = 1 mg/l Controller Output (CO): Increase or decrease signal sent to the chlorinator Manipulated Variable: Chlorine solution flow rate Disturbances (D):Water temperature, water quality, particulates CONTROL LOOPS: Aeration Tank Example Objective: Maintain the temperature of a room despite the falling outside temperature. Process Disturbance DO Level 1 mg/l PLC Controller Output (blower speed) Aeration Tank Chlorine Residual Measured Residual Free Cl: 1.5 mg/l Chlorine Residual Analyzer Process Variable: Chlorine Residual Sensor: Chlorine Residual Analyzer Measured Process Variable (PV) : 4-20 ma signal coming from analyzer Set Point (SP): Desired residual = 1 mg/l Controller Output (CO): Increase or decrease signal sent to the chlorinator Manipulated Variable: Chlorine solution flow rate Disturbances (D):Water temperature, water quality, particulates 3

4 PUMP CONTROL Without control, pump output is fixed. PUMP CONTROL Using a VFD and simple instruments, the range of the pump increases. What about efficiency? 4

5 LEVELS of CONTROL Wastewater Treatment Plant Aeration Systems 1. Airflow-Based Control 2. Simple D.O. Control 3. Ammonia Feed-Forward Control LEVELS of CONTROL Wastewater Treatment Plant Aeration Systems 1. Airflow-Based Control 2. Simple D.O. Control 3. Ammonia Feed-Forward Control 5

6 WASTEWATER 101 BOD Bugs need X amount of air to be able to consume organics. NH 3 If there is more air than they need, the dissolved oxygen (D.O.) increases. TYPICAL AERATION SYSTEM CONFIGURATION The aeration system includes blowers, diffusers, air piping, and instrumentation. DO Probe Diffuser Flow 6

7 Airflow-Based Control Oxygen Demand Deficit Oxygen Provided Excess Time D.O. Time Airflow-Based Control D.O. 7

8 Airflow-Based Control + Simplest + Least Instrumentation - Least Control - High Power Use - High Operating Cost - D.O. instability can lead to effluent violations - (Future): Limits operability of BNR processes Simple D.O. Control The aeration system output is set to maintain a D.O. setpoint. DO Probe Flow 8

9 Simple D.O. Control The aeration system output is set to maintain a D.O. setpoint. Flow Control Valve Flow Simple D.O. Control Airflow D.O. Effluent Ammonia 9

10 Simple D.O. Control + Can rely on limited instrumentation + High Impact on Power Use and Energy Savings - Slow to Respond - Limited Ability to Respond based on Probe Location (i.e. can end up hunting) - Can reduce operational safety factor Simple D.O. Control: The Down Side As an influent load (NH 3 ) comes into the aeration tanks, the blower only responds after DO crashes. Effluent NH 3 peaks. Airflow Influent Ammonia Airflow D.O. Effluent Ammonia NH 3 downstream (EH) 10

11 Ammonia Feed-Forward Control The aeration system output changes based on the rate of change of ammonia influent. DO Probe NH 3 Probe Flow Ion Selective Electrode 5/27/

12 Ammonia Feed-Forward Control: Instrumentation 5/27/ Ammonia Feed-Forward Control Airflow D.O. Influent Ammonia 12

13 Ammonia Feed-Forward Control + High Impact on Power Use and Energy Savings + Maintains operational safety factor + Faster response than simple D.O. control + Minimizes D.O. peaks and valleys + Provides more stable effluent ammonia concentration + Anticipate changes in loading that would otherwise cause D.O. to sag - More instrumentation + (Future): Improved control for BNR and dynamic loads Aeration Control Summary Energy Savings Process Control Airflow-Based Control Simple D.O. Control Ammonia Feed- Forward Better than Nothing 11% more than simple D.O. Even simple control can provide significant energy savings Doesn t have to be at the cost of operational comfort Minimal cost of implementation (in-tank instrumentation, programming) 13

14 QUESTIONS / DISCUSSION Mandy Poole apoole@baxterwoodman.com Dave Green dgreen@bwcsi.com Jason Neighbors neighbors@wsd.dst.il.us 14