Optimizing Coagulation with Streaming Current 2016 Plant Operations Conference Presented by the VA AWWA Plant Operations Committee
Outline Coagulation Background Benefits of SCMs Theory of Operation System Components Modern SCM usage SCM in Process Control Site Selection and Setup Common Sources of Consternation Maintenance Summary
Coagulation Background Chemical Pretreatment is typically applied to sedimentation and filtration to enhance the ability to remove particles Coagulation neutralize charges, form a gelatinous mass to trap or bridge particles Flocculation gentle stirring or agitation to encourage the particles to agglomerate
Colloidal Material Take Tekapo
Adapted from Water Quality and Treatment, 3 rd Edition
Coagulation Background Colloidal Material Large surface area relative to mass Static surface charge, typically Negative Cannot settle They repel one another Expulsion force exceeds gravitational force They attract charges surrounding the colloid, keeping them apart with a double-layer cloud
Coagulation Background Most naturally occurring particles in surface water are negatively charged clays. These like charges repel each other and don t agglomerate and settle out. If this charge is neutralized, the particles can bind together to settle out of suspension
Coagulation Background Purpose of Coagulation is to neutralize the charge causing this effect Positive charged cations are used as coagulants Multivalent Fe and Al are commonly used Polyaluminunum Chloride (PACl) less alk. needed
A word about ph Particle Counts Coagulants consume alkalinity lowering ph ph Control is important for proper coagulant operation A= operation at ph 7.8 B= lowered to ph 7.2 Stabile ph control is important for SCM
Coagulation Background How do we know the right amount of coagulant to add?
Coagulant Background How do we know the right amount of coagulant to add?
Coagulation Background How do we know the right amount of coagulant to add? What if I had a way to measure the effectiveness of our coagulant addition continuously? Streaming Current Monitoring can be that tool
Benefits of SCMs: An optimization tool Reduced coagulant chemical usage Consistent, high quality finished water Longer filter runs Less sludge Rapid response to raw water changes o Automatic dosing adjustments Improved operator awareness Early detection of coagulant feed or ph adjustment failure
The Double Layer Model The Stern Layer ions are tightly bound to the particle. A dynamic equilibrium of negative and positive ions forms outside the Stern layer, known as the Diffuse Layer. The zeta potential is measured outside the Stern layer where shear occurs. The zeta potential can be measured using a microscope to observe particles in an electrophoresis cell. Changes in ion concentration, such as ph changes, affect zeta potential.
Zeta Potential Test Electrophoresis principle Small amount of solution Placed between two opposite polarity electrodes Apply a voltage Measure movement speed toward the poles Speed is related to the strength of the potential Zeta-Meter, Inc.
Streaming Current SC is based on the principle that surfaces quickly take on the charge characteristics of the colloidal particles that flow past. Water flowing rapidly up and down through the annulus results in displacement of the counterions. A current flows through the electrodes to remove the charge separation. Jar testing or Zeta Potential determines approximate or optimum coagulant dosing. SCM tracks changes to this value.
Streaming Current Typical Parameters Small Gap size of 200-500 µm Piston reciprocates at 3-5 times / second Small signal produced is 0.05µA to 5µA
Components of a Streaming Current Monitor Displays Keypad Motor and Drive (inside) Sample Chamber, contains piston Sensor
Components of a Streaming Current Monitor Displays Keypad Motor and Drive Sample Chamber, contains piston Sensor
Typical Water Treatment Process
Coagulant feed Modern SCM Use Feedback control of coagulant dosage The SC set-point is determined by jar tests Mixer Flocculate Clarifier Filtration Disinfection Chemical Pump Controller
So, how do I use this in process control? What number is the right number? How do I calibrate it? Can it be used for enhanced coagulation?
Relationship between SC and settled water turbidity
Relationship between SC and settled water turbidity
What Causes Values to move positive? Increase in coagulant (alum, iron, PACl) Decrease in ph Decrease in raw water flow Decrease in raw turbidity Decrease in color value
What Causes Values to move negative? Decrease in coagulant (alum, iron, PACl) Increase in ph Increase in raw water flow Increase in raw turbidity Increase in color value
So what s the right number? How do I calibrate? Zero the unit when treatment is optimized Periodically test and adjust (or offset) as jar testing and process dictates Raw water calibration of span reading if desired
Jar Testing is Still Key You will still need to effectively jar test Whenever significant changes occur in raw water quality Set pump speed for optimal dosage Wait, Check process Set SCM to 0 at this point
Jar Testing
Site Selection Sample in Sample out Select a representative sample site Ideally 30s after flash mix (well mixed) Protect from the environment Sun/weather shade or protection, ventilated Provide the proper, steady sample flow rate
Walk away for a while at least 30 minutes Ask yourself: Is the process in control today? If so, note the button labeled Set Zero If you like the way the process is running, press and hold Set Zero for about 3 seconds. Proper Set Up
Offset from Zero Point (Drives output) Live actual (raw) streaming current value or PID indicator
Proper Set Up PID Tuning Record Initial SCV reading Change Coagulant 20% up or down Record time to see first change Record time until reading stabilizes Record Final SCV reading Repeat process in reverse Follow formulas in the manuals
Detection of Coagulant Feed Failure
Can I use this for enhanced coagulation?
City of Houston DBP/TOC Study
Common Sources of Consternation No or poor ph control Worn parts Causes drift and inconsistent measurement Poor sample point Incomplete mixing Easily blocked sample lines Too far from the flash mix Controller tuned incorrectly May cycle, esp. at lower flow rates Slow to respond Lack of understanding, such as expecting miracles
Typical Maintenance Summary Item Frequency ~ Time Required Clean strainer, cell and piston Monthly or less (based on experience) 10 min. Replace connecting rod end Yearly 5 min. Replace piston 2 years 10 min. Replace sensor 5-7 years 5 min. Replace linear bearings 5-7 years 30 min.
Routine Maintenance: Cleaning Contamination of the sensor and piston surfaces will result in slower response and drift. Clean with a soft brush wetted with sample water. A toothbrush is ideal. Do not touch any wetted surfaces with bare hands - any oily residue may affect the reading for up to 30 minutes. The reading may be unstable immediately after cleaning, this will normally stabilize within 5 minutes.
Monthly cleaning is a good starting point for most plants when using the grit filter and auto-flush (DW applications) Needs cleaning Does not need cleaning, but shows signs of wear
Symptoms of Worn Parts Zero point drifts away from where it is set Large offset but poor sensitivity to changes Vertical scoring on piston or sensor Significant staining of the piston Looseness of the piston Maintenance and service is key to good operation
Summary A Streaming Current Monitor (SCM) measures the surface charge of suspended particles A SCM will be beneficial in optimizing the control of coagulant dosing It needs to be installed in the correct location and set up correctly to be useful It does need, at least, a little maintenance to continue to be useful
Questions? References Photographs, diagrams, manuals, data sheets and application papers sourced from www.accufloc.com www.chemscan.com www.hach.com www.zeta-meter.com Thank you to Terry Engelhardt, Hach Company For more information contact: Chris Griffin cgriffin@hach.com
Lab Charge Analyzer with Autotitration https://www.youtube.com/watch?v=cdvvoiowfu g