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Streaming Current InstrumentTraining
Chuck VealMicrometrix Corp
Charged Particles Repel
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Coagulant
• A coagulant is a charged chemicalspecies that is added to destabilizecolloidal particles so they are free toaggregate.
• Positively charged metal salts are usedto neutralize negatively charged naturalparticles.
Neutral Particles Flocculate
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Al3+
Monomeric hydrolysis products e.g. Al(OH)2+
Various polymeric species
Colloidal hydroxide precipitate
Precipitated Al(OH)3
Coagulant Particles
Stable
Destabilized
Coagulated
Restabilized
Flocculated
Adsorption
“Sweep Flocculation”
Common Coagulants
• Aluminum sulfate (alum)• Ferric sulfate• Ferric chloride• Polymeric inorganic coagulants• (polyaluminum chloride)
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SCM Charge Measurement
( + ) Positive( + ) Positive
( - ) Negative( - ) Negative
( 0 ) Neutral( 0 ) Neutral Ionic and ColloidalIonic and Colloidal
Background
• Particle charge / distance influences stability.– Like - charged particles repel.– Opposite - charged particles attract.– Neutral Particles are free to collide and
aggregate.– Van Der Waals forces cause particles to
attract.
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Double Layer
• The Double Layer.– The left view shows the change in charge
density around the colloid.– The right shows the distribution of (+ & -) ions
around the charged colloid.
Double Layer ModelDouble Layer Model
Positive Counter-IonNegative Co-Ion
Highly NegativeColloid
Stern Layer
Diffuse Layer
Ions in Equilbriumwith Solution
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Double Layer
• The double layer consists of counter-ions inthe stern layer and the charged atmospherein the diffuse layer.
• The thickness of the layer is determined bythe concentration of ions in the solution andthe type of counter-ion.– Type refers to the valence of the + counter-ion.
• Example:(Al+3) ions are much more effective incharge neutralizing colloidal material than (Na+).
Stabilized Colloids
RepulsiveRepulsiveEnergyEnergy
AttractiveAttractiveEnergyEnergy
Distance Between ColloidsDistance Between Colloids
Net InteractionNet InteractionEnergyEnergy
AttractionAttractionVanVan der Waals der Waals
Electric RepulsionElectric Repulsion
Energy BarrierEnergy Barrier
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Destabilized System
RepulsiveRepulsiveEnergyEnergy
AttractiveAttractiveEnergyEnergy
Distance Between ColloidsDistance Between Colloids
Net InteractionNet InteractionEnergyEnergy
AttractionAttractionVanVan der Waals der Waals
Electric RepulsionElectric Repulsion
Energy BarrierEnergy Barrier
Names for Streaming CurrentDevices
• Streaming Current Meter - SCM• Streaming Current Detector - SCD• Streaming Current Monitor - SCM• Zeta Monitor - ZM• Particle Charge Detector - PCD• Particle Charge Analyzer - PCA
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Applications for SCMs
• Municipal Drinking Water• Papermaking Process• Sludge Dewatering• Industrial Water and High Purity
Drinking Water Treatment
• Plants began using these devices backin the early eighties for coagulantcontrol.
• The American Water Works ResearchFoundation funded a project in 1986 toreport on the satisfaction of users of SCtechnology
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SCM vs. NTUSCMNTU
SCM@Overfeed
SCM@Optimum
ChemicalSavings
ChemicalDosage mg/L
Streaming CurrentResponse
SC Sensor
Stainless Steel ElectrodesStainless Steel ElectrodesReplaceable SleeveReplaceable SleeveTeflon andTeflon and Delrin Delrin Material MaterialSpare Sleeve / PistonSpare Sleeve / Piston
MotorMotor
PistonPiston
SleeveSleeve
ElectrodesElectrodes
InletInletOutletOutletSampleSample
ProbeProbeHousingHousing
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Stationary
++
++
++++++
++ ++++++++
++++
++ ++
++
++++
++++
Liquid / Piston StationaryLiquid / Piston Stationary
Piston Upstroke
++ ++
++++++
++ ++++++
++++ ++
++
++
++++
++++
Liquid / PistonLiquid / Piston
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Background
FlocculatorFlocculator Settling BasinSettling BasinMixingMixingUnitUnit
ControllerControllerPumpPump
SCMSCM
Sample PointSample Point* Well Mixed* Well Mixed* Quick Response* Quick Response
Set Point Set Point ** Emperical Emperical* Based on Process Performance* Based on Process Performance
FiltersFilters
Preventing Upsets
FlowIncrease
CoagulantFeed Loss
Influent NTUIncrease
With SCMWithout SCM
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2
3
4
5
6
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Cla
rifie
d/Se
ttled
Wat
er
Cla
rifie
d/Se
ttled
Wat
er
NTUNTU
Hrs
Maintain Water Quality with SCM ControlMaintain Water Quality with SCM Control
FlowIncrease
CoagulantFeed Loss
Influent NTUIncrease
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Factors Affecting Operation
• Chemical Factors• Response Time and Lag Time• Location of Sample Point
Chemical Factors
• Ionic strength (conductivity)- typically issufficiently constant in most watertreatment applications.
• Ohms law E= I x R• An increase in conductivity decreases the
SCM sensitivity
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Chemical Factors
• pH - similar to the effect on Zeta Potential.• An increase in pH moves the SC reading in
the negative direction for simple systems.• Trends may be more complex for higher
pH values >8.• May see a decrease in sensitivity for very
high pH >10
Response Time
• Lag time is of considerable importance forproper use of the SCM.
• System Lag time equals processdetention/lag time plus sample linedetention/lag time
• The SCM values decreases with time.
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• Feedback signal produced by the SCM• Controller changes the output to the
pump proportionally.• Process controller has programmable
tuning constants that allow the operatorto compensate for lag times
• Built-in alarm functions that alert theoperator immediately if an overfeed orunderfeed of chemicals is encountered
• The setpoint is established empiricallyby the operator to determine whatamount of dosage corresponds tooptimum chemical feed.
• The process of establishing the setpointbegins when a system is producingacceptable water.
• An incremental decrease in dosage ismade and the effect is observed byevaluating the finished quality
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Sample point
• Sample point should follow mixing• Uniform dispersion• Reaction of coagulant• Fluctuations in the SC reading may indicate
incomplete mixing
Sample point
• Sample near the center of a pipe or channel• Avoid sample locations prone to collecting
silt , sand or grit.• Shorten sample lines when possible to
minimize clogging and reduce responsetime.
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Factors Affecting Dose
• Flow Rate• Coagulant Strength• Solids Concentration• pH• Particle Charge• Signal Offset
Flow Rate
• A change in plant flow must correspond to aproportional change in chemical feed
• Automatic flow proportioning can utilizeSCM as a trim adjustment
• Drastic changes in flow can affect lag time.
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Coagulant Strength
• The strength of the coagulant or flocculantmay vary and can be detected with the SCM
• Examples have been seen in slurry orpolymer makedown.
Solids Concentration
• An increase in influent solids, particleconcentration, (turbidity), and organicmatter will increase the coagulant demand.
• The SCM reading will move in the negativedirection under these conditions.
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pH
• Changes in the H+ ion concentration willaffect the SCM.
• Sampling between the coagulant and otherchemical can reduce interference ofchemicals used to adjust pH.
• The chemicals can also be dosedproportionally to minimize effects.
Particle Charge
• Variations in colloidal and sub-colloidal(i.e.color) particle charges are measuredwith the SCM
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Signal Offset
• The absolute SCM reading whichcorresponds to optimum coagulationefficiency is not necessarily Zero.
• The instruments have the capability to forcea “Zero” reading at optimum conditions toallow for an easy reference.
Other Factors
• A periodic “re-optimization” isrecommended to verify the proper SCMsetpoint.
• Downstream process parameters such assettled and filtered turbidity and particlecounts are useful for optimizing.
• The floc meter /PDA can be used as well
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Installation
• Most common problem is clogging ofsample line to SCM.
• Recommend relocation to reduce buildup ofsilt or other debris
• Cyclone separators and “Y” can be used.• Periodic Auto or manual flushing• Increase flow rate• Short sample lines with visible drains
Operational Difficulties
• Location of sensor at sample point vs. in theLab. (remote sensor)
• Signal offset due to sample limitations.• Provides information more rapidly than a
Jar Test, but doesn’t indicate processperformance only deviations from optimum.
• Chemical savings are more substantialunder changing conditions.
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Operational Difficulties
• Excessive lag time can lessen response tochanges in dosage.
• SCM requires scheduled preventativemaintenance.
• Samples high in Iron or manganese requiremore frequent cleaning.
• A loss of response or sensitivity typicallyindicates cleaning is required.
Field Application andExperiences
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Recommendations for SuccessfulOperations
• Sample times from coagulant addition toSCM are typically 3-5 minutes
• SCM must be installed far enoughdownstream to allow proper mixing. (ie.After rapid or inline mixer, venturi’s orseveral pipe bends, etc…)
• Optimize pH control for proper coagulation• Maintain sample flow through sensor and
use flow alarm for automatic dosing.
Recommendations for SuccessfulOperations
• Keep the SCM maintenanced• Get the instrument “tuned up” periodically.
(replace piston and probe components ifnecessary)
• Periodically check optimization of theprocess and verify the SC setpoint
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Determination of Proper Set Point
• First optimize plant process - flocculation,particle and NOM removal
• Minimize filter effluent turbidity/particlecount
• Minimize coagulant dose and maintainwater quality
• Maximizing plant / process efficiency• “Zero” streaming current reading
Maintenance andTroubleshooting
• Source water quality can affect thefrequency of maintenance and cleaning
• High turbidity, high iron and manganeselevels and “foul” the electrode bore faster
• Oxidized iron and manganese can becleaned with a reducing agent.
• Worn probe components must be replaced
Recommended