1

Streaming Current InstrumentTraining

Chuck VealMicrometrix Corp

Charged Particles Repel

2

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

3

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)

4

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.

5

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

6

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

7

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

8

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

9

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

10

Stationary

++

++

++++++

++ ++++++++

++++

++ ++

++

++++

++++

Liquid / Piston StationaryLiquid / Piston Stationary

Piston Upstroke

++ ++

++++++

++ ++++++

++++ ++

++

++

++++

++++

Liquid / PistonLiquid / Piston

11

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

1

2

3

4

5

6

7

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

12

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

13

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.

14

• 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

15

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.

16

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.

17

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.

18

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

19

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

20

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.

21

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

22

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

23

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