The

AccuflocStreaming Current

Monitor

Introduction• Accufloc – Streaming Current Monitor for the monitoring

and control of coagulation

• This presentation covers:– Overview of the relevant parts of water treatment

– Background theory of particle charge distribution

– Streaming current theory and usage

– Benefits of SCM usage and applications overview

– Accufloc installation and use

– Accufloc options and ordering

– Maintenance procedures

Water Treatment• A conventional water treatment plant doses a coagulant

chemical to cause the particles to stick together• The water is then gently mixed to cause the floc size to increase

• The water slowly flows through a clarifier, or settling basin, to

cause the floc to settle out.

Sludge Level

W ater Level

Pre-treatm ent chem icals.eg pH adjustm ent, lim e

Source W ater, possib ly after

pre-sedim entation

Flocculant a id,eg polym er

C oagulant,eg a lum D isinfectant,

eg C hlorine

To storageand

reticu lation

R apid M ixer F locculator C larifier F ilter

Post-treatm ent chem icals,eg pH adjustm ent

Conventional Plant Overview

Typical Rapid Mixers

Mixer and flocculator (gentle mixer)

Clarifier – viewed from top

Typical small, above-ground clarifier

Background Theory• Most naturally occurring particles in surface water are clays

which have a negative surface charge

• Like charges repel each other.

• The main action of the coagulant is to neutralise the charge on the particles to allow them to combine into particles large enough to settle out of suspension

• Like charges repel • Neutral particles can bind together

• The double layer model explains the distribution of ions around each colloidal particle.

• 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 in the lab using a microscope is to observe turbidity particles in an electrophoresis cell.

• Changes in ion concentration, such as pH changes, affect zeta potential.

• The streaming current meter (SCM) was invented around 1966 by F.W Gerdes. It consists of a piston driven up and down into a close-ended chamber

• It is based on the effect where the surfaces which the colloidal particles flow past, quickly take on their charge characteristics

• The water flowing rapidly up and down through the annulus results in displacement of the counter-ions. A current flows through the electrodes to remove the charge separation

Streaming Current

History

• An online version of Zeta-Potential Measurement• Used (badly) on raw water in the 1970s• Started to be used successfully in the late 1980s and 1990s on

dosed water• Some countries, such as New Zealand, now require use of a

streaming current meter for the top grades of water.• Knowledge and correct set-up is important

Modern SCM Usage• Feedback control of coagulant dosage

• The SC set-point is determined by jar tests

• This will compensate for changes in the concentration of particles, their zeta-potential and plant flow

• However periodic jar tests are required because:- The sweep floc effect means that zero zeta-potential does not

always result in the best coagulation, the effect this has will change depending on raw water pH.

– The zero SC reading is typically offset from the zero zeta-potential because of the intrinsic surface charge of the piston itself.

– Wear and contamination of the piston surface gradually changes the SC reading relative to the zeta-potential.

Relationship between SC and settled water turbidity

10 15 20 25 30 35 40-10

-5

0

5

10 15 20 25 30 35 400.2

0.4

0.6

0.8

1

Alum dose (ppm)

Target Turbidity

Minimum Turbidity Point

Target SC

Jar Testing• You will still have to do this,

although not as often.• Weekly to 6 monthly, whenever

significant variation in raw water quality occurs.

• Set the pump speed to the optimal dosage, then set the SCM so it reads zero at this point

• More consistent clarifier operation• Reduced coagulant chemical costs• Longer filter runs• Automatic dosing adjustments• Reduced operator call outs• Better ‘polish’ to the water

Benefits of SCMs

Suitable Applications – almost any coagulant dosing

• Conventional plants with clarifiers. If the jar results have a definite minimum then electrical effects are significant and a SCM can be used.

• Direction filtration plants (no clarifier), here the target SC is typically set to a negative value to ensure pin-floc forms.

• Flotation (DAF) plants.

Less Suitable Applications

• Plants with low turbidity and high organic loading.

• Plants which use a much higher coagulant dosage than normal to compensate for poor pH control. These plants have a high coagulant dose, significantly above the

electrically neutral point. For example 100 ppm.

• Applications where coagulation does not occur

The AccuflocDisplays

Keypad

Terminal Strip

Mounting plate

Motor

Sample Chamber, contains piston

Body, contains bearing

Sensor

Body, contains cam

Sample Flow

Installation

• Design of the sampling is important. – The sample must be completely mixed

– Time for the water to get from the mixer to the sample point should be less than 30sec.

– Sample lines must be resistant to clogging or fouling and easy to clean or flush

• Recommended flow rate is 2-4 L/min, but can go up to >10L/min.

• A constant head is recommended

• The SCM does not pump the sample through itself.

Example Installation

Controller Tuning

• Controller tuning is critical to good results• PID auto tuners – use at your own risk. They are often not well

suited to this type of process.• Controller tuning does not have to be very tight• Tuning can be done by experience or by formula. A simple

procedure is outlined in the manual’s appendices.

The Displays

Man

Auto

ZeroSet Menu

S t r e a m i n g C u r r e n t M o n i t o r

Cancel

Enter

Hold down for 3 secondsto set the set-point to thecurrent reading.Offsetted readingbecomes Zero

Indicators for autoor manual mode.Flashes toindicate pistonstokes.

Upper display. Showsreading - Set-point.(Offset from Set-point)

Lower display. Showsreading with no offset.

Manually adjust set-point.Hold down for 3 secondsto start.

The lower display always shows the unadjusted streaming current reading. Its span can be calibrated but it will always read zero when there is no SC. (This is not necessarily zero zeta-potential)

The upper display shows the difference between the reading and the target SC. This is like single display SCMs. When the target SC is set right, zero on the upper display corresponds to the desired coagulant dosage.

Span and Zero• The span setting affects both displays and determines the sensitivity of

the readings.

• There are no absolute units of Streaming Current, so the span can be calibrated to any value on any sample. For example –10.0 on raw water.

• It is not necessary to change the factory default calibration. The span only needs to be adjusted for conveniance.

• The easiest way is to open the filter drain and pour the sample into the top of the weir. Suggested 5-10 litres of raw water.

• A sample dosed with alum will react with air and the SC reading obtained from the sample will decrease over time. Not advisable for calibration.

• The zero point needs to be set after the span is adjusted. This is set on water with the desired coagulant dosage

Using the Menus

• A full menu map is in back page of the manual

Man

Auto

ZeroSet Menu

S t r e a m i n g C u r r e n t M o n i t o r

Cancel

Enter

Switches betweenautomatic and manualmodes for controller(if fitted)

Go to menu mode.Cancel and go backwhen in menu mode

Enter currentmenu or value.

Step up or down when inmenu mode.

Some Useful Menu Items• Averaging. () A rolling average is performed over a

time period adjustable from 1 to 60 seconds.• Calibration Value. () This is what the reading will be

after a span calibration is performed. – Should be negative if calibration is performed on under dosed

water, or positive if performed on overdosed water.

• Decimal Point Position. () Can be 0.00 or 00.0• Set Defaults. () This returns all settings which affect

the calibration and reading to the factory defaults, and leaves the rest alone.

Common Options• 4-20mA output.

– Corresponds to the zeroed (upper) reading.

– The range of this can be set independently of the calibration range.

– Menu items under are : reading for 4mA () and reading for 20mA ().

• Built in PID Controller. – Manually adjustable P and I settings. Auto/Man button on front.

– Remembers its output when the unit is switched off.

– Menu items under are Proportional Band as a %() and Integral Reset Rate in minutes ()

Common Options 2• Up to 2 alarm relays.

– Each has an independent set-point(), hysteresis () and mode.

– Mode can be configured as high level alarm (), low level alarm() and NC fault alarm () .

• Flushing Mechanism. – Uses an automatically controlled solenoid valve to inject high

pressure clean water into the sample chamber.

– This reduces the need for manual cleaning, but does not remove it.

– Has adjustable interval(), duration() and post flush hold time ().

Ordering InformationDescription

   Part Number                    

                             

Streaming Current Monitor   SCM                    

Analyser Options                          

  4 - 20 mA process output       A                

  Dual Digital Inputs         D                

  Auto flushing mechanism         F              

  1 x relay output             R            

  RS485 (modbus)             M            

  2nd relay               S          

  PID control with 4 - 20mA output             C          

                             

Power Supply                          

  110V power supply                   1      

  230V power supply                   2      

                             

Extra Options                          

  Water fittings, weir and grit filter                     W  

                             

Maintenance : Cleaning• Contamination of the sensor and piston

surfaces will result in slower response and drift.

• Cleaning should be performed with a brush wetted with sample water. A toothbrush is ideal.

• Do not handle any wetted surfaces, any oily residue may affect the reading for up to 30 minutes.

• Immediately after cleaning the reading may be disturbed, this will normally stabilize within 5 minutes.

Really needs cleaning

Does not need cleaning, but shows signs of wear

Weekly cleaning is more than sufficient in most plants

Symptoms of Worn Parts

• Zero point drifts away from where it is set• Large offset, but poor sensitivity to changes• Vertical scouring on the piston or sensor• Significant staining of the piston• Looseness in the piston• Mechanical slackness• A worn out SCM can still be useful with frequent

attention, but why live with it?

Replacement Parts : Piston• Piston may need replacement every 1-2 years.

• This depends on the condition of the surface at the microscopic level

• Extremely dependant on water conditions

Piston (including rod) is replaced by undoing a bolt at the top

Replacement : Sensor• Sensor may need replacement every 2-5 years.

• It is subject to the same wear forces as the piston but is more durable and its condition is less critical

• It is removed and replaced as a unit.

• This takes 5-10 minutes in the field as the front panel of the electronics must be removed to connect the wires.

Replacement : Bearings• Linear bearings may need replacing approx. every 5 years.

• They hold the piston straight and need replacing when it develops horizontal slack

• Water quality affects the life of the bottom bearing

• They can be replaced in the field in 30 minutes. They simply slot into a removable part of the main body.

Common Issues with SCMs

• Worn out parts : causes drift and inconsistency• Controller tuned incorrectly. May cycle, esp. at lower

flow rates, or be slow to respond• Poor sample point, incomplete mixing, easily blocked

sample lines• Lack of understanding, such as expecting miracles

Summary• A Streaming Current Monitor (SCM) measures the charge of

particles in water• A SCM will be useful for the control of coagulant dosing• It needs to be set up correctly to be useful• The Accufloc’s operation is relatively user friendly• It does need, at least, a little maintenance to continue to be useful

Visit our website www.accufloc.com for more information