The Accufloc Streaming Current Monitor

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The AccuflocStreaming Current Monitor
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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

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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.

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• Conventional Plant Overview

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• Typical Rapid Mixers

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• Mixer and flocculator (gentle mixer)

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• Clarifier viewed from top

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• Typical small, above-ground clarifier

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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

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• Neutral particles can bind together

• Like charges repel

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• 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.

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Streaming Current

• 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

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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

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Modern SCM Usage

• Feedback control of coagulant dosage
• The SC set-point is determined by jar tests

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• 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.

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Relationship between SC and settled water
turbidity
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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

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Benefits of SCMs

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

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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.

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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

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The Accufloc
Displays
Keypad
Terminal Strip
Mounting plate
Body, contains cam
Motor
Body, contains bearing
Sample Chamber, contains piston
Sensor
Sample Flow
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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 gt10L/min.
• A constant head is recommended
• The SCM does not pump the sample through itself.

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Example Installation
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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 manuals
  appendices.

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The Displays
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.
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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

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Using the Menus

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

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Some Useful Menu Items

• Averaging. (Asec) A rolling average is performed
  over a time period adjustable from 1 to 60
  seconds.
• Calibration Value. (CaLr) 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. (dP) Can be 0.00 or 00.0
• Set Defaults. (SEt dEF) This returns all settings
  which affect the calibration and reading to the
  factory defaults, and leaves the rest alone.

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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 4-20 are reading for 4mA (Rd4)
  and reading for 20mA (Rd20).
• 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 CtrL are Proportional Band as a
  (Pb) and Integral Reset Rate in minutes (Rt)

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Common Options 2

• Up to 2 alarm relays.
• Each has an independent set-point(SP), hysteresis
  (HySt) and mode.
• Mode can be configured as high level alarm (HI),
  low level alarm(LO) and NC fault alarm (FLt) .
• 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(Int), duration(dur) and
  post flush hold time (PFht).

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Ordering Information
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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.

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Weekly cleaning is more than sufficient in most
plants
Does not need cleaning, but shows signs of wear
Really needs cleaning
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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?

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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
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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.

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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.

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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

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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 Accuflocs 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