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The Accufloc Streaming Current Monitor

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1 The Accufloc Streaming Current Monitor
Intro: A Streaming Current Monitor (or Streaming Current Meter) is an instrument to measure the surface charge on particles suspended in water. This measurement is used in controlling the dosage of coagulant before clarification in water treatment. The exact level of streaming current for optimal settling will depend on the plant. Once this has been determined, by knowledge, experimentation or jar tests, a Streaming Current Monitor can be used to automate the process of coagulant dosing to maintain optimal coagulant dosage levels, even in the presence of raw water changes.

2 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

3 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.

4 Conventional Plant Overview

5 Typical Rapid Mixers

6 Mixer and flocculator (gentle mixer)

7 Clarifier – viewed from top

8 Typical small, above-ground clarifier

9 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 Small particulates suspended in a solution will often have a an electrostatic charge on their surfaces. For example, the majority of naturally occurring particulates in surface waters are Aluminosilicates (clays) which have a negative surface charge. Like charges repel each other and so this means that the particulates will repel each other and therefore they will not be able agglomerate into particles large enough to settle out of suspension. These particles will stay suspended for hours, in some cases forever. A clarifier will be totally useless at removing un-stabilised small suspended particles.

10 Neutral particles can bind together Like charges repel
They probably already know this

11 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.

12 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 A Streaming Current Monitor uses a piston reciprocating into a closed chamber to create a high rate of flow along the chamber wall. You can think of it like the shearing effect of the piston strips away the charged ions in the water which normally surround the particle, leaving only the particles moving along the chamber wall. This moving charge is like a current and it can be measured between two probes at opposite ends of the chamber. This is the streaming current What actually happens is considerably more complicated, but it does not provide any more insight. This signal is amplified and processed to give the output Streaming Current reading. Unfortunately there are no standard units of SC. The units of Zeta-Potential are seldom used in practice, as a known solution is difficult to produce. Also the correspondence between the two types of measurement depends on the precise geometry of the piston and sample chamber. This varies as the unit wears, as you can imagine the movement of particles in this very small gap here 0.25mm, is very dependant on the surface condition of the piston and, to a lesser extent, the surface condition of the sensor. Another problem with SC is that it is slightly offset from the true zero Zeta-Potential. Due to the fact that the piston invariably develops a slight surface charge of its own, there is an apparent offset between the true-zero SC (no current measured) and zero Zeta Potential. This means that neutral surface charge, which is zero Zeta-Potential, will be a slightly positive or negative SC reading.

13 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 SCMs - where did they come from: Originally patented in 1966 by F.W. Gerdes in the USA Evolved from the desire for an online version automatic of Zeta potential meter, Zeta meters were labour intensive and needed specialist personal. A lab technician looking through a microscope with a stop watch. Have been used a little since the 1970's usually applied to look at raw water quality - for predictive water quality control. This had mixed results due to the sample over ranging the sensor - too many variations, sensor saturation, they would have needed constant adjustment In the late 80s and 90s - re introduced as post coagulant feed instrument for controlling coagulant feed rates - this was successful. Their usefulness was generally considered to be conclusively proved by a study done by S.K Dentel published by the American Water Works Association in 1986. They work well, provided: The sales person understood the instruement The client understood the instrument and what it's application All parties were prepared to spend time selecting a dosing point, looking at chemisty and tuning the control loop What did happen often was the reverse - the salesman sold and ran In countries where the sales people and clients understood the SCM, how it Worked and what it was trying to achieve applications were very successful For instance: In many countries around the world the local water quality standards require The plant to have an SCM to achieve a high water quality grading for the treatment plant. Say an "A" or "B" grading - without an SCM it will always be rated lower. Mention NZ specifically (it will be obvious you are from there), and how they are widely used and understood.

14 Modern SCM Usage Feedback control of coagulant dosage
The SC set-point is determined by jar tests Explain the difference between the SCM and the controller (although some SCMs have a controller built in) The dosing pump shown is the one for coagulant dosing. It may be Alum (Aluminium sulphate) or Ferric Chloride or Poly aluminium chloride (PAC) etc. Point out that any pre-pH adjustment will need to be done before the coagulant dosing. The SCM works properly over the same range of pH that coagulation and flocculation does, pH effects the reading in the same way that it effects clarification!! So the SCM will correct the alum dose for small pH changes, but large changes require a new jar test or pH correction. Not shown in this picture is the polymer dosing system. Polymer is dosed at the start of the flocculator and is considered a flocculent aid (not a coagulant). The polymer is intended to help the ALREADY charge-neutrlaised and destabilised particles bind togeather more densly, by tying them up in the long polymer chains. The SC sample point should be before the poly dose point. There is always an optimal coagulant dose, but poly dose depends on dense how they want the sludge blanket.

15 However periodic jar tests are required because:
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.

16 Relationship between SC and settled water turbidity
This is pretty typical. Explain in detail because its important. Note how SC response decreases once we get to a dose that’s much too high or low. Also there is a range of SC values that provide much the same result. The best SP is typically the one with the least dosage, or probably a little higher than that for safety. For example an increase in turbidity will result in this diagram moving rightwards, and the turbidity graph being stretched upwards. This means that the same SC reading will still result in the lowest turbidity, but with a much higher alum dose. It is the responsibility of the controller (in previous diagram) to increase the alum dose to keep the SC reading constant. The units depend on the plant and the unit. This is not relevant to the shape of this diagram. So, one might ask: “how do we determine what is the SC reading for this optimal point here?” Answer….. (next slide)

17 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 ….by jar tests. It is a myth that having an SCM frees you from having to do jar tests. You will still have to do these, although not nearly as often. Probably only every 3-6 months for many plants. Determine the optimal alum dose by a standard jar test, then use the controller to manually set the pump speed to the optimal dosage. Let the change reach the SCM, then set the SCM so it reads zero at this point. Some plants will install a SCM and then never do another jar test for years. This means that if the raw water varies with the season and as the SCM drifts a little, they no longer have the optimal dosage. Normally it is close enough that nobody minds, but for the small amount of effort involved in jar testing the payback is worth it.

18 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 Why are they now mandated? A correctly installed and operating SCM will provide Consistant water quality Reduced coagulant chemical costs, usually in the order of 30% plus as Operators over dose rather than under dose. Longer filter runs with reduced back washes - why you are presenting cleaner Water without floc or dirt Reduced operator call outs Automatic dosing adjustments in the event of a storm event Reduced water colour with some advanced understanding of your plant and chemistry

19 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.

20 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

21 The Accufloc Displays Keypad Terminal Strip Mounting plate
Body, contains cam Motor Body, contains bearing Sample Chamber, contains piston Sensor Sample Flow

22 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.

23 Example Installation After the Accufloc there is an elbow, this is to maintain a 50- 300mm head of water above the sensor. This is necessary for best results. Air bubbles and large flow rate changes have a bad effect on reading stability. The outlet weir is required if the drain pipe is too long for the water to drain freely. A hand valve to limit and turn off the inlet flow is recommended. There is no minimum or maximum flow, although L/min is recommended. Poor mixing of the sample will affect the accuracy of the reading so the sampling point should not be located too close to the mixing point on the flocculator. However, if the AccuFloc is being used for on-line control of flocculation the time for the sample to reach the unit should be kept less than 30 seconds, ideally as low as possible. A good balance must be struck here which will depend on each individual plant. Flocculated particles will not have a significant effect on the condition of the sensor and piston, however sandy or fibrous matter will scratch them and shorten their life considerably. It is therefore necessary to pre-filter the sample if this is likely to be a problem. If sand or grit is present in the sample water then the recommended minimum sample flow rate is 1L/min to prevent grains becoming trapped near the piston and causing deep localised scouring. This means that any grit trap in the line must be kept clean enough that fouling does not reduce the flow below 1L/min. Grit traps tend to become clogged with flocced particles, so there is a trade off between havign to constantly empty the grit trap and premature wear on the piston. Most plants do not need a grit trap, as this stuff should be removed earlier in the process.

24 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. “Tuning” is setting the parameters that determine how the controller responds to process changes. If the controller has an auto-tuning function, this can be tried but it may not always produce good tuning. Many PID controllers do not have a very good autotunign algorithms and will produce a dangerously unstable setting when used with coagulant dosing control. This may not be obvious untill you try a few artificial disturbances (like manually setting the pump speed slower then putting it back into auto) Manual tuning is pretty easy since controller tuning does not have to be very tight. Normally things take hours to change. The Accufloc manual has a simple, one-step, tuning formula in the back.

25 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.

26 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

27 Using the Menus A full menu map is in back page of the manual

28 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.

29 Common Options 4-20mA output. Built in PID Controller.
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)

30 Common Options 2 Up to 2 alarm relays. Flushing Mechanism.
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).

31 Ordering Information Description Part Number Streaming Current Monitor
Part Number       Streaming Current Monitor SCM Analyser Options 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 mA output C Power Supply 110V power supply 1 230V power supply 2 Extra Options Water fittings, weir and grit filter W

32 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.

33 Weekly cleaning is more than sufficient in most plants
Does not need cleaning, but shows signs of wear A dirty piston will cause a slower response and if it gets too bad will cause the zero point to drift away from where it was set to. We find that a medium/hard tooth brush works very well for cleaning the piston and sensor. The piston is much more important to be clean than the sensor. Use sample water and try not to touch piston or sensor with hands as any foreogn residue on them increases the amount of time before it reads correctly again. (The splatter on the background of the right picture is what happened when I cleaned it) Try and point out the vertical scouring on the left picture. If this gets much worse than it will start to drift and need replacing. Really needs cleaning

34 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? If the zero point frequently drifts away from where it is set, but the raw water hasn’t changed, then you have a worn (or possibly dirty) piston. If a 10% change in alum dose away from the ideal one does not produce a significent change in the reading then the sensitivity is too low. Increasing the sensitivity through a knob on the front (if the unit is crude enough to have one) is only a temporary solution, and not a good one. Vertical scouring that can be detected by touch is generally a bad thing. Staining of the piston is not good either, but the unit can operate acceptably like that. The actual damage is at a microscopic level and so there is no hard-and-fast rules on evaluation by inspection. Looseness in the piston means you can move it up, down, forward, back or sideways with your hand. It means that the bearings are shot and will need replacement. This will translate into drift again, sometime very fast drift to a new unwanted zero point. You know it will start to drift out of the proper clarification regime on Friday evening. A worn out SCM can still be useful with frequent attention, but why live with it?

35 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

36 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.

37 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.

38 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 Worn out unit – obvious Controller tuned incorrectly. If it is newly installed and very slow to respond: better not adjust the gain on the SCM – would you do this to a pH meter? Try some manual dose changes, the problem may be in the controller tuning. If cycling: - check the flow rate for cycles. Ensure the controller is tuned to be stable at the lowest plant flow, since at lower flows the same dose rate means a higher ppm alum. Poor sampling: If manual dose changes have no effect, or the reading is very noisey even when everything is in manual. Check sample point is far enough downstream of ANY chemical dosing that mixing is done properly. Lack of understanding The whole deal with zero points, offsets, changeable gains, tuning can be very confusing. A SCM can be made to work well in any water treatment plant that uses coagulation, but a basic understanding of what it can, and cannot, do is necessary.

39 Visit our website for more information
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 for more information

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