- Email: [email protected]
The development of a fine particle monitor based on the method of dynamic light extinction
TheDevelopmentof a Fine Particle MonitorBasedon the Methodof Dynamic light Extinition Richard Kirby, Philip Blakeley* and Doug Spencer Anglian Water Services Ltd, Henderson House, Ermine Business Park, Lancaster Way, Huntingdon PE18 6XQ, UK *Diverse Technologies
& Systems Ltd, Kingfisher House, 7 High Green, Great Shelford, Cambridge CB2 5EG, UK
Presented at the Filtration Society Meeting on ‘Instrumentation in Water Filtration Processes’ at University College, London, UK on 24 June 1997 Since the publication of the 1990 Badenoch report into Cryptosporidium in water supplies, UK water companies have been finding ways to improve monitoring and control systems for water treatment processes. Further improvements are still desirable, and the present paper describes work carried out by Anglian Water, in collaboration with University College, London and Diverse Technologies 81 Systems Ltd of Cambridge, to develop an inexpensive Fine Particle Monitor suitable for routine operational use on water treatment works.
turbidity measurement to monitor the output performance of each filter. Local indication of measured turbidity and remote monitoring via telemetry is provided to allow optimisation of the filter cycle. Turbidity is used as a monitor of performance because it is a well established and trusted technique, and is an effective monitor of filter performance. However, the nature of the measurement makes it most efficient at estimating the concentrations of particles less than 1 pm in diameter.t*l Cryptosporidium oocysts and Giardia cysts are in the range 4-12 Km in diameter. Under stable process conditions the particle size distribution remains constant, so a rise in turbidity reflects a concomitant rise in the concentration of larger particles. Work undertaken at the Water Research Centre (WRc)t31 on behalf of UK Water Industry Research Ltd (UKWIR) has demonstrated that, for rapid gravity filters, the greatest risk from Cryptosporidium is in the first hour of a filter run, and is consistent with turbidity and particle count data. In these studies the vast majority of the filtered water particles were in the smallest size range measured of 2-5 pm. Moreover, no correlation was found between turbidity and particle counts, but particle counts in the range 2-5 urn were found to exhibit much wider fluctuations in values than corresponding values of turbidity.
apid gravity filtration (RGF) is an integral component of drinking water treatment processes, and is used on nearly half of Anglian Water’s 148 source treatment works. Its purpose is to remove suspended material remaining in process water after coagulation and settlement. One potential component of the suspended material is the cysts of pathogenic organisms, particularly Cr~~ptosporidium and Giardia. While the numbers of cysts are very low in comparison with other similarly sized particles, their potential impact on public health in supply is of considerable concern. Infection can be fatal to immuno-suppressed water users, and causes severe symptoms in other water users. As a result it is of paramount importance to preclude such particles from supply. The cysts are extremely resistant to existing disinfection techniques, so the only effective method of removal is the use of efficient physical barrier systems such as slow sand and rapid gravity filtration. Treatment strategies have been developed in response to the first Badenoch report,t’] which was commissioned in response to an outbreak of cryptosporidiosis in the Oxford area in 1989. In addition to providing comprehensive guidelines for all stages of water collection, treatment and supply, it prompted rigid regulations on filter management and monitoring. As a result, every rapid gravity filter in Anglian Water now has dedicated
R
Raw\\eterturbidlty
u-act-95
tMkt-95
15-c.x-9s
)5-0ct-95
%oct-95
Date -
Raw uPlter particle
-
Rawwater
turbidity
counts
_
Filtrate
particle
_
Filtrate
turbidity
cotmts
Figure 1. Effect of change in pre-ozone dose on RGFperformance: comparison of particle count and turbidity (after Chipps eta/.W).
Flpw
Light
Emitting Diode
E LED
q
Photo
Detectc
Dr
:
1
The data produced are very useful in an investigative situation, but are too complex to comfortably integrate into most existing operational control strategies.
A simpler method of measuring fluctuations of particles in suspension has been proposed by Gregory,L61 and is the subject of a UK Patent. The method uses an inexpensive light source and detector system, and exhibits improved sensitivity to particles in the 4-12 pm size range compared with conventional turbidity measurement. Anglian Water has obtained a license from UCLi Ltd to use this technique in the development of a Fine Particle Monitor suitable for on-line operational use(see Figure 2).
1 Clear plastic tubing THEORY
Path length
= L
[ Figure 2. Schematic diagram of fine particle monitor flow-through cell, fixed length passing between,a light-emit&g
diode anda photodetktor.
The theory of particle measurement by dynamic light extinction has been described previously.[61 Salient points are repeated here for the purposes of clarifying the measurement principle. To avoid confusion with conventional techniques for turbidity measurement and particle counting, the new method will be referred to as ‘fine particle monitoring’. Turbidity is defined in terms of the reduction in intensity (extinction) of a beam of light passing through a suspension: I = IOexp( -zL)
(1)
where I, is the intensity of transmitted light, I is the intensity of light after passing through a length L of suspension, and T is turbidity. For a monodisperse suspension, containing N particles per unit volume.
TIME>
Figure 3. Typical sensor output, where J,IVvis transmission through purewater, Iflis the mean output proportional to mean turbidity, DC is the steady (mean) signal, and AC is the fluctuation caused by changes in particle numbers, size distribution or scatter coefficient, the RMS value of which is proportional to the standard deviation of the mean particle number. Other workersr4] have found that the particle size distribution can change dramatically when process conditions, such as the reduction in pre-ozone dose concentration, are altered (Figure 1). They found that while filtrate turbidity remained constant at about 0.45 NTU, the particle count increased from a steady 400 counts/ml to an average 2000 counts/ml, peaking at over 4000 counts/ml. The particle count spanned the size range 2-125 pm, but the most marked increase in counts/ml was shown in the 5-10 pm range. Such data throw into doubt the efficiency of turbidity as a definitive monitor of filter performance. The second Badenochr51 repot-l makes no further firm recommendations for additional moniitoring to be added to treatment plant. However, it points out that: 0 0 0
Studies have shown that the pattern of release of particles from a filter is similar when different sizes of particle are measured, although the sensitivity varies. The most sensitive results have been obtained at the lower end of the detection limits using a particle size range between 2 and 5 pm. Particle size monitoring has been shown to be more sensitive than turbidity monitoring, and can give earlier warning of a possible breakthrough.
The report further states that the role of particle monitoring for particle size needs to be evaluated, taking into account sitespecific factors, and that decisions are required on whether monitors should be used for plant control and/or for diagnostic investigations to identify problems with specific filters. The industry generally recognises that particle counting, while a highly accurate and informative measurement for the purpose of filter management, is not yet viable for routine operational control, for two reasons: q
Particle counters are significantly and operate than turbidimeters.
more expensive
to install
where C is the scattering cross-section of a particle. For a suspension of particles flowing through a cell with a light source and detector positioned as shown in Figure 1, the average number of particles in the beam is v= NAL
(3)
where N is the number per unit volume, A is the effective crosssectional area of the light beam, and L is the optical light path. Combining Eqns. I,2 and 3 we obtain, for a uniform suspension,
where V and V0 are the measured voltages corresponding transmitted and received light intensities, respectively. Variations in the number of particles cause fluctuations value of the detector voltage, which can be expressed as mean-square (RMS) value. The expression for the ratio RMS and the steady-state (DC) component of the detected
to the in the a rootof the light is
R(=E&@ where L is the optical path length, A is the effective crosssectional area of the light beam, N is the number concentration, and C is the light scattering cross-section of the particles. By maintaining the DC component of the detected light at a constant value by suitable design of the electronic circuitry, problems resulting from fouling of the optical surface or electronic drift can be largely eliminated. By measuring variations in the RMS value only, a method for detecting fine particles greater than about l-2 Frn can be achieved which is more sensitive than existing methods of turbidity measurement. Moreover, it can be shown that, for a constant value of scattering coefficient Q and a fixed volume fraction 4, the RMS value increases in proportion to the square-root of the particle size, which is in contrast to the effect of particle size on conventional turbidity measurement. The relevant expression is
(‘3
where $ is the volume fraction of particles in suspension, the particle radius. FINE PARTICLE
MONITOR:
INSTRUMENT
and a is
DESIGN
The user requirement specification for the Fine Particle Monitor was drawn up by Anglian Water’s Innovation Department, and is summarised as follows: Ll 0 0 0 0 0
Simple instrument for on-line detection of fine particles in water in the 2-20 pm size range. Ruggedised construction, suitable for continuous operational use on exposed water treatment works. Local display with alarm outputs. Minimal maintenance. Compliant with relevant EU legislation. Suitable method of in-field calibration.
Following joint discussions between University College London, Anglian Water and Diverse Technologies & Systems, a prototype was produced as shown in Figure 4. The Fine Particle Monitor was housed in an IP65-rated cast aluminium housing. The sensor head was made from a polymer, and was mounted directly onto the side of the main housing. The sample tube could be clipped into the sensing head, which meant that the water-filled sample tube never entered the main electronics housing. The prototype particle monitor used a digital display to indicate the particle count. This was calibrated arbitrarily in order to provide readings over the range of samples used in the initial trials. The front panel also used three LEDs to indicate whether the particle count was low (O-0.99) , medium (0.99--1.99), or high (2.00 and over). Again, the settings were arbitrary, and were used to compare the principle of dynamic light extinction with other particle monitoring techniques. An internal relay was provided to allow persistently high levels of particle concentraGon to be detected by existing data capturing systems. The ‘Test’ button caused a steadily increasing signal to be applied to the detector, so that the displays and the alarm relay could be tested. The ‘Reset’ button was used to return to normal particle monitoring operation. The prototype equipment had internal manual adjustment for the gain and offset levels of the monitored signal. Other manual adjustments were provided to alter the warning levels, and trip
level. The delay on the alarm relay could also be adjusted, to avoid false indications resulting from short-term fluctuations in the particle content. An analogue output was provided to enable chart recordings of the Particle Index to be made. In order to avoid inaccurate readings because of clouding of the sample tube, the front-panel LEDs were flashed if the signal strength became too low. INSTRUMENT
CALIBRATION
The Fine Particle Monitor designed by Diverse Technologies & Systems gives a clear indication of the presence of particles over 1 F-m in diameter. However, the reading itself does not correspond to the number of particles passing through the flow tube. This is because the RMS voltage signal is derived from the scattering cross-section of the particles as well as the number of particles per unit volume. The relationship between these parameters is given in Eqn. 5, which indicates how the RMS value of the detector responds to increasing particle count and to differentsized particles. Thus the RMS value displayed on the monitor is a complex statement of the presence of particles in the flow tube. Since no existing standard of calibration is currently used for this method of detecting particles, we propose a definition which would enable the monitoring equipment to be standardised. As a result of work undertaken by Diverse Technologies & Systems, we propose that the RMS voltage of the detector is referred to as the ‘Particle Index’. In practice the measurement system is adjusted to read zero Particle Index when no particles are flowing through the sample tubing. We further define that the Fine Particle Monitor will be adjusted to read 500 when 500 particles per cm3 of latex with a diameter of 5 km are flowing through the sample tube. This fixes the gain and offset for the monitor. Any other particle sizes and particle concentrations will then give reproducible Particle Index values. (Note that the trials reported below using the prototype monitor used different settings for the gain and offset - later units are calibrated as described here.) The monitor uses an internal reference signal which is available as a reference standard to maintain the initial calibration carried out using the 5 pm particles. This can be used at any time to ensure that the zero point and the gain for the monitor remain the same as when the unit was calibrated using the known particles. In order to prevent changes in the opacity of the sample tube from altering the measured value of the Particle Index, the
.~S~rmoR 3mm Sample tubing
Figure 4. The fine particle monitor. A: General arrangement, B: Sensor head (detail).
HEAD
did eventually affect the measurement, when the output of the LED could no longer be increased. This fouling took approximately four weeks. The fault was rectified by changing the sample tube. It has thus been shown that the Fine Particle Monitor will produce results which correspond broadly with variation in particle count. However, changes in process which alter particle size distribution will alter the relationship between Particle Index and particle count, in a similar manner to the change in the relationship between counts/ml and turbidity reported by other workers. Effect
of variation
in particle
size
Samples of tap water were separately spiked with latex particles 2, 5 and 20 urn in diameter. The samples were then diluted to give high, medium and low concentrations of each, which were then run through the Fine Particle Monitor and Hach 2100an turbidimeter in series, with the Hiac particle counter in parallel. The outputs from each were compared. Results are presented in Figures 7, 8 and 9, with the square of Particle Index and turbidity plotted against counts per ml of latex sphere. The results showed a linear relationship between particle count and the square of the Particle Index for a given size of latex particle. Moreover, the reducing sensitivity of turbidity to increasing particle concentration was apparent as particle size increased. rlgure 3. I~SI rig wow magram. Effect
of sample
flow
rate
Because the unit measures the size of the fluctuating peaks rather than the frequency of the fluctuating signal, theory suggests that the measurement is independent of flow rate. This has potential advantages in the ease of installation and operation of the unit. To confirm this, a sample of filter effluent was recycled through the Fine Particle Monitor at flow rates varying between 20 ml/min and 200 ml/min, and the results compared with the same
lCQO900 :800 ! 700 7+
Figure 6. (Particle Index)2 plotted against particle counter values brightness of the emitter is controlled to give a fixed value for the DC component of the detector voltage. These features allow the Fine Particle Monitor to be recalibrated without further reference to flowing particles. Moreover, by monitoring the voltage required to drive the emitter at a sufficient level to maintain the required DC voltage on the detector, it has been possible to indicate when the sample tube is cloudy and needs to be replaced.
I@** *.*(I b... . ..a / . 8
l
.
.
.
.
. . I .
. . .
Comparison
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. .
.
.
.
.
.
. .
.
:.¶-:. . .
. .
. .
. .
. .
% 2 .
z r
%i LL 2 9 .
6 ‘L 9 x cz ca fi ‘5; s! E
. .
.
E
----y---.-
i
1234
132OQ
0
19500
Particb Cowm per
23369
ml
Figure 7. Response to 2 pm latex spheres.
.
.
l .
oTurbidity 0)
EXPERIMENTAL
.
. .
Wde.x Squared -
.
with
particle
counter
For the initial laboratory evaluation, the unit was placed in series with a Hiac Royce VersaCountTM particle size analyser The unit was tested with samples of tap water, filter effluent and clarifier effluent, and the output compared with the particle counts. The experimental set-up is shown in Figure 5. Several bench tests were carried out, including simulated breakthrough experiments where tap water was spiked with increasing concentrations of clarifier effluent. The squared values of the Fine Particle Monitor output, or Particle Index, are plotted against particle count in Figure 6. Regression plots of the two sets of data showed a positive linear relationship with a significant correlation between the two measurements. All outliers are included in the graph, some of which are attributed to air entrainment in the sample. There is a suspicion that the lower values (tap water and filter effluent) have a steeper gradient on the regression plot than the higher values. This is most likely because of the differing particle size distributions between the two process waters, where filter effluent has proportionately higher numbers of submicron particles, which the particle monitor is more sensitive to than the counter. Some operating problems were encountered during this work. Hardness in the water adhered to the internal walls of the tubing. While this initially was not a problem because the measurement depends on relative fluctuations, not absolute values, the fouling
:igure 6. Response to 5 pm latex spheres 1 4000 3500 g
3ooo
2
2300
f
2000
3 i!E
1500
I
1000 500 0
0.8 a f 0.6 c3 * 0.4
i I 0
50
100
150
200
250
Psrttek Coumtsper ml
Figure 4. Response to 20 pm latex spheres.
300
350
4043
suggested that in process waters with intermittent high particulates, especially adhesive floes such as ferric or aluminium salts, the tubing may require replacement at varying intervals. As a result of these findings, the prototype design has been modified to incorporate a warning signal, to show when the compensation circuit has reached its maximum and the tubing must be changed.
04
0
’ Time (hours)
’
: lo
3
4
- Fine Particle Monlor (varying flowrate) .----. Particle Counter (ftxed flowrate) I
igure 10. Effect of flowrate variation on the Fine Particle Monitor. sample passed through a Hiac particle counter ml/min. The results are shown in Figure IO.
at a rate of 25
PILOT PLANT EVALUATION The Fine Particle Monitor was installed at the Grafham water treatment works pilot plant, on the outlet from one of the RGF columns, to evaluate its performance in comparison with the Hiac particle counter installed on the plant. A major feature of this work was to observe the robustness of the unit and its suitability for on-line measurement. Figure 11 shows the comparison between Particle Index and particle count on the outlet from the filter following a backwash. The results are not directly comparable, since the particle counter results are in the narrow range l-5 urn, whereas the Particle Index values covered the whole range of particle sizes. In addition, the Fine Particle Monitor output value was averaged over a longer period than the particle counter, so some lag in values was expected. It is not surprising, therefore, that the initial high peak of Particle Index is sustained for a longer time than the narrower-band particle counter. However, the trend for each graph provides confidence that the fine particle monitor is accurately reflecting what is actually happening on the plant. Further discoveries were made about the maintenance required by the particle monitor while on trial at the pilot plant. Under normal plant conditions, the Fine Particle Monitor would last one month before requiring routine maintenance. However, on one occasion the plant suffered from a larger than normal carryover of ferric floe; this effectively ‘blocked’ the tube optically within a few days, and the readings began to drop. The sample tube was changed and the readings returned to normal. This
-Particle -Fine
Figure 11. RGFpilot plant performance following backwash.
CONCLUSIONS A practical application of the theory of dynamic light extinction, developed at University College London, has been demonstrated with a prototype Fine Particle Monitor. Experimental evidence to date confirms that the monitor performs in the way predicted. Pilot plant investigations have been carried out to test the monitor’s suitability for operational use in industrial water treatment works. Results are encouraging, and further development of a production version of the monitor is under way. The Fine Particle Monitor provides a practical and economical alternative to existing particle counters for the detection of particles greater than 1 urn. It offers a viable additional measurement to turbidity for protection against Cryptosporidium and Giardia cysts in treated waters. ACKNOWLEDGMENTS The authors wish to thank Anglian Water Services Ltd and Diverse Technologies & Systems Ltd for permission to publish this paper. The views expressed are those of the authors and not necessarily of their respective companies. REFERENCES 1 2
in water supplies’
‘Cryptosporidium
Group
of Experts
Gregory,
Fillration
(HMSO,
J.: ‘Ciyptosporidiom
& Separation,
(The Badenoch
Report).
in water: Treatment
May 1994, 31(3),
and monitoring
4
Boston,
Chipps,
Massachusetts,
M.J. et a/.: ‘Rapid gravity
USA, November filter management’.
control in water Technology
1996. Chartered
of Water & Environmental Management (CIWEM)/SWIG, Symposium Sensors 8 Sensitivity: Water Quality Control through Instrumentation Sensors,
December
Gregory,
Interface
Counter Particle Monitor
in water supplies’. 1995).
J.: ‘Turbidity
Science,
Institution on and
1995.
5 ‘Cfyptosporidium (HMSO, UK, October 6
methods’,
pp. 28s289.
3 Hall, T. and Croll, B.: ‘The UK approach to Cryptosporidium treatment’. American Water Works Association Water Quality Conference,
Report of the
UK, 1990).
fluctuations
June 1985, 105(2).
Second in flowing
Report
of the Group
suspensions’,
of Experts
J. Colloid &
& Systems Ltd, Kingfisher House, 7 High Green, Great Shelford, Cambridge CB2 5EG, UK
Presented at the Filtration Society Meeting on ‘Instrumentation in Water Filtration Processes’ at University College, London, UK on 24 June 1997 Since the publication of the 1990 Badenoch report into Cryptosporidium in water supplies, UK water companies have been finding ways to improve monitoring and control systems for water treatment processes. Further improvements are still desirable, and the present paper describes work carried out by Anglian Water, in collaboration with University College, London and Diverse Technologies 81 Systems Ltd of Cambridge, to develop an inexpensive Fine Particle Monitor suitable for routine operational use on water treatment works.
turbidity measurement to monitor the output performance of each filter. Local indication of measured turbidity and remote monitoring via telemetry is provided to allow optimisation of the filter cycle. Turbidity is used as a monitor of performance because it is a well established and trusted technique, and is an effective monitor of filter performance. However, the nature of the measurement makes it most efficient at estimating the concentrations of particles less than 1 pm in diameter.t*l Cryptosporidium oocysts and Giardia cysts are in the range 4-12 Km in diameter. Under stable process conditions the particle size distribution remains constant, so a rise in turbidity reflects a concomitant rise in the concentration of larger particles. Work undertaken at the Water Research Centre (WRc)t31 on behalf of UK Water Industry Research Ltd (UKWIR) has demonstrated that, for rapid gravity filters, the greatest risk from Cryptosporidium is in the first hour of a filter run, and is consistent with turbidity and particle count data. In these studies the vast majority of the filtered water particles were in the smallest size range measured of 2-5 pm. Moreover, no correlation was found between turbidity and particle counts, but particle counts in the range 2-5 urn were found to exhibit much wider fluctuations in values than corresponding values of turbidity.
apid gravity filtration (RGF) is an integral component of drinking water treatment processes, and is used on nearly half of Anglian Water’s 148 source treatment works. Its purpose is to remove suspended material remaining in process water after coagulation and settlement. One potential component of the suspended material is the cysts of pathogenic organisms, particularly Cr~~ptosporidium and Giardia. While the numbers of cysts are very low in comparison with other similarly sized particles, their potential impact on public health in supply is of considerable concern. Infection can be fatal to immuno-suppressed water users, and causes severe symptoms in other water users. As a result it is of paramount importance to preclude such particles from supply. The cysts are extremely resistant to existing disinfection techniques, so the only effective method of removal is the use of efficient physical barrier systems such as slow sand and rapid gravity filtration. Treatment strategies have been developed in response to the first Badenoch report,t’] which was commissioned in response to an outbreak of cryptosporidiosis in the Oxford area in 1989. In addition to providing comprehensive guidelines for all stages of water collection, treatment and supply, it prompted rigid regulations on filter management and monitoring. As a result, every rapid gravity filter in Anglian Water now has dedicated
R
Raw\\eterturbidlty
u-act-95
tMkt-95
15-c.x-9s
)5-0ct-95
%oct-95
Date -
Raw uPlter particle
-
Rawwater
turbidity
counts
_
Filtrate
particle
_
Filtrate
turbidity
cotmts
Figure 1. Effect of change in pre-ozone dose on RGFperformance: comparison of particle count and turbidity (after Chipps eta/.W).
Flpw
Light
Emitting Diode
E LED
q
Photo
Detectc
Dr
:
1
The data produced are very useful in an investigative situation, but are too complex to comfortably integrate into most existing operational control strategies.
A simpler method of measuring fluctuations of particles in suspension has been proposed by Gregory,L61 and is the subject of a UK Patent. The method uses an inexpensive light source and detector system, and exhibits improved sensitivity to particles in the 4-12 pm size range compared with conventional turbidity measurement. Anglian Water has obtained a license from UCLi Ltd to use this technique in the development of a Fine Particle Monitor suitable for on-line operational use(see Figure 2).
1 Clear plastic tubing THEORY
Path length
= L
[ Figure 2. Schematic diagram of fine particle monitor flow-through cell, fixed length passing between,a light-emit&g
diode anda photodetktor.
The theory of particle measurement by dynamic light extinction has been described previously.[61 Salient points are repeated here for the purposes of clarifying the measurement principle. To avoid confusion with conventional techniques for turbidity measurement and particle counting, the new method will be referred to as ‘fine particle monitoring’. Turbidity is defined in terms of the reduction in intensity (extinction) of a beam of light passing through a suspension: I = IOexp( -zL)
(1)
where I, is the intensity of transmitted light, I is the intensity of light after passing through a length L of suspension, and T is turbidity. For a monodisperse suspension, containing N particles per unit volume.
TIME>
Figure 3. Typical sensor output, where J,IVvis transmission through purewater, Iflis the mean output proportional to mean turbidity, DC is the steady (mean) signal, and AC is the fluctuation caused by changes in particle numbers, size distribution or scatter coefficient, the RMS value of which is proportional to the standard deviation of the mean particle number. Other workersr4] have found that the particle size distribution can change dramatically when process conditions, such as the reduction in pre-ozone dose concentration, are altered (Figure 1). They found that while filtrate turbidity remained constant at about 0.45 NTU, the particle count increased from a steady 400 counts/ml to an average 2000 counts/ml, peaking at over 4000 counts/ml. The particle count spanned the size range 2-125 pm, but the most marked increase in counts/ml was shown in the 5-10 pm range. Such data throw into doubt the efficiency of turbidity as a definitive monitor of filter performance. The second Badenochr51 repot-l makes no further firm recommendations for additional moniitoring to be added to treatment plant. However, it points out that: 0 0 0
Studies have shown that the pattern of release of particles from a filter is similar when different sizes of particle are measured, although the sensitivity varies. The most sensitive results have been obtained at the lower end of the detection limits using a particle size range between 2 and 5 pm. Particle size monitoring has been shown to be more sensitive than turbidity monitoring, and can give earlier warning of a possible breakthrough.
The report further states that the role of particle monitoring for particle size needs to be evaluated, taking into account sitespecific factors, and that decisions are required on whether monitors should be used for plant control and/or for diagnostic investigations to identify problems with specific filters. The industry generally recognises that particle counting, while a highly accurate and informative measurement for the purpose of filter management, is not yet viable for routine operational control, for two reasons: q
Particle counters are significantly and operate than turbidimeters.
more expensive
to install
where C is the scattering cross-section of a particle. For a suspension of particles flowing through a cell with a light source and detector positioned as shown in Figure 1, the average number of particles in the beam is v= NAL
(3)
where N is the number per unit volume, A is the effective crosssectional area of the light beam, and L is the optical light path. Combining Eqns. I,2 and 3 we obtain, for a uniform suspension,
where V and V0 are the measured voltages corresponding transmitted and received light intensities, respectively. Variations in the number of particles cause fluctuations value of the detector voltage, which can be expressed as mean-square (RMS) value. The expression for the ratio RMS and the steady-state (DC) component of the detected
to the in the a rootof the light is
R(=E&@ where L is the optical path length, A is the effective crosssectional area of the light beam, N is the number concentration, and C is the light scattering cross-section of the particles. By maintaining the DC component of the detected light at a constant value by suitable design of the electronic circuitry, problems resulting from fouling of the optical surface or electronic drift can be largely eliminated. By measuring variations in the RMS value only, a method for detecting fine particles greater than about l-2 Frn can be achieved which is more sensitive than existing methods of turbidity measurement. Moreover, it can be shown that, for a constant value of scattering coefficient Q and a fixed volume fraction 4, the RMS value increases in proportion to the square-root of the particle size, which is in contrast to the effect of particle size on conventional turbidity measurement. The relevant expression is
(‘3
where $ is the volume fraction of particles in suspension, the particle radius. FINE PARTICLE
MONITOR:
INSTRUMENT
and a is
DESIGN
The user requirement specification for the Fine Particle Monitor was drawn up by Anglian Water’s Innovation Department, and is summarised as follows: Ll 0 0 0 0 0
Simple instrument for on-line detection of fine particles in water in the 2-20 pm size range. Ruggedised construction, suitable for continuous operational use on exposed water treatment works. Local display with alarm outputs. Minimal maintenance. Compliant with relevant EU legislation. Suitable method of in-field calibration.
Following joint discussions between University College London, Anglian Water and Diverse Technologies & Systems, a prototype was produced as shown in Figure 4. The Fine Particle Monitor was housed in an IP65-rated cast aluminium housing. The sensor head was made from a polymer, and was mounted directly onto the side of the main housing. The sample tube could be clipped into the sensing head, which meant that the water-filled sample tube never entered the main electronics housing. The prototype particle monitor used a digital display to indicate the particle count. This was calibrated arbitrarily in order to provide readings over the range of samples used in the initial trials. The front panel also used three LEDs to indicate whether the particle count was low (O-0.99) , medium (0.99--1.99), or high (2.00 and over). Again, the settings were arbitrary, and were used to compare the principle of dynamic light extinction with other particle monitoring techniques. An internal relay was provided to allow persistently high levels of particle concentraGon to be detected by existing data capturing systems. The ‘Test’ button caused a steadily increasing signal to be applied to the detector, so that the displays and the alarm relay could be tested. The ‘Reset’ button was used to return to normal particle monitoring operation. The prototype equipment had internal manual adjustment for the gain and offset levels of the monitored signal. Other manual adjustments were provided to alter the warning levels, and trip
level. The delay on the alarm relay could also be adjusted, to avoid false indications resulting from short-term fluctuations in the particle content. An analogue output was provided to enable chart recordings of the Particle Index to be made. In order to avoid inaccurate readings because of clouding of the sample tube, the front-panel LEDs were flashed if the signal strength became too low. INSTRUMENT
CALIBRATION
The Fine Particle Monitor designed by Diverse Technologies & Systems gives a clear indication of the presence of particles over 1 F-m in diameter. However, the reading itself does not correspond to the number of particles passing through the flow tube. This is because the RMS voltage signal is derived from the scattering cross-section of the particles as well as the number of particles per unit volume. The relationship between these parameters is given in Eqn. 5, which indicates how the RMS value of the detector responds to increasing particle count and to differentsized particles. Thus the RMS value displayed on the monitor is a complex statement of the presence of particles in the flow tube. Since no existing standard of calibration is currently used for this method of detecting particles, we propose a definition which would enable the monitoring equipment to be standardised. As a result of work undertaken by Diverse Technologies & Systems, we propose that the RMS voltage of the detector is referred to as the ‘Particle Index’. In practice the measurement system is adjusted to read zero Particle Index when no particles are flowing through the sample tubing. We further define that the Fine Particle Monitor will be adjusted to read 500 when 500 particles per cm3 of latex with a diameter of 5 km are flowing through the sample tube. This fixes the gain and offset for the monitor. Any other particle sizes and particle concentrations will then give reproducible Particle Index values. (Note that the trials reported below using the prototype monitor used different settings for the gain and offset - later units are calibrated as described here.) The monitor uses an internal reference signal which is available as a reference standard to maintain the initial calibration carried out using the 5 pm particles. This can be used at any time to ensure that the zero point and the gain for the monitor remain the same as when the unit was calibrated using the known particles. In order to prevent changes in the opacity of the sample tube from altering the measured value of the Particle Index, the
.~S~rmoR 3mm Sample tubing
Figure 4. The fine particle monitor. A: General arrangement, B: Sensor head (detail).
HEAD
did eventually affect the measurement, when the output of the LED could no longer be increased. This fouling took approximately four weeks. The fault was rectified by changing the sample tube. It has thus been shown that the Fine Particle Monitor will produce results which correspond broadly with variation in particle count. However, changes in process which alter particle size distribution will alter the relationship between Particle Index and particle count, in a similar manner to the change in the relationship between counts/ml and turbidity reported by other workers. Effect
of variation
in particle
size
Samples of tap water were separately spiked with latex particles 2, 5 and 20 urn in diameter. The samples were then diluted to give high, medium and low concentrations of each, which were then run through the Fine Particle Monitor and Hach 2100an turbidimeter in series, with the Hiac particle counter in parallel. The outputs from each were compared. Results are presented in Figures 7, 8 and 9, with the square of Particle Index and turbidity plotted against counts per ml of latex sphere. The results showed a linear relationship between particle count and the square of the Particle Index for a given size of latex particle. Moreover, the reducing sensitivity of turbidity to increasing particle concentration was apparent as particle size increased. rlgure 3. I~SI rig wow magram. Effect
of sample
flow
rate
Because the unit measures the size of the fluctuating peaks rather than the frequency of the fluctuating signal, theory suggests that the measurement is independent of flow rate. This has potential advantages in the ease of installation and operation of the unit. To confirm this, a sample of filter effluent was recycled through the Fine Particle Monitor at flow rates varying between 20 ml/min and 200 ml/min, and the results compared with the same
lCQO900 :800 ! 700 7+
Figure 6. (Particle Index)2 plotted against particle counter values brightness of the emitter is controlled to give a fixed value for the DC component of the detector voltage. These features allow the Fine Particle Monitor to be recalibrated without further reference to flowing particles. Moreover, by monitoring the voltage required to drive the emitter at a sufficient level to maintain the required DC voltage on the detector, it has been possible to indicate when the sample tube is cloudy and needs to be replaced.
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.
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.
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Comparison
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19500
Particb Cowm per
23369
ml
Figure 7. Response to 2 pm latex spheres.
.
.
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oTurbidity 0)
EXPERIMENTAL
.
. .
Wde.x Squared -
.
with
particle
counter
For the initial laboratory evaluation, the unit was placed in series with a Hiac Royce VersaCountTM particle size analyser The unit was tested with samples of tap water, filter effluent and clarifier effluent, and the output compared with the particle counts. The experimental set-up is shown in Figure 5. Several bench tests were carried out, including simulated breakthrough experiments where tap water was spiked with increasing concentrations of clarifier effluent. The squared values of the Fine Particle Monitor output, or Particle Index, are plotted against particle count in Figure 6. Regression plots of the two sets of data showed a positive linear relationship with a significant correlation between the two measurements. All outliers are included in the graph, some of which are attributed to air entrainment in the sample. There is a suspicion that the lower values (tap water and filter effluent) have a steeper gradient on the regression plot than the higher values. This is most likely because of the differing particle size distributions between the two process waters, where filter effluent has proportionately higher numbers of submicron particles, which the particle monitor is more sensitive to than the counter. Some operating problems were encountered during this work. Hardness in the water adhered to the internal walls of the tubing. While this initially was not a problem because the measurement depends on relative fluctuations, not absolute values, the fouling
:igure 6. Response to 5 pm latex spheres 1 4000 3500 g
3ooo
2
2300
f
2000
3 i!E
1500
I
1000 500 0
0.8 a f 0.6 c3 * 0.4
i I 0
50
100
150
200
250
Psrttek Coumtsper ml
Figure 4. Response to 20 pm latex spheres.
300
350
4043
suggested that in process waters with intermittent high particulates, especially adhesive floes such as ferric or aluminium salts, the tubing may require replacement at varying intervals. As a result of these findings, the prototype design has been modified to incorporate a warning signal, to show when the compensation circuit has reached its maximum and the tubing must be changed.
04
0
’ Time (hours)
’
: lo
3
4
- Fine Particle Monlor (varying flowrate) .----. Particle Counter (ftxed flowrate) I
igure 10. Effect of flowrate variation on the Fine Particle Monitor. sample passed through a Hiac particle counter ml/min. The results are shown in Figure IO.
at a rate of 25
PILOT PLANT EVALUATION The Fine Particle Monitor was installed at the Grafham water treatment works pilot plant, on the outlet from one of the RGF columns, to evaluate its performance in comparison with the Hiac particle counter installed on the plant. A major feature of this work was to observe the robustness of the unit and its suitability for on-line measurement. Figure 11 shows the comparison between Particle Index and particle count on the outlet from the filter following a backwash. The results are not directly comparable, since the particle counter results are in the narrow range l-5 urn, whereas the Particle Index values covered the whole range of particle sizes. In addition, the Fine Particle Monitor output value was averaged over a longer period than the particle counter, so some lag in values was expected. It is not surprising, therefore, that the initial high peak of Particle Index is sustained for a longer time than the narrower-band particle counter. However, the trend for each graph provides confidence that the fine particle monitor is accurately reflecting what is actually happening on the plant. Further discoveries were made about the maintenance required by the particle monitor while on trial at the pilot plant. Under normal plant conditions, the Fine Particle Monitor would last one month before requiring routine maintenance. However, on one occasion the plant suffered from a larger than normal carryover of ferric floe; this effectively ‘blocked’ the tube optically within a few days, and the readings began to drop. The sample tube was changed and the readings returned to normal. This
-Particle -Fine
Figure 11. RGFpilot plant performance following backwash.
CONCLUSIONS A practical application of the theory of dynamic light extinction, developed at University College London, has been demonstrated with a prototype Fine Particle Monitor. Experimental evidence to date confirms that the monitor performs in the way predicted. Pilot plant investigations have been carried out to test the monitor’s suitability for operational use in industrial water treatment works. Results are encouraging, and further development of a production version of the monitor is under way. The Fine Particle Monitor provides a practical and economical alternative to existing particle counters for the detection of particles greater than 1 urn. It offers a viable additional measurement to turbidity for protection against Cryptosporidium and Giardia cysts in treated waters. ACKNOWLEDGMENTS The authors wish to thank Anglian Water Services Ltd and Diverse Technologies & Systems Ltd for permission to publish this paper. The views expressed are those of the authors and not necessarily of their respective companies. REFERENCES 1 2
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‘Cryptosporidium
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Gregory,
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M.J. et a/.: ‘Rapid gravity
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1996. Chartered
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Gregory,
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Counter Particle Monitor
in water supplies’. 1995).
J.: ‘Turbidity
Science,
Institution on and
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5 ‘Cfyptosporidium (HMSO, UK, October 6
methods’,
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3 Hall, T. and Croll, B.: ‘The UK approach to Cryptosporidium treatment’. American Water Works Association Water Quality Conference,
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fluctuations
June 1985, 105(2).
Second in flowing
Report
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suspensions’,
of Experts
J. Colloid &