Detachment of deposits from sand grains

Colloids and Surfaces, 39 (1989) 239-253 Elsevier Science Publishers B .V., Amsterdam - Printed in The Netherlands

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Detachment of Deposits from Sand Grains K .J . IVES and C .S.B . FITZPATRICK Department Civil and Municipal Engineering, University College London, London (United Kingdom) (Received 20 May 1988 ; accepted 31 August 1988)

ABSTRACT Filtration of suspensions through deep beds of granular material, as in water filtration, leads to accumulation of deposits of particles on the grains . Normally, cleaning of the grains is effected by reverse flow (upward) washing, sometimes accompanied by air bubbling, to scour the deposits away as dirty washwater . Although the processes of deposition have been studied in detail, the mechanisms of detachment during cleaning are less well known . Experiments are described where the detachment phenomena are observed by high speed video recording (200 frames/s) under conditions of sub-fluidisation, and progressive fluidisation of the grains . These observations are of different materials, such as kaolin and kaolin flocculated with aluminium hydroxide, deposited on sand grains, with water and water/air mixtures as the fluidising media . Theoretical cleaning mechanisms include fluid shear, thinning water film shear, grain collision and pulsing collapse following the passage of air bubbles . Some of these have been observed on the video recordings, and preliminary analysis has been made of local fluid shear stresses .

INTRODUCTION

Filtration through sand is a common process for clarifying suspensions, particularly water for public supply . The material removed from suspensions, mainly microscopic, is retained in the filter pores as deposits on the surfaces of the sand grains. As this retention is not straining, the deposits accumulate in depth in the sand, often penetrating distances of 0 .5 to 1.0 m. The accumulation of deposits progressively reduces the permeability of the sand, creating a loss of water pressure so that an adequate flow rate can no longer be sustained . In water filtration this limit, due to clogging of the sand, is reached in about 24 hours, consequently daily cleaning of the filter sand is necessary . The sand is cleaned in situ by an upward flow of clean water, which fluidises the sand and washes out the deposits . In addition, the sand is usually agitated, to dislodge the deposits, either by auxiliary water jets (American practice) or by blowing air bubbles upward through the sand (British and European practice) . The latter procedure is called "air scour" . 0166-6622/89/$03 .50

© 1989 Elsevier Science Publishers B .V .

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The relative merits of water jets and air scour have been debated for more than half a century, but it does appear that air scour is displacing jets in current U.S .A. designs . The way in which air scour is used is also controversial . Some filter designers prefer air then water, others choose water-air-water, others utilise air plus water then water . All have the same objective : to dislodge and remove the deposits from the sand, with minimum energy and use of washwater, to leave the filter clean, ready for the next cycle of filtration . All of these processes work satisfactorily, in various particular circumstances . Consequently, it is appropriate to ask what the mechanisms of cleaning are and how they apply in different circumstances . Filter cleaning is affected by the major variables : (i) the sand ; particularly its size, shape and surface texture (often other granular material is used, such as crushed anthracite or garnet) ; (ii) the water ; its upflow velocity and viscosity (temperatures may range from O'C to 25'C in the U.K.); (iii) the deposits; principally their nature, as they may comprise clays, metal (Al or Fe) hydroxides, natural or synthetic polymers, micro-organisms, plankton and organic debris . Most research on the filter washing process has concentrated on the fluidisation behaviour of the granular bed, backwashed with water only . (See for example Cleasby et al., 1975 .) This accounts for variables (i) and (ii) only . Very little investigation, other than some attempts to obtain overall depositdetachment mass balances (usually not very successful), has been devoted to the actual detachment cleaning process . POSSIBLE MECHANISMS OF DETACHMENT

When investigating fluidisation aspects of filter washing, Amirtharajah (see Cleasby et al ., 1975) proved on theoretical grounds that the maximum shear stress in the water occurred under the following conditions . The mean velocity gradient for non-laminar conditions is given by G= (PI V1c) "'

where P/V is the power dissipated per unit volume of the fluid and {t is the dynamic viscosity . The power is derived from the pressure loss (representing drag) across a fluidised bed, which is a function of the expanded porisity Fe . The relation between upflow superficial velocity v and expanded porosity is given by the Richardson-Zaki equation .

where v, is the settling velocity of a single grain, and n is an empirical exponent

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representing the fluid flow regime, n=5 for laminar flow, n=2 .5 for turbulent flow . By combining these equations and differentiating, the maximum velocity gradient (and therefore the maximum shear stress) occurs when e e = ( n-1) / n, which for water washing of sand was e e =0 .68 . This represents only the drag shear stress for a water fluidised bed . This research has been taken a stage further (Amirtharajah et al ., 1981 ; Amirtharajah, 1984 ; Hewitt and Amirtharajah ; 1984 ; Regan and Amirtharajah, 1984) by considering the action of air scouring dislodging deposits from sand grains prior to their being flushed away by the upflowing water . Amirtharajah concluded that water shear is the principal force acting on deposits on sand grain surfaces . In addition the passage of a sequence of air bubbles creates a series of pulses when the sand-deposit mixture collapses into the wake of a passing bubble . There is little doubt about the presence of shear forces at the sand grain surfaces; they account for all, or most, of the drag forces opposing settling of the grains during fluidisation . Also, in the apparently violent movements of the grains during washing, two grains approaching one another have to displace the water between them . The thinning water film flows radially from the diminishing separation creating a high laminar shear stress . Furthermore, the passage of an air bubble can create a thin water film between the bubble boundary and the gain surface . This again creates a high laminar shear stress . Consequently, the resistance of deposits, and deposit attachments, to shear stress seems to be a fundamental property governing the detachment process . This involves the variable (iii), given previously, concerning the nature and properties of the deposits . There is a widely held view that collisions between the grains during fluidisation are responsible for the dislodgement of deposits . The grains may not actually collide, but only approach extremely closely, due to the water film thinning resistance mentioned above . This is supported to some degree by the observation that experimental filter columns made from perspex (lucite, plexiglass) do not become eroded following extensive sand fluidisation inside . If collisions take place, the hard quartz sand would he expected to erode the relatively soft perspex . However, there is not an instant transition from a packed bed to a fluidised bed condition . The initial reversal of flow with washwater, if there is no prior air scour, detaches deposits by water shear . Then the sand grains commence intermittent movement whilst still in contact, resulting in rotations and other relative motion, which probably removes deposits by a grinding action . Further increases in flow rate intensifies these movements, until the grains are lifted by the water drag into a fluidised mode .

2 42 OBJECTIVES OF THIS RESEARCH

As the use of borescopes and video recording has proved to be useful in studying the mechanisms of deposition in sand filters (Ives and Clough, 1985), and for observing the movement of grains in continuous filtration (Fourie and Ives, 1982), there appears to be an opportunity to similarly observe filter washing mechanisms . Due to the high speed motion of the sand grains during fluidisation, standard TV cameras at 25 frames/s are inadequate and a high speed system operating at 200 or 400 frames/s has been developed . With this high speed facility observations have been made of sub-fluidisation behaviour of clean sand grains, followed by their progressive fluidisation behaviour in water only, giving initial information on their movement patterns, velocities and accelerations . This has been followed by similar experiments, after the sand collected kaolin clay, to determine the critical conditions and to use the detached particles as tracers for local fluid movement . The video recordings have shown that sufficient detail is available ; it has yet to be analysed . A future stage of the experiments will introduce air scour and again investigate both clean and clay-deposited sand behaviour during the passage of air bubbles . From these observations it is planned to consider the possible mechanisms of detachment and to determine their relative importance . This should lead to an appraisal of current practices, to assess their relative merits and possible improvements . EQUIPMENT AND TECHNIQUES OF VISUAL OBSERVATION

Filter system

The model filter apparatus consists of a sand bed contained within a perspex column of 0 .115 m diameter and 1 m height . At varying heights on the column are tapping points for manometers, sampling and visual observation by borescope insertion . The filter system is shown in Fig . 1. The constant head tank supplies a suspension to the filter by gravity and the filtered water emerges at the base of the column . For backwashing and air scour, water and air enter at the base of the column and flow upwards at controlled flow rates . The filter media were Leighton Buzzard sand of two size ranges : 0 .5-1 .0 mm and 1 .0-2 .0 mm, with nominal effective sizes (d, o, i.e. 10% finer by weight) of 0.6 and 1 .13 mm, respectively . The depth of sand could be varied up to a maximum of about 0.85 m, above which media losses would occur during the backwashing process, due to the sand expansion.



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Fig . 1 . Schematic diagram of the experimental filter system . Optical fibre system

A rigid optical fibre endoscope (borescope) was used for internal examination of the filter bed away from the walls of the model filter column . Both 5 and 8 mm borescopes were evaluated for observation of the sand grains but only the 8 mm borescope provides the necessary illumination for high speed video recording . This borescope views the sand bed, in the direction of its axis, through a plane BK7 glass window set in a brass sleeve of 11 mm O .D . This sleeve arrangement provides the necessary minimum focussing distance between the object and borescope . A series of brass sleeves are inserted at different depths within the sand bed, for observation of depth variations, and their bed penetration distance is adjustable . Some of the glass windows have a graticule, consisting of two perpendicular pairs of 1 mm separation parallel lines (25 )um thick chromium), etched on the face in contact with the sand grains . This graticule provides a means of calibrating the optical system .

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For high speed video recording, the light source is a 250 W metal halide lamp . Light from the lamp passes along a flexible, liquid light guide to the borescope where the light is transmitted to the borescope tip via an annular bundle of fibres . The illuminated filter bed is viewed through the eyepiece of the borescope which has adjustable focussing . For sand of size d, 0 =0.56 mm the borescope provides an area of view of approximately ten grains . Further details are given by Clough and Ives (1986) . High speed video system

The NAC HSV400 high speed video system is capable of recording moving objects, in colour, at rates of 200 or 400 pictures/s . The high sensitivity, high resolution, three tube colour camera has a builtin high speed shutter giving very short exposure times to freeze rapid motion . The shutter has four speeds giving 1/1000, 1/2500, 1/5000 and 1/10000 of a second exposure times . The video tape recorder displays scene code and time information for each recording and uses standard VHS cassettes . The recordings are to the NTSC standard but can be downloaded to PAL standard . Recordings can be played back in various modes including normal playback, at 0 .3 times the recording speed, slow motion, still playback (freeze frame), and search and jog (single frame advance) playback in both forward and reverse directions . For use with the borescope, a standard 50 mm SLR camera lens is attached to the video camera which is mounted on a tripod with the lens having a standoff of around 1 cm from the borescope eyepiece. This arrangement allows the camera to be moved to view different locations within the sand bed without mechanical interference or vibration of the bed itself . The experimental setup can be seen in Fig . 2. Magnification obtained by the combined borescope and video system is of the order of 100 times on the video monitor . This could be increased by using a larger monitor, if necessary. XY Coordinator, computer hardware and software The XY Coordinator includes a digitising tablet and pen used to move a cursor, displayed on the HSV400 screen along with its X and Y coordinates . This information together with scene code and time data from the VTR is output to a computer through a built-in interface (RS-232C or GP-IB ) . From X, Y and time data it is possible to calculate object size and motion using a computer . An IBM AT compatible personal computer has been programmed to receive data from the XY Coordinator and software written to obtain particle displace-

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Fig . 2 . High speed camera and video recording system mounted on experimental filter column .

ment, velocity and centre of area, from the two-dimensional view on the video monitor. The computer has also been programmed to provide keyboard control of frame advancement on the VTR and data output from the XY Coordinator, thus facilitating data analysis. EXPERIMENTS Procedure

Initial experiments have been conducted by backwashing a clean filter bed so that the behaviour of the sand grains under sub-fluidisation, progressive fluidisation and fully fluidised conditions could be analysed . Backwash water flow rates are gradually increased, and measured using a rotameter, while headloss over the entire bed and its expanded height are measured so that the degree of fluidisation can be ascertained . High speed video recordings of the sand grains have been made, using the borescope for different backwash water velocities so that their behaviour can be analysed and related to flow rate. For observation of detaching dirt particles, filter runs of about four hours with 100 mg 1 - ' suspension concentration have been performed and then the clogged bed was backwashed and recorded on video .

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Experimental difficulties

Obtaining enough illumination for recording using the high speed shutter remains a problem . For the 1/1000 s shutter speed it is necessary to use the 6 or 12 dB gain available on the video camera which results in a noisy recorded image. This shutter speed is required for freezing some of the more rapid grain movements that occur under fluidisation conditions . Interpretation and analysis of the video data is not straightforward due to the complex and three-dimensional nature of sand grain movement . Rotational and translational velocities and accelerations can only be measured in the two dimensions portrayed on the video screen . Results

Previous work (Ives and Clough, 1985) using conventional video recording equipment (25 frames/s) has shown that the act of flow reversal, as the backwashing cycle begins is sufficient to initiate scour of deposits from sand grains . The deposition of kaolin from a downward flowing suspension results in domes and layers on the upward facing surfaces, including ledges and crevices, on the sand grains . Occasionally deposits adhere to vertical surfaces ; in no cases are pores completely blocked, although many are severely restricted in the size of their openings . If the deposits have accumulated over an extensive period of filter operation (many hours) they may have already been subjected to detachment by the filtration flow . This is because constant volumetric flow rate leads to increases in interstitial (pore) velocities, caused by progressive reduction in the pore sizes by deposits . These increased velocities create higher laminar shear stresses, leading to a limiting stability of deposits . Arrival of freshly depositing particles causes detachment of existing deposit into the flow, sometimes rolling over the deposit surface, or even as an "avalanche" of deposit material . The accumulated deposits at this stage are approximately 100-200 ym in size . Consequently, the deposits are potentially unstable, even in the downflow direction . The reversal of flow for backwashing creates an initial shock movement, but not necessarily detachment, which can be seen on the video recordings . As the flow increases, the unstable deposits detach, apparently instantaneously, and are carried away upwards . Occasionally large deposits detached from below are caught in the field of view, but this is transitory, delaying their upward movement for less than a second . Further increases of upflow, but still at sub-fluidisation rates, detach further deposits, but a certain residual of deposits in crevices, or apparent dead spaces in pores persist while the grains remain immobile . Figures 3 and 4 show deposits of kaolin being scoured from sand grains (d, (,= 1.13 mm) after flow reversal at sub-fluidisation velocities .

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Fig. 3 . Sub-fluidisation detachment of kaolin-aluminium hydroxide deposits in successive video images, showing residual deposits in a nearly stagnant pore .

Fig. 4 . Cloud of kaolin deposits re-suspended after detachment during sub-fluidisation washing .

Beyond this stage, when the grains start to move, the conventional video recording speed cannot freeze the grain movement . Also, in most of the subfluidisation experiments the motion of the detached particles is too rapid for analysis. Consequently, the high speed video equipment was employed . The high speed video equipment, recording at 200 frames/s, is capable of freezing all but the most rapid movement of sand grains (Fig . 5) which could not be frozen by conventional (25 frames/s) video recording . Motion analysis is then possible using the XY Coordinator and computer to obtain position and time information. Observations so far indicate that sand grain motion under progressive and

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Fig. 5 . Successive video frames of fully fluidised sand. The lower white grain has moved approximately 0 .3 mm in 0.2 s from frame (a) to frame (d) . The grid is 1 mm' .

full fluidisation is random and erratic in nature . Circulation patterns are set up, which may be a result of borescope interference, where grains may be stationary for up to six seconds (when the bed is fluidised) or may move at high velocity in a random direction for similar time periods before changing direction. Grains also rotate at varying angular velocities and appear to collide with each other . DISCUSSION

Sub-fluidisation The sub-fluidisation experiments have shown that a considerable amount of deposit can be detached by fluid shear alone . Also they have shown that kaolin clay flocculated with aluminium hydroxide is more readily sheared than kaolin clay alone . The kaolin clay is probably destabilised by the calcium ions in the London tapwater (-80 m 1 - ' as Ca t ) which was used in the preparation of the suspensions to be filtered . Electrostatic forces are thought to be negligible



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in the adhesions of clay to sand, and clay to clay, particularly as both clay and sand have small negative surface potentials, and the electrical double-layer thickness in London tapwater is only 4 am . Therefore, the shear forces are probably breaking van der Waals' bonds, established on rough contact points on the clays and sand surfaces . The clay-hydroxide structures can be seen to he more open and the weak hydroxide links, with relatively large water spaces in the floc structure, are the most likely to be sheared . At present (August 1988) video information has not yet been analysed on the velocities adjacent to the deposits during sub-fluidisation shear . When these are known, velocity gradients and hence shear stresses can be estimated . Mean shear stress can be estimated from overall flow data as follows : Mean velocity gradient G= (P/µV)' 2 (s - ') Mean shear stress r=pG= (P,u/V)'" 2 (N m -2 ) Power/fluid volume P/V=,pgQH/eAL (W m -') From experimental data : p=10' kg m - '; Q/A=3.10 - ' mated) ; H/L=0.75 ; 2=10 -' N s m -` (20`C) From which r=0 .27 N m -2

e=0 .3 (esti-

These are, however, gross estimates with no real definition of stress close to the deposit boundary, where it is higher . An alternative estimate can be made from average interstitial flow velocity, and estimation of local pore dimensions and the assumption of a paraboloid velocity distribution (Poiseuille flow) : For a paraboloid velocity distribution u ; (max) = 2v i (mean) Mean interstitial velocity u ; (mean) =Q/Ae (m s - ' ) At the boundary, velocity gradient du;/dr is a maximum and v i =0 dv i/dr(max)=4v ;(max)/d(s - ')=4 .2Q/Aed (s -') Maximum shear stress r(max) = pdv i/dr (max) =8µQ/Aed (N m -2 ) From experimental data : Q/A=3 . 10-" m s - '; porosity e=0 .3 (estimated) ; pore "diameter" d=0 .20 mm ; absolute viscosity p=10" N s m' (20`C) From which r(max) =0 .40 N M -2 Also from the paraboloid velocity distribution r(mean)=2/3 •r (max)=0 .27 N M -2 Both estimates of mean shear stress, and the further estimate of maximum shear stress are based on assumptions which do not take into account local pore geometry . Therefore they only indicate order-of-magnitude values and require further refinement from observations of local velocity distributions . Another sub-fluidisation phenomenon was the residual deposits which were undisturbed by the upflow . These appeared to lie in sheltered areas of the pores presumably where the water was virtually stagnant with extremely small shear

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stresses . However, when the grains moved, even with small relative motion, these sheltered areas became exposed to the flow with consequent shear . Fluidisation As the upflow velocity was increased, the grains exhibited more violent motion, and the rotations and translations provided mild grinding action . From other testing and experience it is known that this action is not sufficient to wear away the hard surfaces of the quartz sand grains in the operational life of a water filter (e.g. 10 years) . The presence of water as a lubricant has some ameliorating effect, but also the water induced forces on a grain are less than or equal to the weight of the grain (less buoyancy) . Such a force is only about 0.2 mN for a 0 .6 mm sand grain in water . If, however, the grain is accelerated, then greater forces are involved . From accelerations and masses measured from the frame-advance replay on the high speed video, it is planned to estimate these forces . The rubbing together of grains, which are moving but not fully fluidised, is the basis of sand filter cleaning practised by one filter manufacturer . The operating requirement is that all the filter grains should be mobile, but not significantly expanded into a fully fluidised state . In full fluidisation, when all the grains are supported by upward fluid drag, the bed is in an overall steady state, but apparently random motions of grains or clusters of grains are observed inside the bed . These have intermittent periods of rest and vigorous motion, which have proved difficult to analyse so far . No experiments have yet employed air scour, so it is not possible to state whether the high speed video system is suitable to analyse bubble movements . It is probable that the successful removal of clay and clay-hydroxide deposits on the sand grains, with water only, represents a simple case and that the more practical "sticky" deposits involving organic or microbial material will require the extra detachment forces of air scour . This will form part of the future programme of this research . CONCLUSIONS

Video recording through endoscopes in pilot scale sand filters provides some information on the detachment of deposits during reverse flow washing to clean the sand grains . At normal video-framing speeds (25 frames/s), with 100X to 500X magnification on monitor screens, the sub-fluidisation washing with water only, when the grains are not moving, reveals shear induced detachment . Preliminary estimates of shear stress indicate mean values of about 0 .2 N in -', with boundary values of about 0.4 N m - `. Partial fluidisation where there is grain motion, and full fluidisation, require



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high speed video recording to freeze the vigorous grain movements for analysis of position, velocities and acceleration . So far (August 1988) high speed video-recording analysis has been confined to the movements of clean and clay-deposited sand grains backwashed with water only . NOMENCLATURE

• • • • • • • • • • • • • •

plan area of filter, mz pore "diameter", assumed cylindrical, m mean velocity gradient, s - ' head loss through sand bed, m length of sand bed in flow direction, m exponent of Richardson-Zaki equation power dissipated in fluid shear, W volumetric flow rate, m 3 s- ' superficial (approach) velocity, m s - ' interstitial velocity, m s - ' settling velocity of single grain, m s - ' volume of water in filter pores, m3 porosity of expanded (fluidised) bed porosity of sand bed -z dynamic viscosity of water, N s m density of water, kg m -3 -z shear stress, N m

ACKNOWLEDGEMENTS

The expert technical assistance of Mr I . Sturtevant is gratefully acknowledged, for the construction of the filter rig, the manufacture of the borescope insertion sleeves, and the operation of the closed circuit TV and video-recording systems. This research is supported by Science and Engineering Research Council grant No GR/E/10920 .

REFERENCES Amirtharajah, A ., 1984. Fundamentals and Theory of Air Scour . J . Environ . Eng . Div . Am . Soc . Civ . Eng., 110 : 573-590 . Amirtharajah, A ., Morrison, R .J . and Holnbeck, S .R., 1981 . In : N .M. Zeilig (Ed.), The Mechanics of Air Scour during Filter Backwash . 1981 Annual Conference Proceedings, Part 1, American Water Works Assoc ., American Water Works Association, Denver, pp . 209-239 . Cleasby, J .L ., Amirtharajah, A . and Baumann, E .R ., 1975 . Backwash of Granular Filters. In : K .J . Ives (Ed.), The Scientific Basis of Filtration . Noordhoff International, Leyden, pp . 255-272 .

252 Clough, G . and Ives . K .J ., 1986 . Deep Bed Filtration Mechanisms Observed with Fibre Optic Endoscopes and CCTV . 4th World Filtration Congress, Ostend, Belgium, April 1986 . Fourie, J . and Ives, K .J ., 1982 . Continuous Countercurrent Filtration . Proc . Symp . Water Filtration, KVIV, Antwerp, 1982, 3 .7-3.18 . Hewitt, S .R . and Amirtharajah, A ., 1984. Air Dynamics through Filter Media during Air Scour . J . Environ . Eng. Div . Am . Soc . Civ. Eng ., 110: 591-606 . Ives, K .J . and Clough, G ., 1985 . In : R. Drake (Ed.), Optical Fibre Investigations of Filtration Processes . 4th IAWPRC Workshop on Instrumentation and Control of Water and Wastewater Treatment and Transport Systems, Houston, TX, April 1985- Pergamon, Oxford, pp . 69-76 . Regan, M .M . and Amirtharajah, A ., 1984 . In : N .M . Zeilig (Ed .) . Optimization of Particle Detachment by Collapse-Pulsing during Air Scour . Proceedings of Conference of the American Water Works Assoc ., Dallas, TX, American Water Works Association, Denver (16 pp .) .

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DISCUSSION G.C. JEFFREY (BP Research International, Sunbury, United Kingdom) The endoscope allows the bed to be viewed in the vicinity of the probe . Is the probe likely to introduce effects similar to wall effects? K.J. IVES The effect of the probe is to create an atypical void below the cylindrical barrel. Viewing is beyond the tip and therefore, hopefully, beyond such disturbance. The "window" at the endoscope tip is 8 mm diameter, which is considerably less than the wall area . Therefore it is assumed that any wall effect is very much less, in such a restricted area. L.H . LITTLE (University of Western Australia) (1) Is the relative inactivity at the side due to an edge effect or is it a function of the position of the jets on the bottom? (2) What would happen if the pressure was cycled between the operating pressure and zero, instead of maintaining a steady flow? K.J . IVES (1) The bottom of the filter column, which supports the sand, is a metal gauze, without jets, distributing the washwater uniformly . There the lack of movement of particles on the sand surface, next to the wall, is attributed to a wall effect . (2) Pulsation washing would be very effective in cleaning the sand, due to water accelerations. This would introduce an extra technical requirement in practical systems . Air bubbling (scour) produces a similar effect in practice, by a pulsing collapse of the sand into the spaces behind the rising bubbles . J.E . TOBIASON (University of Massachusetts, Amherst, MA, USA What is the particle size resolution of the video apparatus? K.J. IVES On a large monitor screen it is possible to obtain about 500 X magnification, with a resolution of 15-20 µm . Smaller particles of kaolin, being plate-like, and rotating in the flow, are sometimes seen by scattered light .