Clay Minerals (1987) 22, 49-61

F I L T R A T I O N OF C L A Y S U S P E N S I O N S T H R O U G H S A N D

K. J. I V E S

Department of Civil and Municipal Engineering, University College, London WC1E 6BT

(Received 3 March 1986; rev&ed 27 October 1986)

ABSTRACT : The filtration of suspensions containing clay and other particles in water is a common process in drinking water treatment. Such filtration processes are very efficient, producing clear water containing less than 1 mg/1 from suspensions with particle concentrations of up to 100 mg/1. This filtration is not straining, but a process of collection of clay particles on the sand surfaces in the pores. The clays may range in size from sub-micron to ~ 20 #m, and may be flocculated, and are retained in pores ~200/am in size within sand grains ~500 pm in diameter. The collection process has three principal components (i) transport of clay particles across laminar water streamlines by diffusion, gravity and hydrodynamic forces, (ii) attachment by electrical or van der Waals' forces with hydrodynamic forces intervening, (iii) detachment by fluid shear or instabilities caused by arriving particles. Mathematical and physical models relate suspension concentration, quantity of deposit and permeability to depth in a filter, and time of operation. Fibre-optic endoscopes with CCTV enable video recordings to be made of the behaviour of clay particles in the filter pores, at magnifications up to 500 x.

The filtration of clay suspensions through sand may have the same scientific basis as the movement of clays through soils, although the objectives for their study are probably very different. In general, clay suspended in water, due to runoff from land and river flows, is a nuisance to the water purification engineer. His object is to remove the clay from the water with a minimum of cost, to provide drinking water of high clarity. Normally, technologies of sedimentation, flocculation and filtration are utilized, the latter being the final stage of clarification before water is chemically disinfected to provide hygienically safe, potable,

water. Although direct filtration of clays is described in this paper, the more common practice is

to flocculate the clay particles with aluminium sulphate or ferric chloride, which hydrolyse to aluminium and ferric hydroxides respectively. Consequently, the particles to be filtered comprise hydroxide flocs with attached clays, bacteria, algae, organic detritus and other matter suspended in the water. To avoid the complex issues which arise both theoretically and experimentally with such mixtures of natural particles, only the circumstances relating to purified clays suspended in London tapwater will be dealt with here.

Practical waterworks filters are up to 100 m 2 in plan, usually constructed in open concrete tanks, with an underdrain system supporting a depth of about 750 mm of quartz sand with 2- 3 m of water depth over it. The water percolates down through the saturated sand at a rate of

flow between 5 and 15 m3/m2/h. There is an initial head loss of a few cm water gauge due to the permeability of the clean sand. This, however, increases with time, typically over a period of about 24 h, to a limiting value of 2-2.5 m. The progressive clogging in the filter pores due to the accumulating deposits causes the head loss to rise linearly with time. Additionally, an

�9 1987 The Mineralogical Society

50 K. J. Ires

exponential rise of head loss can be caused by deposits accumulating on the inlet face of the sand bed. This has been observed to be a layer with holes in it, which persist in spite of a considerable thickness (several mm) of the surface layer.

During the filter operation the quality of the filtrate changes, first improving, then remaining constant and finally deteriorating quite dramatically to an unacceptable value.

The prediction of the head loss and filtrate quality with time is an objective of filter designers, as the former is constrained by hydraulic conditions and the latter by drinking water quality criteria.

Fortunately, pilot filters of only a fraction of a square metre in plan (about 0.02 m 2 is usual) operate without scale-effect, so experimental tests can be made to select the best sand size, depth, rate of filtration, and pretreatment of water to achieve optimum performance. This paper describes an experimental technique which uses a pilot filter, fitted with fibre-optic endoscopes to observe and record the internal workings of the filter pores.

Further details of the filtration of water are to be found in Ives (1975), Purchas (1977) and Svarovsky (1981).

F I L T R A T I O N C O N D I T I O N S

Clays

Natural clays comprises a range of mineral types, but for experimental purposes similar to those described here kaolinite is frequently used as a 'standard clay'. The source is normally laboratory suppliers, who in the UK derive their stock from English China Clays plc, in Cornwall. If the clay is used as delivered, the size range is from sub-micron to ~20/~m (Coulter Counter). For some experimental purposes the clay is fractionated by differential settlement to restrict the upper size to ~ 3 #m. In London tapwater the clay particles typically carry a surface (zeta) potential of about - 1 5 mV, in common with most other natural particles in water. Also, due to the high calcium content of London tapwater, the clays are flocculated by the Ca 2+. In some water treatment processes the clay may also be flocculated deliberately by the addition of aluminium or ferric salts, to precipitate hydroxides.

Water

Although hydrodynamically water is not a variable substance (at a given temperature), its dissolved chemical constituents affect the behaviour of clay particles and other surfaces. London tapwater contains approximately 350 mg/1 of dissolved salts, principally calcium bicarbonate, giving it a conductivity of about 600/~S/cm. These dissolved salts, in ionized form, suppress the range of the potential field (making a small so-called double-layer) to <0.1/~m from any surface with a (zeta) potential. Other natural waters containing less dissolved salts, for example from upland, rocky or peaty catchments, could have a thicker double-layer, but this is unlikely to exceed 1 #m. The magnitude of the zeta potential, as well as its range, is also diminished by dissolved salts. The pH of the London tapwater is slightly alkaline, ranging from ~7.6 to 8.2.

It should be borne in mind that water has a high relative permittivity (dielectric constant) of ~ 81 compared with air. Consequently, the long-range electrostatic fields experienced in air, such as the several cm of electrostatic precipitators, are not available in water.

The viscosity of water is strongly dependent on temperature; the viscosity increases

Filtration of clay suspensions 51

approximately by 50~ when water temperature drops from 20~ to 0~ a typical British summer to winter variation. Viscosity changes are important in filtration, whereas the relatively small density changes over the same temperature range have little effect.

Suspension

Clay suspension concentrations are dilute in filtration applications, only rarely exceeding 100 mg/1. River waters in flood in Britain rarely reach 1000 mg/1 (although more than 20000 rag/1 can occur in the tropics), but presettlement reduces the suspension concentrations to about 20 mg/1 or less before filtration. The object of drinking water filtration is to reduce these concentrations to 0.1 mg/1 or less. (The unaided eye can detect that 1 mg/1 is very slightly hazy; below that value the turbidity of water is measured by light-scattering photometric instruments.) Consequently, filters have to be highly efficient at removing suspension particles: in excess of 99-5~ efficiency.

Sand

The most usual filter material is sand, with Leighton Buzzard sand often quoted as the standard in the UK. Filter sands are uncemented, rounded quartz grains ranging in size from 0.25 mm to 2.5 mm depending on the particular application. They form packed beds of about 0.4 porosity, and mean pore openings range in size from ~ 100 to 1000 pm. Natural sand carries a surface (zeta) potential in London tapwater of about - 20 mV, although crushing the sand to reveal fresh quartz surfaces can increase the magnitude of the potential to about - 6 5 mV. This does not persist however during prolonged exposure to water and the value returns to - 20 mV.

For operational and technological reasons granular material other than sand is sometimes used: crushed anthracite is most usual. This has a different density, and is more angular, compared to sand. However, its other properties, including surface potential, are very similar.

Flow

The flow of suspensions through sand filters is fully saturated. The deposition of clay from suspension into the sand pores causes a progressive loss of permeability; consequently an increasing pressure difference is necessary to maintain a given flow rate. Flow is laminar, i.e. the flow follows Darcy's Law (flowrate is proportional to pressure difference), and the streamlines are ordered round the grain surfaces and in the pores. Flow is reversible, i.e. the same pattern of streamlines occurs whether flow is upwards, or downwards. There is a maximum fluid shear stress at the boundaries (grain surface) and a shear gradient exists across all pores.

C O L L E C T I O N OF P A R T I C L E S

Particles in suspension flowing through the filter pores are collected on surfaces of the sand grains, gradually filling part of the pore space. The collection, or deposition, of particles is regarded as comprising two stages : transport mechanisms which bring particles adjacent to sand surfaces, and attachment mechanisms securing particles to the surfaces of sand or

52 K. J. Ires

~. . . ~ :.:\~:.'./. ,Diatom 30 ~tm Grain aiarneter 500 p~.(.Asterionella).:.~_....,..__ - PVC microsphere 1.3 F L m - - ~ ~

�9 ':". 7:" 'Silica particle 20 i~m;" .: }:'".:i'~'"~ than . ,. ,,,. ~ ~ : g~i ' ioCfs: rafrat icCel; :r: ~sr~ess

~ ~i)~i, thickness of this line

FIG. 1. Small filter pore with typical particle to be filtered.

existing deposits. Some theoretical concepts combine these into so-called trajectory analyses. In addition, deposited particles may be detached and re-entrained in the flow.

Transport mechanisms

If particles followed the laminar streamlines through the filter pores, only those immediately adjacent (i.e. their centres on a streamline at a distance equal to their radius from the sand surface) would be available for collection. This is a very small fraction of the total suspension, particularly as the particles removed by filtration are much smaller than the pore openings (Fig. 1). It has already been stated that the clays have a size range from sub- micron to ~ 2 0 # m , and pore sizes range from 100 to 1000#m. This also implies that straining is an unlikely phenomenon.

Three forces dominate the transport mechanisms which move particles across the streamlines to arrive at grain surfaces for collection. These are gravity, Brownian motion and hydrodynamic.

Gravity Jbrees act on clay particles in water due to their density difference, and are characterized by their Stokes' settling velocity. This is a constant, in the direction of gravity, and is proportional to the density difference (particle and water), to the square of the particle size and to the inverse of the viscosity. At higher temperatures viscosity is less, particles settle faster and filtration is enhanced. Below about a few microns in size, particles are relatively little affected by gravity forces in filtration.

The gravity effect is often represented by the ratio of the Stokes' velocity vs to the advective velocity v of the suspension flow. Characteristically, the advective velocity is represented by the 'approach velocity' of filtration.

where Q A

= volumetric flowrate (m3/s) = plan area of filter (m:)

Q v = - ( 1 )

A

Filtration o f clay suspensions 53

This is an artifice, as the mean flow rate in the pores is higher, due to the restricted open area between the grains. This is referred to as the interstitial velocity vi.

V v~ - - - ( 2 )

g

where e = porosity. As the porosity is changing, in depth and in time, due to accumulating deposits, v~ is not

used, but the approach velocity v is, as it is more simply defined and is linearly related to vi. In addition v, is only a mean value: it varies from pore to pore due to the packed grain geometry; also the velocity varies across a pore due to the laminar flow velocity distribution.

Stokes' velocity vs is given by

g (P, - p) d 2 vs - ( 3 )

1 8 #

where g = acceleration 9.81 (m/s 2) Ps = density of particles (kg/m 3) p = density of water (kg/m 3) d = particle 'diameter ' (m) # = dynamic viscosity (kg/m/s)

Consequently, the gravity effect, or sedimentation number is given by

S = Vs _ g (Ps - P) d2A (4) v 1 8 # Q

In water filtration S has values lying between 0 (neutrally buoyant particles) and ~ 2. Clay particles ~ 5 #m diameter (Stokes) in water at 20~ filtered at 5 m/h have an S value ~ 1.6, so gravity has a significant effect.

Brownian motion affects small clay particles in water due to the random thermal motion of the water molecules which is directly dependent on the Kelvin temperature. The smaller the particles (< 1 #m) the more the random motion, and the greater is the probability of a particle reaching a grain surface as it is carried by the flow through the filter pores. The Brownian mechanism, often referred to as diffusion, is represented by the Peclet Number, which is the ratio of the advective transport of fluid motion to the random transport of the Brownian motion.

As in the case of the gravity effect, the advective velocity is defined as the approach velocity v= Q/A. All the same arguments apply, as given above, regarding the artificial nature of this characteristic velocity. Brownian motion is defined in terms of the Stokes- Einstein diffusion coefficient D.

k T D - (5)

3~r#d

where k = Boltzmann's constant (1.38 • 10 -z3 J/K) T = absolute temperature K

This diffusion coefficient has dimensions of velocity times distance, so dividing D by filter grain size (diameter) X gives the mean velocity imparted by Brownian motion over a distance of one grain diameter. This velocity can be expressed as a ratio with the advective velocity v.

Brownian velocity D - - - ( 6 )

advective velocity Xv

54 K. J. Ires

This ratio is the reciprocal of the Peclet Number, P, familiar in diffusion processes

1 D k T - - - - ( 7 )

P Xv nit dXv

In water filtration the range of values of lIP is from ~ 10 -8 to 0.5 x 10 -s. Experimentation with such small particles (sub-micron) is very difficult, but there is experimental evidence (Ives, 1975) that the Brownian effeCt is represented by l/P, and that smaller particles are collected more efficiently due to their greater Brownian motion. Sub-micron clay particles with a size of 0.1 itm, filtering through 0.5 mm sand, in water of temperature 20~ at a flow velocity of 5 m/h, have an inverse Peclet Number (l/P) of ~0.5 x 10 -s, just within an efficient filterable range.

Hydrodynamic forces have an effect in two ways. First, the laminar flow velocities are not uniform across a pore opening. The velocity at the centre is the largest, and the velocity diminishes towards the grain surface. Thus, there is a velocity gradient within a pore which causes particles to rotate due to the velocity on one side being greater than that on the other. Secondly, the clay particles are asymmetric and so experience out-of-balance forces which cause them to turn and twist as they move through the water. The combinett rotations, turns and twists cause lateral forces which deflect particles from the streamlines. Due to lack of knowledge of the detailed flow structure, and shapes of the particles, this motion is not predictable, and the particles appear to follow a swerving random path. This path may transport the particles to a grain surface for collection. Although the motion is not predictable it correlates with the Reynolds' Number for the particles, which is a representative of the viscous and inertial hydrodynamic forces on the particles. It is evident that the less spherical the particles, the greater the hydrodynamic effect, so it is significant for the plate-like particles of clay

Particle inertia (the tendency for the particles to continue in straight lines even if the flow lines curve) has been shown to be insignificant in water filtration due to the small mass, diminished density difference due to buoyancy and low velocities. It is, however, a major collection mechanism in air filtration. The transport mechanisms are combined with the velocities of flow to provide the trajectories of the particles through the pores, which, when they intersect grain boundaries, provide opportunities for collection of the particles. Such trajectory analysis has become very sophisticated, but is limited to idealized concepts of the pore geometry, the flow patterns and spherical grains and particles. Consequently, their application is very limited.

Attachment mechanisms

Particles which arrive at a grain surface with a separation of < 1 Itm may come under the influence of surface forces. This is very close approach, and bearing in mind the irregularities of sand grain surfaces and clay particle shapes, the geometry is never clear. Nevertheless, by turning to colloid chemical theories of spherical particle-flat plate interactions at close separation it is possible to see that four factors are likely to most affect particle attachment.

Electrostatic forces occur where particle and grain surfaces carry electrical charges. In water these are of like sign, but their magnitude and range is strongly affected by the ionic strength of the water. The potentials on the surfaces (measured by microelectrophoresis, and streaming potential) are normally less than - 2 0 mV expressed as electrokinetic (zeta) potentials. In the surface and groundwaters of Southeast England their range is of the order of

Filtration of clay suspensions 55

10 nm. Consequently, the electrostatic repulsion is overcome by the highly attractive forces of van der Waals.

Van der Waals' forces are due to the electronic nature of atoms and molecules and are universally attractive and powerful at very close range, consequently, they are the major attachment force.

Charge-carrying polymers (polyelectrolytes), some with molecular weights of several million, can provide bridges between particles and between particles and grains, by adsorbing on one surface and their loops and tails then adsorbing on the other surface. These polymers are still relatively small, much less than 1 #m, so close approach is necessary for their action. Other chemicals can also form bridges; an example is the Ca 2+ ion which can link some clay particles.

Hydrodynamic thinning offers a resistance to the attachment of clays to sand grains. As the clay particles approach the grain surface, impelled by transport mechanisms, or the close proximity of van der Waals' forces, the layer of water between has to be displaced, so it effectively has to flow radially out of the diminishing separation gap. This radial flow experiences viscous resistance and may slow down the approach of the particle so that it is swept past by the porewater flow.

These attachment concepts are indicative of the importance of the four factors, but any precise calculations are not yet possible for practical cases.

Detachment mechanisms

Although the subject of controversy for a long period, it has been observed that clay particles, particularly when flocculated, may be detached from deposits in the pores. If there is an increase in the water flow, the local fluid shear stress at the deposit surface in the pore is increased. If this exceeds the local shear strength of the deposits, some detachment will occur.

In addition, the arrival of particles on an unstable deposit on a sand grain can either detach a particle, or piece of deposit, or lead to an 'avalanche' of detached particles. This is only likely to occur when pores contain considerable amounts of deposited clay, and when the deposit structure consists of flocculated clay.

These detachment mechanisms return particles to the flow, so may be considered as 'anti- filtration'. The detached particles may, of course, be subsequently collected again when flowing through other pores.

The detachment mechanism is particularly important in water filtration, when clogged sand filters have to be cleaned for further operation. This cleaning is effected by reversing the flow upwards, at b_igh velocity, which detaches the deposits into a dirty washwater which is disposed of.

E X P E R I M E N T A L T E C H N I Q U E S

Mathematical modelling

In addition to mathematical statements concerning the physics and chemistry of the transport, attachment and detachment mechanisms, there are overall mathematical models of the filtration process. These relate the quantity of deposited particles in the filters pores to the clarification of the suspension as it flows through the pores. This is a mass balance equation. A kinetic equation describes the fraction of the particles retained in a filter layer

56 K. J. Ires

compared with the amount entering the layer. This is a measure of the filtration efficiency and is dependent on the previously described mechanisms. It is also dependent on the initial state of the filter, particularly size and shape of grains, and its subsequent modification by the deposits which alter the efficiency: first increasing it, and later diminishing it, ultimately to zero.

The mass balance equation states that the mass of particles removed from the flowing suspension equals the mass of particles retained as deposits in the pores.

~Cm 1 ~ % - ( 8 )

~?L v ~?t

where Cm = concentration of particles in the flowing suspension (kg/m 3) o" m = concentration of deposit in the filter pores (kg/m 3) L = distance into the filter from the inlet face (m) t = elapsed time of filtration (s) v = approach velocity of filtration (m/s)

In filtration, it is not the mass but the volume of particles occupying the pores which is important. It is this volume that changes the internal geometry and affects the permeability. Consequently, the balance equation is more usually written in terms of volumetric concentration C ( = Cm/ps) and volumetric deposit % ( = ffm/Ps) which are both dimensionless. In addition, a bulking factor/3 is introduced to allow for the fact that the deposits occupy a volume greater than their solid volume, due to self-porosity of the accumulated deposited particles, so that a = /7%, where a is called the specific deposit.

~C 1 ~?a - ( 9 )

~?L /3v ~3t

This is the form that the balance equation is given in most textbooks (Ives, 1975). The kinetic equation comprises two parts. The first is simply that the removal of particles

from the flow is first-order with respect to depth (i.e. there is an exponential decline in particle concentration with depth).

0C - - - = , t c ( 1 0 )

~L where 2 is the filter coefficient, being the fraction of particles removed per unit depth of the filter. However, the filter coefficient (or 'filter efficiency') varies from its initial state (20) when the sand grains are all clean, through to its final state when the filter is so clogged with deposit that it can retain no more particles (A = 0). Its variation is expressed in terms of internal geometry and flow conditions.

2 = 31. o (1 + ba/%) y (1 -~r/%) z (1 -a /a~) ~ (11)

where b is a geometric packing factor for the filter grains t 0 is initial porosity of the filter a, is the maximum (ultimate) specific deposit

The exponents x, y, z have to be determined empirically because exact definitions are not possible of pore geometry, detailed flow pattern and characteristics of suspended particles. This expression for the filter coefficient matches observed performances in that, judged by filtrate quality, filter efficiency first rises, then is almost constant, then deteriorates, ultimately to zero if the filter is operated for long enough with sufficient pressure head.

Filtration of clay suspensions 57

Consequently, the filtration clarification equations are

~C 1 ~a - ( 9 )

~L fly Ot

OC - 20 (1 + ba/%)y (1 _~/%)z (1 la/Cr,)x C (12)

c3L

The solution of these two equations gives the relationship between clarity of the suspension, and quantity of deposit, as they vary with distance through the filtering layers and with time of the progressive operation of the filter. To the engineer, these are potential design equations.

A third equation describes the changes in permeability of the sand, as it becomes progressively clogged with deposits, which also varies with distance and time.

This third equation is expressed in terms of the hydraulic gradient ?H/OL, where H is the head loss (m of water)

~ H dH

where (dH/dL)o -- the initial, clean sand, hydraulic gradient, which can be calculated from Darcy's Law, or Kozeny's equation

p = head loss coefficient Bearing in mind that ~ varies with distance into the filter (L) and elapsed time of the filter

operation (t) the integration of the hydraulic gradient equation is complex. An approximate solution, which matches practical observations of a linear rise in total

head loss with time is.

H = Ho +pvCot (14)

where /4o = the initial total head loss through the filter (m) Co = the concentration of the incoming suspension to be filtered (vol/vol)

The scientific bases of these various mathematical models is given in Ives (1975), and a more mathematical exposition in Rushton (1985).

Laboratory models

The majority of laboratory model filters are transparent acrylic (perspex, plexiglass, lucite) columns 100-150 mm diameter by 2 m high, typically containing a depth of 0-5-1.0 m of sand. Tanks, tubing and valve connections provide a controlled flow of water through the filter column, both for filtration and reverse-flow washing. Sampling and manometer connections attached to the column at various depth intervals enable concentrations of flow suspension, and sand permeability, to be monitored with respect to distance (depth) in the sand and operation time. Consequently, the mathematical models can be checked experimentally.

If they contain the same depth of sand, such models have been proved to act without scale effect relative to full scale filters which may have dimensions of up to 100 m 2 in plan. A simple criterion to avoid scale effects is to exceed the ratio of 100 : 1 for diameter of column to diameter of sand. Hence for sand of 1 mm grain size a column 100 mm diameter is sufficient (Fig. 2).

Because of the convenience of such model columns they are also frequently used on site as

58 K. J. Ires

FIG. 2. Experimental filter column, with two optical-fibre endoscopes, and CCTV video recording equipment.

pilot filters, although they often are made of opaque material (plastic pipe, for example), and lack the multiple sampling and manometric connections of laboratory models.

Observations of deposition

A key piece of information required in filtration studies is how much deposit is in each layer of the filter. This depends on, and affects, the efficiency of the filter in removing particles from suspensions, and causes the changes in permeability due to pore clogging.

By sampling the flowing suspension at different depth intervals the amount of deposits can be inferred from the balance statement: quantity of particles removed from suspension equals quantity of particles accumulating as deposit. This raises two problems: first, the sampling must be representative and not disturb the filtration; second, the volume of the pore occupied by the deposited particles is greater than their total solid volume as the deposits have a self-porosity, enhanced if the particles are flocculated. A number of experimental

Filtration of clay suspensions 59

solutions to these problems have been tried, including radioactive labelling of the particles, conductimetric in situ measurements with electrodes and electrolytes, and direct observations using scaled-up pore models, or two-dimensional geometric pore models. Since the scaling-up of the transport and attachment mechanisms involves unknown criteria, and two- dimensional models fail to represent the three-dimensional flow pattern as well as imposing severe boundary conditions, these latter approaches are very doubtful.

Fibre-optic probes

The use of fibre-optic endoscopes which penetrate the filter walls and several centimetres into the sand, has allowed direct observation and illumination of pores well away from the wall of the filter. Such rigid endoscopes, which are typically 6 or 8 mm in diameter, are available for direct (180 ~ viewing, and lateral viewing at 90 ~ and 45 ~ and are rotatable (Fig. 3). It is possible to record by camera a volume of the filter containing 5-10 grains and pores. With CCTV and videorecording a continuous record can be made over several hours of filtration so that transitory events and long-term changes in collection and deposit accumulation can be observed and preserved (Figs 4 and 5).

By recording at different depths, through pre-inserted ports in the filter wall, the pattern of deposition can be established and estimates of deposit volumes can be made from prior calibrations. Some observations of particle trajectories are possible, as up to 500 • magnification is possible on a large TV screen, but the passage of observable particles is

FIG. 3. Optical-fibre endoscope. The upper rigid endoscope is used in filtration experiments. The lower flexible endoscope is not optically suitable. The coiled tubes are the light guides

connected from light sources to the endoscopes.

60 K. J. Ires

FIG, 4. TV camera coupled to endoscope viewing inside the sand column.

FIG. 5. Typical view through endoscope; the sand grains are ~0.5 mm in size.

transient, passing across the endoscope field in about 1 s. Detachment events have been readily observed.

Fibre-optic endoscopy with video recording of the filtration of clay through sand is unique to University College London at the time of writing (January 1986) and is part of current research on water filtration. So far, observations have been made of: (i) 'wormholes' through the surface mats of clay deposition on the inlet face of the sand; (ii) the structure of flocculated clay deposits confirming the dominance of the gravity mechanism; (iii) trajectories confirming laminar flow; (iv) an absence of dendritic deposits; (v) detachment and rolling of particles over grain surfaces; (vi) preliminary aspects of reverse-flow scouring of the deposits from the grains.

Future research will continue to quantify these observations and extend to materials other than clay and sand, including microbiological colonization of grains which is important in some aspects of water filtration.

Filtration of clay suspensions 61

C O N C L U S I O N S

Clay suspensions flowing through sand can be clarified to yield water of drinking water standards with respect to turbidity. The retention of the clay particles on the sand grains can be described by the hydrodynamics of laminar flow through the pores, coupled with the physics of particle movement affected by gravity forces, Brownian motion and hydrodyna- mic forces. To these transport mechanisms can be added attachment/detachment mechanisms attributed to electrostatic and Van der Waals' forces, hydrodynamic viscous flow thinning, and shear of deposits.

Mathematical statements of these various mechanisms exist, and overall performance mathematical models allow calculation of the distribution of deposited clay particles through the sand, and the changes in the clay suspension concentration with elapsed time of filtration. The consequent clogging and loss of permeability is also described mathematically, leading to predictions of resistance to flow expressed in terms of pressure head loss.

Experimental tests of these physical, physico-chemical and mathematical concepts have been reported over several decades, but only recently has a reliable technique been developed for direct observation of the clay particles in the filter pores during the process of filtration. This is the use of fibre optic endoscopes which are inserted through the wall of a filter to observe pores within the sand filter, with magnifications of up to 500 times.

These observations have confirmed the significance of gravity forces, laminar flow patterns, the deposition geometry, and detachment mechanisms during the filtration process. Present observations are limited by the framing speed (~ 25 frames/s) of CCTV and video- recording. More refined observations should be possible using high-speed cine-photography (300-500 frames/s), which is planned as the next major experimental development.

ACKNOWLEDGMENTS

The current research on fibre-optic endoscopy is funded by the UK Science and Engineering Research Council. The expertise of Dr G. Clough and Mr I. Sturtevant is gratefuly acknowledged, as well as the pioneer experiments of Mr J. Fourie.

REFERENCES

IrES K.J. (Editor) (1975) The Scientific Basis of Filtration. Noordhoff International, Leyden. IVES K.J. & CLOUGrl G. (1985) Optical fibre investigations of filtration processes. Pp. 69-76 in: Instrumentation

and Control of Water and Wastewater Treatment and Transport Systems (R. A. R. Drake, editor). Pergamon, Oxford.

PURClaAS D. B. (Editor) (1977) Solid/Liquid Separation Equipment Scale-Up. Uplands Press, Croydon. RUSHTON A. (Editor) (1985) Mathematical Models and Design Methods in Solid Liquid Separation. Martinus

Nijhoff, Dordrecht. SVAROVSKY L. (Editor) (1981) Solid-Liquid Separation. Butterworths, London.