EVICE AND METHOD FOR FILTERING WATER, IN PARTICULAR
SURFACE WATER
The present invention relates to a water filtration device, in particular for surface water, comprising a floor above which are positioned a first layer of particles of a first media, of high density, and a second layer of particles of a second media, of lower density than that of the particles of the first media, the average diameter of the particles of the first media being lower than that of the particles of the second media, the device further including a raw water feed in the upper portion of the device above the second layer and a filtered water outlet below the first layer.
The clarification of surface water - removal of turbidity and particles - may be carried out by coagulation and granular media filtration, by the combination of a coagulation - settling or flotation -followed by a granular media filtration. If the quality of the raw water is good, the settling or flotation treatment may be omitted from the treatment plant, in which case the treatment is referred to as "direct filtration".
It is known that filters made up of multimedia layers are characterized by a higher matter retention capacity and a higher filtered water quality than a filter which comprises only a single media layer of equivalent height and particle size and is backwashed simply by fluidization.
The purpose of using multimedia filters comprising more than two different filter medias, typically three to five medias, is to approach an ideal filter in which the particle size distribution varies inversely along the filtration direction, the coarsest particles being
located on the surface of the media, in contact with the raw water, and the particle size progressively decreasing as refining of the filtered water proceeds.
Filters are mainly characterized by their cutoff threshold, this parameter being directly linked to the media particle size involved and to the media height. The choice of filter, and mainly its cutoff threshold, depends on the quality objective and the degree of particle retention during filtration.
When the objective is thorough disinfection, or for feeding clogging-sensitive processes, such as reverse osmosis membranes, surface water treatment requires the use of high-performance clarification processes. The bilayer filter, combined with optimized coagulation, fully meets the above objective. In this case, a media of small particle size will be chosen.
Such a choice goes counter to the suspended matter retention capacity of the filter and results in filtration cycle durations incompatible with industrial operation. In this case, the application of a filter employing several materials of different particle sizes is chosen.
The filter comprises a support, called a floor, on which the granular medias are positioned.
The floor may be of porous nature - a layer of gravel, pipes fitted with nozzles, a floor fitted with nozzles - the latter solution allowing backwashing fluids, air and water to be distributed therein.
The filter bed generally comprises:
- optionally, one or more layers of coarse
particle gravel so as to protect the porous floor
and/or the nozzles from direct contact with the filter
medias;
- a layer of a fine particle media, of high density, usually sand or garnet; and
- a layer of coarser particle media, of lower density, usually anthracite or pumice stone.
A filter of this type is generally backwashed in the following manner:
- after the filtration cycle has been stopped, the level of water above the filter bed is lowered, generally by filtration;
- an air mixing phase is applied, the air being introduced from beneath the floor and distributed over the entire section of the filter by means of the porous floor or the floor fitted with nozzles. This phase is the energetic part of the backwashing operation: the media particles undergo agitation - there is abrasion of the layer of particles/fIocs retained during filtration. During this phase, there is also mixing of the filter media layers; and
- a rinsing phase, using just water, which has the purpose of removing the particles separated beforehand from the water and retained in the filter, and of restratifying the various media so that the filter maintains its cutoff threshold and its retention capacity.
A number of backwashing variants are used, especially backwashing just with water, by simple media expansion: such backwashing has the main drawback of being ineffective for separating floe from media, which rapidly leads to the formation of what are called "mud balls" and to accumulation of matter in the filter to the detriment of the retention capacity of the media and the duration of the filtration cycle. The quality of the filtered water may be impaired by this matter accumulation and there may be premature collapse of the particles, or even the development of a biofilm, i.e. a
film composed of microorganisms that grow on the surface of the filter, owing to the low amount of energy employed and to the absence of abrasion during backwashing.
Another variant consists in applying a backwashing phase using air and water, which phase is interposed between the air-alone phase and the water-alone phase that were described above, thereby improving the effectiveness of mixing and transporting the matter retained during filtration. However, this phase takes place without overflow, so as to prevent loss of media, unless the dirty backwashing water recovery member is equipped with a specific device.
In all these examples, the operation of cleaning the filter (preferably with air) involves substantial mixing of the two media layers. After cleaning, the restratification of the two medias, i.e. reconstitution of the correctly superposed layers, must be able to be carried out by fluidizing the two layers.
The application of coagulation on the filter, or of filtration behind a first clarification step, by coagulation, flocculation, settling or flotation, has limits owing to the necessary compromise when combining two granular medias of different density and particle size, so as to allow the two medias to be restratified and thus meet the two objectives of suspended matter retention and water quality that are required.
According to the prior art, the particle size/density pair is generally chosen in such a way that the desired degree of mixing during the cleaning phase is equivalent for both medias and included within the range from 5 to 30%, typically between 10 and 20%. The degree of mixing is the proportion of the filtering mass in which the two medias are mixed.
The degree of expansion (i.e. the increase in bulk volume during the fluidization phase) of the two materials must be between 10 and 20% so as to ensure proper backwashing (removal of accumulated matter) and correct restratification.
To have a high suspended-matter retention capacity in the filter and consequently a reasonable backwashing frequency, it is necessary to have a large-diameter material in the upper layer so as to store the suspended matter without generating a substantial pressure drop.
To obtain filtered water of the highest possible quality, it is necessary to have a material of small particle size in the lower layer.
However, given the denseness of the materials generally available, especially sand and anthracite, and their density (2.6 and 1.4 kg/1 respectively), it proves to be impossible to have materials of sufficiently different diameters so as to achieve these two objectives at the same time: the sand/anthracite diameter ratio is generally 2 or 2.5. The 2.5 limit for the anthracite/sand diameter ratio is commonly recognized. In this regard, reference may be made to the document "Filter design and application", JAWWA, February 1969, pages 97-101 by W.R. Conley and K.Y. Hsiung.
This is a constraint, given the various granular medias used. The bilayer filter will therefore be either well suited to suspended matter retention, to the detriment of the cutoff threshold, or suited to particle retention, to the detriment of the filtration cycle time. This often leads to the formation of two bilayer filter stages.
The object of the invention is most particularly to combine granular materials which enable the two objectives to be achieved, namely filtered water quality and suspended matter retention, with a long cycle time.
According to the invention, a water filtration device, in particular for surface water, comprising a floor above which are positioned a first layer of a first media, comprising particles of average diameter DA, of high density, and a second layer of a second media, comprising particles of average diameter DB, of lower density than that of the particles of the first media, DB being greater than DA, the device further including a raw water feed in the upper portion of the device above the second layer and a filtered water outlet below the first layer, is characterized in that:
- the ratio of the average particle diameters DB/DA is such that interstices exist between the particles of the second media which have dimensions sufficient to allow, after cleaning, gravity flow of the particles of the first media through the second layer;
- the second media has a fluidization velocity greater than that of the first media; and
- injection means are provided for injecting a cleaning fluid into the floor, the fluid being injected with a velocity such that the second media expands by an amount of between 1% and 10%.
Preferably, the cleaning fluid injection velocity is such that the second media expands by an amount of between 2% and 10%.
During the stirring phase (using air), the degree of mixing is between 70% and close to 100%. According to the invention, during the rinsing or cleaning phase, the first layer of media A, a large portion of which rapidly reforms, will be entirely fluidized, whereas
the second layer B mixed with A will initiate the onset of decompaction corresponding to the onset of fluidization of B, accentuated by the very mixing which, by reducing the apparent porosity of the media (from more than 50% in the case of B down to 3 0% or 40% in the bottom layers of the B+A mixture) reduces the fluidization velocity. In this rinsing phase, there is therefore fluidization of a portion of rapidly reformed A and significant fluidization of the B+A mixture (5 to 15% expansion) in the B+A mixture zone. This fluidization of the B+A mixture allows partial restratification (increase in A and gradient of concentration of A in B decreasing from the bottom of the mixture zone upward) . At the end of the rinsing phase in which the particles of A are again on top of the filter bed, gravity stratification occurs in which the particles of A drop down under the action of gravity through the interstices of the upper layers of B. This therefore results in a kind of (A,B+A,B) trilayer.
Advantageously, the DB/DA ratio is greater than three. This ratio corresponds to a fluidization velocity of the second layer equal to or greater than twice the fluidization velocity of the first layer. Preferably, the DB/DA ratio is between three and six.
The particles of the first media A may have an average diameter of less than or equal to 0.4 mm and a density greater than 2.2 g/cm3.
Advantageously, the particles of the second media B have an average diameter equal to or greater than 1.5 mm and a density of less than 1.6 g/cm3.
The ratio of the density of the particles of the first media A to the density of the particles of the second media B is preferably less than 2.
According to a preferred embodiment, the particles of the first media A have an average diameter of 0.3 mm and a density of 2.5 g/cm3 and the particles of the second media B have an average diameter of 1.5 mm and a density of 1.5 g/cm3, i.e. a diameter ratio of 5 and a density ratio of 1.66. During the backwashing operation, the material of the first media A, or lower material, expands by 10 to 40% and the material of the second media B, or upper material, expands by 2 to 10%, in such a way that there is interpenetration of the lower material in the upper material, followed by partial gravity restratification and thus creation of a trilayer filter consisting of the lower material A, the upper material B and an intermediate mixture zone. This is made possible by the fact that the upper material expands, otherwise there is insufficient interpenetration.
The invention also relates to a process for cleaning such a device.
According to the invention, a water filtration process, in particular for surface water, employs a filtration device comprising a floor above which are positioned a first layer of a first media A, comprising particles of average diameter DA, of high density, and a second layer of a second media B, comprising particles of average diameter DB, of lower density than that of the particles of the first media, DB being greater than DA, a raw water feed being provided above the second layer and a filtered water outlet below the first layer, is characterized in that:
- the ratio of the average particle diameters DB/DA is such that interstices exist between the particles of the second media B which interstices have diameters that are sufficient to allow, on the one hand, during the rinsing phase, a mixture B+A of very low porosity that will undergo an expansion which is greater than
that of B alone and is sufficient to ensure effective rinsing and acceptable restratification and, on the other hand, after rinsing, a gravity flow of the particles of the first media A through the second layer B, making it possible to obtain an upper layer of B devoid of A;
- the second media has a fluidization velocity higher than that of the first media; and
- a cleaning fluid is injected through the floor with a velocity such that the second media (B) expands by an amount of between 1% and 10%.
Advantageously, the fluid is injected with a velocity equal to or greater than the fluidization velocity of the second media B, which ensures fluidization and minimal restratification of the B+A mixture zone and removal of the filtered matter at the top of the filter bed, where B is very predominant. The injected fluid may be air. The fluid may be injected over a period of less than 30 seconds.
The rinsing fluid is filtered water injected for a time of generally less than 10 minutes.
Other features and advantages of the invention will become apparent in the following description of an embodiment with reference to the appended drawings, although this is in no way limiting. In these drawings:
• figure 1 is a sectional view of a bilayer filter according to the invention;
• figure 2 is an enlarged detail of figure 1;
• figure 3 is a detail of figure 1 on a larger scale than figure 2; and
• figure 4 is a graph showing:
- firstly, a curve CM of the variation in the
percentage volume mixing of the two medias during the
cleaning operation, with the mixing percentage plotted
on the Y-axis and the rinsing water velocity plotted on
the X-axis. Initially, after the air stirring phase, the mixing CM is generally between 70% and close to 100%. On the graph, this is the point before the rinsing/restratification phase, therefore at velocity = 0; and
- secondly, two expansion curves CA, CB corresponding to the two medias A and B, with the percentage bulk volume expansion plotted on the Y-axis and the velocity of the backwashing or rinsing water plotted on the X-axis.
To be able to obtain water of very high quality and long cycle times, the invention combines a first layer of particles of a first natural media of small particle size and high density with a second layer of particles of a second natural media of larger particle size and lower density, the fluidization velocities of the two medias not being similar, for a degree of expansion of each media ranging from 5 to 3 0%, typically from 10 to 20%.
To do this, two materials A and B of different densities, with average particle diameters DA, DB such that DB/DA ≥ 3, are used. Typically, DB/DA is between 3 and 6. The fluidization and sedimentation velocities of A are lower than those of B.
In the B+A zone, the porosity decreases (in the case of anthracite, the porosity or void zone between particles is between 50 and 55%, while in the mixture zone it falls below 40%) and therefore, according to the Kozeny-Carman law, the minimum fluidization velocity decreases or else, for a given fluidization velocity, the expansion rate increases.
The velocities in this mixture zone may also be referred to as "supervelocities", which supplement the cleaning and displacement of the filtered matter of the
filter.
The necessary civil engineering structures do not differ appreciably from those of the prior art. The structure of the filter of the invention is illustrated in figure 1. A tank 1 having in its lower portion a floor 2 provided with orifices 3 and injection means I is used.
Placed on the floor 2 is a gravel layer 4 of coarse particle size so as to protect the floor and the nozzles from direct contact with the filter medias.
A first layer 5 of a media A of average diameter DA is placed on top of the layer 4. A second layer 6 of a media B of average diameter DB greater than DA is placed on top of the layer 5. The arrangement of the layers is more particularly visible in the detail illustrated in figure 2.
A raw water feed 7 is provided at the top above the second layer 6. A treated water outlet 8 is provided at the bottom, beneath the floor 2 and the first layer 5.
A waste/floc collection trough 9 is provided above the layer 6 .
The height of the trough 9, relative to the layer 6 at rest, is calculated so as to allow the waste to be removed without loss of materials A and B.
During the cleaning/rinsing, provided by a stream of fluid, generally backwashing water, upwardly injected by the injection means I, the velocity of the injected stream is chosen to be close to (i.e. equal to or slightly greater than) the fluidization velocity of the second layer B.
Based on knowledge of the prior art, a person skilled in the art would not accept such conditions, as they ought to result in the materials of the layers mixing or at least in non-restratification and destructuring of the filter after backwashing, with the layer of fine media A passing above the media B. The objective would therefore not be achieved, since the capacity of the filter would then become very low.
The actual result is surprising and contradicts this primary analysis owing to the operating conditions, but especially owing to the substantial difference between the diameters of the materials. The process is explained below.
The backwashing rate is defined for an A/B pair. Curves CA and CB shown in figure 4 illustrate the percentage expansion of the medias A and B as a function of the backwashing water injection velocity.
Curve CM represent the percentage volume of the zone in which A and B are mixed during the backwashing operation, as a function of the backwashing water injection velocity. The % CM at velocity = 0 corresponds to the state of mixing at the end of the air stirring phase (between 70 and close to 100%). During the backwashing operation, the restratification takes place to a greater or lesser extent depending on the backwashing water velocity. A backwashing velocity is chosen that lies within the zone flanked by the dotted lines, corresponding to the minimum of CM, which also corresponds to the initiation of CB, i.e. the start of fluidization of the second media B, whereas the first media A is already fluidized.
The still relatively high minimum value of CM, about 5 0% (between 30 and 60% depending on the case) , could lead to such an A/B pair not being used.
However, thanks to the gravitational descent of the particles of media A through the second layer B after backwashing, a layer of media B free of A forms at the top of the filter.
Restratification is therefore possible according to the invention, with restructuring of the filter as three layers, namely A, B+A mixture, and B.
For the cleaning operation, nozzles are used to inject filtered water through the floor 2 at a velocity equal to or greater than the fluidization velocity of the media B.
The material A, partially and very quickly reformed, is fluidized first and undergoes an expansion of 50 to 80%.
The B+A mixture zone expands to an amount between the expansion of A and the expansion of B, generally between 5 and 15%. This expansion proves to be sufficient for backwashing and partial restratification.
The material B accumulates on the surface and starts to be fluidized. Its degree of expansion is quite low (from 1% to 10%) , but this is sufficient to ensure decompaction, enabling the matter retained during filtration to be removed. Toward the end of backwashing, a layer of material A may be found that covers the entire surface of the bed.
Complete removal of A from the upper layer of B is achieved gravitationally, upon stopping the backwashing operation.
The two materials A and B have different densities and
particle diameters preferably such that DB/DA > 4. The fluidization and sedimentation velocities of A are lower than those of B.
It might therefore be expected that restratification would be impossible.
Experience has shown that this is not the case, contrary to what a person skilled in the art might have supposed.
During backwashing, a layer of material A rapidly appears, forming what is called a "pseudoliquid" of quite high density with the material A, which tends to make lighter elements or particles of media B present at the interface "float" (T.R. Camp model discussed by J.L. Cleasby in "Intermixing of dual media and multimedia granular filters", JAWWA, April 1975, pp 195-203).
The B+A mixture created by the stirring has a very low overall porosity because of the high DB/DA ratio, which enables the material A to be lodged in the large interstices of the material B. Owing to its low porosity, the fluidization velocity of the mixture is much lower. Consequently, the expansion of the B+A mixture is higher (by a few %) than that of B at the same backwashing speed. This expansion is sufficient for proper rinsing to be carried out. Substantial restratification is observed in the upper layers and partial restratification in the bottom zones of the mixture. This unexpected phenomenon may be explained by the pseudoliquid model (see above) of high density formed by A, which, at the interface, makes the elements of media B float and absorbs the elements of media A. Too high a fluidization velocity results, as indicated by the curve in figure 4, in the medias being remixed by giving preference to the fluidization and
entrainment phenomena to the detriment of the pseudoliquid phenomenon.
When the backwashing is stopped, the bed descends, and when material B has finished settling, the particles are in contact with one another. The particles of A that are still at the top and in the upper layer of the material B descend under gravity into the interstices lying between the particles of B until they are refilled. The average equivalent diameter of the interstices is from two to six times larger than that of the particles of A. It is therefore a gravitational arrangement in the stoppage phase that makes it possible to obtain an upper layer B devoid of elements of media A.
This therefore clearly results in a kind of trilayer, shown more particularly clearly in figure 2.
If the initial filter is a bilayer filter, the first layer A of which represents 45% of the total height and the second layer B of which represents 55% of the total height, the following arrangement may be found after backwashing:
- a lower layer 5 of material A representing 3 0 to 40% of the total height;
- an upper layer 6 of material B representing 10 to 3 0% of the total height; and
- an intermediate zone 10 of an A/B mixture, representing 30 to 60% of the total height, with an A concentration gradient in B, A being highly concentrated at the bottom and weakly concentrated at the top.
Example 1
As a first embodiment, the media A consisted of sand, the particles of which had an average diameter
(effective size) of 0.3 mm, with a minimum fluidization velocity of 5 m/h. The second media B consisted of anthracite, the particles of which had an average diameter (effective size) of 1.5 mm, with a minimum fluidization velocity of 35 m/h. The velocity of the rinsing water to be applied was 35 m/h at 2 0°C.
Example 2
AS a second embodiment, the media A consisted of sand with an effective size of 0.3 mm and a minimum fluidization velocity of 5 m/h. The second media B consisted of pumice stone with an effective size of 1.7 mm and a minimum fluidization velocity of 30 m/h. The velocity of the rinsing water to be applied was 30 m/h at 20°C.
In production, the results were quite astonishing.
Example 3
To illustrate the particles obtained by the invention in terms of filtration effectiveness and productivity, a comparative study was carried out with direct filtration of seawater, after coagulation with an iron salt. The table below summarizes the results of this study.

(Table Removed)
ES = effective size.
Filter 1 provided with a 1.5 anthracite media and a 0.75 sand media had a long filtration cycle time, but the quality of the filtered water was degraded and incompatible with reverse osmosis membrane feedwater.
Filter 3 provided with a media of smaller particle size had improved water quality, but the filtration cycle time was too short and incompatible with industrial operation.
Only filter 2, with materials selected according to the invention, achieved the quality objectives while maintaining long filtration cycle time, generally greater than 24 h in conventional applications.
The effectiveness of the backwashing was demonstrated by monitoring, over several filtration cycles, the pressure drop across the filter media after backwashing. The pressure drop after backwashing remained steady over the course of time, over several cycles, thereby confirming the effectiveness of the media backwashing.
The quality of the filtered water was compatible with the high requirement levels of desalination processes and accomplished in a single filtration state.
This invention has many advantages. In particular, it provides a high level of suspended matter retention, ensuring long filtration cycle times, and a reduction in backwashing water loss and energy consumption.
The invention also permits very low cutoff thresholds, providing excellent removal of floe and particles,
again with a single filtration stage. This contrasts with the current practices in which such effectiveness is generally achieved by employing two filtration stages, the first retaining the maximum amount of suspended matter.
The invention also allows effective backwashing with water velocities lower than those generally employed for expanding all the materials by 10 to 3 0%. This results in smaller backwashing equipment and therefore a lower investment cost.
In particular, the invention makes it possible to provide a reverse osmosis membrane feedwater with a single bilayer filter stage. At least two bilayer filter stages according to the prior art would normally be necessary for directly feeding reverse osmosis membranes. Increasing the number of bilayer filter stages or devices having the same function increases both the investment cost and the footprint of the plant.

CLAIMS
1. A water filtration device, in particular for
surface water, comprising a floor (2) above which are
positioned a first layer (5) of a first media (A) ,
comprising particles of average diameter DA, of high
density, and a second layer (6) of a second media (B) ,
comprising particles of average diameter DB, of lower
density than that of the particles of the first media
(A) , DB being greater than DA, the device further including a raw water feed (7) in the upper portion of the device above the second layer (6) and a filtered water outlet (8) below the first layer (5), characterized in that:
- the ratio of the average particle diameters DB/DA is such that interstices exist between the particles of the second media (B) which have dimensions sufficient to allow, after cleaning, gravity flow of the particles of the first media (A) through the second layer;
- the second media (B) has a fluidization velocity greater than that of the first media (A); and
- injection means (I) are provided for injecting a cleaning fluid into the floor (2), the fluid being injected with a velocity such that the second media (B) expands by an amount of between 1% and 10%.

2. The water filtration device as claimed in claim 1, characterized in that the DB/DA ratio is greater than three.
3. The water filtration device as claimed in claim 2, characterized in that the DB/DA ratio is between three and six.
4. The water filtration device as claimed in any one of the preceding claims, characterized in that the particles of the first media A have an average diameter of less than 0.4 mm and a density greater than
2 . 2 g/cm3 .
5. The water filtration device as claimed in any one of the preceding claims, characterized in that the particles of the second media B have an average diameter equal to or greater than 1.5 mm and a density of less than 1.6 g/cm3 .
6. The water filtration device as claimed in any one of the preceding claims, characterized in that the ratio of the density of the particles of the first media A to the density of the particles of the second media B is less than 2.
7. The water filtration device as claimed in claim 6, characterized in that the particles of the first media A have an average diameter of 0.3 mm and a density of 2.5 g/cm3 and the particles of the second media B have an average diameter of 1.5 mm and a density of 1.5 g/cm3.
8. The water filtration device as claimed in any one of the preceding claims, characterized in that the fluidization velocity of the second layer is equal to or greater than twice the fluidization velocity of the first layer.
9. A water filtration process, in particular for surface water, employing a filtration device comprising a floor above which are positioned a first layer of a first media A, comprising particles of average diameter DA, of high density, and a second layer of a second media B, comprising particles of average diameter DB, of lower density than that of the particles of the first media, DB being greater than DA, a raw water feed being provided above the second layer and a filtered water outlet below the first layer, characterized in that:
- the ratio of the average particle diameters DB/DA
is such that interstices exist between the particles of the second media B which have dimensions sufficient to allow, on the one hand, during the rinsing phase, a mixture B+A of very low porosity that will undergo an expansion which is greater than that of B alone and is sufficient to ensure effective rinsing and acceptable restratification and, on the other hand, after rinsing, a gravity flow of the particles of the first media A through the second layer B, making it possible to obtain an upper layer of B devoid of A;
- the second media has a fluidization velocity higher than that of the first media; and
-. a cleaning fluid is injected through the floor with a velocity such that the second media (B) expands by an amount of between 1% and 10%.
10. The process as claimed in claim 9, characterized in that the fluid is injected with a velocity equal to the fluidization velocity of the second media (B).
11. The process as claimed in claim 9 or 10, characterized in that the injected fluid is water.
12. The process as claimed in claim 9 or 10,
characterized in that the injected fluid is air.
13. The process as claimed in any one of claims 9 to
12, characterized in that the fluid is injected over a
period of less than 30 seconds.