Abstract:
The invention concerns a plant comprising, on a vertical axis, a homogenizing vessel ( 1 ) followed by a settling vessel ( 2 ) and a flocculating vat ( 3 ). The homogenizing vessel ( 1 ) is fed by tangential intake ( 92 ) of the effluent and the input ( 611 ) and the output ( 612 ) of the upper tube ( 61 ) comprise a helical guide ( 613, 614 ) for driving the effluent in rotation. The lower tube ( 62 ) of larger diameter than that of the upper tube ( 61 ) is terminated by an outlet in the form of an upside-down cone ( 622 ) with reduced cross-section ( 621 ). The lower portion ( 31 ) of the flocculation vat ( 3 ) is truncated, with an apex angle, smaller than that of the upper portion ( 32 ), said lower portion ( 31 ) being parallel to the cone of the outlet of the lower tube ( 62 ), and anti-backflow cones ( 7, 8 ) surround the conduit ( 6 ) in the settling vessel ( 2 ).

Description:
FIELD OF THE INVENTION  
   The present invention concerns an effluent purifying plant with centralized structure comprising, on a vertical axis,
         a homogenizing vessel receiving the effluent with the addition of a coagulating or flocculating agent,   a settling vessel beneath the homogenizing vessel,   a flocculating vat comprising a frustoconical upper portion extended by a lower portion, which vat is placed inside the settling vessel and in which sludge flakes form,   a pipe arranged in the axis, leading downwards, connecting the homogenizing vessel to the lower part of the flocculating vat through which the effluent passes from bottom to top and is clarified in contact with the sludge flakes,   the pipe comprising an upper tube, one end of which opens into the homogenizing vessel and the other end of which opens into a lower tube of larger diameter than that of the upper tube, the lower end thereof opening into the lower part of the flocculating vat,   the lower portion of the settling vessel having a frustoconical portion forming the sludge vat which receives the sludge overflowing from the flocculating vat,   an annular vessel surrounding the settling vessel and receiving the filtered water by overflow from the settling vessel.       

   A plant of this type is already known from the document FR 00 07 42 (FR 2 810 310). 
   Aim of the Invention 
   The aim of the present invention is to develop an effluent purifying plant of the type defined above permitting improved management of the dynamic flow by improved distribution and utilisation of the injection of the coagulating agent, acceleration of the coagulation phenomenon by avoiding premature flocculation in the mixing vat, or preventing the bed of sludge from interfering with flocculation and avoiding disturbing flocculation by the rise of the clarified liquid while also permitting bacteriological treatment of the clarified liquid, with very compact means occupying the minimum ground surface area. 
   Statement and Adavantages of the Invention 
   To this end, the invention concerns an effluent purifying plant with centralized structure of the type defined above, characterized in that
         the homogenizing vessel has a tangential intake for the effluent,   the inlet and the outlet of the upper tube include a helical guide for setting the effluent in rotation,   the lower tube of larger diameter than that of the upper tube is terminated by an outlet in the form of an upside-down cone with reduced cross-section,   the lower portion of the flocculating vat is frustoconical with an apex angle smaller than that of its upper portion, the lower portion being parallel to the cone of the outlet of the lower tube, and   at least one anti-backflow cone surrounds the pipe in the settling vessel.       

   In general terms, the plant according to the invention permits excellent management of the flows, no matter what the nature of the effluents, thereby considerably simplifying the setting-up of the plant and its use while reducing the operating costs and the consumption of coagulating and flocculating agents. 
   Owing to the distribution of the effluent at the point of injection of the coagulating and/or flocculating agent, especially by injecting the flocculent upstream of a static mixer and owing to a tangential intake in the homogenizing vessel, the flocculating agent can be distributed more efficiently and therefore the quantities consumed reduced. The means of the invention make it possible to maintain the liquid in rotation on itself by the vortex effect, thereby avoiding any untimely deposit of flakes capable of clogging parts of the plant. The sludge flakes are deposited in the sludge vat without interfering with the rise of the liquid or disturbing the flocculation downstream. 
   The bottom of the flocculating vat facilitates the circulation and rise of the effluent charged with flocculating agents so that the sludge flakes develop, rise towards the upper portion of the flocculating vat and then pass into the sludge vat either by overflow or through openings provided for this purpose. 
   The anti-backflow cone or cones also facilitate the flocculation and release of the sludge flakes towards the sludge vat, preventing the sludge flakes from collecting together in the centre of the plant and gradually clogging the latter by spreading outwards. 
   According to another advantageous feature, a second anti-backflow cone is arranged above the flocculating vat and this second cone is adjustable. The adjustment makes it possible to adapt the effect of the anti-backflow cone to the physical/chemical characteristics of the effluent. 
   The anti-backflow cone or cones also facilitate the preferential circulation of the water first of all charged with sludge flakes and then of the clarified water, in the part above the area of formation and retention of the sludge flakes. The clarified water is then forcibly distributed at the surface of the filter-mass in order to avoid any formation of a preferential passage through the filter-mass. 
   According to another advantageous feature,
         the outlet of the settling vessel has stabilising baffles for the liquid and peripheral distribution of outlet openings,   the annular vessel surrounding the settling vessel contains a filter-mass in the upper portion, its lower portion constituting the storage volume for the filtered water.       

   Owing to the baffles, the regulation of the flow rate and rendering the distribution of the flows towards the filter-mass uniform make it possible to eliminate at least 95% of the residual matter in suspension. 
   According to another advantageous feature, the taper of the upper portion of the flocculating vat is greater than that of the its lower portion. 
   This double frustoconical shape prevents the accumulation of sludge in the angles of the lower truncated cone and the liquid accelerates along the outer wall. Then, the more flared frustoconical shape directs the flow outwards, winding round the area of stagnant liquid around the pipe and beneath the anti-backflow cone. 
   According to another advantageous feature, the lower tube is in the shape of two truncated cones assembled by their major base, the inlet cross-section corresponding to that of the upper tube to continue by flaring out, and an injection means injects a flocculating agent in the middle of this volume. 
   This embodiment is particularly advantageous in the case of weakly charged effluents, such as for example river water to be transformed into potable water. 
   The double cone with a wide central portion at which the flocculating agent is injected makes a thorough mixing of the flocculating agent and the effluent possible so that flocculation takes place within this limited enclosure in which the speed of passage of the effluent is reduced, the effluent being always in rotation about the axis of the plant. 
   Flocculation is substantially terminated at the outlet of the double cone. Then, during the rise of the flow in the vessel, the flakes collect together in order to be deposited in the sludge vat. 
   According to another advantageous feature, a means for metering the reagent or reagents is provided upstream of the homogenising vessel and this metering means is followed by a static mixer. 
   According to another advantageous feature, the annular vessel is composed of a lower portion surrounding the lower portion of the settling vessel, and an upper portion containing the filter-mass installed above and in prolongation of the settling vessel and having the same outer wall, these two portions being connected by a pipe, an annular channel surrounding the pipe to connect the upper volume of the settling vessel to irrigation ramps opening out above the filter-mass. 
   This solution is particularly advantageous because of the compactness of the plant and the volume available for collecting the treated water. 
   The cleansed liquid which rises in the annular channel circulates at an extremely reduced speed. In addition, a cone placed at the inlet of the annular channel prevents any sludge flakes which might float at the upper wall of the flocculating vat from being likely to pass into the annular channel. Thus, this cone has not only a separating and retaining effect, but also a guiding effect, creating a very slow circulation like a damming means, so that the flakes which may be floating in the upper portion of this volume are redirected downwards and gradually descend into the sludge vat while adhering to any other possible sludge flakes. 
   In the case of this improvement, advantageously, a collecting chamber is provided which separates the upper portion from the upper volume of the settling vessel, the collecting chamber being connected to the lower portion by a pipe equipped with a valve. 
   It is also of advantage to treat the effluent by means of a pre-treatment before introducing it into the actual plant described above, in order to avoid causing excessive stress on the plant. 
   To this end, the plant includes an annular pre-treatment vessel, surrounding the outer vessel, and composed of a lower volume in which the effluent arrives with the addition of a coagulating and/or flocculating agent in order to deposit the sludge flakes there, and of an upper volume in which the partially clarified liquid rises in order to be transferred into the distribution vessel. 
   In particular, the annular pre-treatment vessel comprises a flocculating vat formed of an annular plate provided with frustoconical cavities bordered by passage openings, a pipe coming from an effluent distribution chamber opening into each cavity, and the lower volume is bordered by a bottom in the shape of an annular collar constituting the sludge storage vat. 
   According to another feature, the lower volume of the vessel is separated from its upper volume by a partition provided with openings, the inlet of which is delimited by divergent cones, these openings being offset angularly in relation to the angular position of the flocculation cavities so as to occupy an intermediate position in each case. 
   This pre-treatment with decantation makes it possible not only to eliminate a considerable part of the charge of the effluent, but above all such elimination facilitates the treatment of the effluent by the central portion of the plant already mentioned above, since the effluents arriving in this central portion will thus all have substantially related characteristics. 
   The plant also permits bacteriological treatment of the liquid at the outlet of the decanting and filtering portion, and for this purpose the plant includes a complementary bacteriological treatment portion composed of a series of annular chambers surrounding the outer vessel and receiving the cleansed liquid, which passes through these chambers connected in a cascade by communication pipes, being provided with helical guides for setting in rotation the stream of liquid and suspended particles constituting the charge of each chamber, the communication pipes being U-shaped tubes, the inlet arm of which faces downstream in the upstream chamber in order to open out through an outlet facing downstream in the following annular chamber, in the direction of flow of the liquid in these chambers. 
   The effectiveness of the bacteriological treatment plant permits a significant reduction in the materials used, and consequently a reduction in the operating costs of the plant. 

   
     DRAWINGS 
     The present invention will be described below in more detail by means of exemplary embodiments shown diagrammatically in the appended drawings, in which: 
       FIG. 1  is a view in axial section of a first embodiment of an effluent purifying plant, 
       FIG. 2  shows a first variant of the effluent purifying plant, especially for the production of potable water, 
       FIG. 3  is a sectional view analogous to that of the preceding figures of another embodiment of an effluent purifying plant including a pre-treatment section, 
       FIG. 3A  is a simplified view in axial section showing the pre-treatment section of the variant of  FIG. 3 , 
       FIG. 3B  shows in a top view the annular plate supporting the flocculating vats, 
       FIG. 3C  is a top view of the annular plate separating the lower volume from the upper volume of the outer annular vessel, 
       FIG. 4  is a view in axial section of another embodiment of an effluent purifying plant equipped not only with the pre-treatment section but also with a bacteriological treatment means, 
       FIG. 4A  is an extract from  FIG. 4  showing the bacteriological treatment portion of the plant, 
       FIG. 4B  is a partial view of the connection between two annular chambers of the bacteriological treatment portion of the embodiment of the effluent purifying plant according to  FIG. 4 . 
   

   DESCRIPTION OF EMBODIMENTS 
   According to  FIG. 1 , the invention concerns an effluent purifying plant with centralized structure. The plant, distributed schematically about a vertical axis ZZ, preferably in rotational symmetry, is composed of a homogenizing vessel  1  receiving the effluent to be treated. Beneath the homogenizing vessel there is a settling vessel  2  in which a flocculating vat  3  is located, and the space between the settling vessel  2  and the flocculating vat  3  constitutes the sludge vat  4 , the upper portion  21  forming the actual settling volume; around the settling vessel  2 , an annular vessel  5  receives the filtered water by overflow from the settling vessel  2 . An immersion pipe  6  arranged in the axis ZZ descends through the homogenizing vessel  1  as far as the lower part of the flocculating vat  3 . The effluent receives flocculating and coagulating agents. By the effect of rotation it circulates in the homogenizing vessel  1  then into the pipe  6  descending through the settling vessel  2  and the flocculating vat  3 , in order to rise up there, release sludge flakes FB and allow the filtered water to be collected. The settling vessel  2  has at least one anti-backflow cone  7 ,  8  surrounding the pipe  6  and pushing back the sludge flakes, floating in the ascending flow of filtered effluent/water, which rises to the top  21  of the settling vessel  2 . 
   The circulation of the liquid (effluent or filtered liquid, filtered water) is indicated by arrows without reference numbers. Circulation takes place either on a more or less straight path, or according to curved arrows indicating a rotational movement of the liquid. There are also return or winding movements for example below the anti-backflow cone or cones  7 ,  8 . 
   The different vessels  1 ,  2 ,  3  and pipe  6  are connected and supported by means that are neither shown nor described, since they are customary means of construction. All that is important is the arrangement grouped and centred about the axis ZZ so that the plant occupies the least possible ground surface area and volume for a maximum capacity of effluents and filtered liquids. 
   In more detail, the effluent purifying plant shown in section, especially in axial section in  FIG. 1 , will be explained hereinafter. 
   The homogenizing vessel  1  located in the upper portion of the plant is equipped with an effluent intake  9 . The pipe  91  of this intake opens tangentially into the homogenizing vessel  1  in order to induce a rotational movement of the liquid therein. Upstream of the homogenizing vessel  1 , the effluent pipe  91  coming, for example, from a pump  92 , has an injection point  93  for coagulant/flocculant. The injection point  93  is followed by a static mixer  94  in the form of a chamber which slows down the fluid flow rate by means of an increase in the cross-section in order to facilitate mixing between the effluent and the injected coagulating/flocculating agents. 
   The settling vessel  2  which is located beneath the homogenizing vessel  1  also has a rotationally symmetrical shape with respect to the axis ZZ. The vessel  2  is composed overall of a substantially cylindrical, circular, upper portion  21 , and of a frustoconical lower portion  22 . 
   The lower portion  22  houses the flocculating vat  3 , and the upper portion  21  forming the volume providing the actual settling function houses one, and preferably two, anti-backflow cones  7 ,  8 . One ( 7 ) of the cones is fixed, the other ( 8 ) is adjustable. The two cones  7 ,  8  are centred on the axis ZZ and surround the effluent pipe  6  descending from the homogenizing vessel  1  almost to the bottom of the flocculating vat  3 . 
   The flocculating vat  3  does not abut with the upper edge of its side  31  the side of the lower portion  22  of the settling vessel  2 , but leaves a gap and an annular volume as well as a lower volume beneath the flocculating vat  3  (this latter rests on feet in the settling vessel) to form a sludge collecting chamber or sludge vat  4 . The bottom of this vat  4  is connected to a sludge discharge pump  10 . The pump  10  is controlled or programmed to discharge the sludge according to its accumulation or periodically. 
   The effluent pipe  6  is preferably installed according to the axis ZZ, that is, according to the axis of symmetry of the structure of the plant. The descending pipe  6  is composed of a first tube  61  opening into the homogenizing vessel  1 , rising up in this latter so as to delimit therein an annular volume facilitating the setting in rotation of the effluent arriving and rotating in the homogenizing vessel  1 . 
   The upper tube  61  descends through the upper portion  21  of the settling vessel  2  to meet a lower tube  62  forming part of the pipe  6 . The lower tube  62  opens out close to the bottom of the flocculating vat  3 . The lower tube  62  extends the upper tube  61  and has a diameter significantly larger than that of the upper tube. Both at its entry  611  into the homogenizing vessel  1  and at its outlet  612  into the lower tube  62 , the upper tube  61  includes in each case a helical guide  613 ,  614  in the form of a vane, incorporated in the end of the tube  61  so as to impart a rotational impulse to the effluent which passes through the upper tube  61 . 
   This rotational movement of the effluent continues through the lower tube  62  which is terminated by a narrowed outlet  621  constituted by a truncated cone, the apex of which faces downwards. At the base of the truncated cone  622 , the lower tube  62  has a support  623  equipped with an injection point for a flocculating agent  624 . 
   The flocculating vat  3  into which the pipe  6  opens and, more precisely, its rotationally symmetrical lower tube  62 , is formed by the combination of two frustoconical shapes  31 ,  32 , a very flared frustoconical upper portion  31  and a less flared frustoconical lower portion  32 . The truncated cone  622  terminating the lower portion of the lower tube  62  of the pipe  6  and the truncated cone  32  constituting the lower portion of the flocculating vat  3  are preferably parallel, so as to regularize the flow emerging from the opening  622  of the tube  62  in order to rise up in the flocculating vat. On the other hand, the upper truncated cone  31  of the flocculating vat  3  is very widely flared, so as to slow down the flow velocity of the ascending flow in order to facilitate the formation of sludge flakes which collect together and gradually rise up, entrained by the liquid of the flocculating vat. Then, the large sludge flakes FB pass over the edge of the vat  3  and, since the speed of the liquid at this location is very slow, the flakes descend into the sludge vat  4  and accumulate at the bottom. 
   One ( 7 ) of the anti-backflow cones is located just above the flocculating vat  3 . It is a frustoconical surface engaged both on the lower tube  62  of the pipe  6  and around the lower end of the upper tube  62 . This anti-backflow cone  7  serves to stabilise the flow of effluent rising up in the flocculating vat  3 , in proximity to the outer wall of the lower tube  62 . 
   The cone  7  thus creates a ring of more or less immobile liquid surrounding the lower tube so that any flakes located in this area circulate very slowly and therefore have a tendency to drop down again, while the flakes entrained by the circulation of the liquid arriving from the pipe  6  pass preferentially towards the collar of the flocculating vat  3 . 
   Above this first cone  7  there is a second cone  8  located clearly in the upper volume  21  of the settling vessel  2 , that is, the actual settling volume, again in such a way as to stabilise the flow in order to facilitate the circular rising/falling movement of any streams of cleansed liquid, in order to eliminate therefrom the last particles or flakes of sludge. This cone  8  is adjustable in height in the settling volume  21  of the vessel  2 . 
   The vertical positioning of the cone  8  depends on the residual density of the sludge flakes entrained in the ascending flow of the clarified water and is intended to regulate the ascending speed of the flow in order to obtain the precipitation of the flakes. 
   For the following adjustment, the type of effluent to be treated, knowing the behaviour of the flakes, and according to the flow rate to be treated, pre-adjustment is carried out in the workshop and the final positioning is carried out in situ after the setting up of the whole of the plant. 
   In its upper portion  21 , the settling vessel  2  is equipped with a peripheral distribution of overflow openings  23 . Beneath these openings  23 , inside the settling vessel  2 , there is an assembly of very slightly inclined baffles  24 . The filtered water which rises up again in the settling vessel  2  passes between the baffles  24  then through the openings  23 . 
   In order to facilitate the distribution of the filtered water in the vessel  5 , the ejection openings  23  each open preferably into an elbow  27  terminated by a sprinkler rose so as to cause the liquid to drip or trickle onto a relatively large surface area of the filter-mass  511  and avoid preferential passages. 
   The settling vessel is in fact surrounded by the annular vessel  5 , the lower portion  52  of which makes it possible to receive the filtered water, and the upper portion  51  contains a filter-mass supported by a partition  53  separating the upper portion  51  and the lower portion  52 . 
   The annular vessel  5  substantially follows the contours of the settling vessel  2 . It is bounded by an outer wall, the lower portion  55  of which is frustoconical and the upper portion  54  cylindrical. 
   The horizontal partition  53  in the form of an annular collar is pierced by holes to allow the water which has passed through the filter-mass  511  to descend into the lower portion  52 , whence it can be extracted by a pump  12 . 
   The baffles  24  of the settling vessel  2  are equipped with cleaning ramps  26  connected to a water source, for example of filtered water, and below the filter-mass  511 , above the partition  53 , strainers  56  allow water to be injected, if necessary, charged with gas for cleaning the filter-mass  511  by reverse circulation. 
     FIG. 2  shows a first variant of the plant of  FIG. 1 . This variant repeats the main elements of the first embodiment, and these elements bear the same references without their description being repeated in detail. The elements which are analogous but particular to this variant will bear the same references with the addition of the suffix A. 
   Thus, the lower tube  62 A of the pipe  6  is formed by a flared portion in the shape of a truncated cone to which is connected, in an inverted position, the truncated cone  622 ; these two truncated cones joined by their major base include at the join the support  623  equipped with the injection point for flocculating agent  624 . 
   The flocculating vat  3  has the same shape as that of the first embodiment, except that, close to its upper edge, it has openings  33  for the passage of the sludge flakes which will no longer pass over the upper edge. These openings  33  open into the sludge vat  4  formed as previously between the flocculating vat  3  and the lower portion  22  of the settling vessel  2 . The extraction of the sludge is again effected by a pump or a valve  10  in the lower portion of the sludge vat  4 . 
   The principal modifications of this variant concern the vessel receiving the filtered effluent or clarified water. This vessel is divided into two parts, an upper portion  51 A installed above the settling vessel  2  and in prolongation of this latter, sharing with the upper volume  21  of the settling vessel  2  the same circular cylindrical wall. The second part or lower portion  52 A of the vessel receiving the filtered water is located in the lower part around the lower portion  22  of the settling vessel. The lower portion  52 A shares with the settling vessel the common wall  22 . It is bounded externally by a circular cylindrical wall  55 A with a bottom and a top. 
   The lower portion  52   a  is connected to the pump  12  for drawing off the filtered liquid (filtered water). 
   The intake for the upper portion  51   a  of the vessel is provided by an annular channel  211 , surrounding the upper tube  61  of the pipe  6 . The annular channel  211  has an inlet in the upper volume  21  formed by a cone  212  also intended to stabilise the flows like the anti-backflow cones  7  and  8 . In the upper portion the annular channel  211  has openings  23 A comparable to the overflow openings  23  of  FIG. 1  which open into irrigation ramps  27 A distributing the filtered liquid over the filter-mass  511 A. The filter-mass  511 A is located above the partition bounding the collecting chamber  213  collecting the filtered liquid at the outlet of the upper portion  511 A. This chamber  213  is connected by a pipe  214 , equipped with a valve  215 , to the lower portion  52 A. The valve  215 , preferably remote-controlled, makes it possible to discharge the liquid emerging from the filter-mass  511 A, continuously or discontinuously. 
   The cleansing of the filter-mass  511 A is carried out by countercurrent or reverse flow by means of strainers  56 A that are installed at the base of the filtering material in the upper portion  51 A and are fed by a pipe  122  coming from a three-way valve  121  at the outlet of the draught pump  12 . The valve  121 , normally connected to the outlet  124  for drawing off filtered water, may be switched in order to send the filtered water back through the strainers  56 A and the filter-mass  511 A, the water then circulating in the reverse direction being discharged by means not shown. 
   The plant thus described is suited quite particularly to the production of potable water. 
     FIG. 3  shows an alternative embodiment corresponding substantially to  FIG. 1 , with the addition of means making it possible to decant the effluent before its entry into the actual plant through the homogenizing vessel  1 . 
   Thus, the concentric structure of the effluent purifying plant described above is surrounded externally by another annular structure forming an outer vessel  11  for pre-treatment and partial settling. This annular outer vessel  11  is bounded by an outer, circular cylindrical wall  111  forming an upper volume  112  and a lower volume  113 , this latter benefitting from the increase in volume provided within the circumscribed perimeter, owing to the frustoconical shape of the lower portion  55  of the annular chamber  5  surrounding the settling vessel  2 . 
   The outer annular vessel  11  makes it possible to decant a significant portion of the solid matter in the form of sludge or flakes and, for this purpose, its lower volume  113  is bounded by an annular collar  114  forming the bottom of the outer annular vessel. Above the annular collar  114 , for example toroidal in form and of semi-circular cross-section, there is a flocculating vat  115  formed by an annular plate  115 A equipped with frustoconical cavities  115 B, into which descending pipes  116  open. The frustoconical cavities  115 B are bordered by openings  115 C through which the sludge flakes descend into the sludge vat  114 . For the purpose of simplification, these different means will be termed: flocculating vat  115 . The pipes  116  are connected in the upper portion of the plant to a distribution vessel  13 , itself fed with effluent by a pump  14  and a pipe  15  with, preferably, an injection point  151  for coagulating agent, followed by a chamber forming a static mixer  152 , then an injection point  153  for flocculating agent, followed also by a chamber forming a static mixer  153  for opening into the distribution vessel  13 . A plurality of pipes  116  depart from the distribution chamber  13  in radial directions to descend to the bottom of the flocculating vat  115  through a slightly divergent form of pipe  117  so as to slow down the speed of the effluent. 
   At this level, significant flocculation occurs, and may represent from 30 to 40% of the sludge in suspension in the liquid to be cleansed. The sludge overflows from the flocculating vat  115  to reach the sludge storage vat  114  formed by the annular collar whence the sludge is pumped. 
   The effluent is fed in by the pump  14  and the purified water is extracted by the pump  12 . The lifting pump  92  takes up the effluent that has undergone the first decantation of its sludge flakes in the outer annular vat  11 . The effluent arriving in the lower portion  113  of the annular vessel  11  rises up to pass into the upper volume  112  forming the annular buffer chamber located above the partition  118  dividing the outer annular vessel  11 . Communication between these two annular volumes is provided by openings  119  distributed in the partition  118  and the inlet of which is preceded by a divergent cone  119 A. 
   The cleansing of the filter-mass  511  of the annular chamber  5  receiving the filtered water is effected by sending filtered water in the reverse direction from the normal direction of passage of the water through the filter-mass. This water is introduced, in the lower part at the lower partition  53 , through the strainers  56 . The water thus injected for cleansing the filter-mass  511  is preferably water recovered in the filtered water storage area. The water is fed in preferably by the same pump as that which extracts the filtered water from the enclosure  52 . A system of valves makes it possible to circulate the water for cleansing the filter-mass solely through the mass and not through the homogenizing vessel. 
     FIG. 3A  shows in isolation the addition to the plant of  FIG. 3  in relation to the basic plant shown in  FIG. 1 . 
     FIG. 3B  shows in more detail the shape of the flocculating vat  115  provided in the actual annular plate  115 A. The frustoconical cavities are each fed by a pipe  116  departing from the distribution chamber  13 . This view shows the distribution of the openings  115 C around each cavity  115 B for the passage of the sludge flakes emerging from the cavity  115 B. 
   In its portion separating two cavities  115 B, the collar  115 A is traversed by oblong and/or round passages permitting the calm rise of the liquid of the sludge vat  115 . 
     FIG. 3C  shows the outer vessel  11  sectioned above the partition  118 , showing this latter and also the openings  119  bordered by the inlet cones  119 A. The figure also shows the pipes  116 , emphasising the angular off-set between on the one hand the pipes  116  (the cavities  115 B associated but not shown in this figure) and on the other hand the openings  119  occupying intermediate positions. 
     FIG. 4  shows another variant of the plant of  FIG. 1  or of that with the addition of  FIG. 3  with, in addition, means for bacteriological treatment of the filtered effluent, at the outlet of the plant of  FIG. 3 . As above, the elements identical or analogous to those already described bear the same references and their description will not be repeated. 
   Half of the complementary part added to the plant of  FIG. 3A  is shown in isolation in  FIG. 4A . 
   This complementary part  200  is composed of a series of annular or toroidal chambers  201 ,  202 ,  203 , surrounding the outer vessel  11  and arranged in a cascade to receive the purified liquid coming from the lower portion  52  of the vessel  5 . The liquid (water) is pumped by the pump  12 , but instead of being supplied outside, it feeds the cascade of toroidal chambers  201 - 203  in succession from bottom to top. The toroidal chambers have inner helical guides for imparting to the stream of fluid a rotational movement about its axis of movement. These chambers contain charges which remain in suspension owing to this movement of displacement and rotation of the fluid which passes through them. The chambers communicate with one another through a U-shaped communicating pipe  205 ,  206 ,  207 , the inlet of which in each case faces downstream according to the circulating movement of the liquid (arrow L) and its charge of particles in the corresponding annular chamber in order to extract liquid therefrom according to the arrival of untreated liquid in the annular chamber without removing from it the charge of particles in suspension and to pass it into the following chamber. 
   Finally, the last chamber  204  located in the top part serves for drawing off the purified liquid. 
     FIG. 4B  shows the detail of the joining of two annular chambers, for example the chambers  201 ,  202 , by a U-shaped pipe  205 , the inlet  204 E of which opens into the chamber  201  facing downstream in the direction of circulation of the liquid represented by the arrow L inside the chamber  201 . Upstream of the inlet  205 E, the chamber  201  has a helical guide  210  in the shape of an insert intended to impart to the stream of fluid circulating in the chamber a rotational component about the axis of the chamber. A plurality of such guides may be distributed inside the chamber  201 , as also in the other chambers such as the chamber  202 . 
   The connecting pipe  205  opens into the downstream chamber  202  through its outlet  205 S, also preferably provided with a helical guide  211 , imparting to the liquid opening into the chamber  202  a rotational movement about its axis. The rotational movement is amplified by the helical guide  10  which is located downstream.