according to fig1 , 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 fig1 , 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 . fig2 shows a first variant of the plant of fig1 . 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 fig1 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 . fig3 shows an alternative embodiment corresponding substantially to fig1 , 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 . fig3 a shows in isolation the addition to the plant of fig3 in relation to the basic plant shown in fig1 . fig3 b 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 . fig3 c 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 . fig4 shows another variant of the plant of fig1 or of that with the addition of fig3 with , in addition , means for bacteriological treatment of the filtered effluent , at the outlet of the plant of fig3 . 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 fig3 a is shown in isolation in fig4 a . 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 . fig4 b 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 .