Abstract:
A process for water treatment, including a combination of methods from the group comprising coagulation, sedimentation, flocculation and ballast flocculation, which is further improved by the addition of a simplified sludge recirculation system. The recirculation system corresponding to this process allows higher sludge density as well as less significant water volume losses by making the sludge accumulating at the bottom of the sedimentation zone go through a hydro cyclone a certain number of times in repeated cycles thus augmenting the solid particles density of the extracted sludge. The system may also be controlled by a suspended solid analyser, a flow meter and/or a timer. The present invention also includes a method of producing specific fluid flow control behavior with this simplified sludge recirculation system, which furthermore improves the efficiency of the process.

Description:
FIELD OF THE INVENTION 
     This invention relates to a simplified sludge recirculation system to be added to a system for potable, or industrial water or waste water treatment, which may include a combination of methods from the group comprising coagulation, sedimentation, flocculation and ballast flocculation, in order to improve its efficiency by reducing ballast and water loss. It also relates to a specific fluid flow behaviour rendered possible specifically due to the addition of the simplified sludge recirculation system, and which furthermore improves the efficiency of the process. 
     BACKGROUND OF THE INVENTION 
     Water treatment facilities are indispensable to the purification of potable, used and industrial water, wherein the water has been exposed to contaminants of various size and composition. The purification process is thus intended to remove those contaminants with the use of appropriately selected methods, which are generally relying on the containment of water in large tanks in order to apply the treatment. Some contaminants are dense enough to sink and accumulate at the bottom of those tanks, depending of the flow rate, while others are big enough to be successfully sieved from the water with a filter. However, some contaminants, called colloids, are microscopic particles evenly distributed inside a mixture that cannot be separated effectively from the hydrocolloid solution, which is the water and colloid mixture, by physical means and thus require specific treatment methods. 
     In order to separate the water from those unwanted pollutants, it must go through certain steps of purification. Pre-treatment can be made in order to retrieve large debris and adjust the pH of the water to facilitate ulterior steps of the treatment. To eliminate the smaller particles in suspension and thus clarify the water, water treatment facilities generally comprise a flocculation zone where a flocculating agent, either a polymer (like modified polyacrylamides), a chemical product (like sodium silicate) or in rare occasions a natural product with the same properties, is introduced within the water. With the addition of such a flocculating agent, flocs particle aggregates) of contaminants start to form out of the colloids. A mixer with rotative blades generally stirs the mixture located inside the flocculation zone in order to maximise the contact between the flocculating agent and the contaminants, thus enabling the creation of bigger flocs. 
     This first step process, called flocculation, can be further improved with the addition of a ballasted material, like micro-sand, which acts as ballast and contact mass catalyzing the flocculation reaction inside the water and contaminants solution. When ballast is added, the aforementioned flocculating agent bonds it together with flocs of colloids and other particles, thus creating even bigger and heavier flocs by agglomerating previously created flocs along with sand particles. This in turn has the advantage of making the flocculation and the next step of the treatment happen faster. 
     The next step of the water treatment process is called sedimentation. It takes place in the sedimentation zone and capitalizes on the fact that gravity pulls every object toward the surface of the earth with a force proportional to its weight. Therefore, heavier particles are more easily dragged toward the bottom of this containment zone so the addition of granular ballast like sand, while not essential, can make a worthy addition to the process, reducing the time needed for the flocs to settle down at the bottom of the zone. The flocculation process is thus essentially a means of reducing the amount of colloids in suspension inside the liquid solution, creating relatively heavy flocs out of colloids which do not effectively sink to the bottom of the sedimentation zone with the influence of gravity as would the bigger particles in suspension inside the liquid solution. Purified water is subsequently collected when it overflows from the sedimentation zone. If ballast is used in the flocculation zone, ballasted flocs then accumulate at the bottom of the sedimentation zone and comprise both sand and particulate contaminants, further requiring to be treated to separate the sand from the pollutants. 
     The mixture comprising contaminants, colloïds, water and also sometimes sand form what is generally called “sludge”, which is to be removed from the system after the extraction of as much of the sand and water as possible in order to maximize the efficiency of the process. The extracted sand can be used again and again in the process without the need to add much more throughout the course of action, depending on the effectiveness of the aforementioned extraction. 
     A non-essential additional step, called coagulation, can be added to the water treatment process in order to further improve its efficiency. If included in the process, it is generally the first step by which the polluted water begins its purification after pre-treatment. It consists in the addition of trivalent metallic salts to the water and contaminants solution. The salts (generally iron or aluminium composites) dissolve in water releasing ions with three positive charges which bind with colloids and then form small aggregates. Those aggregates are combined into flocs when a flocculating agent is added to the solution and because they are bigger particles than the colloids themselves, they make the agglomeration of aggregates into flocs relatively easier than the process without prior coagulation and thus augment the efficiency of the procedure at the cost of the inclusion of another zone to the facilities, the coagulation zone, and added expenses for the trivalent metallic salts. 
     The purified water is generally filtered after the sedimentation zone in order to remove unsettled flocs and particles which could still be in suspension inside the water. Water concentration of the sludge produced after sedimentation is still too high and thickening means are therefore needed to reduce it enough to facilitate transport, for example to landfill sites. This added process takes a lot of time to be efficient and often necessitates large amounts of space, as in the case of open air evaporation sites (or drying beds). An alternative is the method of pressing which requires the sludge to be pressed against textile filters to extract as much liquid as possible after what a compact residual cake is made out of the remaining solid contaminants. The method of centrifugation uses centrifugal force to extract water from the sludge, and as for pressing the residual contaminants are shaped in a compact cake. On the other hand, these methods require specialized machinery or vast open spaces to be efficient, which are costly and may be impractical depending of the economic and geographic situation of the community requiring them. 
     Another common problem of actual water treatment facilities is the extraction of sand ballast from the produced sludge which results in needless waste of material. 
     OBJECTS OF THE INVENTION 
     A first object of this invention is to reduce the volume of the sludge rejected by water treatment facilities which typically make use of a combination of water treatment methods comprising coagulation, flocculation, sedimentation and ballast flocculation, by providing an enhanced means of progressively purging water from said sludge through the use of an improved sludge recirculation system. 
     A second object of this invention is to present means to enhance water treatment processes which can be retro-fitted to existing facilities as well as newly constructed ones at minor costs. 
     A third object of this invention is to reduce the size of sludge water purging means in such facilities. 
     A fourth object of this invention is to eliminate the need for an exterior sludge water purging basin, used by some treatment facilities, thus reducing operating costs and duration of the water treatment process in such facilities. 
     A fifth object of this invention is to reduce the amount of ballast lost during water treatment processes which may include ballast flocculation. 
     SUMMARY OF THE INVENTION 
     The present invention represents a solution for already existing and future water treatment facilities necessitating means of reducing the amount of water contained inside the residual sludge retrieved after water treatment in order to reduce the volume of waste to be disposed subsequently. It also reduces the cost and size of the apparatus needed to further concentrate the sludge. The present invention also reduces the loss of ballast in concerned facilities with certain types of liquid and solid separation means by augmenting its recuperation rate, accomplished by multiple repeated cycles of sludge reinsertion in those means which is rendered possible by the present invention. 
     A water treatment process comprising coagulation, ballast flocculation and sedimentation typically allows sludge solid matter concentration between 0.05% and 0.1% (0.5 to 1.0 gram/litre). When combined to the simplified sludge recirculation system of this invention, extensive testing shows that the concentration proves to augment to above 30 g/L with rejected sludge volumes down by a percentage between 30 and 97 percents and necessitating smaller sludge thickening equipment. 
     The present invention also allows ballast recuperation with a rate equivalent to the one of water recuperation. The following table compares the performance results of the present invention with those of typical water treatment systems: 
     The combination of elements of embodiments one, two and three, as described latter on, makes possible the creation of a complex three-dimensional flow preventing the sludge from re-entering the topmost part of the sedimentation zone. This flow further improves the efficiency of the present invention and is a result of the particular designs described in the detailed description of the embodiments. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                 System 
                   
               
               
                 Rise Rate/ 
                 Matter in 
                 Typical Prior 
                 performance 
                 Perfor- 
               
               
                 Recirculation 
                 suspension 
                 Art system 
                 according to the 
                 mance 
               
               
                 ratio 
                 (Raw water) 
                 performance 
                 present invention 
                 gain 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 20 m/h 
                  20 mg/L 
                 96 
                 m 3 /hour 
                 3.2 
                 m 3 /hour 
                 97% 
               
               
                 and 3% 
                   
                 0.6 
                 g/L 
                 20 
                 g/L 
               
               
                 recirculation 
               
               
                 40 m/h 
                 200 mg/L 
                 96 
                 m 3 /hour 
                 47.1 
                 m 3 /hour 
                 51% 
               
               
                 and 3% 
                   
                 9.82 
                 g/L 
                 20 
                 g/L 
               
               
                 recirculation 
               
               
                 40 m/h 
                 600 mg/L 
                 192 
                 m 3 /hour 
                 133.6 
                 m 3 /hour 
                 30% 
               
               
                 and 6% 
                   
                 13.9 
                 g/L 
                 20 
                 g/L 
               
               
                 recirculation 
               
               
                 60 m/h 
                 200 mg/L 
                 192 
                 m 3 /hour 
                 133.6 
                 m 3 /hour 
                 30% 
               
               
                 and 6% 
                   
                 13.9 
                 g/L 
                 20 
                 g/L 
               
               
                 recirculation 
               
               
                 80 m/h 
                 200 mg/L 
                 192 
                 m 3 /hour 
                 94 
                 m 3 /hour 
                 51% 
               
               
                 and 3% 
                   
                 9.82 
                 g/L 
                 20 
                 g/L 
               
               
                 recirculation 
               
               
                   
               
             
          
         
       
     
     The invention consists of a sludge recirculation system to be added to a sedimentation zone of a water treatment facility using at least one purification method selected from the group consisting of flocculation, sedimentation, coagulation and ballast flocculation, said sludge recirculation system for repeated cycling of said sludge in a progressively water purging fashion, said system comprising:
         a downstream sludge recovering vessel, including a sludge recovering cavity located at a bottom portion of said vessel, said cavity defining a certain volume of said sedimentation zone wherein sludge may accumulate therein under the influence of gravity;   a recirculation apparatus, comprising:
           i. liquid and solid separation means that allows the purification of a liquid solution by removing solid contaminants located therein;   ii. recirculation means, comprising:
               1. a recirculation line connected at an intake end thereof to said sludge recovering cavity and connected to said liquid and solid separation means at an outlet end thereof;   2. a reinsertion line operatively connected at an intake end thereof to said liquid and solid separation means and to said sludge recovering vessel at an outlet end thereof; and   3. an elimination line connected at an intake end thereof to said reinsertion line and rejecting high density sludge outside of said water treatment facility at a downstream outlet end thereof;   
               iii. means active during said repeated cycling of said sludge through the sludge recirculation system for progressively eliminating said sludge from said sludge recirculation system through said elimination line;   iv. means to drive said sludge into said recirculation apparatus during said repeated cycling.   
               

     Preferably, said means to drive said sludge into said recirculation apparatus is a pump located downstream on said recirculation line. Also, said means for progressively eliminating said sludge from said sludge recirculation system includes means to monitor the solid constituents concentration of said sludge. 
     Preferably, said liquid and solid separation means is a hydro cyclone mounted downstream of said recirculation line relative to said pump, which comprises an overflow outlet and an underflow outlet, said overflow outlet connected to said recirculation means and said underflow outlet pouring inside a flocculation zone. 
     Preferably, a control means selected from the group comprising flow control means and solids concentration control means is further provided to regulate the speed of said liquid solution flowing through said recirculation apparatus in such a fashion as to optimize the efficiency of said hydro cyclone. 
     Preferably, said means for progressively eliminating said sludge from said sludge recirculation system through said elimination line is a suspended solid analyser which works in conjunction with said flow control device to further optimize the efficiency of said hydro cyclone by adjusting the flow speed to said solid constituents concentration of said sludge. 
     Preferably, the sedimentation zone comprises a rotating scraper, comprising a top part and a bottom part relative to the plane of said sedimentation zone and rotating in said plane, which guides said sludge deposited at said bottom of said sedimentation zone toward said sludge recovery cavity in such a fashion as to keep it grounded and effectively separates said sedimentation zone in a first upper section and a second lower section relative to the plane of the scraper, thus isolating said sludge recovery cavity, said recirculation line intake end and said reinsertion line outlet end located within said second lower part from said first upper part of said sedimentation zone. 
     Preferably, said rotating scraper is hollow-centered forming a hollow shaft and coincides with a downstream end portion of said reinsertion line of said recirculation apparatus pouring inside said sludge recovering cavity. 
     Preferably, an inverted cone is embossed on said bottom part of said scraper coaxially to said hollow shaft, substantially preventing said liquid solution located in said sludge recovering cavity from dynamically back flowing into said reinsertion line and maximising flow through said recirculation line. 
     Preferably, said recirculation apparatus extends externally to said sludge recovering vessel. 
     Preferably, said reinsertion line outlet of said recirculation apparatus opens inside of said sludge recovering cavity. 
     Preferably, a sand sedimentation chamber is further included in said recirculation apparatus and mounted upstream of said reinsertion line and of said elimination line and downstream of said hydro cyclone, enabling sand-like granular material recuperation within said recirculation apparatus where the sludge contains sand-like material. 
     Preferably, said recirculation apparatus comprises a hydro cyclone, a recirculation flow control valve mounted to said reinsertion line and a suspended solid analyser also mounted to said reinsertion line, controlling the opening and closing of said flow control valve depending on concentration of said sludge inside said recirculation apparatus. 
     Preferably, a suspended solid analyser is installed at an entry pipe feeding said water treatment facility with water, thus allowing the water flow through said recirculation apparatus to be controlled depending on the colloidal contaminants concentration inside the water. 
     Preferably, said hydro cyclone wherein said hydro cyclone rejects recirculated sludge in the sludge recirculation system. 
     The invention also consists of a method of creating a particular fluid flow behaviour making use of said sludge recirculation, preventing the sludge located in said second lower section of said sedimentation zone of coming back in said first upper section of the sedimentation zone as well as maximizing the flow from said reinsertion line to said recirculation line, comprising the following steps:
         a) a mixture of water and contaminants flocs enters a sedimentation zone;   b) the flocs then drop to a downstream sludge recovering vessel, including a sludge recovering cavity, located at a bottom portion of said vessel, said cavity defining a certain volume of said sedimentation zone under the influence of gravity, forming sludge;   c) a rotating scraper, comprising a bottom end and a hollow center, guides said sludge deposited at said bottom of said vessel toward said sludge recovery cavity in such a fashion as to keep it grounded;   d) a recirculation line having an intake mouth thereof located inside said sludge recovering cavity and operatively connected to a pump, drives sludge into said recirculation apparatus;   e) a certain amount of sludge is reinserted within said sludge recovering cavity through a reinsertion line having an outlet end thereof located inside said hollow-center of said scraper;   f) the resulting stream of sludge flows back toward said recirculation line without back flowing inside the reinsertion line due to the specific combination of:
           i. said rotating scraper;   ii. said sludge recovering cavity;   iii. said recirculation apparatus;   iv. said recirculation line; and   v. said reinsertion line of said recirculation apparatus located inside said hollow center of said rotating scraper and pouring into said sludge recovering cavity.   
               

     Preferably, an inverted cone is added to said bottom end of said scraper, further enabling the flow of said sludge pouring from said reinsertion line through said recirculation line without backflowing inside said reinsertion line. 
     The invention also consists of a method of creating a particular fluid flow behaviour that prevents the sludge located in said second lower section of said sedimentation zone of coming back in said first upper section of the sedimentation zone as well as maximizes the flow from said reinsertion line to said recirculation line, comprising the following steps:
         a) a mixture of water and contaminants flocs enters a sedimentation zone;   b) the flocs then drop to a downstream sludge recovering vessel, including a sludge recovering cavity, located at a bottom portion of said vessel, said cavity defining a certain volume of said sedimentation zone under the influence of gravity, forming sludge;   c) a rotating scraper, comprising a bottom end and a hollow center, guides said sludge deposited at said bottom of said vessel toward said sludge recovery cavity in such a fashion as to keep it grounded;   d) a recirculation line having an intake mouth thereof located inside said sludge recovering cavity and operatively connected to a pump, drives sludge into said recirculation apparatus;   e) a certain amount of sludge is reinserted within said sludge recovering cavity through a reinsertion line having an outlet end thereof located inside said hollow-center of said scraper;   f) the resulting stream of sludge flows back toward said recirculation line without back flowing inside the reinsertion line due to the specific combination of:
           i. said rotating scraper;   ii. said sludge recovering cavity;   iii. said recirculation apparatus   iv. said recirculation line; and   v. said reinsertion line of said recirculation apparatus located on the wall of said sludge recovering cavity.   
               

     Preferably, an inverted cone is added to said bottom end of said scraper, further enabling the flow of said sludge pouring from said reinsertion line through said recirculation line without backflowing inside said reinsertion line. 
     The invention also consists of a method of creating a particular fluid flow behaviour that prevents the sludge located in said second lower section of said sedimentation zone of coming back in said first upper section of the sedimentation zone as well as maximizes the flow from said reinsertion line to said recirculation line, comprising the following steps:
         a. a mixture of water and contaminants flocs enters a sedimentation zone;   b. the flocs then drop to a downstream sludge recovering vessel, including a sludge recovering cavity, located at a bottom portion of said vessel, said cavity defining a certain volume of said sedimentation zone under the influence of gravity, forming sludge;   c. a recirculation line having an intake mouth thereof located inside said sludge recovering cavity and operatively connected to a pump, drives sludge into said recirculation apparatus;   d. a certain amount of sludge is reinserted within said sludge recovering cavity through a reinsertion line having an outlet end thereof located on the wall of said sludge recovering cavity;   e. the resulting stream of sludge flows back toward said recirculation line without back flowing inside the reinsertion line due to the specific combination of:
           i. said sludge recovering cavity;   ii. said recirculation apparatus;   iii. said recirculation line; and   iv. said reinsertion line of said recirculation apparatus located on the wall of said sludge recovering cavity.   
               

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings illustrating the preferred embodiment of the invention: 
         FIG. 1  is a schematic elevational view of a water treatment facility with the present invention relying on an exterior sludge recirculation system, further known as embodiment one; 
         FIG. 2  is a schematic elevational view of a water treatment facility where a sludge recirculation system reinserts the sludge at the bottom of the sedimentation zone with a conduit going down through the hollow center of the rotating scraper, further known as embodiment two, and where the sludge recirculation flow is regulated by a suspended solid analyzer; 
         FIG. 3  is another schematic elevational view of a water treatment facility comprising the second embodiment of the sludge recirculation system of  FIG. 2  where the sludge recirculation flow is instead regulated by a flowmeter; 
         FIG. 4  is another schematic elevational view of a water treatment facility where the sludge recirculation flow is controlled by a timer; 
         FIG. 5  is another schematic elevational view of a water treatment facility showing the different emplacements wherein recirculation flow control or suspended solid analysis components may be located; 
         FIG. 6  shows another schematic elevational view of a water treatment facility where the recirculation flow control is done by suspended solid analysis of the water flowing through the feeding pipe; and 
         FIG. 7  shows a graphic representation at an enlarged scale relative to  FIGS. 2 to 4 ,  5  and  6 , of the simulation of the specific flow dynamic of the sludge going through the sludge recirculation system of the present invention, in this case the sludge recirculation system of embodiment two, and taken at the bottom right hand side portion of  FIG. 2-4 ,  5  or  6 . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  generally shows a water treatment facility  01  comprising 3 main zones inside which water is circulating with an added sludge recirculation system corresponding to embodiment one of the present invention. Water flows from left to right, in order to be progressively purified from its contaminants. The upstream coagulation zone  10  is defined as an upright containment vessel  100  for example of cubic shape receiving water through a conduit that may be a conventional pipe and which is not shown on the figure. A coagulant, preferably a trivalent metallic salt is added to the water flowing in coagulation zone  10  in order to initiate the formation of aggregates of contaminants amidst the water. The small pollutant particles in suspension within this water are generally negatively charged and thus are attracted by the trivalent metallic salts, which dissolve in the water leaving ions with three positive charges. A motor  11  fixed above the coagulation zone allows the rotation of the mixer  12 , to which it is connected by the rotary shaft  11   a . Mixer  12  includes a number of rotatable blades  12   a ,  12   b , . . . extending generally horizontally in operative condition, spacedly over the flooring  102  of the first upstream containment vessel  100 . 
     The shaft  11   a  is long enough for the mixer  12  to rest at a certain depth below water surface inside the coagulation zone  10  and rotates along with the mixer  12  because of the rotary motion transmitted by the motor  11 . The mixer blades  12   a ,  12   b , . . . rotate in a plane generally parallel to the plane of vessel flooring  102  and stirs the water and trivalent metallic salts solution in order to maximize the contact surface between the two reactants and thus the ionic attraction between said ions with positive charges and the contaminants within the water. This step is not compulsory to the achievement of proper water purification but may improve the efficiency of the water treatment. 
     The water, now containing small aggregates of contaminants, is then poured inside a second upright containment vessel  104 , which is called the flocculation zone  15 . A motor  110  fixed above the flooring  106  of vessel  104  also allows a second mixer  108  to rotate at a certain depth below the water surface spacedly over the plane of the flooring  106  of vessel  104  by the inclusion of the rotary shaft  110   a . A flocculating agent is mixed to the water in vessel  104 , which is already containing aggregates formed in the coagulation zone. 
     This flocculating agent is mixed thoroughly inside the water by mixer  108  and allows the formation of flocs of particles inside the flocculation zone  15  when combining with contaminants. The formation rate and size (and thus the weight) of the flocs can preferably be further augmented by the addition of ballast. The most commonly used ballast is micro-sand (for example between 50 μm and 150 μm in diameter), due to its general availability and relatively cheap cost. 
     The water then enters a third zone called the sedimentation zone  16  located in another upright containment vessel  112 . The flocs and aggregates that were created inside the two preceding zones  10  and  15  are attracted by gravity toward the funnel-shaped flooring  22  of downstream vessel  112 . Heavier particles are therefore more likely to sink to the flooring  22  of the sedimentation zone  16  and do so more quickly than lighter ones, which is the interest of coagulation and ballast flocculation in order to improve the efficiency of the water treatment system. A scraper  20 , which may carry a device such as an inverted cone  21  at its center, is given a rotational movement along the plane of the sedimentation zone  16  through a motor  18  driving a rotatable upright shaft  17 . 
     The purpose of shaft  17  is to rake the flocs of contaminants which have deposited on the radially inwardly downwardly sloped walls of flooring  22  of a sludge recovering cavity  19  located beneath inverted cone  21  in the center of the sedimentation zone  16 . The flocs of contaminants thus gather inside the sludge recovering cavity  19 , the mass of which consequently forming sludge. 
     The inverted cone  21  may be replaced by other suitable structures, for example by a horizontal perforated plate, spacedly supported over pit flooring  22 . The perforations of such a perforated plate would enable free passage of the sludge at such a flow rate that the sludge would not be returned to a state of suspension. 
     This sludge, which contains a relatively large volume of water, shall hereinafter be called diluted sludge. To optimize the operation of this water treatment, this diluted sludge needs to be treated in order to purge as much water as possible from the diluted sludge. To achieve this goal, the diluted sludge is sucked into the recirculation intake line  39  of the recirculation apparatus by the action of the pump  38 . The sludge then goes through outlet line  33  and enters a hydro cyclone  30 , which conventionally serves as a liquid and solid separation means. The hydro cyclone  30  is made in such a way that sludge with a higher concentration of contaminants needs a slower flow rate through the hydro cyclone  30  to achieve high separation rates, and inversely sludge with a lower concentration of contaminants requires faster flow rates to achieve good separation rates, due to its centrifugal functioning. 
     The overflow material, containing the lower density particles, exits the hydro cyclone  30  through outlet pipe  32  and the underflow material, containing the higher density particles, goes through the bottom opening to be reused. The service water input  31  enables cleansing of recirculated ballasted material. A sand sedimentation air vent chamber  34  may be connected to pipe  32  as it allows better recuperation of the sand still found inside the overflow provided by the hydro cyclone  30 . Also, we have found after several testings that the and sedimentation chamber is a good place to add an air vent. This air vent facilitates separation of air from recirculated sludge, and thus prevents air from being introduced at the sludge recirculation pit. The resulting sludge is then either sent through the elimination outlet line  35  of the recirculation apparatus out of the water treatment facility, or the sludge goes back to the sludge recovering cavity  19  by reinsertion line  40 . A device  36  controlling the opening of flow control valve  37  selects lines  35  or  40 , if the concentration of solid contaminants inside the sludge reaches a predetermined level or if the flow rate reaches a specified value or after a certain amount of time. Device  36  may consist for example of a suspended solid analyser, a flow meter or a timer, respectively. The higher contaminants concentration sludge thereafter reinserted inside the sludge recovering cavity  19  mixes with diluted sludge resulting from the sedimentation of the flocs in the sedimentation zone  16  and the cycle starts over again, gradually increasing the solid constituents concentration of the sludge being progressively purged of water. 
     Also, the combination of the scraper  20 ″″″, the upwardly pointed embossed cone  21 ″″″, the sludge recovering cavity and both the recirculation line  39 ″″″ and reinsertion in the central tube line  40 ″″″ create a particular fluid flow behaviour inside the sludge recovering cavity  19 ″″″ as seen on  FIG. 7 . This flowing behaviour guides the sludge from the reinsertion line  40 ″″″ to the recirculation line  39 ″″″ while also incorporating to the concentrated sludge coming from the reinsertion line  40 ″″″ the diluted sludge which accumulates inside sludge recovering cavity  19 ″″″ due to sedimentation. 
     Once again, the combination of the scraper  20 ′,  20 ″,  20 ′″ the inverted cone  21 ′,  21 ″,  21 ′″ the sludge recovering cavity and both the recirculation line  39 ′,  39 ″,  39 ′″ and reinsertion line  40 ′,  40 ″,  40 ′″ located inside the rotating shaft  17 ′,  17 ″,  17 ′″ on  FIGS. 2 ,  3  and  4 , respectively creates a particular fluid flow behaviour inside the sludge recovering cavity  19 ′,  19 ″,  19 ′″. This flowing behaviour guides the sludge from the reinsertion line  40 ′,  40 ″,  40 ′″ to the recirculation line  39 ′,  39 ″,  39 ′″ while also incorporating to the concentrated sludge coming from the reinsertion line  40 ′,  40 ″,  40 ′″ the diluted sludge which accumulates inside sludge recovering cavity  19 ′,  19 ″,  19 ′″ due to sedimentation. In this case, however, the inverted cone  21 ′,  21 ″,  21 ′″ is much preferred as it prevents concentrated sludge coming from the reinsertion line  40 ′,  40 ″,  40 ′″ from back flowing. 
     This particular fluid flow behaviour has been simulated using state of the art computer programs following known principles of fluid mechanics and the result is thus shown on  FIG. 7 . It shows the concentrated sludge downwardly pouring from the reinsertion line  40 ″″″ into the sludge recovering cavity  19 ″″″ and either going straight to the recirculation line  39 ″″″ or being redirected by the inverted cone  21 ″″″ in order to keep this sludge in the cavity  19 ″″″ of the sedimentation zone  16 ″″″. This fluid flow behaviour thus maximizes recirculation of the concentrated sludge, while mixing it with diluted sludge continually depositing due to gravity, through the recirculation apparatus with the inverted cone  21 ″″″ substantially controlling backflow of the concentrated sludge toward the top part of the sedimentation zone  16 ″″″, thus effectively separating the sludge recovering cavity from the top part of the sedimentation zone  16 ″″″. 
       FIGS. 2 ,  3  and  4  essentially show the water treatment facility of  FIG. 1  but with embodiment two of the present invention and different means of controlling the flow throughout the recirculation apparatus. In those embodiments, the rotating shaft  17 ′ of the sedimentation zone  16 ′, which rotates the scraper  20 ′, has a hollow center, allowing the outlet end mouth of reinsertion line  40 ′ from embodiment one to be located inside thereof. This configuration allows for a better integration of the sludge recirculation system of the present invention inside the water treatment facility, necessitating less space to operate. 
     On  FIG. 2 , a suspended solid analyser  41  is installed on line  42 ′ connecting the sand sedimentation chamber  34 ′ and the flow control valve  37 ′, allowing the selective opening of the latter depending on the concentration of solid constituents of the sludge inside the sand sedimentation chamber  34 ′. If this concentration is below a predetermined threshold value, then the suspended solid analyser  41  controls the flow control valve  37 ′ by sending a signal through a communicating means  42 ′, which is in this case a cable. The sludge is then reinserted inside the sludge recovering cavity  19 ′ so it can mix with the diluted sludge that gradually accumulates due to sedimentation. When the concentration exceeds said threshold value, the flow control valve  37 ′ closes and the highly concentrated sludge can exit the sludge recirculation system through the elimination line  35 ′. 
     On  FIG. 3 , a flow meter  43 ″ is installed at the same position as the suspended solid analyser  41  of  FIG. 2  which it replaces. In this case, the flow meter  43 ″ also dictates to the flow control valve  37 ″ whether it should be opened or closed, depending on the predetermined threshold values of flow rates. 
     On  FIG. 4  a timer  44  can replace the suspended solid analyser  41  of  FIG. 2 . In this case, the timer  44  is used to punctually operate the flow control valve  37 ′″, depending on the predetermined time value inputted. 
       FIG. 5  shows the preferred positions for the flow control devices of  FIGS. 2 and 3 , respectively a suspended solid analyser  41  or a flow meter  43 ″. In this case, the superfluous sand sedimentation chamber  34 ″″ has been omitted for clarity of the view. The devices are still used to manage the flow control valve  37 ″″, while the different positions shown each have their particular advantages depending on the intended use of the sludge recirculation system. The flow control device position  45   a  is connected to the overflow outlet line  32 ″″ of the hydro cyclone  30 ″″ and located downstream of the junction  46 ″″ between reinsertion line  40 ″″ and elimination line  35 ″″. At position  45   a , the flow control device effectively changes the configuration of the flow control valve  37 ″″ before the desired concentration crosses the y-junction  46 ″″. This position  45   a  allows the sludge recirculation system to reinsert only sludge with lower concentrations than the threshold value inside the sludge recovering cavity  19 ″″, which in turn allows for a certain time saving. 
     Actually, if the flow control device is located at position  45   b , only sludge with a concentration equal to or above the threshold value will be eliminated from the system, thus assuring a minimum efficiency. However, sludge with a concentration high enough to be eliminated through the elimination line  35 ″″ will be reinserted inside the sludge recovering cavity  19 ″″ because of its position upstream of the y-junction  46 ″″ thus requiring unneeded recirculation of the sludge and in turn more time to treat it. A combination of the two systems  45   a  and  45   b  of flow control devices, however, allows for the qualities of both to be used to maximize the efficiency of the system. A flow control device located at position  45   c  could further be used in combination with either a flow control device at position  45   a  or  45   b  or both  45   a  and  45   b  in order to stop the output of concentrated sludge in the event of a breakdown or failure of the system that could send diluted sludge accidentally toward the elimination line  35 ″″ even though it doesn&#39;t meet the concentration requirements of the predetermined threshold value. 
     The water treatment facility of  FIG. 6  comprises a suspended solid analyser  48  mounted on the input line  47  which brings water to the coagulation zone  10 ″″′ which manages the flow control valve  37 ″″′ of the sludge recirculation system in such a manner that the concentration of the sludge coming out of the sludge recirculation system through the elimination line  40 ″″′ is concentrated enough based on the water input concentration. This system can be used in combination with those of the preceding figures in order to further optimize the efficiency of treatment of the extracted sludge.