Abstract:
A method is provided for introducing a fresh sludge flux whose concentration in terms of dry material is less than 40 g/l into the upstream part of digester, maintaining a thin sludge layer in the digester, circulating substantially horizontally effluents above and across the sludge layer and removing the effluents from the digester simultaneously with soluble products of sludge digestion. The cuvette of the digester is divided into sections by transverse walls, which are provided with communication openings in the lower part thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of copending International Application No. PCT/FR2004/001271 filed on May 21, 2004, which designates the United States and claims the benefit of French Application No. 0307631 filed Jun. 24, 2003. 

   BACKGROUND 
   The invention relates to a method for anaerobic digestion of sludge from the treatment of sewage effluents, an anaerobic digester of primary sludge containing effluents, as well as treatment plant comprising mainly such a digester. 
   Anaerobic digestion of sludge from the treatment of sewage effluents, for example household effluents, consists of hydrolysis and methane fermentation with considerable cell-destroying power, such as to be able to eliminate a considerable amount of the organic material contained in this sludge. 
   The sludge results, for example, from the settling of raw sewage. At the outlet of the digester, digested sludge is obtained on the one hand, together with effluents on the other, these two parts can then be separately subjected to a number of additional treatments. 
   Document U.S. Pat. No. 6,673,243 presents an anaerobic sludge digester comprising a tank divided into several successive compartments by means of transversal walls that do not reach the top of the tank. In the first upstream compartment, at the bottom, the sludge to be treated is inserted through a feeding pipe, the treated effluents being drained off from the final downstream compartment through a discharge pipe. 
   The median transversal wall comprises an opening at the bottom that allows the sludge and effluents to pass from the upstream compartment to the downstream compartment it defines. 
   On the other hand, none of the other transversal walls have a bottom opening. It follows that, on the one hand, the flow of the effluents alternates between rising and falling and, on the other hand, the sludge can build up in a compartment but cannot flow downstream after being digested. The effectiveness of the digester is therefore considerable diminished. 
   The invention aims to solve these problems. 
   SUMMARY 
   For this purpose, according to a first aspect, the invention relates to a method for anaerobic digestion of sludge, which comprises the following steps:
         inserting a flow of primary sludge in the upstream part of an anaerobic sludge digester, the said primary sludge containing effluents having a dry matter concentration of less than 40 g/l;   maintaining a layer of sludge in the digester with a thickness of less than 0.6 m, the sludge being transformed by anaerobic digestion;   making the said effluents circulate within the digester, above and through the layer of sludge, in a substantially horizontal manner at least in the useful part of the digester, from the upstream part towards the downstream part of the digester, and draining off the said effluents from the digester, so as to drain off the soluble products of sludge digestion from the digester;   draining off the sludge from the digester.       

   Thus, from the start of the digestion by hydrolysis of the dry matter, the soluble components formed are extracted from the layer of sludge. In this way, the phenomena of inhibition or at least of slowing down of the digestion of the sludge in the tank are prevented or considerably reduced. The digestion therefore continues to take place at an optimized speed, and the necessary retention of the sludge in the digester is greatly reduced. 
   According to a possible embodiment of the invention, a layer of sludge with a thickness of less than 0.5 m is maintained in the digester. For example, the primary sludge has a dry matter concentration of less than 20 g/l. 
   According to a second aspect, the invention relates to an anaerobic digester of primary sludge containing effluents, with the aim implementing the method as described previously. 
   The digester comprises a tank with a substantially horizontal bottom, a pipe for feeding the primary sludge into the tank, a pipe for discharging the digested sludge from the tank, the said tank comprising at least one wall positioned transversally to the flow of the effluents, the wall defining an upstream and compartment and a downstream compartment, in order for the tank to have a first upstream compartment, into which the pipe for feeding primary sludge flows, and a final downstream compartment, from which the effluent discharge pipe flows. 
   According to the invention, the wall has, at the bottom, an opening to link the upstream compartment with the downstream compartment, so as to allow the passage of the sludge and the circulation of the effluents, above and through the layer of sludge maintained on the bottom of the tank, in a substantially horizontal manner from the first upstream compartment to the final downstream compartment. 
   The transversal wall extends from the bottom of the tank to a height that is less than the height of the tank, so as to allow the floating matter in the effluents to overflow from one compartment to another. 
   For example, the digester comprises at least two transversal walls, each of which has a linking opening at the bottom, such as to define at least three successive compartments in the tank. 
   Finally, according to a third aspect, the invention relates to a sewage effluent treatment plant comprising at least one such digester. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further characteristics of the invention can be seen from the following description of its embodiments, made in reference to the appended figures, in which: 
       FIG. 1  is a schematic section view, along a median longitudinal vertical plane, of a digester according to the invention; 
       FIG. 2  is a schematic perspective view of the digester of  FIG. 1 , seen along the same section; 
       FIG. 3  is a schematic view of a treatment plant according to the invention, comprising mainly a settling tank, a filter or a filtering facility, and an anaerobic sludge digester, also showing the steps in a method for treating the sewage effluent in the said treatment plant; 
       FIG. 4  is a schematic cross-section view of a settling tank in the treatment plant, according to a possible embodiment of the invention; 
       FIG. 5  is a section view, along the line AA, of the  FIG. 6  settling tank in  FIG. 4 ; 
       FIG. 6  is a section view of the settling tank in  FIG. 4 , along the line BB of  FIG. 5 ; 
       FIG. 7  is a schematic longitudinal vertical section view of a filter in the treatment plant, according to a possible embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Referring initially to  FIGS. 1 and 2 , a digester  200  according to the invention, intended for the treatment of primary sludge containing effluents is shown. This sludge is the result of a prior treatment of sewage effluents, for example, resulting from a settling process. 
   The digester  200  comprises a tank  201  with an axis  202 , which is used here to define the term “longitudinal”. The following description is made in relation to a tank  201  with a substantially cylindrical overall shape. However, the tank can also have a substantially rectangular vertical section: it is therefore possible to provide a very wide tank (on the horizontal plane), while keeping a reduced height. 
   The tank  201  is sealed, with the exception of three main openings and one or several inspection traps, as can be seen below, such as to guarantee the absence of oxygen in order to enable the methane fermentation. The tank  201  has a relatively low height (matching the diameter of the cylinder), of between 50 and 70 cm, and length comprised between 2 and 3 m. For example, the tank can have a volume of 250 to 300 1, for example, 270 1. In the case of a tank with a rectangular section, the volume increases according to its width, and can reach several thousand litres. 
   The tank  201  is positioned that the axis  202  is substantially horizontal. It comprises a substantially cylindrical side wall defining a bottom  203  and a top wall  204 , as well as an upstream end wall  205  and a downstream end wall  206 , which are substantially disc-shaped. 
   An opening is made at the top of the upstream wall  205  to allow the primary sludge to enter the digester  200  by means of a feeding pipe. The feeding pipe  207  extends inside the tank  201 , substantially parallel to the axis  202  for a short length, and then vertically downwards for an approximate length of 10 cm, for example, such as to form inside the tank  201  and near the upstream wall  205 , an elbow  208  that opens towards the bottom  203  of the tank  201 . The opening of the feeding pipe  207  may be above or below the level of the effluents in the tank  201 . 
   The downstream wall  206  has, substantially halfway up, an opening that allows the effluents to be drained off from the digester  200 , by means of a discharge pipe  209 . 
   The discharge pipe  209  extends inside the tank  201 , substantially parallel to the axis  202  for a short length, and then vertically downwards for an approximate length of 10 cm, for example, such as to form, inside the tank  201  and near the downstream wall  206 , an elbow  208  that opens towards the bottom  203  of the tank  201 , forming a siphon-shaped inlet. The siphon-shaped inlet is located substantially halfway between the bottom  203  of the tank  201  and the level of the effluents maintained in the tank  201 , so as not to allow the floating matter or the matter resting on the bottom  203  to escape. 
   The top wall  204  of the tank  201  can comprise at least one inspection trap  211 ; three inspection traps in the embodiment of the invention shown. Devices for sampling the gas produced by methane digestion can also be connected to the top wall  204 . 
   Finally, an orifice for draining off digested sludge is made in the bottom  203  of the tank  201 , near the downstream wall  206 , the said orifice being connected to a discharge pipe  212  for the said sludge, with means for cutting off the said discharge pipe  212  being provided. 
   The digester  200  also comprises three transversal walls, respectively upstream  213   a , intermediate  213   b  and downstream  213   c . According to other foreseeable embodiments of the invention, the digester may comprise a different number of transversal walls, for example comprised between 1 and 6, as required in each case. 
   The walls  213   a, b  and  c  have substantially the same shape, namely that of a portion of a disc. The annular contour  214  of the walls matches the shape of the inside of the tank  201 , from the bottom  203  to a height that is lower than the height of the tank, so that each wall  213   a, b  and  c  extends along the entire width of the tank  201 . The walls  213   a, b  and  c  also have a top edge  215  that is substantially flat and horizontal, located at a distance from the bottom  203  of the tank  201  comprised between 40 and 70% of the height of the tank  201 . For example, the height at the walls is somewhere between 20 and 40 cm, mainly 30 cm. 
   A linking opening  216 , substantially disc-shaped, is made at the bottom of each wall  213   a, b  and  c . The opening  216  comprises a substantially annular top edge  217  and extends from the bottom  203  of the tank  201  to a height comprised between 30 and 60% of the height of the transversal wall  213   a, b  and  c . For example, the opening  216  extends to a height of approximately 10 to 15 cm. 
   Each transversal wall  213   a, b  and  c  defines an upstream compartment and a downstream compartment, so that the tank  201  of the digester  200  is divided longitudinally into four compartments  218   a ,  218   b ,  218   c  and  218   d:  
         a first upstream compartment the  218   a , into which the primary sludge feeding pipe  207  flows, for example at the top;   two consecutive intermediate compartments  218   b ,  218   c;      a final downstream compartment  218   d , from which the effluent discharge pipe  209  flows.       

   The walls  213   a, b  and  c  make it possible to keep the sludge formed by agglomeration of the suspended matter in each compartment  218   a  to  218   d , while the excess floating matter is able to pass by overflowing, from an upstream, compartment to a downstream compartment, or vice-versa. 
   The axial lengths of the compartments can be similar or, as an alternative, different. In the example shown, the first three compartments  218   a, b  and  c  have substantially identical lengths, while the final downstream compartment  218  is shorter, approximately half the length of the other compartments. 
   The number of compartments and the length of the digester can be reduced or, on the contrary, increased to refine the treatment. 
   The following is a description of the operation of the digester  200 . 
   The primary sludge containing effluents is inserted in the tank  201  of the digester  200  by means of the feeding pipe. 
   “Primary sludge” is used here to refer to sludge that has a relatively low concentration of dry matter, lower than 25 g/l, or even lower than 20 g/l. It is possible particularly to insert sludge with a concentration comprised between 5 and 10 g/l into the digester. A lower concentration is possible, but the process becomes less cost-effective (larger digester volume, larger pumping flows, etc.). 
   This sludge can be the result of a settling process, and thus still obviously contain effluents. As an alternative, it can be previously concentrated sludge (dry matter concentration of 10 to 15%), mainly in order to facilitate transporting it from one treatment site to another. 
   In this case, the concentrated sludge is diluted with diluting effluents—for example, settled effluents that result from settling sewage effluents—before being inserted in the digester, so as to obtain a dry matter concentration that is compatible with the process. 
   The sludge settles on the bottom  203  of the tank  201  in the form of a thin stable layer, for example, less than 0.5 m, or even less than 0.3 m. According to a specific embodiment, the thickness of the layer of sludge can be less than 0.2 m. A level  219  of effluents is maintained in the tank  201 , as shown in  FIGS. 1 and 2 . The level  219  of effluents is substantially at the same height as the top end of the same as discharge pipe  209  and the top edge  215  of the walls  213   a, b  and  c.    
   The sludge is gradually broken down inside the digester  200 . As it is digested, the sludge liquefies and passes into the consecutive compartments  218   a  to  218   d , through the linking openings  216  made in the transversal walls  213   a  to  c.    
   As regards the effluents, they flow substantially parallel to the axis  202 , at least in the useful part of the tank  201 , in other words, excluding the feeding and draining areas, where the effluents respectively flow in a locally downward direction from the feeding pipe  207  towards the bottom  203  of the tank and in an upward direction from the bottom  203  of the tank towards the discharge pipe  209 . 
   The effluents therefore circulate substantially horizontally through the openings  216  between the bottom  203  of the tank  201  and the top edge  217  of the said openings  216 , and do so from the first upstream compartment  218   a  towards the final downstream compartment  218   d . During this flow, the effluents circulate above and through the layer of sludge. In this way, on the one hand, they carry with them the soluble products that result from the digestion of the sludge and, on the other hand, they force the sludge to move towards the compartments that are further downstream. 
   The effluents and the soluble products are then drained off from the tank  201  through the discharge pipe  209  and transported, as required, to another treatment unit. 
   The digested sludge contained in the final downstream compartment  218   d  is drained off through the discharge pipe  212 , through the simple action of gravity or with the help of a suitable pumping device. The emptying processes can be either partial or complete, and either regular (periodic emptying of the digester, or consisting of periodic or continuous emptying, daily for example. As an alternative, the tank  201  is unequipped with the sludge discharge pipe, the sludge therefore being temporarily stored in the digester  200  and periodically drained off via the traps  211 . 
   The digester according to the invention makes it possible to eliminate by liquefaction or gasification at least 40% of the dry matter in the sludge and up to over 90%, and to do so with a retention of the sludge in the digester comprised between 5 and 15 days, without needing to heat the digester, as long as its temperature remains above 10.C. Shorter retention is possible if the digester is heated (for example to a temperature comprised between 20 and 35° C.) since the performance of the digester is improved. 
   In these conditions, the concentration of the digested sludge is greatly facilitated, since only 50% to under 10% of sludge remains. 
   This can be compared with the methods of the previous technique, in which the sludge is concentrated prior to entering the digester. Its retention, in the form of a relatively thick layer is generally comprised between 12 and 40 days, the digester must be heated to between 25 and 35° C., and all this to obtain elimination of around 30% of the dry matter in the sludge. 
   Next is a description of a sewage effluent treatment plant  46 , made in reference to  FIG. 3 . 
   The raw sewage is passed through a feeding pipe  107  into a primary settling tank  100 , such as to be rid of a considerable part of its suspended matter. 
   The primary settling tank  100  comprises a discharge pipe  109  for the settled effluents connected, at the inlet of the effluents, to a filter  1  or a filtering facility  34  comprising several filters  1  operating in parallel, by means of a feeding pipe  4 , and means for draining off the sludge that results from settling. 
   The sludge that results from the primary settling is drained off towards the digester via draining means, such as a discharge pipe  111 , connected to the pipe  207  that feeds primary sludge into the digester  200 . 
   The effluent discharge pipe  209  is connected to the feeding inlet of effluents to be settled in the primary settling tank  100 . The evacuation outlet of the digested sludge is connected, via the discharge pipe  212  to the inlet of a sludge concentrator  48 . 
   The sludge concentrator  48  comprises a wastewater outlet  49  connected to the feeding inlet of effluents to be settled in the primary settling tank  100 , possibly via the pipe  209  for discharging the effluents from the digester  200 . 
   The sludge concentrator  48  also comprises a concentrated sludge outlet  50 . This sludge can be suitably conditioned and treated subsequently with a view to being used for the purpose of enriching soil. It can also be sent to a dump or to an incineration unit. 
   The effluents filtered by the filter  1 , or the filtering facility  34 , are drained off through a discharge pipe  15 . If the filter  1  is able to retain the suspended matter contained in the effluents to be filtered, the filtered effluents are directly rejected. In the opposite case, the filtered effluents are directed towards the inlet of a secondary settling tank  51 . 
   The filter  1  also comprises drawing-off outlets that allow the biomass to be drained off through drawing-off pipes  24 , which may be connected either to the inlet to the secondary settling tank  51  or, when the plant is not equipped with a secondary settling tank, to the feeding inlet of effluents to be settled in the primary settling tank  100  or to the feeding inlet of primary sludge to the digester  200 . 
   The secondary settling tank  51  comprises means for draining off the sludge that results from settling, connected either to the feeding inlet of effluent to be settled in the primary settling tank  100 , via a discharge pipe  52 , which may flow into the pipe  209 , or to the feeding inlet of primary sludge to the digester  200 . A part of this sludge can also be sent back to the top of the filter  1  so as to maintain the biomass at a desired level. 
   In addition, the secondary settling tank  51  comprises a discharge pipe  53  for the settled effluent. 
   In a possible embodiment of the invention, shown in  FIGS. 4 to 6 , the primary settling tank  100  and/or the secondary setting tank  51  comprise, according to a general definition:
         a tank  101  with a bottom  103  and which has, in relation to the direction of flow of the effluents, an upstream part into which a sewage effluent feeding pipe  107  flows and a downstream part from which the settled effluent discharge pipe  109  flows.   a settling surface arranged in the tank  101 , formed by the top face of at least one settling panel  114 ,  114 ′, the said panel having a medium plane that is substantially parallel to the direction of flow of the effluents and tilted, according to a plane that is a transverse to the flow of the effluents, and in relation to the orthogonal projection of the vertical of the said transverse plane, at an angle.       

   A first assembly of at least one settling panel  114  is tilted at a first angle α of between approximately 15° and 60°, and at least one second assembly of at least one settling panel  114 ′ is tilted at a second angle β of between approximately 15° and 60°, the angles α, β, the surface finish and the friction coefficient of the panels being chosen so that, when the effluents flow into the tank, the sludge settles on the settling surface and then slides towards the bottom of the tank, at least one draining passage  117  for the sludge being provided between the panels of the two assemblies, so as to allow the sludge collected on the top faces of the panels to fall through the action of gravity towards the bottom  103  of the tank  101 . 
   The tank  101  is substantially cylindrical, buried so that the axis  102  is horizontal, and. sealed with the exception of four openings:
         an opening for the inlet of effluents to be settled, made in the upstream end wall  105 , the feeding pipe  107  extending into the tank by means of an elbow  108  that is open towards the upstream direction;   an opening for discharging the settled effluents, made in the downstream end wall  106 , a siphon-shaped wall  119  equipped with an opening  120  being provided so as to keep the floating matter inside the tank;   an inspection trap  110  in the top wall  104 ;   possibly, an orifice for discharging the sludge that results from settling, connected to a discharge pipe  111 , cut-off means  112  also being provided. As an alternative, the evacuation means can comprise force pumps submerged in the tank  101  or suction pipes that rise back up to the top of the tank  101  and are connected to a suction pump.       

   The tank comprises assemblies of two panels, arranged symmetrically in relation the vertical, longitudinal, median plane  113  of the tank. 
   The first such assembly (on the left) comprises five panels  114   a  to  114   e  that are substantially parallel, vertically stacked and separated from each other by a substantially constant distance L 1  of approximately 30 cm. The panels  114   a  to  114   e  are tilted from top to bottom from the substantially vertical side wall of the tank towards the vertical plane  113 , at an angle of approximately 45 20  . 
   The panels  114  have different widths  1  depending on their distance from the bottom  103  of the tank, but lengths L (parallel to the flow of the effluents) that are substantially identical. The panels  114  are separated from the vertical side wall of the tank  101  by a horizontal distance L 2  of approximately 10 cm, for the suspended matter to pass in between them, and from the bottom  103  by a vertical height H of almost 30 cm, so as to create a space  115  for the accumulation of the sludge that results from settling. Finally, the top end of the panels is located at a distance d above the level  116  of the effluents inside the tank. 
   The second assembly (on the right) is symmetrical to the first in relation to the vertical plane  113 , and comprises five panels  114 ′ a  to  114 ′ e . The horizontal separation L 3  between the first and second panel assemblies is around 30 cm, so as to create a space  117  that allows the sludge collected in the panels to fall towards the bottom, being directed and collected in the central area of the bottom  103  of the tank  101 . 
   Support and attachment means  118  of the panels  114 ,  114 ′ to the tank  101  are placed in the proximity of the upstream and downstream ends of the said panels. 
   According to a possible embodiment of the invention, the settling tank  100  comprises, in relation to the flowing direction of the effluents, at least a first and second series of panels  114 ,  114 ′, the second series being located downstream from the first series, the said series each comprising at least a first and second assembly made up of at least one panel, so as to further improve the extraction of the sludge. 
   In a possible embodiment of the invention, shown in  FIG. 7 , the filter  1  comprises, according to a general definition, a reaction chamber  2  in which the flow of effluents to be filtered is arranged so as to flow from the bottom up, the said reaction chamber  2  comprising:
         at the bottom, an inlet  3  for effluents to be filtered and an inlet  6  for oxygenated gas;   and, at the top, an outlet  14  for the filtered effluents;   filtering means  18  comprising layers of particles of a solid material forming supports and a biomass stuck to the surface of the said supports, the said filtering means  18  having a density that is lower than the density of the effluents to be filtered and being placed between the inlet  3  of the effluents to be filtered and the outlet  14  of the filtered effluents.       

   The reaction chamber  2  is subdivided into at least three stacked compartments forming levels in between at least two walls  19   a ,  19   b ,  19   c  provided with openings, the said openings being disposed such as to retain the filtering means  18 , so as to create the following inside the reaction chamber:
         at least two filtering levels  20   a ,  20   b ,  20   c;      and a top outlet level  17 , the outlet  14  of the filtered effluents flowing from the top outlet level  17 .       

   Each filtering level  20   a ,  20   b ,  20   c  is provided with a layer of its own filtering means  18  and comprises, at the bottom, an outlet  22   a ,  22   b ,  22   c  for drawing off the excess biomass, the amount and the density of the filtering means in each filtering level  20   a ,  20   b ,  20   c  being such that, in drawing-off mode, the bottom part of at least the bottom level or levels  20   a ,  20   b  from which the drawing-off outlet  22   a ,  22   b  flows, is unequipped with filtering means, so as to enable the recovery of the excess biomass. 
   The reaction chamber  2  defines a main axis X with a vertical orientation. The inlet  3  of the effluents to be filtered is connected to a feeding pipe  4  provided with a gate  5 , and the oxygenated gas inlet  6  is connected to a feeding pipe  7  provided with a gate  8 . According to an embodiment of the invention, the two inlets  3 ,  6  are made in the bottom wall  9  of the reaction chamber  2  and flow into an inlet compartment  10  delimited by a part of the side wall  11  of the reaction chamber and by a top inner wall  12  that is permeable to the effluents and to the oxygenated gas. 
   The outlet  14  of the filtered effluents is connected to a discharge pipe  15  provided with a gate  16 , and flows from a top outlet level  17  of the reaction chamber. 
   The reaction chamber  2  is subdivided into stacked compartments forming filtering levels by means of grilles  19   a ,  19   b ,  19   c , the top outlet level  17 , formed between the top wall  21  of the reaction chamber and the top wall  19   c  placed opposite, being unequipped with filtering means. 
   Each filtering level  20   a ,  20   b ,  20   c  comprises at the bottom an outlet  22   a ,  22   b ,  22   c  for drawing off the excess biomass, connected to a drawing-off pipe  24  equipped with a gate  25 . A compartment  23   a ,  23   b ,  23   c  may be provided for recovering the said excess biomass. 
   The filtering means  18  are designed to float, at least in certain compartments, in drawing-off mode, such as to form a free bottom space  31  unequipped with filtering means. The grilles  19   a ,  19   b ,  19   c  make it possible to retain the filtering means  18 . A device  26  for detecting the amount of excess biomass can also be provided. 
   The filter  1  can comprise means  27  for returning the filtered effluents, comprising a pipe  28  connected to a pump  29 , for additional treatment of at least part of the filtered effluents. 
   During the filtering phase, the effluents and the gas circulate in an upward direction through the various filtering levels. The filtered effluents are drained off through the pipe  15  and the biomass remains on the supports. The feeding of effluents and/or gas can be continuous or intermittent. 
   During the washing phase, the excess biomass of the chosen level is recovered via the relevant drawing-off pipe  24  by means of a downward flow of the effluents contained in the reaction chamber  2 .