FR2815957A1 - Liquid effluent treatment procedure with recycling of biomass uses two-stage digestion with circulation loop and separation stage - Google Patents

Abstract

The treatment procedure for a liquid e.g. water-based effluent containing a fermentable organic charge uses an e.g. bacterial biomass (1) fed into an input flow (2) of the effluent, and extracting from the biomass a water-based output flow, poor or rich in the biomass. The procedure uses two aerobic or anaerobic digestion stages: one (A) in which the input flow (2) is mixed with a flow (4) of recycled liquid rich in the biomass to produce a liquid flow (51) poor in organic charge, and a residual gas flow (61) separated from it, and a final stage (Z) in which a liquid circulation loop (7) is established, the latter being enriched with biomass in a separation stage (8). The separation stage, using permeation or centrifugal decantation produces a flow (3) of water poor in or exempt of biomass, and a flow of recycled liquid (4) rich in biomass. A residual gas, e.g. methane, is removed in intermediate stages (B, C).

Description

<Desc / Clms Page number 1>

 The present invention relates to the treatment of a liquid effluent, namely an aqueous effluent, comprising an organic filler, or fermentable substrate, with a view to obtaining purified water, relatively poor or depleted in said organic filler, for example water practically free of organic and biomass load.

 More particularly, the invention relates to treatment methods according to which the organic load is digested with a biomass, for example bacterial, and this aerobically or anaerobically, for example with an activated sludge in the case of '' an aerobic fermentation.

 By "biomass" is meant the mass of cellular organic material, the growth of which is supplied by the fermentable organic charge.

 For example, in a process as defined above, the biomass contained in a digester is supplied with an input current obtained from the liquid effluent, and at least one is withdrawn from said biomass, and therefore from the digester. aqueous output current, depleted or enriched in biomass.

 In order to bring the organic charge to ferment or digest, or fresh substrate, into contact with biomass of good biological quality, it is known to recycle part of the biomass at the inlet of the digester, which mixes and enters into contact with the input current.

 As the recycled biomass is suspended in a relatively large amount of water, and therefore diluted, this requires reintroducing a large amount of water into the digester, which requires, on the one hand, having a large internal volume, all other things being equal, and on the other hand, to put large liquid flow rates into forced circulation.

 The present invention therefore aims to remedy this drawback.

 More specifically, the subject of the present invention is a solution making it possible to reduce the useful volume of any digester, without however affecting the biodegrading power of the biomass present within said digester, and this for the same input current to the digester.

 According to the invention, for this purpose, digestion is carried out in at least two successive digestion steps (A, Z), namely:

<Desc / Clms Page number 2>

a) une première étape, au cours de laquelle on mélange le courant d'entrée avec un flux liquide recyclé, enrichi en biomasse, et on obtient un flux liquide appauvri en charge organique, et un flux gazeux résiduel séparé dudit flux liquide ; b) une dernière étape, au cours de laquelle on établit une boucle de circulation d'un flux liquide recirculé, on enrichit ce dernier en biomasse dans un étage de séparation, on alimente la boucle de circulation, en amont de l'étage de séparation, avec le flux liquide appauvri en charge organique provenant de l'étape précédente, et on extrait de ladite boucle de circulation, d'une part, grâce à l'étage de séparation, un courant de sortie aqueux, appauvri en biomasse, et d'autre part, en aval de l'étage de séparation, ledit flux liquide recyclé, enrichi en biomasse.

a) a first step, during which the input stream is mixed with a recycled liquid stream, enriched in biomass, and a liquid stream depleted in organic charge is obtained, and a residual gas stream separated from said liquid stream; b) a final step, during which a circulation loop of a recirculated liquid flow is established, the latter is enriched in biomass in a separation stage, the circulation loop is supplied, upstream of the separation stage , with the liquid flow depleted in organic charge coming from the preceding stage, and from said circulation loop, on the one hand, thanks to the separation stage, an aqueous output current, depleted in biomass, and d on the other hand, downstream of the separation stage, said recycled liquid stream, enriched in biomass.

 Thanks to the invention, the recycled liquid stream at the inlet of the digester is concentrated or enriched in biomass, and correspondingly the amount of water reintroduced into the digester is reduced.

 If necessary, another aqueous output stream enriched in biomass is extracted from the circulation loop, the quantity of which is determined for example to deplete or deconcentrate the biomass in one or more pollutants, for example of the heavy metal type.

 Thanks to the invention, the aqueous volume reintroduced at the inlet of the digester is limited, per unit of quantity of recycled biomass, without modifying the biological residence time of the biomass within the reactor. As a result, the hydraulic residence time is essentially determined by the quantity or flow rate of the input current, which makes it possible to size the digester according to a much smaller useful volume, all other things being equal.

The solution according to the invention also provides the following essential advantage:
In the recirculation loop associated with the last digestion step, the biomass is relatively deprived of fermentable substrate, compared to the first digestion step.

Correlatively, in order to feed, the biomass partially self-destructs, which has two consequences:

<Desc / Clms Page number 3>

3 - on the one hand, the quantity of biomass produced decreases significantly; ultimately, it can be envisaged to evacuate a small quantity of biomass from the digester, for example sufficient to deconcentrate the biomass into certain pollutants, such as phosphorus and heavy metals; - on the other hand, the biomass surviving at the end of the observed phenomenon of self-destruction has a much better bacteriological quality.

It also results therefrom that the recycled biomass at the entrance to the digester is of much better quality than that normally reintroduced in contact with the fresh substrate to be fermented or digested.

 The present invention is now described with reference to the accompanying drawing, the single figure of which represents a digester according to the invention, associated with an external separation stage, and this schematically.

 In general, the digester 13 shown and associated with an external stage 8 of separation, makes it possible to treat a liquid effluent, namely aqueous, comprising an organic charge, or fermentable substrate, the quantity or concentration of which is expressed in biochemical oxygen demand (DB 0) and / or chemical oxygen demand (COD), in order to obtain, via line 3, purified water with an organic charge, having a COD much lower than that of the liquid effluent. This treatment takes place by digestion of the organic load, or substrate, in the digester 13, with a biomass, or activated sludge, aerobically or anaerobically, depending on the bacteria used for the purposes of digestion. An excess biomass is thus obtained, which is recycled and / or removed from the digester, via the conduit 10 on the circulation loop 7 described below.

 In all of the description below, by liquid)) is meant any current, flow or other static or moving volume, of an aqueous medium, in which the biomass is suspended, in a more or less concentrated manner, without however prevent the ability of said aqueous medium to be pumped with traditional means, or generally put into forced circulation.

 The digester 13 shown comprises: - an outer wall 15, having the shape of a cylindrical shell, and delimiting a chamber 16 for digestion;

<Desc / Clms Page number 4>

this wall 16 comprises a lower part 15b, or bottom, as well as an upper part 15a, or roof, of frustoconical shape; apart from the inputs and outputs described below, the digestion chamber 16 is sealed vis-à-vis the outside, with respect to the gas and liquids circulating therein; at least three, and for example in this case five continuous interior walls, namely five interior ferrules 21 to
25, arranged concentrically (except 21) one inside the other, and inside the outer shroud 15, so that each said shroud is inside an outer shroud; these continuous interior walls determine together and between them, at least three annular or cylindrical compartments, and in this case and for example five compartments, namely respectively
Y, A, B, C and Z whose functions will be described below; the central ferrule 25 has the shape of a pot with a frustoconical bottom; an inlet 17 for a stream 2 obtained from the liquid effluent to be treated, communicating with the base of the buffer compartment Y described below; an outlet 18 for an aqueous current, depleted in biomass, communicating with the base of the central pot delimited by the ferrule 25; an inlet 27 for an aqueous stream enriched in biomass, communicating with the upper part of the aforementioned central pot; an orifice 29 for venting, compartments C and Z only, disposed on the upper part 15a, or roof of the digester; an outlet 28 for a residual gas flow, or biogas, disposed on the aforementioned roof of the digester for the compartments Y, A and B only; a pipe 31 for overflow of a liquid stream enriched in biomass; means 14 of forced circulation, consisting of a pump, the suction of which communicates with the buffer compartment Y described below, and the discharge of which communicates with the bottom of the first digestion compartment A described below; the means 14 of forced circulation make it possible to circulate the same liquid flow in the process of enrichment in biomass, within the digestion chamber 16,

<Desc / Clms Page number 5>

 vertically and alternately from top to bottom and from bottom to top.

It follows from the above description that the digester comprises at least three digestion compartments arranged in series, in this case and for example: a first compartment A delimited between the ferrules 15 and
23, in which the liquid flow circulates from bottom to top a first intermediate compartment B, delimited between the ferrules 23 and 24, in which the liquid flow circulates from top to bottom; and a second intermediate compartment C in which the liquid flow circulates from bottom to top; - and a last compartment Z belonging to the recirculation loop 7 described below, and delimited by the central pot defined by the central ferrule 25.

 Each compartment described above therefore forms a reservoir for alternating storage and circulation of the liquid stream being enriched in biomass. These compartments are closed together and in common by the upper part 15a, or roof, of the wall 15 of the digester. In the direction of circulation of the aforementioned liquid flow, and thanks to the means 14 of forced circulation, a given compartment communicates, on the one hand with the following compartment, by means of a weir, for example 191 between the first compartment A and the first intermediate compartment B, and 192 between the second intermediate compartment C and the last compartment Z, and on the other hand, with the preceding compartment, by a passage in the partition separating the given compartment from the preceding compartment, for example by a passage 20 disposed at the bottom of the shell 24 forming a partition between the compartments B and C.

obtenu à partir de l'effluent, et d'autre part avec le refoulement de la pompe 14, assure une fonction de réservoir tampon. The compartment Y disposed inside the shell 21, communicating at its base, on the one hand with the inlet 2 for the liquid stream

obtained from the effluent, and on the other hand with the discharge of the pump 14, provides a buffer tank function.

 The last digestion compartment Z communicates, on the one hand by an outlet 18 with the inlet 26 of a stage 8 of separation, which makes it possible to enrich in biomass a recycled liquid stream described below, and on the other hand by an entrance 27 with the exit of the same separation stage.

This separation stage 8 can work by permeation or centrifugation,

<Desc / Clms Page number 6>

 according to the equipment installed in said floor. Another means of forced circulation, or pump 15, makes it possible to recirculate the liquid flow in the circulation loop 7, in which are included and the central pot determined by the ferrule 25, and the separation stage 8, as well as the pump 15.

A recycling duct 4 connects the outlet of the separation stage 8 and the aspiration of the means 14 of forced circulation by a branch, which also communicates with the inlet 2 for the inlet current obtained from the effluent. liquid, through the bottom of the Y buffer compartment
The last digestion compartment Z and / or the penultimate digestion compartment C are arranged relative to the upper part of the wall 15 of the digester, or its roof, to form a capacity 12, closed by a removable cover, for collecting of a residual gas stream 62, or biogas. This capacity 12 communicates freely through the orifice 29 with the outside of the digester 13, being separated by a watertight partition 30, formed by an extension of the shell 24 upwards, with respect to another collection capacity 11 of another residual gas flow 61, or biogas. This other capacity 11 is common to the other compartments A, B, Y, and is formed or delimited with respect to the roof 15a of the digester. As shown in the appended figure, the level of discharge in the last compartment Z and / or the penultimate compartment C of digestion is greater than the level of discharge in a said other digestion compartment, for example from compartment A to compartment B.

 The compartment Z is supplied with a liquid stream depleted in organic load by spillage 192 above the partition 25.

 The pipe 31 for discharging the liquid flow enriched in biomass is arranged to communicate with the last digestion compartment Z, when the liquid flow in this compartment passes above the level chosen for the liquid reserve in compartment 31.

 The operation of the previously described digester, in association with the separation stage 8, is deduced from the preceding description.

 The fermentable organic load is digested in at least two successive stages, and in this case and by way of example in four successive digestion stages A, B, C, Z, namely:

<Desc / Clms Page number 7>

a first step A during which, thanks to the circulation means 14, an input stream 2 obtained from the liquid effluent is mixed with a liquid stream recycled through the conduit 4, enriched in biomass, and one obtains a liquid flow
51 depleted in organic load, and a residual gas flow
61, or biogas, separated from the liquid flow in the capacity 11; at least two intermediate stages B and C of digestion, and / or degassing between the first stage of digestion A and the last stage Z of digestion, described below; at the start or at the end of each intermediate digestion step, a liquid stream, for example 51 or 52, comprising the biomass, is run in contact with a gaseous atmosphere contained in a capacity, for example 11 or 12, by means of what is separated in each said capacity a residual gas stream, for example 61 or 62, or biogas, enriched in non-fermentable compounds, for example methane; a last fermentation step Z, during which a previously described circulation loop 7 is established, making it possible to recirculate a liquid stream, said recirculated liquid is enriched in biomass in the separation stage 8; the circulation loop 7, upstream of the separation stage 8, is supplied with the liquid stream 52 depleted in organic load coming from the preceding digestion step C, and the circulation loop 7 is extracted from a on the one hand, thanks to the separation stage 8, an aqueous outlet stream 3, depleted in biomass, or clear juice, and on the other hand, downstream of the separation stage 8, a liquid stream recycled through the conduit
4 enriched, in biomass, and a stream 10 also enriched in biomass (or excess sludge produced) corresponding to the deconcentration purge.

 If necessary, an aqueous outlet stream 9 enriched in biomass is extracted from the circulation loop 7, by the discharge pipe 31 or overflow.

 As already indicated, in stage 8, the separation is carried out either by permeation or by centrifugal decantation.

<Desc / Clms Page number 8>

 Any excess of the input current 2 relative to the output current 3, depleted in biomass, generates a liquid overflow flow, derived from the liquid input current 2, in compartment Z.

 Depending on the case, all or part of the digestion, in compartments A, B, C and Z is carried out aerobically or anaerobically.

 Not shown, if necessary, during each digestion step A, B, C, except during the last digestion step Z, part of the output liquid stream 51 or 72 is extracted, depleted in organic load, obtained at term of the digestion step considered, and this part is recycled and mixed with the input liquid stream, enriched in organic load, introduced during the same digestion step considered.

Claims (5)

 CLAIMS 1. Method for treating a liquid effluent comprising a fermentable organic charge, according to which the latter is digested with a biomass (1), by supplying said biomass with an input current (2) obtained from liquid effluent, and by drawing from said biomass at least one aqueous outlet stream (3), depleted or enriched in biomass, characterized in that digestion is carried out in at least two successive digestion steps (A, Z), namely: a) a first step (A), during which the input stream (2) is mixed with a recycled liquid stream (4) enriched in biomass, and a liquid stream is obtained (51 ) depleted in organic load, and a residual gas flow (61) separated from said liquid flow; b) a last step (Z), during which a loop (7) for the circulation of a recirculated liquid flow is established, the latter is enriched with biomass in a separation stage (8), the loop (7) is fed ) of circulation, upstream of the stage (8) of separation, with the liquid flow (52) depleted in organic load coming from the preceding stage (C), and extract from said circulation loop (7), d on the one hand, thanks to the separation stage (8), an aqueous outlet current (3), depleted, or even free of biomass, and on the other hand, downstream of the separation stage (8), said stream (4) recycled liquid, enriched in biomass.  2. Method according to claim 1, characterized in that extracts from the circulation loop (7), another stream (10) of aqueous outlet enriched in biomass, the amount of which is determined for example to deplete the biomass in a or several pollutants, for example of the heavy metal type.  3. Method according to claim 1, characterized in that, in the separation stage (8), said separation is carried out by permeation or centrifugal decantation.  4. Method according to claim 1, characterized in that at least one intermediate stage (B, C) of digestion exists between the first (A) and the last (Z) stages of digestion, at the beginning or at the end of which is made to flow a liquid stream (51,52) comprising the biomass in contact <Desc / Clms Page number 10>  a gaseous atmosphere (11,12), whereby a residual gas flow (61,62), for example methane, is separated.  5. Method according to claim 1, characterized in that the digestion is carried out aerobically or anaerobically.