Sewage treatment plant digester

Abstract

Sewage treatment plant digester Sewage treatment plant digester (1) comprising: A tank (10) arranged to receive organic matter, comprising a skirt (15), and a base (11), a dome (20) resting on the skirt (15) to close the tank (10) in the upper part, this dome (20) comprising a plurality of sectors (23) prefabricated from cementitious material, at least one of said sectors (23) comprising at least one preferential deformation zone (27) near the interface (21) with the skirt (15), allowing at least a part of this sector (23) to rise outwards under the effect of accidental overpressure in the digester. Figure for the abstract: Fig. 2

Description

Title of the invention: Sewage treatment plant digester Technical field

[0001] The present invention relates to digesters for wastewater treatment plants.

[0002] The digesters of wastewater treatment plants are enclosures where flammable gases from the digestion of organic matter are generated.

[0003] The consequences of an accidental explosion within the enclosure must be controlled.

[0004] Today, regulations require compliance with danger circles corresponding to authorized pressure levels depending on the distance from the center of the digester, the site of the explosion.

[0005] Consequently, the design of the structure must limit the external pressure levels resulting from the explosion generated by the quantity of gas ejected, while respecting the danger circles defined by the regulations. The design of the structure must also limit, or even avoid, the projection of debris that could injure site personnel.

[0006] The domes of some reinforced concrete digesters may no longer comply with changing regulations. Indeed, during an explosion, the pressure inside the digester increases very rapidly. The tensile forces in the dome are taken up by the reinforcement of the latter. This can withstand up to a pressure inside the digester typically of the order of 250 to 300 mbar, known as "rupture pressure". From this pressure, the digester ruptures and the secondary release of a significant quantity of gas causes a secondary explosion generating pressures around the digester that are too high to comply with the regulations.

[0007] The danger circle calculated from this burst pressure is all the greater as the burst pressure is high. To comply with the new regulations, it is therefore essential to reduce the burst pressure to reduce the danger circle.

[0008] In order to achieve such a reduction, it is known to use commercially available explosion vents, having a burst pressure of between 100 and 150 mbar.

[0009] However, this solution is not entirely satisfactory.

[0010] First of all, it requires the use of a large number of vents. This can result in sealing and maintenance problems. In addition, it involves complicated structures, particularly with raised beams, which can be expensive. Finally, the risk of gas leaks is increased. Statement of the invention

[0011] There is therefore a need to improve wastewater treatment plant digesters, particularly in order to reduce the diameter of the danger circle while minimizing maintenance and gas leakage problems, and having a structure that is relatively simple to manufacture. Summary of the invention

[0012] The invention aims to meet all or part of this need and has as its subject a wastewater treatment plant digester comprising:

[0013] - a tank arranged to receive organic materials, comprising • a skirt, and • a raft - a dome resting on the skirt to close the tank in the upper part, this dome comprising a plurality of prefabricated sectors made of cementitious material, at least one of said sectors having at least one preferential deformation zone near the interface with the skirt, allowing at least part of this sector to lift outwards under the effect of accidental overpressure in the digester.

[0014] In normal conditions, the dome of the invention can withstand the service loads of the digester just as well as a conventional dome.

[0015] In an accidental situation, that is to say when an explosion occurs, due on the one hand to the non-continuity of the cementitious material of the dome, and on the other hand to the presence of the aforementioned preferential deformation zone, the dome can open at least at the level of one sector, thus allowing part of the overpressure gas to escape. The invention thus makes it possible to reduce the quantity of ejected gas, called secondary cloud, escaping when the dome of the digester ruptures. Also, the digester according to the invention has the advantage of implementing a simple structure which makes it possible to reduce the circles of danger compared to a conventional digester.

[0016] By "accidental overpressure" is meant a pressure inside the digester equal to or greater than the gravity pressure acting on the dome.

[0017] By "service loads" is meant all the actions acting on the digester in normal conditions. The service loads may correspond to the forces exerted due to the self-weight of the digester dome, to the operating loads, i.e. the loads coming from the use of the digester, to the climatic loads, in particular linked to wind and / or snow, and / or to the thermal loads, linked to the expansion of the materials.

[0018] By "longitudinal axis of the sector" is meant the line joining all the barycenters of the cross-sections of the sector. Preferably, the dome has a general shape of a spherical cap or any other shape promoting the appearance of a vault effect, for example conical, truncated, ellipsoidal or prismatic. Sectors

[0019] The sectors can each correspond to a single-block unit element made of cementitious material.

[0020] Preferably, the sectors are assembled with a dry seal at their interface.

[0021] Advantageously, the sectors are embedded at their base in the skirt.

[0022] The sectors are preferably made of reinforced concrete, presenting a continuity of reinforcements.

[0023] Preferably, a plurality, in particular all of the sectors, is produced with at least one preferential deformation zone allowing at least part of the sector to lift outwards under the effect of accidental overpressure in the digester.

[0024] Preferably, the dome comprises alternating sectors of different widths. This makes it possible to have sectors of lesser width, which are lighter and therefore more capable of lifting in the event of accidental overpressure. The sectors of greater width may be formed from a single element or from several rigidly assembled elements.

[0025] The width of a sector is measured along an axis perpendicular to the longitudinal axis of the sector, following a meridian. It corresponds to the distance separating the longitudinal edges of the sector.

[0026] The sectors of smaller width can each have an angular extent of less than 30°, better still less than 20°, for example of the order of 15° (+ / - 5°).

[0027] The sectors of greater width preferably have an angular extent of less than 75°, better still less than 60°, for example of the order of 45° (+ / - 5°).

[0028] The dimensions of the sectors advantageously depend on the radius of the digester. For example, the sectors may each have a length of between 5 m and 20 m, better still between 5 and 15 m, even better still between 5 and 10 m. The length of the sector is measured along its longitudinal axis, following a meridian.

[0029] The sectors may have a thickness, measured at their base, of between 20 cm and 50 cm. The thickness of the sectors is preferably chosen as a function of the internal pressure. Preferential deformation zone

[0030] By "preferential deformation zone" is meant a deformable zone, preferably plastically, under the action of forces generated by accidental overpressure inside the digester. The preferential deformation zone makes it possible to control the opening pressure and the quantity of gas released into the secondary cloud, locate areas of rupture of the cementitious material, thus limiting uncontrolled ruptures of the cementitious material in the event of an explosion. This makes it possible in particular to limit, or even avoid, the ruin of the skirt and the uncontrolled projection of the cementitious material from the dome.

[0031] The preferential deformation zone may comprise at least one engraving in the cementitious material, in particular an engraving opening onto the upper face of the sector, and the depth of which preferably represents at least 25% of the maximum thickness of the sector, better still at least 30% of the maximum thickness of the sector. The presence of the engraving locally reduces the resistance of the sector to mechanical stresses, in particular bending and shearing.

[0032] Alternatively or additionally, the preferential deformation zone comprises a lower density of reinforcements and / or reinforcements of lower mechanical strength. For example, the reinforcements used in the preferential deformation zone are of smaller diameter.

[0033] The preferential deformation zone may be located at a distance from the skirt which corresponds to less than one fifth of the length of the sector. During an explosion, the deformation of the sector has little impact, or even no impact, on the skirt. Since the dome behaves like a fuse, this makes it possible to replace only the dome, or even the sector(s) which have reacted, following an explosion, in order to obtain a digester which is once again functional.

[0034] The preferential deformation zone can extend over the entire width of the sector, in particular by following a circular line centered on the axis of the skirt. Brief description of the drawings

[0035] The invention may be better understood by reading the description which follows, a non-limiting example of its implementation, and by examining the attached drawing, in which:

[0036] [Fig.l] schematically and partially represents a wastewater treatment plant digester according to the invention,

[0037] [Fig.2] schematically and partially represents a dome according to the invention, in front view,

[0038] [Fig.3] and [Fig.4] schematically represent the forces exerted on the dome of [Fig.l] in a normal situation,

[0039] [Fig.5] and [Fig.6] are views similar to those of figures 2 and 3, in an accident situation,

[0040] [Fig.7] represents a detail of the preferential deformation zone of a sector of the dome of [Fig.l],

[0041] [Fig.8] and [Fig.9] illustrate calculation models of the dome, in normal situations and accidental, respectively, and

[0042] [Fig. 10] illustrates the temporal evolution of the pressure inside the digester. Detailed description

[0043] Figures 1 to 6 illustrate an example of a wastewater treatment plant digester according to the invention.

[0044] As illustrated, the digester 1 comprises a tank 10 arranged to receive organic matter, comprising a skirt 15, which is for example generally cylindrical, with a vertical axis X. In this example, the skirt 15 is provided, on its upper part, with a belt 18. The tank 10 also comprises a base 11 of substantially conical shape, centered on the axis X of the skirt 15.

[0045] Means 13 for stirring the organic materials are arranged in the center of the tank 10, their axis of rotation being the same as the axis X of the skirt.

[0046] The digester 1 comprises a dome 20 resting on the skirt 15 to close the tank in the upper part. The dome comprises a through opening 22 for the recovery of the gas resulting from the fermentation of the organic matter, preferably centered on the axis X of the skirt, as visible in particular in [Fig.3] or 5. As can be seen in [Fig.l], the opening 22 receives for this purpose a gas outlet pipe connection 40.

[0047] In the example illustrated, the dome 20 has a general shape of a spherical cap, centered on the axis X of the skirt, but it does not go beyond the scope of the invention if the dome has another shape, preferably generally concave towards the inside of the tank, in particular conical, truncated, ellipsoidal or prismatic.

[0048] The dome 20 comprises a plurality of sectors 23 which are prefabricated from cementitious material. A dry joint 25 is present at the interface between the sectors.

[0049] The sectors 23 each comprise a curved body 30, of generally trapezoidal shape. The body 30 comprises a lower base 31, an upper end 33 and two lateral sides 35 converging towards the center of the dome and forming an angle α between them. The bodies of the sectors 23 may be solid or may comprise orifices, in particular for observation ports, valves, access hatches, etc.

[0050] In the example illustrated, the dome is formed by the assembly of 8 sectors embedded, at their base 14, in the belt 18 of the skirt. The upper ends 16 are aligned so as to delimit the gas outlet opening 22.

[0051] As can be seen in [Fig.2] in particular, the dome comprises an alternation of sectors of lesser width 23a and of greater width 23b. In a variant not illustrated, the sectors are of identical width.

[0052] The sectors of smaller width each have, for example, an extent angular a of the order of 15° (+ / - 5°).

[0053] The sectors of greater width have, for example, an angular extent a of the order of 75° (+ / - 5°).

[0054] The sectors 23 each have a preferential deformation zone 27 near the belt 18, allowing them to lift outwards under the effect of accidental overpressure in the digester 1. This preferential deformation zone 27 extends over the entire width l of the sector 23, as illustrated.

[0055] In this example, and so as to preserve the skirt 15 in the event of an explosion, the preferential deformation zone 27 is distant from the belt 18, for example by a distance d which is between 10% and 15% of the length L of the sector 23.

[0056] Preferably, the preferential deformation zone comprises at least one engraving 29 in the cementitious material, as illustrated in [Fig.7]. In this example, the engraving 29 opens onto the upper face of the sector 23. Preferably, the depth p thereof represents at least 30% of the maximum thickness H of the sector 23. In the example illustrated, the depth p represents approximately 35% of the thickness H.

[0057] The weight of the sector 23 can be between 100 and 105% of the thrust resulting from the maximum operating pressure. In the example illustrated, the maximum thickness is approximately 32 cm and the depth p is approximately 12 cm.

[0058] [Fig.7] also shows an example of a reinforcement cage 50 for reinforcing the concrete of the sectors.

[0059] As illustrated, the sector 23 comprises, on its external face at the level of the area of ​​the engraving 29, pairs of reinforcements 52a; 52b each having a curved end 54a; 54b on either side of the engraving. The pairs of reinforcements are connected to each other by connecting reinforcements 55, which allows continuity of reinforcements on the aforementioned face. With such an assembly of reinforcements, damage to the reinforcements of the reinforcement cage is avoided during the production of the engraving. The presence of visible portions of reinforcements at the level of the engraving which would be subject to corrosion problems, in particular, is also avoided. Finally, the deformation of the sector is facilitated due to the reduction in thickness and the lower density of reinforcements in the area of ​​the engraving.

[0060] On the internal face side, the cage comprises continuous reinforcements 56.

[0061] The set of reinforcements has a diameter of approximately 10 mm, in the example illustrated.

[0062] The preferential deformation zone 27 may also comprise, as a variant, reinforcements of lesser mechanical resistance.

[0063] The operating principle of the dome 20 is described below.

[0064] Normally, service loads create mainly loads of compression and the dry joints 25 at the interface of the sectors 23 are compressed, as illustrated in figures 3, 4 and 8.

[0065] In the service configuration, the pressure inside the digester 1 is approximately 45 mbar. In an accident situation, the pressure inside the digester 1 increases very quickly and exceeds a predetermined threshold, for example approximately 80 mbar.

[0066] From this threshold, the forces exerted on the dome are reversed, as illustrated in Figures 4 and 5. Consequently, the dry joints open. The different sectors of the dome then work in console with respect to the junction with the skirt, the zone of preferential deformation forms a plastic ball joint, and the sectors lift, thus allowing the excess pressure to escape, as illustrated in particular in [Fig.9]. In the example illustrated, the sectors of lesser width 23a lift first.

[0067] [Fig.9] shows the time evolution of the pressure inside the digester. As illustrated, the pressure rises to 300 mb. The opening of the dome sectors in this case corresponds to approximately 10 cm, allowing the gas to escape from the digester.

[0068] Then, the internal pressure decreases to approximately 100 mb and the sectors 23 of the dome 20 close under their own weight. Since the combustion inside the digester is not complete, the pressure of the internal gas then rises to approximately 280 mb, causing the sectors 23 to reopen. An oscillatory phenomenon therefore occurs during the few seconds of the explosion.

[0069] Of course, the invention is not limited to the example which has just been described.

[0070] In particular, modifications can be made to the production of the sectors.

Claims

Claims

1. A wastewater treatment plant digester (1) comprising: - a tank (10) arranged to receive organic matter, comprising • a skirt (15), and • a base (11), - a dome (20) resting on the skirt (15) to close the tank (10) in the upper part, this dome (20) comprising a plurality of sectors (23) prefabricated from cementitious material, at least one of said sectors (23) comprising at least one preferential deformation zone (27) near the interface (21) with the skirt (15), allowing at least part of this sector (23) to rise outwards under the effect of accidental overpressure in the digester.

2. Digester according to the preceding claim, a plurality, in particular all of the sectors (23), having at least one preferential deformation zone (27) allowing at least part of the sector (23) to lift outwards under the effect of accidental overpressure in the digester.

3. Digester according to one of the preceding claims, the preferential deformation zone (27) comprising at least one engraving (29) in the cementitious material (C), in particular an engraving opening onto the upper face of the sector, and the depth (p) of which preferably represents at least 30% of the maximum thickness (H) of the sector.

4. Digester according to any one of the preceding claims, the preferential deformation zone (27) comprising a lower density of reinforcements and / or reinforcements of lower mechanical resistance.

5. Digester according to any one of the preceding claims, the preferential deformation zone (27) being located at a distance from the skirt (15) which corresponds to less than one fifth of the length (L) of the sector (23).

6. Digester according to any one of the preceding claims, the preferential deformation zone (27) extending over the entire width (Z) of the sector (23).

7. A digester according to any preceding claim, the sectors (23) being assembled with a dry seal (25) at their interface.

8. Digester according to any one of the preceding claims, the sectors (23) being embedded at their base in the skirt (15).

9. Digester according to any one of the preceding claims, the dome (20) comprising an alternation of sectors of different widths (Z).

10. Digester according to any one of the preceding claims, the dome (20) having a general shape of a spherical cap.

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