WO2008046927A1

WO2008046927A1 - Biofilm support component for a biological water purification reactor, having a series of corrugations, and corresponding reactor

Info

Publication number: WO2008046927A1
Application number: PCT/EP2007/061262
Authority: WO
Inventors: Jens Meinhold; Caroline Dale; Kieran Healey; Gianfranco Favali; Juan Ochoa
Original assignee: Otv Sa
Priority date: 2006-10-20
Filing date: 2007-10-22
Publication date: 2008-04-24
Also published as: FR2907447A1; FR2907447B1

Abstract

The subject of the invention is a biofilm support component intended to be immersed in a biological water purification reactor, comprising a tubular envelope having two end openings that communicate with each other via a channel, said envelope having an inner surface delimiting said channel and an outer surface, said envelope having a series of corrugations that form deformations of said inner surface and of said outer surface, characterized in that said envelope has a length less than or equal to 15 mm and in that the volume of said corrugations that form deformations of said inner surface (131) and the volume of said corrugations that form deformations of said outer surface (132) each represent between 20 % and 29% of the total volume of said envelope.

Description

Biofilm support element for biological water purification reactor, having a series of corrugations, and corresponding reactor.

The field of the invention is that of water treatment. More specifically, the invention relates to the biological treatment of effluents for their purification or purification, such as including but not limited to urban or industrial wastewater.

There are various types of biological treatments for water pollution. These treatments are based on the ability of biomass (fixed or mobile) to eliminate biodegradable pollution, thanks to the different microorganisms it contains.

In these treatments, it is extremely difficult to obtain a high concentration of the different species of microorganisms in the case of free biomass processes, such as activated sludge processes (this being related to the limits of solid liquid separation processes such as clarification). This is because the biomass is maintained in the system by a gravity separation process (secondary clarifiers).

This technique is also limited by the applicable load in terms of BOD (biological oxygen demand), COD (chemical oxygen demand), but also with respect to nitrogen or other inorganic pollutants. In systems with fixed biomass such as processes known as the biofiltration process, the concentration of biomass (in bacteria) is achieved by adhering the bacteria to a support element. The clarification capacity to maintain the biomass in the system is no longer an essential criterion and this technique has a purification potential much higher than conventional methods.

Most biofilm processes rely on the implementation of a reactor incorporating support elements in the form of hollow bodies or blocks fixed stationarily in the reactor.

This technique involves a risk of clogging of the support elements by biomass or other materials, which can lead to the formation of inactive areas or "shortcuts" in the processing unit. Such a phenomenon then results in an unsatisfactory contact between the water to be treated and the microorganisms, as well as a reduction in the active volume of the unit.

Another technique consists in suspending the support elements in the reactor.

Such a method has in particular been described in the early 1990s in the patent document published under number WO-91/011396. According to the method described, the support elements have: a biofilm support surface at least 1.5 times greater than that of smooth support elements of the same dimensions;

a density of between 0.9 and 1.2; an outer surface partially protected against wear likely to be caused by contact with other support elements; walls allowing a large passage of water through the support element.

These elements are essentially in the form of tubular portions having internal partitions and external fins.

Due to their inherent mobility in the so-called MBBR (Mobile Bed Biofilm Reactor) technique, the support elements are exposed to repeated collisions with each other.

This has the consequence that the surfaces of the supports exposed to those of the other support elements tend to remain cleaned of any growth of biofilm. The effectiveness of the treatment is then considerably dependent on the protected areas of the collisions, these being constituted in particular by the internal passages and the compartments of the support elements. Thus, the objective of forming a large protected support surface initially led to the implementation of support elements of small dimensions (smaller than 15 mm) in the MBBR processes. The following developments have identified the critical conditions to ensure a large support surface on larger support elements while having a large protected area.

In this context, a support element has been described by the patent document published under the number WO-95/25072 in which the support element takes the form of a cylindrical support consisting of radial walls and having a central recess, the walls being interconnected to form a structure close to that of a turbine.

The support elements of this type, whose size varies between 30 and 60 mm and having a protected surface of about 300 m ² / m ³ of overall volume, have been used many times.

However, in practice, it has not been possible to use such support elements (greater than 15 mm) intended to achieve a protected surface area greater than 400 m ² / m ³ of overall volume without causing problems relating to the transfer of mass to and into the biofilm, such as clogging by biomass of passages through the elements.

More recently, a support member combining large dimensions in length and diameter with a protected surface greater than 1000 m ² / m ³ of overall volume has been described by the patent document published under number EP-1340720.

However, apart from size considerations and protected surfaces, the existing support elements have a common point with regard to their mechanical fragility due to the design of their multiple openings and the problems related to the transfer to and in the biofilm. Also known is a support element in the form of a corrugated plastic tube, developed in the 90s. This support element was used inter alia for studies related to the evolution of wastewater treatment processes due to the implementation of MBBR reactor construction ("Upgrading of a small WWTP in a cold climate region using a MBBR System", Water Science and Technology (2000), Vol 41 No. 1, pp 177-185, and "Dairy wastewater treatment in a moving bed biofilm reactor ", Water Science and Technology (2002), Vol 45 No. 12, pp 321-328).

However, the size of these support elements, despite dimensions exceeding 20 mm (length = 25 mm and outer diameter of 20 mm), makes them particularly difficult to use because of the particularly low protected surface they have in comparison other existing support elements.

In addition, the design of these support elements induces hydraulic conditions that do not support the development of optimized biofilm.

In addition, the protected area is relatively small (160 m ² / m ³ ) and the ripple depth is less than 20% (about 18%) of the outer radius of the element.

In other words, it is therefore noted that, in general, despite the variety of shapes and sizes of the existing support elements, they have two disadvantages in common.

One of these disadvantages is related to their mechanical strength. This is due on the one hand to the material used for the production of support elements (flexible plastic whose density is close to that of water to allow a good distribution of the elements in the reactor) and, on the other hand, to their design to give them a large protected area, this by the implementation of partitions and multiple openings. The combination of these two points makes the support elements fragile.

The other of these drawbacks is related to the objective of providing, at least partially, suitable conditions for the development of a thick biofilm structure without causing clogging of the elements.

In fact, in general, an active thick biofilm is characterized by a distribution of nutrients and, in the case of aerated systems, by the oxygenation of biofilm. This makes it possible to establish greater variations in environmental conditions for different types of bacteria and, therefore, increase the diversity of microbial fauna in biofilm. This contributes to improving the performance of the treatment. On the other hand, the absence of the possibility of obtaining, at least partially, a thick biofilm leads to a decrease in the actual sludge age, limiting or even suppressing the development of bacteria with a low growth rate in the biofilm.

The known support elements tend to limit or even eliminate the development of a thick biofilm due to the implementation of multiple openings (resulting in continually young sludge due to the excessive and permanent abrasion of the biofilm by air and water) and / or to cause clogging of the support elements likely to block access to the protected surfaces.

The invention particularly aims to overcome the disadvantages of the prior art.

More specifically, the object of the invention is to provide a biofilm support element for the biological treatment of water which makes it possible to offer protected surfaces for the growth of biomass which are substantially greater than those of the prior art.

The invention also aims to provide such a support element that allows a homogeneous distribution of biomass in a reactor. The invention also aims to provide such a support member which has a good mechanical strength.

Another object of the invention is to provide such a support element which is not, or little, subject to clogging phenomena.

Yet another object of the invention is to provide such a support element which is simple in design, easy to implement and inexpensive to produce.

These objectives, as well as others which will appear later, are achieved thanks to the invention which relates to a biofilm support element intended to be immersed in a biological water treatment reactor, comprising a tubular envelope having two opening ends communicating between them by a channel, said envelope having an inner surface delimiting said channel and an outer surface, said envelope having a series of corrugations forming deformations of said inner surface and said outer surface, characterized in that said envelope has a shorter length or equal to 15 mm and in that the volume of said corrugations forming deformations of said inner surface and the volume of said corrugations forming deformations of said outer surface each represent between 20% and 29% of the total volume of said envelope, advantageously between 25% and 29% and preferably 29%. Such characteristics of the support according to the invention generate unique hydrodynamic phenomena within the support element.

Thanks to a Computational Fluid Dynamics (CFD) analysis, an optimal ratio between the geometry of a support element, the protected surface and the volume of the corrugations has been determined. According to these tests, a corrugated protected volume of 29% allows a maximum liquid flow inside the support element. These conditions guarantee an optimal transfer of material while keeping a significant stress in the longitudinal axis of the support (thus avoiding its clogging).

Thus, the invention overcomes the drawbacks of the prior art, in that it proposes a support element combining the advantages of: providing a satisfactory mechanical resistance (due to the presence of the corrugations); provide niches for the development of a thick active biofilm (inside undulations, either on the inner or outer surface of the envelope) and aged sludge; ensure good transport of water and oxygen; avoid clogging phenomena.

It is noted that the formed niches form protected surfaces both with respect to collisions with other support elements and with respect to the abrasion due to the flow of water and air, regardless of the flows generated by the system for aeration and mechanical mixing, or the additional forces for the control of the film.

An optimized development is thus obtained with a biofilm active on the entire surface and partially thick.

According to an advantageous solution, said envelope has a length of between 7 mm and 15 mm, and preferably of 10.4 mm.

According to another advantageous characteristic, said envelope has an outer diameter of between 8 mm and 12 mm and preferably of 10 mm. According to yet another advantageous characteristic, said envelope has an internal diameter of between 5 mm and 7 mm, and preferably of 6 mm.

Such dimensions contribute to the distribution of stresses within the element so that it is limited at the level of the protected surfaces and greater in approaching the longitudinal axis. The relatively large dimensions of the corrugated element of the prior art does not allow such a result. These facts make it possible to optimize the specific surface while maintaining a large opening in order to limit clogging. According to a preferred embodiment, said envelope is made of a plastic or composite material.

According to an advantageous solution, said envelope has a thickness of between 0.2 mm and 0.7 mm, and preferably of 0.5 mm.

According to another advantageous characteristic, said envelope has a density of between 0.92 g. cm ³ and 1.1 g cm ^-1 , and preferably

0.95 g.cm ³ .

According to an advantageous solution, the biofilm support element comprises three to six corrugations, and preferably four corrugations.

Preferably, said undulations each have a depth of at least 30% of the radius of said envelope. The invention also relates to a biological reactor for the treatment of water incorporating a plurality of biofilm support elements as described above.

According to a first embodiment, said support elements are mobile.

According to a second embodiment, said support elements are fixed.

According to an advantageous characteristic, said internal surfaces of said support elements represent a protected surface for the development of a biofilm greater than 200 m ² for 1 m ³ of volume of said reactor, and preferably 300 m ² for 1 m ³ or 350 m ² .

Other features and advantages of the invention will emerge more clearly on reading the following description of a preferred embodiment of the invention and of several of its variants, given by way of illustrative and nonlimiting example, and attached drawings of which: Figure 1 is a schematic representation of a support member according to the invention; Figure 2 is an end view of a support member according to the invention; FIG. 3 is a view of a support element according to the invention according to section A-A indicated in FIG. 2; FIG. 4 is a graphical representation of stress distributions over the length of a support element according to the invention and of a support element according to the prior art; FIGS. 5 and 6 are abatement readings respectively of the soluble COD and the total COD with a support element according to the invention and a support element according to the prior art.

With reference to FIG. 1, a support element according to the invention is of the type consisting of a plastic envelope 1 having corrugations 11 identical on the inner wall and on the outer wall of the envelope, the envelope having at each of its ends an opening 12, the openings being connected by a channel 13.

Note that the openings 12 have, as illustrated in Figure 1, identical diameters, these diameters may however be different from one another according to another possible embodiment.

Each undulation (here four in number but possibly three to six according to a feasible embodiment) provides a protected area 131 inside the envelope, and a protected area 132 outside the envelope, to inside which the biofilm is intended to grow by adhering to the corresponding protected surface.

For each element, the sum of the protected volumes 131, 132 represents at least 58% of the total volume of the envelope (the sum of the volumes 131 representing 29% and the sum of the volumes 132 representing 29% of the total volume).

The total volume of the envelope is defined as the sum of the interior volume of the envelope and the volume defined by the thickness B of the envelope.

According to the embodiment illustrated in FIGS. 2 and 3, a support element has the following dimensions: a length L of about 10.4 mm (and more generally between 7 and 15 mm, and even more generally less than 15 mm ); an outside diameter D _OUT of about 10 mm (more generally between 8 and 12 mm); an internal diameter D _INT of about 6 mm (more generally between 5 and 7 mm); a thickness B of the envelope of about 0.5 mm (more generally between 0.2 mm and 0.7 mm). The plastic material in which the envelope is made preferably a density of about 0.95 g. cm ^"3 (more generally between 0.92 g. cm- ^'3 and 1.1 g. ^{cm-' 3).}

The dimensions of the casing are further preferably selected such that the corrugations have a depth being at least 30% of D _or τ / 2.

In the determination of the parameters which have just been listed, it is sought to obtain a protected area which represents 350 m ² / m ³ , considering the overall volume of the reactor (or even 300 m ² / m ³ only, and at the very least at least 200 m ² / m ³ depending on the dimensions of the support used), such values providing a sufficient surface for a good development of the biofilm and allowing a powerful treatment.

Hydraulic modeling tests were carried out by computer according to a recognized technique for evaluating the dynamic fields and the mechanical stresses of a system. According to these tests, under identical hydraulic conditions, the analysis

CFD (Computational Fluids Dynamics) clearly reveals the low level of stress exerted on the inner surface of a support element according to the invention compared to those acting on a corrugated support element according to the prior art. In parallel, as illustrated by the graphs of FIG. 4, the support element A according to the invention has a higher stress level than the corrugated element B of the prior art when approaching the longitudinal axis.

This is important insofar as one avoids the clogging phenomena. The support element according to the invention therefore promotes the development of biomass while avoiding that this development causes clogging, which significantly differentiates the element according to the invention from the corrugated element of the prior art.

Preliminary tests were also conducted in a MBBR-type pilot scale configuration (it is recalled that support elements according to the invention can also be used in fixed bed reactors).

Two units were tested in parallel, one equipped with the support element A according to the invention, and the other with a reference support element C.

The operational parameters that have been used are those conventionally encountered in the context of wastewater treatment.

As shown in Figure 5, the results obtained in terms of the reduction of the so-called COD are identical for the two units.

On the other hand, as shown in FIG. 6, the unit using the support elements according to the invention has better performances in terms of total COD reduction than the unit using the reference support elements.

In addition, the examination of the support elements after treatment shows: a good adhesion of the biomass to the surfaces of the support elements according to the invention; the maintenance of a central passage with the support elements according to the invention, while an almost total clogging appears on the reference support elements. Indeed, as illustrated in FIG. 7, for corrugated protected volumes of the order of 20%, the support element according to the invention has a relatively high level of stress in the longitudinal axis and the order of 0.24 Pa. However, for a protected corrugation volume of 29%, a stress level is observed on the longitudinal axis close to the maximum value (0.28 Pa). This is of importance insofar as this avoids clogging phenomena inside the support. On the other hand, the CFD simulation shows that the use of a protected corrugation volume greater than 29% can significantly reduce this constraint. At the same time, looking at the stress exerted on the internal surface of the support, we observe very low values (<0.0005 Pa) for protected volumes of corrugations lower than 29%. Above this value, the constraint is strongly incremented at the expense of biomass development.

Claims

Biofilm support element intended to be immersed in a biological water purification reactor, comprising a tubular envelope (1) having two end openings (12) communicating with each other via a channel (13), said envelope ( 1) having an inner surface delimiting said channel (13) and an outer surface, said envelope (1) having a series of corrugations (11) forming deformations of said inner surface and said outer surface, characterized in that said envelope ( 1) has a length less than or equal to 15 mm and in that the volume of said corrugations forming deformations of said inner surface (131) and the volume of said corrugations forming deformations of said outer surface (132) each represent between 20% and 29% of the total volume of said envelope (1). 2. biofilm support element according to claim 1, characterized in that the volume (131, 132) of said corrugations (11) is between 25% and 29%.

3. biofilm support element according to claim 2, characterized in that the volume (131, 132) of said corrugations (11) is 29%.

4. biofilm support element according to any one of claims 1 to 3, characterized in that said casing (1) has a length (L) of between 7 mm and 15 mm.

5. biofilm support element according to any one of claims 1 to 4, characterized in that said casing (1) has an outer diameter (D _out ) of between 8 mm and 12 mm. 6. biofilm support element according to claim 5, characterized in that said casing (1) has an outer diameter (D _out ) of 10 mm. 7. Biofilm support element according to any one of claims 1 to

Characterized in that said envelope (1) has an internal diameter (D _1n ) of between 5 mm and 7 mm. Biofilm support element according to any one of claims 1 to 7, characterized in that said envelope (1) is made of a plastic or composite material.

9. biofilm support element according to claim 8, characterized in that said casing (1) has a thickness (B) of between 0.2 mm and 0.7 mm.

10. biofilm support element according to any one of claims 1 to 9, characterized in that said envelope (1) has a density of between 0.92 g. cm ³ and 1.1 g cm ^-1 .

11. biofilm support element according to any one of claims 1 to 10, characterized in that it comprises three to six corrugations.

12. biofilm support element according to any one of claims 1 to 11, characterized in that said corrugations each have a depth at least equal to 30% of the radius of said envelope.

A biological water treatment reactor incorporating a plurality of biofilm support members according to any one of claims 1 to 12.

14. Biological reactor according to claim 13, characterized in that said support elements are movable.

15. Biological reactor according to claim 13, characterized in that said support elements are fixed. 16. Biological reactor according to any one of claims 13 to 15, characterized in that said internal surfaces of said support members represent a biofilm support surface greater than 200 m ² for 1 m ³ of volume.