WO1998019970A1

WO1998019970A1 - Method for fighting against the presence of scum in plants for the treatment of effluents by activated sludge and plant for implementing such a method

Info

Publication number: WO1998019970A1
Application number: PCT/FR1997/002014
Authority: WO
Inventors: Pascal Jouaffre; Geneviève Boissonnade
Original assignee: Otv Omnium De Traitements Et De Valorisation
Priority date: 1996-11-07
Filing date: 1997-11-07
Publication date: 1998-05-14
Also published as: AU5059198A; FR2755436A1; FR2755436B1

Abstract

The invention concerns a method for fighting against the formation of scum in a plant for the treatment of effluents including at least one tank (3) of activated sludge with a plurality of partitions (4, 5) defining a plurality of compartments (3a, 3b, 3c) ventilated or not inside said tank (3) and at least a final clarification basin (6) separated from said tank (3) by a wall (7), and means for recirculating the sludge coming from said final clarification basin at the top of said tank. The invention is characterised in that it comprises steps consisting in allowing the passage of at least part of the effluent from one compartment of said tank to the other over each of said partitions (4, 5) for allowing the scum likely to be formed in the different compartments to flow freely towards a single final zone, and in treating mechanically and/or biologically and/or chemically the scum present in said single final zone, occasionally or continuously.

Description

Process for combating mosses appearing in effluent treatment plants with activated sludge and plant for implementing such a process.

The invention relates to the field of treatment for the purification of effluents such as mainly waste water, by activated sludge.

More specifically, the invention relates to the field of methods used to combat the foams appearing during the implementation of such treatments.

One of the main problems posed by the operation of activated sludge treatment plants is the appearance in certain circumstances of large quantities of biological foam on the surface of the various activated sludge basins and of the clarifiers constituting them. . Such biological mosses, which are in the form of very stable floating clumps, brown, viscous and thick, affect the proper functioning of these stations and moreover imply a significant additional cost of their exploitation linked to the unblocking of the pipes, the cleaning of the overflows. , etc. Microscopic examination of such biological foams reveals that they are made up of filamentous bacteria among which we mainly find Microthήx parvicella and Elm Nocardiaf bacteria. These microorganisms induce sludge settling difficulties, hence the possibility of deterioration in the quality of the effluent discharged into the natural environment. Such a phenomenon can be expressed either by a "swelling" of the sludge which will remain in the water and decant very difficultly, or by sludge which will rise to the surface, hence a development of foam or foam at the surface of basins and clarifiers.

The factors favoring the appearance of these mosses have been widely studied and are mainly constituted by the significant age of the sludge, the lack of substrate provided to them, the nature of the water to be treated, the favorable surface environment. .

The problem constituted by such foams is therefore particularly present in wastewater treatment plants using "low charge" activated sludge which combines high sludge age and low substrate supply. However, such activated sludges at low loads are increasingly used, in particular in the context of the treatment of nitrogen pollution and the treatment of phosphorus pollution. Note also that this problem is increased by the use of more and more frequent means of aeration of sludge activated by fine air bubbles.

Numerous techniques have already been proposed in the prior art for combating these biological foams. Such techniques are described in particular in the "Guide to the fight against stable biological mosses in activated sludge treatment plants", published by the Moss Scientific Interest Group in October

1993 (technical document FNDAE special issue).

Thus, it has been proposed to combat these biological foams by using chemical reagents of various natures, namely mainly coagulants constituted in particular by iron or aluminum salts and / or anti-foaming products mainly based on poly -glycol and / or silicone (polyalkylene glycol ester, aqueous emulsion of polydimethylsiloxamic oil, etc.). Such chemical reagents are generally used by injection at the level of the sludge recirculation in an attempt to eliminate the foams that have formed on the surface of the activated sludge basins or of the clarifiers. However, this is not very effective because such foams reappear a few days, even a few weeks after stopping treatment. As the continuous use of such chemicals cannot be envisaged as an economically acceptable solution, the use of such chemical reagents is generally limited to the treatment of emergencies intended to eliminate very significant foaming.

It has also been proposed in the state of the art to use biological reagents to combat such foams. Such reagents are constituted by specific bacteria having an action on fats, surfactants and reduced sulfur products, that is to say on substances capable of promoting the appearance of biological foams. However, various experiments have shown that such biological reagents are very ineffective.

It has also been proposed to treat the foams by chlorination and in particular by spraying chlorine on the surface of the basins or by injecting chlorine at the level of the sludge recirculation. However, it is known that the hydraulic design of aeration basins intervenes on the quality of rejection. In order to obtain good hydraulic distribution, these frequently consist of several compartments, the hydraulic flow and the homogenization of the effluent in these compartments being ensured by mechanical means such as agitators. The effluent then passes successively above or below the partitions delimiting these compartments, the separation partitions which make it possible to circulate the effluent below the partitions being raised relative to the water level. Such a configuration causes blocking of the foams between the compartments. The chlorination operation therefore has the drawback of having to be carried out by spraying chlorine over all the compartments of the basins. Another solution for combating the appearance of mosses consists in providing a contact zone between the effluent and the sludge at the head of the installation. In a large number of cases, the use of such a contact zone makes it possible to suppress the appearance of biological foams without harming the general functioning of the station. Such a contact area can be produced from an existing structure of the station, for example by fitting the degreaser thereof. However, it is not always possible to develop such an upstream area, either for reasons of space or for economic reasons. In addition, such a contact area is not sufficient to eliminate any risk of foaming.

The objective of the present invention is to propose a method making it possible to fight effectively against the biological mosses appearing during the purification of the effluents by activated sludge, method which does not have the drawbacks mentioned above.

An objective of the invention is in particular to describe such a process which is particularly effective for stations operating with low charges of activated sludge. Another objective of the present invention is to propose such a method which can be easily implemented during the rehabilitation of an existing station.

These various objectives, as well as others which will appear subsequently, are achieved thanks to the invention which relates to a method for combating the mosses appearing in an effluent treatment installation including on the one hand at least a sludge basin activated provided with a plurality of partitions delimiting a plurality of ventilated or non-ventilated compartments within said basin and on the other hand at least one clarifier separated from said basin by a wall, and means for recirculating sludge from said clarifier at the head of said basin; characterized in that it comprises the steps consisting in: - authorizing the passage of at least part of the effluent from one compartment to the other of said basin over each of said partitions, so as to allow the free flow of foams which can form in said different compartments towards a single final zone, and;

- mechanically and / or biologically and / or chemically treating said foams present in said single final zone, episodically or continuously.

In an original way, the invention therefore proposes, during the implementation of such an installation to allow the passage of the effluent over all the partitions defining the different compartments of a pool of activated sludge. In fact, in all the installations known from the state of the art, the passage from one compartment to the other is carried out either alternately from above and from below such partitions, or only from below these partitions, or over the entire height of these, these partitions then forming baffles. There are therefore always areas favorable to the accumulation of sludge located near the partitions over which the effluent is not allowed to pass. Such zones, multiple on most installations, contribute to making the problems associated with the appearance of mosses more acute.

The only characteristic according to the invention authorizing the passage of the foams over the partitions delimiting the various compartments of the basin alone makes it possible to reduce the quantity of biological foams produced. These are then more easily eliminated thanks to the step of mechanical and / or biological and / or chemical treatment of these at the level of the final zone. It will be noted that this final zone may be located in the last compartment of the aeration basin, however, preferably, the passage of the effluent over said wall separating the basin of activated sludge from said clarifier will also be authorized and said final zone will then be located in said clarifier. The method according to the invention can also be applied to an installation including at least one degassing tank provided between said activated sludge tank and said clarifier, said degassing tank being separated from said tank by a first wall and from said clarifier by a second wall. , the method comprising a step of authorizing the passage of the effluent also over said first wall and optionally also over said second wall, said final zone then being located in said degassing tank or in said clarifier.

According to a preferred variant of the invention, the passage of the effluent from an aerated compartment to a non-ventilated compartment is carried out both above and below said partition, the bulk of said effluent passing over said partition.

Also preferably, the passage of the effluent from an unventilated compartment to an aerated compartment is carried out both above and below said partition, the bulk of said effluent passing beneath said partition.

Consequently, according to the invention, there will always preferably be a passage for the effluent at the bottom of the partitions, in order to facilitate the operations for emptying the basin.

In order to allow the water to benefit from the residence time provided in each compartment and to avoid by-pass, stirring means may advantageously be provided in the compartments. Such means may for example be in the form of mechanical means (agitators, aerators specially adapted for ventilated compartments ...)

Thus, the method preferably comprises a step consisting in agitating the effluent present in the non-aerated compartment or compartments by mechanical means and / or a step consisting in agitating the effluent present in the aerated compartment or compartments thanks to the means of ventilation with which this or these compartments are provided, by judiciously installing these ventilation means. The effluent contained in the ventilated compartment (s) may also be agitated by mechanical means.

Preferably, said step of treating the foams present in said single final zone includes chlorination. Such an operation may in particular be carried out by spraying chlorine on said foams, at a rate of chlorine of between 100 and 4000 g, preferably between 150 and 1000 g of chlorine per cubic meter of foam, or according to another variant by injection of chlorine into said sludge recirculation means, due to a dose of chlorine between 1 and 15 grams, preferably between 2 and 15 grams of chlorine per kilogram of suspended matter present in said basin and per day.

In this regard, it should be noted that the spraying may take place directly in the final zone and / or just immediately upstream thereof, in the last compartment of the activated sludge tank or even at the level of the degassing tank if a such a tarpaulin is provided.

When the method according to the invention will be implemented on an installation including a primary decanter, it will preferably also include an additional step consisting in at least partially bypassing the primary decanter and / or an additional step consisting in recirculating all or part of the primary sludge from said primary settling tank in the first compartment of said activated sludge tank. Such characteristics make it possible to continuously adjust the content of suspended matter in the effluent entering the activated sludge tank. It has in fact been observed that the development of foams is often accentuated by an insufficient concentration of such suspended matter. The invention also covers any installation for implementing a method as described above comprising at least one activated sludge tank provided with a plurality of partitions delimiting a plurality of aerated or non-aerated compartments within said basin , at least one clarifier and possibly a degassing tank provided between said tank and said clarifier, said clarifier and / or said tank being separated from said tank by one or more walls, and means for recirculating sludge from said clarifier at the head of said tank , characterized in that said partitions and possibly the or at least some of said walls are designed so as to allow the passage of at least a portion of the effluent over all of said partitions and possibly over or at least some of said walls to allow the accumulation of foam that may form in said basin in a final area as well as means allowing episodic or continuous treatment of said foams which may be present in said final zone.

Preferably, said means for processing said foams include chlorination means. Advantageously, when the installation includes a primary settling tank, it also includes means for recirculating the primary sludge in the first compartment of said basin and / or means for bypassing the primary settling tank.

Also preferably, the installation will include adjustable aeration means provided in said normally non-aerated compartments, making it possible to transform them into aerated compartments, so as to be able to vary the aerated volume of the basin as a function of the effluent to treat, and so as not to freeze the different compartments in anoxic, anaerobic or aerobic zones. In this way, we will be able to fight against the development of filamentous bacteria, the addition of oxygen in an unventilated zone making it possible to avoid the appearance of zones in prolonged anaerobic which cause phenomena of stable foams. To combat such mosses, it will thus be possible to provide an aeration ramp in the non-aerated basins. Such aeration means may be provided adjustable and subject to means for controlling the redox potential or to a continuous measurement of the nitrate content of the effluent.

The invention, as well as the various advantages which it presents, will be more easily understood thanks to the description which follows of a nonlimiting embodiment of the latter with reference to the single figure schematically representing an installation putting in implements the method according to the invention.

Referring to the single figure, the wastewater treatment installation shown includes an inlet pipe 1 for the water to be treated which has previously undergone a conventional pretreatment (screening, degreasing, sand removal), a primary decanter 2 receiving the water from from line 1, a treatment tank 3 in which activated sludge is used, said tank being divided into three compartments 3a, 3b, 3c by two partitions 4,5, a final clarifier 6 separate from the activated sludge tank 3 by a wall 7 and an outlet 8 of the treated water. The installation further comprises a pipe 9 for sludge recirculation coming from the final clarifier 6 at the head of the installation, a pipe 10 for internal recycling of the sludge coming from the last aerated compartment of the basin 3 (that is to say compartment 3b or compartment 3c if one chooses to ventilate this compartment 3c normally not ventilated as will be explained below) to the first compartment 3a of this same basin.

The activated sludge of the basin 3 is implemented according to a low charge process, to effect the elimination of carbon pollution and nitrogen pollution of the effluent. To this end, the compartments 3a, 3b and 3c define a succession of ventilated or non-ventilated zones. For this purpose, the compartment 3b is provided with ventilation means 11, while the compartments 3a and 3c, which are not aerated, are provided with means for deep stirring 12,13. Such aeration and agitation means also make it possible to facilitate the flow of water from one compartment to the other, avoiding any hydraulic short circuit. In this regard, it will be noted that the non-aerated compartments 3a and 3c are also provided with aeration ramps 16, 17 slaved to a measurement of the redox potential (or to the nitrate content) making it possible to temporarily ventilate these compartments and thus avoid any prolonged anaerobic stay in order to fight against the appearance of filamentous bacteria capable of causing the appearance of mosses.

According to the present invention, the partitions 4 and 5 are designed so as to allow the flow of at least part of the effluent from the compartments which they delimit above them. More specifically, the partition 4 is designed to allow the passage of a small part of this effluent both from above and most of the effluent from below. The partition 5 is in turn positioned to allow the passage of the effluent essentially above it, a small part of the effluent being able to pass from below. In the same spirit, the wall 7 allows the effluent to pass over it to reach the clarifier 6. Such characteristics make it possible to reduce the stagnation of mosses forming in the installation and to allow them to flow. towards a final zone located on the surface of the effluent present in the final clarifier 6.

The installation also includes means for treating the foam which can accumulate in the final clarifier 6. These treatment means consist of means for spraying chlorine 14 at the rate of a dose of chlorine between 100 and 4000 g, preferably between 150 and 1000 g of chlorine per cubic meter of foam. Such spraying means are designed to put the spraying of chlorine both above the last compartment 3c of the basin 3 and above the clarifier 6. It will be noted that, in addition to or in place of such means of spraying, it could also be envisaged to use means for injecting chlorine at a rate of chlorine of between 1 and 15 grams, preferably between 2 and 15 grams, of chlorine per kilogram of suspended matter present in the basin 3 and per day, in the pipe 9 for the return of the sludge. Finally, in order to further reduce the appearance of foam, the installation also includes a pipe 15 allowing the sludge from the primary settling tank 2 to be brought into contact with the effluent present in the first compartment 3a of the activated sludge tank 3.

The embodiment of the invention described here is not intended to reduce the scope of the invention. The method according to the invention can therefore in particular be implemented on installations including more than one activated sludge pool and / or delimiting more than three compartments.

Claims

1. Method for combating mosses appearing in an effluent treatment installation including on the one hand at least one basin (3) of activated sludge provided with a plurality of partitions (4,5) delimiting a plurality of compartments ( 3a, 3b, 3c) aerated or non-aerated within said tank (3) and on the other hand at least one clarifier (6) separated from said tank (3) by a wall (7), and means for recirculating sludge from said clarifier at the head of said basin; characterized in that it comprises the stages consisting in:

- authorize the passage of at least part of the effluent from one compartment to the other of said basin over each of said partitions (4,5), so as to allow the free flow of the foams that can form in said different compartments to a single final area, and;

2. Method according to claim 1 characterized in that it comprises a step consisting in authorizing the passage of the effluent over said wall (7) separating the basin (3) of activated sludge from said clarifier (6), said final zone being located in said clarifier.

3. A method according to claim 1 characterized in that said installation includes at least one degassing tank provided between said activated sludge tank and said clarifier, said degassing tank being separated from said tank by a first wall and from said clarifier by a second wall, the process comprising a step consisting in authorizing the passage of the effluent over said first wall and optionally also over said second wall, said final zone being located in said degassing tank or in said clarifier.

4. Process according to any one of Claims 1 to 3, characterized in that the passage of the effluent from an aerated compartment to a non-ventilated compartment is carried out both above and below said partition, most of said effluent passing over said partition.

5. Process according to any one of Claims 1 to 4, characterized in that the passage of the effluent from an unventilated compartment to an aerated compartment is carried out both above and below said partition, the bulk of said effluent passing beneath said partition.

6. Method according to one of claims 1 to 5 characterized in that it comprises a step consisting in agitating the effluent present in the non-aerated compartment (s) (3a, 3c) by mechanical means (12,13) .

7. Method according to any one of Claims 1 to 6, characterized in that it comprises a step consisting in agitating the effluent present in the aerated compartment or compartments (3b) by means of the aeration means (11) which are provided with this or these compartments.

8. Process according to any one of Claims 1 to 7, characterized in that it comprises a step consisting in agitating the effluent present in the aerated compartment or compartments by mechanical means.

9. Method according to one of claims 1 to 8 characterized in that said step of treating the foams present in said single final zone includes chlorination.

10. Method according to claim 9 characterized in that said chlorination step is carried out by spraying chlorine on said foams, at a rate of chlorine dose between 100 and 4000 g, preferably between 150 and 1000 g of chlorine per meter foam cube.

11. Method according to claim 9 characterized in that said step of chlorinating the sludge is carried out by injecting chlorine into said sludge recirculation means, at a rate of a dose of chlorine between 1 and 15 grams, preferably between 2 and 15 grams of chlorine per kilogram of suspended matter present in said basin and per day.

12. Method according to one of claims 1 to 11 characterized in that said installation comprises a primary settling tank, said process comprising an additional step consisting in at least partially passing the primary settling tank.

13. Method according to one of claims 1 to 12 characterized in that said installation comprises a primary settling tank, said process comprising an additional step consisting in recirculating all or part of the primary sludge from said primary settling tank in the first compartment of said basin.

14. Installation for the implementation of a method according to one of claims 1 to 13 comprising at least one basin (3) of activated sludge provided with a plurality of partitions (4,5) delimiting a plurality of ventilated compartments or not aerated (3a, 3b, 3c) within said tank (3), at least one clarifier (6) and possibly a degassing tank provided between said tank (3) and said clarifier (6), said clarifier (6) and said tank being separated from said tank by one or more walls (7), and means for recirculating sludge from said clarifier at the head of said tank, characterized in that said partitions and possibly the or at least some of said walls are designed so authorizing the passage of at least part of the effluent over all of said partitions (4,5) and optionally over the or at least some of said walls (7) to allow the accumulation of foams that can form in said low sin (3) in a final zone as well as means (14) allowing episodic or continuous treatment of said foams which may be present in said final zone.

15. Installation according to claim 14 characterized in that said processing means (14) of said foams include chlorination means.

16. Installation according to one of claims 14 or 15 characterized in that it includes a primary settling tank (2) and recirculation means (15) of all or part of the primary sludge in the first compartment (3a) of said basin.

17. Installation according to one of claims 14 to 16 characterized in that it includes a primary settling tank (2) and at least partially bypass means thereof.

18. Installation according to one of claims 14 to 16 characterized in that it includes ventilation means (16,17) provided in said initially non-ventilated compartments making it possible to transform them into ventilated compartments.

19. Installation according to claim 18 characterized in that said aeration means (16,17) are adjustable and subject to means for controlling the oxy-reduction potential or to a continuous measurement of the nitrate content.