MXPA00001138A - Method and device for purifying waste water comprising an additional sludge treatment by ozonation - Google Patents

Abstract

The invention concerns a method for purifying waste water, whereby the waste water is subjected to a biological treatment (in 3) producing sludge, part of this sludge being recycled towards the biological treatment while being subjected to ozonation combined with mechanical stirring (in 9). A mechanical stirring energy is brought in to attack the walls of the microorganisms.

Description

METHOD AND DEVICE DS WASTE WASTE DISPOSAL COMPRISING AN ADDITIONAL TREATMENT OF MUD FOR OZONIZATION FIELD OF THE INVENTION The present invention relates to methods and devices for the purification of wastewaters that comprise an additional treatment of sludge by ozonation, in such a way that it significantly reduces the quantities of sludge produced by a biological treatment system, in particular to reduce the cost of the treatment of this mud, resulting from new regulations.

BACKGROUND OF THE INVENTION One of the advantages of said invention is to allow the improvement of the sedimentation capacity of the sludge after the treatment. More particularly, the invention relates to a method of purifying wastewater charged with organic matter, the method comprising a step during which the wastewater is stagnated in a biological treatment device, called the main biological treatment device (constituted by one or several reactors such as ventilation vessels, bacterial beds, anaerobic digesters, clarifiers, etc., performing a biological treatment possibly associated with a physico-chemical treatment), where said organic matter is degraded by the microorganisms when the sludge is produced, subjecting a part of the sludge to ozonization combined with mechanical agitation before being sent in the main biological treatment device, the sludge thus subjected to ozonation, hereinafter being called "recycled sludge". EP-A-0 645 347 discloses a method, in which ozonation takes place after the acidification of the recycled mud at a pH of less than 5, with mechanical stirring being carried out, that is, during the acidification methods for mixing the recycled sludge in an acidification reagent, that is by pumping in order to spray a part of the recycled sludge in the ozonation reactor. The method described in this document especially has the disadvantages of needing relatively high amounts of ozone, and of disturbing the operation of the main biological treatment device due to the acidification of the recycled sludge. The object of the present invention is especially to overcome these drawbacks. For this purpose, the invention proposes a method essentially characterized in that sufficient mechanical energy is supplied to said recycled sludge to attack the walls of the microorganisms contained in this recycled sludge. Thanks to these provisions, the effectiveness of the ozonation treatment is improved in relation to the method described in the above mentioned document, by the fact that the mechanical stirring energy provided to the recycled sludge is sufficient to weaken the codend and the cell walls of the microorganisms contained in said recycled sludge, in order to allow a more effective attack of these microorganisms by ozone. The deconstruction of the codend results from the attack of the exopolymers that ensure the cohesion of said codend and cause the fragmentation of various bacteria and protozoa. Thus, ozone amounts clearly lower than in the method described in the aforementioned document can be used. In addition, it is not necessary to acidify the recycled sludge, so that the proper functioning of the main biological treatment device is not disturbed. On the other hand, the fact of not having to acidify the recycled mud further improves the performance of ozonation processes. Finally, thanks to the recirculation of the sludge towards the main biological treatment device, the additional pollution generated at the time of the destructuring of the cell walls of the microorganisms (increase of the chemical demand in oxygen [DCO], of the demand) is absorbed. oxygen biochemistry [DBO] and dissolved organic carbon), in order to finally obtain a significant reduction in the volumes and masses of the sludge, a significant improvement of the mud indices (especially the Mohlman index) and a minimization of eventual biological disorders (especially "bulging" due to filamentous bacteria).

SUMMARY OF THE INVENTION In the preferred embodiments of the invention, one may optionally resort to one and / or another of the following provisions: the pH of the recycled sludge is always comprised between 6 and 9; - the mechanical agitation of the recycled sludge takes place before its ozonation; - the mechanical agitation of the recycled sludge takes place after its ozonation; - the mechanical agitation and ozonation of the recycled sludge takes place in the same reaction room; - a certain flow of recycled sludge, called first flow, is taken at the outlet of an ozonation reactor of the recycled sludge, then subjecting this first flow to mechanical agitation, said first flow being then rerouted to the ozonation reactor with a certain flow rate supplementary mud from the main biological reactor, called the second flow, the second flow being lower than the first flow; - the recycled sludge is subjected to aerobic or anaerobic digestion in addition to ozonation and mechanical agitation; aerobic or anaerobic digestion takes place after ozonation and mechanical agitation; - a certain volume of recycled mud is taken, at the outlet of a digester where the aerobic or anaerobic digestion of the recycled sludge is carried out, this flow of recycled sludge then being subjected to mechanical agitation and ozonation before being returned to the digester with a certain supplementary flow of sludge from the main biological reactor; only part of the recycled sludge that has been subjected to anaerobic digestion is sent to the main biological treatment device, and another part of the recycled sludge from the anaerobic digester is evacuated; the ozonation step is carried out in an ozonation reactor that includes at least one vent from which a gaseous effluent containing at least ozone and oxygen leaves, the method also includes a step consisting in collecting this gaseous effluent, and in reusing said effluent gaseous to treat wastewater or other liquid resulting from the treatment of this wastewater; - the ozone contained in the gaseous effluent collected at the outlet of the vent is destroyed before reusing said gaseous effluent, being able to effect the destruction of the ozone by thermal or catalytic means; - a mechanical agitation energy of between 10 and 20,000 kJ per kg of recycled mud dry matter is added to the recycled sludge; - the mechanical energy of agitation is between 50 and 3000 kJ per kg of dry matter of recycled mud. On the other hand, the invention also aims at a device for carrying out a previously defined method, this device comprising an ozonation reactor working under pressure, in which ozonation of the recycled sludge is carried out. Other features and advantages of the invention will appear in the course of the following description of some of its embodiments, given by way of non-limiting examples, with respect to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: - Figure 1 is a schematic view of a wastewater treatment installation performing a method according to an embodiment of the invention, - Figures 2 to 4 are block diagrams showing in more detail the contents of the assembly 9 of figure 1, - figures 5 to 8 are schematic views showing in more detail the content of the device 10 of figures 2 to 4, in which the combined treatment of ozonation and of mechanical agitation of the recycled sludge, - and Figure 9 is a view similar to Figure 1, for another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES In the different figures, the same references designate the identical or similar elements. Figure 1 very schematically represents a sewage treatment station 1 comprising: - an inlet 2 of waste water loaded with organic matter, - one or more reactors for biological treatment, for example a ventilation tank 3 in which it is degraded said organic matter by the microorganisms when producing the sludge, this ventilation tank 3 being able, if necessary, to be associated to or replaced by one or several other biological treatment devices such as reactors in fixed cultures, anaerobic digesters, or others, - a clarifier 4, or any other sol-liquid separation system, which receives the waste water after it passes through the ventilation tank 3 and separates the water and the sludge, - a treated water outlet 5, which collects the water the output of the clarifier 4, - an evacuation of the sludge 6, which collects the sludge at the outlet of the clarifier 4, - a first recycling loop 7, which collects a part of the sludge at the level of the evacuation 6 and recycling this sludge in the upper part of the ventilation tank 3, this first recycling loop 7 being able to be suppressed if necessary (this recycling loop can, for example, ensure a flow representing 50 to 300% of the nominal flow of urban or industrial effluent treated by the depuration station), - a second loop 8 of degradation of the mud, which also collects a part of the mud at the level of the evacuation 6 and which forwards this mud to the upper part of the ventilation tank 3 after passage in a sludge treatment assembly 9 where said sludge is subjected to at least a combined treatment of ozonization and mechanical agitation. As shown in Figures 2 to 4, the mud treatment assembly 9 can comprise: - either only an ozonation and mechanical agitation device 10 (Figure 2), - either an aerobic or anaerobic digester 11 placed under an ozonization and mechanical agitation device 10 (FIG. 3), a fraction of the sludge projecting from the digester 11 and it being then possible to evacuate instead of recycling all of this sludge in the upper part of the ventilation tank 3, either an anaerobic digester 11 including a recirculation loop 12 (figure 4) in which an ozonization and mechanical agitation device 10 is integrated, the flow rate of mud Q4 taken at the level of the mud evacuation 6 being generally lower than the flow rate sludge Q3 running through the recycling loop 12 between the inlet and the outlet of the digester 11 (as in the case of figure 3, a sludge fraction leaving the digester may then come, evacuate Instead of recycling all the mud in the upper part of the ventilation tank 3). As shown in Figure 5, the ozonation and mechanical agitation device 10 comprises a mechanical agitator 13 which generally consists of an enclosure 14 comprising one or more turbines 15, or possibly dynamic mixers, hydroeyectors, grinders or any other agitation system mechanics. It will be noted that ultrasounds are considered as a physical phenomenon (generation of acoustic waves) and do not fall into the category of so-called mechanical phenomena in the sense of the present invention, by starting the moving devices. The power of the mechanical agitation system is chosen so that the sludge treatment unit 9 preferably dissipates a mechanical agitation energy comprised between 10 and 2000 kJ per kg of recycled mud dry matter (optionally 600 to 3600 kJ per m 3 of recycled mud), this energy can reach up to 20,000 kJ / kg of dry matter (eventually 14,000 kJ / m3 of recycled mud), the preferred range of mechanical energy is 500 to 3000 kH / kg of dry matter. In addition, the ozonation and mechanical agitation device 10 comprises an ozonation reactor 16 which generally consists of a closed chamber 17 which receives the sludge to be recycled and in which the ozone coming from an ozonator is injected., in the middle of injection nozzles 19 (possibly replaced by porous diffusers, hydroeyectors or others), these nozzles being coupled, if necessary, to static or dynamic mixers. The ozonation globally consumes preferably 0.001 to 0.02 g of ozone per g of dry matter contained in the recycled sludge passing through the sludge treatment assembly 9 (in the case where the recycled sludge passes several times in the reactor). ozonation 16, the ozone consumption mentioned above will be the total consumption in the set of steps of the sludge in the ozonation reactor). The enclosure 17 may, if necessary, be pressurized and in that case be the object of appropriate structure calculations. In addition, this enclosure 17 includes a vent 20 from which a gaseous effluent that comprises at least oxygen and ozone not consumed by the sludge treatment leaves, this vent 20 being able to be connected, if necessary, to a device 21 that destroys the ozone by heating or by passing over the activated carbon, or else said gaseous effluent can then be reused at any point of the purification station, for example by injection into the waste water in the upper part of the ventilation tank 3, or by the contact with any other liquid resulting from the treatment of wastewater (treated water at the outlet of the ventilation tank or at the outlet of the clarifier, or others). The mechanical stirrer 13 and the ozonation reactor 16 are generally fed with sludge by means of a pump 22 which may if necessary participate in the mechanical agitation of the sludge, in which case the pump 22 may advantageously be of the centrifugal type. In this case, the mechanical stirring energy provided to the sludge by the agitator 13 may, if necessary, be less than 10 kJ per kg of recycled mud dry matter (or, if necessary, 600 kJ per m 3 of recycled sludge), assuming that the sum of this mechanical agitation energy with the mechanical energy contributed to the sludge by the pump 22 is between 10 and 2000 kJ per kg of dry matter of recycled mud (in the case of 600 to 14400 kJ per m3 of recycled mud ). Of course, the mechanical agitator 13 and the ozonation reactor 16 are not necessarily placed as in figure 5, as shown in figures 6 to 8, it is possible to: - place the ozonation reactor 16 above the mechanical stirrer 13 (figure 6), placing the turbine 15 or other mechanical agitation system by itself in the ozonation reactor 23 (figure 7), said reactor having similar characteristics, to the ozonation reactor 16 described above, placing the mechanical agitator 13 in a recirculation loop 24 which takes a flow Ql of sludge at the level of the outlet of the ozonation reactor 16 and which forwards that flow Ql to the inlet of said reactor, the flow rate Q2 of mud that is taken at the level of the evacuation of sludge 6 and which gathers the flow Ql at the inlet of the ozonation reactor, being generally lower than the flow rate Ql, and the recirculation loop 24 being generally provided with a pump 25 that can, if necessary, parti cipate in the mechanical agitation of sludge, as described above for pump 22.

Finally, as shown in FIG. 9, the sludge treatment unit 9, with all its variants described above, can eventually take sludge in the ventilation tank 3, and send the recycled sludge to the same ventilation tank. More generally, the sludge treatment assembly 9 can take the sludge to be recycled at any site of the purification station 10 after at least one biological treatment of wastewater, and forward at least a portion of recycled sludge to that sludge. biological treatment Finally, it will be noted that the recycled sludge is not subjected to any acidification at any time, the pH of this sludge always remaining above 5, and preferably between 6 and 9, so that the re-injection of this sludge in the treatment The biological treatment of the purification station does not disturb said biological treatment. * £ & > .

Claims (14)

1. A method of purifying wastewater loaded with organic matter, characterized in that it comprises a stage during which the wastewater is staked in a biological treatment device, called the main biological treatment device, where said organic matter is degraded by microorganisms when producing sludge, a part of this sludge being subjected to ozonization combined with mechanical agitation before being returned to the main biological treatment device, the sludge thus being subjected to "recycled sludge" ozonization, characterized in that during the mechanical stirring stage, it is said recycled sludge sufficient mechanical energy to attack the cell walls of bacteria and other microorganisms contained in this recycled sludge, said mechanical energy being comprised between 50 and 3000 kj per kg of dry matter of recycled sludge, and characterized in that during the ozonation stage, it is consume from 0.001 to 0, 2 g of ozone per g of dry matter of recycled mud. The method according to claim 1, characterized in that the pH of the recycled sludge is always comprised between 6 and 9. The method according to any of claims 1 and 2, characterized in that the mechanical stirring of the recycled sludge is carried out before its ozonation 4. The method according to any of claims 1 and 2, characterized in that the mechanical agitation of the recycled sludge is carried out after its ozonation. The method according to any of claims 1 and 2, characterized in that the mechanical agitation and ozonation of the recycled sludge is carried out in the same reaction chamber. The method according to any of claims 1 and 2, characterized in that a certain volume of recycled sludge, called the first flow, is taken at the outlet of an ozonation reactor where ozonation of the recycled sludge is carried out, and this first flow is then submitted. to mechanical agitation, said first flow being rerouted to the ozonation reactor with a certain additional flow of sludge from the main biological reactor, called the second flow rate, the second flow being lower than the first flow rate. The method according to any of the preceding claims, characterized in that the recycled sludge is subjected to aerobic or anaerobic digestion in addition to ozonation and mechanical agitation. 8. The method according to claim 7, characterized in that the aerobic or anaerobic digestion is carried out before ozonation and mechanical agitation. The method according to claim 7, characterized in that a certain volume of recirculated sludge is taken at the outlet of a digester where the aerobic or anaerobic digestion of recycled sludge is carried out, this sludge flow then being subjected to mechanical agitation and ozonation before Forwarded to the digester with a certain additional flow of sludge from the main biological reactor. The method according to any of claims 7 to 9, characterized in that only one piece of recirculated mud that has been subjected to aerobic or anaerobic digestion is sent to the main biological treatment device, and in which another is evacuated. part of recycled mud SS £ = Si2 £ KS; outgoing from the digester. The method according to any of the preceding claims, characterized in that the ozonation step is carried out in an ozonation reactor that includes at least one vent from which a gaseous effluent that comprises at least ozone and oxygen emerges, the method further comprising a step which consists in collecting this gaseous effluent, and in reusing said gaseous effluent to treat the wastewater or other liquid that results from the treatment of this wastewater. The method according to claim 11, characterized in that the ozone contained in the gaseous effluent collected at the outlet of the vent is destroyed before reusing said gaseous effluent. The method according to any of the preceding claims, characterized in that the ozonation of the recycled sludge is carried out in an ozonation reactor that works under pressure. The method according to any of the preceding claims, characterized in that after its passage in the main biological treatment device, the wastewater is subjected to a clarification stage during which at least the sludge destined to be recycled by ozonation and mechanical agitation it is separated from said wastewater.