EP2173672A2

EP2173672A2 - Method and installation for biologically treating waste water

Info

Publication number: EP2173672A2
Application number: EP08837949A
Authority: EP
Inventors: Laure Graveleau
Original assignee: Degremont SA
Current assignee: Suez International SAS
Priority date: 2007-07-30
Filing date: 2008-07-25
Publication date: 2010-04-14
Also published as: US8382984B2; CA2694375A1; WO2009047406A3; WO2009047406A2; CN101795981A; FR2919601A1; CN101795981B; FR2919601B1; US20100200496A1

Abstract

The invention relates to a method for biologically treating waste water, using a set of micro-organisms having different metabolic spectra, in order to eliminate carbon and nitrogen, even phosphorus. Some of the micro-organisms are fixed to mobile solid carriers (2) and form a fluidised fixed biomass. Some of the micro-organisms are free to be used in an activated mud treatment. To this end, a first non-aerated treatment zone (21a) is followed by a second aerated treatment zone (21b); the treated effluent is subjected to a solid/liquid separation by flotation (6) at a speed higher than 10m/H; and part of the mud recovered by flotation is recirculated (8) towards the activated mud treatment, said recirculation being controlled (9, 10, 11) so that the MES concentration of the effluent subjected to the liquid/solid separation remains compatible with the retained flotation.

Description

METHOD AND PLANT FOR PURIFYING WASTEWATER BY BIOLOGICAL TREATMENT

The invention relates to a process for the purification of wastewater by a biological treatment using a set of micro-organisms for the removal of carbon and nitrogen, or even phosphorus, a part of the microorganisms being fixed on mobile solid supports and constituting a fluidized fixed biomass.

The biological treatment of water aims to eliminate carbon, nitrogen or even phosphorus, thanks to the metabolic activity of a set of bacteria with different metabolic spectra. These characteristics impose operating conditions and in particular specific hydraulic and biological residence times requiring the multiplication of the number of structures. This entails, as disadvantages, high construction costs and a large footprint.

The development of biological processes allowing bacterial growth in the form of biofilm fixed on mobile solid supports, in particular as shown in EP 0 575 314, made it possible to envisage conditions of low mass loading with smaller treatment structures. . The eliminated charge is thus expressed in kgDCO / m ³ of materials / J. The supply of oxygen is also increased beyond the metabolic needs to allow good homogenization of the supports in the reactor. The high cost of the supports represents a very important part of the total cost of construction of the installation and very often constitutes a limit to the diffusion of the process. The nitrogen removal conditions also impose the addition of external carbon (methanol type) to satisfy denitrification yields, which penalizes the cost of operation.

Various alternatives to overcome the disadvantages mentioned above are envisaged in the conventional activated sludge process: for example the addition of membrane processes in the biological reactor or on the re-circulation of sludge.

These processes require major washing phases penalizing their overall profitability. The carbon fraction is present in water in soluble form and dissolved and estimated by the COD (chemical oxygen demand). Heterotrophic bacteria are able to assimilate this carbon under aerobic conditions or in the presence of nitrates, thus allowing the denitrification of the aquatic environment. According to the COD / biomass ratio present in the reactor, their oxygen requirement is more or less important: when this ratio is low (low mass load), the oxygen requirement to degrade the same amount of COD is nearly 40% higher than that necessary in case of high ratio (high mass load). Thus the assimilation of carbon is more economical under conditions of high mass load. Note that under these conditions, the carbon removal efficiency is of the order of 75 to 85%.

Nitrogen treatment requires a first step called nitrification which consists of biologically oxidizing ammonium in the form of nitrites and / or nitrates. The bacteria performing this step have a low growth rate, requiring their maintenance in the system for a long time and thereby causing conditions of low mass load. The second step called denitrification requires carbon available to achieve the reduction of nitrites and / or nitrates to nitrogen gas. The kinetics of denitrification are of the same order of magnitude as that of nitrification.

Advanced carbon treatment under low mass loading conditions is not always compatible with denitrification and causes the addition of external carbon in the form of methanol, for example, which represents, in addition to an additional operating cost, significant constraints in security term near the station.

The purpose of the invention is, above all, to optimize the volumes of the reactors and the supply of oxygen.

This optimization is obtained by coupling conditions of high mass loading in free culture and a nitrogen treatment under low mass load conditions by the fixed fluidized biomass. The free crop is re-circulated from a sludge flotation system, realized in a structure with a speed greater than 10m / H. The re-circulation is controlled so that the concentration is compatible with the flotation system selected.

According to the invention, the process for purifying wastewater by a biological treatment using a set of microorganisms having different metabolic spectra, for the removal of carbon and nitrogen, even phosphorus, a part of the microorganisms being fixed on mobile solid supports and constituting a fluidized fixed biomass, is characterized in that:

another part of the microorganisms is free for an activated sludge type treatment,

a first non-aerated treatment zone is provided followed by a second aerated treatment zone,

the treated effluent is subjected to a solid / liquid separation by flotation at a speed greater than 10 m / h, and

a portion of the sludge recovered by flotation is re-circulated to the activated sludge treatment, this re-circulation being controlled so that the concentration of MES (suspended solids) of the effluent subjected to the liquid / solid separation remains compatible with the flotation retained.

Preferably, conditions of high mass loading in free culture and a nitrogen treatment under conditions of low mass loading by the fluidized fixed biomass are coupled. The high mass load preferably corresponds to a load greater than 0.4 kgDB0 ₅ .kg ^-1 MV.j ^-1 .

Advantageously, the recirculation of sludge recovered by flotation to the activated sludge treatment is controlled so that the MES concentration of the effluent subjected to flotation remains between 0.3 g / l and 1.5 g / l.

It can be expected that the first non-aerated treatment zone is a high load anoxic treatment zone, for removal of most of the carbon and a part of the nitrogen mainly by the action of heterotrophic bacteria, followed by the second aerated treatment zone for nitrogen removal.

Both treatment areas can be physically separated.

A fraction of the nitrates produced in the second aerated treatment zone can be re-circulated to the first anoxic treatment zone.

Advantageously, the concentration of MES is measured in the first high-load anoxic treatment zone, and the recirculation rate of the sludge recovered by flotation is controlled so as to maintain the concentration of MES in the desired range.

The invention also relates to an installation for the purification of wastewater comprising a biological reactor containing a set of microorganisms having different metabolic spectra, for the elimination of carbon and nitrogen, or even phosphorus, a part of the microorganisms being fixed on mobile solid supports and constituting a fluidized fixed biomass, characterized in that:

the biological reactor comprises a first non-aerated treatment zone followed by a second aerated treatment zone.

the plant comprises, downstream of the biological reactor, a float with a separation rate greater than 10 m / H which receives the effluent leaving the biological reactor,

a circuit is provided for re-circulating a portion of the sludge recovered from the float to the biological reactor,

and means are provided to maintain the MES concentration of the effluent in a range compatible with flotation.

Preferably, the MES concentration of the effluent subjected to flotation is maintained between 0.3 g / L and 1.5 g / L.

Means for maintaining the MES concentration of the effluent under flotation in a given range may include:

a means for measuring the concentration of MES in the biological reactor,

- a controller to which the result of the measurement is sent,

and a pump, controlled by the controller, for the recirculation flow.

Advantageously, the first non-aerated treatment zone is an anoxic zone with a high load, for removal of most of the carbon and a part of the nitrogen, mainly by the action of heterotrophic bacteria, followed by a second aerated zone. treatment for nitrogen removal.

Both treatment areas can be physically separated. A fraction of the nitrates produced in the second aerated treatment zone can be re-circulated to the first anoxic treatment zone.

On the one hand, this system has the advantage of producing, in a natural way, little excess sludge, these sludges being on the other hand highly fermentable and therefore a potential source of carbon that can be used for denitrification after specific treatment. The invention proposes a new solution combining the advantages of the fixed culture for the treatment of nitrogen (nitrification and denitrification) and those of a high load activated sludge at the top, to improve the overall treatment. This association allows a significant reduction in the size of structures and oxygen requirements and also allows the use of a high-speed biomass separation system by sludge flotation, compatible with the reduction objective of the size of the treatment works and the quality of the discharges on the phosphorus concentration.

The supports of the fixed biomass are retained in the reactor while the treated water and the free biomass are oriented on the high-speed separation flotation, according to the requirements of the treated water quality.

The float has a significantly lower volume than a conventional clarifier and the possible addition of reagent ensures optimal quality of treatment and offers the possibility of precipitation removal of excess phosphorus.

The zone of high head load is useful to allow hydrolysis of the dissolved organic matter, or particulate, brought by the wastewater. This pre-hydrolysis under anoxic conditions produces easily assimilable organic matter necessary for the denitrification stage and improves the kinetics of nitrification by autotrophic bacteria in the aerated zone.

In addition, some of the nitrates are degraded in this area of high head load thereby reducing the amount of supports in anoxic zone.

The complete treatment of nitrogen (nitrification and denitrification) can be advantageously obtained either by sequential aeration or by a channel-like configuration allowing the use of the residual carbon after the high charge for denitrification.

In addition to the gain in terms of structure size and oxygen supply, the invention makes it possible to clog, thanks to the high load, the organic load variations often observed and conducive to malfunctions of the filamentous type on the aeration treatments. prolonged, and on fixed cultures to limit the development of a loose biofilm little resistant to abrasion forces implemented in the aerated or anoxic reactor.

Depending on the operating conditions and the standards of rejection of the treated water, it is possible to separate the zone of high load from that of fixed culture and to establish preferential circulation of the supports, the liquid phase and the solid phase. The invention consists, apart from the arrangements described above, in a certain number of other arrangements which will be more explicitly discussed hereinafter with regard to embodiments described with reference to the appended drawing, but which are not in no way limiting. On this drawing :

Fig.1 is a diagram of an installation according to the invention and

Fig.2 is a diagram of an alternative embodiment of the installation according to the invention.

Referring to Fig.1 of the drawing, one can see an installation E for the purification of waste water which comprises a biological reactor 1 containing a set of microorganisms having different metabolic spectra for the removal of carbon and nitrogen or even phosphorus. Part of the microorganisms is fixed on mobile solid supports 2 schematically represented by circles in the drawing. The arrival of water to be treated is provided by a pipe 3. The output of the treated effluent is provided by a pipe 4. A retention grid (not shown) is provided on the outlet of the treated effluent to maintain the supports 2 in the reactor 1.

Another part of the microorganisms is free in the reactor 1 liquor for an activated sludge type treatment.

The reactor 1 comprises a first non-aerated treatment zone 1a situated on the inlet pipe 3 side. This zone is advantageously provided for an anoxic treatment with a high load. This anoxic zone does not comprise, in the bottom of the reactor, a means of aeration for injecting air.

The high load corresponds to a mass load greater than 0.4 kgDBO ₅ .kg ^"1 MV.j ^'1. The letter j corresponds to" day ", BOD corresponds to the biochemical oxygen demand, and BOD ₅ corresponds to the quantity. of oxygen consumed after 5 days of incubation. MV is the volatiles content that is an approximate measure of organic matter (see "technical Digest water" ^10th edition Degrémont, Volume 1, pages 418- 419, and pages 534-535).

This first high-load anoxic treatment zone allows removal of most of the carbon and a portion of the oxidized nitrogen mainly by the action of heterotrophic bacteria. The first zone 1a is followed by a second aerated treatment zone 1b for the oxidation of nitrogen, essentially by autotrophic bacteria. The second zone 1b comprises, in the bottom, aeration means 5 constituted for example by air blowing ramps in the liquor located above.

The treated effluent leaving the line 4 of the reactor 1, is sent into a float 6 whose solid / liquid separation rate is greater than 10m / H. Floats of this type are marketed by the plaintiff company, and are described in particular in the already cited work "Memento technique of water" 10 ^th edition, Volume 2, pages 876-877. The solid particles collect in the form of a mud bed 7 at the top of the float. Part of the sludge is re-circulated through a pipe 8 to the inlet pipe 3 of the reactor 1. The other part of the sludge is evacuated.

The re-circulation of the sludge is controlled so that the suspended solids concentration (MES) of the effluent sent via line 4 into the float 7 remains between 0.3 g / l and 1.5 g / l in order to ensure a good Operation of this float 6. To exercise control, at least one probe 9 for measuring the concentration of MES, in particular in the anoxic treatment zone 1a, is provided. The result of the measurement is transmitted to a controller 10 or microcomputer which, depending on the desired concentration of MES in the float 7, controls the rate of recirculation of the sludge by adjusting the speed of a pump 11 installed on the pipeline 8.

Fig.2 shows an alternative embodiment of the installation according to which the biological reactor 21 comprises two treatment zones 21a, 21b physically separated.

Zone 21a is a non-ventilated zone, not including a means of aeration. This zone 21a, advantageously anoxic zone, comprises at least one stirring means 12, in particular of the spiral type.

The elements of Fig.2 identical or playing a role similar to elements already described about Fig.1 are designated by the same reference numerals without their description is repeated. The ventilated zone 21b, provided in the lower part of aeration means 5, communicates with the zone 21a by a non-visible passage in FIG. 2 provided with a grid for retaining the solid supports 2 in their respective portions 21a and 21b.

A fraction of the nitrates produced in the second aerated zone 21b of treatment is re-circulated by a line 13 from zone 21b to zone 21a.

In the installation of Figs. 1 and 2 the treated water is removed from the float 6 by a pipe 14. Zone 1a or 21a at the head of the installation allows hydrolysis of dissolved organic matter, or particulate, brought by the wastewater. This pre-hydrolysis under anoxic conditions produces easily assimilable organic matter necessary for the denitrification stage and improves the kinetics of nitrification by autotrophic bacteria in aerated zone 1b.

In addition, some of the nitrates are degraded in this area of high head load, thereby reducing the amount of supports 2 in the anoxic zone.

Complete nitrogen treatment (nitrification and denitrification) can be achieved either by sequenced aeration or by a channel-like configuration allowing the use of residual carbon after the high load for denitrification.

Example

The data presented below correspond to a daily hydraulic flow of 420 m ³ / H, a MES concentration of 250 mg / L with a MV content (volatile matter) of 80%. The COD of the effluent is 500 mg / L, 40% of which is BOD. The nitrogen concentration is 45 mg / L.

The water treatment objectives are: TWA of 20 mg / L, BOD of 20 mg / L, NGL (global nitrogen) of 14 mg / L and PT (total phosphorus) of 2 mg / L.

The presence of a biomass with a high mass load (and thus a low sludge age, less than 3 days) under anoxic conditions allows a prehydrolysis of nearly 22% of the incoming COD, which represents a contribution of Easily assimilable DBO of the order of 33mg / L. This BOD addition is necessary to ensure total denitrification since the BOD intake easily assimilated by the water to be treated (without pre-hydrolysis step) is estimated at 70 mg / L whereas the needs to ensure total denitrification are of 98 mg / L.

In the case of a conventional configuration, without re-circulation of high load free biomass, it would have been necessary to provide this additional BOD in the form of methanol, for example.

The presence in the anoxic zone of a free culture under high load conditions also contributes to the elimination of nitrates: this contribution is estimated between 10 and 20%, which represents a saving in support in the anoxic zone. In conclusion, in the numerical example chosen, the advantages of the proposed solution are presented in the table below:

The step of pre-hydrolysis of the organic matter in anoxic condition will promote the kinetics of denitrification by providing easily assimilable BOD (abbreviated: DBOfa) and nitrification kinetics by limiting the carbon input in the aerated zone.

This action on carbon treatment will also play an important role in case of load surges since the concentration of biodegradable carbon will be lower in the aerated zone.

It should be noted that the water treatment plant requires an overall structure size of the order of 40% compared to a conventional solution.

The invention applies, in particular, to any effluent treatment having a carbon concentration which may vary from 100 mg / l to 800 mg / l and containing a proportion of assimilable carbon of 20 to 100%, a carbon mass ratio. / nitrogen above 3.5 and no limit for the carbon / phosphorus ratio.

According to these characteristics, the treated effluent can be waste water of residual, industrial or agricultural origin.

The suspended matter concentration of the effluent can be controlled by the addition of a decantation step at the head of any biological treatment.

Claims

1. A process for the purification of waste water by a biological treatment using a set of microorganisms having different metabolic spectra, for the removal of carbon and nitrogen, or even phosphorus, a part of the micro-organisms organisms being fixed on mobile solid supports and constituting a fluidized fixed biomass, characterized in that:

a first non-aerated treatment zone (1a, 21a) is provided, followed by a second aerated treatment zone (1b, 21b),

a portion of the sludge recovered by flotation is re-circulated to the activated sludge treatment, this re-circulation being controlled so that the MES concentration of the effluent subjected to the liquid / solid separation remains compatible with the flotation retained.

2. Method according to claim 1, characterized in that one couples conditions of high mass load in free culture and a nitrogen treatment in low mass load condition by the fixed fluidized biomass.

3. Method according to claim 1 or 2, characterized in that the recirculation of sludge recovered by flotation to the activated sludge treatment is controlled so that the concentration of MES flotation effluent remains between 0.3 g / L and 1.5 g / L.

4. Method according to any one of claims 1 to 3, characterized in that it is expected that the non-aerated treatment zone is an anoxic treatment zone (1a, 21a) at high load, for elimination of the major part of the carbon and a part of the nitrogen mainly by action of heterotrophic bacteria, followed by the second aerated treatment zone (1b, 21b) for the removal of nitrogen.

5. Method according to claim 4, characterized in that the two treatment zones (21a, 21b) are physically separated.

6. Process according to claim 5, characterized in that a fraction of the nitrates produced in the second aerated treatment zone is re-circulated to the first anoxic treatment zone.

7. Method according to one of claims 4 to 6, characterized in that the concentration of MES is measured in the first high-load anoxic treatment zone, and the rate of recirculation of the sludge recovered by flotation to maintain the SS concentration in the desired range.

8. Installation for the purification of wastewater comprising a biological reactor containing a set of microorganisms having different metabolic spectra, for the elimination of carbon and nitrogen, or even phosphorus, a part of the microorganisms being fixed on mobile solid supports and constituting a fluidized fixed biomass, characterized in that:

the biological reactor (1, 21) comprises a first non-aerated treatment zone (1a, 21a) followed by a second aerated treatment zone (1b, 21b),

the plant comprises, downstream of the biological reactor (1, 21), a float (6) with a separation rate greater than 10 m / H which receives the effluent leaving the biological reactor,

a circuit (8) is provided to re-circulate a portion of the sludge recovered from the float to the biological reactor,

and means (9, 10, 11) are provided for maintaining in a given range the concentration of MES in the effluent subjected to the liquid / solid separation.

9. Installation according to claim 8, characterized in that the MES concentration of the effluent subjected to liquid / solid separation is maintained between 0.3 g / L and 1.5 g / L.

10. Installation according to claim 8 or 9, characterized in that the means for maintaining in a determined range the MES concentration of the effluent subjected to the liquid / solid separation comprise:

a means (9) for measuring the concentration of MES in the biological reactor, a controller (10) to which the result of the measurement is sent,

and a pump (11), controlled by the controller, for the recirculation flow rate.

11. Installation according to any one of claims 8 to 10, characterized in that the first non-aerated zone of treatment (1a, 21a) is a high-load anoxic zone, for removal of most of the carbon and part of the nitrogen mainly by action of heterotrophic bacteria, followed by the second aerated treatment zone (1b, 21b) for nitrogen removal.

12. Installation according to claim 11, characterized in that the two treatment zones (21a, 21b) are physically separated.

13. Installation according to claim 12, characterized in that a fraction of the nitrates produced in the second aerated treatment zone (21b) is re-circulated (8) to the first anoxic treatment zone (21a).

14. Installation according to any one of claims 11 to 13, characterized in that the high load of the first anoxic treatment zone (1a, 21a) is greater than 0.4 kgDBO ₅ .kg ^"1 MV.j ^{" 1} .