Method for a water treatment in a system comprising at least one sequencing batch reactor and a moving bed biofilm reactor

Abstract

The present invention concerns a method for biological treatment of carbon, nitrogen and optionally phosphorus in water, in a reactor system (1) comprising a sequencing batch reactor (SBR) (2) and a moving bed biofilm reactor (MBBR) (3). The method comprises a step (10) of filling said SBR reactor (2) with water to be treated (5), a step (20) of anoxic/aerobic biological treatment in said reactor system (1) and a step (30) of discharging treated water (35) from said SBR reactor (2). The anoxic/aerobic biological treatment step (20) comprises: a biological treatment (210) under largely anoxic conditions in the SBR reactor (2), producing a first effluent (215), a biological treatment (220) under aerobic conditions in the MBBR reactor (3), producing a second effluent (225), and a continuous recirculation of the first and second effluents. The present invention also concerns a corresponding facility.

Claims

1. A method of biologically treating water containing carbon and nitrogen in a system of reactors comprising at least first and second sequencing batch reactors (SBR) disposed in parallel and one moving bed biofilm reactor (MBBR), the method comprising the steps of: filling each of the SBRs with water to be treated; biologically treating the water under predominantly anoxic conditions by a first biomass in the first and second SBRs, with each SBR producing a first effluent, and wherein said first biomass comprises mainly heterotrophic denitrifying microorganisms; biologically treating the first effluent continuously and solely under aerobic conditions in the MBBR, the MBBR producing a second effluent and directing the second effluent back to the first SBR or the second SBR, and wherein said second biomass comprises a biofilm with predominantly nitrifying biofilm carried on media; continuously recirculating the first and second effluents between said MBBR and each of said first and second SBRs sequentially, wherein in one sequence the continuously recirculating of the first and second effluents is between the first SBR and the MBBR and in another sequence the continuously recirculating of the first and second effluents is between the second SBR and the MBBR, such that the biologically treating under anoxic conditions and the biologically treating under aerobic conditions take place by turns between said MBBR and each of said first and second SBRs sequentially.

2. The method of claim 1 wherein the step of filling each of the first and second SBRs is implemented under anaerobic conditions and wherein a step of biologically treating under anaerobic conditions in the first and second SBRs is implemented before said steps of biologically treating under anoxic conditions and biologically treating under aerobic conditions.

3. The method of claim 2 wherein said step of filling each of said first and second SBRs has a duration of ten minutes to four hours and wherein said step of biologically treating under anaerobic conditions has a duration of ten minutes to three hours.

4. The method of claim 1 wherein said first biomass comprises denitrifying polyphosphate accumulative organisms.

5. The method of claim 1 wherein said steps of biologically treating under anoxic conditions and biologically treating under aerobic conditions have a total duration of one hour to eight hours.

6. The method of claim 1 wherein said first biomass is a suspended or granular biomass or a biofilm carried on media.

7. The method of claim 1 wherein the concentration in diatomic oxygen dissolved in said second effluent is 1 milligram to 6 milligrams per liter.

Description

5. LIST OF FIGURES

(1) The invention, as well as its different advantages, shall be understood more clearly from the following description of a non-restrictive embodiment, given with reference to the appended figures, of which:

(2) FIG. 1 represents a system of reactors according to the present invention comprising an MBBR reactor and three SBMBBR reactors disposed in parallel;

(3) FIG. 2A represents the steps of the method according to the present invention in the system of reactors according to FIG. 1;

(4) FIG. 2B provides a detailed view of the step of anoxic/aerobic biological treatment in the system of reactors according to FIG. 1;

(5) FIG. 3 represents the steps of the method according to the present invention furthermore comprising complementary steps of biological treatment;

(6) FIG. 4 represents a time lagging phasing that is particularly suited to the different steps of the method according to FIG. 3 for each of the reactors of the system of reactors of FIG. 1.

6. DESCRIPTION OF DETAILED EMBODIMENTS OF THE INVENTION

(7) Referring to FIG. 1, the system of reactors 1 comprises three sequencing batch moving-bed biofilm reactors 2, SBMBBR.sub.1, SBMBBR.sub.2, and SBMBBR.sub.3 and one moving-bed biofilm reactor 3, MBBR. Referring to FIGS. 2a and 2b, the method of biological treatment of carbon, nitrogen and, optionally, phosphorous in water, in the system of reactors 1 comprises the steps of:

(8) filling 10 each of the three sequencing batch moving-bed biofilm reactors 2 with water to be treated 5;

(9) carrying out anoxic/aerobic biological treatment 20, in the system of reactors 1, of the water present in the three sequencing batch moving-bed biofilm reactors 2; and

(10) removing 30 treated water 35 from each of the sequencing batch moving-bed biofilm reactors 2.

(11) The step of anoxic/aerobic biological treatment 20 in the MBBR reactor 3 and one of the SBMBBR reactors 2, for example SBMBBR.sub.1, comprises:

(12) biological treatment 210 in predominantly anoxic conditions by a first biomass fixed on media in the SBMBBR 2, producing a first effluent 215, the first biomass comprising heterotrophic micro-organisms with nitrifying effect;

(13) biological treatment 220 in aerobic conditions by a second biomass in the MBBR 3 producing a second effluent 225, the second biomass comprising a biofilm with nitrifying effect carried on media; and

(14) continuous recirculation of the first effluent 215 towards the MBBR reactor 3 and of the second effluent 225 towards the reactor SBMBBR.sub.1 2.

(15) The step of anoxic/aerobic biological treatment 20 also takes place firstly in the MBBR reactor 3 and the reactor SBMBBR.sub.2 and secondly in the MBBR reactor 3 and the reactor SBMBBR.sub.3 simultaneously or not simultaneously. One particular mode where the biological treatment is implemented sequentially is presented here below with reference to FIG. 4.

(16) The reactors SBMBBR 2 (SBMBBR.sub.1, SBMBBR.sub.2, SBMBBR.sub.3) are initially empty of water.

(17) The filling step 10 can be carried out via a tap at a low point of the reactors SBMBBR 2 or by a siphoning system in a low central position of the reactors SBMBBR 2 until the high filling level is reached. The filling step 10 can be implemented simultaneously or else sequentially in the three SBMBBR reactors 2. For each SBMBBR reactor 2, for example, the reactor SBMBBR.sub.1, a mechanical stirring can be activated as soon as all the media of the first biomass have been sufficiently immersed to favor the stirring. Mechanisms of biological dephosphatation can then start through the presence of PAO bacteria, preferably DPAO bacteria present in the media of the first biomass. A degradation or reduction of soluble organic carbon present in the water during treatment is observed along with a releasing of soluble inorganic phosphorous. The filling step 10 can be rapid, lasting several (about 10) minutes or it can be slow, lasting up to 4 hours.

(18) Should the filling step 10 be rapid, and depending on the characteristics of the water to be treated for each SBMBBR reactor 2, referring to FIG. 3, the method may furthermore include a step of biological processing in anaerobic conditions 18 in the SBMBBR reactors 2 after the end of the filling step 10 for filling the SBMBBR reactors 2. This additional step is implemented in anaerobic conditions and with stirring. It enables an increased activation of the mechanisms of biological dephosphatation. It can be implemented for several tens of minutes up to three hours depending on the characteristics of the water to be treated 5.

(19) The step of anoxic/aerobic biological treatment 20 relies on the working of the SBMBBR reactors 2 under predominantly anoxic conditions, the working of the MBBR reactor 2 under aerobic conditions and the continuous recirculation of each of the first effluents 215 of the SBMBBR reactors 2 with the effluent 225 of the MBBR reactor 3. In the present case, the first effluents 215 include the liquid fraction of water under biological treatment in anoxic conditions in the SBMBBR reactor 2 and generally include particles of suspended matter but do not contain the carrier media of the first biomass. Similarly, the second effluent 225 includes the liquid fraction of water under biological treatment in the MBBR reactor 3 and generally includes particles of suspended solids but does not contain the carrier media of the second biomass. The carrier media of the first biomass and the second biomass can easily be retained within their respective reactors 2, 3 during the continuous recirculation of the effluents 215 and 225 through perforated screens disposed in each of the reactors 2, 3. The diameter of the holes of the perforated screens makes it possible to let through the liquid fraction of the effluents 215, 225 as well as the suspended solids but retains the biomass carrier media. The step of anoxic/aerobic biological treatment 20 in the SBMBBR.sub.1 2 and MBBR 3, SBMBBR.sub.2 2 and MBBR 3 and SBMBBR.sub.3 2 and MBBR 3 can be implemented simultaneously or else sequentially in the three SBMBBR reactors 2. The total duration of the step of anoxic/aerobic biological treatment 20 between one of the SBMBBR reactors 2, for example SBMBBR.sub.1 and the MBBR reactor 3, is variable and can be adjusted according to the characteristics of the water to be treated 5 or the water under treatment and on the nature of the first biomass and the second biomass. It could generally last from one hour to 8 hours.

(20) The biological treatment 210 under predominantly anoxic conditions in the SBMBBR reactors 2 enables a denitrification to be carried out. The nitrates then present in the water under treatment in the SBMBBR reactors 2 are converted into diatomic nitrogen by means of the denitrifying bacteria at least partly forming the first biomass. If necessary, it is also this biological treatment that predominantly carries out a biological dephosphatation. The phosphorous/phosphate ions are absorbed by over-accumulation by the PAO micro-organisms, especially DPAO micro-organisms at least partly forming the first biomass. An elimination of soluble carbon is also observed. During the biological treatment 210, in predominantly anoxic conditions, the SBMBBR reactor 2 can be stirred by means of a mechanical stirrer. An aeration can also be activated temporarily during the treatment in order to ensure greater refinement of the phosphorous treatment.

(21) The biological treatment 220 under aerobic conditions in the MBBR reactor 3 obtains a nitrification. The oxygen concentration could be included between 1 and 6 mg/L in the MBBR reactor 3 depending on the characteristics of the water to be treated and the nature of the last biomass.

(22) Once the step of anoxic/aerobic biological treatment 20 is completed, the water treated by anoxic/aerobic biological treatment 20 is brought together in the SBMBBR reactors 2.

(23) Referring to FIG. 3, a refining step of biological treatment 28 in the SBMBBR reactors 2 can sometimes be necessary after the step of anoxic/aerobic biological treatment 20. This additional step finalizes the denitrification and/or dephosphatation in the SBMBBR reactors 2. It generally takes place under anoxic and/or aerobic conditions. An aeration can also be activated during the treatment in order to ensure greater refinement of the phosphorous treatment.

(24) The mechanical stirring, if it exists, of the SBMBBR reactors 2 is stopped. The draining step 30 in the SBMBBR reactors 2 is implemented by gravity of pumping at a low point of the reactor or again through a siphoning system at a central low position of the reactors. The carrier media of the first biomass are then maintained in the SBMBBR reactors 2 through the placing of perforated screens, the diameter of the holes of which enables the removal of the treated water 35 while at the same time preventing the passage of the carrier media.

(25) An additional step of clarification (not shown) of the drained treated water 35 can follow the draining step 30. The structure for the additional step of clarification is however very compact because of the low concentration in suspended solids (SS).

(26) Referring to FIG. 4, the three SBMBBR reactors 2 are used with the method according to FIG. 3. The step of biological treatment 20 is implemented sequentially by the MBBR reactor 2 and in turn the reactors SBMBBR.sub.1, SBMBBR.sub.2 and SBMBBR.sub.3. The SBMBBR reactors 2, for example SBMBBR.sub.1, are placed in anaerobic conditions during the filling step 10 and the biological treatment step in anaerobic conditions 18. The SBMBBR reactors 2, for example SBMBBR.sub.1, are then placed in anoxic conditions during the step of anoxic/aerobic biological treatment 20 (biological treatment in anoxic conditions 210) and then during the refining step of biological treatment 28. The start and the end of continuous recirculation of the effluents 215, 225 between the SBMBBR reactors 2 and the MBBR reactor 3 are indicated by dotted lines. Finally, the removal step 30 enables treated water 35 to be recovered. The steps of the method are shifted or lagged for the reactors SBMBBR 1, SBMBBR 2 and SBMBBR 3 in such a way that the step of anoxic/aerobic biological treatment (20) is implemented in alternation and successively in the reactor SBMBBR.sub.1 2 and MBBR 3, SBMBBR.sub.2 2 and MBBR 3 and SBMBBR.sub.3 2 and MBBR 3. The MBBR reactor 3 then works in a way that is on the whole continuous and only under aerobic conditions. The MBBR 3 thus provides optimally for optimized nitrification for the water to be treated in the reactors SBMBBR.sub.1, SBMBBR.sub.2 and SBMBBR.sub.3.

(27) The characteristics of the water to be treated 5 or the water under treatment can especially be measured by a certain number of indicators (the French measurement standards are indicated in brackets); the chemical oxygen demand (DCO or COD-NF T 90-101), the biological oxygen demand (DBO or BOD-NF EN 1899-1), the suspended matter or suspended solids (MES or SS NF T 90-105(2), NF EN 872), Kjeldahl nitrogen corresponding to the sum of the nitrogen in ammonia and organic form (NF EN25663), the quantity of ammonium (NF T 90-015), the quantity of nitrate ions (NF T 90-045), the quantity of phosphorous (NF EN ISO6878). In order to optimize performance during each of the steps of the method, the different reactors 2, 3 can be equipped with specific sensors or probes, especially to measure the redox potential, the diatomic oxygen concentration, the nitrate concentration, the ammonium concentration, and the phosphate concentration.

(28) In the embodiment described here above, the first biomass and the second biomass are both carried on media. These media have a shape and surface characteristics specifically chosen to enable efficient adhesion of the first biomass, and the second biomass respectively to said carrier media. They are manufactured from synthetic materials and have a density close to that of water, preferably ranging from 0.9 to 1.1. This is a preferred embodiment of the invention since the biomass carried on media makes it possible to obtain especially robust biofilms resistant to variations in charge with an increased selection of micro-organisms of interest, and grows with a high concentration of micro-organisms. The method obtained is therefore flexible with respect to variations in charge, and is stable and robust. The corresponding installation is therefore very compact. It makes it possible especially to obtain an energy-competitive method adapted to the simultaneous treatment of carbon, nitrogen and phosphate, given that nitrification can thus be optimized and that dephosphatation can be mainly coupled with denitrification.

(29) As an alternative to the embodiment described here above, the first biomass can be an activated-sludge type or granular type of suspended biomass. In this case, the SBR reactors 2 are not SBMBBR type reactors but suspended-biomass SBR type reactors. Although this is not a preferred embodiment, since the corresponding installation is in principle be less energy competitive and less compact, it can be envisaged especially in the case of the rehabilitation of already existing installations. Certain differences to be taken into account are especially the fact that the first suspended biomass or granular biomass type of biomass is present in the suspended biomass or granular biomass SBR reactors 2 as well as in the MBBR reactor 3 during the step of anoxic/aerobic biological treatment 20. The first suspended biomass or granular biomass type of biomass is less robust, less resistant to variations in charge with a less substantial selection of micro-organisms of interest since it is alternately in aerobic and in anoxic conditions and grows in lower concentrations of micro-organisms. In addition, a decantation or settling step, which may be lengthier or shorter depending on the speed of settling of the first suspended biomass or granular type of biomass, must be carried out before the draining step 30. The draining step 30 can be implemented by means of buckets. In this alternative embodiment, the second biomass is a biofilm on media remaining permanently in the MBBR reactor 3, thus giving optimized nitrification and therefore giving installations of smaller volume than in the prior art.

7. CONCLUSIONS ON ADVANTAGES PROVIDED BY THE INVENTION AS COMPARED WITH THE PRIOR ART TECHNIQUES

(30) Compared with the prior-art methods that use only a suspended-biomass SBR reactor (of the conventional or hybrid type), the method according to the invention has especially the following advantages (in the case especially where the SBR reactors of the reactor system are suspended biomass or granular biomass SBR reactors):

(31) It enables better nitrification because the MBBR is aerated continuously and includes a dedicated nitrifying biomass on biofilm. The consequence of this is that the cycle in the SBR reactors are shorter, inducing a gain in volume of the corresponding installations.

(32) It reduces the minimum sludge age of the suspended biomass. Indeed, the system of MBBR-SBR reactors according to the present invention comprises on the whole less suspended biomass than an prior art SBR reactor with suspended or granular biomass. In addition, the MBBR reactor makes it possible to maintain high nitrification performance through its nitrifying biofilm. This induces a gain in volume of the corresponding installations;

(33) The nitrifying biofilm of the MBBR reactor is more robust than the suspended biomass of the prior-art suspended biomass or granular biomass SBR reactors and enables better management of sudden increases in charge to be nitrified since it is almost exclusively dedicated to nitrification. This implies a greater robustness and a better management of ammonia load peaks by the corresponding installations.

(34) As compared with the prior-art methods using only an IFAS-SBR reactor, the method according to the present invention has especially the following advantages (in the case especially where the SBR reactors of the system of reactors are suspended biomass or granular biomass SBR reactors):

(35) The MBBR reactor works continuously without a settling phase in which the media take a great deal of place and leads to a limit on the volume of water that can be treated. This induces a gain in volume of the corresponding installations;

(36) An installation can be easily rehabilitated by simple modification of configuration between reactors, the SBR reactors having no need of being drained and immobilised during the reconfiguration. This therefore means that the method according to the present invention can be easily implemented in an existing installation.

(37) In comparison with the prior art methods using only an SBMBBR reactor, the method according to the present invention has especially the following advantages (when the SBR reactors of the system of reactors are SBMBBR type reactors): the presence of carriers to support the first biomass makes it possible to remove a step of settling the sludges in the SBMBBR reactors; the biological dephosphatation (in SBMBBR) and the nitrification (in MBBR) are uncoupled thus eliminating competition between the dephosphating and nitrifying populations relative to diatomic oxygen. Thus, the nitrification can be implemented more efficiently and can be completed more rapidly, implying shorter cycles of operation for the SBMBBR reactors and therefore a gain in volume of the corresponding installations; in the same way, the SBMBBR reactors have shorter cycles, the organic loads applied are greater, the biofilm is therefore thicker, thus favoring the obtaining of an anoxic zone in the biofilm and therefore favoring denitrification; the recirculation of the nitrate ions produced in the MBBR reactor towards the SBMBBR reactors enables the dephosphatation to be mainly carried out by consumption of the nitrate ions by DPAO micro-organisms present in the SBMBBR reactors rather than by consumption of diatomic oxygen by PAO micro-organisms. This implies lower consumption of diatomic oxygen and therefore a gain in energy for the corresponding installations.