Water softening treatment using in-situ ballasted flocculation system

Abstract

The present invention concerns a process for treating waters containing at least two different dissolved inorganic salts which do not precipitate and/or crystallize in the same conditions by precipitation and ballasted flocculation, in which the ballast is produced in situ.

Claims

1. A process for treating waters containing dissolved inorganic salts by precipitation and ballasted flocculation comprising the following steps: asupplying water containing dissolved inorganic salts, said inorganic salts comprising at least two different inorganic salts which do not precipitate and/or crystallize in the same conditions; bin a first reactor precipitating and/or crystallizing a first inorganic salt in order to obtain particles having a controlled size whose D50 in volume measured by a Coulter granulometer is within 10 to 2500 m, and separating on the one hand water depleted in said first inorganic salt and on the other hand precipitated/crystallized particles of said first inorganic salt having the controlled size, wherein said first reactor is capable of simultaneously carrying out the precipitation and/or crystallization and a classification of the size of the particles; cin a second reactor precipitating a second inorganic salt from the water depleted in said first inorganic salt and collecting water depleted in said first and second inorganic salts; dflocculating by addition, in the water depleted in said first and second inorganic salts, of a flocculant, and of a ballast which is stable in the flocculation conditions, said ballast being constituted by some or all the precipitated/crystallized particles of said first inorganic salt having a controlled size obtained in step (b); and eseparating the treated water from the solid contained therein and collecting said treated water.

2. The process according to claim 1, comprising a coagulation intermediary step (b1) between steps (b) and (c) or between steps (a) and (b), by addition of a coagulant.

3. The process according to claim 1, wherein the water of step (a) is an industrial, municipal, surface or underground water.

4. The process according to claim 1, wherein the reactor of step (b) is a high solid reactor with integrated solid-liquid separation or a fluidized bed.

5. The process according to claim 1, wherein the two different inorganic salts precipitate in different pH conditions and/or in different temperature conditions and/or by addition of different precipitation reagents and/or by addition of another solvent and/or in different redox conditions.

6. The process according to claim 1, wherein said first inorganic salt is selected from calcium carbonate, calcium sulfate, barium sulfate and mixture thereof.

7. The process according to claim 1, wherein said second inorganic salt is selected from silica salts, fluorides salts, phosphates salts, strontium salts, metallic salts and mixture thereof.

8. The process according to claim 1, wherein: said first inorganic salt is calcium carbonate and said second inorganic salt is selected from silica salts, metallic salts and mixture thereof or said first inorganic salt is calcium sulfate and said second inorganic salt is selected from fluorides salts, phosphates salts and mixture thereof or said first inorganic salt is barium sulfate and said second inorganic salt is strontium salts.

9. The process according to claim 1, wherein the flocculant is brought into contact with the ballast before their use in step (d).

10. The process according to claim 1, wherein part of the water of step (a) is directly added in step (c), without being pre-treated in step (b).

11. The process according to claim 1, wherein step (e) is carried out in a lamellar clarifier.

12. The process according to claim 1, wherein the reactor of step (c) is stirred.

13. The process according to claim 1, wherein steps (c) and (d) are carried out simultaneously in the same reactor.

14. The process according to claim 1, wherein steps (c) and (d) are carried out successively in different reactors.

15. The process according to claim 14, wherein the reactor of step (d) is stirred.

16. The process according to claim 4, wherein the reactor of step (b) is a high solid reactor with integrated solid-liquid separation.

17. The process according to claim 1, wherein the particles of step (b) have a controlled size whose D50 in volume measured by a Coulter granulometer is within 50 to 1000 m.

18. The process according to claim 1, wherein the flocculant is a polyacrylamide polymer.

19. The process according to claim 2, wherein the coagulant is a trivalent metal salt.

20. The process according to claim 3, wherein the water of step (a) is a waste water or an industrial water.

Description

(1) The invention will be better understood in view of the description of the figures and the examples which are given in a non-limitative way.

(2) FIG. 1 represents an example embodiment of the method according to the present invention in which steps (a), (b), (b1), (c), (d) and (e) are carried out.

(3) FIG. 2 represents another example embodiment of the method according to the present invention in which steps (a), (b), (b1), (c), (d) and (e) are carried out and only part of the water of step (a) is directly added in step (c).

(4) FIG. 3 represents a schematic view of the system used in example 1 for carrying out the process according to the present invention.

(5) FIG. 4 represents the cumulative particle size distribution (m, log scale) according to the % vol of the ballast for example 1.

(6) FIG. 5 represents the particle size distribution (m, log scale) according to the % vol of the particles in the flocculation tank compared to ballast and particles precipitated in step (c) for example 1.

(7) FIG. 6 represents a schematic view of the system used in example 2 for carrying out the process according to the present invention.

Example 1: process according to the present invention containing steps (a), (b), (b1), (c) and (d)

(8) In this example, carbonates and silica contained in water are removed in a single process. The schematic view of the system used in this example is represented in FIG. 3.

(9) Calcium carbonate is the first inorganic salt according to the present invention and silica salts the second inorganic salt according to the present invention. Water of step (a) is the water from the Seine River. 850 L/h of this water is added in the reactor of step (b).

(10) Step (b) is carried out in reactor (tank) #1. The reactor used is the reactor described in WO2013/150222 having a size: lLh=5005001300 mm including a precipitation volume of 130 L with a Total Suspended Solids content of 73 g/L and a hydraulic residence time of 9 minutes.

(11) Lime is added as a reagent in order to obtain a pH of between 9.5 and 9.9 and to obtain the precipitation and crystallization of CaCO.sub.3 in the reactor of step (b).

(12) The coagulant FeCl.sub.3, in an amount of 40 mg/L is added in the pipe between the reactor of step (b) and the reactor of step (c).

(13) The coagulation step (b1) is carried out at the same time as step (c) in the same reactor: reactor (tank) #2, which is a Turbomix reactor fully agitated. MgCl.sub.2 at a concentration of 50 mg/L and NaOH in order to obtain a pH of 10.7 are added in the reactor of step (c) in order to obtain the precipitation of silica. The hydraulic residence time in the reactor of step (c) is 13.1 min.

(14) The flocculant which is an anionic polyacrylamide polymer, at a concentration of 0.6 mg/L, is added to the ballast before their addition in the flocculation step (d).

(15) The flocculation step (d) is carried out in reactor (tank) #3, which is a Turbomix reactor fully agitated.

(16) The ballast has a D50 in volume measured by a Beckman Coulter granulometer LS13 320 used with software LS3 series of 480 m and is added in the flocculation step (d) at flowrate of 5.4 L/h.

(17) The treated water is recovered from the reactor #3 outflow (flocculation step (d) outlet).

(18) The chemical characteristics of the effluent before and after treatment are as follow:

(19) TABLE-US-00002 TABLE 2 Results obtained on chemistiy in configuration of example 1 Raw Reactor Reactor #1 Reactor Reactor Reactor #3 water #1 Outflow #2 #3 Outflow pH 7.9 9.9 9.6 10.7 10.7 10.6 Ca (mg/L) 90 25 20 Mg (mg/L) 4 5 10 TAC (eq. mg 165 60 65 CaCO.sub.3/L) SiO.sub.2 (mg/L) 33 28 10

(20) The process according to the invention therefore allow in the case of example 1 the elimination of 70% by weight of the silica contained in the effluent to be treated while 78% by weight of calcium and 61% of alkalinity is also eliminated despite their differences in elimination conditions (pH, reagents . . . ).

(21) Particle size analysis of the ballast created in Tank#1 is carried out with a Beckman Coulter granulometer LS13 320 used with software LS3 series and presented on FIG. 4. The obtained cumulative curve shows a D50 for the particles equals to 480 m.

(22) As can be observed on the graph of FIG. 5, the particle size distribution obtained in a sample of the sludge from the flocculation vessel (Reactor #3) is a mixture of the particle size distribution of the ballast and of the small particles produced during the elimination step of the second pollution (the second inorganic salt).

(23) Jar-tests on ballasted flocculation are carried out on softened Seine river water to demonstrate the advantage of a ballasted flocculation with in-situ ballast production compared to simple flocculation. Flocculation is carried out with single addition of polymer and the ballasted flocculation by addition of polymer and ballast produced in step (b) of example 1.

(24) TABLE-US-00003 TABLE 3 Results of lab-scale jar-tests After ballasted Seine Softened flocculation River Seine After with in-situ water water flocculation produced ballast pH 8.3 9.3 10.7 10.7 SiO2 (mg/L) 33 33 11 9 Turbidity (NTU) 39 1.3 1.4 Settling velocity 4.4 9 (m/h)

(25) As we can see in table 3, the ballasted precipitated particles settle twice faster than precipitated particles. An optimized ballasted flocculation will produce better residual turbidity after the clarification (i.e. settling) step.

Example 2: process according to the present invention containing steps (a), (b1), (b), (c), (d) and (e)

(26) In this example, carbonates and silica contained in water are removed in a single process.

(27) The schematic view of the system used in this example is represented in FIG. 6. In this configuration, coagulation (step b1) is performed first, before step (b).

(28) Calcium carbonate is the first inorganic salt according to the present invention and silica the second inorganic salt according to the present invention.

(29) Water of step (a) is the water from the Seine River. 850 L/h of this water is added in the reactor of step (b).

(30) The coagulation step (b1) is carried out directly in the pipe which feeds the reactor of step (b). The coagulant FeCl.sub.3 is added in an amount of 40 mg/L. Step (b) is carried out in reactor (tank) #1. The reactor used is the reactor described in WO2013/150222 (lLh=5005001300 mm with a precipitation volume of 130 L) with a Total Suspended Solids content of 27 g/L and a hydraulic residence time of 9 minutes.

(31) Lime is added as a reagent in order to obtain a pH of between 9.5 and 9.9 and to obtain the precipitation and crystallization of CaCO.sub.3 in the reactor of step (b).

(32) MgCl.sub.2 at a concentration of 50 mg/L and NaOH in order to obtain a pH of 10.7 are added in the reactor of step (c) (tank #2) in order to obtain the precipitation of silica. The hydraulic residence time in the reactor of step (c) is 13 min.

(33) The flocculant which is an anionic polyacrylamide polymer, at a concentration of 0.6 mg/L, is added to the ballast before their addition in the flocculation step (d). The flocculation step (d) is carried out in reactor (tank) #3, which is a Turbomix reactor fully agitated.

(34) The ballast has a D50 in volume measured by a Beckman Coulter granulometer LS13 320 used with software LS3 series of 475 m and is added in the flocculation step (d) at flowrate of 5.4 L/h.

(35) The water treated after the solid-liquid separation step (e) has a mirror velocity of 40 m/h.

(36) The chemical characteristics of the effluent before and after treatment are as follow:

(37) TABLE-US-00004 TABLE 4 Results obtained in process configuration of example 2 Raw Reactor Reactor #1 Reactor Treated water #1 Outflow #2 water pH 8 9.8 9.7 10.7 10.6 Turbidity 9 280 18 (NTU) TSS (mg/L) 8 27600 260 478 18 Ca (mg/L) 94 30 32 Mg (mg/L) 3 4 22 TAC (eq. mg 150 40 60 CaCO.sub.3/L) SiO.sub.2 (mg/L) 31 25 10

(38) The process according to the invention therefore allow in the case of example 2 the elimination of 67% by weight of the silica contained in the effluent to be treated while 65% by weight of calcium and 60% of alkalinity is also eliminated despite their differences in elimination conditions (pH, reagents . . . ).