Method for treating an effluent supersaturated with calcium carbonate in the presence of phosphonate precipitation-inhibiting products

Abstract

The present invention relates to a method for treating an aqueous liquid effluent containing calcium and carbonate ions and containing precipitation-inhibiting products, said process comprising the following successive steps: a) providing an aqueous liquid effluent supersaturated with CaCO.sub.3 and containing precipitation-inhibiting products; b) having the effluent obtained in step a) pass into a reactor with high solid content with a solid content maintained between 20 and 800 g/l and integrated solid-liquid separation, at a pH comprised between 8 and 9.2 allowing in a single step precipitation in situ of the aragonite polymorph of calcium carbonate and removal of the precipitation-inhibiting products; c) recovering an aqueous liquid supernatant containing a suspended solids content of less than or equal to 0.1% by mass of the solid content in the reactor, advantageously a suspended solids content of less than 50 mg/l, the precipitation-inhibiting products being phosphonates.

Claims

1. A method of removing contaminant compounds from a feedwater in a membrane separation system while increasing water recovery rate, the method comprising: injecting a precipitation inhibitor reagent into the feedwater; directing the feedwater containing the contaminant compounds and the precipitation inhibitor reagent to a membrane separation unit and producing a permeate and a first concentrate supersaturated with the contaminant compounds and containing the precipitation inhibitor reagent; directing the first concentrate supersaturated with the contaminant compounds and containing the precipitation inhibitor reagent to a reactor containing suspended solids and having an integrated first solids-liquid separator, a mixing zone in the lower portion of the reactor, and a tranquilization structure in an upper portion of the reactor; maintaining a heavy suspended solids concentration of 20-800 g/L in the reactor for the purpose of facilitating the de-supersaturation of the contaminant compounds in the reactor; mixing the supersaturated contaminant compounds, the precipitation inhibitor reagent, and the suspended solids in the mixing zone of the reactor; wherein mixing the supersaturated contaminant compounds with the heavy concentration of suspended solids in the reactor gives rise to de-supersaturating the supersaturated contaminant compounds in the reactor by continuously precipitating in situ the supersaturated contaminant compounds; removing sludge from the reactor wherein the sludge comprises the precipitated contaminant compounds, precipitation inhibitor reagent, and some of the suspended solids; recovering from the reactor a treated supernatant substantially free of suspended solids; and enhancing the water recovery rate of the membrane system by continuously recycling the supernatant to the membrane separation unit, or continuously directing the treated supernatant to a second solids-liquid separator and producing a first filtrate substantially free of suspended solids and a second concentrate and recycling the first filtrate to the membrane separation unit.

2. The method of claim 1 wherein the contaminant compounds include calcium sulfate and wherein the first concentrate is supersaturated with calcium sulfate; and mixing the supersaturated calcium sulfate with the heavy concentration of suspended solids in the reactor causes the supersaturated calcium sulfate to precipitate and crystallize onto the surfaces of the suspended solids.

3. The method of claim 1 including lowering the pH of the feedwater prior to reaching the membrane separation unit and raising the pH of the first concentrate at a point between the membrane separation unit and the reactor.

4. The method of claim 1 wherein the feedwater directed to the membrane separation unit includes contaminant compounds chosen from the group consisting of chloride, boron, magnesium, sulfate, barium, silica, fluoride, strontium, cesium, phosphate ions, metals, and a mixture thereof.

5. The method of claim 1 further including maintaining the concentration of the suspended solids in the reactor between 25 and 200 g/L.

6. The method of claim 1 including maintaining a hydraulic residence time in the reactor at between three minutes and two hours.

7. The method of claim 1 further including maintaining a water recovery rate of at least 95% by continuously recycling the treated supernatant or the first filtrate to the membrane separation unit.

8. The method of claim 1 wherein reactions taking place in the reactor occur in the absence of a coagulating agent or a flocculating agent.

9. The method of claim 1 wherein the method is carried out in the absence of a precipitation reagent.

10. The method of claim 1 including: continuously directing the treated supernatant to the second solids-liquid separator and producing the first filtrate substantially free of suspended solids and the second concentrate and recycling the first filtrate to the membrane separation unit; directing the sludge to a dewatering device and producing a second filtrate and a solids residue; and further enhancing the water recovery rate by continuously directing the second filtrate back to the membrane separation unit.

11. The method of claim 10 including recycling the second concentrate back to the reactor and mixing the second concentrate with the supersaturated contaminant compounds and the suspended solids in the mixing zone of the reactor.

12. A method of removing contaminant compounds from a feedwater in a membrane separation system while increasing water recovery rate, the method comprising: directing the feedwater containing the contaminant compounds to a membrane separation unit and producing a permeate and a first concentrate supersaturated with the contaminant compounds; directing the first concentrate supersaturated with contaminant compounds to a reactor containing suspended solids and having an integrated first solids-liquid separator, a mixing zone in a lower portion of the reactor, and a tranquilization structure in an upper portion of the reactor; maintaining a heavy suspended solids concentration of 20-800 g/L in the reactor for the purpose of facilitating the de-supersaturation of the contaminant compounds in the reactor; mixing the supersaturated contaminant compounds and the suspended solids in the mixing zone of the reactor; wherein mixing the supersaturated contaminant compounds with the heavy concentration of suspended solids in the reactor gives rise to de-supersaturating the supersaturated contaminant compounds in the reactor by continuously precipitating in situ the supersaturated contaminant compounds; recovering from the reactor a treated supernatant substantially free of suspended solids; enhancing the water recovery rate of the membrane system by continuously directing the treated supernatant to a second solids-liquid separator and producing a first filtrate substantially free of suspended solids and a second concentrate and recycling the first filtrate to the membrane separation unit; directing sludge from the reactor to a dewatering device, the sludge comprising the precipitated contaminant compounds and at least some of the suspended solids; wherein the dewatering device produces a second filtrate substantially free of suspended solids and a solids residue; and further enhancing the water recovery rate of the membrane system by continuously recycling the second filtrate from the dewatering device to the membrane separation unit.

13. The method of claim 10 including controlling the water recovery rate by purging a portion of the first filtrate or a portion of the second filtrate.

14. The method of claim 12 including controlling the water recovery rate by purging a portion of the first filtrate or a portion of the second filtrate.

Description

(1) The invention will be understood more clearly in the light of the description of the figures and of the examples that follow.

(2) FIG. 1 represents the scheme of a device for performing the method according to the present invention in which steps i), ii), iii), α), a), b), c), d) and e) are represented.

(3) FIG. 2 represents a 2.sup.nd scheme of a device for performing the method according to the present invention in which steps i), ii), iii), α), a), b), c), d) and e) are represented.

(4) FIG. 3 represents the scheme of a 3.sup.rd device for performing the method according to the present invention in which steps i), ii), iii), α), a), b), c), d), e), f), k) and h) are represented.

(5) FIG. 4 represents the scheme of a 4.sup.th device for performing the method according to the present invention in which steps i), ii), iii), α), a), b), c), d), e), f), k) and h) and the steps for adjusting the pH p1, p2 and p3 are represented.

(6) FIG. 5 represents the scheme of a 5.sup.th device for performing the method according to the present invention in which steps i), ii), iii), iiii), α), a), b), c), d), e), f), k), g) and h) and the steps for adjusting the pH p1, p2 and p3 are represented.

(7) FIG. 6 represents the summary of the principle of the test method of Example 2 on a nanofiltration concentrate with a high content of sulfates (mine).

(8) FIG. 7 represents the SO.sub.4.sup.2− content (in g/L) as a function of the time (in minutes) in the nanofiltration concentrate with a high sulfate content of Example 2 during the implementation of the method illustrated in FIG. 6 as a function of the amount of seeds used.

(9) FIG. 8 represents the summary of the principle of the test method of Example 2 on a nanofiltration concentrate with a high content of carbonates.

(10) FIG. 9 represents the scheme of the device for performing Comparative Example 3.

(11) FIG. 10 represents the results for desupersaturation (with seeding of gypsum at 420 g/l) of the synthetic effluent containing Ca.sup.2+ & SO.sub.4.sup.2− (content in g/L) in the presence of precipitation-inhibiting product (antiscaling agents: mixture of ATMP and of HDTMPA in ppm of PO.sub.4.sup.2−) in the reactor according to Comparative Example 3 as a function of the operating time of the reactor.

(12) FIG. 11 represents an image obtained with a scanning electron microscope at the 10 μm scale of calcium carbonate particles serving for seeding according to Example 4 (synthetic CaCO.sub.3: FIG. 11A and CaCO.sub.3 formed beforehand experimentally during the tests: FIG. 11B).

(13) FIG. 12 represents the residual contents of phosphonates (PO.sub.4.sup.3− in mg/L) as a function of the amount of seeding (in g/L) and of the type of seeding (CaCO.sub.3 or gypsum) and of the pH in the course of the various laboratory tests of Example 4.

(14) FIG. 13 represents the measurement of the content of calcium ions (in mg/L) at the reactor inlet and in the supernatant (reactor outlet), the total alkalinity (in mg/L equivalent of CaCO.sub.3) at the reactor inlet and in the supernatant (reactor outlet) and the phosphonate content (in ppm equivalent of PO.sub.4.sup.3−) at the reactor inlet as a function of the number of days of operating of the method, no phosphonate having been injected during the first two days of operating, in the context of the implementation of the method according to Example 5.

(15) FIG. 14 represents the measurement of the content of residual phosphonate (in mg/L PO.sub.4.sup.3−) at the reactor inlet and in the supernatant (reactor outlet) and the percentage of phosphonate removed (in %) as a function of the number of days of operating of the method, no phosphonate having been injected during the first two days of operating, in the context of the implementation of the method according to Example 5.

(16) FIG. 15 represents the measurement of the particle size (in μm) by analysis via Beckmann laser granulometry for obtaining the particle size distribution of the particle suspension: D10 (D10 is the size distribution diameter of the finest particles, i.e. 10% by volume of the particles have a diameter less than this diameter and 90% by volume of the particles have a diameter greater than this diameter), D50 (D50 is the median size distribution diameter of the particles, i.e. 50% by volume of the particles have a diameter smaller than this diameter and 50% by volume of the particles have a diameter greater than this diameter) and D90 (D90 is the size distribution diameter of the coarsest particles, i.e. 90% by volume of the particles have a diameter smaller than this diameter and 10% by volume of the particles have a diameter greater than this diameter) as a function of the number of days of operating of the method, no phosphonate having been injected during the first two days of operating, and 1.8 mg/l of phosphonate expressed as P-PO4, being injected thereafter, in the context of the implementation of the method according to Example 5.

(17) FIG. 16 represents an image obtained with a scanning electron microscope at the 1 mm scale of calcium carbonate particles in the reactor of Example 5: FIG. 16A: at the start; FIG. 16B: after 2 days of operating; FIG. 16C: after 7 days of operating.

(18) FIG. 17 represents an image obtained with a scanning electron microscope at the 100 μm scale (FIG. 17A) and 30 μm (FIG. 17B) of the precipitated calcium carbonate particles in the reactor in the context of the method according to Example 6.

(19) FIG. 18 represents an image obtained with a scanning electron microscope at the 10 μm scale of the surface of the precipitated calcium carbonate particles in the reactor in the context of the method according to Example 6.

(20) FIG. 19 represents a scheme of the method (line) used in Example 7.

(21) FIG. 20 represents the scheme of the method (line) used in Example 7 with the simulated flow rate, pH, SC (solid content) and SSM data.

(22) FIG. 21 represents the decrease over time of the ATMP and HDTMA mixture (% of removal of PO.sub.4.sup.3−) used as precipitation-inhibiting product and of removal of HCO.sub.3 in the context of the method according to Example 7.

(23) Batch tests made it possible to demonstrate the possibility of spontaneous desaturation of solution in the presence of a high solid content by trapping the precipitation-inhibiting products on the crystals in suspension.

(24) In point of fact, to increase the suspension rate of the membrane lines, in particular reverse osmosis (RO) and nanofiltration (NF), desupersaturation may be used between two stages. Upstream of the NF or RO equipment, precipitation-inhibiting products are added so as to prevent the formation of insoluble precipitates. To precipitate the salts and thus achieve the solubility of a solution, the precipitation-inhibiting products must be inhibited or removed.

Comparative Example 1

(25) Preliminary studies of this question were conducted on a concentrate of first step of nanofiltration (NF) originating from a metallurgical site, i.e. an industrial effluent obtained after a nanofiltration membrane treatment. This effluent is supersaturated with CaSO.sub.4 and contains precipitation-inhibiting products consisting of a mixture of ATMP and HDTMPA with a corresponding content of phosphonates of 1.5 ppm which inhibit the precipitation of the supersaturation salts (CaSO.sub.4). The reagents tested have the purpose of accelerating this rate of precipitation reaction inhibited by the precipitation-inhibiting products.

(26) The following table summarizes the results obtained in terms of time required and of amounts of chemical reagents to initiate the precipitation reaction of the supersaturation compounds (CaSO.sub.4), i.e. the induction time. The tests performed are laboratory tests based on the addition of said reagents in the concentration indicated into beakers containing the effluent with stirring for the time indicated.

(27) TABLE-US-00001 Method Time & consumption Conclusions Precipitation with CaCl.sub.2 Stoichiometric conditions: several days Low and requires a (formation of gypsum) With 14 times the stoichiometry: 2.5 h large amount of without pretreatment chemical reagents Precipitation by formation of Stoichiometric conditions: several days Low and requires a ettringite (addition of lime With 30 times the required dose: 1 h large amount of and Al.sub.2O.sub.3) chemical reagents Phosphonate oxidation via 40 min Energy consumption potassium persulfate + CaCl.sub.2 (heating to 120° C.) Phosphonate oxidation with No effect Not effective ozonation + CaCl.sub.2 Phosphonate complexation 1 h High risk of fouling of with ferric ions + CaCl.sub.2 the membrane

Example 2

(28) The first tests were performed at the laboratory scale in batch mode with 2 different NF concentrates 1.sup.st stage of NF with high contents of sulfates (mine)

(29) Analyses of the NF Concentrate

(30) TABLE-US-00002 Concentration Total phosphorus (P.sub.total) <0.5 mgP/L Orthophosphates (PO.sub.4) <2.5 mgP/L Sulfates 4530 mgSO.sub.4/L Total barium <50 μg/L Calcium (Ca.sup.2+) 680 mg/L Total iron <50 μg/L Potassium 15 mg/L Magnesium 520 mg/L Sodium 570 mg/L Strontium 11 000 μg/L Alkalinity  0.0° F. Total alkalinity 43.1° F. Conductivity (20° C.) 5.6 mS/cm Chlorides 55 mg/L pH 7.90 Turbidity 0.6 FAU An NF concentrate with high contents of carbonates

(31) Analyses of the NF Concentrate with a High Content of Carbonates

(32) TABLE-US-00003 concentration in (mg/L) Ca.sup.2+ 485 Mg.sup.2+ <10 Alkalinity (eq. CaCO.sub.3 mg/L) 701 P-PO.sub.4.sup.3− <0.05 P.sub.total 0.287 ΔP 0.282

(33) To evaluate the change of the phosphonates in the effluent, analysis of the total phosphorus (P.sub.total) and of the phosphates (PO.sub.4) is performed. The phosphonate content is considered as being proportional to the difference between the concentrations of total P and PO.sub.4 (ΔP in the table).

(34) The principle of this method is illustrated in FIG. 6.

(35) Protocol of the NF Concentrate Method (Sulfates)

(36) This method consists in trapping the precipitation-inhibiting products (mixture of ATMP and HDTMPA with a corresponding content of phosphonates of 1.5 ppm) on solid seeds (identical in nature to the salt inhibited by the action of the precipitation-inhibiting products) and in inducing precipitation by adding CaCl.sub.2 to the concentrate in stoichiometric amount. Various amounts of sludges as seed were tested.

(37) The results are represented on the graph in FIG. 7. In this figure, the ratios 1:1 and 10:1 correspond to the solid seed on the amount of solid produced by the reaction.

(38) Equilibrium is reached with a ratio of 10:1. The kinetics are fast with equilibrium reached in 3 minutes.

(39) These tests were later performed on NF concentrates with a high content of carbonate. The precipitation-inhibiting products are first removed by seeding with calcium carbonate particles. Next, desaturation is performed without adding reagents, spontaneously. The protocol is represented in FIG. 8.

(40) The results are collated in the table below.

(41) TABLE-US-00004 NF Desupersaturated concentrate concentrate Ca.sup.2+ (mg/L) 485 312 Mg.sup.2+ (mg/L) <10 <10 Alkalinity (eq. CaCO.sub.3 mg/L) 701 263 P-PO.sub.4.sup.3− (mg/L) <0.05 <0.05 P.sub.total (mg/L) 0.287 <0.05 ΔP (mg/L) 0.282 <0.05

(42) The precipitation-inhibiting products were indeed removed and spontaneous desupersaturation takes place without addition of additional reagents.

Comparative Example 3

(43) Thereafter, continuous pilot tests were thus performed. The treated effluent (nanofiltration concentrate) has the characteristics indicated in the table below:

(44) TABLE-US-00005 Raw water Parameters (NF concentrate) pH 7.3 Conductivity (mS/cm) 8 Ca.sup.2+ (g/L) 0.8 Mg.sup.2+ (g/L) 0.66 SO.sub.4.sup.2− (g/L) 5.9 SiO.sub.2 (mg/L) 38 Phosphonates (mg/L) 1.7 Total alkalinity (ppm of CaCO.sub.3) 1560 Turbidity (NTU) <1 NTU Al (μg/L) <25 Ba (μg/L) <50 Fe (μg/L) <50 Mn (μg/L) <10 Sr (μg/L) 16 000 K (mg/L) 19 Na (mg/L) 760 Cl (mg/L) 82 F (mg/L) <1 P (mg/L) 0.6

(45) A synthetic concentrate was used to perform certain tests, with the main ions present as indicated in the table below.

(46) TABLE-US-00006 Parameters Synthetic effluent Ca.sup.2+ 1 g/L SO.sub.4.sup.2− 6 g/L Total alkalinity 1560 ppm eq. CaCO.sub.3

(47) The precipitation-inhibiting products (antiscaling agent AS) added are ATMP (1 mg/L of PO.sub.4.sup.3−) or a mixture of ATMP and of HDTMPA (1.8 mg/L of PO.sub.4.sup.3). The doses of precipitation-inhibiting product are expressed in phosphate equivalent stemming from the phosphonate analysis. During the tests, the residual precipitation-inhibiting product is expressed as residual phosphonate expressed as mg/L eq PO.sub.4.sup.3−.

(48) The general operating conditions of the continuous tests performed are collated below. Depending on the parameters tested, certain conditions were liable to change from one test to another. Flow rate: 10 l/h Reaction time: 30 min Seeding Gypsum SS target in the reactor: >100 g/L Stirring speed: 950 rpm Test time: several days

(49) Compositions (may change depending on the test): [Total alkalinity].sub.reactor=1 594 ppm eq CaCO.sub.3 [SO.sub.4.sup.2−].sub.reactor=6 g/L [Ca.sup.2+].sub.reactor=1 g/L

(50) Precipitation-inhibiting product: phosphonate (e.g. ATMP and HDTMPA at 1.8 mg/L eq PO.sub.4.sup.3− of phosphonates)

(51) The diagram of the pilot device is illustrated in FIG. 9. The precipitation unit is a continuous pilot composed of a reactor with a high solid content with a stream guide including a reaction zone and a separation zone integrated into the same unit. Homogenization is performed by a mechanical stirrer. The pH is controlled with a probe on an external loop and the regulation reagents are introduced into the reactor. The control of the solid content in the reactor is performed by an extraction pump coupled to a clock.

(52) Tests in the presence of ATMP and HDTMPA are performed on the test bed with a concentration of suspended solids (SS) of 120 g/l. During these tests, it was observed that after one day of operating, the phosphonates were virtually no longer adsorbed. The calcium and sulfate residuals in overflow were similar to the inlet concentrations (zero decrease). As a result, no solid was formed. The concentration of solid in the reactor was divided by 3, decreasing from 120 g/l to 44 g/l.

(53) After one day of operating, the solid in the reactor no longer makes it possible to retain the precipitation-inhibiting product (antiscaling agent). It is possible to envisage that once the solid is saturated with precipitation-inhibiting product (antiscaling agent), the latter inhibits the precipitation, which entails a reduction in the concentration of solid in the reactor. A test performed with higher concentrations of gypsum in the reactor (up to 420 g/l) confirmed this, as illustrated in FIG. 10.

(54) Specifically, after 4 hours of operating (in the presence of ATMP and HDTMPA˜1.3 ppm PO.sub.4.sup.3−), the residual phosphonate after treatment reaches 0.2 mg/L PO.sub.4.sup.3−, which confirms the efficacy of phosphonate reduction (˜85%). Moreover, the residual concentration of calcium and sulfate leaving the reactor shows that spontaneous precipitation does indeed take place.

(55) However, after 24 hours, the residual phosphonate becomes equal to the inlet content. Since the gypsum which seeded the reactor is saturated and the in situ gypsum production does not allow a sufficient rate of renewal with fresh gypsum to continuously remove the phosphonates.

Example 4

(56) Laboratory tests made it possible to demonstrate the key role of calcium carbonate in the trapping of the precipitation-inhibiting products.

(57) The operating conditions used during the tests are as follows (the conditions being adapted as a function of the test performed): precipitation-inhibiting products (antiscaling agents): ATMP+HDTMPA at an equivalent content of PO.sub.4 of 2 ppm total alkalinity as equivalent CaCO.sub.3=1594 ppm Ca.sup.2+=1 g/l reaction time: 5 min initial pH=7.8 pH regulation=addition of NaOH (30%) CaCO.sub.3 synthetic=1.5 g/l d50=28.7 μm(analysis with the Beckmann laser granulometer) CaCO.sub.3 experimental=1.5 g/l d50=6.7 μm(analysis with the Beckmann laser granulometer)

(58) Two types of seeds were tested: commercial lime (calcite: synth. CaCO.sub.3) and lime formed experimentally beforehand during tests (exp. CaCO.sub.3).

(59) The seeding with synthetic calcium carbonate at pH 8.8 did not allow adsorption of the phosphonates, whereas calcium carbonate originating from the method (experimental) shows that 52% of the phosphonates are adsorbed.

(60) The two types of calcium carbonate do not have the same size distributions, morphology and nature (polymorphism).

(61) The synthetic calcium carbonate has a calcite form as illustrated in FIG. 11A, whereas the experimental calcium carbonate appears to have more of an aragonite form as illustrated in FIG. 11B.

(62) It was confirmed that the increase in pH had no effect on the phosphonates alone.

(63) The summary conclusion of the laboratory tests is collated in the table below:

(64) TABLE-US-00007 Without seeds With CaCO.sub.3 seeds pH < 8.4 8.4 < pH < 8.5 pH > 8.5 pH < 8.4 8.4 < pH < 8.5 pH > 8.5 Precipitation No Yes Yes Yes but Yes Yes CaCO.sub.3 only incomplete Adsorption of No No No No Yes Yes phosphonates (only with (only with only CaCO.sub.3exp) CaCO.sub.3exp) Precipitation No No Yes No Yes Yes CaCO.sub.3 and adsorption phosphonates

(65) The removal of the phosphonates is explained by the adsorption on the CaCO.sub.3 particles (without degradation). The nature of the crystalline form of CaCO.sub.3 greatly influences the adsorption rate of the phosphonates onto the crystals. In addition, the pH has an impact on the adsorption rate during the in situ formation of the adsorbent medium.

(66) These tests thus make it possible to demonstrate that the adsorption of the precipitation-inhibiting products necessary for spontaneous precipitation of the supersaturated species is possible only by maintaining the formation of a polymorphic form of CaCO.sub.3 which will be dependent on the pH of the reaction.

(67) The adsorption rates of gypsum and lime during the various laboratory tests are compared in FIG. 12.

Example 5

(68) Following the laboratory tests concerning the adsorption of phosphonates of Example 4, continuous pilot tests were performed at a pH of 8.5 to observe the removal of phosphonates and the precipitation of calcium carbonate.

(69) The diagram of the pilot device is the same as that presented previously in Comparative Example 3 and illustrated in FIG. 9 and the operating procedure is the same as in Comparative Example 3. The operating conditions are as follows: precipitation-inhibiting product (antiscaling agent): ATMP+HDTMPA at an equivalent content of PO.sub.4 of 1.8 ppm total alkalinity: equivalent CaCO.sub.3=1594 ppm Ca.sup.2+=1 g/l the calcium concentration is adjusted by adding CaCl.sub.2 reaction time: 30 min regulated pH=8.5 (by adding 3% NaOH solution) total flow rate: 10 l/h maintenance of a solid content of CaCO.sub.3 in the reactor at 20 g/L CaCO.sub.3 seeding=20 g/l stirring speed: between 750 rpm and 950 rpm

(70) During the first two days of intervention, the precipitation-inhibiting product was not injected so as to study the behavior of the calcium carbonate precipitation at a pH close to 8.5.

(71) Under these working conditions, as may be seen in FIG. 13, about 60% of Ca.sup.2+ and 90% alkalinity are removed. The precipitation reaction generates 1.7 g/l of CaCO.sub.3. The results are collated in the table below:

(72) TABLE-US-00008 Parameters Inlet concentration Outlet concentration [Ca.sup.2+] (g/L) 1.35 0.682 [Ca.sup.2+] (mol/L) 0.034 0.0171 [HCO.sub.3.sup.−] (g/L) 0.949 0.077 [HCO.sub.3.sup.−] (mol/L) 0.0156 0.0013 Precipitated CaCO.sub.3 — 1.7 [CaCO.sub.3].sub.soluble (mol/L) 0.023 0.0047

(73) In the presence of precipitation-inhibiting product, the spontaneous precipitation of the calcium carbonate is maintained. Between 90% and 100% of the precipitation-inhibiting products are trapped in the reactor.

(74) During the test, the SS fluctuate between 2 g/l and 35 g/l in the reactor. Slight scaling is observed in the sludge extraction pipework and also wall clogging which reduces the concentration of suspended solids in the reactor. Although the concentration of SS in the reactor is really high to maintain the stability, the adsorption of phosphonates remains efficient, as may be seen in FIG. 14.

(75) As may be seen in FIG. 15, without the precipitation-inhibiting product, the mean particle sizes (D50) were close to values of between 40 μm and 160 μm.

(76) It is observed that the D50 and the D90 increase greatly after injection of the precipitation-inhibiting product, respectively from 70 μm to 250 μm and from 400 μm to 800 μm. The D10 does not appear to be affected, and there is no further production of fine particles.

(77) The precipitation-inhibiting product has an effect on the particle sizes and appears to greatly improve the agglomeration of the particles (as illustrated in FIG. 16).

(78) These tests at continuous flow rate reveal that the precipitation of calcium carbonate is capable of removing 90% of the phosphonates at a pH regulated between 8.4 and 8.5, allowing desupersaturation of the CaCO.sub.3. The precipitated calcium carbonate tends to reach the reaction equilibrium between the CaCO.sub.3 seeds required for the adsorption of the phosphonates and the precipitation of the CaCO.sub.3 particles. The CaCO.sub.3 particles created with the phosphonates are agglomerated.

Example 6

(79) Thereafter, tests were conducted in the presence of carbonates and sulfate (operating conditions similar to those described previously in Example 5):

(80) Operating conditions: flow rate: 10 l/h reaction time: 30 min seeding: gypsum (6 g/l) and CaCO.sub.3 (100 g/l) SS target in the reactor: >100 g/L stirring speed: 950 rpm test time: several days

(81) Compositions (may vary slightly depending on the test): [Total alkalinity].sub.reactor=1 594 mg/L eq CaCO.sub.3 [SO.sub.4.sup.2−].sub.reactor=6 g/L [Ca.sup.2+].sub.reactor=1 g/L

(82) Precipitation-inhibiting product (antiscaling agent): phosphonate (ATMP and HDTMPA at 1.8 mg/L eq PO.sub.4.sup.3− of phosphonates).

(83) In the same manner as during the preceding tests, more than 90% of the phosphonates are removed and 88% of the alkalinity is removed, this being achieved after more than 10 days of functioning. The table below collates a few results.

(84) TABLE-US-00009 Inlet Outlet Parameters concentration concentration [Ca.sup.2+] (g/L) 1.082 0.636 [Ca.sup.2+] (mol/L) 0.027 0.016 [HCO.sub.3.sup.−] (g/L) 2.115 0.217 [HCO.sub.3.sup.−] (mol/L) 0.034 0.0035 [SO.sub.4.sup.2−] (g/L) 6.420 5.718 [SO.sub.4.sup.2−] (mol/L) 0.067 0.0595 CaCO.sub.3 (g/L) precipitate — 0.35 Gypsum (g/L) precipitate — 1.29 [CaCO.sub.3].sub.soluble (mol/L) 0.030 0.007 [CaSO.sub.4, 2H.sub.2O].sub.soluble (mol/L) 0.043 0.031

(85) The very satisfactory performance in terms of removal of phosphonates throughout the tests confirms that the amount of CaCO.sub.3 produced by the reaction makes it possible to provide a sufficient amount of seeds to maintain the removal of the precipitation-inhibiting products.

(86) The SEM analyses represented in FIGS. 17A and 17B show that the morphology of the precipitated particles is in the form of spherical particles. Moreover, an FX analysis makes it possible to demonstrate the lime CaCO.sub.3. A more detailed SEM analysis suggests that aragonite is indeed present. Specifically, a magnification on the surface of the formed particles (FIG. 18) reveals that they are composed of agglomerates of needles that are characteristic of the aragonite form.

Example 7: Continuous Pilot Tests of Lines with a High Conversion Rate

(87) Tests were conducted so as to validate in continuous pilot functioning the line with a high conversion rate on the basis of the following modeling:

(88) The scheme of the line tested is represented in FIG. 19.

(89) Prior to the tests, a digital simulation was performed using calculation models, the scheme of which is presented in FIG. 20. The modeling of the line on the basis of a real effluent (a mine drainage water) gives the results that are collated in the following table:

(90) TABLE-US-00010 Feed Inlet Inlet Treated water of the reverse desupersaturation Inlet water Parameter approach osmosis reactor MF/UF (RO permeate) Flow rate m3/h 446.85 564.62 141.15 138.55 423.46 Average temperature C. 13.00 13.00 13.00 13.00 13.00 Pressure bar 3.50 0.00 28.60 0.00 1.00 SC mg/L 3374.43 13181.60 52580.71 50447.02 67.08 COT mg/L 0.00 0.00 0.00 0.00 0.00 SSM mg/L 2.00 3.67 14.67 10.00 0.00 pH — 7.35 6.80 8.03 8.45 5.22 Conductivity uS/cm 2949.11 8844.18 27924.26 27398.48 112.88 CO2 mg/L 38.20 82.30 18.68 0.16 48.06 Silica mg/L 12.90 60.67 241.89 241.89 0.26 Boron mg/L 0.00 0.00 0.00 0.00 0.00 Dissolved O2 mg/L 10.29 9.76 9.76 7.73 9.76 Calcium mg/L 340.00 309.51 1234.92 193.81 1.04 Magnesium mg/L 230.00 1084.89 4328.32 4328.32 3.75 Sodium mg/L 301.00 2127.15 8490.37 9054.82 13.73 Potassium mg/L 7.30 33.23 131.58 131.58 0.45 NH4(+) mg/L 0.14 0.31 0.92 0.90 0.10 NH3 mg/L 0.00 0.00 0.01 0.03 0.00 Bicarbonates mg/L 547.22 368.34 1469.93 31.97 4.25 Carbonates mg/L 0.87 0.24 24.24 1.36 0.00 Chlorides mg/L 26.58 122.74 487.50 487.50 1.15 Sulfates mg/L 1900.00 9064.66 36133.26 35959.55 41.80 Nitrates mg/L 1.00 2.80 9.63 9.63 0.53 Total sulfites mg/L 0.00 0.00 0.00 0.00 0.00 Fluoride mg/L 0.50 0.77 3.07 1.82 0.01 PO4(−−−) mg/L 0.00 0.00 0.00 0.00 0.00 HPO4(−−) mg/L 0.07 0.04 0.31 0.00 0.00 H2PO4(−) mg/L 0.03 0.04 0.01 0.00 0.00 Total sulfides mg/L 0.00 0.00 0.00 0.00 0.00 Barium mg/L 0.02 0.02 0.07 0.00 0.00 Strontium mg/L 6.60 5.24 20.92 0.09 0.02 Dissolved Fe mg/L 0.10 0.47 1.88 1.88 0.00 Total Fe mg/L 0.66 0.92 3.66 1.88 0.00 Dissolved Mn mg/L 0.05 0.24 0.94 0.94 0.00 Total Mn mg/L 0.21 0.36 1.45 0.94 0.00

(91) The results obtained in these continuous tests over several days are collated in the table below:

(92) TABLE-US-00011 Reverse osmosis Desupersaturation unit RO Treated water Reactor MF/UF Parameter RO feed concentrate (RO permeate) feed feed SC mg/L 14472.00 42860.00 173.00 SSM mg/L 0.00 0.00 0.00 0.00 300.00 pH — 8.68 8.51 8.51 8.60 8.45 Conductivity uS/cm 16410.00 48600.00 196.00 45169.00 44792.00 Calcium mg/L 300.00 1022.00 3.00 1140.00 750.00 Bicarbonates mg/L 400.00 1470.00 16.50 1160.00 630.00 Sulfates mg/L 8840.00 31900.00 <40 30000.00 28700.00 P—PO4 (HEDP and DTPMPA) mg/L 1.37 5.60 0.05 5.53 0.38 P—PO4 (ATMP and HDTMPA) mg/L 6.82 0.80

(93) A good quality of permeate is thus obtained and, during the desupersaturation step, the removal of the precipitation-inhibiting products (antiscaling agents) is obtained, demonstrated by the reduction in P-PO.sub.4. Moreover, some of the carbonates, calcium and sulfates precipitate in the reactor.

(94) In the supernatant, these dissolved salts thus stem from the desupersaturation at a content not leading to precipitation. This is confirmed by the behavior of the ceramic membrane which shows no clogging.

(95) The behavior of an HEDP and DTPMPA mixture was followed over the entire line. That of the ATMP and HDTMPA mixture alone during the desupersaturation step since it was confirmed that when the latter is effective, the rest of the line is validated.

(96) Each of the steps of the method was specifically monitored.

(97) 1. Description of the reverse osmosis membrane unit:

(98) The membrane used for these tests is a reverse osmosis membrane, of BW30-400 type (DOW®). A simulation and then monitoring of the experimental tests with a focus on a possible clogging potential of the polymerization layer was performed.

(99) Results obtained on the reverse osmosis membrane unit: the dosing of precipitation-inhibiting products was effective for all the test products (HEDP+DTPMPA and ATMP+HDTMPA): no clogging observed at these dosages on the polymerization layer of the membrane with recycling of the desupersaturated effluent. the quality of the permeate obtained experimentally is in accordance with the modeling the operating conditions were validated (the rejection rate, feed pressure, streams of permeate; etc.).

(100) 2 Description of the concentrate desupersaturation unit (SAPHIRA reactor)

(101) The operating conditions are as follows: seeding of the reactor with a mixture of CaCO.sub.3 in the form of the aragonite polymorph+CaSO.sub.4.2H.sub.2O (150 g/L) residence time=30 min (10-11 l/h) regulation of the pH with sodium hydroxide to between 8.5 and 9.0 composition of the concentrate: Ca.sup.2+: 1.2 g/L; total alkalinity (TAC): 1 g/L eq CaCO.sub.3; SO.sub.4.sup.2−: 30 g/L-35 g/l. Solid content in the reactor: maintained between 100 and 250 g/L

(102) Experimental line: 2 precipitation-inhibiting products tested/concentrations: a mixture of ATMP and HDTMPA with an equivalent content of P-PO.sub.4 of 7.4 mg/l a mixture of HEDP and DTPMPA tested at equivalent contents of P-PO.sub.4 of 4.6 and then 5.7 ppm

(103) The results obtained in terms of decrease of the precipitation-inhibiting products are collated in FIG. 21 for the mixture of ATMP and HDTMPA.

(104) It is observed that continuously about 90% of the precipitation-inhibiting products are removed. Moreover, the precipitation rate of the lime is about 50%: 90% of the precipitation-inhibiting products removed with CaCO.sub.3 at a pH of between 8.4 and 9.

(105) The trapping rate of the precipitation-inhibiting products depends on the type of polymorphism of the CaCO.sub.3 (the aragonite particles have a better adsorption rate). Specifically, the reduction rate is no longer as stable when calcite is present. The mean reductions obtained during the tests are collated in the table below:

(106) TABLE-US-00012 Desupersaturation unit Parameter Reactor feed MF/UF feed SC mg/L ss mg/L 0.00 300.00 pH — 8.60 8.45 Conductivity uS/cm 45169.00 44792.00 Calcium mg/L 1140.00 750.00 Bicarbonates mg/L 1160.00 630.00 Sulfates mg/L 30000.00 28700.00 Phase 1: P—PO.sub.4 (HEDP mg/L 5.53 0.38 and DTPMPA) Phase 2: P—PO.sub.4 (ATMP mg/L 6.82 0.80 and HDTMPA)

(107) 3. Description of the microfiltration/ultrafiltration (UF) membrane unit:

(108) Characteristics of the UF modules tested: material: ceramic membrane

(109) The operating conditions and experimental lines: feed: overflow of the SAPHIRA reactor (target on the suspended solids (SS)˜50 mg/l) concentration factor=20

(110) Results obtained on the microfiltration/ultrafiltration (UF) membrane unit: the hydraulic performance qualities are satisfactory the quality of the permeate is in accordance with the simulation (no suspended solids after filtration)

(111) These tests thus allowed us to validate functioning of this line with a conversion rate of 95%.