Method for purifying water as well as plant suitable for said method

Abstract

The present invention relates to a method for purifying water using at least one unit for trapping one or more ions and at least one membrane filtration unit, wherein an aqueous stream is withdrawn from a source and is fed to the aforementioned unit for trapping one or more ions, wherein the aqueous stream leaving the aforementioned unit is fed to the aforementioned membrane filtration unit, and at least one concentrate stream and one permeate stream are obtained in said membrane filtration unit, wherein at least part of the aforementioned one or more ions that are trapped is added to the aforementioned permeate stream to obtain a permeate stream enriched with one or more ions.

Claims

1. A method for purifying water using at least one unit for trapping one or more ions and at least one membrane filtration unit, wherein an aqueous stream is withdrawn from a source and is fed to the unit for trapping one or more ions, the aqueous stream leaving the unit for trapping one or more ions is fed to the membrane filtration unit, at least one concentrate stream and one permeate stream are obtained in said membrane filtration unit, wherein at least part of the one or more ions that are trapped is added to the permeate stream to obtain a permeate stream enriched with one or more ions and the concentrate stream is fed to a unit for generating a basic stream and an acid stream, said unit performs bipolar membrane electrodialysis, the acid stream is applied in a regeneration step of the unit for trapping one or more ions, an effluent stream obtained in the regeneration step is fed to a second unit for trapping one or more ions, wherein at least part of the effluent stream obtained from the second unit for trapping one or more ions is fed to the permeate stream, wherein the effluent stream is stored and is fed to the permeate stream, said second unit for trapping one or more ions performs a regeneration step, and in said regeneration step the basic stream is fed to the second unit for trapping one or more ions.

2. The method according to claim 1, wherein the aqueous stream obtained in the regeneration step is led away.

3. The method according to claim 1, wherein the basic stream and acid stream are stored.

4. The method according to claim 1, wherein the units for trapping one or more ions comprise ion exchangers or zeolites, and within which an operation selected from the group consisting of crystallization, precipitation, membrane filtration, electrodialysis, or a combination thereof is performed, and wherein the unit for trapping one or more ions to which the aqueous stream derived from a source is fed comprises a cationic ion exchanger.

5. The method according to claim 4, wherein the second unit for trapping one or more ions, to which unit the basic stream is fed in the regeneration step, comprises an anion exchanger.

6. The method according to claim 1, wherein the one or more ions are selected from the group consisting of Ca, Mg, Fe, Mn and NH.sub.4.

7. The method according to claim 1, wherein at least one aqueous stream enriched with Ca and Mg is added to the permeate stream to obtain a permeate stream enriched with Ca and Mg.

8. The method according to claim 7, wherein the permeate stream and/or the permeate stream enriched with one or more ions undergo one or more post-treatment steps selected from the group consisting of adjusting the buffer capacity, adjusting the acidity (pH) and adjusting the saturation index (SI).

9. The method according to claim 5, wherein the unit for trapping one or more ions comprises treatment with an eluting agent, wherein an eluent obtained with the treatment is enriched with one or more ions, wherein the eluent is submitted to a fractionation treatment to obtain one or more product streams enriched with ions, wherein the fractionation treatment is selected from the group consisting of eutectic freezing, nanofiltration (NF), gradient elution, zeolites and processes of crystallization, precipitation, electrodialysis, or a combination thereof.

10. The method according to claim 7, wherein the permeate stream enriched with one or more ions is fed to one or more consumer units connected together via a central pipeline network, wherein the consumer units are fed with the permeate stream enriched with one or more ions, wherein the one or more consumer units are connected via one or more branch pipelines to the central pipeline network, wherein the consumer units are selected from the group consisting of households, businesses, and industrial locations.

11. The method according to claim 1, wherein the water to be fed to at least one membrane filtration unit is derived from a source selected from the group consisting of surface water, groundwater, brackish groundwater, natural groundwater, infiltrated surface water, infiltrated seawater, infiltrated sewage treatment effluent, bank-filtered water, wastewater, effluent from wastewater treatment, seawater, process water, or a combination thereof.

12. The method according to claim 1, wherein the at least one membrane filtration unit is assembled from one or more membranes of the reverse osmosis type, wherein the at least one membrane filtration unit further comprises one or more membranes of the microfiltration, ultrafiltration or nanofiltration type, wherein the membranes are connected in series, in parallel, or in an combination, wherein the membranes are tubular, sheet-like or spiral-wound.

Description

(1) The accompanying FIGS. 1 and 2 show a schematic representation of a plant according to the present invention. For clarity, measurement and control systems, pumps and valves have been omitted, as an expert in this field will easily discern the position of the aforementioned units in the diagrams.

(2) In FIG. 1, plant 10 for purifying an aqueous stream comprises a source 1 of raw water, for example surface water. The raw water to be treated 2 is led to a unit 3 for trapping one or more ions, for example a cation exchanger. The aqueous stream 4 leaving unit 3 during operation, which is thus stripped of one or more ions, is fed to a membrane filtration unit 5, for example a RO plant. The RO plant is operated at high recovery and a concentrate stream 16 and a permeate stream 6 are obtained. Permeate stream 6 undergoes post-treatment in unit 7 and the resultant post-treated aqueous stream 8 is fed to unit 9. After unit 3 for trapping one or more ions has been in operation for a certain length of time, a regeneration step is carried out, so that a stream 11 is obtained. Stream 11 is enriched with ions that are derived from unit 3, said ions originating in the raw water 2. In unit 12, fractionation takes place, to obtain a stream 15, and in stream 15 the content of one or more ions is higher than the content thereof in stream 11. For example, stream 15 is a stream that has a high content of Ca and/or Mg. In the fractionation in unit 12, other streams 13 are also obtained, for example streams enriched with Fe, streams enriched with NH.sub.4, and streams enriched with Mn, which are suitable for various final applications 14, for example as soil improver, but also as precipitation agent for the precipitation of phosphate in for example wastewater treatment works and odour control at wastewater treatment works by binding H.sub.2S. Stream 15 is mixed in unit 9 with stream 8 to obtain a permeate stream enriched with one or more ions 10. Stream 10 is supplied to end users and may undergo optional post-treatment steps (not shown).

(3) Although in the accompanying figure we refer to a plant for preparing drinking water, it must be clear that the present invention is not limited to this. Plant 10 may also be employed for preparing other types of water, for example process water.

(4) Moreover, it should be noted that the present invention is not limited to the application of fractionation, but that in an embodiment, the stream 11 obtained in unit 3 is added directly to for example stream 6 and/or stream 8.

(5) Furthermore, the presence of unit 7 must be regarded as optional, because in particular embodiments the stream 6 emerging from unit 5 can be offered to end users directly, after mixing with stream 11 and/or stream 15 in unit 9.

(6) FIG. 2 shows a schematic representation of a plant 20 according to the present invention. In plant 20, an aqueous stream to be purified 22 is withdrawn from source 21, for example anaerobic groundwater. The raw water to be treated 22 is led, via valve 24 in the open position, to a unit 23 for trapping one or more ions, for example a cation exchanger. The aqueous stream 26 leaving unit 23 during operation, which is thus stripped of one or more ions, is fed to a membrane filtration unit 25, for example a RO plant. The RO plant is operated at high recovery and a concentrate stream 38 and a permeate stream 28 are obtained. Permeate stream 28 may be submitted to one or more post-treatments (not shown). In this kind of operation, in which purified drinking water is produced, valves 24, 27 are open, whereas valves 30 and 35 remain in the closed position.

(7) The concentrate stream 38 is fed to a unit 39, for example BMED, for generating a basic stream 41 and an acid stream 40, wherein valve 36 is thus open. Basic stream 41 is stored in tank 43 and an acid stream 40 is stored in tank 42.

(8) After some time, saturation of trapped ions occurs in cation exchanger 23, so that a regeneration step is desirable. In regeneration mode, supply of the raw water to be treated 22 is stopped, by closing valve 24, valve 27 also being closed. Cation exchanger 23 is washed with acid stream 40, by opening valve 35. The effluent 29 obtained in this washing step is led to anion exchanger 31, with valve 30 in the open position. In anion exchanger 31, stream 29 rich in CaCl.sub.2 and MgCl.sub.2 is converted into stream 46, which is rich in Ca(HCO.sub.3).sub.2 and Mg(HCO.sub.3).sub.2. Stream 46 is stored, via valve 33 in the open position, in tank 47. Dosing from tank 47 takes place by opening valve 37, after which stream 46, which is rich in Ca(HCO.sub.3).sub.2 and Mg(HCO.sub.3).sub.2, is added to permeate stream 28 to obtain drinking water 48, which can be supplied to the desired end user.

(9) After some time, a regeneration step is also desirable for anion exchanger 31, regeneration taking place by feeding basic stream 41 into anion exchanger 31. During this regeneration, valve 34 is open, and valves 33 and 30 are closed. The effluent 45 obtained in this regeneration step is sent, via valve 32 in the open position, to a wastewater treatment plant 44.

(10) Although FIG. 2 only shows a single unit for membrane filtration unit 25, it must be clear that membrane filtration unit 25 may comprise several membrane filtration units. The same reasoning applies to anion exchanger 31, cation exchanger 23 and BMED 39. The flowsheet in FIG. 2 thus clearly shows that effective remineralization is possible, wherein the minerals already present in the water to be treated are removed and are mixed again, in the desired amount, with the treated water. The addition of a unit for generating a basic stream and an acid stream, for example BMED, and the other unit for trapping ions, the anion exchanger, mean that it is possible for both units for trapping ions, namely the cation exchanger and the anion exchanger, to be submitted to a regeneration step in an advantageous manner, the necessary chemicals for said regeneration steps being generated in the process itself. Extra dosage of chemicals thus becomes unnecessary. In addition, minerals previously removed from the water to be treated are used for remineralizing the permeate stream.