DE19603494A1 - Water softening process - Google Patents

Abstract

In a water softening process, alkali esp. calcium and/or magnesium salts is added to the raw water in a first stage, to precipitate most of the hardening agents including iron, silicates, and any organic agents. In a second stage, the water is treated in a weak acid cation exchanger removing most of the multivalent cations. The water then passes through a membrane separation stage producing partly softened water as the permeate, a concentrated saline solution with a low-level content of calcium, magnesium, iron, silicates and organic materials. The novelty is that the concentrate is then sub-divided into (a) a mixture of acids (b) a mixture of alkalis and (c) a weak saline solution, by electrodialysis using a bipolar membranes. The mixed alkalis and/or mixed acids have a concentration of between 0.5 and 6 per cent by weight. The pH value in the first-stage of the process is > 10. Between the first and second stages, water is passed through a filter bed to reduce the cloudiness arising from precipitation.

Description

The invention relates to a process for the partial desalination of Water, especially with recycling of concentrates, as waste in membrane-operated water desalination plants product.

Mainly power plants, the electronics industry, pharmaceutical and chemical industry, paper mills, the electroplating industry as well as food and beverage industries use as a pro zeßwasser, for example production water, boiler feed water demineralized water with a electrical conductivity usually between 0.055 and 10 µS / cm.

The surface and river usually available In addition to suspended, d. H. undissolved inorganic and dissolved in numerous dissolved and polyvalent ions. The use of such waters in the Technology as process water therefore reduces this dissolved and undissolved impurities to uncritical values advance in the respective admissibility area.

Various technologies are available for the necessary water treatment cal process known, usually from a Kombina tion of a pre-treatment level and a partial or full desalination level.

Usually, the raw water to be treated first runs through a first pretreatment stage, in which Alka lien - mostly lime milk Ca (OH) ₂ - metal hydroxides and calci  umcarbonate can be largely precipitated. At the same time co-precipitating organic colloids can additionally Addition of coagulants, e.g. B. iron salts to untreated water The sink time of the precipitated carbonates and hydroxides leaves yourself by adding flocculants, e.g. B. Organi cal polyelectrolytes, improve. A subsequent bed fil tration ensures water that is low in particles and carbonate.

It is also known to process ions from process water exchanger, possibly together with other separation pro zessen, use.

In a known method for demineralizing water Ion exchange passes through the raw water after the pretreatment depending on the raw water composition a Kombina weak and strong cation and anion exchange shear and mixed bed exchangers. The ion exchanger scoop during this loading cycle. To make the they must correspond to the ability to work again by adding the acids and bases are regenerated. Usually the cation exchange resins with strong acids, such as. B. Hydrochloric acid, and the anion exchangers with strong bases, like sodium hydroxide, regenerated. This regeneration requires however the use and storage of large amounts of acids and lyes. After use for regeneration, the sau neutralized. The waste water thus obtained leads depending on the salinity of the raw water and the process parameters a salt load of up to 80 g / l with it when discharged in the natural water cycle, among other things, long-term to change the biological balance in na leads to natural waters. It is therefore often required not simply to derive this concentrated saline solution, son to reduce or process them in a suitable manner.

In order to extend the service life of the ion exchanger and thus to reduce the necessary regeneration chemicals, it is known to provide a partial desalination stage between the pretreatment stage and the ion exchanger stage. Membrane separation processes such as reverse osmosis (RO) and electrodialysis (ED) are used for partial desalination. Since the water after the first pretreatment stage still contains considerable amounts of hardness and fouling agents, various precipitation inhibitors and acid for reducing HCO₃⁻ and CO₃ ^{2- are added} before the RO / ED stage in a second pretreatment stage to block the membranes to avoid.

The diluate thus partially desalinated is usually used in further Process steps to the required purity oniert. In addition to the demineralized water, these produce memes industry-specific concentrated salt solutions, the mono- and polyvalent ions, silicates, organic substances as well Inhibitors and acid contain and usually as not usable waste water are disposed of.

Another method of electrodia is known from US Pat. No. 4,880,513 lysis with bipolar membranes (EDBM) known, in which salt solution solutions of NaCl in the corresponding acid HCL and the Corresponding lye NaOH split and then to the Re generation of ion exchangers can be used.

Based on this state of the art, it is the task of present invention, a process for the partial desalination of Water using a membrane separation process far that is economically feasible and where the additional dosage of chemicals in the membrane separator drive upstream pretreatment levels can be reduced can.

This task is accomplished through a procedure according to the teaching of Pa claim 1 solved.

Advantageous embodiments of the invention are the subject of Subclaims.

First of all, in a manner known per se, when Ver drive to partial desalination of water according to the invention in a first pretreatment stage by adding alkalis a considerable reduction in hardness formers in particular, Iron, silicate and possibly organic raw materials  water by precipitation to uncritical values. In a two The pretreatment stage then becomes at least weak in one acidic cation exchanger with extensive distance more valuable cations that come from the first pretreatment stage water so conditioned that after passing one after switched membrane separation stage as a demineralized water Permeate and a concentrated saline solution with low levels calcium, iron, silicates and organic substances will hold.

According to the invention, the concentrate obtained in membrane separation processes Electrodialysis with bipolar membranes to form a Mixed acid, a mixed liquor and a depleted brine solution.

Although the method of electrodialysis with bipolar Membranes for the separation of concentrate solutions in mixed acids and mixed lye was known in principle, it found the still practically no application as it has been used for this mixing acids and alkalis were not used.

According to a particularly preferred embodiment of the Er The solution is the alkali in the first preparation action level and / or the mixed acid for regeneration of the second pretreatment level used. This will make the crowd the chemicals required in the two pretreatment stages lien, namely significantly reduced acids and alkalis. This Circulation management requires a coordinated pretreatment, which is capable of the polyvalent metal cations and Si remove likate, carbonate and organic contaminants nen, so that for the most part monovalent cations the membrane procedures are supplied.

In the following, the invention is based on only one Exemplary embodiment illustrating the drawing. It shows the only one  

Fig. 1 in a schematic block diagram-like presen- tation a method according to the present inven tion.

The raw water, for example what was taken from a river ser, is first sieved in a manner not shown to remove coarse particles, suspended matter and the like.

This is followed by VB1 in a first pretreatment stage an alkaline conditioning of the raw water to one pH value <10 in a sedimentation tank with the addition of mixed lye, Coagulant and flocculant to precipitate more term metal hydroxides and metal carbonates. This is done at the same time a partial reduction of silicates and organic colloids by co-precipitation in flakes of CaCO₃, Magnesium and iron hydroxide. The precipitated sludge will dissipated.

If the concentration of calcium and magnesium salts in the Raw water is not sufficient, a sufficiently large amount of Mg (OH) ₂ to deliver as an adsorbent and to achieve an optimal To achieve the ratio of CaCO₃ / Mg (OH) ₂ in the precipitation stage, can in the first pre-treatment stage VB1 an additional Dosage of calcium and magnesium salts, e.g. B. CaCl₂ or MgCl₂ take place.

After leaving the first pretreatment stage VB1, a bed filtration BF takes place in the method shown in FIG. 1 to remove residual turbidity from the precipitation stage.

In a second pretreatment stage VB2 there is another Pretreatment to remove residual hardness and egg sen. This second pretreatment stage VB2 comprises a weak one acidic cation exchanger in particular for the removal of Residual contents of divalent metal ions (calcium, magnesium) and Iron to produce a mixed salt solution, its cations are predominantly monovalent. The pretreatment described in this way reduces susceptibility, especially blocking gung, the following membrane separation stage for partial water desalination  against hardness formers, silicate scaling and organic Fouling.

This is dealt with after the second pre-treatment stage VB2 Water supplied to a membrane system for partial water desalination. This membrane separation plant works in a manner known per se using reverse osmosis or electrodialysis. The partially demineralized water is removed as the permeate of the membrane system leads and subjected to a pure water level; the received and also removed concentrate does not already contain salt load removed by the pretreatment.

While in the previously known processes the concentrate is disposed of, the concentrate or the concentrate solution by means of electrodialysis with bipolar Split EDBM membranes in mixed lye and mixed acid. Here are preferably alkali and / or acid concentrations between 0.5 and 6 wt .-% set.

The mixed liquor obtained in the EDBM electrodialysis process is in the illustrated embodiment, a storage container V1 fed and cached there. In an analogous way he the mixed acid obtained is temporarily stored in a storage container V2.

The further as the end product of the EDBM electrodialysis process depleted salt solution can again as the raw water portion Circuit or the first pretreatment stage VB1 for renewed Cleaning can be supplied.

From the storage container V1 takes place continuously and / or be controlled feeding of the stored mixed liquor into the sales basin of the pre-treatment level VB1 for pH-on position of the raw water to be treated there. The regenerative the mixed liquor obtained replaces the usually required Dosage of additional alkalis.

The mixed acid temporarily stored in storage tank V2 is weak cation exchange during the regeneration phase shear of the pre-treatment stage VB2. This eliminates  the dosage of regeneration usually required Additional acids or is at least significantly reduced.

The regenerant solution obtained during the regeneration is in disposed of in the usual way, e.g. B. supplied to the sewer.

Claims (11)

1. A process for the partial demineralization of water, the raw water being largely freed in a first pretreatment stage by adding alkalis, in particular hardness formers, iron, silicate and optionally organic substances by means of precipitation, and in a second pretreatment stage in an at least weakly acidic cation exchanger with extensive removal of multivalent cations is conditioned in such a way that after passing through a downstream membrane separation stage, partially demineralized water is obtained as permeate and a concentrated salt solution with low levels of calcium, magnesium, iron, silicates and organic substances, characterized in that the concentrate is split by electrodialysis with bipolar membranes to form a mixed acid, a mixed alkali and a depleted saline solution. 2. The method according to claim 1, characterized, that the mixed liquor and / or mixed acid concentrations have between 0.5 and 6 wt .-%. 3. The method according to claim 1 or 2, characterized, that in the first pre-treatment stage a pH value <10 is set.   4. The method according to any one of claims 1 to 3, characterized, that in the first stage of pretreatment an additional Calcium and / or magnesium salts are added. 5. The method according to any one of claims 1 to 4, characterized, that between the first and second pretreatment stages a bed filtration to reduce turbidity from the Precipitation takes place. 6. The method according to any one of claims 1 to 5, characterized, that the mixed liquor as alkali in the first pretreatment level is used. 7. The method according to any one of claims 1 to 6, characterized, that the mixed acid for regeneration of the second vorbe action level is used. 8. The method according to any one of claims 1 to 7, characterized, that mixed acid and / or mixed liquor before further processing in each case in a storage container be saved. 9. The method according to any one of claims 1 to 8, characterized, that the membrane separation process is a reverse osmosis driving. 10. The method according to any one of claims 1 to 8, characterized, that the membrane separation process is an electro dialysis procedure. 11. The method according to any one of claims 1 to 10, characterized, that the depleted salt solution as raw water he is fed again to the first pretreatment stage.