MXPA00002792A - Method for inhibiting growth of bacteria or sterilizing around separating membrane - Google Patents

Abstract

A method for surely sterilizing a separating membrane in a membrane separation apparatus which comprises adding an inexpensive acid such as sulfuric acid intermittently to a liquid feed after being pretreated in such a manner that the feed has a pH of 4 or less, to thereby intermittently sterilize the membrane and the piping in the vicinity thereof.

Description

METHOD OF BACTERIOSTASIS OR DISINFECTION FOR PERM-SELECTIVE MEMBRANE TECHNICAL FIELD The present invention relates to a method for the pretreatment of raw water in membrane separation, especially for that in reverse osmosis for the desalination of seawater, with a bacteriostatic or disinfection method for membranes, and with a device for them. BACKGROUND OF THE INVENTION Membrane separation is widely used in different fields of desalination of seawater and saltwater, production of pure water and ultrapure water for medical and industrial use, treatment of industrial drainage, etcetera. In that membrane separation, contamination of the membrane separation apparatus with microorganisms worsens the quality of the permeate, and decreases the permeability and the separability of the membrane due to the growth of microorganisms in and around the membranes, or adhesion of microorganisms and their metabolites on them. Specifically, the influences of the microorganisms result in the degradation of the quality of the permeate, the reduction in the amount of permeate, the increase in operating pressure or in the increase in pressure loss. In order to avoid these serious problems, different techniques and methods have been proposed up to now for bacteriostasis and even for disinfecting microbes in membrane separation units. For example, microbicides are used. More generally, a microbicide containing chlorine is added, from which the effect has been verified, and which has the advantages of low cost and easy manageability, to the membrane separation units, in a concentration of from 0.1 to 50 ppm more or less. A general method for using that microbicide comprises the addition of a microbicide to a pre-treatment zone in a membrane separation apparatus, in which the water previously treated, having been subjected to disinfection with sodium hypochlorite, and then to flocculation and Filtration, before being fed into the membrane separation units, is stored once in a tank, and then processed to remove the free chlorine from it, through reduction with sodium bisulfite before the safety filter, as it is arranged in the area before the membrane treatment units. Chlorine-containing microbicides chemically degrade reverse osmosis membranes. Therefore, when these are used, the free chlorine of them must be reduced with a reducing agent, before they reach the reverse osmosis membranes. As the reducing agent which is generally used is sodium bisulfite in an amount of from 1 to 10 folded equivalents. The concentration of the reducing agent is determined in consideration of its ability to completely remove the remaining microbicide, and the probability of its reaction with dissolved oxygen in the system being treated. However, even when a membrane separation apparatus is operated in a continuous operating manner, in accordance with that method for using that chlorine-containing microbicide, its membrane capabilities often worsen, and it has been found that the method It is not always satisfactory to disinfect microorganisms in the apparatus. In this connection, it is said that chlorine, as added in the method, oxidizes the organic carbons that exist in the raw water that is being treated, whereby the organic carbons thus oxidized are converted to compounds that microorganisms decompose rapidly. (see AB Hamida and T. Moch, Jr., Desalination &Water Reuse, 6/3, 40-45, 1996), but so far their theory has not been verified. Given this situation, another method has been developed for the disinfection of membranes, which comprises intermittently adding sodium bisulfite to a membrane separation system, generally at a concentration of 500 ppm. This method has come to be used in practice, but, in some cases, it is still not effective. Those who have tried the method have often experienced the deposition of microorganisms in perm-selective membranes. Objective of the Invention In the conventional pretreatment method, the water previously treated, having been subjected to disinfection and to flocculation and filtration, is stored in a tank for a time, which, therefore, is frequently contaminated with some external contaminants. , by which the microorganisms grow much in the stagnant water so contaminated, to make the quality of the water even worse. The disinfecting effect of sodium bisulfite to be used in the method is to remove oxygen from the raw water that is being processed, and to reduce the pH value of the raw water. However, although a perm-selective membrane apparatus is operated in accordance with the method, the intermittent addition of sodium bisulfite to the apparatus is not always effective to disinfect the membrane in the apparatus. We, the present inventors, have studied the reason, and we have found that ordinary aerobic bacteria growing in a neutral or alkaline condition can be prevented from growing in an anaerobic environment to a certain degree, but can not be eliminated in that environment ambient. Having noted that, we have come to the conclusion that the depression of the pH in the system where the bacteria can live, is rather the most effective to disinfect the bacteria in it. This conclusion is not contradictory for the microbio logical point of view in this respect. On the other hand, we have also found that, even when a high concentration of sodium bisulfite of 500 ppm is added to raw water, which has a high concentration of salt, such as seawater, the pH value of the water system does not it could be reduced to such a degree that the ordinary bacteria existing in it could be eliminated. Therefore, it is understood that sodium bisulfite added to raw water having a lower salt concentration, may exhibit its disinfecting effect, not in an anaerobic condition, but rather in a reduced pH condition. In accordance with the above, we have found that the addition of a high concentration of expensive sodium bisulfite to the membrane separation units is not necessary to disinfect them, but simply the addition of inexpensive or similar sulfuric acid thereto, for reducing the pH value in the system around them, it is satisfactory to disinfect the units, and that, when the previously treated water is prevented from remaining for a time in a tank or the like, in a water treatment apparatus, it is then It can inhibit the growth of microorganisms in the device. On the basis of these findings, we have finished the present invention. Description of the Invention The object of the invention can be achieved by the constitution mentioned later. Specifically, the invention provides "a bacteriostatic or disinfecting method for a perm-selective membrane in a membrane separation apparatus for water purification, comprising a step to treat raw water with an acid at a pH of at most 4" , and also provides a method for purifying water, which essentially comprises the method of disinfection and an apparatus for the method. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow chart showing the constitution of the essential parts of a seawater desalination apparatus. 1: pretreatment unit 2: reverse osmosis membrane treatment unit after treatment unit membrane wash unit first duct flocculation agent feeder sand filter (primary filter) 9 safety filter 10 second duct 11 agent feeder pH controller 12 third pipeline 13 microbicide feeder. BEST MODE FOR CARRYING OUT THE INVENTION The membrane separation unit for the invention is one for production, concentration, separation of water or the like, in which the raw water to be treated is fed into a membrane module under pressure, and separated in a permeate and a concentrate, by means of the membrane. The membrane module includes a reverse osmosis membrane module, an ultrafiltration membrane module, a precision filtration membrane module, and the like. Depending on the type of membrane module to be used therein, the membrane separation unit is grouped into a reverse osmosis membrane unit, an ultrafiltration membrane unit, and a precision filtration membrane unit. A reverse osmosis membrane unit is specifically mentioned herein. The reverse osmosis membrane is a semipermeable membrane through which a mixed liquid that is going to separate partially passes through it, for example, a solvent of the liquid can pass through it, but the other components that constitute the liquid may not pass. They are also within the scope of a broad meaning of the reverse osmosis membrane, a nanofiltration membrane and a loose RO membrane. Cellulose acetate polymer polymer materials, polyamides, polyesters, polyimides, vinyl polymers and the like, are widely used for the reverse osmosis membrane. Depending on its structure, the membrane is grouped in an asymmetric membrane having a dense layer on at least one surface, in which the pore size is gradually increased from the dense layer towards the inside of the membrane, or towards the opposite surface of the same, and a composite membrane having an extremely thin active layer of a different material, formed in the dense layer of the asymmetric membrane. Depending on its shape, the membrane is grouped into a hollow fiber membrane and a flat sheet membrane. The thickness of the hollow fiber membrane and the flat sheet membrane can fall between 10 μm and 1 mm; and the outer diameter of the hollow fiber membrane can fall between 50 μm and 4 mm. The flat, asymmetric or composite sheet membrane is preferably supported with a woven cloth substrate, mesh fabric, non-woven fabric, or the like. The disinfection method of the invention, in which a mineral acid is used, can be applied effectively to any and all types of reverse osmosis membranes, without depending on the material, structure or shape of the membranes. Typical reverse osmosis membranes to which the invention is applied are, for example, asymmetric membranes of cellulose acetate or polyamide, and composite membranes having an active layer of polyamide or polyurea. Of these, the method of the invention is especially effective for asymmetric membranes of cellulose acetate, and membranes composed of polyamide, and is more effective for membranes composed of aromatic polyamide (see JP-A 62-121603, 8-138653, patent of the United States No. 4,277,344). The reverse osmosis membrane module is of a form that can be practiced of any of the reverse osmosis membranes indicated above, for which a flat sheet membrane is combined with a spiral, tubular or plate-and-frame module , and hollow fiber membranes are bonded and combined with it. However, the invention does not depend on the shape of the reverse osmosis membrane module. Regarding its capabilities, the reverse osmosis membrane module for use in the invention has a desalination rate of 98 percent to 99.9 percent, and a water production rate of 10 to 25 cubic meters. day, in a standardized size of 1 meter (in length) x 20 centimeters (in diameter), when evaluated for raw seawater that has a salt concentration of 3.5 percent (this is the most general seawater concentration ), as applied to it under a pressure of 5.5 MPa, and at a temperature of 25 ° C, for a recovery of 12 percent; or has a desalination index of 98 percent to 99.9 percent, and a water production rate of 10 to 25 cubic meters / day, in a standardized size of 1 meter (in length) x 20 centimeters ( in diameter), when evaluated for raw seawater having a salt concentration of 5.8 percent, as applied to it under a pressure of 8.8 MPa, and at a temperature of 25 ° C, for a recovery of 12 percent . Preferably, it has a desalination index of 99 percent to 99.9 percent, and a water production rate of 12 to 23 cubic meters / day, in a standardized size of 1 meter (in length) x 20 centimeters (in diameter), when evaluated for raw seawater having a salt concentration of 3.5 percent, as applied to it under a pressure of 5.5 MPa, and at a temperature of 25 ° C, for a recovery of 12 percent; or has a desalination index of 99 percent to 99.9 percent, and a water production rate of 12 to 23 cubic meters / day, in a standardized size of 1 meter (length) x 20 centimeters ( in diameter), when evaluated for raw seawater having a salt concentration of 5.8 percent, as applied to it under a pressure of 8.8 MPa, and at a temperature of 25 ° C, for a recovery of 12 percent . More preferably, it has a desalination rate of from 99.3 percent to 99.9 percent, and a water production rate of 14 to 20 cubic meters / day, in a standardized size of 1 meter (in length) x 20 centimeters (in diameter), when evaluated for raw seawater that has a salt concentration of 3.5 percent, as applied to it under a pressure of 5.5 MPa, and at a temperature of 25 ° C, for a recovery of 12 percent; or has a desalination index of from 99.3 percent to 99.9 percent, and a water production rate of from 14 to 20 cubic meters / day, in a standardized size of 1 meter (in length) x 20 centimeters (in diameter), when evaluated for raw seawater that has a concentration of salt of 5.8 percent, as applied to it under a pressure of 8.8 MPa, and at a temperature of 25 ° C, for a recovery of 12 percent. The reverse osmosis membrane module having a spiral shape comprises other members of a water supply duct, a permeate extraction duct and others, wherein the other members can be made of any materials. Especially preferably, the module is at least partially designed so that it can be applied to high concentration raw water having a salt concentration of at least 3.5 percent, and which can be applied to high pressure operation. , at a pressure of at least 7.0 MPa (see JP-A 9-141060, 9-141067). The operating pressure to be applied to the reverse osmosis membrane unit, for use in the invention, may fall between 0.1 MPa and 15 MPa, and may vary depending on the type of raw water to be treated in the unit , and the operating mode of the unit. For example, raw water having a low osmotic pressure, such as salt water, ultrapure water or the like, can be applied to the unit under a relatively low pressure. However, for the desalination of seawater, for drainage treatment, and for the recovery of useful substances, the raw water to be treated is applied to the unit under relatively high pressure. The temperature at which the reverse osmosis membrane unit is operated can fall between 0 ° C and 100 ° C. If this is lower than 0 ° C, the raw water that is being treated will freeze, so that the unit can not be operated; but if it is higher than 100 ° C, the raw water applied to the unit will vaporize and can not be treated. The recovery in the separation unit can be suitably determined within a range of from 5 to 100 percent, depending on the mode of operation of the unit for separation, and the type of the unit. The recovery in the reverse osmosis membrane unit can be suitably determined within a range of from 5 to 98 percent. For this, however, the condition of the previous treatment and the operating pressure of the unit must be taken into consideration, depending on the properties of the raw water to be treated and the concentration of the same, in its concentrations, and of the osmotic pressure in the unit (see JP-A 8-108048). For example, for the desalination of seawater, the recovery in the unit that has an ordinary efficiency can fall between 10 and 40 percent, but that in the unit that has a high efficiency can fall between 40 and 70 percent. For saltwater desalination or for the production of ultrapure water, the unit can be driven to achieve a recovery of at least 70 percent, for example, 90 to 95 percent. Regarding its configuration, the reverse osmosis membrane module can be arranged in a single stage, but if desired, plural reverse osmosis membrane modules can be arranged in series or in parallel, in relation to the direction in which the membrane runs. raw water that is going to be treated with them. It is desirable to provide plural reverse osmosis membrane modules in series, in relation to the direction in which the raw water to be treated runs, since the raw water may have contact with the membrane modules for a long period. of time. In that condition, the method of the invention produces better results. Where plural membrane modules are arranged in series, relative to the raw water running through them, the pressure to the raw water can be suitably increased between the adjacent stages of the modules. The increase in pressure can be made within a range of 0.1 to 10 MPa, for which a pressure pump or a booster pump can be used. In addition, plural reverse osmosis membrane modules can also be arranged in series with respect to the direction in which the permeate passes through them. This method is favorable when it is desired to further improve the quality of the permeate, or when it is desired to recover the solute in the permeate. Where the plural reverse osmosis modules are connected in series with respect to the permeate passing through them, a pump can be arranged between the adjacent membrane modules, by means of which the pressure to the permeate can be increased, or the permeate having been pressurized excessively in the previous step, it can be subjected to the following membrane separation under back pressure to it. In this condition where the plural reverse osmosis membrane modules are connected in series with respect to the permeate passing through them, an acid feeder is disposed between the adjacent membrane modules, such that the membrane module in the last stage can be disinfected with acid from it. The fraction of raw water that has not passed through the reverse osmosis membrane unit is removed from the membrane module, and this is a concentrate of the raw water. Depending on its use, the concentrate is further treated and its waste is discarded, or it can be re-concentrated in any desired method. A part of, or all of the concentrate can be circulated to, and combined with, the raw water that is being treated in the unit. Also depending on its use, the fraction of raw water that has passed through the membrane can be discarded as is, or it can be used directly as it is, or a part of, or the entire fraction can be circulated to, and combined with the raw water that is being treated in the unit. In general, the concentrate formed in the reverse osmosis membrane unit has pressure energy, and it is desirable to recover the energy to reduce the operating cost of the unit. For this, an energy recovery unit can be adapted to a high pressure pump as it is available in any stage, by means of which the concentrate pressure energy can be recovered. Preferably, a specific turbine type energy recovery unit is provided before or after the high pressure pump, or between adjacent modules, by means of which the concentrate pressure energy can be recovered. With respect to these capabilities, the membrane separation unit can treat water at a rate of 0.5 cubic meters / day to 1,000,000 cubic meters / day. In the invention, the raw water to be treated should have a pH value of at most 4, and pH control is extremely important to safely disinfect the perm-selective membranes that are used. In particular, when treating raw seawater, through membrane filtration, the effect of the invention is outstanding. The pH value at which microorganisms must die is specific to the type of microorganisms. For example, the lower limit of the pH value at which Escherichia coli could grow is 4.6, but Escherichia coli dies at a pH of 3.4 or less. On the other hand, in sea water there are many types and varieties of microorganisms, and these die at different pH values. However, in the invention, when the seawater containing those many types and varieties of living microorganisms is kept at a pH of at most 4, during a predetermined period of time, 50 to 100 percent of the water can be removed from the water. those microorganisms. For this, a pH acidity of at most 3.9 is preferred, and pH acidity of at most 3.7 is more preferred. In seawater that contains many types and varieties of live microorganisms, some of these microorganisms will be resistant to acids. Even in that case, at least 98 percent of microorganisms can be eliminated therein, when the seawater is maintained at a pH of 2.6 or less, for a predetermined period of time. Therefore, the method of the invention can generally produce better results when the raw water to be treated therein is maintained at a pH of at most 4, for a previously determined period of time, and is occasionally maintained at a pH of 2.6 or less. For the desired pH control in the method, an acid is generally employed. The acid can be any of the organic acids and the inorganic acids. From the economic aspect, however, sulfuric acid is preferred. The amount of sulfuric acid that is to be added is proportional to the concentration of salt in the raw water to be treated. For example, the addition of 50 ppm of sulfuric acid to a physiological saline solution (having a salt concentration of 0.9 percent), which has been subjected to pressure disinfection (at 120 ° C for 15 minutes), may decrease the pH of the solution at 3.2. However, the addition of up to 100 ppm of sulfuric acid to each of three samples of seawater collected at different locations, and one sample of commercially available artificial seawater (which has a salt concentration of 3.5 percent), which were all subjected to pressure disinfection (at 120 ° C for 15 minutes), reduced the pH of these seawater samples only to the range between 5.0 and 5.8. This is probably due to the fact that the pH of these seawater samples could vary greatly, depending essentially on the M alkalinity of the seawater. To further reduce the pH of these seawater samples, the addition of at least 120 ppm of sulfuric acid thereto is necessary to achieve a pH of 4 or less, or the addition of at least 250 ppm of sulfuric acid thereto is necessary to achieve a pH of 2.6 or less. In consideration of the economic aspect, and the influence on the equipment including the pipes, the amount of the acid to be added will preferably be 400 ppm, more preferably 300 ppm. The additional increase in the concentration of sulfuric acid added to the samples of seawater and artificial seawater, indicated above, at 150 ppm, 200 ppm, 250 ppm and 300 ppm resulted in the reduction in the pH change in samples from a pH of 3.2 to 3.6, from a pH of 2.8 to 2.9, a pH of 2.6, and a pH of 2.4, respectively, in accordance with the increase in the concentration of the added acid. If the pH of the seawater to be treated is maintained at all times at 2.6, almost all bacteria, including acid-resistant bacteria, will be completely eliminated in the seawater. However, the proportion of bacteria resistant to acid in seawater is small. Therefore, in the method of the invention, it is desirable that seawater is generally disinfected at a pH of from 2.7 to 4, but occasionally at a pH of 2.6 or less, to disinfect the acid-resistant bacteria in the the same, to save the costs of the chemical products that are going to be used, and to reduce the influences of the chemical products that are used in the pipes. To disinfect the membranes in the method of the invention, an acid can be intermittently added to the raw water, after the raw water has been previously treated, and before it is applied to the membrane module. Where plural membrane modules are arranged in series with respect to the direction in which the permeate passes through them, an acid can be intermittently added for disinfection of the membrane to the site between the adjacent membrane modules , in order to disinfect the last membrane module. The time for the addition of acid, and the frequency of acid addition will vary greatly, depending on the site to which the acid is added, and the condition for the addition of acid. For example, the addition of acid can be carried out for a period of from 0.5 to 2.5 hours, once a day, a week or a month. The same will apply to the case of disinfecting acid resistant bacteria. However, when the acid addition is directed to achieve the different pH conditions in two steps, it is desirable that the acid treatment step, for a pH range of from 2.7 to 4 (step A), be carried out at a frequency from once in a period from one day to 30 days, and that the step of acid treatment, for a pH range of at most 2.6 (step B), is carried out at a frequency of one time in a period of from two days to 180 days. When step A and step B are carried out several times within a previously determined period of time, it is desirable that the ratio of the total time for step A (TA) to that for step B (TB), TA / TB, drop between 1 and 100. The operation for step A can be changed directly to that for step B, and vice versa. However, it is desirable that the raw water that is not subjected to the pH control, or the raw water having a pH of from 6.5 to 7.5, be fed into the system between step A and step B. In this case , raw water not subjected to pH control, or having a pH from 6.5 to 7.5, can be treated in ordinary membrane separation, and the resulting permeate or concentrate can be used for its intrinsic purposes. The amount of additional raw water may vary, depending on the decrease in the amount of the permeate, the increase in the number of live bacteria in the concentrate and in the organic carbon content of the concentrate, and the increase in membrane pressure. Where the membrane separation method of the invention is performed in a discontinuous manner, the membranes can be immersed in an acid to disinfect them, while the apparatus is stopped. The disinfection method of the invention can be combined with any other disinfection with chlorine or the like. The membrane disinfection method of the invention can be applied not only to the membrane separation units, but also to water separation systems that partially comprise membrane separation units. For example, the invention can be applied to the constitution of the following systems. A. Water intake device. This is an apparatus for taking raw water, and generally includes water inlet pumps, chemical feeds, and so on. B. Pre-treatment apparatus connected to the water intake apparatus: This is an apparatus for pretreating raw water to be fed to a perm-selective membrane apparatus, in which the raw water is purified to a certain degree previously, and comprising, for example, the following units connected in that order. B-l: Flocculation and filtration unit. B-2: Polishing filtration unit. Instead of B-1 and B-2, an ultrafiltration unit, or a precision filtration unit, can be used. B-3: feeders of chemical products, to feed flocculation agents, microbicides, pH controlling agents, etcetera. C. Optional intermediate tank connected to the pre-treatment apparatus.

This is to control the water level, and to regulate the pH of the water quality. D. Filter connected to intermediate tank C, or directly to the pre-treatment apparatus in the absence of intermediate tank C. This is to remove solid impurities from the water to be fed into the membrane separation apparatus.

E. Membrane separation apparatus. This comprises high pressure pumps and perm-selective membrane modules. In this, the plural membrane separation units can be arranged in series or in parallel. Where these are connected in series, a pump can be arranged between the adjacent membrane separation units, by means of which the pressure of the water to be applied to the last unit can be increased. F. Subsequent treatment apparatus connected to the membrane separation apparatus at the outlet, through which the permeate flows outwards. For example, it comprises any of the following units. F-l: Degassing unit, which is for decarbonization. F-2: Tower of calcium. F-3: Chlorine feeder. G. Subsequent treatment apparatus connected to the membrane separation apparatus, at the outlet through which the raw water flows out. For example, it comprises any of the following units. C-1: Unit for treating raw water having a pH of 4, for example, a neutralization unit. C-2: Drainage. H. Any other optional apparatus for treating waste water. In these systems, the pumps can be arranged in any desired zone. It is desirable that chemical products or chemical solutions be added, to make the raw water have a pH of at most 4, to the systems in the water intake apparatus A, or in the pre-treatment apparatus B, or before of the pre-treatment apparatus B, or before the filter D, or after the filter D. In order to further improve the effect of the invention, it is desirable that the acid feeder can be controlled automatically. For example, the acid feeder is preferably equipped with a pump capable of controlling the amount of the acid to be fed. To control the amount of acid, it is also desirable to provide a pH meter, to measure the pH of the raw water and the concentrate at any desired site in the system. To control the intermittent addition of acid, it is still desirable to have a stopwatch in the system. In addition to preference, the system is equipped with an automatic controller to ensure automatic operation of the system. The members that constitute the apparatus of the invention, such as tubes, valves, and others, are preferably made of materials that will not degrade to a pH of 4 or less. For example, those that can be used are stainless steel members, members coated on their inner surface, resin members, and so on. The control of the pH of the raw water so that it is at most 4, ensures the good disinfection of the perm-selective membranes, and, in addition, the raw water thus controlled is additionally effective to remove scale in the pipes. Sodium bisulphite will have to be added to prevent the perm-selective membranes from being degraded by the chlorine oxides and the like, and their amount will have to be increased when the microorganisms (including sulfur bacteria, etc.) that have adhered on them increase. membranes, or when increasing the metal salts that have also adhered on them. However, in the invention, where the raw water that is to be treated is acidified, the amount of sodium bisulfite that is to be added in that condition can be significantly reduced. The method of the invention is favorable for membrane separation, especially for that of aqueous solutions. In particular, it is especially effective for the separation of liquids-solids, and the concentration of liquids with precision filtration membranes, for the separation of impurities, and for the concentration of permeate with ultrafiltration membranes, and for the separation of solutes and the concentration of permeate with reverse osmosis membranes. More specifically, the invention is effective for seawater desalination, salt water desalination, industrial water production, production of ultrapure water and pure water, medical water production, concentration of food and beverages, Urban water purification, quality improvement in urban water. In addition, to separate and concentrate organic substances that degrade rapidly with conventional oxidizing microbicides, the method of the invention is effective. In accordance with the method of the invention, these organic substances are not degraded through oxidation, and can be concentrated and recovered safely. By producing water for drinking in the invention, trihalomethanes which can be formed in disinfection with chlorine are not formed. For the disinfection of raw water in the pretreatment, in general, a microbicide containing chlorine is added continuously to it, as mentioned hereinabove. In accordance with the method of the invention, raw water is disinfected almost completely, as long as acid-resistant bacteria do not grow in it. Since the microbicide chemically degrades the reverse osmosis membranes, a reducing agent, such as typically sodium bisulfite, is added to the raw water, before the membrane separation unit. However, in the raw water from which the microbicide is removed in the pre-treatment step, microorganisms can easily grow. In addition, it has become clear that raw seawater to which no microbicide is added, contains specific groups of microorganisms of many types and varieties of microorganisms, and some of those microorganisms that exist in raw seawater, Not disinfected, they are resistant to acid. In addition, it has also been made clear that, when a satisfactory amount of a reducing agent, such as typically sodium bisulfite, is not added to the raw water to which a microbicide containing chlorine has been added, the microbicide remaining in the Raw water may not be completely removed from it, but if too much reducing agent is added, some types of microorganisms will grow in the raw water. For these reasons, it is desirable not to add a microbiotide containing chlorine to the raw water to be treated in accordance with the method of the invention. If done, however, microorganisms will grow in the raw water in the pre-treatment step. The problem could be solved by intermittently adding a microbicide and a reducing agent to the raw water. In this condition, microorganisms that have adhered to, and that have been deposited on the inner walls of the tubes and the filtration tanks in the pre-treatment step, in the absence of a microbicide, could be eliminated by the microbicide intermittently added to them. The mode of intermittent addition is preferable, since it does not degrade the membranes. The interval for the intermittent addition of microbicide can be determined, depending on the quality of the raw seawater to be treated, and the condition of the microorganisms that grow in it. For example, a microbicide may be added once, at intervals of from 1 day to 6 months, and the time for addition may be from 30 minutes to 2 hours or so. Depending on the range, the membranes can be disinfected in accordance with the method of the invention. The intermittent addition of chlorine-containing microbicide signi fi cantly reduces the cost of treatment, which is ensured for the first time only by the membrane disinfection method of the invention, but not at all by the conventional membrane disinfection method of use. a high concentration of sodium bisulfite. This is because the conventional membrane disinfection method is not satisfactory to completely disinfect microorganisms. To prevent adhesion and deposition of microorganisms in the absence of a microbicide, and to improve the disinfection effect of the invention, a favorable system is mentioned later, as illustrated in Figure 1. The system of Figure 1 is for desalination of sea water, comprising a pre-treatment unit 1 , a reverse osmosis unit 2, a subsequent treatment unit 3, and a membrane washing unit 4. The pre-treatment unit 1 comprises a flocculation agent feeder 7, through which a solution of flocculating agent is added. to sea water (raw water), which runs through the first pipeline 6; a sand filter 8 which is a primary filter element; a security filter 9 which is a secondary filter element; a pH controlling agent feeder 7, through which a pH-controlled mineral acid solution is added to the primary filtrate, which runs through the second pipe 10; and a microbicide feeder 13, through which a microbicidal solution is added to the secondary filtrate running through the third conduit 12. The first conduit 6 is connected to the water intake pump 14, and to the sand filter 8.; the second duct 10 is connected to the sand filter 8, and to the security filter 9; the third duct 12 is connected to the high pressure pump 15, and to the first stage membrane module 17 in the reverse osmosis unit 2. Accordingly, seawater can be fed to the sand filter 8, by actuating the water inlet pump 14, and secondary filtering can be pressurized to a high degree, and fed to the reverse osmosis membrane unit 2, by operating the high pressure pump 15. In this step , ferric chloride having a pre-determined concentration to seawater is added through the flocculating agent feeder 7 via the pipeline 18, while sulfuric acid is added thereto through the controller's agent feeder. pH 11, by means of line 19; and a solution of sulfuric acid is added intermittently thereto, through the microbicide feeder 13, by means of the pipeline 20. The pipeline 20 can be connected to the pipeline 12, or, as the case may be, can be integrated into a unit the pH controlling agent feeder 11 and the microbicide feeder 13. From the tank 22 of the flocculation agent feeder 7, a ferric chloride solution is fed into the raw seawater being treated, by means of actuating the pump 21; and from the tank 24 of the pH 11 controlling agent feeder, sulfuric acid is fed into the raw seawater, by means of operating the pump 23. In the system of Figure 1, the pipe from the water inlet pump 14 to the first stage membrane module 17, in the reverse osmosis membrane unit 2, is a closed pipe. In other words, this is not a so-called open pipe, where the raw water being treated is temporarily stored in a tank, as in a conventional system, but it is a closed pipe, not open. The system of the invention may comprise a raw water tank, a sand filtration tank, and a feed pump, in which, however, the pipe from the water intake unit to the reverse osmosis membrane module is preferably a closed pipe, not open. In the closed, not open pipe, the raw water being treated is protected from being contaminated with any external contaminants, and can be treated continuously. The change in flow velocity after the high pressure pump 15 can be avoided by controlling the flow velocity in the units constituting the pre-treatment unit 1. In this condition, the raw water can be kept flowing all the time through the pipe, without stopping anywhere in the pipeline, and can be continuously treated on the line. The sand filter 8 can always be actuated stably. In the pretreatment unit, a polishing filter can be arranged after the sand filter. If desired, a UF or MF membrane having a pore size of from 0.01 to 1.0 μm can be used, instead of the sand filter or the polishing filter, or in place of both. In the system illustrated, the raw water being treated does not remain in a tank or the like, and therefore, adhesion and deposition of microorganisms in the pipeline can be prevented, even in the absence of a microbicide in the pipeline. same. Thus, the disinfecting effect of the invention in the system of that type can be improved. Examples The invention is described specifically with reference to the following Examples, which, however, are not intended to restrict the scope of the invention. In these Examples, the disinfecting effect is represented by the number of living microorganisms, the loss of pressure in the membrane modules, and the consumption of sodium bisulfite (SBS). Reference Example 1: A previously determined amount of a suspension of live cells of Escherichia coli K12 IFO 3301 was added to a physiological saline solution (having a salt concentration of 0.9 percent), which had been subjected to pressure disinfection (at 120 ° C for 15 minutes), and then to a pH control with sulfuric acid added thereto, and kept at 20 ° C for a previously determined period of time, and the survival rate of the cells was obtained by dividing the number of living cells still remaining in the solution by the number of cells added to the solution. As a result, the survival rate of the cells was not less than 90 percent, when the solution to which 10 ppm of sulfuric acid had been added, and which had a pH of 4.7, was kept under the condition for 2.5 hours. However, the survival rate of the cells in the solution having a pH of 3.2, to which 50 ppm of sulfuric acid had been added, was 90 percent after maintaining it for 0.5 hours, 20 percent after keep it for 1 hour, and 1 percent or less after keeping it for 2.5 hours. When 100 ppm of sulfuric acid was added to the solution, the survival rate of the cells in the solution was 1 percent or less after 0.5 hours. Reference Example 2: To commercially available 3.5% artificial seawater, which had been subjected to pressure disinfection (at 120 ° C for 15 minutes), and then to pH control with sulfuric acid added to the same, a pre-determined amount of the same Escherichia coli cell suspension as in Example 1, or a previously determined amount of a suspension of a solid deposit in a reverse osmosis membrane that had been used in the desalination was added. of sea water, or a previously determined amount of unidentified bacteria, as separated from the suspension of solid deposits, of which the number was the largest among all bacteria separated from the suspension. Then, each seawater was maintained as such at 20 ° C for a previously determined period of time, and the survival rate of the cells in it was measured. The data are shown in Table 1. For comparison, 500 ppm of sodium bisulfite was added, instead of sulfuric acid, and also in Table 1 the data obtained are shown. From the data in Table 1, it is understood that the cells in the seawater were removed to an extremely high degree, when the seawater was maintained at a pH of 4.0 or lower, for a period of 0.5 hours or longer .

Table 1 Example 1: Two membrane separation units were activated, each having a polyamide reverse osmosis membrane, for the desalination of seawater through reverse osmosis filtration, to produce fresh water. One of the two units was applied daily raw seawater, which had been previously treated and subjected to a pH control to have a pH of 3.5 to 4.0, with sulfuric acid added to it, during a period of 30 minutes a day. In that condition, the two units were continuously activated for 1 month. As a result, the pressure loss in the unit to which no sulfuric acid had been added was increased, but the loss of pressure in the other unit to which sulfuric acid had been added did not change. While the units were activated under the condition, the number of living cells in the concentrate that had passed through each unit was counted. As a result, the number of living cells in the concentrate in the unit that had been subjected to the sulfuric acid treatment was reduced to 1/100 or less, compared to that of the living cells in the concentrate in the other unit not submitted to the treatment with sulfuric acid. Example 2: Raw seawater was applied, in which the number of live cells was 200 cells / milliliter, as counted with an agar plate counter, to a membrane separation unit having a reverse osmosis membrane of polyamide, in which the raw seawater was subjected to reverse osmosis separation. In the pretreatment unit, before the membrane separation unit, a microbicide containing chlorine was continuously added to the raw seawater, such that the concentration of chlorine remaining therein could be 1 ppm. Just before the reverse osmosis membrane module in the separation unit, sodium bisulfite was added to the raw seawater that was being treated. The amount of sodium bisulfite added was controlled in such a way that the concentration of sodium bisulfite remaining in the saline solution that would be allowed to settle through the module could be at least 1 ppm. The consumption of sodium bisulfite was 5 ppm in the initial stage. However, after the system was operated continuously for 10 days, the sodium bisulfite consumption was increased to 35 ppm. Within those 10 days, the pressure loss in the membrane module increased by approximately 0.01 MPa. Then, raw seawater that had been subjected to pH control, to have a pH of from 3 to 4, with sulfuric acid added thereto, was passed through the membrane separation unit, over a period of 30 minutes a day. As a result, the consumption of sodium bisulfite decreased to 8 ppm. In this case, the pressure loss was increased by 0.01 MPa, compared to the original value, and remained thereafter as such.

Example 3: Raw seawater was applied, in which the number of live cells was 200,000 cells / ml, as counted with an agar plate counter, to a membrane separation unit that had an osmosis membrane reverse polyamide, in which the raw seawater was subjected to separation by reverse osmosis. In the pretreatment unit, before the membrane separation unit, a microbicide containing chlorine was continuously added to the raw seawater, such that the concentration of chlorine remaining in it could be at least 1 ppm, and also 6 ppm of a sodium bisulfite dechlorinating agent was added thereto. In the membrane separation unit, 500 ppm of sodium bisulfite was added to the raw seawater, for a period of 1 hour a week. After the system was operated for approximately 1 month, the pressure loss in the membrane separation unit was increased by approximately 0.02 MPa, compared to the initial value. The same raw seawater was treated in the same system as before. In this case, however, 1 ppm of the chlorine-containing microbicide was added intermittently to the raw seawater, in the pretreatment unit, for a period of 1 hour a day, and 6 ppm of bisulfite was added thereto. sodium, for a period of 3 hours a day; and raw seawater that had been subjected to pH control was applied, to have a pH of 4, with sulfuric acid added thereto, to the membrane separation unit for a period of 1 hour a day. After about 1 month, the loss of pressure in the membrane separation unit changed little. Example 4: The same raw seawater was previously treated in the same manner as in the last process of Example 3. Then, the raw seawater was treated in the same membrane separation unit, as in Example 3. In This, however, did not disinfect the membrane in the unit, and the system was operated for 50 days. As a result, the pressure loss in the membrane separation unit was increased by 0.03 MPa. After this step, the raw seawater that had been subjected to the pH control was applied, to have a pH of 3, with sulfuric acid added thereto, to the membrane separation unit, during a period of 1 hour a day After 8 days, the pressure loss decreased by 0.015 MPa. Then, the system was operated for another 20 days, without disinfecting the membrane separation unit. As a result, the pressure loss was increased by 0.02 MPa. After this step, raw seawater that had been subjected to pH control was applied, to have a pH value of 4, with sulfuric acid added thereto, to the membrane separation unit, over a period of 1 hour. hour a day. After 12 days, the pressure loss again decreased by 0.012 MPa. Example 5: In a system comprising a pretreatment unit and a membrane separation unit, which had a polyamide reverse osmosis membrane module, raw seawater was desalinated by reverse osmosis filtration, to fresh water . In the pretreatment unit, chlorine was continuously added to the raw seawater, in such a way that the remaining chlorine concentration in the seawater could be 1 ppm, and sodium bisulfite was added to the raw seawater, before the module of reverse osmosis membrane. The amount of sodium bisulfite added was controlled in such a way that the concentration of sodium bisulfite remaining in the saline solution to be removed from the reverse osmosis membrane module could be at least 1 ppm. After the system was powered, the consumption of sodium bisulfite was increased. After 10 days, sodium bisulfite (this was obtained by subtracting the amount of sodium bisulfite remaining in the saline, from that which was added to the raw seawater) reached 21 ppm. After this, raw seawater that had been subjected to pH control was passed, to have a pH of 2.5, with sulfuric acid added thereto, through the membrane unit, for a period of 30 days in the day 1, day 2 and day 10, and raw seawater that had also been subjected to pH control was passed, to have a pH of 3, with sulfuric acid added to it, through it, during a period of 30 minutes on day 14 and day 27. In this stage, the consumption of sodium bisulfite decreased to 10 ppm.

Table 2 15 20 * "Excessive 1 ppm" means that the remaining chlorine concentration in the raw seawater, as treated in the pretreatment unit, was 1 ppm, and that the reduction agent that had remained in the saline solution, as was taken from the reverse osmosis membrane module was 1 ppm.

Comparative Example 1: 1 percent of seawater was added to 3.5 percent artificial seawater, commercially available, which had been subjected to pressure disinfection (at 120 ° C, for 15 minutes), and the pH was measured of the resulting seawater mixture as of 8.5. After it was kept at 20 ° C for 2 hours, 0.1 milliliter of the seawater mixture was applied on a medium for marine bacteria agar, from which the pH value had been controlled to be 7, and then it was kept warm at 20 ° C. After incubating it for a few days, the medium had 200 colonies formed therein. Reference Example 3: 1 percent of seawater was added to 3.5 percent artificial seawater, commercially available, which had been subjected to pressure disinfection (at 120 ° C, for 15 minutes), and then to control of the pH with 200 ppm of sulfuric acid added to it. The pH of the resulting seawater mixture was 2.8. After it was kept at 20 ° C for 2 hours, 0.1 milliliter of the seawater mixture was applied on a medium for marine bacteria agar, from which the pH value had been controlled to be 7. After After incubation for a few days, the medium had 3 colonies formed in it. The data in this Reference Example 3 are shown in Table 3, along with those in Comparative Example 1. The microbes that had formed the colonies on the agar medium are acid-resistant microbes that could not be removed at a pH of 2.8, and it is believed that 1.5 percent of those acid-resistant microbes existed in the seawater tested here. Table 3 Reference Example 4: To commercially available 3.5% artificial seawater, which had been subjected to pressure disinfection (at 120 ° C for 15 minutes), and then to pH control with sulfuric acid added thereto , a predetermined amount of acid-resistant, unidentified bacteria (3 strains taken together) that had been obtained in Example 7 was added, and kept at 20 ° C for a previously determined period of time. Afterwards, the survival rate of the bacteria in the artificial seawater, controlled in its pH, was obtained, and in Table 4 the data are shown. From Table 4, it is understood that seawater is disinfected well when maintained at a pH of 2.6 or less, for a period of 0.5 hours or longer.

Table 4 Example 6: Two membrane separation systems (systems A and B) were operated, each comprising a pretreatment unit and a membrane separation unit, having a polyamide reverse osmosis membrane module, for the desalination of seawater through reverse osmosis filtration, to produce fresh water. In these, a culture of the acid-resistant bacteria that had been obtained in Reference Example 3 was added to the seawater previously treated. Seawater that had been subjected to pH control, to have a pH of 3.5 to 4.0, was passed through both systems for a period of 30 minutes a day. These systems having thus been subjected to pH control were operated more stably, in comparison with others not subjected to pH control. However, after these systems were continuously operated for 30 days in that condition, the pressure loss in the membrane separation unit was increased. After this state, the seawater that had been subjected to the pH control was passed, to have a pH of 2.6, through system A for a period of 30 minutes a day, while the seawater that had been subjected to pH control to have a pH of from 3.5 to 4.0, was passed through system B, also during a period of 30 minutes a day. Through system B, seawater that had been subjected to pH control to have a pH of 2.6 was further passed for a period of 30 minutes per day, but once at 5 day intervals. Under these conditions, the two systems were continuously operated for 30 days. As a result, the loss of pressure in the membrane separation unit in the two systems did not change. Although the systems were operated under the defined conditions, the number of living cells in the concentrate was counted. The number of live cells in the concentrate in the two systems decreased to 1/100 or less, compared to that in the concentrate in those systems, where only seawater having a controlled pH value of from 3.5 to 4.0. The data are shown in Table 5. From Table 5, it is understood that the disinfecting effect of seawater having a controlled pH value of from 3.5 to 4.0 is not good, but the disinfecting effect of seawater having a controlled pH value of 2.6 is satisfactory . In addition, it is also understood that the disinfecting effect of seawater can be satisfactorily improved only when the pH value of seawater is decreased to 2.6 once, at 5 day intervals.

Table 6 INDUSTRIAL APPLICABILITY To disinfect microorganisms that exist in and around the membranes in a membrane separation apparatus for water purification, the method of the invention is better than conventional methods of intermittently adding sodium bisulfite in high concentration to the apparatus. In accordance with the method of the invention, all microorganisms in the apparatus are safely disposed of.

Claims (13)

1. CLAIMS 1. A bacteriostatic or disinfecting method for perm-selective membranes in a membrane separation apparatus for water purification, comprising raw water to acid treatment at a pH of 4 or lower.
2. 2. The bacteriostatic or disinfecting method for perm-selective membranes, as defined in claim 1, wherein the raw water has a pH of 3.4 or less.
3. 3. The bacteriostatic or disinfecting method for perm-selective membranes, as defined in claim 2, wherein the raw water has a pH of 2.6 or less.
4. 4. The method of bacteriostasis or disinfection for perm-selective membranes, as defined in claim 1, wherein the acid treatment is carried out intermittently, and the time for treatment falls between 0.5 and 2.5 hours.
5. 5. The bacteriostatic or disinfecting method for perm-selective membranes, as defined in claim 4, wherein the frequency of the acid treatment is once at intervals of 1 day to 1 month.
6. 6. The bacteriostatic or disinfecting method for perm-selective membranes, as defined in claim 4, which comprises additional acid treatment for pH control at a pH of 2.6 or less, and in which the additional acid treatment is carried out once at intervals of 2 to 180 days.
7. 7. The bacteriostatic or disinfecting method for perm-selective membranes, as defined in claim 1, wherein the perm-selective membranes are reverse osmosis membranes.
8. 8. The bacteriostatic or disinfecting method for perm-selective membranes, as defined in claim 1, wherein the raw water to be treated is seawater.
9. 9. The bacteriostatic or disinfecting method for perm-selective membranes, as defined in claim 1, wherein the acid treatment is carried out with at least 120 ppm of sulfuric acid added to the raw water.
10. 10. A pre-treatment apparatus for a reverse osmosis membrane treatment apparatus, which comprises a first duct for feeding raw water to a sand filter, a second duct for feeding it from said sand filter to a safety filter , a third duct for feeding from said safety filter to a reverse osmosis membrane treatment apparatus, a flocculant feeder for feeding a flocculant to said first duct, a pH controlling agent feeder for feeding a pH controlling mineral acid to said second duct, and a microbicide feeder for feeding a microbicidal mineral acid to any of said ducts first to third.
11. 11. A method for separating or purifying water in a membrane separation apparatus, for which the method of bacteriostasis or disinfection for perm-selective membranes, as defined in any of claims 1 to 9, or the apparatus of Treatment for reverse osmosis membrane treatment apparatus, as defined in claim 10, is indispensable.
12. 12. The method for separating and purifying water, as defined in claim 11, wherein the raw water to be treated is seawater. The method for separating and purifying water, as defined in claim 11 or 12, wherein the raw water to be treated is previously subjected to intermittent disinfection with chlorine.