HK1032761B - Method for inhibiting growth of bacteria or sterilizing around separating membrane - Google Patents

Description

Method for inhibiting proliferation or sterilization of bacteria on separation membrane

Technical Field

The present invention relates to membrane separation, and more particularly to a method for pretreating raw water, a method for sterilizing a membrane, and an apparatus therefor, in desalination and separation by a reverse osmosis method, for example, in desalination of sea water by a reverse osmosis method.

Background

Membrane separation techniques are widely used in the fields of desalination of seawater or brackish water, production of pure water and ultrapure water for medical and industrial use, treatment of industrial wastewater, and the like. In these membrane separation techniques, the quality of the obtained permeated water is deteriorated due to contamination of the separation apparatus by microorganisms, or the permeability and separability of the membrane are lowered due to proliferation of microorganisms on the membrane surface or adhesion of microorganisms and metabolites thereof to the membrane surface. The influence of these microorganisms is specifically expressed by deterioration of the permeated water quality, reduction of the permeated water amount, increase of the operation pressure, or increase of the pressure loss. In order to avoid such important problems, various techniques for inhibiting the proliferation of bacteria in a membrane separation apparatus or sterilization methods have been proposed. As the bactericide, a chlorine-containing bactericide having practical effects and advantageous in terms of price and handling is added to a concentration of about 0.5 to 50ppm as the most common method. In a method generally employed, a disinfectant is injected into a pretreatment portion of a membrane separation apparatus, and the pretreatment water subjected to sodium hypochlorite sterilization and flocculation filtration is temporarily stored in a temporary storage tank before being supplied to the membrane separation apparatus, and free chlorine is reduced and removed with sodium bisulfite before a safety filter belonging to a preceding stage of the membrane separation apparatus.

Since the chlorine-containing bactericide chemically deteriorates the reverse osmosis membrane, it is necessary to reduce free chlorine with a reducing agent before supplying the bactericide to the reverse osmosis membrane. Sodium bisulfite, which is a reducing agent, can be added in an amount of usually 1 to 10 times by equivalent. The reason for using the above-mentioned concentration of the reducing agent is to consider that not only the residual sterilizing agent is completely eliminated but also the reaction with the dissolved oxygen is carried out. However, even if the operation is continued in this method, the membrane performance is lowered, because it has been found that the sterilization effect of the operation method on microorganisms is not necessarily sufficient. In this regard, it is considered that the addition of chlorine can oxidize organic carbon present in the feed liquid to convert it into a compound easily decomposed by microorganisms (a.b. hamida and i.moch.jr., Desalination and Reuse of Water (desalinization & Water Reuse), 6/3, 40-45, (1996)) but this has not been confirmed.

In the conventional pretreatment method, the pretreated water subjected to the sterilization treatment and the flocculation filtration treatment is stored in a temporary storage tank, and contamination and retention of contaminants from the outside easily cause an increase in microorganisms and deterioration in water quality. Further, although the bactericidal effect of sodium hydrogen sulfite is such that it can remove oxygen in the feed liquid and lower the pH of the feed liquid, it is difficult to say that the bactericidal effect is exerted when sodium hydrogen sulfite is intermittently added when the membrane device is in operation, which is the current situation. The present inventors have studied the cause of this problem and have come to the conclusion that, in aerobic bacteria that generally live under neutral to weakly alkaline conditions, even if the anaerobic state can inhibit the growth of the aerobic bacteria, the anaerobic state cannot be an environment that leads to bacterial death, and that lowering the pH is the most effective sterilization measure. This conclusion is not contradictory to the microbiological findings. On the other hand, it was found that even when sodium hydrogen sulfite was added at a high concentration of 500ppm to a feed liquid having a high salt concentration such as seawater, the pH could not be lowered to such an extent that bacteria were killed. Therefore, even in the case of a feed liquid containing a relatively low salt concentration, the sterilization effect of sodium bisulfite is not caused by the anaerobic state but by the decrease in pH, and therefore, it is not necessary to add a high concentration of expensive sodium bisulfite, and it is possible to achieve a sufficient sterilization effect by merely decreasing pH by adding an inexpensive acid such as sulfuric acid, and it is possible to suppress the growth of microorganisms by preventing the retention of pretreatment water.

Disclosure of the invention

The purpose of the invention can be achieved according to the following technical scheme. That is, the present invention relates to a method for sterilizing a separation membrane, which is characterized by having an acid treatment step for adjusting the pH of a feed liquid to be fed to a membrane separation device to 4 or less, a method for purifying water in which the sterilization method is essentially used, and an apparatus therefor.

Brief description of the drawings

Fig. 1 shows the constitution of the main part of a seawater desalination plant.

1: pretreatment device

2: reverse osmosis membrane treatment device

3: post-processing device

4: membrane washing device

6: no. 1 pipeline

7: flocculating agent adding device

8: sand filter (one-time filtering means)

9: safety filter

10: 2 nd pipeline

11: PH adjustment adding device

12: no. 3 pipeline

13: sterilizing agent adding device

Best Mode for Carrying Out The Invention

In the present invention, the purpose of the membrane separation device is to produce water, concentrate, separate, etc., and the function thereof is to supply a liquid to be treated to a membrane module under pressure to separate the liquid to be treated into a permeate and a concentrate. The membrane module includes a reverse osmosis membrane module, an ultrafiltration membrane module, a microfiltration membrane module, etc., and the membrane separation apparatus is classified into a reverse osmosis membrane apparatus, an ultrafiltration membrane apparatus, a microfiltration membrane apparatus, etc., mainly depending on the kind of the membrane module used, and specifically, a reverse osmosis membrane apparatus can be exemplified.

The reverse osmosis membrane is a semipermeable membrane which allows a part of components such as a solvent in a mixed liquid to be separated to permeate therethrough and prevents other components from permeating therethrough. Nanofiltration membranes (ナノフイルトレ - ション membranes) and ル - ス RO membranes are also broadly classified as reverse osmosis membranes. As the raw material, a polymer material such as a cellulose acetate polymer, polyamide, polyester, polyimide, or polyethylene can be used. In addition, the membrane is structured as an asymmetric membrane having a dense layer on at least one side thereof and having a plurality of micropores therein whose pore diameters gradually expand as they transition from the dense layer to the inside of the membrane or the other side thereof, or a composite membrane having a very thin active layer formed of other raw materials on the dense layer of the asymmetric membrane. The membrane is in the form of hollow fiber or flat membrane. The thickness of the hollow fiber and the flat membrane is 10 mu M-1 mm, and the outer diameter of the hollow fiber is 50 mu M-4 mm. In addition, the asymmetric membrane and the composite membrane in the flat membrane are preferably supported by a base material such as a woven fabric, a knitted fabric, or a nonwoven fabric. According to the inorganic acid sterilization method of the present invention, any raw material, membrane structure or membrane form of a reverse osmosis membrane can be used, and any method is effective. Typical examples of the reverse osmosis membrane include an asymmetric membrane of cellulose acetate or polyamide, and a composite membrane having an active layer of polyamide or polyurea. Among these membranes, cellulose acetate-based asymmetric membranes and polyamide-based composite membranes are effective for the process of the present invention, while the effect is greater with aromatic polyamide-based composite membranes (Japanese patent laid-open Nos. 62-121603, 8-138658, and 4277344).

The reverse osmosis membrane module is a member having a certain shape formed by the reverse osmosis membrane in actual use, and may be used in a form of a module in which a flat membrane is assembled into a spiral roll, a tube or a plate frame, or a module in which a hollow fiber bundle is assembled.

The reverse osmosis membrane module used in the present invention has a desalination rate of 98% to 99.9% and a water yield of 10 to 25m when evaluated under conditions of 5.5MPa, 25 ℃ and a recovery rate of 12% with seawater having a salt concentration of 3.5% (the most common seawater concentration) as a feed solution, and normalized by a length of 1m × a diameter of 20cm³A day; or a salt rejection ratio of 98 to 99.9% and a water yield normalized by a length of 1m x a diameter of 20cm of 10 to 25m when evaluated under conditions of a salt concentration of 5.8% in seawater as a feed solution at 25 ℃ and a recovery rate of 12% at 8.8MPa³The day is. Preferably, the salt concentration of 3.5% sea water as the supply liquid and at 5.5MPa, 25 ℃, recovery rate of 12% under the conditions of performance for 99% -99.9% desalination, length of 1m x diameter of 20cm normalized water yield of 12-23 m²A day; or the salt concentration of 5.8% seawater as the feed solution and the recovery rate of 12% under 8.8MPa, 25 deg.C, the desalting rate is 99-99.9%, and the water yield is 12-23 m when normalized by length of 1m × diameter of 20cm³A day; more preferably, the salt removal rate is 99.3 to 99.9% and the water yield is 14 to 20m when normalized by the length of 1m × the diameter of 20cm, when the performance is evaluated under the conditions of 5.5MPa, 25 ℃ and the recovery rate of 12% using seawater having a salt concentration of 3.5% as a feed solution³A day; or the salt concentration of 5.8% seawater as the feed solution and the recovery rate of 12% under 8.8MPa, 25 deg.C, the desalting rate is 99.3-99.9%, and the water yield is 14-20 m when normalized by length of 1m × diameter of 20cm³The day is. Further, the reverse osmosis membrane module can be spirally assembled as a member for supplying a water flow path material, a permeated water flow path material or the like, and each of the devices composed of these members can be used, and particularly, it is more effective when a module designed for high concentration of 3.5% or more and high pressure of 7.0MPa or more is used for at least a part thereof (Japanese patent application laid-open No. 9-141060, Japanese patent application laid-open No. 9-141067).

The operating pressure of the reverse osmosis membrane device is 0.1 MPa-15 MP: the number can be appropriately selected and used according to the type of the liquid to be supplied, the operation method, and the like. A lower pressure can be used when a solution having a low osmotic pressure such as salt water or ultrapure water is used as the feed liquid, and a higher pressure can be used when the solution is used for desalination of sea water or treatment of waste water and recovery of useful substances.

The reverse osmosis membrane device can be used because the operation temperature is in the range of 0-100 ℃, the feed liquid is frozen and can not be used when the operation temperature is lower than 0 ℃, and the feed liquid is evaporated when the operation temperature is higher than 100 ℃, so the reverse osmosis membrane device can not be used.

The recovery rate of the separation apparatus is 5 to 100%, which can be set according to the separation operation and the separation apparatus. The recovery rate of the reverse osmosis membrane device can be selected appropriately between 5% and 98%. However, the pretreatment and the operation pressure must be considered in accordance with the properties, concentration, osmotic pressure, etc. of the feed solution or the concentrated solution (Japanese patent application laid-open No. 8-108048). For example, in the case of desalination of sea water, the recovery rate is usually 10 to 40%, and the recovery rate when using a high-efficiency apparatus is 40 to 70%. When desalination of salt water or production of ultrapure water is carried out, the operation can be carried out at a recovery rate of 70% or more or 90 to 95%.

The structure of the reverse osmosis membrane module may be used in a single stage, but the feed water may be arranged in a plurality of stages in series or in parallel. When the feed liquid is arranged in a tandem manner, the contact time between the membrane module and the feed water is long, and therefore the effect of the method of the present invention can be improved. When a tandem configuration is used for the feed liquid, a means for pressurizing the feed liquid may be used between appropriate sections. The pressure can be increased to a pressure range 0.1-10 MPa higher than that of the previous stage by using a booster pump or a boosting pump. In addition, the reverse osmosis membrane module can also adopt a tandem configuration for the permeated water. The tandem arrangement is a preferable method in the case where the quality of the permeated water is not good enough or in the case where it is desired to recover solute components in the permeated water. In the case of the in-line configuration of the permeated water, a pump may be provided between the two apparatuses to repressurize the permeated water, or a back pressure may be applied by using the residual pressure of the previous stage to perform membrane separation. In the case of the tandem arrangement of the permeated water, an acid adding device may be provided between the membrane modules in order to sterilize the subsequent membrane module parts.

In the reverse osmosis membrane apparatus, a part of feed water that does not permeate the permeable membrane is taken out of the membrane module as concentrated water. The concentrated water may be disposed of after being treated according to the purpose, or may be further concentrated by another method. In addition, part or all of the concentrate may be recycled to the feed water. Even the part having permeated the permeable membrane may be discarded, directly used, or partially or entirely recycled to the feed water depending on the use.

Typically, the concentrate in the reverse osmosis unit contains pressure energy, which is preferably recovered in order to reduce operating costs. As the energy recovery method, energy can be recovered at any part of the high-pressure pump using an energy recovery device included in the high-pressure pump, but it is preferable to install a dedicated turbine type energy recovery pump before and after the high-pressure pump or between the membrane modules to recover energy. In addition, the membrane separation device has a treatment capacity of 0.5m per day³100 ten thousand meters³A water device.

In the present invention, it is very important to provide a high bactericidal effect to the membrane by adjusting the pH to 4 or less, and particularly, the effect is more remarkable in a membrane filtration apparatus using seawater as feed water. The pH value capable of killing microorganisms is different for each microorganism, for example, the lower limit for inhibiting the propagation of Escherichia coli is pH4.6, and the pH value for killing Escherichia coli is 3.4 or less. On the other hand, various microorganisms are present in seawater, and the pH at which these microorganisms can be killed is different. However, in the present invention, if seawater containing a plurality of living bacteria is kept at a pH of 4 or less for a certain period of time, the living bacteria therein can be killed by 50 to 100%. The acidity is preferably kept at a pH of 3.9 or less, more preferably at a pH of 3.7 or less. In addition, when acid-resistant microorganisms are present in seawater containing many kinds of viable bacteria, these microorganisms can be killed by 98% or more by maintaining the seawater at a ph of 2.6 or less for a certain period of time. Therefore, high effects can be obtained by treating the mixture at a pH of 4 or less for a certain period of time and, in some cases, additionally treating the mixture to a pH of 2.6 or less. In order to maintain the pH at the desired value, an acid may generally be used. As the acid, any of organic acids and inorganic acids can be used, but sulfuric acid is preferably used from the economical viewpoint. The amount of sulfuric acid added should be proportional to the salt concentration of the feed solution. For example, although 50ppm of sulfuric acid was added to normal saline (common salt concentration 0.9%) which had been subjected to pressure sterilization (120 ℃ C., 15 minutes) to lower the pH to 3.2, the pH was lowered to 5.0 to 5.8 by adding 100ppm of sulfuric acid to seawater at three locations subjected to pressure sterilization (120 ℃ C., 15 minutes) and artificial seawater (salt concentration of about 3.5%) on the market. This is believed to be due to a large change in the M alkalinity of the seawater. In order to lower the pH to 4 or less, it is necessary to add 120ppm or more of sulfuric acid, and in order to lower the pH to 2.6 or less, it is preferable to add 250ppm or more of sulfuric acid. However, the maximum amount of sulfuric acid added is preferably 400ppm, more preferably 300ppm, from the viewpoint of economic efficiency and influence on facilities such as piping. Further, when further sulfuric acid of 150ppm, 200ppm, 250ppm and 300ppm is added to the seawater and the artificial seawater, the pH value is lowered to pH3.2 to 3.6, pH2.8 to 2.9, pH2.6 and pH2.4, and the change in pH value is reduced as the concentration of the added sulfuric acid increases. Generally, a high bactericidal effect is exhibited on all bacteria including acid-resistant bacteria contained in water by lowering the pH to 2.6, but since the proportion of acid-resistant bacteria in seawater bacteria is small, it is generally necessary to sterilize the seawater bacteria only under the conditions of pH2.7 to 4, and further to sterilize the acid-resistant bacteria under the conditions of pH2.6 or less, which is preferable in view of reducing the chemical cost required for making the feed liquid acidic and the influence on piping facilities.

The membrane sterilization operation of the present invention may be performed in a step of supplying the pretreated water to the membrane modules after the pretreatment of the water, or may be performed intermittently between the membrane modules in order to sterilize the membrane modules at the rear in the case of treating the permeated water in a serial arrangement. The addition time and frequency of the acid vary greatly depending on the place of use and the conditions of use. For example, it is conceivable to perform the acid addition operation for 0.5 to 2.5 hours every 1 day, 1 week or 1 month. In the case of treating with two types of pH combinations, the same is true in the case of killing acid-resistant bacteria, and the treatment step (step A) at pH2.7 to 4 is preferably performed at a frequency of once every 1 to 30 days, while the treatment step (step B) at pH2.6 or less is preferably performed at a frequency of once every 2 to 180 days. When the process A and the process B are required to be carried out a plurality of times within a fixed time, the ratio (TA/TB) of the total Time (TA) of the process A to the total Time (TB) of the process B is preferably in the range of 1/100 to 100. Furthermore, from the viewpoint of the processing cost and the durability of the apparatus, it is preferable that TA/TB (Process A: pH2.7 to 4.0, Process B: < pH2.6) is in the range of 1 to 100. The operation of step a may be directly transferred to the operation of step B, or conversely, the operation of step B may be directly transferred to the operation of step a, but it is preferable to supply a supply liquid having a pH value of 6.5 to 7.5 without pH adjustment between step a and step B. In this case, as the feed liquid which is not subjected to pH adjustment or has a pH of 6.5 to 7.5, a permeate or a concentrate obtained by separating ordinary water by a membrane separation operation may be selected depending on the intended purpose. These factors may be changed depending on the decrease in the water permeability of the membrane, the number of viable bacteria in the concentrated solution, the increase in the organic carbon contained in the water, the increase in the membrane pressure, and the like. In addition, when the film is not continuously used, the film may be subjected to a dipping treatment at the time of shutdown.

The sterilization method of the present application may be used in combination with other sterilization methods such as a chlorine treatment method.

The membrane sterilization method of the present invention is applicable not only to a membrane separation device but also to a water separation system including a membrane separation device.

The water separation system has, for example, the following configuration.

A. A water intake device. This is a device for sucking raw water, and is generally composed of a water suction pump, a chemical injection device, and the like.

B. A pretreatment device communicated with the water intake device. Which purifies the water supplied to the separation membrane device to a desired degree by pretreatment. For example, it may be constituted in the following order.

B-1, a flocculation filtration device.

B-2, cleaning (ポリツシング) the filtering device. However, it is also possible to replace the devices B-1 and B-2 with ultrafiltration devices or microfiltration devices.

And B-3, a medicine injection device for injecting a flocculating agent, a bactericide, a pH regulator and the like.

C. An intermediate tank communicating with the pretreatment apparatus may be provided as required. It can provide a function of regulating water quantity and buffering water quality change.

D. And (3) a filter. When C is provided, it communicates with the intermediate tank, or, when C is not provided, it communicates with the pretreatment apparatus. Which is capable of removing solid impurities from water supplied to the membrane separation device.

E. A membrane separation device. It consists of a high-pressure pump and a separation membrane component. The membrane separation device may be provided in plural, and they may be provided in parallel or in series. When arranged in series, a water pump may be provided between the two membrane separation devices to increase the water pressure supplied to the subsequent membrane separation device.

F. A post-treatment device communicating with the membrane permeation side outlet portion of the membrane separation device. It may be the following device, for example.

F-1, a degassing device. It has the function of removing carbon dioxide.

F-2 and a calcium tower.

F-3, a chlorine injection device.

G. And an aftertreatment device communicated with the raw water side outlet part of the membrane separation device. It may be the following device, for example.

G-1, treating the supply liquid with the pH of 4. Such as a neutralization device.

G-2, releasing equipment.

H. Other wastewater treatment devices may be suitably provided.

In these apparatuses, a water pump may be provided at any position. In addition, in order to reduce the pH to 4 or less, a chemical or a solution of a chemical may be added thereto, and the addition position is preferably in the water intake device of a or the pretreatment device of B, or before the pretreatment device or the filter of D, or after the filter.

In addition, in order to further enhance the effect of the present invention, it is preferable to use an automatically controlled acid adding device, and more preferably, an automatically controlled acid adding device equipped with a metering pump capable of controlling an appropriate injection amount. For control, it is preferable to install a device capable of measuring the pH of the feed liquid or the concentrated liquid at an appropriate position in the device. In addition, in order to control the intermittent addition operation, it is preferable to provide a device capable of measuring time. More preferably, the system includes an automatic control device capable of automatic operation.

As the components of the apparatus of the present invention, for example, piping, valves, etc., those which are not easily changed even under the condition of pH4 or less should be used. For example, stainless steel, lined materials, resins, and the like may be used.

By lowering the pH to 4 or less, not only a high sterilizing effect but also an effect of removing scale in the pipe can be obtained. In addition, in order to prevent the deterioration of the film due to the chlorine-based oxide, it is necessary to add sodium bisulfite thereto, but the amount added increases with the influence of microorganisms (sulfur bacteria and the like are considered) adhering to the film surface, metal salts and the like, and the amount added can be significantly reduced by the method of acidic water treatment.

The method of the present invention is suitable for a process of separation using a membrane, and is more effective particularly when used in an aqueous solution separation process. Further, as an application of separation, separation and concentration of liquid-solid components using a microfiltration membrane; separation and concentration of the turbid component using an ultrafiltration membrane; separation and concentration of dissolved components using a reverse osmosis membrane are effective. Especially has good effects in seawater desalination, salt water desalination, industrial water production, ultrapure water and pure water production, medical water production, food concentration, tap water raw water turbidity removal, tap water high-level treatment and the like. In the case where it is necessary to separate and concentrate organic substances and the like which are easily decomposed by conventional oxidizing fungicides, the method of the present invention does not undergo the oxidative decomposition reaction, and therefore, can concentrate and recover them very efficiently. In addition, in the case of producing drinking water, trihalomethanes generated at the time of sterilization with chlorine can be effectively prevented from being generated.

In general, sterilization in the pretreatment step is performed by continuously injecting a chlorine-containing bactericide as described above. According to this method, although it is possible to almost completely kill the bacteria unless drug-resistant bacteria are present in the feed water, since the reverse osmosis membrane is chemically deteriorated by the bactericide, a reducing agent such as sodium hydrogen sulfite must be added before the membrane separation apparatus. However, according to this step, the feed water after the removal of the residual bactericide is in a state in which microorganisms are easily propagated. Further, this water supply does not have various microorganisms as in the case of raw seawater before addition of a bactericide but has only certain specific microorganisms, and it has been found that most of them are acid-resistant bacteria. In addition, when the amount of the reducing agent such as sodium hydrogen sulfite added is insufficient, the chlorine-containing bactericide cannot be completely removed, which results in deterioration of the film, while when the amount is too large, some kinds of bacteria gradually and remarkably grow. Therefore, in the case of carrying out the sterilization method of the present invention, it is preferable not to add a chlorine-containing bactericide, but in contrast to this, microorganisms are propagated in the pretreatment step described above. In order to solve this problem, the above problem is solved by intermittently injecting the bactericide and the reducing agent to kill microorganisms which have adhered to and accumulated on the equipment such as piping and a filtration tank in the pretreatment step during the non-injection period, and the deterioration of the membrane can be effectively prevented. The time interval of injecting the bactericide is suitably selected to be once every 1 day to 6 months, depending on the quality of the original seawater, that is, the existing state of the living things, and the time of each injection is about 30 minutes to 2 hours. Sterilization of the membrane of the present invention may be carried out at such intervals. This method of intermittently injecting a chlorine-containing bactericide can bring about an effect of remarkably reducing the cost of disposal, but this effect can be attained only by the film sterilization method according to the present invention, which has not been carried out finally because the sterilization effect was not good enough according to the film sterilization method in which sodium hydrogen sulfite was added at a high concentration in the past. In addition, in order to prevent the adhesion and accumulation of microorganisms during the non-injection period of the bactericide and to improve the bactericidal effect of the acid, it is effective to use the following apparatus.

Fig. 1 shows a seawater desalination plant having a pretreatment apparatus 1, a reverse osmosis apparatus 2, a post-treatment apparatus 3, a membrane washing apparatus 4, and the like. The pretreatment device 1 is composed of the following devices: a flocculant adding device 7 for adding a flocculant solution to seawater (raw water) flowing into the first pipeline 6, a sand filter 8 as primary filtration means, a safety filter 9 as secondary filtration means, a pH adjuster adding device 7 for adding an inorganic acid for pH adjustment to the primary filtered water flowing into the second pipeline 10, a bactericide adding device 13 for adding a bactericide solution to the secondary filtered water flowing into the third pipeline 12, and the like.

The first line 6 is connected to the suction pump 14 and the sand filter 8, the second line 10 is connected to the sand filter 8 and the guard filter 9, and the third line 12 is connected to the high-pressure pump 15 and further connected to the high-pressure pump 15 and the first-stage membrane module 17 of the reverse osmosis apparatus 2.

The seawater can be sent to the sand filter 8 by the operation of the water suction pump 14, and the secondarily filtered water can be supplied to the reverse osmosis membrane treatment apparatus 2 by the operation of the high pressure pump 15 while applying a high pressure to the secondarily filtered water. At this time, ferric chloride of a predetermined concentration is added to the system through the line 18 by the flocculant adding device 7, sulfuric acid is added to the system through the line 19 by the pH adjuster adding device 11, and a sulfuric acid solution is intermittently added to the system through the line 20 by the bactericide adding device 13. In this case, the line 20 may be connected to the line 12, or the pH adjuster addition device 11 and the bactericide addition device 13 may be shared as the same device as each other in some cases.

In addition, the flocculant adding device 7 can draw out the ferric chloride solution from the reservoir 22 by the operation of the pump 21 and add it into the system, and the pH adjuster adding device 11 can draw out the sulfuric acid from the reservoir 24 by the operation of the pump 23.

In fig. 1, a pipeline from the water suction pump 14 to the first-stage membrane module 17 of the reverse osmosis membrane treatment apparatus 2 is a closed pipeline. That is, the open pipe is not an open pipe, as in the conventional case, in which the temporary raw water tank is provided. The apparatus of the present invention may have a raw water tank, a sand filtration water tank, a liquid feed pump, etc., and it is preferable that a pipeline from the suction pump to the reverse osmosis membrane module is designed as a non-open pipeline.

The non-open type pipeline can prevent the contamination from the outside and can carry out the treatment while continuously sending water. Further, since the flow rate change after the high-pressure pump 15 can be adjusted by changing the flow rate in each component in the pretreatment device 1, the state of the water being constantly supplied can be maintained, and the sand filter 8 can be stably operated while the treatment is performed in a continuous operation without generating a stagnant portion.

As the pretreatment device, a cleaning filter may be provided after the sand filter. In addition, a UF membrane or MF membrane having a pore size of 0.01 to 1.0 μ M may be used in place of the sand filter or the cleaning filter or in place of both of them.

The use of this device can prevent the treated water from being retained in a storage tank or the like, and therefore, can prevent microorganisms from attaching to and accumulating in the apparatus during the period of non-injection of the bactericide, and can improve the bactericidal effect of the present invention.

Examples

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples. In these examples, the bactericidal effect is expressed in terms of the number of living bacteria, the pressure loss of the membrane module, and the consumption of Sodium Bisulfite (SBS).

Reference example 1

After a physiological saline solution (salt concentration 0.9%) was sterilized under pressure (120 ℃ C., 15 minutes), sulfuric acid was added thereto to adjust the pH thereof, and a suspension of a predetermined amount of Escherichia coli (Escherichia coli K12 IFO 3301) was added thereto and kept at 20 ℃ for a predetermined time, the ratio of the number of remaining viable bacteria to the number of viable bacteria at the time of addition was determined, and the ratio was used as the survival rate. The results are as follows: when 10ppm of sulfuric acid (pH4.7) was added, the survival rate of Escherichia coli was 90% or more even when kept for 2.5 hours; however, when 50ppm of sulfuric acid (pH3.2) was added, the retention time was 0.5 hour, and the survival rate was 90%, 20% for 1 hour, and 1% or less for 2.5 hours. On the other hand, if 100ppm sulfuric acid is added, the survival rate can be reduced to 1% or less by keeping for only 0.5 hour.

Reference example 2

Commercially available 3.5% artificial seawater was subjected to pressure sterilization (120 ℃ C., 15 minutes), sulfuric acid was added thereto to adjust the pH thereof, and then a certain amount of Escherichia coli used in example 1, a suspension of a deposit on a reverse osmosis membrane used in seawater desalination, and the most unidentified bacteria among the bacteria separated from the suspension were added thereto, and the bacteria were stored at 20 ℃ for a certain period of time, and then determinedSurvival, the results in table 1 were obtained. In addition, for comparison, the results obtained when 500ppm of sodium hydrogen sulfite was added instead of sulfuric acid are also shown in Table 1. As is apparent from Table 1, a high bactericidal effect can be obtained by maintaining the pH at 4.0 or lower for 0.5 hours or longer. TABLE 1

Additive agent	Additive concentration (ppm)	Time (h)	pH	Survival Rate (%)
							Escherichia coli	Suspension of film deposits	Bacteria from membrane deposits
				Sodium bisulfite free sulfuric acid	-500100120120150200300	2.52.52.50.52.50.50.50.5				8.55.95.14.04.03.32.92.5	1009810710593＜1＜1＜1	10090603715＜1＜1＜1	1008681＜1＜1＜1

Example 1

Two membrane separation apparatuses using a polyamide reverse osmosis membrane were arranged in parallel and operated simultaneously using seawater as feed water, and reverse osmosis filtration was performed on fresh water. In one of the apparatuses, sulfuric acid is added to pretreated seawater every day to adjust the pH of the seawater to 3.5 to 4.0, and the resultant is used as feed water to be passed through for 30 minutes. After 1 month of continuous operation, the apparatus to which no sulfuric acid was added found to have an increased pressure loss, but the apparatus to which sulfuric acid was added had no change in pressure loss. Further, when the number of viable bacteria in the filtered and concentrated water was measured under the conditions of normal operation, it was found that the number of viable bacteria in the former was reduced to 1/100 or less in the latter in the case of the apparatus subjected to the persulfate treatment as compared with the apparatus not subjected to the sulfuric acid treatment.

Example 2

A membrane separation apparatus using a polyamide reverse osmosis membrane was operated to perform reverse osmosis separation using, as feed water, seawater having a viable cell count of 200 per 1ml as measured by the agar smear method. Chlorine-containing bactericide was continuously added to the feed seawater in the pretreatment process to maintain the residual concentration of chlorine at 1ppm, and sodium hydrogen sulfite was added before the reverse osmosis membrane module. The concentration of sodium hydrogen sulfite added is adjusted so that the residual concentration in the salt water discharged from the reverse osmosis membrane module is 1ppm or more. The consumption of sodium hydrogen sulfite was initially 5ppm, but increased to 35ppm when the operation was continued for 10 days. Within the 10 days, the pressure loss of the membrane module increased by about 0.01 MPa.

Then, the feed water adjusted to pH 3-4 by adding sulfuric acid was run for 30 minutes per day, and the consumption of sodium hydrogen sulfite was reduced to 8 ppm. The pressure loss value at this time was maintained at an increased level of 0.01MPa from the initial value.

Example 3

A membrane separation apparatus using a polyamide reverse osmosis membrane was operated to perform reverse osmosis separation using, as feed water, seawater having a viable cell count of 20 ten thousand per 1ml as measured by the agar application method. In the pretreatment step, chlorine-containing bactericide was continuously injected so that the residual concentration in the feed seawater became 1ppm or more, and sodium hydrogen sulfite as a dechlorinating agent was continuously injected so that the residual concentration became 6ppm, and in the membrane separation step, 500ppm of sodium hydrogen sulfite was added for 1 hour within one week. After one month had elapsed, the pressure loss increased by about 0.02MPa from the initial stage.

Using the same apparatus and the same supply water, 1ppm of a chlorine-containing bactericide was intermittently added for 1 hour per day and 6ppm of sodium hydrogen sulfite was intermittently added for 3 hours per day, respectively, in the pretreatment step; in the membrane separation step, feed water adjusted to pH4 with sulfuric acid was fed daily for 1 hour. So that the pressure loss hardly changes even after about one month has elapsed.

Example 4

Since the same conditions as in the latter half of example 3 were applied up to the pretreatment step and sterilization was not performed in the membrane separation step, the pressure loss increased by 0.03MPa after 50 days of operation. From this point on, the feed water adjusted to pH3 with sulfuric acid was fed to the membrane separation step for 1 hour a day, and as a result, the pressure loss was reduced by 0.015MPa after 8 days. Then, sterilization in the membrane separation step was stopped, and the operation was continued for 20 days, whereby the pressure loss increased by 0.02 MPa. From this point on, the feed water adjusted to pH4 with sulfuric acid was fed to the membrane separation step for 1 hour a day, and as a result, the pressure loss was reduced by 0.012MPa after 12 days.

Example 5

A pretreatment apparatus for performing a pretreatment step and a membrane separation apparatus having a membrane module containing a polyamide reverse osmosis membrane are operated using seawater as feed water to perform reverse osmosis filtration to fresh water. In the pretreatment step, chlorine was continuously added to the feed seawater to maintain a residual concentration of 1ppm, and sodium hydrogen sulfite was added before the reverse osmosis membrane module. The concentration of sodium hydrogen sulfite added is adjusted so that sodium hydrogen sulfite remains in the salt water discharged from the reverse osmosis membrane moduleThe concentration is above 1 ppm. The consumption amount of sodium hydrogen sulfite increased after the start of the operation, and the consumption amount of sodium hydrogen sulfite (the addition concentration minus the residual concentration in the salt water) reached 21ppm after 10 days of the operation. Then, on day 1, day 2 and day 10, feed water adjusted to pH2.5 with sulfuric acid was fed in for 30 minutes, and on day 14 and day 27, feed water similarly adjusted to pH3 was fed in for 30 minutes, resulting in a reduction in the amount of sodium bisulfite consumed to 10 ppm. TABLE 2

	Number of viable bacteria	Sterilizing in pretreatment		RO sterilization			Days of operation (sky)	Effect
												Sterilizing and reducing agent	The injection time is divided into one minute per day	Bactericide	The injection time is divided into one minute per day	pH	Pressure loss rise	Number of viable bacteria	NaHSO3Consumption of
	Example 1		Is free of	-	Sulfuric acid	30		3.5-4	1-30	Is free of										Reduced to below 1/100
Is free of							-				6.5	Is provided with
					Example 2	200		Cl/NaHSO3(each 1ppm excess)		(Continuous)								0			5ppm
Is free of	-		1-10	0.01MPa					35ppm
											Sulfuric acid	30	3.4	11-11	0.01MPa		8ppm
Example 3	20 ten thousand	Cl/NaHSO3(1ppm excess/6 ppm)	(Continuous)	NaHSO3500ppm			60/7 (60 minutes in 1 week)											1-30	0.02MPa
					60/180	Sulfuric acid		60		4.0	1-30	Is free of
			Example 4	20 ten thousand			Cl/NaHSO3(1ppm excess/6 ppm)		60/180						Is free of	-		1-50	0.03MPa
Sulfuric acid	60	3.0			51-58	-0.015MPa
											Is free of	-		59-78	0.020MPa
Sulfuric acid	60	4.0			79-90	-0.012MPa
											Example 5		Cl/NaHSO3(each 1ppm excess)	(Continuous)	Is free of	-
Sulfuric acid	30	2.5	Days 11, 12 and 20			21ppm
							3	Days 14 and 27							10ppm

Note that: the term "1 ppm excess" means that the residual concentration of chlorine in the sea water supplied is 1ppm, or that the residual concentration of the reducing agent in the salt water discharged from the reverse osmosis membrane module is 1 ppm.

Comparative example 1

To commercially available 3.5% artificial seawater which had been sterilized under pressure (120 ℃ C., 15 minutes), 1% seawater was added, and the pH was measured to be 8.5. After culturing at 20 ℃ for 2 hours, 0.1ml of the culture was applied to an agar medium for marine bacteria adjusted to pH7, and the culture was incubated at 20 ℃. After several days of culture, 200 colonies were found on agar medium.

Reference example 3

Commercially available 3.5% artificial seawater was sterilized under pressure (120 ℃ C., 15 minutes), then sulfuric acid was added to a concentration of 200ppm to adjust its pH, and then 1% seawater was added to the artificial seawater, at which pH 2.8. After keeping at 20 ℃ for 2 hours, 0.1ml of the agar medium for marine bacteria adjusted to pH7 was applied and incubated at 20 ℃. After several days of culture, 3 colonies appeared on the agar medium. The results are shown in table 3 together with the results of comparative example 1. These bacteria appeared as a type of acid-resistant bacteria which could not be killed at an acidity of ph2.8, and it is considered that 1.5% of such bacteria were present in seawater. TABLE 3

	Conditions of treatment	Number of colonies present
			Comparative example 1	pH8.52 hours	200
Reference example 3	pH2.82 hours	3

Reference example 4 example

Carrying out the preparation of 3.5% artificial seawaterAfter sterilizing under pressure (120 ℃ C., 15 minutes), the pH was adjusted with sulfuric acid, and 3 strains of the unidentified acid-resistant bacterium obtained in example 7 were added to the artificial seawater in an amount of one portion and kept at 20 ℃ for a certain period of time, the survival rate of the bacteria was determined, and the results shown in Table 4 were obtained. As can be seen from Table 4, a high bactericidal effect can be obtained by keeping the pH below 2.6 for 0.5 hour. TABLE 4

	Concentration of addition	Time of addition	pH	Survival Rate (%)
							Additive agent	(ppm)	(h)	pH	Acid-resistant bacterium 1	Acid-resistant bacterium 2	Acid-resistant bacterium 3
Sulfuric acid without sulfuric acid	-200250250250300300	110.512.50.51	8.02.82.62.62.62.62.4	745017＜1＜18＜1	8922332＜11＜1	29＜11＜1＜1＜1＜1

Example 6

Two membrane separation apparatuses (apparatus a and apparatus B) each having a pretreatment apparatus for performing a pretreatment step and a membrane module including a polyamide reverse osmosis membrane were simultaneously operated in parallel using seawater as feed water to perform reverse osmosis filtration into fresh water. The culture solution of acid-fast bacteria obtained in reference example 3 was added to the seawater after the pretreatment, and water was passed through the culture solution. The operation can be stabilized by supplying water adjusted to pH 3.5-4.0 for 30 minutes every day, but the pressure loss is found to increase after 30 days of continuous operation, compared with the case where the low pH treatment is not performed. Then, feed water having a pH value adjusted to 2.6 was fed to the membrane separation device A for 30 minutes per day, feed water having a pH value adjusted to 3.5 to 4.0 was fed to the membrane separation device B for 30 minutes per day, and further, the pH value was adjusted to 2.6 once per 5 days and water was fed to the membrane separation device B for 30 minutes per day. As a result of the continuous operation for 30 days, the pressure loss of each membrane separation apparatus did not change. In addition, when the number of viable bacteria in the filtered and concentrated water is measured under the condition of normal operation, the number of viable bacteria is reduced to below 1/100 compared with the case of using only the feed water with the pH value adjusted to 3.5-4.0 for water passing through the membrane separation device. The results are shown in Table 5. As can be seen from table 5, although the supply water adjusted to pH3.5 to 4.0 had insufficient bactericidal effect, the supply water adjusted to pH2.6 had sufficient bactericidal effect, and sufficient bactericidal effect could be obtained by adjusting the pH to 2.6 once every 5 days. TABLE 5

Acidic Water treatment Process	The ratio of viable count of the filtered and concentrated water	Ratio of sulfuric acid
			30 minutes per day at pH 3.5-4.0	100	1
pH2.6 daily 30 minutes	＜1	2
			pH 5-4.0 pH2.6 30 min daily 30 min (adjusted only once every 5 days)	＜1	1.2

Industrial applicability

When water is purified using a membrane separation apparatus, as a method of sterilizing microorganisms present on the membrane surface or in the vicinity of the membrane, the method according to the present invention can obtain a reliable sterilization effect as compared with the conventional method of intermittently adding high-concentration sodium hydrogen sulfite.

Claims (10)

1. A method for sterilizing a separation membrane, characterized by comprising intermittently subjecting feed water to an acid treatment for a treatment time of 0.5 to 2.5 hours to reduce the pH to 3.9 or less at a frequency of 1 time per 1 day to 1 month when purifying the water by a membrane separation apparatus.

2. The method of sterilizing a separation membrane according to claim 1, wherein the pH of the feed water is 3.4 or less.

3. The method of sterilizing a separation membrane according to claim 2, wherein the pH of the feed water is 2.6 or less.

4. The method for sterilizing a separation membrane according to claim 1, wherein the acid treatment with a pH of 2.6 or less is performed 1 time in 2 to 180 days.

5. The method for sterilizing a separation membrane according to claim 1, wherein the separation membrane is a reverse osmosis membrane.

6. The method for sterilizing a separation membrane according to claim 1, wherein the feed water is seawater.

7. The method for sterilizing a separation membrane according to claim 1, wherein the acid treatment is addition of 120ppm or more of sulfuric acid.

8. A method for purifying water by separating purified water using a membrane separation apparatus, which requires the use of a method for sterilizing the separation membrane according to any one of claims 1 to 7.

9. The method of claim 8, wherein the feed water is seawater.

10. The method for separating and purifying water according to claim 8 or 9, wherein the method is performed in advance with a batch-wise chlorine sterilization treatment.