AU2007205782B2

AU2007205782B2 - Washing method and apparatus of separation membrane

Info

Publication number: AU2007205782B2
Application number: AU2007205782A
Authority: AU
Inventors: Koichiro Kando; Motoharu Noguchi
Original assignee: Metawater Co Ltd
Current assignee: Metawater Co Ltd
Priority date: 2007-08-14
Filing date: 2007-08-14
Publication date: 2012-04-26
Anticipated expiration: 2027-08-14
Also published as: AU2007205782A1

Abstract

Abstract of the disclosure: When a separation membrane (3) is washed with a chemical solution that is a sodium hypochlorite solution, which has a high concentration, diluted with membrane treated water stored in a chemical solution tank (6), an ammonic nitrogen concentration in membrane-treated water is monitored with a monitoring device (9), and a control device (8) controls a chemical solution dosing pump (7) using the concentration measured by the device (9) to adjust a dilution rate. Thereby, even when water quality of membrane-treated water varies, a sodium hypochlorite concentration in backwashing water can be retained constant, and fluctuation in the chemical washing effect can be prevented. $ 0 $ S A I I" Cr) 0ta

Description

AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT ORIGINAL Applicant(s): NGK INSULATORS, LTD Actual Inventor(s): Koichiro Kando and Motoharu Noguchi Address for Service: PATENT ATTORNEY SERVICES 26 Ellingworth Parade Box Hill Victoria 3128 Australia Title: WASHING METHOD AND APPARATUS OF SEPARATION MEMBRANE The following statement is a full description of this invention, including the best method of performing it known to me/us: 1 WASHING METHOD AND APPARATUS OF SEPARTION MEMBRANE Field of the Invention [0001] The present invention relates to a washing method and an apparatus of separation membranes, which are used in water and waste water treatment and the like. Background of the Invention (0002] In water treatment using a separation membrane, in order to remove deposits on a membrane surface and suppress a rise in a transmembrane pressure difference, backwashing using membrane-treated water is performed. However, in order to degrade and remove deposits, which cannot be removed simply by backwashing, chemical washing using a sodium hypochlorite solution is performed periodically. When a separation subject is surface water, since raw water is clear, an interval of chemical washing is relatively long, but when a separation subject is sewage water, since much contaminant is contained in raw water, for example, chemical washing is performed at a frequency of around once per 2 hours. [0003] This chemical washing is normally performed using washing water obtained by diluting a sodium hypochlorite solution, which has having a high concentration, with membrane-treated water

I

adjusted to be a predetermined hypochlorite concentration, as shown in Japanese Patent Application Laid Open (JP-A) No.09-313902. As this dilution rate, a fixed value is used as far as a concentration of a normally used high concentration sodium hypochlorite solution is not changed. However, although a separation membrane is washed using diluted water, which has been diluted at a prescribed rate, the sufficient chemical washingeffects cannotbe obtainedin some cases. Insuchcases, although chemical washing is performed, recovery of a transmembrane pressure difference of a separation membrane becomes insufficient, and stable operation cannot be performed. When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. The above references to and descriptions of prior proposals or products are not intended to be, and are not to be construed as, statements or admissions of common general knowledge in the art in Australia. Summary of the Invention [0004] According to a first aspect of the invention, there is 2 provided, a washing method of a separation membrane using washing water obtained by diluting a sodium hypochlorite solution with membrane-treated water, comprising monitoring an ammonia nitrogen concentration in membrane-treated water, and adjusting a dilution rate of a sodium hypochlorite solution, such that when an ammonic nitrogen concentration in membrane-treated water is high, a dilution rate is reduced to increase a sodium hypochlorite concentration in washing water and, conversely, when an ammonia nitrogen concentration in membrane-treated water is low, a dilution rate is increased to reduce a sodium hypochlorite concentration in washing water. According to a second aspect of the invention, there is provided a washing apparatus of a separation membrane comprising a separationmembrane, abackwashing pump for driving membrane-treated water on a filtrate side of the separation membrane, a chemical solution tank for storing a sodium hypochlorite solution, a chemical solution dosing pump for injecting a sodium hypochlorite solution in the chemical solution tank into backwashing water to dilute the sodium hypochlorite solution, a device for monitoring an ammonic nitrogen concentration in membrane-treated water, and a control device for controlling the chemical solution dosing pump using an ammonic nitrogen concentration measured with the monitoring device to adjust a dilution rate, wherein the control device has the function of adjusting the dilution rate so that when the ammonic nitrogen concentration is high, the dilution rate 3 is decreased, and, conversely, when the ammonic nitrogen concentration in membrane-treated water is low, the dilution rate is increased. According to a an embodiment of the invention, a total organic carbon concentration together with an ammonic nitrogen concentration in membrane-treated water are measured, and a dilution rate of a sodium hypochlorite solution is adjusted using the total organic carbon concentration together with the ammonic nitrogen concentration. Accordingly, embodiments of the invention provide a washing method and an apparatus of a separation membrane, which usually allow for stable chemical washing, and which can assuredly remove deposition from a membrane surface and thereby to restore a transmembrane pressure difference. [0005] In order to solve the aforementioned problems, the present inventors thoroughly investigated a cause by which the chemical solution washing effect varies irrespective of washing of a separation membrane using diluted water having a prescribed dilution rate. As a result, we discovered that water quality of membrane-treated water varies depending on the operation condition such as a biological treatment tank and a coagulation mixing tank placed in its early stage, and is not necessarily constant and, when ammonic nitrogen and an organic nitrogen compound are contained in membrane-treated water, they react with free chlorine to produce chloramines such as NH 2 Cl, NHCl, 4 NCl 3 and the like as shown in the following equation, reducing the membrane washing effect of sodium hypochlorite, Since a reaction speed of chloramines are slow, and are a substances, which have the poor membrane washing effect, compared with free chlorine, when free chlorine is consumed in production of chloramines, the effect on removing a membrane clogging substance is reduced. That is, when ammonic nitrogen or an organic nitrogen compound is contained in membrane-treated water, a concentration of a washing solution used at chemical washing becomes lower than a predetermined concentration, and the chemical washing effect is deteriorated in some cases.

NH

3 +HClO - NH 2 CltH 2 O

NH

2 Cl+HClO - NHC12+H 2 0 NHCl 2 +HClO - NC1 3

+H

2 0 [0006] The present invention was concluded based on the discoveries above, and is a method of washing a separation membrane using washing water obtained from diluting a sodium hypochlorite solution with membrane-treated water, monitoring an ammonic nitrogen concentration in membrane-treated water, and adjusting a dilution rate of a sodium hypochlorite solution using the concentration as an index. In the present invention, when a concentration of ammonic nitrogen in membrane-treated water is high, a dilution rate is lowered to increase a sodium hypochlorite concentration in washing water and, conversely, when an ammonic nitrogen concentration in membrane-treated 5 water is low, a dilution rate is increased to reduce a sodium hypochlorite concentration in washing water. [0007] This adjustment of a dilution rate keeps a free chlorine concentration in backwashing water almost constant, and allows for stable chemical washing. It is preferable that a concentration of diluted sodium hypochlorite is in a range of 20 to 1000mg/L. It is preferable that a dilution rate ofasodium hypochlorite solution is adjusted by controlling an injection amount of sodium hypochlorite with a chemical solution injecting pump. In addition, it is also preferable that after chemical solution washing of a separation membrane is performed like this, stop a chemical solution dosing pump, and perform the normal backwashing with membrane-treated water to eliminate the remaining chemical solution. [0008] The inventive feature of this washing apparatus a separation membrane of the present invention comprises a separation membrane, a backwashing pump to feed membrane-treated water on a filtrate side of the separation membrane, a chemical solution tank for storing a sodium hypochlorite solution, a chemical solution injecting pump for injecting the sodium hypochlorite solution in the chemical solution tank into backwashing water to dilute it, a device for monitoring an ammonic nitrogen concentration in membrane-treated water, to a control device for controlling the 6 chemical solution-injecting pump using an ammonic nitrogen concentration measured with the monitoring device as an index, to adjust a dilution rate. The control device has the function of adjusting a dilution rate so that, when an ammonic nitrogen concentration is high, a dilution rate is decreased and, conversely, when an ammonic nitrogen concentration in membrane-treated water is low, a dilution rate is increased. In a preferable embodiment, the separation membrane is a monolith-type MF membrane or UF membrane made of a ceramic, and the separation membrane is arranged at a stage after a biological treatment tank or a coagulation mixing tank. [0009] According to the present invention, since an ammonic nitrogen concentration in membrane-treated water is monitored, and a dilution rate of a sodium hypochlorite solution is adjusted using the concentration monitored, when a concentration of ammonic nitrogen or an organic nitrogen compound in membrane-treated water is high, a dilution rate can be lowered to increase a sodium hypochlorite concentration in washing water and, conversely, when a concentration of an ammonia or an organic nitrogen compound in membrane-treated water is low, a dilution rate can be increased to reduce a sodium hypochlorite concentration in washing water. For this reason, even when an amount of free chlorine in sodium hypochlorite to be consumed - for production of chloramines varies with variation in water quality of membrane-treated water, it becomes possible to 7 maintain a free chlorine concentration in backwashing water approximately constant, and the stable chemical solution washing effect can be exerted. Brief Description of the Drawings [0010] Preferred embodiments of the invention will thereinafter be described, by way of example only, with reference to the drawings in which: Fig.1 is a block drawing that shows an embodiment of the present invention. Fig. 2 is a graph showing a relationship between an ammonic nitrogen concentration in membrane-treated water, and a dilution rate. Fig. 3 is a graph showing a relationship among concentrations of TOC, ammonic nitrogen in membrane-treated water, and a dilution rate. Detailed Description of the Invention [0011] A preferable embodiment of the present invention will be shown below. In Fig.1, 1 is a biological treatment tank for treating sewage water. Water in the tank is supplied to a separation membrane 3 by means of a circulation pump 2 and membrane separation is performed in a cross flow filtration or a dead 8 end filtration. As the separation membrane 3, an MF membrane or a UFmembrane is used, and amaterial thereformaybe a polymer or a ceramic. The shape may be arbitrary such as a tubular membrane, a flat membrane and a hollow fibermembrane in addition to the monolith membrane, and may be an external pressure filtration or an internal pressure filtration. In this embodiment, a monolith membrane made of a ceramic is used. Membrane-treated water from the separation membrane 3 is stored in a filtrate water tank 4. Number 5 is a backwashing pump that, at normal backwashing, drives membrane-treated water in the treating water tank 4 on a secondary side of the separation membrane 3 and performs backwashing. Alternatively, backwashing may be performed using pressured water tank to store backwash water without using backwash pump. [0012] Like this, in the case of normal backwashing, backwashing with backwashing water without containing chemical solution is performed, but upon periodic chemical washing, a sodium hypochlorite solution with a high concentration in a chemical solution tank 6 is injected into backwashing water with a chemical solution dosing pump 7. A concentration of sodium hypochlorite in a chemical solution tank 6 is usually about 130000mg-Cl/L, and the chemical solution is diluted with membrane-treated water to a concentration of around 20 to 1000mg-Cl/L to prepare a washing solution. Then, this washing solution is supplied from a filtrate side of a separation 9 membrane 3 with a pump or the compressed air to extend the washing solution all over a membrane including a raw water side of the membrane. Thereafter, by immersing the separation membrane 3 in the washing solution for certain amount of time, the separation membrane 3 is chemically washed. A dilution concentration thereupon is determined by an injection amount of a chemical solution dosing pump 7. The method of immersing the separation membrane 3 in the washing solution may be performed by flowing the washing solution from a raw water side to a filtrate side of the membrane. In this case, for diluting a high concentration sodium hypochlorite solution, raw water is used. As a procedure, first, a high concentration sodium hypochlorite solution is injected into a raw water supply line, and diluted washing water is extended over the membrane with a raw water pump. After washing water is extended over the membrane, the raw water pump is stopped, and the membrane is immersed in washing water for certain amount of time, After certain amount of time has passed, washing water is discharged to an outside of a system. [0013] Previously, this dilution rate has been set to be, for example, around 1000 times, but in the present invention, a control device 8 is placed for controlling a chemical solution-injecting pump 7 that adjusts a dilution rate. In addition, apart from this, a device 9 for monitoring an ammonic nitrogen concentration in membrane-treated water is placed, and 10 the control device 8 changes a dilution rate using an ammonic nitrogen concentration measured by the monitoring device 9. [0014] Currently, a device can directly automatically measure a concentration of a sodium hypochlorite solution used in backwashing has not been developed yet, however, a device for automatically measuring an ammonic nitrogen concentration water has already been developed and sold, for example, as a trade name of automatic ammonium ion monitors (Model AMNA-101/102) from HORIBA, Ltd. Then, an ammonic nitrogen concentration in membrane-treated water is monitored and, when the concentration is high, since free chlorine in sodium hypochlorite easily reacts with ammonic nitrogen to produce chloramines, the control device 8 is set to reduce a dilution rate to increase a concentration of sodium hypochlorite in backwashing water as shown in Fig.2. Conversely, when an ammonic nitrogen concentration in membrane-treated water is low, by increasing a dilution rate, a concentration of sodium hypochlorite in backwashing water is reduced and consumption of sodium hypochlorite is suppressed in order to reduce the running cost. [0015] As a result, even when an amount of free chlorine in sodium hypochlorite to be used to produce chloramines varies in accordance with variations in water quality of membrane-treated water, it becomes possible to maintain a free chlorine 11 concentrations in backwashing water approximately constant, and the constant chemical washing effects can usually be obtained. Thereby, it becomes possible to prevent a rise in a transmembrane pressure difference to stably operate the separation membrane 3. Generally, an amount of backwashing water to be used in chemical washing is around 1 to 2 times a volume of a casing accommodating the separation membrane and, a chemical solution injecting pump 7 is stopped after the chemical washing, and normal backwashing is performed to prevent a chemical solution from remaining inside the separation membrane 3. [0016] The present invention can be widely applied to a membrane separation system targeting sidestreams, industrial waste water, garbage-leaching water, night soil, agricultural waste water, livestock waste water, aquafarming sewage water, and seawater in addition to the aforementioned water and waste water. Examples of the specific application system including sewage water tertiary treatment, membrane separation activated sludge treatment(final sedimentation tank effluent treatment), membrane bioreactor treatment, and pre-treatment for seawater desalination RO membrane. [0017) It is further preferable that a dilution rate is controlled bymeasured values of COD (chemical oxygen demand) and TOC (total organic carbon concentration) that can be automatically measured in addition to an ammonia nitrogen concentration in 12 membrane-treated water, This is because COD and TOC consume free chlorine. In this case, the control device 8 may be operated so that, when a COD or TOC value is high, a dilution rate is further decreased and, when a COD or TOC value is low, a dilution rate is further increased. A graph is shown in Fig. 3 that shows the case of the dilution rate in which TOC concentration is changed from 5mg/L to 40mg/L. As shown in the graph, a change of the dilution rate due to the ammonia nitrogen concentration is large when TOC value is low. However, the effect of the TOC becomes dominant as the amount increases, and therefore changes in the dilution rate due to the ammonia nitrogen concentration becomes smaller. Moreover, COD has indicators, such as potasiumpermanganate (CODMn) and potassium bichromates (CODCr), and either one can be used as a COD indicator. [Examples] [0018] Water in a biological treatment tank for performing activated sludge treatment of sewage water was circulated into a ceramic monolith membrane placed outside the tank, to perform a cross flow filtration. The membrane-separated water was stored in a treated water tank, backwashing was performed with a backwashing pump every 40 minutes, and chemical washing was performed every 120 minutes. A chemical washing was performed for 2 minutes using a washing solution obtained by diluting by injecting a sodium hypochlorite solution having a concentration 13 of 130000 mg-Cl/L stored in a chemical solution tank into backwashing water with a chemical solution injecting pump. Upon this chemical washing, the washing solution was flown from a filtrate side to a raw water side of the separation membrane. And, at a stage at which the separation membrane was immersed in the washing solution, the state thereof was retained for 2 minutes. [0019] Since a dilution rate was previously fixed at 1000, the chemical washing effect varied greatly and, for example, a rising rate of a trasmembrane pressure difference was increased 2 times more than that of at an ammonic nitrogen concentration of 0mg-N/L in some cases. To the contrary, according to the present invention, an ammonic nitrogen concentration in membrane-treated water was monitored and, when the concentration was increased to 2mg-N/L, a dilution rate was controlled to be 860 times and, when the concentration was reduced to Omg-N/L, a dilution rate was controlled to be 1000 times, thereupon, the chemical washing effect was stabilized and, even when an ammonic nitrogen concentration in membrane-treated water varied between Omg-N/L and 2mg-N/L, a rising rate of a trasmembrane pressure difference became equivalent as compared with the case where an ammonic nitrogen concentration was usually Omg-N/L. A trasmembrane pressure difference is a difference in a pressure difference between a raw water side and a filtrate side of a membrane, and a rising 14 rate of a trasmembrane pressure difference is a rate of increase of a membrane differential pressure. Therefore, a lower rising rate of a trasmembrane pressure difference means that stable membrane filtration can be performed for a long period of time. [0020] In addition, since an organic substance varied greatly in raw water used, previously, when a dilution rate was fixed at 1000 times, the chemical washing effect varied greatly and, for example, a rising rate of a trasmembrane pressure difference greatly differed in some cases between the case where a TOC concentration was 6mg/L, and the case where a TOC concentration was 8mg/L. To the contrary, according to the present invention, an ammonic nitrogen concentration and a TOC concentration in membrane-treated water were monitored, a dilution rate was changed as described above in response to variations in an ammonic nitrogen concentration and, when a TOC concentration was increased to 8mg/L, a dilution rate was controlled to be 830 times and, when the concentration was reduced to 6mg/L, a dilution rate was controlled to be 1000 times, thereby, the chemical washing effect was stabilized. The aforementioned dilution rate is one example of dilution rates used when an ammonic nitrogen concentration was constant as Omg-N/L. As a result, even when a TOC concentration varied between 6mg/L and 8mg/L, a rising rate of a trasmembrane pressure difference became equivalent as compared with the case where a TOC concentration was usually 6mg/L. 15 [0021] According to the present invention, even when the operation condition, such as a biological treatment tank and an coagulation mixing tank placed at pretreatment stage of a separation membrane varies, it becomes possible to operate membrane filtration stably and, moreover, the running cost can be reduced by making an amount of sodium hypochlorite to be used reasonable. 16

Claims

1. A washing method of a separation membrane using washing water obtained by diluting a sodium hypochlorite solution with membrane-treated water, comprising monitoring an ammonic nitrogen concentration in membrane-treated water, and adjusting a dilution rate of a sodium hypochlorite solution, such that when an ammonic nitrogen concentration in membrane-treated water is high, a dilution rate is reduced to increase a sodium hypochlorite concentration in washing water and, conversely, when an ammonic nitrogen concentration in membrane-treated water is low, a dilution rate is increased to reduce a sodium hypochlorite concentration in washing water.

2. The washing method according to claim 1 wherein a total organic carbon concentration together with an ammonic nitrogen concentration in membrane-treated water are measured, and a dilution rate of a sodium hypochlorite solution is adjusted using the total organic carbon concentration together with the ammonic nitrogen concentration.

3. The washing method according to claim 1 or claim 2 wherein the adjustment of the dilution rate of the sodium hypochlorite solution is performed by controlling an amount of sodium hypochlorite to be injected with a chemical solution dosing pump.

4. The washing method according to any one of the preceding claims wherein a free chlorine concentration in backwashing 17 water is retained constant by adjusting the dilution rate.

5. The washing method according to any one of the preceding claims wherein the diluted sodium hypochlorite concentration is in a range of 20 to 1000mg/L.

6. A washing method for a separation membrane wherein after washing the separation membrane with the washing water obtained by diluting the sodium hypochlorite solution with membrane-treated water according to claim 1, the method further comprises stopping a chemical solution dosing pump, and performing normal backwashing with the membrane-treated water.

7. A washing apparatus of a separation membrane comprising a separation membrane, a backwashing pump for driving membrane-treated water on a filtrate side of the separation membrane, a chemical solution tank for storing a sodium hypochlorite solution, a chemical solution dosing pump for injecting a sodium hypochlorite solution in the chemical solution tank into backwashing water to dilute the sodium hypochlorite solution, a device for monitoring an ammonic nitrogen concentration in membrane-treated water, and a control device for controlling the chemical solution dosing pump using an ammonic nitrogen concentration measured with the monitoring device to adjust a dilution rate, wherein the control device has the function of adjusting the dilution rate so that when the ammonic nitrogen concentration is high, the dilution rate is decreased, and, conversely, when the ammonic nitrogen concentration in membrane-treated water is low, the dilution 18 rate is increased.

8. The washing apparatus of a separation membrane according to claim 7, wherein the separation membrane is arranged after the stage of a biological treatment tank or a coagulation mixing tank.

9. The washing apparatus of a separation membrane according to claim 7 or claim 8, wherein the separation membrane is a monolith-type MF membrane or UF membrane made of a ceramic.

10. A washing method substantially as hereinbefore described with particular reference to the accompanying drawings.

11. A washing apparatus substantially as hereinbefore described with particular reference to the accompanying drawings . 19