【０００１】
【発明の属する技術分野】
本発明は、水処理に用いられる濾過膜モジュールの洗浄方法に関するものであり、さらに詳しくは二酸化塩素を利用してファウリング物質により低下した膜濾過モジュールの濾過性能を回復させるための膜濾過モジュールの洗浄方法に関する。
【０００２】
【従来の技術】
近年、濾過膜を用いた分離技術の開発が進み、浄水処理及び下水処理を含む水処理の分野への膜処理の採用が拡がっている。膜濾過の過程では、濾過の継続に伴って被濾過水中の懸濁物質等のファウリング物質が膜表面に付着、または膜孔に侵入し、透過流束の低下が生じる。このため、透過流束を回復するために濾過膜の洗浄が必要となる。洗浄方法として、気泡によるスクラビング洗浄、逆流水洗浄、逆流気体洗浄、オゾンガスの曝気による洗浄、スポンジボールなどの洗浄用小体を使用した洗浄、薬品を使用する薬液洗浄など多くの洗浄方法が行われている。洗浄に用いる薬品についても、次亜塩素酸ナトリウム水溶液などの酸化剤、酸水溶液、アルカリ水溶液、食塩水、界面活性剤など種々の薬品が使用されている。また、必要に応じてこれらの洗浄方法を複数組み合わせて洗浄を行っている。
【０００３】
通常、中空糸膜モジュールの洗浄においては、原水を一次側に入れて空気で曝気する物理洗浄や濾過水を用いた逆流洗浄を高い頻度で行い、それでも閉塞が進んだ場合に、薬品を使用した薬品洗浄を行っている。薬品洗浄は主に、次亜塩素酸ナトリウム、オゾン、二酸化塩素などの酸化剤による有機物の酸化と、酸溶液による鉄やマンガンなどの金属のファウリング物質の洗浄を組み合わせている場合が多い。
【０００４】
二酸化塩素溶液を使用する膜の洗浄方法は、特開平５−１６８８７３号公報および特開２００１−７９３６６号公報に記載がある。特開平５−１６８８７３公報には、二酸化塩素を混合した濾過処理水を逆圧洗浄時に濾過膜に接触させて洗浄する方法が記載されている。特開２００１−７９３６６号公報には、濾過側から加圧した次亜塩素酸ナトリウム、二酸化塩素、過酸化水素の少なくとも１以上を含む液体または気体で逆流洗浄する膜の洗浄方法が記載されている。さらに、逆流洗浄に併用して原水側に気泡を導入して洗浄すると洗浄効果が大きくなることが示されている。
【０００５】
【特許文献】
特開平５−１６８８７３号公報
特開２００１−７９３６６号公報
【０００６】
【発明が解決しようとする課題】
従来、膜の薬品洗浄においては、次亜塩素酸ナトリウムのような酸化剤を含む液で膜の閉塞の原因となる有機物を酸化して剥がれやすくするとともに、硫酸、塩酸、硝酸、クエン酸、シュウ酸などの酸を含む薬液により膜の閉塞の原因となる鉄やマンガン化合物を溶解させて除去していた。この場合は酸化剤を含む薬液と酸を含む薬液の２種類を使用する必要があった。
【０００７】
膜洗浄用酸化剤として二酸化塩素を単独で使用する場合、二酸化塩素の酸化力により、膜に付着した物質表面の有機物が酸化分解あるいは変性され、膜表面から剥離し易くなる。通常、二酸化塩素は酸を使用して製造するため、二酸化塩素溶液は酸を含んでおり、酸溶液としての洗浄効果を持つが、酸化剤である二酸化塩素が含まれているために、ファウリング物質である鉄やマンガンの酸化物を溶解する効果は少なかった。
【０００８】
【課題を解決するための手段】
本発明者は、二酸化塩素による膜の洗浄方法について検討した結果、二酸化塩素溶液から二酸化塩素を除けば、酸溶液となり酸による金属系のファウリング物質の溶解に利用でき、一種類の薬液で酸化剤による洗浄効果と酸による洗浄効果が得られると考え、以下の発明を完成するに至った。
【０００９】
第１の観点では、膜モジュールの洗浄方法において、二酸化塩素を含む薬液に膜を浸漬することによって膜を洗浄して膜に付着した有機物を酸化し、その後、曝気により薬液の二酸化塩素を気散させ、二酸化塩素が気散した後に残る酸溶液でさらに膜を洗浄する方法を提供する。この方法によれば、一種類の薬液で酸化剤による洗浄と酸による洗浄が可能となり、薬液タンクを減らすことができ、洗浄が容易で廃液量が少なくなる。
【００１０】
第２の観点では、膜モジュールの洗浄方法において、二酸化塩素を含む薬液に膜を浸漬することによって膜を洗浄して膜に付着した有機物を酸化し、その後、薬液に還元剤を二酸化塩素の分解に必要な量よりも過剰に添加することにより二酸化塩素を分解し、残った酸と還元剤の効果により鉄やマンガンなどの金属酸化物を原因とするファウリング物質を溶解し、膜の濾過性を回復する方法を提供する。この方法によれば、一種類の薬液で酸化剤による洗浄と酸による洗浄が可能となり、薬液タンクを減らすことができ、廃液量が少なくなり、さらに還元剤の作用で金属酸化物の洗浄が容易となる。
【００１１】
第３の観点では、外圧式中空糸膜濾過モジュールの洗浄方法において、膜モジュールの一次側（原水側）に二酸化塩素を含む薬液を注入して循環させ膜に付着した有機物を酸化し、その後、曝気により膜を揺動させ洗浄効果を高めるとともに、薬液の二酸化塩素を気散させ、二酸化塩素が気散した後に残る酸溶液でさらに膜を循環洗浄する方法を提供する。この方法によれば、一種類の薬液で酸化剤による洗浄と酸による洗浄が可能となり、薬液タンクを減らすことができ、洗浄が容易で廃液量が少なくなる。
【００１２】
第４の観点では、外圧式中空糸膜濾過モジュールの洗浄方法において、膜モジュールの一次側に二酸化塩素を含む薬液を注入して循環させ膜に付着した有機物を酸化し、その後、薬液に還元剤を二酸化塩素の分解に必要な量よりも過剰に添加することにより二酸化塩素を分解し、後に残る酸と還元剤を含む薬液でさらに膜を循環洗浄することにより鉄やマンガンなどの金属酸化物を原因とするファウリング物質を溶解し、膜の濾過性を回復する方法を提供する。この方法によれば、一種類の薬液で酸化剤による洗浄と酸による洗浄が可能となり、薬液タンクを減らすことができ、廃液量が少なくなり、さらに還元剤の作用で金属酸化物の洗浄が容易となる。
【００１３】
【発明の実施の形態】
以下、図１に示す一実施形態により本発明をさらに詳細に説明する。なお、これにより本発明が限定されるものではない。
【００１４】
二酸化塩素溶液で膜を洗浄する手順を以下に示す。膜濾過時は原水タンク２の原水８を原水ポンプ３により膜の一次側（原水側）１ａに導き、濾過水９を二次側（濾過水側）１ｂから取り出す。薬品洗浄は膜濾過を中断して行う。薬品洗浄の際は、原水ポンプ３を停止し、一次側１ａの原水を排水し、薬液ポンプ５を稼動して、膜モジュール１の一次側１ａに二酸化塩素を含有する薬液を入れ、薬液ポンプ５、膜モジュール１、薬液タンク４の間を循環させて二酸化塩素による循環洗浄を行う。この洗浄において、薬液を一時側１ａに充満させ、循環しないで洗浄しても良いが洗浄効果は低下する。
【００１５】
薬品洗浄用の薬液の二酸化塩素濃度は、１０ｍｇ／Ｌ以上１０００ｍｇ／Ｌ以下の範囲が好ましい。これより濃度が低い場合は、酸化が十分に進まないため、洗浄効果が十分に得られ難い。一方、濃度がこの範囲を超えると、薬品のコストが高くなりすぎる。濃度範囲は、好ましくは１００ｍｇ／Ｌ以上５００ｍｇ／Ｌ以下である。
【００１６】
循環洗浄後に、循環を継続しながら一次側１ａに圧力空気をコンプレッサ６により導入しエアスクラビング洗浄を行い膜面を振動させることにより膜を洗浄するとともに二酸化塩素を気散させる。圧力空気の供給量は適宜決めればよい。二酸化塩素の曝気による気散は膜モジュール以外の薬液タンクなどで行ってもよいが、膜を振動させて洗浄効果を上げるために膜モジュール内で行うことが望ましい。
二酸化塩素が気散した後に、残った酸溶液を薬液ポンプ５により膜モジュール１内を循環させ酸洗浄を行う。酸洗浄の後、酸溶液を一次側１ａから排出する。酸洗浄の間は圧力空気による曝気を継続しても停止しても良い。
【００１７】
二酸化塩素を発生させる方法としては、（１）亜塩素酸ナトリウム溶液と塩酸を混合する方法、（２）亜塩素酸ナトリウム溶液、次亜塩素酸ナトリウムおよび塩酸を混合する方法、（３）塩素酸ナトリウム溶液、硫酸および過酸化水素水を混合する方法、（４）塩素酸ナトリウム溶液を電解する方法、の４種類の方法が一般的に使用されている。また、二酸化塩素は不安定なガス態であり、発生後直ちに水に吸収させ、溶液として使用するのが望ましいとされている。前述の４種類の発生方法のうち（１）、（３）の方法で発生させた二酸化塩素溶液のｐＨは１〜２程度、（２）、（４）の方法で発生させた二酸化塩素溶液のｐＨは４〜５程度であった。よって、本発明で使用する二酸化塩素は（１）または（３）の方法で発生させた二酸化塩素を用いるのが好ましい。
【００１８】
二酸化塩素溶液の二酸化塩素を除去するためには、二酸化塩素濃度に対して過剰な還元剤を添加してもよい。アスコルビン酸や亜硫酸ナトリウムなどの還元剤を使用することで、二酸化塩素を気散させるまでに必要な時間を短縮することができる。還元剤の添加は、曝気により二酸化塩素をある程度気散させた後に添加すれば使用する還元剤の量が少なくなる。還元剤を含む残った酸溶液で膜を洗浄することで、鉄、マンガンなどの金属酸化物のファウリング物質を酸溶液のみの場合よりも効果的に溶解除去できる。
【００１９】
二酸化塩素溶液への還元剤の添加方法は、固体や液体の状態で薬液タンクなどに直接投入して撹拌しても良いし、あるいは水溶液として、薬液タンクから多孔膜に至る配管の途中でエジェクタやラインミキサを用いて添加しても良い。
【００２０】
【実施例１】図１を用いて実施例１を示す。原水８には富栄養化した湖水を使用した。通常の濾過運転時には、原水タンク２から原水８が原水ポンプ３により膜に導入され、一部は膜を通過して濾過水９となり、残りは原水タンク２に戻り、循環を繰り返す。使用した膜は外圧式中空糸膜でポリ弗化ビニリデン製のＭＦ膜であり、濾過膜モジュールは内径７６ｍｍ、長さ１ｍのものである。
濾過は、膜濾過流束３．０ｍ３ ／ｍ２ ／日、濾過水量２０ｍ３／日の定量濾過とし、膜濾過水量と循環水量の比をおおよそ１対１としたクロスフロー方式で行った。運転条件は、濾過を３０分間行った後、濾過水９による逆流洗浄と１モジュールあたり空気量２ｍ３／ｈの空気による一次側１ａのエアスクラビング洗浄を２０秒間行うという操作を繰り返した。上記運転条件で１ヶ月間運転した後の膜間差圧は、１７０ｋＰａであった。
【００２１】
膜の閉塞が進み、膜間差圧が約２００ｋＰａに上昇した段階で、濾過を停止し本発明による洗浄を行った。洗浄には、二酸化塩素を亜塩素酸ナトリウムと塩酸を混合して発生させ、その後直ちに水に溶解した二酸化塩素溶液を使用した。はじめに一次側１ａの原水８を排水した後、薬液タンク４から二酸化塩素濃度５００ｍｇ／Ｌの二酸化塩素溶液を膜モジュール１の一次側１ａに充満させ、一次側１ａと薬液タンク４を９０分間循環させた。次にコンプレッサ６により加圧空気を一次側１ａに導入し、二酸化塩素を気散するために空気量２ｍ３／ｈで曝気し、あらかじめ測定しておいた二酸化塩素の気散に必要な時間である４０分経過後からさらに曝気を２０分間行い、その後、二酸化塩素気散後に残った酸溶液による循環洗浄を６０分間行った。実施例１による洗浄後の濾過流束の回復を図２に示す。洗浄後の膜間差圧は７８ｋＰａになった。
【００２２】
【実施例２】図１を用いて実施例２を示す。実施例１と同様の条件で膜濾過を行い、膜の閉塞が進み、膜間差圧が２００ｋＰａに上昇した段階で、濾過を停止し本発明による洗浄を行った。はじめに一次側１ａの原水８を排水した後、薬液タンクから二酸化塩素濃度５００ｍｇ／Ｌの二酸化塩素溶液を膜モジュール１の一次側１ａに充満させ、一次側１ａと薬液タンク４を９０分間循環させた。次にコンプレッサ６により加圧空気を一次側１ａに導入し、二酸化塩素を気散するために空気量２ｍ３／ｈで３０分間曝気した後、還元剤を薬液タンクに二酸化塩素を分解するのに必要な量よりも過剰に添加して、還元剤を含む酸溶液による循環洗浄を６０分間行った。今回使用した還元剤はアスコルビン酸で０．５重量％になるように添加した。実施例２による洗浄後の濾過流束の回復を図２に示す。洗浄後の膜間差圧は６１ｋＰａになった。
【００２３】
【比較例１】図１を用いて比較例１を示す。実施例１と同様の条件で膜濾過を行い、膜の閉塞が進み、膜間差圧が２００ｋＰａに上昇した段階で、濾過を停止し薬液洗浄を行った。比較例１では、二酸化塩素で洗浄した後曝気は行わなかった。はじめに一次側１ａの原水を排水した後、薬液タンク４から二酸化塩素濃度５００ｍｇ／Ｌの二酸化塩素溶液を膜モジュール１の一次側１ａに充満させ、一次側１ａと薬液タンク４を２１０分間循環させ、その後排水した。比較例１による洗浄後の濾過流束の回復を図２に示す。洗浄後の膜間差圧は１０９ｋＰａになり、実施例１および実施例２の洗浄後の膜間差圧にくらべかなり高い値となった。
【００２４】
【発明の効果】
本発明によれば、一種類の薬液で酸化による洗浄と酸による洗浄を実施できるため、薬液タンクを減らすことができ、洗浄が容易で廃液量が少なくなる。
【図面の簡単な説明】
【図１】本発明を実施する一般的な膜処理装置を示す構成図である。
【図２】実施例および比較例の洗浄前後における膜間差圧を示す表である。
【符号の説明】
１ 膜濾過モジュール
１ａ 原水側（一次側）
１ｂ 濾過水側（二次側）
２ 原水タンク
３ 原水ポンプ
４ 薬液タンク
５ 薬液ポンプ
６ コンプレッサ
７ エア抜き
８ 原水
９ 濾過水
１０ 排水[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for cleaning a filtration membrane module used in water treatment, and more particularly, to a method for recovering filtration performance of a membrane filtration module reduced by a fouling substance using chlorine dioxide. It relates to a cleaning method.
[0002]
[Prior art]
In recent years, the development of separation technology using a filtration membrane has progressed, and the use of membrane treatment in the field of water treatment including water purification treatment and sewage treatment has been expanding. In the process of membrane filtration, a fouling substance such as a suspended substance in the water to be filtered adheres to the membrane surface or penetrates into the pores of the membrane as the filtration is continued, and the permeation flux is reduced. Therefore, it is necessary to wash the filtration membrane in order to recover the permeation flux. Many cleaning methods are used, such as scrubbing cleaning using air bubbles, backflow water cleaning, backflow gas cleaning, cleaning by aeration of ozone gas, cleaning using cleaning bodies such as sponge balls, and chemical cleaning using chemicals. ing. Various chemicals such as an oxidizing agent such as an aqueous sodium hypochlorite solution, an acid aqueous solution, an alkaline aqueous solution, a saline solution, and a surfactant are also used for the cleaning. Cleaning is performed by combining a plurality of these cleaning methods as needed.
[0003]
Normally, in the cleaning of the hollow fiber membrane module, physical cleaning in which raw water is put into the primary side and aeration with air and backwashing using filtered water are performed at a high frequency, and when the clogging still proceeds, a chemical is used. Chemical cleaning is performed. Chemical cleaning often mainly combines the oxidation of organic substances with an oxidizing agent such as sodium hypochlorite, ozone, and chlorine dioxide, and the cleaning of metal fouling substances such as iron and manganese with an acid solution.
[0004]
Methods for cleaning a film using a chlorine dioxide solution are described in JP-A-5-168873 and JP-A-2001-79366. Japanese Patent Application Laid-Open No. 168873/1993 discloses a method in which filtration treatment water mixed with chlorine dioxide is brought into contact with a filtration membrane at the time of back-pressure washing for washing. Japanese Patent Application Laid-Open No. 2001-79366 describes a method for cleaning a membrane which is backwashed with a liquid or gas containing at least one of sodium hypochlorite, chlorine dioxide and hydrogen peroxide pressurized from the filtration side. . Further, it is shown that the cleaning effect is enhanced when air bubbles are introduced into the raw water side for cleaning together with the backwashing.
[0005]
[Patent Document]
JP-A-5-168873 and JP-A-2001-79366.
[Problems to be solved by the invention]
Conventionally, in chemical cleaning of a membrane, a liquid containing an oxidizing agent such as sodium hypochlorite oxidizes an organic substance that causes blockage of the membrane to make it easily peeled off, and also removes sulfuric acid, hydrochloric acid, nitric acid, citric acid, and sulfur. Iron or manganese compounds that cause blockage of the film are dissolved and removed by a chemical solution containing an acid such as an acid. In this case, it was necessary to use two types of chemical solutions, one containing an oxidizing agent and the other containing an acid.
[0007]
When chlorine dioxide is used alone as an oxidizing agent for cleaning a film, the oxidizing power of chlorine dioxide oxidizes or decomposes organic substances on the surface of a substance adhering to the film and makes it easy to peel off from the film surface. Normally, chlorine dioxide is produced using acid, so chlorine dioxide solution contains acid and has a cleaning effect as an acid solution, but fouling because chlorine dioxide, which is an oxidizing agent, is contained. The effect of dissolving the oxides of iron and manganese as substances was small.
[0008]
[Means for Solving the Problems]
The present inventor has studied a method of cleaning a film with chlorine dioxide.As a result of removing chlorine dioxide from the chlorine dioxide solution, the solution becomes an acid solution, which can be used for dissolving metal-based fouling substances by the acid, and oxidized with one kind of chemical solution. It is believed that the cleaning effect by the agent and the cleaning effect by the acid can be obtained, and the following invention has been completed.
[0009]
According to a first aspect, in a method for cleaning a membrane module, the film is washed by immersing the film in a chemical solution containing chlorine dioxide to oxidize organic substances attached to the film, and then the chlorine dioxide in the chemical solution is diffused by aeration. And a method for further cleaning the membrane with an acid solution remaining after chlorine dioxide has been diffused. According to this method, cleaning with an oxidizing agent and cleaning with an acid can be performed with one type of chemical solution, the number of chemical solution tanks can be reduced, cleaning is easy, and the amount of waste liquid is reduced.
[0010]
According to a second aspect, in the method for cleaning a membrane module, the film is washed by immersing the film in a chemical solution containing chlorine dioxide to oxidize organic substances attached to the film, and then a reducing agent is added to the chemical solution to decompose the chlorine dioxide. Chlorine dioxide is decomposed by adding it in excess of the amount needed, and fouling substances caused by metal oxides such as iron and manganese are dissolved by the effect of the remaining acid and reducing agent. Provide a way to recover. According to this method, cleaning with an oxidizing agent and cleaning with an acid can be performed with one type of chemical solution, the number of chemical solution tanks can be reduced, the amount of waste liquid can be reduced, and metal oxides can be easily cleaned by the action of a reducing agent. It becomes.
[0011]
According to a third aspect, in a method for cleaning an external pressure type hollow fiber membrane filtration module, a chemical solution containing chlorine dioxide is injected into the primary side (raw water side) of the membrane module and circulated to oxidize organic substances attached to the membrane, and thereafter, Provided is a method of oscillating a film by aeration to enhance a cleaning effect, diffusing chlorine dioxide of a chemical solution, and further circulating and cleaning the film with an acid solution remaining after the chlorine dioxide is diffused. According to this method, cleaning with an oxidizing agent and cleaning with an acid can be performed with one type of chemical solution, the number of chemical solution tanks can be reduced, cleaning is easy, and the amount of waste liquid is reduced.
[0012]
According to a fourth aspect, in the method for cleaning an external pressure type hollow fiber membrane filtration module, a chemical solution containing chlorine dioxide is injected into the primary side of the membrane module and circulated to oxidize organic substances attached to the membrane. Is added in excess of the amount required for the decomposition of chlorine dioxide to decompose chlorine dioxide, and the remaining membrane is circulated and washed with a chemical solution containing the remaining acid and reducing agent to remove metal oxides such as iron and manganese. Disclosed is a method for dissolving a fouling substance causing a problem and restoring the filterability of a membrane. According to this method, cleaning with an oxidizing agent and cleaning with an acid can be performed with one type of chemical solution, the number of chemical solution tanks can be reduced, the amount of waste liquid can be reduced, and metal oxides can be easily cleaned by the action of a reducing agent. It becomes.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to an embodiment shown in FIG. Note that the present invention is not limited by this.
[0014]
The procedure for cleaning the membrane with a chlorine dioxide solution is described below. At the time of membrane filtration, the raw water 8 in the raw water tank 2 is guided to the primary side (raw water side) 1a of the membrane by the raw water pump 3, and the filtered water 9 is taken out from the secondary side (filtrated water side) 1b. Chemical cleaning is performed by interrupting membrane filtration. At the time of chemical cleaning, the raw water pump 3 is stopped, the raw water on the primary side 1a is drained, the chemical liquid pump 5 is operated, and a chemical liquid containing chlorine dioxide is put into the primary side 1a of the membrane module 1. Circulating between the membrane module 1 and the chemical solution tank 4 to perform circulating cleaning with chlorine dioxide. In this cleaning, the chemical solution may be filled in the temporary side 1a and cleaned without circulation, but the cleaning effect is reduced.
[0015]
The chlorine dioxide concentration of the chemical solution for chemical cleaning is preferably in the range of 10 mg / L to 1000 mg / L. If the concentration is lower than this, oxidation does not proceed sufficiently, so that it is difficult to obtain a sufficient cleaning effect. On the other hand, if the concentration exceeds this range, the cost of the chemical becomes too high. The concentration range is preferably from 100 mg / L to 500 mg / L.
[0016]
After the circulation cleaning, pressurized air is introduced into the primary side 1a by the compressor 6 while the circulation is continued, and air scrubbing cleaning is performed to vibrate the film surface to clean the film and diffuse chlorine dioxide. The supply amount of the compressed air may be determined as appropriate. Air diffusion by aeration of chlorine dioxide may be performed in a chemical solution tank or the like other than the membrane module, but is preferably performed in the membrane module in order to vibrate the membrane and increase the cleaning effect.
After the chlorine dioxide is diffused, the remaining acid solution is circulated in the membrane module 1 by the chemical pump 5 to perform acid cleaning. After the acid washing, the acid solution is discharged from the primary side 1a. During the acid cleaning, the aeration with the pressurized air may be continued or stopped.
[0017]
As methods for generating chlorine dioxide, (1) a method of mixing a sodium chlorite solution and hydrochloric acid, (2) a method of mixing a sodium chlorite solution, sodium hypochlorite and hydrochloric acid, (3) a chloric acid Four methods are generally used: a method of mixing a sodium solution, sulfuric acid and aqueous hydrogen peroxide, and a method of (4) electrolyzing a sodium chlorate solution. Further, chlorine dioxide is in an unstable gas state, and it is considered that it is desirable to absorb chlorine dioxide into water immediately after generation and use it as a solution. Of the above four generation methods, the pH of the chlorine dioxide solution generated by the methods (1) and (3) is about 1-2, and the pH of the chlorine dioxide solution generated by the methods (2) and (4) is pH was about 4-5. Therefore, as the chlorine dioxide used in the present invention, it is preferable to use chlorine dioxide generated by the method (1) or (3).
[0018]
In order to remove chlorine dioxide from the chlorine dioxide solution, an excess reducing agent may be added to the chlorine dioxide concentration. By using a reducing agent such as ascorbic acid or sodium sulfite, the time required for diffusing chlorine dioxide can be reduced. If the reducing agent is added after chlorine dioxide is diffused to some extent by aeration, the amount of the reducing agent used is reduced. By cleaning the film with the remaining acid solution containing the reducing agent, fouling substances of metal oxides such as iron and manganese can be dissolved and removed more effectively than in the case of using only the acid solution.
[0019]
The method of adding the reducing agent to the chlorine dioxide solution may be a solid or liquid state, which may be directly charged into a chemical tank or the like and stirred, or as an aqueous solution, an ejector or the like may be provided in the middle of a pipe from the chemical tank to the porous membrane. You may add using a line mixer.
[0020]
Embodiment 1 Embodiment 1 will be described with reference to FIG. Eutrophic lake water was used for the raw water 8. During a normal filtration operation, raw water 8 from the raw water tank 2 is introduced into the membrane by the raw water pump 3, a part of which passes through the membrane to become filtered water 9, and the rest returns to the raw water tank 2 and repeats circulation. The membrane used was an external pressure type hollow fiber membrane, an MF membrane made of polyvinylidene fluoride, and the filtration membrane module had an inner diameter of 76 mm and a length of 1 m.
Filtration was performed by a quantitative flow of 3.0 m 3 / m 2 / day with a membrane filtration flux of 20 m 3 / day and a cross-flow method in which the ratio of the amount of membrane filtration water to the amount of circulating water was approximately 1: 1. The operation conditions were as follows: the operation of performing filtration for 30 minutes, then performing backwashing with filtered water 9 and air scrubbing cleaning of the primary side 1a with air having an air amount of 2 m 3 / h per module for 20 seconds was repeated. The transmembrane pressure after operating for one month under the above operating conditions was 170 kPa.
[0021]
At the stage where the closure of the membrane progressed and the transmembrane pressure rose to about 200 kPa, the filtration was stopped and the washing according to the present invention was performed. For cleaning, chlorine dioxide was generated by mixing sodium chlorite and hydrochloric acid, and immediately thereafter, a chlorine dioxide solution dissolved in water was used. First, after the raw water 8 on the primary side 1a is drained, a chlorine dioxide solution having a chlorine dioxide concentration of 500 mg / L is filled in the primary side 1a of the membrane module 1 from the chemical liquid tank 4, and the primary side 1a and the chemical liquid tank 4 are circulated for 90 minutes. Was. Next, pressurized air is introduced into the primary side 1a by the compressor 6 and aerated at an air volume of 2 m 3 / h in order to diffuse chlorine dioxide. After a certain 40 minutes, aeration was further performed for 20 minutes, and then circulating cleaning was performed for 60 minutes with the acid solution remaining after the chlorine dioxide gas was diffused. The recovery of the filtration flux after washing according to Example 1 is shown in FIG. The transmembrane pressure difference after washing was 78 kPa.
[0022]
Embodiment 2 Embodiment 2 will be described with reference to FIG. Membrane filtration was performed under the same conditions as in Example 1. At the stage where the closure of the membrane progressed and the transmembrane pressure increased to 200 kPa, the filtration was stopped and the washing according to the present invention was performed. First, after draining the raw water 8 on the primary side 1a, a chlorine dioxide solution having a chlorine dioxide concentration of 500 mg / L was filled in the primary side 1a of the membrane module 1 from the chemical tank, and the primary side 1a and the chemical tank 4 were circulated for 90 minutes. . Next, pressurized air is introduced into the primary side 1a by the compressor 6 and aerated at an air volume of 2 m 3 / h for 30 minutes to diffuse chlorine dioxide. It was added in excess of the required amount, and circulating washing was performed for 60 minutes with an acid solution containing a reducing agent. The reducing agent used this time was added in an amount of 0.5% by weight ascorbic acid. The recovery of the filtration flux after washing according to Example 2 is shown in FIG. The transmembrane pressure difference after washing was 61 kPa.
[0023]
Comparative Example 1 Comparative Example 1 will be described with reference to FIG. Membrane filtration was performed under the same conditions as in Example 1. At the stage where the closure of the membrane progressed and the transmembrane pressure rose to 200 kPa, the filtration was stopped and the chemical solution was washed. In Comparative Example 1, aeration was not performed after washing with chlorine dioxide. First, after draining the raw water on the primary side 1a, a chlorine dioxide solution having a chlorine dioxide concentration of 500 mg / L is filled in the primary side 1a of the membrane module 1 from the chemical liquid tank 4, and the primary side 1a and the chemical liquid tank 4 are circulated for 210 minutes. Then drained. FIG. 2 shows the recovery of the filtration flux after washing according to Comparative Example 1. The transmembrane pressure difference after washing was 109 kPa, which was considerably higher than the transmembrane pressure difference after washing in Examples 1 and 2.
[0024]
【The invention's effect】
According to the present invention, since cleaning by oxidation and cleaning by acid can be performed with one kind of chemical solution, the number of chemical solution tanks can be reduced, cleaning is easy, and the amount of waste liquid is reduced.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing a general film processing apparatus for implementing the present invention.
FIG. 2 is a table showing a transmembrane pressure difference before and after cleaning in Examples and Comparative Examples.
[Explanation of symbols]
1 Membrane filtration module 1a Raw water side (primary side)
1b Filtered water side (secondary side)
2 Raw water tank 3 Raw water pump 4 Chemical liquid tank 5 Chemical liquid pump 6 Compressor 7 Air release 8 Raw water 9 Filtrated water 10 Drainage
