JP2004073950A

JP2004073950A - Membrane washing method

Info

Publication number: JP2004073950A
Application number: JP2002235486A
Authority: JP
Inventors: Tomotaka Hashimoto; 橋本　知孝; Yoshihiko Mori; 森　吉彦
Original assignee: Asahi Kasei Chemicals Corp; Asahi Chemical Co Ltd
Current assignee: Asahi Kasei Chemicals Corp; Asahi Chemical Co Ltd
Priority date: 2002-08-13
Filing date: 2002-08-13
Publication date: 2004-03-11

Abstract

<P>PROBLEM TO BE SOLVED: To maintain a high filtration speed by washing a membrane filter effectively when raw water such as river water, lake water, ground water, stored water, sewage secondary treatment water, industrial wastewater, sewage, or the like is filtered with the membrane filter, when valuables are separated, or when filtration is done by the membrane filter for concentration. <P>SOLUTION: A method of washing the membrane filter, in which a membrane penetration pressure is applied to a backwashing medium by a pressure generating device to supply it to the filtrate side of the membrane filter, is characterized in that, in backwashing for removing substances adherent to the membrane filter by ejecting the backwashing medium which passed through the membrane to the raw water side of the membrane filter, a membrane penetration pressure is applied pulsewise to the backwashing medium. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、上水道や工業用水、その水源の河川水、湖沼水、地下水、貯水の濾過処理、下水二次処理水の濾過処理、下水、排水の濾過処理、あるいは有価物の分離または濃縮に用いられる濾過膜の洗浄方法に関するものである。
【０００２】
【従来の技術】
種々の原水の濾過に用いられる濾過膜は、濾過精度に優れること、設置スペースが少なくて済むこと、運転管理が容易であることなどの理由から、各種の濾過装置に用いられている。しかし、濾過の継続に伴い原水中の除去対象物質が膜表面に付着し、孔を閉塞するため徐々に濾過性能が低下し、ついには濾過できなくなってしまう。そこで、濾過性能を維持するために、濾過方向とは逆方向に濾水側から濾水あるいは清澄水等の逆洗媒体を噴出させて膜の濾過面の付着物を除去する逆流洗浄（逆洗）が一般的に行われている。
【０００３】
また逆洗効果を高めるため、逆洗媒体に酸化作用のある次亜塩素酸ソーダを添加したり、特開平４−３１０２２０号公報に示されているように、オゾン水を用いて逆洗する方法や特開昭６０−５８２２２号公報に開示されているオゾン化加圧空気で逆洗する方法が知られている。さらには空気を濾過膜の原水側に気泡として導入する方法や、特開昭６３−４２７０３号公報に開示されているように、オゾン化空気を濾過膜の原水側に気泡として注入する方法が知られている。
【０００４】
【発明が解決しようとする課題】
前述した次亜塩素酸ソーダを添加した逆洗水やオゾン水などの酸化剤を用いた逆洗方法、空気やオゾン化空気を濾過膜の原水側に気泡として導入する方法等は逆洗効果を高める上で有効であるが、原水の濁質等の条件によっては必ずしも十分安定な濾過流束が得られない。
膜表面の付着物を除去し高い濾過流束を維持するためには、逆洗時の圧力を高くしたり、逆洗時間を長くすることが有効である。しかし、これらは濾水あるいは清澄水を多く使用することになり、得られる濾水量の低下や高価な清澄水を使用することから、逆洗に要するコストを増加させることになる。また逆洗圧力を高くすると濾過膜やモジュール、配管に負荷を掛け、濾過寿命が短くなるだけでなくこれらの破裂、漏液をもたらす恐れがある。
【０００５】
【課題を解決するための手段】
本発明者らは、濾過膜の洗浄方法について鋭意検討した結果、以下の発明を完成するに至った。すなわち本発明は、
（１）圧力発生装置により逆洗媒体に膜透過圧力を与えて濾過膜の濾液側に供給し、濾過膜の原水側に膜透過した逆洗媒体を噴出させて濾過膜の付着物を除去する濾過膜の逆洗方法において、逆洗媒体に与える膜透過圧力をパルス状に与えて逆洗を行うことを特徴とする膜洗浄方法、（２）逆洗媒体が１種類以上の酸化剤を含む液体または気体であることを特徴とする（１）記載の膜洗浄方法、（３）濾過膜の原水側に気泡を導入して濾過膜を揺動させつつ行なうことを特徴とする（１）記載の膜洗浄方法である。
【０００６】
以下に本発明の詳細を述べる。
本発明の対象となる原水は、河川水、湖沼水、地下水、貯水、下水二次処理水、工場排水、あるいは下水などである。従来、上記の様な原水を膜で濾過すると、該原水中に含まれる懸濁物質や使用する膜の孔径以上の大きさの物質は膜で阻止され、いわゆる濃度分極やケーク層を形成すると同時に、膜を目詰まりさせたり、あるいは膜内部の網状組織に吸着される。その結果、原水を濾過した際の膜の濾過流束は、清澄水を濾過した際のそれに比べて数分の１から数十分の１にまで低下してしまい、また濾過の継続に従って濾過流束は徐々に低下していく。
これを防止し、膜濾過流束を維持するために、膜の濾過方向とは逆方向から濾水あるいは清澄水を噴出させて膜の濾過面の付着物を除去する逆洗が一般に用いられる。しかし、前述したように逆洗条件は濾過膜、モジュール、配管の耐久圧力、または濾水回収率の面からの制約で必ずしも十分な効果が得られていなかった。
【０００７】
本発明は濾過膜の洗浄方法として逆洗媒体に与える膜透過圧力をパルス状に与えて逆洗を行う方法である。膜透過圧力をパルス状に与えると濾過膜表面に堆積した付着物層に逆洗媒体を噴出させる事で歪み応力を繰り返し与えることができ、連続した一定の圧力を与える従来の場合に比べ効率よく付着物層を膜表面より剥離させることができる。しかも使用する逆洗媒体の量は連続的な一定の圧力を与える場合に比べ少なくて済むため濾液回収率の面でも有効である。
【０００８】
また本発明の逆洗媒体にパルス状に膜透過圧力を与えて逆洗を行う際に、逆洗媒体として次亜塩素酸ナトリウム、二酸化塩素、過酸化水素、オゾン含有水、オゾンガスなどの酸化剤を少なくとも１つ以上を含む液体或いは気体を用いる方法、あるいは濾過膜の原水側から空気、オゾンガス等の気泡を導入して膜を揺動させる方法を併用すると一層の逆洗効果を得ることができる。酸化剤を添加した逆洗媒体による逆洗の効果が高い理由は酸化剤が逆洗中のパルス状の急激な圧力変化によって付着物の内部まで入れ換わり易くなり洗浄効果が高くなるものと考えられる。また本発明の逆洗中に原水側から空気、オゾンガス等の気泡を導入して膜を揺動させると逆洗効果が高い理由としては逆洗中のパルス状の急激な圧力変化のため濾過膜モジュールの２次側（濾過側）容積や気泡径が変化することにより大きな膜の振動を得ることができるためと考える。
【０００９】
以上のように、逆洗を行う際に逆洗媒体にパルス状にの圧力を与える方法、更には逆洗媒体として酸化剤を少なくとも１種類以上を含む液体または気体を用いる方法、あるいは同時に原水側から気泡を導入して膜を揺動させる、または両方を行う方法により効果的に膜付着物を効果的に除去することができる。これにより、濾過流束を高い状態に維持でき、ランニングコストを低減することができる。
【００１０】
本発明の逆洗媒体にパルス状に膜透過圧力を与えるとは図１または図２のごとく、逆洗工程中に逆洗媒体に、高い膜透過圧力をかけた状態（Ａ）と、これより低い膜透過圧力もしくは膜透過圧力をかけない状態（Ｂ）を複数回繰り返すことを意味する。（Ａ）の最高膜透過圧力は（Ｂ）の最低膜透過圧力の２倍以上が好ましく、さらに３倍以上がより好ましい。また、（Ａ）の最高膜透過圧力は濾過膜が著しい変形や破断に至らないように、膜破裂圧力より低いことが好ましい。
【００１１】
複数回繰り返す際の隣り合う（Ａ）と（Ａ）のずれ時間は０．１秒以上、３０秒以下、同じく隣り合う（Ｂ）と（Ｂ）のずれの時間は０．１秒以上、３０秒以下が好ましい。この時間内であれば（Ａ）、（Ｂ）各々の圧力は時間と共に変化してもよい。また複数回繰り返す（Ａ）、（Ｂ）ともに毎回同じ時間、同じ圧力である必要はない。
また、逆洗媒体に圧力を与える際には図１のごとく、瞬時に昇圧して与える方が洗浄効果が高いのでより望ましいが、図２のごとく与えても構わない。
【００１２】
逆洗工程の時間は、濾過流束の回復性と濾過水の回収率を勘案して適宜決めれよい。逆洗媒体は液体、気体いずれも用いることができるが、酸化剤を広範囲に添加できる点で液体の方が好ましい。
逆洗媒体に膜透過圧力を加えるための圧力発生装置としてはポンプ、コンプレッサーあるいは加圧気体容器等があり、パルス状の圧力を逆洗媒体に与える方法としては例えばポンプ、コンプレッサー等を断続的に動かす事が挙げられる。もしくは逆洗媒体を送液するポンプあるいは加圧気体を逆洗媒体に連続的に作用させておき、原水側もしくは濾水側の配管に設けた弁を断続的に開閉することで与えることができる。
【００１３】
本発明の濾過膜は特に限定されないが、例えば、ポリエチレン、ポリプロピレン、ポリブテン等のポリオレフィン；テトラフルオロエチレン−パーフルオロアルキルビニルエーテル共重合体（ＰＦＡ）、テトラフルオロエチレン−ヘキサフルオロプロピレン共重合体（ＦＥＰ）、テトラフルオロエチレン−ヘキサフルオロプロピレン−パーフルオロアルキルビニルエーテル共重合体（ＥＰＥ）、テトラフルオロエチレン−エチレン共重合体（ＥＴＦＥ）、ポリクロロトリフルオロエチレン（ＰＣＴＦＥ）、クロロトリフルオロエチレン−エチレン共重合体（ＥＣＴＦＥ）、ポリフッ化ビニリデン（ＰＶＤＦ）等のフッ素系樹脂；ポリスルホン、ポリエーテルスルホン、ポリエーテルケトン、ポリエーテルエーテルケトン、ポリフェニレンスルフィド等のスーパーエンジニアリングプラスチック；酢酸セルロース、エチルセルロース等のセルロース類；ポリアクリロニトリル；ポリビニルアルコールの単独及びこれらの混合物が挙げられる。
【００１４】
さらにオゾン等の強力な酸化剤を併用する場合は、セラミック等の無機膜、ポリフッ化ビニリデン（ＰＶＤＦ）膜、ポリ４フッ化エチレン（ＰＴＦＥ）膜、エチレン−テトラフルオロエチレン共重合体（ＥＴＦＥ）膜、テトラフルオロエチレン−パーフルオロアルキルビニルエーテル共重合体（ＰＦＡ）膜等のフッ素系樹脂膜等の有機膜を適用することが出来る。特にポリフッ化ビニリデン（ＰＶＤＦ）膜を使用すれば好ましい。このような濾過膜のうち、その孔径領域がナノ濾過（ＮＦ）膜から精密濾過（ＭＦ）膜であるものが使用し得る。特に分画分子量が１００程度のＮＦ膜から平均孔径が１０μｍ以下のＭＦ膜が好ましい。
【００１５】
濾過膜の形状としては、中空糸状、ウェーブをつけた中空糸状、平膜状、プリーツ状、スパイラル状、チューブラー状など任意の形状を用いることができるが、単位体積当たりの膜面積が大きくとれる中空糸状がより好ましい。一般に、濾過は膜を収納したモジュールを用いて行われる。濾過方式としては、全量濾過方式でもクロスフロー濾過方式でもよい。加圧濾過方式あるいは陰圧濾過方式でもよい。また中空糸状膜の場合内圧濾過、外圧濾過のどちらでもよい。
【００１６】
逆洗媒体に含まれる酸化剤は、次亜塩素酸ナトリウム、二酸化塩素、過酸化水素、オゾンが挙げられ、少なくともこれらを１種類以上逆洗媒体に含有させて逆洗を行う。逆洗媒体に含有させる酸化剤の濃度は次亜塩素酸ソーダ、二酸化塩素、過酸化水素は　０．０５ｍｇ／リットル以上５０重量％以下の濃度が好ましく、さらには０．１ｍｇ／リットル以上２０重量％以下が好ましく、さらには１０重量％以下が好ましい。　これ以下では、有機物の溶解が十分に進まないため、洗浄効果が十分に得られず、濃度が高すぎると洗浄効果は十分に得られるが、薬品代が高くなり経済的でない。逆洗媒体へのこれらの添加方法は、濾水タンクなどに直接投入しても良いし、あるいは水溶液として、濾水タンクから濾過膜に至る配管の途中でラインミキサーを用いて添加しても良い。
【００１７】
逆洗媒体が水の場合、逆洗水に含有させるオゾン濃度は０．０５ｍｇ／リットル以上、５０ｍｇ／リットル以下が好ましい。さらに好ましくは、０．１ｍｇ／リットル以上、１０ｍｇ／リットル以下が好ましい。オゾン濃度がこの範囲内であるとオゾンによる酸化が十分に進むため、洗浄効果が高い。また、オゾン濃度を過度に高くすることは、オゾン発生に関わる費用が上がり現実的ではない。逆洗水に添加するオゾンは、オゾン単体でもオゾン化空気でも良い。逆洗水へのオゾンの導入は、逆洗タンクの適宜位置に設けた散気管等を介して行えば良い。あるいは濾水タンクから濾過膜に到る配管の途中でエジェクター及びラインミキサーを用いて添加してもよいし、あるいはＵチューブ方式を用いることもできる。
【００１８】
オゾン発生方法として放電によるオゾン発生を行う場合、原料は空気でもよく、あるいは酸素でも良い。また、水の電気分解によってオゾンを発生する方法を用いてもよい。
濾過膜の原水側に気泡を導入する方法は（ａ）常に本発明の逆洗と同時に行うと洗浄効果が高いが、（ｂ）気泡の導入（同時に逆洗）に先立ち本発明の逆洗のみを行っても良い。あるいは（ｃ）気泡の導入（同時に逆洗）を行った後本発明の逆洗のみを行っても良い。さらに、（ｄ）原水を導入しながら気泡を導入し同時に本発明の逆洗を行っても良いし、さらには、（ａ）〜（ｄ）を交互に組み合わせても良い。
【００１９】
気泡の種類としては、空気またはオゾンガスが望ましい。気泡の導入量は、単位時間当たりの濾過流量の０．５〜２０倍の体積流量を供給するのが好ましく、１〜１０倍の体積流量であることがより好ましい。本発明は、上述のごとく構成したので、長期間にわたって高い膜濾過流束を維持することができる。
【００２０】
【発明の実施の形態】
以下に、本発明の実施例を示す。
原水１として、濁度が５〜２０度、水温が１８〜２５℃の河川表流水を用いた。図３に示すように、原水１は循環タンク２を経て原水供給ポンプ３により膜モジュール４へ圧送され、得られた濾過水は逆洗タンクを兼用する濾水タンク５に貯められる。逆洗時に、濾水タンク５中の濾過水は逆洗ポンプ６により膜モジュール４へ送られ逆洗が行われるが、ここで逆洗ポンプ６から膜モジュール４へ至る配管の途中に酸化剤タンク７の酸化剤を、酸化剤送液ポンプ８により逆洗水に添加することができる。また、膜モジュール４に気泡を導入するエアーバブリングは、コンプレッサー１１で圧縮した空気を、膜モジュール４の原水側へ供給して行われる。パルス状の膜透過の圧力はタイマーより開閉する電磁弁１０を短時間に開閉を繰り返すことで逆洗媒体に与えることができる。
【００２１】
膜モジュール４は、特開平３−２１５５３５号公報に基づいて作製した内径が１．０ｍｍφ、外径が１．９ｍｍφ、平均孔径０．６μｍのＰＶＤＦ（ポリフッ化ビニリデン）製中空糸状精密濾過（ＭＦ）膜を１ｍ長、３インチ径のＰＶＣ（ポリ塩化ビニル）ケーシングに納めた外圧式モジュールである。当該モジュールの膜面積は４．７ｍ^２　、モジュール濾過圧が５０ｋＰａの時の清澄水濾過流束は毎時５．０ｍ^３である。
【００２２】
【実施例１】
濾過は膜モジュール４へ原水１を一定圧力で供給する定圧濾過とし、また、膜濾過水量と循環水量の比を５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２０分間行った後、逆洗を３０秒間の繰り返しでを行った。この逆洗の際に電磁弁１０を１秒間開き、４秒間閉じる周期を６回繰り返してパルス状の膜透過圧力を逆洗水に与えた。電磁弁が開いた際の濾過膜側の最高膜透過圧力は１１０ＫＰａ、電磁弁が閉じた状態では０ＫＰａであった。この時の濾水回収率（得られた慮水量／濾過した原水量）は９８％であった。上記運転条件で３ヶ月間運転した後の膜濾過流量は、３．１ｍ^３／ｍ^２／日であった。
【００２３】
【比較例１】
実施例１において、逆洗の際に電磁弁１０を常時開とし、一定の圧力で実施例１と同量の逆洗水を使って逆洗を行った以外は実施例１と同様に膜濾過運転を実施した。この時の濾水回収率は実施例１と同じく９８％であった。３ヶ月後の膜濾過流量は１．１ｍ^３／ｍ^２／日であった。
【００２４】
【実施例２】
実施例１において、逆洗を３０秒間の繰り返しでを行う際に電磁弁１０を２．５秒間開き、１２．５秒間閉じる周期を２回繰り返してパルス状の膜透過圧力変化を逆洗水に与えた。電磁弁が開いた際の濾過膜側の最高膜透過圧力は１２０ＫＰａ、電磁弁が閉じた状態では０ＫＰａであった。この時の濾水回収率は実施例１と同じく９８％であった。上記運転条件で３ヶ月間運転した後の膜濾過流量は、３．０ｍ^３／ｍ^２／日であった。
【００２５】
【実施例３】
実施例１において、逆洗を３０秒間の繰り返しで行う際に電磁弁１０を０．５秒間開き、２秒間閉じる周期を１２回繰り返してパルス状の膜透過圧力を逆洗水に与えた。電磁弁が開いた際の濾過膜側の最高膜透過圧力は１００ＫＰａ、電磁弁が閉じた状態では０ＫＰａであった。この時の濾水回収率は実施例１と同じく９８％であった。上記運転条件で３ヶ月間運転した後の膜濾過流量は、３．２ｍ^３／ｍ^２／日であった。
【００２６】
【実施例４】
実施例１において、濾過を６０分間行った後、逆洗を１２０秒間の繰り返しを行った。この逆洗の際に電磁弁１０を２０秒間開き、２０秒間閉じる周期を３回繰り返してパルス状の膜透過圧力を逆洗水に与えた。電磁弁が開いた際の濾過膜側の最高膜透過圧力は９０ＫＰａ、電磁弁が閉じた状態では０ＫＰａであった。この時の濾水回収率（得られた慮水量／濾過した原水量）は９６％であった。上記運転条件で３ヶ月間運転した後の膜濾過流量は、２．８ｍ^３／ｍ^２／日であった。
【００２７】
【比較例２】
実施例４において、逆洗の際に電磁弁１０を常時開とし、一定の圧力で実施例４と同量の逆洗水を使って逆洗を行った以外は実施例４と同様に膜濾過運転を実施した。この時の濾水回収率は実施例１と同じく９６％であった。３ヶ月後の膜濾過流量は０．９ｍ^３／ｍ^２／日であった。
【００２８】
【実施例５】
実施例１において、逆洗水が５ｍｇ／リットル濃度の次亜塩素酸ソーダ水になるよう酸化剤送液ポンプ８より次亜塩素酸ソーダを注入した以外は実施例１と同様に膜濾過運転を実施した。この時の濾水回収率は実施例１と同じく９８％であった。３ヶ月後の膜濾過流量は３．６ｍ^３／ｍ^２／日であった。
【００２９】
【実施例６】
実施例１において、濾過を２０分間行った後、電磁弁１０を１秒間開き、４秒間閉じる周期を６回繰り返してパルス状の膜透過圧力を逆洗水に与える逆洗と同時にモジュール下部から毎時２Ｎｍ^３の空気を濾過膜の原水側に導入してエアーバブリングを３０秒間行うという操作を繰り返した。３ヶ月後の膜濾過流量は３．５ｍ^３／ｍ^２／日であった。
【００３０】
【実施例７】
実施例１において、濾過を２０分間行った後、電磁弁１０を１秒間開き、４秒間閉じる周期を６回繰り返してパルス状の膜透過圧力を逆洗水に与える逆洗の際に逆洗水が５ｍｇ／リットル濃度の次亜塩素酸ソーダ水になるよう酸化剤送液ポンプ８より次亜塩素酸ソーダを注入し、さらにモジュール下部から毎時２Ｎｍ^３の空気を濾過膜の原水側に導入してエアーバブリングを３０秒間行うという操作を繰り返した。３ヶ月後の膜濾過流量は４．５ｍ^３／ｍ^２／日であった。
【００３１】
【実施例８】
実施例６において、コンプレッサー９を空気源を用いたオゾン発生機につなぎ変えて実施例３の空気をオゾンガスに変えた以外は実施例３と同条件で膜濾過装置の運転を行った。この時のオゾンガス濃度は、２０ｇ／ｍ^３であった。３ヶ月後の膜濾過流量は４．３ｍ^３／ｍ^２／日であった。
【００３２】
【発明の効果】
本発明によれば、効果的に洗浄を行う事ができ、この結果、長期間に亘って高い膜濾過流束を維持することが可能である。
【図面の簡単な説明】
【図１】本発明での逆洗媒体に与えるパルス状の膜透過圧力の一例を示す模式図。
【図２】本発明での逆洗媒体に与えるパルス状の膜透過圧力の別の例を示す模式図。
【図３】本発明の膜の洗浄方法を組み込んだ処理フローの一例を示したものである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention is used for water supply and industrial water, river water, lake water, groundwater, storage water filtration, sewage secondary treatment water filtration, sewage, wastewater filtration, or separation or concentration of valuable resources. The present invention relates to a method for washing a filtration membrane.
[0002]
[Prior art]
BACKGROUND ART Filtration membranes used for filtration of various raw waters are used in various filtration devices because of their excellent filtration accuracy, small installation space, and easy operation management. However, with the continuation of the filtration, the substance to be removed in the raw water adheres to the membrane surface and closes the pores, so that the filtration performance gradually decreases, and finally the filtration becomes impossible. Therefore, in order to maintain the filtration performance, backwashing (backwashing) in which a backwash medium such as filtrate or clear water is jetted from the filtrate side in a direction opposite to the filtration direction to remove deposits on the filtration surface of the membrane. ) Is commonly done.
[0003]
In order to enhance the backwashing effect, sodium hypochlorite having an oxidizing effect is added to the backwashing medium, or the backwashing is carried out using ozone water as disclosed in JP-A-4-310220. And a method of backwashing with ozonized pressurized air disclosed in JP-A-60-58222. Further, a method of introducing air into the raw water side of the filtration membrane as air bubbles and a method of injecting ozonized air as air bubbles into the raw water side of the filtration membrane as disclosed in JP-A-63-42703 are known. Have been.
[0004]
[Problems to be solved by the invention]
The above-described backwashing method using an oxidizing agent such as backwashing water or ozone water to which sodium hypochlorite has been added, and a method of introducing air or ozonized air as bubbles into the raw water side of the filtration membrane have a backwashing effect. Although effective in increasing the pressure, a sufficiently stable filtration flux cannot always be obtained depending on conditions such as turbidity of raw water.
In order to remove deposits on the membrane surface and maintain a high filtration flux, it is effective to increase the pressure during backwashing and lengthen the backwashing time. However, these use a large amount of filtered water or clarified water, and increase the cost required for backwashing due to the reduction in the amount of obtained filtrate and the use of expensive clarified water. When the backwash pressure is increased, a load is applied to the filtration membrane, the module, and the piping, so that not only the filtration life is shortened, but also there is a possibility that these may burst or leak.
[0005]
[Means for Solving the Problems]
The present inventors have conducted intensive studies on a method for cleaning a filtration membrane, and as a result, have completed the following invention. That is, the present invention
(1) The backwashing medium is given a membrane permeation pressure by a pressure generator and supplied to the filtrate side of the filtration membrane, and the backwashing medium that has permeated the membrane is ejected to the raw water side of the filtration membrane to remove deposits on the filtration membrane. In the method of backwashing a filtration membrane, a backwashing is performed by applying a membrane permeation pressure applied to the backwash medium in a pulsed manner, (2) the backwash medium contains one or more oxidizing agents. (1) The method for cleaning a membrane according to (1), wherein the method is a liquid or a gas, and (3) the method is performed while introducing a bubble into the raw water side of the filtration membrane to oscillate the filtration membrane. Is a method for cleaning a film.
[0006]
The details of the present invention are described below.
The raw water that is the object of the present invention is river water, lake water, groundwater, storage water, sewage secondary treatment water, industrial effluent, or sewage. Conventionally, when raw water as described above is filtered through a membrane, suspended substances contained in the raw water and substances having a size larger than the pore size of the membrane to be used are blocked by the membrane, so-called concentration polarization and a cake layer are formed. Clogging the membrane or being adsorbed by the network inside the membrane. As a result, the filtration flux of the membrane at the time of filtering the raw water is reduced from a fraction to one-tenth of that at the time of filtering the clarified water, and the filtration flux is increased as the filtration is continued. The bundle gradually decreases.
In order to prevent this and maintain the membrane filtration flux, backwashing is generally used in which filtered water or clear water is jetted from the direction opposite to the filtration direction of the membrane to remove deposits on the filtration surface of the membrane. However, as described above, the backwashing conditions have not always been sufficiently effective due to limitations in terms of the durable pressure of the filtration membrane, module, and piping, or drainage recovery.
[0007]
The present invention is a method for performing backwashing by applying a membrane permeation pressure to a backwash medium in a pulsed manner as a method for washing a filtration membrane. When the membrane permeation pressure is applied in a pulsed form, the backwash medium is jetted onto the deposit layer deposited on the filtration membrane surface to apply strain stress repeatedly, which is more efficient than the conventional case where a constant pressure is continuously applied. The deposit layer can be separated from the film surface. Moreover, the amount of the backwashing medium to be used is smaller than when a continuous and constant pressure is applied, which is effective in terms of the filtrate recovery rate.
[0008]
When the backwashing medium of the present invention is subjected to backwashing by applying a membrane-permeable pressure in a pulsed manner, an oxidizing agent such as sodium hypochlorite, chlorine dioxide, hydrogen peroxide, ozone-containing water, or ozone gas is used as the backwashing medium. A further backwashing effect can be obtained by using a method of using a liquid or gas containing at least one of the above, or a method of introducing bubbles such as air and ozone gas from the raw water side of the filtration membrane to oscillate the membrane. . The reason why the backwashing effect by the backwashing medium to which the oxidizing agent is added is high is considered to be that the oxidizing agent is easily exchanged into the inside of the deposit by the rapid pulse-like pressure change during the backwashing, and the cleaning effect is enhanced. . In addition, when the membrane is shaken by introducing air bubbles such as air and ozone gas from the raw water side during the backwashing of the present invention, the reason why the backwashing effect is high is that a rapid change in pulsed pressure during the backwashing causes a filtration membrane to be used. It is considered that a large membrane vibration can be obtained by changing the secondary side (filtration side) volume or bubble diameter of the module.
[0009]
As described above, a method of applying a pulsed pressure to the backwash medium when performing the backwash, a method of using a liquid or a gas containing at least one oxidizing agent as the backwash medium, or simultaneously a raw water side The method of introducing air bubbles from the substrate to oscillate the film or both of them can effectively remove film deposits effectively. Thereby, the filtration flux can be maintained at a high state, and the running cost can be reduced.
[0010]
The application of the membrane permeation pressure to the backwash medium of the present invention in a pulsed manner means, as shown in FIG. 1 or FIG. 2, a state (A) in which a high membrane permeation pressure is applied to the backwash medium during the backwash step. This means that the state (B) in which the low transmembrane pressure or the low transmembrane pressure is not applied is repeated a plurality of times. The maximum membrane permeation pressure of (A) is preferably at least twice, more preferably at least three times, the minimum membrane permeation pressure of (B). Further, the maximum membrane permeation pressure of (A) is preferably lower than the membrane rupture pressure so that the filtration membrane will not be significantly deformed or broken.
[0011]
The shift time between adjacent (A) and (A) when repeating a plurality of times is 0.1 seconds or more and 30 seconds or less, and the shift time between adjacent (B) and (B) is 0.1 seconds or more and 30 seconds or less. Seconds or less are preferred. Within this time, each of the pressures (A) and (B) may change with time. Further, it is not necessary that the same pressure and the same pressure are used for both (A) and (B) which are repeated plural times.
Also, when applying pressure to the backwash medium, it is more desirable to increase the pressure instantaneously as shown in FIG. 1 because the washing effect is high, but it is also possible to apply it as shown in FIG.
[0012]
The time of the backwashing step may be appropriately determined in consideration of the recovery of the filtration flux and the recovery rate of the filtered water. As the backwash medium, any of a liquid and a gas can be used, but a liquid is preferable because an oxidizing agent can be added in a wide range.
Pumps, compressors, or pressurized gas containers are examples of pressure generating devices for applying a membrane permeation pressure to the backwash medium. As a method of applying pulsed pressure to the backwash medium, for example, a pump, a compressor, etc. are intermittently operated. Moving it. Alternatively, it can be provided by a pump for feeding the backwashing medium or a pressurized gas continuously acting on the backwashing medium, and intermittently opening and closing a valve provided on a raw water side or a drainage side pipe. .
[0013]
Although the filtration membrane of the present invention is not particularly limited, for example, polyolefins such as polyethylene, polypropylene, and polybutene; tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA) and tetrafluoroethylene-hexafluoropropylene copolymer (FEP) , Tetrafluoroethylene-hexafluoropropylene-perfluoroalkylvinyl ether copolymer (EPE), tetrafluoroethylene-ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), chlorotrifluoroethylene-ethylene copolymer (ECTFE), fluororesins such as polyvinylidene fluoride (PVDF); polysulfone, polyethersulfone, polyetherketone, polyetheretherketone, polyphenylenesul Super engineering plastics such as I de; cellulose acetate, cellulose such as ethyl cellulose; polyacrylonitrile; alone and mixtures of these polyvinyl alcohol.
[0014]
Further, when a strong oxidizing agent such as ozone is used together, an inorganic film such as a ceramic, a polyvinylidene fluoride (PVDF) film, a polytetrafluoroethylene (PTFE) film, and an ethylene-tetrafluoroethylene copolymer (ETFE) film are used. And an organic film such as a fluororesin film such as a tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) film. It is particularly preferable to use a polyvinylidene fluoride (PVDF) film. Among such filtration membranes, those whose pore size region is from a nanofiltration (NF) membrane to a microfiltration (MF) membrane can be used. In particular, an NF membrane having a molecular weight cut-off of about 100 to an MF membrane having an average pore size of 10 μm or less is preferable.
[0015]
As the shape of the filtration membrane, any shape such as a hollow fiber shape, a hollow fiber shape with a wave, a flat membrane shape, a pleated shape, a spiral shape, and a tubular shape can be used, but a large membrane area per unit volume can be obtained. A hollow fiber shape is more preferred. Generally, filtration is performed using a module containing a membrane. The filtration method may be a full filtration method or a cross flow filtration method. A pressure filtration method or a negative pressure filtration method may be used. In the case of a hollow fiber membrane, either internal pressure filtration or external pressure filtration may be used.
[0016]
The oxidizing agent contained in the backwash medium includes sodium hypochlorite, chlorine dioxide, hydrogen peroxide, and ozone, and at least one or more of these are contained in the backwash medium to perform backwash. The concentration of the oxidizing agent contained in the backwash medium is preferably from 0.05 mg / L to 50% by weight for sodium hypochlorite, chlorine dioxide and hydrogen peroxide, and more preferably from 0.1 mg / L to 20% by weight. Or less, more preferably 10% by weight or less. Below this, the dissolution of organic substances does not proceed sufficiently, so that a sufficient cleaning effect cannot be obtained. If the concentration is too high, a sufficient cleaning effect can be obtained, but the cost of chemicals is high and it is not economical. These addition methods to the backwash medium may be directly added to a drainage tank or the like, or may be added as an aqueous solution using a line mixer in the middle of a pipe from the drainage tank to the filtration membrane. .
[0017]
When the backwash medium is water, the concentration of ozone contained in the backwash water is preferably 0.05 mg / L or more and 50 mg / L or less. More preferably, it is 0.1 mg / liter or more and 10 mg / liter or less. When the ozone concentration is within this range, the oxidation by ozone sufficiently proceeds, and the cleaning effect is high. Further, if the ozone concentration is excessively high, the cost associated with the generation of ozone is increased and is not realistic. Ozone added to the backwash water may be ozone alone or ozonized air. The introduction of ozone into the backwash water may be performed via an air diffuser or the like provided at an appropriate position in the backwash tank. Alternatively, it may be added using an ejector and a line mixer in the middle of the pipe from the drainage tank to the filtration membrane, or a U-tube method may be used.
[0018]
When ozone is generated by discharge as an ozone generation method, the raw material may be air or oxygen. Alternatively, a method of generating ozone by electrolysis of water may be used.
The method of introducing bubbles into the raw water side of the filtration membrane has a high washing effect when (a) the washing is always performed simultaneously with the backwashing of the present invention, but (b) only the backwashing of the present invention prior to the introduction of bubbles (simultaneously backwashing). May be performed. Alternatively, (c) after introducing the bubbles (simultaneously backwashing), only the backwashing of the present invention may be performed. Furthermore, (d) bubbles may be introduced while introducing raw water, and the backwash of the present invention may be performed at the same time, or (a) to (d) may be alternately combined.
[0019]
Air or ozone gas is desirable as the type of bubble. It is preferable to supply a volume flow rate of 0.5 to 20 times the filtration flow rate per unit time, and it is more preferable that the volume flow rate of the bubbles is 1 to 10 times the volume flow rate. Since the present invention is configured as described above, it is possible to maintain a high membrane filtration flux over a long period of time.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, examples of the present invention will be described.
As raw water 1, river surface water having a turbidity of 5 to 20 ° C and a water temperature of 18 to 25 ° C was used. As shown in FIG. 3, the raw water 1 is pumped through a circulation tank 2 by a raw water supply pump 3 to a membrane module 4, and the obtained filtered water is stored in a filtered water tank 5, which also serves as a backwash tank. At the time of backwashing, the filtered water in the drainage tank 5 is sent to the membrane module 4 by the backwash pump 6 for backwashing. Here, an oxidizing agent tank is provided in the pipe from the backwash pump 6 to the membrane module 4. The oxidizing agent 7 can be added to the backwash water by the oxidizing agent feed pump 8. Air bubbling for introducing bubbles into the membrane module 4 is performed by supplying air compressed by the compressor 11 to the raw water side of the membrane module 4. The pulsed membrane permeation pressure can be applied to the backwash medium by repeatedly opening and closing the electromagnetic valve 10 which is opened and closed by a timer in a short time.
[0021]
The membrane module 4 is a hollow fiber microfiltration (MF) made of PVDF (polyvinylidene fluoride) having an inner diameter of 1.0 mmφ, an outer diameter of 1.9 mmφ, and an average pore diameter of 0.6 μm manufactured based on JP-A-3-215535. This is an external pressure type module in which the membrane is housed in a 1 m long, 3 inch diameter PVC (polyvinyl chloride) casing. The membrane area of the module is 4.7 m ² , and the clarified water filtration flux when the module filtration pressure is 50 kPa is 5.0 m ³ per hour.
[0022]
Embodiment 1
Filtration was carried out by constant-pressure filtration in which raw water 1 was supplied to the membrane module 4 at a constant pressure, and a cross-flow method in which the ratio of the amount of membrane filtration water to the amount of circulating water was 5: 1. The operating conditions were as follows: after performing filtration for 20 minutes, backwashing was repeated for 30 seconds. At the time of this backwashing, a cycle of opening the electromagnetic valve 10 for 1 second and closing it for 4 seconds was repeated 6 times to give a pulsed membrane permeation pressure to the backwash water. The maximum membrane permeation pressure on the filtration membrane side when the solenoid valve was opened was 110 KPa, and was 0 KPa when the solenoid valve was closed. At this time, the drainage recovery rate (the amount of water obtained / the amount of filtered raw water) was 98%. The membrane filtration flow rate after operating for 3 months under the above operating conditions was 3.1 m ³ / m ² / day.
[0023]
[Comparative Example 1]
In Example 1, membrane filtration was performed in the same manner as in Example 1 except that the electromagnetic valve 10 was always opened at the time of backwashing, and backwashing was performed using the same amount of backwash water as in Example 1 at a constant pressure. Operation was carried out. The drainage recovery rate at this time was 98% as in Example 1. After 3 months, the membrane filtration flow rate was 1.1 m ³ / m ² / day.
[0024]
Embodiment 2
In Example 1, when performing backwashing by repeating for 30 seconds, the electromagnetic valve 10 is opened for 2.5 seconds, and a cycle of closing for 12.5 seconds is repeated twice to change the pulse-like membrane permeation pressure change to the backwash water. Gave. The maximum membrane permeation pressure on the filtration membrane side when the solenoid valve was opened was 120 KPa, and was 0 KPa when the solenoid valve was closed. The drainage recovery rate at this time was 98% as in Example 1. The membrane filtration flow rate after operating for three months under the above operating conditions was 3.0 m ³ / m ² / day.
[0025]
Embodiment 3
In Example 1, when performing backwashing repeatedly for 30 seconds, the electromagnetic valve 10 was opened for 0.5 seconds, and a cycle of closing for 2 seconds was repeated 12 times to apply a pulsed membrane permeation pressure to the backwash water. The maximum membrane permeation pressure on the filtration membrane side when the solenoid valve was opened was 100 KPa, and was 0 KPa when the solenoid valve was closed. The drainage recovery rate at this time was 98% as in Example 1. After operating for 3 months under the above operating conditions, the membrane filtration flow rate was 3.2 m ³ / m ² / day.
[0026]
Embodiment 4
In Example 1, after performing filtration for 60 minutes, backwashing was repeated for 120 seconds. At the time of this backwashing, a cycle of opening the electromagnetic valve 10 for 20 seconds and closing it for 20 seconds was repeated three times to apply a pulsed membrane permeation pressure to the backwash water. The maximum membrane permeation pressure on the filtration membrane side when the solenoid valve was opened was 90 KPa, and was 0 KPa when the solenoid valve was closed. At this time, the drainage recovery rate (the obtained amount of filtered water / the amount of filtered raw water) was 96%. After operating for 3 months under the above operating conditions, the membrane filtration flow rate was 2.8 m ³ / m ² / day.
[0027]
[Comparative Example 2]
In Example 4, membrane filtration was performed in the same manner as in Example 4 except that the electromagnetic valve 10 was always opened at the time of backwashing, and backwashing was performed using the same amount of backwash water as in Example 4 at a constant pressure. Operation was carried out. The drainage recovery rate at this time was 96% as in Example 1. After 3 months, the membrane filtration flow rate was 0.9 m ³ / m ² / day.
[0028]
Embodiment 5
In Example 1, the membrane filtration operation was performed in the same manner as in Example 1 except that sodium hypochlorite was injected from the oxidizing agent sending pump 8 so that the backwash water became sodium hypochlorite water having a concentration of 5 mg / liter. Carried out. The drainage recovery rate at this time was 98% as in Example 1. After 3 months, the membrane filtration flow rate was 3.6 m ³ / m ² / day.
[0029]
Embodiment 6
In Example 1, after performing filtration for 20 minutes, the solenoid valve 10 is opened for 1 second, and a cycle of closing for 4 seconds is repeated 6 times, and a pulse-like membrane permeation pressure is applied to the backwash water, and at the same time from the bottom of the module at the same time as the backwash. The operation of introducing 2 Nm ³ of air into the raw water side of the filtration membrane and performing air bubbling for 30 seconds was repeated. After three months, the membrane filtration flow rate was 3.5 m ³ / m ² / day.
[0030]
Embodiment 7
In Example 1, after performing filtration for 20 minutes, the solenoid valve 10 is opened for 1 second, and a cycle of closing for 4 seconds is repeated six times to apply a pulsed membrane permeation pressure to the backwash water. Is injected from the oxidizing agent feed pump 8 so that the concentration becomes 5 mg / liter sodium hypochlorite water, and 2Nm ³ of air per hour is introduced from the lower part of the module into the raw water side of the filtration membrane. The operation of performing air bubbling for 30 seconds was repeated. After 3 months, the membrane filtration flow rate was 4.5 m ³ / m ² / day.
[0031]
Embodiment 8
In Example 6, the operation of the membrane filtration device was performed under the same conditions as in Example 3 except that the compressor 9 was connected to an ozone generator using an air source to change the air of Example 3 to ozone gas. The ozone gas concentration at this time was 20 g / m ³ . After 3 months, the membrane filtration flow rate was 4.3 m ³ / m ² / day.
[0032]
【The invention's effect】
According to the present invention, cleaning can be performed effectively, and as a result, a high membrane filtration flux can be maintained over a long period of time.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an example of a pulsed membrane permeation pressure applied to a backwash medium in the present invention.
FIG. 2 is a schematic diagram showing another example of a pulsed membrane permeation pressure applied to a backwash medium in the present invention.
FIG. 3 shows an example of a process flow incorporating the film cleaning method of the present invention.

Claims

A pressure generating device applies a membrane permeation pressure to the backwashing medium, supplies the backwashing medium to the filtrate side, ejects the backwashing medium that has permeated the membrane to the raw water side of the filtration membrane, and removes deposits on the filtration membrane. In the backwashing method, a backwash is performed by applying a membrane permeation pressure applied to the backwash medium in a pulsed manner.

2. The method according to claim 1, wherein the backwash medium is a liquid or a gas containing at least one oxidizing agent.

2. The method for cleaning a membrane according to claim 1, wherein the filtration is performed while introducing a bubble into the raw water side of the filtration membrane to oscillate the filtration membrane.