【０００１】
【発明の属する技術分野】
本発明は廃水処理方法に係り、特に有機物と固形物を含む被処理水を最初沈殿池で沈殿処理した後に、後段に設けた生物反応槽で生物処理するようにした廃水処理方法に関する。
【０００２】
【従来の技術】
従来のこの種の廃水処理方法としては図６〜図８に示したものが周知である。図６に示したものは基本的な活性汚泥処理法であり、有機物と固形物を含む被処理水１は最初沈殿池２で沈殿処理を受け、初沈汚泥３が分離される。固形物が除去された被処理水４は曝気槽５で活性汚泥処理を受け、有機物が酸化分解される。活性汚泥処理後の汚泥混合液６は最終沈殿池７で沈殿処理を受け、処理水８と終沈汚泥９とに分離される。終沈汚泥９の一部は生物処理のための活性汚泥９Ａとして曝気槽５に返送され、残部は余剰汚泥９Ｂとして系外に排出される。
【０００３】
図７に示したものはＡＯ法と通称されており、図６の最初沈殿池２と曝気槽５との間に嫌気槽４Ａを設けている。嫌気槽４Ａに返送された活性汚泥は嫌気条件下で有機物を取り込むとともに、過剰に蓄積していたリンを放出する。曝気槽５では活性汚泥は好気条件下で被処理水中の有機物を酸化分解するとともに、前記嫌気槽４Ａで放出したリン及び被処理水中のリンを過剰に取り込む。この結果、余剰汚泥９Ｂはリンを過剰に取り込んだ状態で系外に排出されることになり、被処理水中の有機物とリンを同時に除去することができる。
【０００４】
図８に示したものはＡ２Ｏ法と通称されており、図７の嫌気槽４Ａと曝気槽５との間にさらに無酸素槽４Ｂを設けている。この方法は曝気槽５で被処理水中のアンモニア態窒素が好気条件下で硝化されて硝酸態窒素となる。この硝酸態窒素を含む硝化処理水５Ａを前段の無酸素槽４Ｂに循環させることによって、無酸素槽４Ｂでは硝酸態窒素が脱窒反応によって窒素ガスとなる。したがって、このＡ２Ｏ法によれば前記ＡＯ法による有機物とリンの除去作用に加えて、アンモニア態窒素も除去できるので、被処理水中の有機物とリンと窒素とを同時に除去することができる。
【０００５】
前記従来の技術に関して、最初沈殿池２で発生した初沈汚泥３に着目した場合に、主に二つの問題点がある。第１に初沈汚泥３は通常かなりの量の有機物を含んでおり、腐敗しやすいので処理・処分が厄介である。第２に前記ＡＯ法やＡ２Ｏ法では嫌気槽４Ａでのリン放出や無酸素槽４Ｂでの脱窒反応に有機物が必要である。ところが、最初沈殿池２で初沈汚泥３を除去するとＳＳ性の有機物が初沈汚泥３に移行するため、有機物が不足してリン放出や脱窒反応が十分に進行しない場合がある。このような問題点を改善するために、特にＡＯ法やＡ２Ｏ法では種々の提案がされている。例えば、特許文献１には最初沈殿池で分離した初沈汚泥の一部を嫌気槽に送り、有機物の不足分を補給する方法が開示されている。また、特許文献２には最初沈殿池で分離した初沈汚泥の一部を酸発酵槽に送り、酸発酵によって発生した溶解性の有機酸を後段の生物反応槽に導入する方法が開示されている。
【０００６】
【特許文献１】
特開２００１−２５９６８２号公報
【特許文献２】
特開２００１−９４９８号公報
【０００７】
【発明が解決しようとする課題】
しかしながら、特許文献１に記載された方法は補給された初沈汚泥中のＳＳ性の有機物が溶解性の有機物に比べて生物学的に安定しており、リン放出や脱窒反応のために必要な有機物としては即効性に乏しく、利用する有機物としては余り有効ではないという問題点がある。また、特許文献２に記載された方法は新たに酸発酵槽が必要であり、その運転，制御が複雑となる問題点がある。
本発明の目的は上記従来技術の問題点を改善し、酸発酵槽を設置することなく、被処理水中のＳＳ性の有機物を簡単な手法で溶解性の有機酸に移行させることによって、初沈汚泥の処理，処分を容易にするとともに、得られた溶解性の有機酸をリン放出や脱窒反応に有効利用することができる廃水処理方法を提供することにある。
【０００８】
【課題を解決するための手段】
上記の課題を解決するために、本発明に係る廃水処理方法は、有機物と固形物を含む被処理水を最初沈殿池で沈殿処理した後に、後段に設けた生物反応槽で生物処理するようにした廃水処理方法において、前記最初沈殿池で分離された初沈汚泥を当該最初沈殿池で還流させることを特徴とする。
【０００９】
また、本発明に係る廃水処理方法は、有機物と固形物とリンを含む被処理水を最初沈殿池で沈殿処理した後に、嫌気槽、好気槽、最終沈殿池の順に通水し、最終沈殿池で分離した終沈汚泥を前記嫌気槽に返送するようにした廃水処理方法において、前記最初沈殿池で分離した初沈汚泥を当該最初沈殿池で還流させることを特徴とする。
【００１０】
また、本発明に係る廃水処理方法は、有機物と固形物とリンとアンモニア態窒素を含む被処理水を最初沈殿池で沈殿処理した後に、嫌気槽、無酸素槽、好気槽の順に通水して生物処理し、前記好気槽で得られた硝化処理水の一部を前記無酸素槽に循環させるとともに、残部の硝化処理水を最終沈殿池で沈殿処理し、分離した終沈汚泥を前記嫌気槽に返送するようにした廃水処理方法において、前記最初沈殿池で分離した初沈汚泥を当該最初沈殿池で還流させることを特徴とする。
【００１１】
また、本発明に係る廃水処理方法は、前記初沈汚泥を還流させる手段が、最初沈殿池から引き抜いた初沈汚泥を最初沈殿池に流入する被処理水と合流させる手段であり、この初沈汚泥と被処理水との合流水に凝集剤を添加した後に、当該合流水を最初沈殿池に導入することを特徴とする。
【００１２】
【発明の実施の形態】
図１は本発明の第１実施形態を示す装置系統図である。有機物と固形物を含む被処理水が管路１０から最初沈殿池１２に供給される。最初沈殿池１２では被処理水が沈澱処理を受け、上澄水１４と初沈汚泥１６とに分離される。上澄水１４、すなわち固形物が除去された被処理水は管路１８から曝気槽２０に導入される。曝気槽２０は散気手段２２からの曝気によって槽内が好気条件に維持されている。被処理水はこの曝気槽２０で活性汚泥処理を受け、被処理水中の有機物が酸化分解される。活性汚泥処理後の汚泥混合液は最終沈殿池２４で沈殿処理を受け、上澄水２６と終沈汚泥２８とに分離される。上澄水２６は処理水として管路３０から系外に排出される。終沈汚泥２８の一部は生物処理のための活性汚泥として管路３２から曝気槽２０に返送される。また、終沈汚泥２８の残部は余剰汚泥として管路３４から系外に排出される。
【００１３】
最初沈殿池１２では初沈汚泥１６は最初沈殿池１２の下部で数時間，滞留した後に引き抜かれる。この初沈汚泥１６の滞留層は嫌気条件下に置かれており、酸発酵菌が繁殖している。このため、初沈汚泥１６中のＳＳ性有機物の少なくとも一部が酸発酵菌の作用によって酸発酵処理を受け、溶解性の有機酸となる。生成した有機酸はその一部が拡散して上澄水１４側に移行する。また、最初沈殿池１２の低部に接続された汚泥引抜き用の管路３６には、還流ポンプ３８を備えた汚泥還流用の管路４０の一端が分岐し、管路４０の他端は管路１０に接続している。管路３６から引き抜かれた有機酸を含む初沈汚泥１６の一部は還流ポンプ３８により管路４０を介して管路１０を通る被処理水と合流し、最初沈殿池１２に還流される。この初沈汚泥１６の還流過程でも生成した有機酸が被処理水に溶け込み、上澄水１４側に移行する。また、還流する初沈汚泥１６には多数の酸発酵菌が存在しているので、還流によって最初沈殿池１２内で繰り返し沈殿する初沈汚泥１６は速やかに酸発酵が進行する。また、管路３６から引き抜かれた初沈汚泥１６の残部は汚泥引抜きポンプ４２を間欠的に運転することにより、余剰の初沈汚泥として系外に排出される。
【００１４】
還流させる初沈汚泥の最初沈殿池１２内での滞留時間は、汚泥引抜きポンプ４２の運転間隔によって任意に調整できる。汚泥引抜きポンプ４２の運転間隔は、一定間隔の運転としたり、最初沈殿池１２内での汚泥界面が所定レベルに達する毎に間欠的に運転したり、最初沈殿池１２内の上澄水１４の有機物濃度が所定値に達する毎に間欠的に運転するなどの方法を単独に行うか、又は組み合わせることによって制御することができる。
【００１５】
なお、本実施形態では初沈汚泥と被処理水との合流水に対して、例えば高分子系の凝集剤１１を添加することが好ましい。この凝集剤１１の添加により、固形物の沈降性が高まる。このため、最初沈殿池１２内での汚泥界面を低く維持しつつ、初沈汚泥の滞留時間を増加させることができ、最初沈殿池１２の運転の安定性と酸発酵の促進を図ることができる。
【００１６】
このように、本実施形態によれば初沈汚泥中のＳＳ性の有機物が還流の過程で酸発酵を受け溶解性の有機酸に移行する。このため、系外に排出する初沈汚泥の減容化を図ることができる。また、系外に排出する初沈汚泥は無機性固形物の相対比率が大きくなり、腐敗し難くなるので、処理，処分が容易である。なお、ＳＳ性の有機物が溶解性の有機酸に移行することによって、曝気槽２０における被処理水の有機物負荷が増加するが、生成した有機酸は生物学的に分解し易い低分子のものが多いので格別の問題はない。したがって、例えば既設の廃水処理設備に本実施形態に係る初沈汚泥の還流機構を追設して、容易に初沈汚泥の減容化と改質を図ることができる。
【００１７】
図２は本発明の第２実施形態を示す装置系統図である。この実施形態はＡＯ法に適用した場合であり、図１の最初沈殿池１２と曝気槽２０との間に嫌気槽６０が設けられる。図２において、図１と同一符号の要素は図１に示したものと実質的に同一か又は類似の要素であるので、詳細な説明を省略する。最初沈殿池１２には有機物と固形物とリンを含む被処理水が供給され、沈殿処理を受ける。初沈汚泥は管路４０を介して最初沈殿池１２に還流され、ＳＳ性の有機物が溶解性の有機酸となる。この有機酸を溶解した上澄水１４は、被処理水として管路１８から嫌気槽６０に導入される。嫌気槽６０では被処理水が管路３２から返送された活性汚泥と混合され、活性汚泥は嫌気条件下で有機物を取り込むとともに、過剰に蓄積していたリンを放出する。この際、被処理水に溶解した前記有機酸がリン放出に必要な有機物源として有効に利用される。
【００１８】
好気槽である曝気槽２０では活性汚泥が好気条件下で被処理水中に残存する有機物を酸化分解するとともに、前記嫌気槽６０で放出したリン及び被処理水中に元々含まれていたリンを過剰に取り込む。この結果、余剰汚泥はリンを過剰に取り込んだ状態で管路３４から系外に排出されることになり、被処理水中の有機物とリンを同時に除去することができる。
【００１９】
図３は本発明の第３実施形態を示す装置系統図である。この実施形態はＡ２Ｏ法に適用した場合であり、図２の嫌気槽６０と曝気槽２０との間にさらに無酸素槽７０が設けられる。図３において、図２と同一符号の要素は図２に示したものと実質的に同一か又は類似の要素であるので、詳細な説明を省略する。最初沈殿池１２には有機物と固形物とリンとアンモニア態窒素を含む被処理水が供給され、沈殿処理を受ける。初沈汚泥は管路４０を介して最初沈殿池１２に還流され、ＳＳ性の有機物が溶解性の有機酸となる。この有機酸を溶解した上澄水１４は、被処理水として嫌気槽６０に導入される。嫌気槽６０では被処理水が管路３２から返送された活性汚泥と混合され、活性汚泥は嫌気条件下で有機物を取り込むとともに、過剰に蓄積していたリンを放出する。この際、被処理水に溶解した前記有機酸がリン放出に必要な有機物源として有効に利用される。嫌気槽６０でリンを放出した被処理水は無酸素槽７０を経て曝気槽２０に導入される。
【００２０】
好気槽である曝気槽２０では活性汚泥が好気条件下で被処理水中に残存する有機物を酸化分解するとともに、前記嫌気槽６０で放出したリン及び被処理水中に元々含まれていたリンを過剰に取り込む。さらに、活性汚泥中に存在する硝化菌によって被処理水中のアンモニア態窒素が好気条件下で硝化されて硝酸態窒素となる。この硝酸態窒素を含む硝化処理水は管路７２を介して無酸素槽７０に循環される。無酸素槽７０では活性汚泥中に存在する脱窒菌によって硝酸態窒素を窒素ガスにする。この脱窒反応も被処理水中に有機物が必要であり、前記被処理水に溶解した有機酸が脱窒反応に必要な有機物源として直接に利用されるか、又は脱窒反応に必要な有機物源を間接的に補う。この結果、前記ＡＯ法による有機物とリンの除去作用に加えて、アンモニア態窒素も除去できる。
【００２１】
上述のとおり、ＡＯ法又はＡ２Ｏ法が適用された本発明においては、最初沈殿池で初沈汚泥を還流させることによって生成した有機酸が、リン放出や脱窒反応の際に必要な有機物として有効に利用される。このため、リン濃度やアンモニア態窒素濃度が比較的高く、溶解性の有機物濃度が比較的低い被処理水をＡＯ法又はＡ２Ｏ法で処理する場合に、有機物の不足分を補う意味で本発明は特に有効である。
【００２２】
図４は初沈汚泥の還流機構の変形例を示す模式図である。管路３６には初沈汚泥１６の還流と引抜きを兼用するポンプ４４が設けられている。また、初沈汚泥を還流させる管路５０の途中には弁４６を介して酸発酵槽５２が設けられている。初沈汚泥を酸発酵槽５２で所定時間，滞留させた後に、被処理水と合流して循環させる。この方法によれば有機酸の発生をより一層，促進させることができる。最初沈殿池でも酸発酵が進行するので、酸発酵槽５２は補助的な役割となり、その容量は小さくてもよい。なお、初沈汚泥の還流量と引抜き量の調整は、汚泥引抜き用の管路４８に設けた弁４９と前記弁４６の開閉を交互に切り替えるか、または弁４６、弁４９の開度を調節することによって行うことができる。
【００２３】
図５は初沈汚泥の還流機構の他の変形例を示す模式図である。最初沈殿池８０にはパドル型の撹拌機８２が配設されている。この撹拌機８２を連続的又は間欠的に稼動することによって、初沈汚泥を適宜に混合撹拌する。この混合撹拌によって初沈汚泥の滞留層に繁殖した酸発酵菌が上下左右にむらなく分布することになり、滞留層全体でＳＳ性有機物の酸発酵が効率よく進行する。この変形例は図１に示したポンプ３８による初沈汚泥の外的な還流機構に替えて、初沈汚泥を最初沈殿池内で内的に還流させるものであり、同等の効果を達成することができる。撹拌機８２としてはパドル型のものに限らず、インペラ型や撹拌ポンプ型でもよい。なお、余剰の初沈汚泥はポンプ８３を備えた管路８４から引き抜き、系外に排出する。余剰の初沈汚泥の引抜き量が、例えば最初沈殿池８０における汚泥界面を指標として管理されることは、図１で説明した場合と同様である。
【００２４】
【発明の効果】
本発明によれば、最初沈殿池で分離した初沈汚泥を当該最初沈殿池で還流させることによって、この還流の過程で被処理水中のＳＳ性の有機物が酸発酵を受け溶解性の有機酸に移行する。このため，格別の酸発酵槽などを設置することなく、最初沈殿池のみで初沈汚泥の減容化を図ることができるとともに、初沈汚泥は無機性固形物の相対比率が大きくなり、処理，処分が容易になる。また、ＡＯ法又はＡ２Ｏ法に適用した場合には、酸発酵によって得られた溶解性の有機酸をリン放出や脱窒反応に有効利用することができる。
【図面の簡単な説明】
【図１】本発明の第１実施形態を示す装置系統図である。
【図２】本発明の第２実施形態を示す装置系統図である。
【図３】本発明の第３実施形態を示す装置系統図である。
【図４】初沈汚泥の還流機構の変形例を示す模式図である。
【図５】初沈汚泥の還流機構の他の変形例を示す模式図である。
【図６】従来の基本的な活性汚泥処理法を示す装置系統図である。
【図７】従来のＡＯ法を示す装置系統図である。
【図８】従来のＡ２Ｏ法を示す装置系統図である。
【符号の説明】
１０……管路（被処理水の流入用）
１１……凝集剤
１２……最初沈殿池
１４……上澄水
１６……初沈汚泥
２０……曝気槽（好気槽）
２４……最終沈殿池
５２……酸発酵槽
６０……嫌気槽
７０……無酸素槽[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a wastewater treatment method, and more particularly, to a wastewater treatment method in which water to be treated containing organic matter and solid matter is first subjected to sedimentation treatment in a sedimentation basin, and then biologically treated in a biological reaction tank provided at a subsequent stage.
[0002]
[Prior art]
As conventional wastewater treatment methods of this type, those shown in FIGS. 6 to 8 are well known. FIG. 6 shows a basic activated sludge treatment method, in which water to be treated 1 containing organic matter and solid matter is first subjected to a sedimentation treatment in a sedimentation basin 2, and primary sludge 3 is separated. The water 4 from which solids have been removed is subjected to activated sludge treatment in the aeration tank 5, and organic matter is oxidatively decomposed. The sludge mixture 6 after the activated sludge treatment is subjected to a sedimentation treatment in a final sedimentation basin 7 and separated into a treated water 8 and a final settled sludge 9. Part of the final sludge 9 is returned to the aeration tank 5 as activated sludge 9A for biological treatment, and the remaining part is discharged out of the system as excess sludge 9B.
[0003]
The one shown in FIG. 7 is commonly called the AO method, and an anaerobic tank 4A is provided between the first sedimentation tank 2 and the aeration tank 5 in FIG. The activated sludge returned to the anaerobic tank 4A takes in organic substances under anaerobic conditions and releases excessively accumulated phosphorus. In the aeration tank 5, the activated sludge oxidizes and decomposes organic matter in the water to be treated under aerobic conditions, and also takes in excessively the phosphorus released in the anaerobic tank 4A and the phosphorus in the water to be treated. As a result, the excess sludge 9B is discharged out of the system in a state in which phosphorus is excessively taken in, so that organic matter and phosphorus in the water to be treated can be simultaneously removed.
[0004]
The one shown in FIG. 8 is commonly called the A2O method, and further includes an oxygen-free tank 4B between the anaerobic tank 4A and the aeration tank 5 in FIG. In this method, ammonia nitrogen in the water to be treated is nitrified under aerobic conditions in the aeration tank 5 to become nitrate nitrogen. By circulating the nitrification-treated water 5A containing nitrate nitrogen to the anoxic tank 4B in the preceding stage, the nitrate nitrogen becomes nitrogen gas in the anoxic tank 4B by a denitrification reaction. Therefore, according to the A2O method, in addition to the action of removing organic substances and phosphorus by the AO method, ammonia nitrogen can also be removed, so that organic substances, phosphorus, and nitrogen in the water to be treated can be removed at the same time.
[0005]
Regarding the conventional technique, there are mainly two problems when attention is paid to the initial settling sludge 3 generated in the first settling tank 2. First, the initial sludge 3 usually contains a considerable amount of organic matter and is easily rotted, so that it is troublesome to treat and dispose. Secondly, in the AO method and the A2O method, organic substances are required for phosphorus release in the anaerobic tank 4A and denitrification in the anoxic tank 4B. However, when the primary sludge 3 is first removed in the sedimentation basin 2, the SS organic matter is transferred to the primary sediment sludge 3, so that the organic matter may be insufficient and the phosphorus release or the denitrification reaction may not proceed sufficiently. In order to improve such a problem, various proposals have been made especially for the AO method and the A2O method. For example, Patent Literature 1 discloses a method in which a part of primary sludge separated first in a sedimentation basin is sent to an anaerobic tank to supply a shortage of organic matter. Patent Document 2 discloses a method in which a part of primary sludge separated first in a sedimentation basin is sent to an acid fermentation tank, and a soluble organic acid generated by acid fermentation is introduced into a subsequent biological reaction tank. I have.
[0006]
[Patent Document 1]
JP 2001-259682 A [Patent Document 2]
JP 2001-9498 A
[Problems to be solved by the invention]
However, in the method described in Patent Document 1, the SS organic matter in the replenished primary sludge is biologically more stable than the soluble organic matter, and is necessary for phosphorus release and denitrification. However, there is a problem that it is poor in immediate effect as an organic substance and not very effective as an organic substance to be used. Further, the method described in Patent Document 2 requires a new acid fermentation tank, and has a problem that its operation and control are complicated.
An object of the present invention is to solve the above-mentioned problems of the prior art, and to convert the SS organic matter in the water to be treated into a soluble organic acid by a simple method without installing an acid fermentation tank. It is an object of the present invention to provide a wastewater treatment method capable of facilitating the treatment and disposal of sludge and effectively using the obtained soluble organic acid for phosphorus release and denitrification.
[0008]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, the wastewater treatment method according to the present invention is to perform a biological treatment in a biological reaction tank provided in a subsequent stage, after first subjecting treated water containing organic matter and solid matter to a sedimentation treatment in a sedimentation tank. In the wastewater treatment method described above, the initial settling sludge separated in the first settling tank is refluxed in the first settling tank.
[0009]
Further, in the wastewater treatment method according to the present invention, after the water to be treated containing organic matter, solid matter, and phosphorus is first settled in a sedimentation basin, an anaerobic tank, an aerobic tank, and a final sedimentation basin are passed in that order, and the final sedimentation A wastewater treatment method in which the final settled sludge separated in the pond is returned to the anaerobic tank, wherein the initial settled sludge separated in the first settling tank is refluxed in the first settling tank.
[0010]
Further, in the wastewater treatment method according to the present invention, after the water to be treated containing organic matter, solid matter, phosphorus and ammonia nitrogen is first settled in a sedimentation tank, the anaerobic tank, the oxygen-free tank, and the aerobic tank are passed in this order. Biological treatment, and a part of the nitrification treatment water obtained in the aerobic tank is circulated to the anoxic tank, the remaining nitrification treatment water is subjected to precipitation treatment in the final sedimentation tank, the separated final sludge separated In the wastewater treatment method for returning the wastewater to the anaerobic tank, the primary sludge separated in the first sedimentation basin is refluxed in the first sedimentation basin.
[0011]
Further, in the wastewater treatment method according to the present invention, the means for refluxing the initial settled sludge is a means for joining the initially settled sludge drawn from the first settling tank with the water to be treated flowing into the first settling tank. After adding a coagulant to the combined water of the sludge and the water to be treated, the combined water is first introduced into a sedimentation basin.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is an apparatus system diagram showing a first embodiment of the present invention. To-be-treated water containing organic matter and solid matter is first supplied to the sedimentation basin 12 from the pipe 10. In the first sedimentation basin 12, the water to be treated undergoes a sedimentation treatment, and is separated into supernatant water 14 and primary sludge 16. The supernatant water 14, that is, the water to be treated from which solids have been removed, is introduced into an aeration tank 20 through a pipe 18. The aeration tank 20 is maintained under aerobic conditions by aeration from the aeration means 22. The water to be treated undergoes activated sludge treatment in the aeration tank 20, and organic substances in the water to be treated are oxidatively decomposed. The sludge mixture after the activated sludge treatment is subjected to a sedimentation treatment in the final sedimentation basin 24 and is separated into a supernatant water 26 and a final sedimentation sludge 28. The supernatant water 26 is discharged out of the system from the pipe 30 as treated water. Part of the final settling sludge 28 is returned to the aeration tank 20 from the pipeline 32 as activated sludge for biological treatment. Further, the remaining part of the final settled sludge 28 is discharged out of the system from the pipe 34 as surplus sludge.
[0013]
In the first sedimentation basin 12, the initial sedimentation sludge 16 stays at the lower part of the first sedimentation basin 12 for several hours and is then pulled out. The stagnant layer of the primary sludge 16 is placed under anaerobic conditions, and acid fermentation bacteria are breeding. Therefore, at least a part of the SS organic matter in the initial settled sludge 16 undergoes acid fermentation treatment by the action of the acid fermentation bacterium, and becomes a soluble organic acid. The generated organic acid partially diffuses and moves to the supernatant water 14 side. In addition, one end of a sludge recirculation pipe 40 provided with a recirculation pump 38 branches off into a sludge withdrawal pipe 36 connected to the lower part of the first settling basin 12, and the other end of the pipe 40 is connected to a pipe. It is connected to road 10. A part of the initial sludge 16 containing the organic acid extracted from the pipe 36 is combined with the water to be treated passing through the pipe 10 via the pipe 40 by the reflux pump 38, and is first returned to the sedimentation basin 12. The organic acid generated even in the refluxing process of the initial settling sludge 16 dissolves in the water to be treated and moves to the supernatant water 14 side. Further, since a large number of acid-fermenting bacteria are present in the recirculated initial sludge 16, the acid-fermentation of the initial sludge 16 that is repeatedly settled in the sedimentation basin 12 by the recirculation proceeds rapidly. The remaining portion of the initial sludge 16 drawn out from the pipe 36 is discharged out of the system as surplus initial sludge by operating the sludge drawing pump 42 intermittently.
[0014]
The residence time of the recirculated primary sludge in the primary sedimentation basin 12 can be arbitrarily adjusted by the operation interval of the sludge withdrawal pump 42. The operation interval of the sludge withdrawing pump 42 is set to a fixed interval operation, an intermittent operation every time the sludge interface in the first settling tank 12 reaches a predetermined level, or an organic matter in the supernatant water 14 in the first settling tank 12. Control can be performed by performing a method such as intermittent operation every time the concentration reaches a predetermined value, alone or in combination.
[0015]
In the present embodiment, it is preferable to add, for example, a polymer-based flocculant 11 to the combined water of the initial sludge and the water to be treated. The addition of the coagulant 11 enhances the sedimentation of solids. For this reason, the residence time of the initial settling sludge can be increased while keeping the sludge interface in the first settling tank 12 low, and the operation stability of the first settling tank 12 and the promotion of acid fermentation can be promoted. .
[0016]
As described above, according to the present embodiment, the SS organic matter in the initial settled sludge undergoes acid fermentation in the process of reflux and shifts to a soluble organic acid. Therefore, it is possible to reduce the volume of the initial sludge discharged outside the system. In addition, the initial sludge discharged to the outside of the system has a large relative ratio of inorganic solids and is hardly putrefactive, so that it is easy to treat and dispose. In addition, although the organic matter load of the to-be-processed water in the aeration tank 20 increases by migrating the SS organic substance to the soluble organic acid, the generated organic acid has a low molecular weight that is easily decomposed biologically. There are no special problems because there are many. Therefore, for example, the volume of the initial settled sludge can be easily reduced and reformed by adding the reflux mechanism of the initial settled sludge according to the present embodiment to the existing wastewater treatment equipment.
[0017]
FIG. 2 is an apparatus system diagram showing a second embodiment of the present invention. This embodiment is a case where the present invention is applied to the AO method, and an anaerobic tank 60 is provided between the first sedimentation tank 12 and the aeration tank 20 in FIG. In FIG. 2, elements denoted by the same reference numerals as those in FIG. 1 are elements substantially the same as or similar to those shown in FIG. 1, and a detailed description thereof will be omitted. First, water to be treated containing organic matter, solid matter and phosphorus is supplied to the sedimentation basin 12 and undergoes a sedimentation treatment. The primary sludge is returned to the primary sedimentation basin 12 via the pipe 40, and the SS organic matter becomes a soluble organic acid. The supernatant water 14 in which the organic acid is dissolved is introduced into the anaerobic tank 60 from the pipe 18 as the water to be treated. In the anaerobic tank 60, the water to be treated is mixed with the activated sludge returned from the pipe line 32, and the activated sludge takes in organic substances under anaerobic conditions and releases excessively accumulated phosphorus. At this time, the organic acid dissolved in the water to be treated is effectively used as a source of an organic substance necessary for releasing phosphorus.
[0018]
In the aeration tank 20, which is an aerobic tank, the activated sludge oxidizes and decomposes organic substances remaining in the water to be treated under aerobic conditions, and removes the phosphorus released in the anaerobic tank 60 and the phosphorus originally contained in the water to be treated. Take in too much. As a result, the excess sludge is discharged out of the system through the pipe 34 in a state where phosphorus is excessively taken in, and the organic matter and the phosphorus in the water to be treated can be simultaneously removed.
[0019]
FIG. 3 is an apparatus system diagram showing a third embodiment of the present invention. This embodiment is a case where the present invention is applied to the A2O method, and an oxygen-free tank 70 is further provided between the anaerobic tank 60 and the aeration tank 20 in FIG. In FIG. 3, elements denoted by the same reference numerals as those in FIG. 2 are elements substantially the same as or similar to those shown in FIG. 2, and a detailed description thereof will be omitted. First, water to be treated containing organic matter, solid matter, phosphorus, and ammonia nitrogen is supplied to the sedimentation basin 12, and undergoes sedimentation treatment. The primary sludge is returned to the primary sedimentation basin 12 via the pipe 40, and the SS organic matter becomes a soluble organic acid. The supernatant water 14 in which the organic acid is dissolved is introduced into the anaerobic tank 60 as water to be treated. In the anaerobic tank 60, the water to be treated is mixed with the activated sludge returned from the pipe line 32, and the activated sludge takes in organic substances under anaerobic conditions and releases excessively accumulated phosphorus. At this time, the organic acid dissolved in the water to be treated is effectively used as a source of an organic substance necessary for releasing phosphorus. The water to be treated, which has released phosphorus in the anaerobic tank 60, is introduced into the aeration tank 20 via the anoxic tank 70.
[0020]
In the aeration tank 20, which is an aerobic tank, activated sludge oxidizes and decomposes organic matter remaining in the water to be treated under aerobic conditions, and removes the phosphorus released in the anaerobic tank 60 and the phosphorus originally contained in the water to be treated. Take in too much. In addition, the nitrifying bacteria present in the activated sludge nitrify the ammonia nitrogen in the water to be treated under aerobic conditions into nitrate nitrogen. The nitrification-treated water containing nitrate nitrogen is circulated to the oxygen-free tank 70 via the pipe 72. In the anoxic tank 70, nitrate nitrogen is turned into nitrogen gas by denitrifying bacteria present in the activated sludge. This denitrification reaction also requires organic matter in the water to be treated, and the organic acid dissolved in the water to be treated is directly used as a source of organic matter necessary for the denitrification reaction, or an organic material source necessary for the denitrification reaction. Supplement indirectly. As a result, in addition to the action of removing organic substances and phosphorus by the AO method, ammonia nitrogen can also be removed.
[0021]
As described above, in the present invention to which the AO method or the A2O method is applied, the organic acid generated by refluxing the primary sludge in the primary sedimentation tank is effective as an organic substance necessary for phosphorus release and denitrification. Used for For this reason, in the case of treating the water to be treated having a relatively high concentration of phosphorus or ammonium nitrogen and a relatively low concentration of a soluble organic substance by the AO method or the A2O method, the present invention is intended to compensate for the shortage of the organic substance. Especially effective.
[0022]
FIG. 4 is a schematic diagram showing a modified example of the recirculation mechanism of the initial settled sludge. The pipe 36 is provided with a pump 44 which is also used for recirculating and extracting the initial sludge 16. An acid fermentation tank 52 is provided via a valve 46 in the middle of a pipe 50 for refluxing the initial sludge. After the primary sludge is retained in the acid fermentation tank 52 for a predetermined time, it is combined with the water to be treated and circulated. According to this method, generation of an organic acid can be further promoted. Since acid fermentation proceeds in the first settling basin, the acid fermentation tank 52 plays an auxiliary role, and its capacity may be small. Adjustment of the recirculation amount and the withdrawal amount of the initial sludge is performed by alternately switching the opening and closing of the valve 49 and the valve 46 provided in the pipe 48 for sludge withdrawal, or by adjusting the opening degrees of the valves 46 and 49. Can be done by doing
[0023]
FIG. 5 is a schematic view showing another modified example of the recirculation mechanism of the initial sludge. A paddle type stirrer 82 is provided in the first settling basin 80. By operating the stirrer 82 continuously or intermittently, the initial sludge is appropriately mixed and stirred. By this mixing and stirring, the acid-fermenting bacteria that have propagated in the stagnant layer of the initial settled sludge are evenly distributed up, down, left and right, and the acid fermentation of the SS organic matter proceeds efficiently in the entire stagnant layer. In this modification, the initial sludge is first internally refluxed in the sedimentation basin instead of the externally-recirculated mechanism of the initial sludge by the pump 38 shown in FIG. 1, and the same effect can be achieved. it can. The stirrer 82 is not limited to the paddle type, but may be an impeller type or a stirring pump type. The surplus initial sludge is withdrawn from a pipe 84 provided with a pump 83 and discharged out of the system. As in the case described with reference to FIG. 1, the amount of surplus primary sludge withdrawn is managed using, for example, the sludge interface in the initial sedimentation basin 80 as an index.
[0024]
【The invention's effect】
According to the present invention, primary sludge separated in the primary sedimentation basin is refluxed in the primary sedimentation basin, so that SS organic matter in the water to be treated undergoes acid fermentation in the course of the reflux to form a soluble organic acid. Transition. For this reason, the volume of primary sludge can be reduced only by the primary sedimentation basin without installing a special acid fermentation tank, etc., and the relative ratio of inorganic solids in the primary sedimentation sludge increases. ， Disposal becomes easy. When applied to the AO method or the A2O method, a soluble organic acid obtained by acid fermentation can be effectively used for phosphorus release and denitrification.
[Brief description of the drawings]
FIG. 1 is an apparatus system diagram showing a first embodiment of the present invention.
FIG. 2 is an apparatus system diagram showing a second embodiment of the present invention.
FIG. 3 is an apparatus system diagram showing a third embodiment of the present invention.
FIG. 4 is a schematic view showing a modified example of a recirculation mechanism for initial sludge.
FIG. 5 is a schematic view showing another modified example of the recirculation mechanism of the initial settled sludge.
FIG. 6 is an apparatus system diagram showing a conventional basic activated sludge treatment method.
FIG. 7 is a system diagram showing a conventional AO method.
FIG. 8 is an apparatus system diagram showing a conventional A2O method.
[Explanation of symbols]
10 Pipe line (for inflow of treated water)
11 ... flocculant 12 ... first sedimentation basin 14 ... supernatant water 16 ... first sedimentation sludge 20 ... aeration tank (aerobic tank)
24 Final sedimentation tank 52 Acid fermentation tank 60 Anaerobic tank 70 Oxygen-free tank
