JP3722686B2

JP3722686B2 - Organic wastewater treatment method and equipment

Info

Publication number: JP3722686B2
Application number: JP2000308728A
Authority: JP
Inventors: 栄小三田; 裕一府中; 尚広立石
Original assignee: Ebara Corp
Current assignee: Ebara Corp
Priority date: 2000-10-10
Filing date: 2000-10-10
Publication date: 2005-11-30
Anticipated expiration: 2020-10-10
Also published as: JP2002113478A

Description

【０００１】
【発明の属する技術分野】
本発明は、有機性排水の処理に係り、特に、下水、排水、汚濁の進んだ河川水、湖沼水等の有機性排水を生物学的に処理する有機性排水の処理方法と装置に関する。
【０００２】
【従来の技術】
従来の固定床式の下向流式好気性生物膜ろ過法においては、ろ材層は単一性状の粒状ろ材で構成されている。また、粒子径が異なる複数の粒状ろ材層を有する場合においては、粒状ろ材の真密度は水よりも大きく、或いは、上層粒状ろ材の粒子径は下層粒状ろ材の平均径よりも大きい。
生物膜ろ過法では、被処理水に含まれるＳＳが除去される。更に、被処理水に含まれる溶解性有機成分が生物膜により酸化される過程で、生物の増殖が生じて生物膜が増大する。この為、処理を継続するに従い、捕捉ＳＳと過剰生物膜によるろ材層の閉塞が生じ、ろ過抵抗が上昇する。この対策として、特定の周期で、或いは、ろ過抵抗が一定の値に達した時点で、ろ材層の洗浄を行い、捕捉ＳＳと過剰生物膜の排出を行うことにより、ろ過抵抗を回復させる。ろ材層の洗浄は、ろ材層下方から空気或いは水を注入することにより行う。
【０００３】
このような従来の処理法では、以下に示す問題があった。
ろ材層の閉塞は、被処理水に含まれるＳＳとＢＯＤの両者に起因し、どちらの場合もろ材層下部に比べてろ材層上部の閉塞が顕著であり、ろ材層上部の閉塞を防止して抵抗を低減することを目的として、ろ材層洗浄を行う。
被処理水に含まれるＳＳは、ろ材層上部で捕捉され、ろ材層下部に比べてろ材層上部の閉塞が進行する。この為、ろ材層全体が有効に利用されず、ろ材層上部のろ過抵抗が上昇した時点でろ材層洗浄を行う。
【０００４】
一方、生物によるＢＯＤ酸化分解は、一次反応で進行する。即ち、生物量が同一の場合、ＢＯＤ濃度が高いほど単位時間あたりに酸化分解されるＢＯＤ量は多い。この為、下向流式好気性生物膜ろ過法では、ＢＯＤ濃度が高いろ材層上部ほど、酸化分解量が多く、生物の増殖は顕著となる。この結果、ろ材層下部に比べてろ材層上部で、過剰生物膜による閉塞が進行する。この為、ろ材層全体が有効に利用されず、ろ材層上部のろ過抵抗が上昇した時点でろ材層洗浄を行う。
ところで、ろ材層が単一の粒状ろ材で構成されている場合、ろ材層洗浄時の粒状ろ材の流動状態は、ろ材層上部とろ材層下部で同様であり、閉塞の顕著なろ材層上部が、ろ材槽下部に比べて効率的に洗浄されることはない。
このように、ろ材層上部のろ過抵抗の上昇時には、ろ材層全体の洗浄が必要であり、洗浄頻度が高い場合には、洗浄排水発生量の増加と洗浄排水の処理装置への水量負荷の増加、更に水回収率の低下を招くことになる。
【０００５】
【発明が解決しようとする課題】
本発明では、上記従来技術の問題点を解消し、ろ材層上部を下部よりも効率よく洗浄することができ、更に、被処理水中のＳＳ捕捉及び生物膜の増殖による閉塞がろ材層上部に偏らず、ろ材層全体を有効に利用することでろ過抵抗の上昇を抑制し、ろ材層の洗浄頻度と洗浄排水発生量を減少することができる有機性排水の処理方法と装置を提供することを課題とする。
【０００６】
【課題を解決するための手段】
上記課題を解決する為に、本発明では、有機性排水を、好気性条件で浄化する固定床式の下向流式生物膜ろ過法による処理方法において、前記生物膜ろ過法は、ろ材層上方で且つ水面下に多孔部材を有し、該多孔部材の下方にろ材の真密度が異なる多層のろ材層を有し、最上層を構成するろ材層は、粒状ろ材の真密度を水よりも小さくし、それにより、最上層は、通水時には水面下に浮上固定ろ層を形成し、洗浄時には個々の粒状ろ材が独立に動き、更に、最上層のろ材層は、ろ材層の空隙の平均寸法が、下層のろ材層の空隙の平均寸法より大きく、かつ、ろ材層の空隙率が、下層の空隙率より大きいことを特徴とする有機性排水の処理方法としたものである。
また、本発明では、有機性排水を、好気性条件で浄化する固定床式の下向流式生物膜ろ過槽を有する処理装置において、前記生物膜ろ過層が、ろ材層上方で且つ水面下に多孔部材を配設し、該多孔部材の下方にろ材の真密度が異なる多層のろ材層を配し、最上層のろ材層は、真密度が水より小さい粒状ろ材で構成され、ろ材層の空隙の平均寸法及び空隙率が、下層のろ材層より大きく、より下層のろ材層は、真密度が最上層のろ材より大きい粒状ろ材で構成され、ろ材層の空隙の平均寸法及び空隙率が、最上層よりも小さく、更に、ろ材層下方には散気手段を有することを特徴とする有機性排水の処理装置としたものである。
【０００７】
【発明の実施の形態】
本発明は、生物膜ろ過槽において、ろ材層を多層構造とし、真密度が異なる粒状ろ材を用いており、最上層は、通水時には水面下で浮上固定層を形成することで生物膜ろ過機構によりＳＳとＢＯＤの低減に寄与し、洗浄時には粒状ろ材が独立に流動することで効率良い洗浄が行われる。最上層を水面下で浮上固定層とする為に、最上層の粒状ろ材の真密度を水よりも小さくすると共に、最上層上方で且つ水面下に多孔部材を配設している。
更に、最上層は空隙寸法が大きい粗粒層、下層は空隙寸法が小さい細粒層とし、被処理水を粗粒の層で処理した後に細粒の層で処理することで、ろ材層全体を有効に使ってＳＳ捕捉を行い、ろ過抵抗の上昇を抑制する。最上層の空隙寸法を下層のそれよりも大きくする手段としては、最上層粒状ろ材の粒径を下層粒状ろ材の粒径より大きくする等が有効である。
ここで、粒状ろ材の形状が同一として、粗粒層と細粒層を比較すると、粗粒層は細粒層に比べて比表面積が小さく、ろ材層容積あたりの生物膜量が少ない。最上層での生物膜による有機成分の酸化分解は、下層に比べて劣ることになる。
【０００８】
この欠点を補う為に、本発明では、最上層の空隙率を下層の空隙率よりも大きくする。これにより、ろ材層容積あたりの水理学的滞留時間を比較すると、最上層は下層よりも長く、最上層では生物膜量が少なくても良好な生物処理が行われる。また、空隙率を大きくすると、ろ材層に対する処理水の線流速が同じでも、空隙を流れる水の速度は低下する。この結果、空隙率の増大は、ろ過抵抗を下げる効果を併せ持つ。最上層の空隙率を下層のそれよりも大きくする手段としては、最上層粒状ろ材の形状を中空とする等が有効である。
このように、有機性排水を、粒状媒体を充填した槽内で、下向流により好気性生物膜ろ過処理する際に、ろ材層を粒状ろ材の真密度が異なる多層構造とし、最上層上方で且つ水面下に多孔部材を配設し、最上層を構成する粒状ろ材の真密度を水より小さくすることで、最上層は、通水時には水面下に浮上固定ろ層を形成し、洗浄時には、個々の粒状ろ材が独立に動く。最上層の空隙寸法は、下層の空隙寸法よりも大きく、且つ最上層の空隙率は、下層の空隙率よりも大きい、となるような寸法と形状の粒状ろ材を用いる。
【０００９】
ろ材層が閉塞した場合には、槽内の水を多孔部材上部まで一旦排出した後に、充填層下部から空気或いは空気と水を同時に噴射し、ろ材層が捕捉したＳＳと粒状ろ材に過剰に付着した生物膜の剥離を行う。その後に、水による逆洗浄を行い、剥離したＳＳと生物膜を槽外に排出する。この洗浄操作において、空気或いは空気と水の同時噴射を行うことにより、浮上固定ろ層を形成していた最上層の粒状ろ材が独立に動く。この時に粒状ろ材同士が衝突することで、捕捉ＳＳと過剰生物膜の効率良い剥離が行われる。
また、より効果的な洗浄方法として、次の操作も可能である。即ち、洗浄前の水位を最下層上端まで下げた後に、充填層下部から空気と水を同時に噴射し、洗浄中の槽内水位を上方ろ材層の下端から上端へゆるやかに移動させる。この洗浄方法では、水面での発泡作用による洗浄効果をろ材層全体に与えることになり、捕捉ＳＳと過剰生物膜のより効率的な洗浄が可能である。
【００１０】
次に、本発明を図面を用いて詳細に説明する。
図１は、本発明の有機性排水の処理装置の一例を示す概略構成図であり、図１ではろ材層が２層のものを示しているが、３層以上の多層のものにも同様に適用可能である。
図１において、下水などの有機性排水である被処理水１１は、生物膜ろ過槽である処理槽１の上部から入り、生物膜処理された処理水１２は、処理槽下部から流出する。
処理槽１には、水よりも真密度が小さい粒状ろ材４から成るろ材層３と、水よりも真密度が大きい粒状ろ材７から成るろ材層６が充填され、ろ材層３上部に多孔部材２を配設することで、ろ材層３は水面下に浮上固定ろ層を形成している。洗浄時に粒状ろ材４が流動可能なように、ろ材層３とろ材層６の間には空間部５を設けている。
また、ろ材層６の下層には支持層８が配され、支持層８には散気管９が設けられており、その散気管９の下方に集水管１０が配されており、処理水１２が抜き出される。
【００１１】
【実施例】
以下、本発明を実施例により具体的に説明する。
実施例１
図１の装置を用いて、排水を処理した。
用いた粒状ろ材４と粒状ろ材７の材質等の例を表１に示す。処理条件は表２に示す通りである。なお、従来法は、ろ材として粒状ろ材７を一層で用いたものである。
【表１】

【００１２】
【表２】

【００１３】
その結果を、本発明による有機排水処理時の処理生能と、従来法の性能とを比較して表３に示す。
処理水水質が同等である一方、ろ層当たりのＳＳ捕捉量は、本発明の方が、約２．３倍に向上している。
【表３】

【００１４】
【発明の効果】
本発明によれば、従来法に比較して以下の効果を奏することができる。
（１）有機物除去能力が高い。
形状が同一の場合、粒径の大きな粒状ろ材を用いると比表面積は小さくなる。しかし、空隙率を大きくすることで、滞留時間が長くなり、空隙寸法を大きくすることでろ材の単位面積当たりの生物量が多くなり、有機物除去能力が高くなる。
（２）ＳＳ捕捉量が高い。
空隙率と空隙寸法の大きいろ材により被処理水中のＳＳを除去する為、ＳＳの除去により生じるろ過抵抗の上昇が少ない。よって、逆洗を要するまでろ過抵抗が上昇する間のＳＳ捕捉量が高くなる。
これらの効果から、被処理水中のＳＳ捕捉によるろ過抵抗上昇と、ＢＯＤの分解によって生じる過剰生物膜によるろ過抵抗上昇を、低く抑えることができる。この為、ろ材層容積あたりの捕捉ＳＳ量が大きく、ろ材層の洗浄頻度を低くすることが可能であり、故に、ろ材層洗浄により発生する排水量の低減と水回収率の向上が可能である。
【図面の簡単な説明】
【図１】本発明の処理装置の一例を示す概略構成図。
【符号の説明】
１：処理槽、２：多孔部材、３：ろ材層、４：ろ材、５：空間部、６：ろ材層、７：ろ材、８：支持層、９：散気管、１０：集水管、１１：被処理水、１２：処理水、１３：空気[0001]
BACKGROUND OF THE INVENTION
The present invention relates to organic wastewater treatment, and more particularly, to an organic wastewater treatment method and apparatus for biologically treating organic wastewater such as sewage, wastewater, polluted river water and lake water.
[0002]
[Prior art]
In the conventional fixed-bed downflow aerobic biofilm filtration method, the filter medium layer is composed of a single granular filter medium. In the case of having a plurality of granular filter media layers having different particle diameters, the true density of the granular filter media is larger than that of water, or the particle diameter of the upper granular filter media is larger than the average diameter of the lower granular filter media.
In the biofilm filtration method, SS contained in the water to be treated is removed. Furthermore, in the process in which the soluble organic component contained in the water to be treated is oxidized by the biofilm, the organism grows and the biofilm increases. For this reason, as the treatment is continued, the filter medium layer is blocked by the trapped SS and the excess biofilm, and the filtration resistance increases. As a countermeasure, the filtration resistance is recovered by washing the filter medium layer at a specific cycle or when the filtration resistance reaches a certain value, and discharging the trapped SS and the excess biofilm. The filter medium layer is washed by injecting air or water from below the filter medium layer.
[0003]
Such a conventional processing method has the following problems.
The clogging of the filter medium layer is caused by both SS and BOD contained in the water to be treated. In both cases, the clogging of the upper part of the filter medium layer is remarkable compared to the lower part of the filter medium layer, and the clogging of the upper part of the filter medium layer is prevented. In order to reduce the resistance, the filter medium layer is washed.
SS contained in the water to be treated is captured at the upper part of the filter medium layer, and the upper part of the filter medium layer is blocked as compared with the lower part of the filter medium layer. For this reason, the entire filter medium layer is not effectively used, and the filter medium layer is washed when the filtration resistance at the upper part of the filter medium layer is increased.
[0004]
On the other hand, BOD oxidative degradation by living organisms proceeds in a primary reaction. That is, when the biomass is the same, the higher the BOD concentration, the more BOD is oxidatively decomposed per unit time. For this reason, in the downflow aerobic biofilm filtration method, the upper part of the filter medium layer having a higher BOD concentration, the greater the amount of oxidative degradation, and the more the organism grows. As a result, the clogging with the excess biofilm proceeds in the upper part of the filter medium layer compared to the lower part of the filter medium layer. For this reason, the entire filter medium layer is not effectively used, and the filter medium layer is washed when the filtration resistance at the upper part of the filter medium layer is increased.
By the way, when the filter medium layer is composed of a single granular filter medium, the flow state of the granular filter medium at the time of cleaning the filter medium layer is the same at the upper part of the filter medium layer and the lower part of the filter medium layer, It is not cleaned more efficiently than the lower part of the filter medium tank.
Thus, when the filtration resistance at the top of the filter media layer increases, the entire filter media layer needs to be washed. If the frequency of washing is high, the amount of washing wastewater generated will increase and the load on the wastewater treatment equipment will increase. In addition, the water recovery rate is further reduced.
[0005]
[Problems to be solved by the invention]
In the present invention, the above-mentioned problems of the prior art can be solved, and the upper part of the filter medium layer can be washed more efficiently than the lower part. Further, the clogging due to SS trapping and biofilm growth in the treated water is biased to the upper part of the filter medium layer. It is an object to provide an organic wastewater treatment method and apparatus that can suppress the increase in filtration resistance by effectively using the entire filter medium layer and reduce the frequency of washing the filter medium layer and the amount of generated waste water. And
[0006]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, in the present invention, in the treatment method using a fixed bed type downflow biofilm filtration method for purifying organic wastewater under an aerobic condition, the biofilm filtration method is performed above the filter medium layer. In addition, a porous member is provided below the water surface, a multilayer filter medium layer having different true densities of the filter medium is provided below the porous member, and the filter medium layer constituting the uppermost layer has a true density of the granular filter medium smaller than that of water. and, thereby, the top layer, at the time of passing water to form a floating fixed filtration layer under water, dynamic-out to separate the individual particulate filtration media at the time of washing, further, the top layer of the filter medium layer has an average void filter media layer The organic wastewater treatment method is characterized in that the size is larger than the average size of the voids of the lower filter medium layer, and the porosity of the filter medium layer is larger than the porosity of the lower layer .
Also, in the present invention, the organic waste water, in the processing apparatus having a downflow-type biofilm filtration tank of fixed bed to clean under aerobic conditions, the biofilm filtration layer, and underwater in filter medium layer above A multi-layer filter medium layer having a different true density of the filter medium is disposed below the porous member, and the uppermost filter medium layer is composed of granular filter medium having a true density smaller than water . The average dimension and porosity of the voids are larger than the lower filter medium layer, and the lower filter medium layer is composed of granular filter medium having a true density larger than that of the uppermost filter medium, and the average dimension and void ratio of the voids of the filter medium layer are The organic waste water treatment apparatus is characterized in that it is smaller than the uppermost layer and further has an air diffuser below the filter medium layer.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
In the biofilm filtration tank, the present invention uses a granular filter medium having a multilayer structure and a different true density, and the uppermost layer is a biofilm filtration mechanism by forming a floating fixed layer below the surface of the water when water flows. This contributes to the reduction of SS and BOD, and the granular filter medium flows independently during cleaning, so that efficient cleaning is performed. In order to make the uppermost layer a floating fixed layer below the water surface, the true density of the granular filter medium of the uppermost layer is made smaller than that of water, and a porous member is disposed above the lower layer and below the water surface.
Furthermore, the uppermost layer is a coarse-grained layer with a large void size, the lower layer is a fine-grained layer with a small void size, and the water to be treated is treated with a coarse-grained layer and then treated with a fine-grained layer. Effectively captures SS and suppresses increase in filtration resistance. As a means for increasing the void size of the uppermost layer larger than that of the lower layer, it is effective to make the particle size of the uppermost granular filter medium larger than the particle size of the lower granular filter medium.
Here, when the shape of the granular filter medium is the same and the coarse particle layer and the fine particle layer are compared, the coarse particle layer has a smaller specific surface area than the fine particle layer, and the amount of biofilm per filter medium layer volume is small. Oxidative decomposition of organic components by the biofilm in the uppermost layer is inferior to the lower layer.
[0008]
In order to compensate for this drawback, in the present invention, the porosity of the uppermost layer is made larger than the porosity of the lower layer. Thereby, when the hydraulic residence time per volume of the filter medium layer is compared, the uppermost layer is longer than the lower layer, and the uppermost layer performs good biological treatment even if the amount of biofilm is small. Further, when the porosity is increased, the speed of water flowing through the voids is decreased even if the linear flow velocity of the treated water with respect to the filter medium layer is the same. As a result, the increase in porosity has the effect of reducing the filtration resistance. As a means for making the porosity of the uppermost layer larger than that of the lower layer, it is effective to make the shape of the uppermost granular filter medium hollow.
Thus, when organic wastewater is subjected to an aerobic biofilm filtration process by downward flow in a tank filled with a granular medium, the filter medium layer has a multilayer structure in which the true density of the granular filter medium is different, and above the uppermost layer. And by disposing a porous member under the water surface and making the true density of the granular filter medium constituting the uppermost layer smaller than water, the uppermost layer forms a floating fixed filter layer under the water surface when water flows, Individual granular filter media move independently. A granular filter medium having a size and shape such that the void size of the uppermost layer is larger than the void size of the lower layer and the porosity of the uppermost layer is larger than the void rate of the lower layer.
[0009]
When the filter medium layer is clogged, the water in the tank is once discharged to the upper part of the porous member, and then air or air and water are injected simultaneously from the lower part of the packed bed, and excessively adhere to the SS and the granular filter medium captured by the filter medium layer. The biofilm is peeled off. Thereafter, reverse cleaning with water is performed, and the peeled SS and biofilm are discharged out of the tank. In this washing operation, by performing simultaneous injection of air or air and water, the uppermost granular filter medium forming the floating fixed filter layer moves independently. At this time, the particulate filter media collide with each other, so that the captured SS and the excess biofilm are efficiently separated.
Moreover, the following operation is also possible as a more effective cleaning method. That is, after the water level before washing is lowered to the uppermost lowermost layer, air and water are simultaneously jetted from the lower part of the packed bed, and the water level in the tank being washed is gradually moved from the lower end to the upper end of the upper filter medium layer. In this cleaning method, a cleaning effect by foaming action on the water surface is given to the entire filter medium layer, and the trapping SS and excess biofilm can be cleaned more efficiently.
[0010]
Next, the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic configuration diagram showing an example of an organic wastewater treatment apparatus according to the present invention. FIG. 1 shows a filter medium layer having two layers, but the same applies to a multilayer of three or more layers. Applicable.
In FIG. 1, the to-be-processed water 11 which is organic waste_water | drains, such as sewage, enters from the upper part of the processing tank 1 which is a biofilm filtration tank, and the treated water 12 by which the biofilm process was carried out flows out from the lower part of a processing tank.
The treatment tank 1 is filled with a filter medium layer 3 made of a granular filter medium 4 having a true density smaller than that of water and a filter medium layer 6 made of a granular filter medium 7 having a true density larger than that of water. The filter medium layer 3 forms a floating fixed filter layer below the water surface. A space 5 is provided between the filter medium layer 3 and the filter medium layer 6 so that the granular filter medium 4 can flow during washing.
Further, a support layer 8 is disposed below the filter medium layer 6, and a diffusion tube 9 is provided on the support layer 8. A water collection tube 10 is disposed below the diffusion tube 9, and treated water 12 Extracted.
[0011]
【Example】
Hereinafter, the present invention will be specifically described by way of examples.
Example 1
Waste water was treated using the apparatus of FIG.
Table 1 shows examples of the material of the granular filter medium 4 and the granular filter medium 7 used. The processing conditions are as shown in Table 2. Note that the conventional method uses a granular filter medium 7 as a filter medium.
[Table 1]

[0012]
[Table 2]

[0013]
The results are shown in Table 3 comparing the processing viability during the organic wastewater treatment according to the present invention and the performance of the conventional method.
While the treated water quality is the same, the amount of SS trapped per filter bed is improved about 2.3 times in the present invention.
[Table 3]

[0014]
【The invention's effect】
According to the present invention, the following effects can be obtained as compared with the conventional method.
(1) High organic substance removal ability.
When the shape is the same, the specific surface area becomes small when a granular filter medium having a large particle size is used. However, increasing the porosity increases the residence time, and increasing the pore size increases the amount of organisms per unit area of the filter medium and increases the organic matter removal ability.
(2) SS capture amount is high.
Since SS in the water to be treated is removed by a filter medium having a large porosity and void size, the increase in filtration resistance caused by the removal of SS is small. Therefore, the amount of trapped SS increases while the filtration resistance increases until backwashing is required.
From these effects, the increase in filtration resistance due to the capture of SS in the water to be treated and the increase in filtration resistance due to excess biofilm caused by the decomposition of BOD can be kept low. For this reason, the amount of captured SS per volume of the filter medium layer is large, and the frequency of cleaning the filter medium layer can be reduced. Therefore, the amount of waste water generated by the filter medium layer cleaning can be reduced and the water recovery rate can be improved.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing an example of a processing apparatus of the present invention.
[Explanation of symbols]
1: treatment tank, 2: porous member, 3: filter medium layer, 4: filter medium, 5: space, 6: filter medium layer, 7: filter medium, 8: support layer, 9: air diffuser, 10: water collecting pipe, 11: Water to be treated, 12: treated water, 13: air

Claims

In the treatment method by a fixed bed type downflow biofilm filtration method for purifying organic wastewater under aerobic conditions, the biofilm filtration method has a porous member above the filter medium layer and below the water surface, The filter medium layer having a multilayer filter medium having different true densities of the filter medium is provided below the porous member, and the filter medium layer constituting the uppermost layer makes the true density of the granular filter medium smaller than that of water. forming a floating fixed filtration layer under the water surface, when flushing-out movement independent individual granular filter material, further, the top layer of the filter medium layer has an average dimension of the voids of the filter medium layer has an average dimension of the voids of the lower layer of the filter material layer A method for treating organic wastewater, which is larger and the porosity of the filter medium layer is larger than the porosity of the lower layer .

In a treatment apparatus having a fixed bed type downflow biofilm filtration tank for purifying organic wastewater under aerobic conditions, the biofilm filtration layer has a porous member disposed above the filter medium layer and below the water surface. A multilayer filter medium layer having a different true density of the filter medium is disposed below the porous member, and the uppermost filter medium layer is composed of granular filter medium having a true density smaller than water, and the average size and porosity of the voids of the filter medium layer Is larger than the lower filter medium layer, the lower filter medium layer is composed of granular filter medium having a true density larger than that of the uppermost filter medium, and the average size and porosity of the voids of the filter medium layer are smaller than that of the uppermost filter layer. An organic wastewater treatment apparatus having a diffuser means below the filter medium layer.