JP2008253994A - Method for biologically treating organic wastewater - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for biologically treating organic wastewater capable of allowing the discharged amount of sludge to approach zero by digesting the highly concentrated sludge employing a small-sized apparatus. <P>SOLUTION: Raw wastewater 2 is introduced into an aeration tank 1 for the biological treatment. The wastewater biologically treated 9 is separated by a membrane separator 7 to discharge a permeating solution as treated water, and a part of a concentrated solution 12 is returned to the aeration tank 1 as a return solution 3. The other part of the concentrated solution 12 is introduced as excess sludge into an aerobic digestion tank 21 having an immersion-type membrane separator 22 and digested by diffused air from an air diffuser 27. The solution in the tank is separated by the immersion-type membrane separator 22, and a permeating solution is discharged as treated water. A part 32 of aerobically digested solution is treated with ozone in an ozone treatment tank 31 to modify the sludge therein, which is circulated to the aerobic digestion tank 21. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  INDUSTRIAL APPLICABILITY The present invention is suitably used for a biological treatment method of organic effluent, in which organic effluent is biologically treated, and excess sludge is reformed and aerobic digested to reduce the volume, particularly biological nitrogen removal. The present invention relates to a method for biological treatment of organic effluents.

  In the method of aerobically biologically treating organic wastewater in the presence of activated sludge, a large amount of hardly activated dehydrated excess activated sludge is produced. In addition, a large amount of excess digested sludge is also produced by a method of anaerobically treating in the presence of anaerobic sludge. In order to reduce the volume of such excess sludge, a method of aerobically or anaerobically digesting excess sludge is performed.

JP-A-8-299995 describes an organic wastewater treatment method in which surplus sludge is introduced into a sludge digestion tank separate from a biological treatment tank, and aerobic digestion is performed. FIG. In the figure, 101 is a biological treatment tank, 102 is a solid / liquid separation tank, 103 is a sludge digestion tank, 104 is a solid / liquid separation tank, 105 is an ozone treatment tank, and 106 is an ozone generator.
The biological treatment tank 101 has an air diffuser 107 inside, and is configured to aerobically biologically treat organic drainage. The sludge digestion tank 103 also has an air diffuser 108 inside, and is configured to aerobically digest excess sludge.

  In the organic waste liquid treatment method using the above-described apparatus, the waste liquid 111 and the return sludge 112 are introduced and mixed in the biological treatment tank 101 as a biological treatment process, and then diffused from the air diffuser 107 to be aerobically biological. Process. A part 113 of the mixed liquid is extracted into the solid-liquid separation tank 102 and subjected to solid-liquid separation, and the separated liquid is discharged as the processing liquid 114. A part of the separated sludge 115 is returned to the biological treatment tank 101 as a return sludge 112, and the remaining part is sent to the excess sludge 116 and the sludge digestion tank 103.

  In the sludge digestion step, surplus sludge 116 and circulating sludge 117 are introduced and mixed in the sludge digestion tank 103, and aerobic digestion is performed by aeration from the aeration device 108. A part of the digested liquid 118 is introduced into the solid-liquid separation tank 104 and separated into solid and liquid, and the separated liquid 119 is discharged. The separated digested sludge 121 is circulated in the sludge digesting tank 103 as a part of the circulating sludge 117.

  In the ozone treatment process, the other part of the digested sludge 121 is introduced into the ozone treatment tank 105 as a drawn sludge 122, and ozone treatment is performed by blowing ozone gas 123 from the ozone generator 106. It is circulated and used for digestion. If necessary, another part of the digested sludge 121 is discharged as discharged sludge 125 in order to prevent an increase in the inorganic substance concentration.

  In the above treatment, a part of the digested sludge 121 is ozone-treated as the extracted sludge 122 and returned to the sludge digestion tank 103, whereby the ozone-treated sludge 124 is digested as a substrate and reduced in volume.

However, in the above conventional method, the volume of sludge can be reduced. However, when the solid-liquid separation of the digested liquid 118 is performed by gravity sedimentation, the solid-liquid separation is difficult if the sludge concentration is high. The sludge concentration is about 5000 mg / l, which is a practical limit. Therefore, digestion must be performed in a state where the sludge concentration is low. As a result, the volume of the sludge digestion tank 103 is increased, which increases the construction cost and increases the installation area.
In particular, when trying to mineralize almost all of the sludge by aerobic digestion so that excess sludge is not discharged out of the system as waste, the volume of the sludge digestion tank 103 is the same as that of the biological treatment tank 101 for biological treatment, There was a problem of the above volume. Therefore, the sludge digestion tank 103 required for reducing the volume of excess sludge to a high level is an apparatus having a size nearly twice that of the conventional apparatus and is not practical.

  On the other hand, a method has also been proposed in which surplus sludge is readily biodegradable (substrate) using ozone, etc., and recycled to the main body of the biological treatment process to reduce the amount of surplus sludge generated and, in some cases, to reduce the generated sludge amount to zero. (For example, JP-A-6-206088). This method solves the above-mentioned problem by sharing a digestion tank that should be separately required with the biological treatment process main body.

  However, this method also has the problems that the excess sludge is easily biodegraded and becomes a burden on the biological treatment process body, so that the sludge concentration becomes high, the sludge retention time (SRT) becomes short, and the treated water quality deteriorates. is there. Therefore, this method is limited to devices having sufficient load and sludge concentration, for example, devices having a BOD-sludge load of 0.2 kg / kgSS / d or less and devices having an MLSS of 3000 mg / l or less.

Further, when the biological treatment process main body is a device that performs nitrification, it is important to maintain the SRT at a certain value or more, not simply determining the volume of the nitrification tank based on the tank load or sludge load. That is, when the surplus sludge generation amount (= sludge extraction amount) is X [kg / d] and SRT is θ [day], the sludge amount retained in the nitrification tank is X × θ [kg] or more. There is a need. The value of θ is usually about 7 to 15 days, although it varies depending on the treated water temperature and the margin rate. This is because the growth of nitrifying bacteria that perform the nitrification reaction is slow, and if the amount of excess sludge drawn is too large, the growth of nitrifying bacteria cannot catch up and a sufficient amount of nitrifying bacteria cannot be retained in the nitrifying tank.
Here, if the sludge concentration in the nitrification tank is M [mg / l] = 10 ⁻³ M [kg / m ³ ], the required nitrification tank volume is Xθ / 10 ⁻³ M [m ³ ].

  Also, in the method of easily degrading surplus sludge as described above and circulating it to the main body of the biological treatment process, a part of the easily degradable sludge becomes surplus sludge again, so that the amount of surplus sludge generated is easily biodegradable. There is a need to. Usually, when excess surplus sludge of X [kg / d] is readily biodegradable by, for example, ozone treatment and circulated to the main body of the biological treatment process, about 0.33X [kg / d] sludge is mineralized and disappears, and the remaining 0 .67X [kg / d] is a new excess sludge. Therefore, in order to reduce the generation amount of excess sludge to zero, if 3X [kg] is easily decomposed and circulated to the biological treatment process body, the amount of excess sludge to be mineralized is 3 × 0.33X [kg. / D] ≈X [kg / d], and surplus sludge becomes zero.

At this time, microorganisms in 3X [kg / d] sludge that are biodegradable by ozone etc. are almost completely killed and lose their activity, so 3X [kg / d] of excess sludge is extracted for the microorganisms. The same effect as that. Since almost all of the nitrifying bacteria in 3X [kg / d] are also killed, if the SRT of the nitrifying tank is not set assuming that 3X [kg / d] is the amount of sludge extraction, the growth of the nitrifying bacteria cannot catch up, A sufficient amount of nitrifying bacteria cannot be retained in the nitrification tank. That is, the amount of sludge that must be retained in the nitrification tank is (3X) × θ [kg] or more, and the required nitrification tank volume is (3X) θ / 10 ⁻³ M [m ³ ], that is, volume reduction is performed. Three times as much as there is no.

  Therefore, in the case of an aeration tank in which nitrification is performed, it is more efficient to reduce the excess sludge in a separate aerobic nitrification tank as disclosed in JP-A-8-299995. However, even in this case, the volume of the aerobic nitrification tank is equal to or more than that of the biological treatment process main body as described above.

  In particular, if the biological treatment process main body is a membrane activated sludge method that uses a membrane such as an ultrafiltration membrane or a microfiltration membrane for solid-liquid separation of treated water and activated sludge, the biological treatment tank has a sludge concentration. The biological treatment process is originally miniaturized because it can be kept high and consequently the load per volume can be taken high. When the total amount of excess sludge discharged from such a biological treatment process is to be reduced in an aerobic digestion tank as in the conventional method, the volume of the aerobic digestion tank is 2 to 2 compared to the aeration tank of the biological treatment process main body. Since it becomes 3 times, it becomes less practical.

  In addition, since the sludge concentration is originally high (for example, 10,000 mg / l) and is a highly viscous liquid, even if the sludge concentration is reduced to a concentration that allows precipitation separation by aerobic digestion (for example, 5000 mg / l), Since the viscosity is high and precipitation separation is difficult, it is necessary to use a precipitation tank having a large area. Accordingly, the digestion process is further enlarged, the construction cost is increased, the installation area is increased, and the practicality is lowered.

  Further, surplus sludge is not always generated continuously, and the amount of surplus sludge generated in the biological treatment process is reduced when the load of raw biological treatment water is low. In such a case, if the sludge in the aerobic digester is sent to the reforming process, and it is recirculated in the digester again after the reforming process to biodegradability, the sludge concentration in the aerobic digester Decreases rapidly, and the ability to digest sludge decreases rapidly. In order to avoid this, it is necessary to reduce the amount of reforming sludge by reducing the amount of circulation to the reforming process, etc., but since this reduces the substrate given to the sludge in the digester, so-called empty The floc is dismantled in the aerated state and becomes sludge with poor sedimentation. As a result, if solid-liquid separation cannot be completed in the sedimentation tank and flocs of digested sludge flow out of the system, the sludge concentration in the aerobic digester also decreases, and the ability to digest sludge decreases.

In this way, even if the sludge concentration in the aerobic digestion process decreases, if the sludge from the biological treatment process is introduced as seed sludge, aerobic digestion can be performed again, but aerobic digestion is required until the sludge is acclimatized. There are problems such as poor efficiency, sludge is not easily reduced in volume, and foaming becomes intense. To avoid this, it is important not to excessively reduce the sludge in the aerobic digestion tank. For this reason, when the load is reduced, the sludge digestion tank should not be over-aerated and not excessively reduced in volume. Need to maintain.
JP-A-8-299995 JP-A-6-206088

  The object of the present invention is to solve the above-mentioned conventional problems, without reducing the quality of the treated water in the biological treatment process, and without increasing the size of the aerobic digestion process. It is to propose a method for treating organic waste liquid that can be reduced in size, and that the amount of discharged sludge can be brought close to zero, and that can be easily maintained at low load.

The present invention is the following organic drainage treatment method.
(1) a biological treatment process for biologically treating organic effluent,
A reforming process for reforming surplus sludge and / or its aerobic digested liquid discharged from the biological processing process to be readily biodegradable;
Aerobic digestion process in which surplus sludge discharged from biological treatment process and / or its reforming treatment liquid is introduced into aerobic digestion tank and aerobic digestion is performed. Organic drainage, including a circulating process that circulates, and an aerobic digestion solution that separates the aerobic digestion solution by membrane separation, returns the concentrated solution to the aerobic digestion step, and discharges the permeate as digested water Biological treatment method of liquid.
(2) The method according to (1) above, wherein the biological treatment step is a biological treatment step including a nitrification step and a denitrification step.
(3) The method according to (1) or (2) above, wherein the biological treatment step performs membrane separation as a solid-liquid separation means.
(4) The method according to any one of (1) to (3), wherein the aerobic digestion step comprises aeration of at least a part with oxygen-enriched air.

  The organic drainage to be treated in the present invention is a drainage containing organic matter, ammoniacal nitrogen compound, organic nitrogen compound, nitrate nitrogen, nitrite nitrogen, etc. that are treated by biological treatment. A non-biodegradable organic substance or inorganic substance may be contained. Such organic effluents include sewage, human waste, landfill leachate, food factory effluent, and other industrial effluents.

  The biological treatment process for biologically treating such organic drainage is an aerobic biological treatment, an anaerobic biological treatment, or a biological treatment combining these. Examples of the aerobic biological treatment include an activated sludge method and a biofilm method. The activated sludge method is an aerobic biological treatment method for treating organic wastewater in the presence of activated sludge. Organic wastewater is mixed with activated sludge in a biological treatment tank (aeration tank) and aerated. A standard activated sludge method is generally used in which solid-liquid separation is performed with a solid-liquid separator and a part of the separated sludge is returned to the aeration tank. However, other treatment methods obtained by modifying this method may be used. The biofilm method is a treatment in which a biofilm is formed on a carrier and contacted with drainage under aerobic conditions. Examples of the anaerobic treatment include an anaerobic digestion method and a high-load anaerobic treatment method. As the solid-liquid separation device, any solid-liquid separation means such as sedimentation separation, filtration, and membrane separation is used. In particular, a membrane activated sludge method in which membrane separation means is provided as a solid-liquid separation device is preferred.

  The biological treatment process includes a general aerobic treatment and / or anaerobic treatment, as well as a nitrification step in which ammonia nitrogen is nitrified (oxidized) into nitric acid or nitrite nitrogen under aerobic conditions by nitrifying bacteria, And a biological treatment process including a denitrification process in which nitric acid or nitrite nitrogen is reduced under anaerobic conditions by denitrifying bacteria. In particular, the organic effluent and the nitrification liquid returned from the nitrification process are mixed, and the denitrification process in which denitrification is performed while maintaining an anaerobic state, the denitrification liquid of this denitrification process is introduced into the nitrification tank, and nitrification is performed. A nitrification process in which nitrification is carried out by mixing with biological sludge containing bacteria, a nitrification liquid separation process in which the nitrification liquid is membrane-separated by membrane separation means and the permeate is discharged as treated water, and a nitrification liquid containing concentrated sludge is denitrified. A biological treatment process including a return process for returning to the process is preferable, but not limited thereto.

When performing more advanced nitrogen removal, after introducing the nitrification solution of the nitrification tank into the second denitrification tank and performing denitrification, introduce the submerged membrane separation means into the membrane separation tank provided in the tank. A method is preferred in which re-aeration is performed, activated sludge is separated into membranes, the permeate is discharged as treated water, and at least a part of the sludge in the membrane separation tank is returned to the denitrification tank.
In addition, after the nitrification and denitrification processes are performed by the intermittent aeration method in which the nitrification process and the denitrification process are alternately performed in one tank without separately providing the nitrification tank and the denitrification tank in this way, the intermittent aeration treatment liquid May be membrane separated.

  When the biological treatment process is a biological treatment process that includes a nitrification process and a denitrification process, if the modified treatment liquid is treated in the nitrification and denitrification processes, the burden on the biological treatment process is greatly increased and the generation of modified excess sludge The amount increases about 3 times. For this reason, if the treatment liquid is treated in the nitrification and denitrification process, the burden on the biological treatment process is greatly increased, and the nitrification tank needs to be about three times larger to ensure an SRT above a certain value. Since the method of the invention digests surplus sludge in a system separate from the biological treatment process, it does not affect the nitrification and denitrification processes at all, does not cause deterioration of the treated water, and is preferable under optimum conditions. Temper digestion can be performed.

In the case of using a membrane separation device as a solid-liquid separation means in the biological treatment process, a conventionally known membrane separation device can be used.
As a membrane separator, use a pump-circulating cross-flow filtration type membrane separator that circulates activated sludge at a high flow rate on one side of the membrane surface using a pump or other liquid feeding means, and takes out the filtrate from the other side. Can do. As the driving pressure for filtration, the pressure of the circulating pump may be used, or the permeate side of the filtrate may be sucked.

Further, as another membrane separation means, the membrane is immersed in the membrane immersion tank, and a circulating water flow caused by aeration caused by the air diffuser provided in the tank is applied to the membrane surface. An immersion type membrane separator that performs filtration while preventing the concentration of suspended substances can be used. As the driving pressure for filtration, the inside of the tank may be pressurized by using a membrane immersion tank as a sealed type, or the filtrate side may be sucked. The suction means may be a pump or a so-called gravity filtration that only discharges the filtrate that has oozed out due to the water pressure in the membrane immersion tank. The membrane immersion tank is generally provided with a partition wall such as a baffle plate to distinguish between the upflow caused by the aerated water flow and the lower part of the non-aerated portion (the upflow and the downward flow are collectively called swirl flow). Usually, a membrane is installed in the upward flow.
Both the cross flow filtration method and the immersion membrane method can be used, but the immersion membrane is preferable from the viewpoint of construction cost and operation power cost.

  When solid-liquid separation is performed using these membranes, the sludge concentration in the biological treatment tank can be higher than that for solid-liquid separation using a sedimentation tank, so the BOD load and nitrogen load per sludge are the same as before. Or even if it is below conventional, since BOD load and nitrogen load per volume can be taken high, a biological treatment tank can be reduced in size.

The membrane separation device can take a high load per sludge, but the membrane is more likely to be contaminated as the load is higher. Therefore, the BOD-sludge load is usually 0.3 kgBOD / kgSS / d or less, preferably 0.05-0. .15 kg BOD / kg VSS / d. Also, the BOD load in the denitrification process is set to the same value.
In order to maintain these values, it is necessary to take a long SRT in the biological treatment process, and the SRT is usually 5 days or longer, preferably 10 to 30 days.

  As described above, when biologically degrading surplus sludge that has been biodegradable, some of it becomes mineralized and disappears, but the rest becomes new surplus sludge, which increases the sludge concentration and makes membrane separation difficult. As a result, the membrane filtration efficiency decreases. On the other hand, if the amount of sludge to be mineralized is increased by increasing the circulation amount to the reforming process step in order to keep the same sludge concentration, the SRT becomes shorter, the sludge load becomes higher, and the membrane is easily contaminated. For this reason, the method of treating surplus sludge in another series of aerobic digesters as in the present invention is also effective for preventing membrane contamination and for stable operation.

  As the membrane type of the membrane separation device, an ultrafiltration (UF) membrane, a microfiltration (MF) membrane, or the like can be used. As a material of the membrane, a membrane of any material such as cellulose acetate (CA) membrane, polyamide (PA) membrane, aramid membrane, polysulfone membrane, hydrophilic polyethylene can be used. The membrane can be of any shape such as a flat membrane, spiral membrane, tubular membrane, hollow fiber membrane.

In the present invention, surplus sludge discharged from the biological treatment process as described above and / or aerobic digestion liquid obtained by digesting this surplus sludge in the aerobic digestion process described later is changed to easily biodegradable in the reforming process. Quality.
The amount of sludge to be reformed is preferably not less than the amount of sludge flowing into the reforming step or the aerobic digestion step as surplus sludge, particularly 2 to 5 times the amount.

  When the modified sludge is treated again in the aerobic digestion tank, some of the sludge is oxidized and decomposed by the action of microorganisms, mineralized into carbon dioxide, water, dissolved salts, etc., and disappears as solid matter. Is assimilated into microorganisms and becomes excess sludge again. Here, if the ratio of the surplus sludge that has been reformed to become surplus sludge again after aerobic digestion (the sludge conversion rate of the reformed sludge) is y1, the ratio of mineralization per reformed sludge is (1- y1). Therefore, when the amount of surplus sludge generated from the biological treatment process is X [kg / d], the sludge of X / (1-y1) [kg / d] is reformed and regenerated in the aerobic digester. If treated, the amount of sludge to be mineralized becomes (1−y1) × {X / (1−y1)} = X, and the amount of surplus sludge generated can be made zero. Accordingly, the amount of sludge to be reformed is preferably 1 / (1-y1) times the amount of excess sludge generated from the biological treatment process. The value of y1 varies depending on the reforming method, the aerobic digestion condition, the treated water temperature, etc., but is usually about 0.5 to 0.8. Therefore, the amount of the modified sludge is preferably 2 to 5 times the excess sludge amount.

  As modification treatment, killing microorganisms in sludge such as ozone treatment, acid treatment, alkali treatment, heat treatment and a combination of these, and destroying cell membranes and cell walls to improve biodegradability. Any known reforming treatment that can be used can be employed without limitation, but ozone treatment is preferred.

  The ozone treatment is performed by contacting surplus sludge or aerobic digestion liquid with ozone. As a contact method, a method of introducing excess sludge or an aerobic digestion liquid into an ozone treatment tank and blowing ozone, a method of mechanical stirring, a method of using a packed bed, or the like can be employed. As ozone, ozone gas, ozone-containing air, ozonized air, or the like can be used.

The conditions for the ozone treatment are not particularly limited, but the pH is adjusted to 5 or less, and the amount of ozone used is 0.002 to 0.05 g-O ₃ / g-VSS, preferably 0.005 to 0.03 g-O ₃ / It is desirable to perform processing as g-VSS.

  The surplus sludge reforming treatment is performed by introducing surplus sludge discharged from the biological treatment process into a reforming treatment tank. In the reforming process of the aerobic digestion liquid, at least a part of the aerobic digestion liquid in the aerobic digestion process is introduced into the reforming treatment tank and reformed. The reforming treatment liquid is circulated to the aerobic digestion process described later as a circulation process, and used for aerobic digestion.

  When introducing the aerobic digestion liquid into the reforming treatment tank and modifying it, the aerobic digestion liquid can be collected from any position in the aerobic digestion tank. Is preferably collected from the second tank. This is because in the first tank, there are many broken organic substances such as cell walls, which are decomposed by digested sludge without any modification treatment.

  When there is a margin in the biological treatment process, a part of the modified treatment solution can be sent to the biological treatment process for treatment. In this case, the volume of the aerobic digester can be further reduced. In particular, when the biological treatment process includes a denitrification process, by sending a part of the reforming treatment liquid to the denitrification process for processing, the organic matter contained in the reforming treatment liquid is removed by an electron donor for denitrification. Since it can be effectively used as, it is preferable.

In the aerobic digestion process of the present invention, surplus sludge discharged from the biological treatment process and / or a reforming treatment liquid obtained by modifying the surplus sludge in the reforming process step is introduced into an aerobic digestion tank, Digest by aeration.
In this case, it is necessary to dissolve a large amount of oxygen in order to oxidize and decompose (mineralize) sludge into carbon dioxide gas and water by applying a high load to the aerobic digester. That is, in terms of the amount of oxygen required per volume, oxygen of about 1.5 to 2.5 kgO / m ³ / d is required.

For this reason, as means for supplying oxygen, it is more preferable to perform aeration with a high dissolution efficiency such as deep layer aeration or aeration with oxygen-enriched air (including oxygen aeration). In particular, when sludge reforming treatment is performed using ozone, it is efficient to generate ozone using oxygen-enriched air. In this way, sludge was reformed with oxygen-enriched air containing ozone. In this case, oxygen-enriched air remains after ozone is consumed for sludge reforming. It is more efficient to supply this oxygen-enriched air to the aerobic digester, and it is preferable to use it in combination with an aeration apparatus having a high dissolution efficiency such as deep aeration. When the oxygen-enriched air discharged from the reforming process lacks the pressure for aeration, the pressure may be increased by using a blower or a compressor as needed. Normally, in order to increase the ozone dissolution efficiency in the reforming process, the reforming process is performed with ozone by applying a pressure of about 1 kgf / cm ² , and therefore has sufficient pressure for aeration.

  Thus, when high concentration oxygen gas is dissolved with high dissolution efficiency, carbon dioxide gas generated by aerobic digestion may be dissolved in the mixed solution without being deaerated, and the pH may be lowered. That is, when air is aerated, nitrogen gas occupying 80% by volume is released without being dissolved, so that almost all of the nitrogen gas is released, so that carbon dioxide gas is deaerated together with this nitrogen gas, and no extreme pH drop occurs. On the other hand, for example, when 90% by volume of oxygen-containing gas is dissolved in the mixed solution in the aerobic digestion tank with 90% dissolution efficiency, the gas released without being dissolved is about 19% by volume of oxygen gas. Therefore, the deaeration effect is low, and therefore, carbon dioxide accumulates in the mixed solution and the pH is lowered. As one means for preventing this, it is possible to provide a deaeration tank that deaerates carbon dioxide in the liquid by aeration with normal air to raise the pH. It is preferable to use an immersion membrane because carbon dioxide can be degassed by aeration for membrane filtration. In this case, in the oxygen-enriched air dissolution process, the pH decreases due to the dissolution of carbon dioxide, but in the membrane separation process using the submerged membrane, the pH increases due to degassing of the carbon dioxide, and liquids having different pH are mixed with the return sludge. Therefore, no extreme pH drop or pH rise occurs. In this case, in order to effectively mix liquids having different pHs, it is preferable to increase the amount of returned sludge and make it 5 to 30 times the amount of surplus sludge that flows in. However, if the return sludge is excessively excessive, the power cost is wasted. Therefore, it is preferable to change the return magnification and change it according to the amount of surplus sludge flowing in while observing the change in pH.

  Even in the case of using a normal air aeration method, it is preferable to use a fine bubble diffusing tube in order to increase dissolution efficiency. Examples of such a diffuser tube include fine holes of 1 mm or less, slits provided in a large number of elastic bodies, and air aerated (or oxygen-enriched air) through a porous ceramic pore. It is done.

  Aerobic digestive fluid containing sludge that has been modified to be easily degradable contains a large amount of high-molecular organic substances derived from broken cell walls, etc., so it takes some time before it is hydrolyzed and utilized by microorganisms. Take it. As described above, the undecomposed fine organic matter is usually viscous and finer than bacteria, and therefore has a property of being easily clogged in the pores of the membrane for solid-liquid separation. Therefore, the aerobic digestion tank is preferably an aeration tank connected in two or more stages in series, and the raw water for membrane separation is preferably taken from the aeration tanks after the second stage. However, the present invention is not limited to this, and a sufficient effect is exhibited even in a single aeration tank or an aeration tank provided with a plurality of tanks in parallel.

As the membrane separation means in the aerobic digestion liquid separation step, the same membrane separation means as the solid-liquid separation means in the biological treatment step can be used. In the cross flow type and the immersion film type, the immersion film is preferable from the viewpoint of cost.
When the membrane separation is performed, the sludge is concentrated in the membrane separation raw water tank (or the membrane soaking tank), so the concentrated liquid is returned from the membrane separation raw water tank to the upstream portion of the aerobic digestion tank. The return amount of the concentrate is at least twice, preferably 3 to 7 times the amount of excess sludge discharged from the biological treatment step and flowing into the aerobic digester. However, when the membrane separation tank and the aerobic digestion tank are integrated, it is not particularly necessary to return the sludge.

  The aerobic digestion liquid in the present invention is originally a high-concentration excess sludge, so it has a high viscosity, and since it is processed at a high load, the activity of microorganisms is high, the microorganisms slime and adhere to the membrane surface, and the membrane is blocked. It's easy to do. In addition, since it contains a large amount of sludge that has been modified to be biodegradable, it contains a large amount of undegraded polymer organic matter derived from a broken cell wall or the like. For this reason, the membrane used for solid-liquid separation in the aerobic digester is very easily contaminated, and it is important to take measures against contamination of the membrane.

As a measure against contamination of the membrane, if it is a pump circulation type cross-flow filtration device, the permeation flux of the filtrate is usually 1 to 2 m ³ / m ² / d, 1 m ³ / m ² / d or less, and more Preferably it is set to a low value of 0.3 to 0.7 m ³ / m ² / d, or the membrane surface flow rate by pump circulation is usually 1 to 2 m / s, more preferably 2 m / s or more, more preferably 2 It is effective to increase it to ˜3 m / s. In the case of a submerged membrane separator, the permeation flux of the filtrate is usually 0.2 to 0.5 m ³ / m ² / d, and is 0.2 m ³ / m ² / d or less, more preferably 0.8. When it is set to a low value such as 05 to 0.2 m ³ / m ² / d, or the amount of aerated air from below the membrane is usually 30 to 80 m ³ / m ² / hour per swirling flow rising portion bottom area, It is effective to set a high value of 100 m ³ / m ² / hour or more, more preferably 100 to 200 m ³ / m ² / hour. In this way, more membrane area is required than usual, and the power required for filtration increases, but the amount of water that is subject to solid-liquid separation is the amount of water corresponding to excess sludge discharged from the biological treatment process. Since this is usually about 1/5 to 1/100 of the amount of raw water for biological treatment, there is no particular problem.

  In the case of the submerged membrane separator, the diffuser that aerates from below is preferably one that generates coarse bubbles, and preferably a diffuser having a diffused pore diameter of about 3 to 10 mm. Therefore, it is preferable to clearly distinguish the aeration tube for supplying oxygen to the aerobic digestion process and the aeration tube for supplying the aerated water flow to the submerged membrane, and to use different types. By aeration with coarse bubbles in this manner, the oxygen dissolution efficiency in the membrane immersion tank is reduced, but the amount of aeration is excessive, so that the inside of the film immersion tank is sufficiently aerobic. Therefore, the membrane immersion tank also has an aerobic digestion action and can be handled as a part of the aerobic digestion tank.

  Another measure against film contamination is to make it easy to clean the film. In particular, since the amount of membrane filtration water is small as described above, a membrane filtration device having a capacity of 50% or more, preferably 100%, in addition to a constantly operating membrane is placed as a preliminary series and the membrane is washed. It is preferable to switch the operation to the preliminary series. Normally, having a spare membrane of 50% or more in this way is not feasible because the cost of the membrane and its associated equipment is high, but in the present invention, a membrane filtration device necessary for solid-liquid separation of aerobic digestion Therefore, it is practical enough to provide an excessive spare sequence. In particular, since the film is easily contaminated for the above-mentioned reason, for example, if the capacity of the preliminary film is set to 25% and the film is washed by 1/4, it may not be in time, so 50% or more, preferably 100%. It is effective to have a preliminary film.

  When the membrane separation device is an immersion membrane, it is possible to install an immersion membrane in the aerobic digestion tank and integrate the aerobic digestion membrane and the membrane separation tank. It is preferable that the aerobic digestion tank and the membrane immersion tank can be easily shut off. Moreover, it is preferable to provide two or more such film immersion tanks in parallel and at least one of them as a reserve. In this case, the aerobic digestion liquid in the series to be cleaned is transferred to the membrane immersion tank in the preliminary series, and then the cleaning chemical liquid is allowed to flow into the film immersion tank (or the cleaning chemical liquid is adjusted in the film immersion tank). It is preferable to perform the cleaning. The cleaning chemical solution can be treated by neutralizing it by an appropriate means and then mixing with the raw water of the biological treatment process.

  The aerobic digestion of excess sludge involves aerobic digestion by introducing excess sludge discharged from the biological treatment process into an aerobic digester. In the aerobic digestion of the reforming treatment liquid, the reforming treatment liquid circulated from the reforming treatment step is introduced into an aerobic digestion tank to perform aerobic digestion.

  The sludge concentration (MLSS) in the aerobic digestion step is preferably high, and is generally 10,000 mg / l or more, preferably 15000 to 30000 mg / l. The SRT is preferably short, generally 15 days or less, preferably 2 to 5 days. Moreover, although HRT with respect to the inflow excess sludge of an aerobic digestion process changes with the density | concentrations of the excess sludge discharged | emitted, about 0.5 to 10 days is preferable. The pH is about 4-9, preferably 5-7.

  In the aerobic digestion liquid separation step in the present invention, the aerobic digestion liquid is subjected to membrane separation by membrane separation means, and the permeate is discharged out of the system as treated water (digestion treated water). Since a COD component, nitrogen component, phosphorus component and other harmful substances are contained, it is preferable to discharge after performing a treatment such as a coagulation separation treatment, a crystallization treatment, an activated carbon adsorption treatment, and a nitrification denitrification treatment. In addition, the digested water can be returned to the biological treatment step for biological treatment. When it is desired to reduce the nitrogen concentration in the digested water, the aerobic digestion process can be a flow of denitrification and nitrification.

[Action]
The reason why the aerobic digester can be miniaturized in the method of the present invention will be described.
In the method of introducing surplus sludge into an aerobic digester and treating it, the sludge concentration of surplus sludge discharged from the biological treatment process is x ₀ [kg / m ³ ], and the discharged amount is q ₀ [m ³ / d]. To do. Further, the volume of the aerobic digestion tank is v [m ³ ], and the sludge concentration of the liquid in the tank (aerobic digestion liquid) is x [kg / m ³ ]. The sum of the amount of aerobic digested liquid sent to the reforming tank and the amount of sludge drawn out of the system is defined as q ₁ [m ³ / d]. At this time, the SRT (denoted as θ) of the aerobic digester is represented by θ = v / q ₁ [day].

A normal SRT is defined as follows.
SRT [day] = (Amount of sludge retained in the aerobic digester [kg]) / (Amount of extracted sludge [kg / day])
Therefore, when the generation of surplus sludge is brought close to zero, the SRT becomes infinite, but the microorganisms in the sludge modified as described above are almost killed and the activity becomes almost zero. It has the same effect as extracting excess sludge out of the system. Therefore, in the present invention, for the sake of convenience, the amount of sludge extracted and circulated to the reforming process is not discharged out of the system, but can be considered as the amount of extracted sludge. Therefore, SRT in the present invention is strictly defined as follows.
SRT [day] = (Retained sludge amount in aerobic digester [kg]) / (Extracted sludge amount [kg / day] + Reformed sludge amount [kg / day])

The sludge conversion rate when the excess sludge is reformed is y ₀ [−], and the sludge conversion rate when the aerobic digestion liquid is reformed is y ₁ [−].
When surplus sludge is introduced into an aerobic digester and processed, the following relational expression holds.
Increase in sludge = q ₀ x ₀ (1)
Sludge reduction = (1-y ₁ ) q ₁ x (2)
Here, since (1) = (2) holds in the steady state, the following relational expression holds.
Aerobic digester volume = {[θq ₀ x ₀ ] / [(1-y ₁ ) x]} (3)

Further, when surplus sludge is introduced into the reformer and treated, the following relational expression is established.
Increase in sludge = y ₀ q ₀ x ₀ (4)
Sludge reduction = (1-y ₁ ) q ₁ x (5)
Here, since (4) = (5) holds in the steady state, the following relational expression holds.
Aerobic digester volume = {[θy ₀ q ₀ x ₀ ] / [(1-y ₁ ) x]} (6)

As can be seen from the above formulas (1) to (6), in order to reduce the aerobic digester volume, the inflow sludge amount (q ₀ x ₀ ) is decreased, or the sludge concentration in the aerobic digester (x ) Is increased or SRT (θ) is decreased.
In the present invention, since the in-vessel liquid (digested liquid) in the aerobic digestion tank is membrane-separated by the membrane separation means, the sludge concentration (x) can be increased as compared with the case of solid-liquid separation by gravity sedimentation separation. Therefore, the aerobic digester volume can be reduced. Moreover, in the case of the method of introducing the excess sludge into the reforming treatment tank and performing the reforming treatment, the substantial sludge load applied to the aerobic digestion tank can be reduced by y ₀ times, and usually y ₀ ≈y ₁ ≈0. Since it is 6 to 0.7, it is possible to reduce the digester volume to 60 to 70%.

Here, the effect of the present invention will be described by design examples.
It is assumed that the drainage amount is 1000 m ³ / d, the BOD concentration is 1000 mg / L, and the NH ₄ —N is 200 mg / L.
<< A1. Biological treatment using a sedimentation tank (BOD removal / nitrogen removal) >>
Flow from denitrification tank → nitrification tank → precipitation tank. The mixed liquid (nitrification liquid) in the nitrification tank is circulated to the denitrification tank 8 times the amount of raw water, and nitrate nitrogen is denitrified in the denitrification tank. In addition, the sludge will be returned from the sedimentation tank to the denitrification tank by one time the amount of raw water. The MLSS concentration in the denitrification tank and nitrification tank is 4000 mg / L. Sludge extraction is performed from the sedimentation tank, and the extraction amount is set so that the SRT of the nitrification tank is 10 days. The drawn sludge concentration is assumed to be 8000 mg / L. The denitrification tank is designed so that the denitrification rate per sludge is 0.08 kgN / KgSS / d.

  It is assumed that 60% of BOD is generated as excess sludge. The surplus sludge derived from nitrifying bacteria is negligible and is ignored for convenience. The amount of nitrogen to be denitrified in the denitrification tank is 90% of the inflow nitrogen amount. This is because of the nitrogen nitrified in the nitrification tank, 9 times the amount of raw water is returned to the denitrification process in combination with nitrification liquid circulation and return sludge, and the remaining 10% flows out into the treated water. Actually, there is a nitrogen content that is assimilated as surplus sludge together with BOD, so the amount of nitrogen to be denitrified is smaller, but here it is ignored for convenience.

The inflow BOD amount is 1000 [m ³ / d] × 1000 [mg / L] ÷ 1000 = 1000 [kg / d]
The amount of excess sludge generated is 1000 [kg BOD / d] × 60 [%] ÷ 100 = 600 [kg / d]
The amount of NH ₄ —N flowing in is 1000 [m ³ / d] × 200 [mg / L] ÷ 1000 = 200 [kg / d].
The volumetric load of the denitrification tank is 0.08 [kgN / kgSS / d] × 4000 [mgSS / L] ÷ 1000 = 0.32 [kgN / m ³ / d]
The amount of nitrogen to be denitrified is 200 [kg N / d] × 90 [%] ÷ 100 = 180 [kg N / d].
Denitrification tank volume is 180 [kg N / d] ÷ 0.32 [kg N / m ³ / d] = 563 [m ³ ]

Since the nitrification tank volume needs to be set to 10 days for the surplus sludge amount of 600 kg / d, it is necessary to hold 600 [kg / d] × 10 [d] = 6000 [kg] of sludge, From the sludge concentration in the nitrification tank, 6000 [kg] / (4000 [mg / L] / 1000) = 1500 [m ³ ]
The total volume of the nitrification tank and the denitrification tank is 2063 m ³ .

<< A2. Biological treatment using sedimentation tank (only BOD removal) >>
The BOD sludge load is 0.2 kg BOD / kg SS / d. Other conditions are the same as A1,
BOD volumetric load is 0.2 [kg BOD / kg SS / d] × 4000 [mg / L] ÷ 1000 = 0.8 [kg BOD / m ³ / d]
The aeration tank volume is 1000 [kg BOD / d] ÷ 0.8 [kg BOD / m ³ / d] = 1250 [m ³ ].

<< B1. Biological treatment using membrane separation (with nitrogen removal) >>
Flow from denitrification tank to nitrification tank / membrane separation tank. The mixed solution in the nitrification tank is circulated to the denitrification tank 9 times the amount of raw water, and nitrate nitrogen is denitrified in the denitrification tank. The amount of nitrogen to be denitrified is 90% of the amount of inflowing nitrogen as in A1.
The MLSS concentration in the nitrification tank is 10000 mg / L. Since solid-liquid separation by membrane separation is performed in the nitrification tank, the sludge concentration is concentrated compared to the denitrification tank. Therefore, the sludge concentration in the denitrification tank is lower than that in the nitrification tank, and the denitrification tank MLSS concentration is 9000 mg / L in the case of the current circulation rate.
The denitrification rate per sludge is 0.08 kgN / KgSS / d, which is the same as A1. The sludge extraction is performed from the nitrification tank, and the extraction amount is set so that the nitrification tank SRT has the same 10 days as A1.

The volumetric load of the denitrification tank is 0.08 [kgN / kgSS / d] × 9000 [mg / L] ÷ 1000 = 0.72 [kgN / m ³ / d]
The amount of nitrogen to be denitrified is 200 [kg N / d] × 90 [%] ÷ 100 = 180 [kg N / d].
Denitrification tank volume is 180 [kg N / d] ÷ 0.72 [kg N / m ³ / d] = 250 [m ³ ]
Since the nitrification tank volume needs to hold 6000 kg of sludge as in A1 above,
The nitrification tank volume is 6000 [kg] / (10000 [mg / L] / 1000) = 600 [m ³ ]
The total volume of the nitrification tank and the denitrification tank is 850 m ³ .

<< B2. Biological treatment using membrane separation (BOD removal only) >>
The aeration tank and the membrane separation tank are used together, and the aeration tank MLSS concentration is 10000 mg / L. BOD sludge load is 0.2 kgBOD / kgSS / d, the same as A2, BOD volume load is 0.2 [kgBOD / kgSS / d] × 10000 [mg / L] ÷ 1000 = 2 [kgBOD / kgSS / d]
Aeration tank volume is 1000 [kg BOD / d] ÷ 2 [kg BOD / m ³ / d] = 500 [m ³ ]

<< C1. Aerobic digester using precipitation tank for solid-liquid separation >>
The sludge conversion rate (y1) of the reformed sludge is set to 0.7. Since the ratio of mineralization per reformed sludge is (1-0.7) = 0.3, 1 / 0.3 = 3.333 times the amount of surplus sludge generated in the biological treatment process. The generation of excess sludge can be made zero by reforming and aerobic digestion. The surplus sludge discharged out of the system as waste is designed to be zero. The surplus sludge amount is 600 kg / d as described in A1.
The SRT of the aerobic digester is 4 days, and the digester MLSS concentration is 4000 mg / L.

The amount of the modified sludge is 600 [kg / d] × 3.333 = 2000 [kg / d].
In order to keep the SRT of the aerobic digester for 4 days, it is necessary to hold 2000 [kg / d] × 4 [day] = 8000 [kg] sludge in the aerobic digester. From the concentration
The digester volume is 8000 [kg / d] / (4000 [mg / L] / 1000) = 2000 [m ³ ].
As the SS load, 2000 [kg / d] ÷ 2000 [m ³ ] = 1 [kg / m ³ / d]
Assuming that the VSS / SS ratio is 0.8, the organic substance load (VSS load) is 1 [kgSS / m ³ /d]×0.8=0.8 [kgVSS / m ³ / d]
However, the modified sludge was also considered as SS load and organic matter load.

Since the organic load of the conventional aerobic digestion method is 0.8 to 1.0 kg VSS / m ³ / d (from JP-A-8-299995), it can be seen that the values are almost the same.
This volume is 0.97 times as large as the biological treatment tank of A1, and is almost the same size.
Moreover, it is 1.6 times the biological treatment tank of A2.
Moreover, if the surplus sludge of the biological treatment tank using a film | membrane like said B1 and B2 is aerobically digested, it will become 2.4 times and 4 times, respectively, and it turns out that it cannot be said that it is practical.

<< C2. Aerobic digester using membrane for solid-liquid separation (invention) >>
The sludge conversion rate (y1) of the reformed sludge is set to 0.7, which is the same as C1. Therefore, like C1, it is necessary to aerobic digestion by modifying 3.333 times the amount of excess sludge generated in the biological treatment process. The SRT of the aerobic digester is 4 days, the same as C1, and the digester MLSS concentration is 20000 mg / L.
In order to keep the SRT of the aerobic digester for 4 days, it is necessary to keep 8000 kg of sludge in the aerobic digester as in the case of the C1, and from the sludge concentration of the digester,
The digester volume is 8000 [kg / d] / (20000 [mg / L] / 1000) = 400 [m ³ ].
SS load is 2000 [kgSS / d] ÷ 400 [m ³ ] = 5 [kgSS / m ³ / d]
Assuming that the VSS / SS ratio is 0.8, the organic matter (VSS) load is 5 [kgSS / m ³ /d]×0.8=4 [kgVSS / m ³ / d].

This value greatly exceeds the conventional organic substance load of 0.8 to 1.0 kg VSS / m ³ / d, and it can be seen that a very small aerobic digester is realized. Moreover, almost no excess sludge is generated.
The aerobic digester volume is 0.19 times and 0.32 times that of the biological treatment tank, respectively, when used as an aerobic digestion tank for the biological treatment steps A1 and A2 using a sedimentation tank. Generation of excess sludge can be reduced to zero in the aerobic digester. In addition, the aerobic digestion tanks for biological treatment processes B1 and B2 using membrane separation are 0.47 times and 0.8 times, respectively, and the amount of excess sludge generated in the aerobic digestion tank is also smaller than the biological treatment process. Can be zero.

  The reason why such a small aerobic digester can be realized is that, as described above, the amount of excess sludge or more is modified to be biodegradable and circulated in the aerobic digester, and the membrane flux is lowered. Keeping the flow rate higher for the pump circulation type membrane and increasing the amount of aeration for the submerged membrane, the hard-to-filter aerobic digested sludge is concentrated to a high concentration of, for example, 20000 mg / L. The structure is easy to clean the membrane, and if necessary, aerobic digesters are aerated with oxygen-enriched air or fine bubble diffusing tubes to supply sufficient oxygen to high organic loads. And so on.

Regarding the membrane area, for example, in the biological treatment process of B1 or B2, if a submerged membrane that performs intermittent filtration with a flux of 0.3 m ³ / m ² / d at the time of membrane filtration, 8-minute filtration and 2-minute stop is installed. The average flux is 0.3 × 0.8 = 0.24 [m ³ / m ² / d], and the amount of treated water is 1000 [m ³ / d].
Membrane area, 1000 [m ³ /D〕÷0.24〔M ³ / m ² / d] = 4167 [m ^2] is required.

On the other hand, the membrane required for the C2 aerobic digester is, for example, as an immersion membrane that performs intermittent filtration with a flux of 0.2 m ³ / m ² / d during membrane filtration, 6-minute filtration, and 2-minute stoppage. The flux is 0.2 × 0.6 = 0.12 [m ³ / m ² / d], and the amount of treated water is 600 kg / d of excess sludge and 10000 mg / L of excess sludge concentration.
Amount of treated water 600 [kg / d] ÷ (10000 [mg / L] ÷ 1000) = 60 [m ³ / d]
Membrane area 60 [m ³ /D〕÷0.12〔M ³ / m ² / d] = 500 [m ^2]
Thus, even if the average flux is half that of the biological treatment process, the required membrane area may be 1/8 or less of the biological treatment process. Even if 100% of the preliminary membrane is prepared, the required membrane area is 1000 m ² in total, and it is sufficient that the membrane area is about 25% of the membrane area required for the biological treatment process.

  Intermittent filtration is said to be effective for preventing the concentration and sticking of suspended solids to the membrane surface. In particular, the intermittent filtration is difficult by designing a longer pause time for the filtration process of aerobic digested sludge. Membrane contamination by filterable aerobic digested sludge is prevented.

<< C3. A method of using a membrane for solid-liquid separation and modifying surplus sludge before introducing it into an aerobic digester (the present invention) >>
Although a sufficiently small aerobic digester can be realized even with the C2, the aerobic digester is particularly suitable for a compact biological treatment process such as the B2 when the biological treatment process uses membrane separation. Since the problem that the volume of occupies 80% of the biological treatment tank remains, it is necessary to make the aerobic digestion tank more compact.
Therefore, if 600 kg / d of excess sludge is reformed and then introduced into the aerobic digester, and 1400 kg / d of digested sludge is circulated from the aerobic digester to the reforming process, the total amount of reformed sludge The aerobic digester SRT should be set for 1400 kg / d sludge that circulates from the aerobic digester to the reforming process even though the excess sludge is expected to be 2000 kg / d. The digester can be made smaller.

That is, in order to maintain the SRT of the aerobic digester for 4 days, it is sufficient to hold 1400 [kg / d] × 4 [day] = 5600 [kg] sludge in the aerobic digester. From the sludge concentration,
The digester volume is 5600 [kg] ÷ (20000 [mg / L] ÷ 1000) = 280 [m ³ ].
It becomes a digestive tank. This digestion tank is 0.56 times the B2 biological treatment tank, and is a smaller aerobic digestion tank than the biological treatment tank. Compared to the biological treatment tanks of A1, A2 and B1, it can be seen that smaller volumes of 0.14 times, 0.22 times and 0.33 times can be realized, respectively.
At this time, the SS load on the aerobic digester is 2000 [kgSS / d] ÷ 280 [m ³ ] = 7.1 [kgSS / m ³ / d],
The organic load is 5.7 kg VSS / m ³ / d, which is even higher.

  In the present invention, since the solid-liquid separation of the aerobic digestion liquid is performed by the membrane, there is no fear that the digestion liquid causes sedimentation failure and the sludge flows out of the system. Therefore, even if the amount of reforming treatment is reduced or stopped and the aerobic digestion liquid is left in an aerated state even during periods when the load of raw biological treatment water is low and surplus sludge is hardly generated, Biological sludge can be easily maintained in the system without sludge being reduced excessively and without floc being dismantled and discharged by air aeration. That is, it is not necessary to perform advanced maintenance management even when the load is reduced, and it is sufficient to reduce or stop the circulation amount to the reforming process and leave it after that, and anyone can manage it.

  In this way, biological sludge well-adapted to the aerobic digestion reaction can be kept in the system at all times, so even if surplus sludge is generated again and a load is applied to the aerobic digestion process, this acclimatized digestion By holding highly efficient biological sludge, the aerobic digestion process can be restarted without any problems.

The organic wastewater biological treatment method of the present invention is aerobic digestion in an aerobic digestion tank when reducing the volume of excess sludge by reforming treatment and aerobic digestion. Since membrane separation is performed, digestion can be performed with a high sludge concentration, and the volume of excess sludge can be reduced using a small aerobic digester compared to biological treatment processes. The efficiency can be increased and the amount of discharged sludge can be brought close to zero, and it can be easily maintained even during periods when the load is low and the amount of surplus sludge generated is small.
Moreover, since the volume of excess sludge is reduced without applying an excessive load to the body of the biological treatment process, it is not necessary to make the biological treatment process, particularly the nitrification tank larger than before, and the quality of the treated water does not deteriorate. . Even if the biological treatment step includes a membrane separation step, an excessive sludge load is not applied, and the membrane separation device can be stably operated as much as the conventional one.

Next, embodiments of the present invention will be described with reference to the drawings.
FIGS. 1 to 4 are system diagrams showing biological wastewater treatment apparatuses according to different embodiments. FIG. 1 shows the case where surplus sludge is introduced into an aerobic digester for treatment, and FIG. FIG. 3 shows an example in which a biological treatment process is a nitrification process and a denitrification process, and FIG. 4 shows an example in which an aerobic digestion tank and a membrane separation tank are provided separately. is there.

  In FIG. 1, 1 is an aeration tank, the raw water channel 2 and the return liquid channel 3 communicate, the aeration apparatus 4 is provided in the bottom part, and the air supply channel 5 communicates. A communication path 9 having a pump 8 communicates from the aeration tank 1 to the membrane separation device 7.

  The membrane separation device 7 is divided into a permeate chamber 11 and a concentrate chamber 12 by a separation membrane 10, a treatment water channel 13 communicates with the permeate chamber 11, and a return fluid channel 3 communicates with the concentrate chamber 12. . As the membrane separation device 7, a device provided with an arbitrary separation membrane 10 such as a flat membrane, a spiral membrane, a tubular membrane, a hollow fiber membrane or the like can be used.

  21 is an aerobic digestion tank, and the surplus sludge path 14 branched from the return liquid path 3 communicates, and the submerged membrane separation apparatus 22 is provided in the tank. The submerged membrane separation device 22 is configured such that the separation membrane 23 is immersed in the liquid, and the liquid in the tank (aerobic digestion liquid) passes through the separation membrane and is collected on the permeate chamber 24 side. A treatment water channel 25 communicates with the permeate chamber 24 and a pump 26 is provided. An air diffuser 27 is disposed below the submerged membrane separator 22 and communicates with an air supply path 28. Reference numeral 29 denotes a partition wall.

  The aerobic digestion liquid path 32 communicates from the aerobic digestion tank 21 to the ozone treatment tank 31, and the ozone treatment liquid path 33 communicates from the ozone treatment tank 31. An ozone supply path 36 communicates with the ozone treatment tank 31 from an ozone generator 35.

  In order to treat the organic drainage (raw water) with the apparatus of FIG. 1, the raw water is introduced into the aeration tank 1 from the raw water channel 2, and the return sludge in the concentrated liquid returned from the return liquid channel 3 and the aeration tank 1 The activated sludge is mixed, and the air supplied from the air supply path 5 is diffused from the aeration device 4 to be treated aerobically. As a result, the organic matter in the raw water is decomposed by the biooxidation reaction.

  A part of the liquid in the tank of the aeration tank 1 is taken out from the communication path 9, pressurized by the pump 8, introduced into the membrane separation device 7, and separated into a permeate and a concentrated liquid by membrane separation. The permeate that has permeated through the separation membrane 10 is discharged from the treatment water channel 13 as treated water, and a part of the concentrated liquid in which activated sludge and other solid components are concentrated is returned to the aeration tank 1 from the return liquid channel 3.

  The concentrated liquid discharged as surplus sludge is introduced into the aerobic digestion tank 21 from the surplus sludge path 14, and air (preferably oxygen-enriched air) supplied from the air supply path 28 is diffused from the aeration device 27. Perform aerobic digestion.

  Further, the pump 26 is driven to discharge the permeated liquid from the treatment water channel 25, thereby performing membrane separation of the liquid in the tank (aerobic digestive liquid) in the aerobic digestive tank 21.

  A part of the aerobic digestion liquid is introduced into the ozone treatment tank 31 from the aerobic digestion liquid path 32, ozone generated by the ozone generator 35 is supplied from the ozone supply path 36 and brought into contact with the ozone treatment (reforming treatment). )I do. As a result, the sludge in the aerobic digestive fluid is converted to BOD. The ozone treatment liquid circulates from the ozone treatment liquid path 33 to the aerobic digestion tank 21 and is subjected to aerobic digestion.

  Thus, the surplus sludge can be reduced in volume by aerobic digestion of the surplus sludge and the ozone treatment liquid. In this case, since the aerobic digestion is performed in a system different from the biological treatment, the quality of the treated water is not affected, and the membrane separation is performed by the submerged membrane separator 22 in the aerobic digester 21. Therefore, the sludge concentration in the aerobic digestion tank 21 can be increased, thereby increasing the digestion efficiency and reducing the size of the apparatus, but the digestion efficiency is further increased by using oxygen-enriched air. .

  The apparatus of FIG. 2 is configured in the same manner as in FIG. 1 except that the excess sludge path 14 communicates with the ozone treatment tank 31. The processing method using the apparatus shown in FIG. 2 is the same as that shown in FIG. 1 except that the concentrated liquid concentrated in the membrane separator 7 and discharged as excess sludge is introduced into the ozone treatment tank 31 from the excess sludge passage 14 and subjected to ozone treatment. . In the case of FIG. 2, since the excess sludge is reformed in addition to the aerobic digestion liquid, the digestion efficiency can be further increased as compared with the case of FIG. 1, and the aerobic digestion tank 21 can be further downsized. .

  FIG. 3 shows an example of nitrification and denitrification as biological treatment. In FIG. 3, reference numeral 41 denotes a denitrification tank. The raw water channel 42, the return liquid channel 43, the communication channels 44 and 45, the ozone treatment liquid transfer channel 46 and the electron donor supply channel 47 communicate with each other, and a stirrer 48 is provided. .

  Reference numeral 51 denotes a nitrification tank. A return liquid path 43, a communication path 44, a chemical injection path 52, and an excess sludge path 53 communicate with each other, and a submerged membrane separation device 22a similar to that shown in FIG. 1 is provided in the tank.

Reference numeral 61 denotes a crystallization treatment tank. The communication paths 45 and 62, the drug injection path 63, and the discharge path 64 communicate with each other, introduce the permeate of the submerged membrane separation device 22 and perform crystallization, and salts in the permeate Etc. can be separated.
The ozone treatment tank 31 communicates with an ozone treatment liquid transfer path 46 so that a part of the ozone treatment liquid can be transferred to the denitrification tank 41.
Other configurations are the same as those in FIG.

  In order to biologically process the raw water using the apparatus of FIG. 3, the raw water is introduced into the denitrification tank 41 from the raw water channel 42, the activated sludge returned from the return liquid channel 43, the ozone processing liquid transferred from the ozone processing liquid channel 46, and The mixture is mixed with denitrifying bacteria in the tank, and an electron donor such as methanol is supplied from the electron donor supply path 47 to maintain anaerobic properties. Decompose.

  A part of the denitrification liquid is introduced into the nitrification tank 51 from the communication path 44 and mixed with the activated sludge containing nitrifying bacteria, and the pH is adjusted to 7-8 by adding acid or alkali from the chemical injection path 52, Air diffuses from the air diffuser 27a, decomposes organic matter to remove BOD, and decomposes organic nitrogen into ammoniacal nitrogen. In this case, aeration in excess of the aeration for removing the BOD makes the nitrifying bacteria dominant.

  The liquid in the nitrification tank 51 (nitrification liquid) is membrane-separated by the submerged membrane separation device 22a, and the permeate is discharged from the treatment channel 25a as treated water. A part of the nitrification liquid in which the sludge is concentrated is returned to the denitrification tank 41 from the return liquid passage 43. The nitrification liquid corresponding to the excess sludge is introduced into the aerobic digestion tank 21 from the excess sludge passage 53 and digested.

  The permeate separated by the submerged membrane separator 22 is introduced into the crystallization tank 61 from the communication path 62, and a crystallization process is performed by injecting a chemical such as magnesium chloride from the drug injection path 63. The treated water is returned from the communication path 45 to the denitrification tank 41, and the crystal is discharged from the discharge path 64.

Part of the ozone treatment liquid ozone-treated in the ozone treatment tank 31 is transferred from the ozone treatment liquid transfer path 46 to the denitrification tank 41. Thereby, the organic substance in the ozone treatment liquid is used as an electron donor necessary for denitrification.
Other processes are the same as those in FIG.

In the case of FIG. 3, surplus sludge can be aerobically digested under optimum conditions, for example, with optimum SRT without any influence on nitrification and denitrification as a biological treatment step, and there is no waste in the treatment process. .
Although the aerobic digestion tank 21 is omitted in FIG. 3 and the solution in the nitrification tank 51 is directly ozone-treated in the ozone treatment tank 31, the entire amount is returned to the denitrification tank 41. In addition, it is necessary to increase the volume of the nitrification tank 51, in particular, the volume of the nitrification tank 51, which increases the size of the apparatus. When the nitrification tank 51 is equipped with the submerged membrane separation device 22a, the sludge concentration in the nitrification liquid is already high, and it is difficult to reduce the size of the device by increasing the sludge concentration. Therefore, as shown in FIG. 3, the entire apparatus can be miniaturized by providing an aerobic digestion tank 21 equipped with a submerged membrane separation apparatus 22 and treating excess sludge.

The apparatus of FIG. 4 has a structure in which the submerged membrane separation apparatus 22 and the partition wall 29 are taken out from the aerobic digestion tank 21 of the apparatus of FIG. 1 and accommodated in a separate membrane separation tank 71.
A part of the aerobic digestion liquid is introduced into the ozone treatment tank 31 from the aerobic digestion liquid path 32 by driving the pump 78 to perform ozone treatment (reforming treatment). The ozone generator 35 is connected to an oxygen-enriched air supply path 73 from an oxygen enricher 72, and supplies oxygen-enriched air containing ozone gas to the ozone treatment tank 31 using the oxygen-enriched air supplied therefrom. is doing. The oxygen-enriched air containing ozone gas is supplied to the air diffuser 27b of the aerobic digestion tank 21 through the oxygen-enriched air supply path 73a after the ozone is consumed in the ozone treatment tank 31, thereby digesting. Increase efficiency. As described above, when oxygen-enriched air is effectively used, it is preferable that the aerobic digestion tank 21 be a deep aeration tank or the like to improve dissolution efficiency.

The aerobic digestion tank 21 and the membrane separation tank 71 communicate with each other by an air lift pump 74 and a return path 75. The air lift pump 74 has an air diffuser 76 in the lower part and a digested sludge inflow valve 77 in the middle.
The air lift pump 74 transfers the digested liquid in the aerobic digestion tank 21 to the membrane separation tank 71 using the air blown out from the air diffuser 76 as a driving force. This transfer amount is at least 3 times, preferably 4 to 8 times the amount of excess sludge flowing from the excess sludge passage 14.
Digested liquid that has not been membrane-filtered is returned as a return sludge from the return path 75 at least twice the amount of excess sludge, preferably 3-7 times, and undergoes aerobic digestion again.

The membrane separation tank 71 has substantially the same structure as the aerobic digestion tank 21 in FIG. 1, but the amount of air diffused from the air diffuser 27 is set to prevent contamination of the submerged membrane separator 22. ing.
A plurality of membrane separation tanks 71 are preferably provided, at least one of which is a preliminary series, and when the separation membrane 23 in the membrane separation tank 71 becomes heavily contaminated, the operation of the corresponding membrane separation tank is performed. And the digested sludge in the membrane separation tank is transferred to the preliminary series membrane separation tank to start the preliminary series operation, and the cleaning membrane is washed by injecting a cleaning chemical into the stopped membrane separation tank. I do. After washing, the membrane separation tank is usually filled with clean water, industrial water, or biologically treated water, and used as a new preliminary series.

The operation of the air lift pump 74 is stopped or the digested sludge inflow valve 77 is closed so that the sludge in the aerobic digester does not flow into the series where the operation is stopped.
Other configurations and operations are the same as those in FIG. In the case of the apparatus of FIG. 4, instead of taking out the digested liquid in the aerobic digestion tank 21 and performing ozone treatment, the digested liquid in the membrane separation tank 71 can be taken out and ozone treated.

Example 1:
The organic effluent was biologically treated with the apparatus of FIG. The volume of the denitrification tank 41 was 40 liter, and the volume of the nitrification tank 51 was 40 liter. As the submerged membrane separation device 22a provided in the nitrification tank 51, a hollow fiber immersion membrane made of hydrophilic polyethylene was used as the separation membrane 23a, and the hollow fiber membrane stretched in a flat plate shape. The membrane area is 0.5 m ² . The hollow fiber membrane is an MF membrane having a pore diameter of 0.1 μm, and has an outer diameter of 410 μm and an inner diameter of 270 μm.

The raw water used for the test is tap water added with sodium acetate as a BOD source and ammonium sulfate as a nitrogen source. The raw water concentration was adjusted so that BOD was 300 mg / l and NH ₄ -N was 300 mg / l. The addition of phosphoric acid as PO ₄ -P is 9 mg / l. The amount of raw water was 100 liter / d.

Denitrification was performed by adding methanol to the denitrification tank 41 at 600 mg / l per raw water.
In the nitrification tank 51, air was diffused from the air diffuser 27a, a cross flow velocity was given to the membrane surface of the separation membrane 23a, and oxygen was supplied into the tank for nitrification. The amount of air was 100 liter / min.

Further, in conjunction with the pH meter, sodium hydroxide was added from the drug injection channel 52 to adjust the pH to 7.2 to 7.3.
The amount of liquid returned from the nitrification tank 51 to the denitrification tank 41 was 1000 liter / d. From the nitrification tank 51, the liquid in the tank (excess sludge) was quantitatively extracted, and the SRT in the nitrification tank 51 was maintained on the 15th. That is, the liquid in the tank was drawn out at a flow rate of 2.67 liter / d. The MLSS concentration in the nitrification tank 51 was approximately constant at about 10,000 mg / l.

  The effective volume of the aerobic digestion tank 21 was 10 liters, and the same immersion membrane separator 22 as the nitrification tank 51 was installed in the tank, and aerobic treatment was performed by aeration from the aeration device 27. In this case, it is preferable because aeration that gives a cross flow to the membrane surface of the separation membrane 23 and aeration for sludge digestion can be used in combination.

The aerobic digestion liquid was introduced into the ozone treatment tank 31 with an amount of water of 3.33 liter / d, and subjected to ozone treatment with an ozone injection rate of 0.03 g-O ₃ / g-VSS. The ozone treatment liquid was circulated to the aerobic digester 21 with an amount of water of 3.33 liter / d (SRT of the aerobic digester 21 was 3 days). In addition, the ozone treatment liquid was not transferred to the denitrification tank 41.

Thus, when the biological treatment test of raw | natural water was conducted, the MLSS density | concentration of the aerobic digester 21 became about 27000 mg / l, and was stabilized.
In this case, the liquid in the tank in the aerobic digestion tank 21 has a high concentration of sludge, and solid-liquid separation could not be performed by gravity sedimentation. In order to perform solid-liquid separation by gravity sedimentation, a dilution of at least about 3 times was necessary. This indicates that a water tank volume of about 3 times is required for solid-liquid separation by gravity sedimentation.

The treated water quality discharged from the submerged membrane separator 22 through the communication path 62 was 500 to 600 mg / l for NH ₄ -N, 70 to 100 mg / l for PO ₄ -P, and 150 to 200 mg / l for COD _Mn . .
This water was introduced into the crystallization treatment tank 61, 100 mg-Mg / l of magnesium chloride was added, the pH was adjusted to 9 with sodium hydroxide, and crystallization treatment was performed. NH ₄ -N and magnesium ammonium phosphate PO ₄ -P was removed. As a result, the NH ₄ —N concentration decreased to 470 to 580 mg / l, and the PO ₄ —P concentration decreased to 10 mg / l or less.

When this crystallized treated water was returned from the communication channel 45 to the denitrification tank 41 and passed therethrough, the TN concentration of the treated water discharged from the treated water channel 25a was 1-2 mg / l, and the PO ₄ -P concentration was about 0. .2 mg / l and COD _Mn increased by 2 to 5 mg / l, both of which are within the error range and are acceptable.

Example 2:
Since the biological treatment process of Example 1 has a sludge concentration of 10,000 mg / l and sufficient room for a membrane separation apparatus, the ozone treatment liquid is supplied from the ozone treatment tank 31 to the denitrification tank 41 at a flow rate of 0.5 liter / d. Otherwise, the same procedure as in Example 1 was performed.
As a result, the MLSS concentration in the aerobic digester 21 was reduced from about 27000 mg / l to about 14000 mg / l, and a margin was generated in the treatment.

  However, the MLSS concentration in the denitrification tank and the nitrification tank increased in the range of 2000 to 3000 mg / l, and reached about 13000 mg / l in the nitrification tank. In this case, the membrane separation apparatus was designed with a margin and was not particularly problematic. However, if the ozone treatment liquid is further transferred to the biological treatment tank, it is clear that the sludge concentration becomes higher and membrane separation becomes difficult.

Example 3:
In Example 1, it carried out similarly to Example 1 except having introduce | transduced into the ozone treatment tank 31 instead of introducing the liquid (nitrification liquid) in the tank of the nitrification tank 51 into the aerobic digestion tank 21. FIG. That is, in the ozone treatment tank 31, a total of 6.0 liters of the nitrification liquid 2.67 liter / d introduced from the excess sludge passage 53 and the aerobic digestion fluid 3.33 liter / d introduced from the aerobic digestion passage 32. / D sludge was treated with ozone. As a result, the MLSS concentration in the aerobic digester 21 was about 19000 mg / l and stabilized.

This is a sludge concentration of about 70% compared to 27000 mg / l in Example 1, indicating that a 30% margin has occurred in the treatment. For example, even if the aerobic digester is further reduced to 7 liters, it is carried out. It shows that it is possible to operate at an MLSS concentration of 27000 mg / l, which is the same condition as in Example 1.
At this time, when the aerobic digestion liquid was taken out and the oxygen consumption rate was measured, it was ³ to 3.5 kgO / m ³ / d. In this way, it is difficult to supply oxygen with high efficiency by using a normal air diffuser in an actual apparatus, and thus it became clear that a highly efficient oxygen supply means is necessary.

It is a systematic diagram which shows the biological treatment apparatus of the organic waste liquid of embodiment. It is a systematic diagram which shows the biological treatment apparatus of the organic waste liquid of other embodiment. It is a systematic diagram which shows the biological treatment apparatus of the organic waste liquid of another embodiment. It is a systematic diagram which shows the biological treatment apparatus of the organic waste liquid of another embodiment. It is a systematic diagram which shows the biological treatment apparatus of the conventional organic drainage.

Explanation of symbols

1 Aeration tank 2, 42 Raw water channel 3, 43 Return liquid channel 4, 27, 27 a, 27 b, 107, 108 Aeration device 5, 28, 28 a Air supply channel 7 Membrane separation device 8, 26, 26 a, 78 Pump 9, 44, 45, 62, 65 Communication path 10, 23, 23a Separation membrane 11, 24, 24a Permeate chamber 12 Concentrate chamber 13, 25, 25a Treatment channel 14, 53 Excess sludge path 21 Aerobic digester 22, 22a Immersion Mold membrane separators 29 and 29a Partition walls 31 and 105 Ozone treatment tank 32 Aerobic digestion liquid path 33 Ozone treatment liquid path 35 and 106 Ozone generator 36 Ozone supply path 41 Denitrification tank 46 Ozone treatment liquid transfer path 47 Electron donor supply path 48 Stirrer 51 Nitrification tank 52, 63 Chemical injection path 61 Crystallization tank 64 Discharge path 71 Membrane separation tank 72 Oxygen enrichment apparatus 73, 73 a Oxygen enriched air supply path 74 Air lift pump 7 Return path 76 Air diffuser 77 Valve 101 Biological treatment tank 102, 104 Solid-liquid separation tank 103 Sludge digestion tank 111 Waste liquid 112 Return sludge 113 Part of mixed liquid 114 Treatment liquid 115 Separation sludge 116 Surplus sludge 117 Circulation sludge 118 Digestion liquid 119 Separation liquid 121 Digested sludge 122 Extracted sludge 123 Ozone gas 124 Ozone-treated sludge 125 Discharged sludge

Claims (4)

Biological treatment process for biological treatment of organic effluent,
A reforming process for reforming surplus sludge and / or its aerobic digested liquid discharged from the biological processing process to be readily biodegradable;
An aerobic digestion process in which surplus sludge discharged from the biological treatment process and / or its modified treatment liquid is introduced into an aerobic digestion tank to perform aerobic digestion;
A circulation process that circulates the modified treatment liquid of the reforming process to the aerobic digestion process, and the aerobic digestion liquid is solid-liquid separated by membrane separation and the concentrated solution is returned to the aerobic digestion process, and the permeate is digested. A biological treatment method for organic effluent, which includes a separation step of aerobic digestive juice discharged as water.   The method according to claim 1, wherein the biological treatment step is a biological treatment step including a nitrification step and a denitrification step.   The method according to claim 1 or 2, wherein the biological treatment step performs membrane separation as solid-liquid separation means. The method according to any one of claims 1 to 3, wherein at least a part of the aerobic digestion step is aerated with oxygen-enriched air.

Images