JPH0437760B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to a method for removing phosphorus and nitrogen from BOD-containing water. Conventional technology The presence of phosphorus and nitrogen is said to be the main cause of eutrophication in closed water bodies such as lakes, and there is an urgent need to develop technology to limit the inflow of phosphorus and nitrogen into such water bodies. be. On the other hand, urban sewage and industrial wastewater are generally treated using the activated sludge method.
Although BOD has been sufficiently removed, the current situation is that phosphorus and nitrogen have not been sufficiently removed. Development of methods for removing nitrogen from wastewater is progressing, and the cyclic nitrification-denitrification method that effectively utilizes BOD in wastewater is becoming an established technology as an economical and effective method. On the other hand, development has been rapidly progressing in recent years regarding the removal of phosphorus from wastewater. As a conventionally well-known method for removing phosphorus, there is a coagulation-sedimentation method using a coagulant, but this method cannot be said to be a satisfactory technique from an economic standpoint. Another method that has been developed in recent years and is attracting attention is a method that utilizes the phenomenon of excessive phosphorus intake in activated sludge. This method utilizes the phosphorus metabolism function of microorganisms to incorporate phosphorus into the sludge by repeatedly exposing activated sludge to anaerobic and aerobic conditions. On the other hand, as an economical method for removing phosphorus and nitrogen from organic wastewater, a method combining biological denitrification and biological dephosphorization has been developed, with complete anaerobic-anaerobic-aerobic activity. The sludge method is a typical example. The wastewater and returned sludge flow into a completely anaerobic tank and are kept in a completely anaerobic state in the absence of dissolved oxygen and NO _x (nitrogen oxides) such as nitrous acid and nitric acid, and the activated sludge releases PO ₄ -P. The mixed liquid that flows out of the completely anaerobic tank flows into the anaerobic tank where there is no dissolved oxygen, and comes into contact with and mixes with the circulating nitrification solution (aerobic treatment circulating solution).
At this time, NO _x in the circulating nitrification liquid reacts with organic matter (BOD) in the mixed liquid effluent from the complete anaerobic tank,
It is reduced and denitrified. The effluent mixed liquid from the anaerobic tank flows into the aerobic tank, where ammonia is oxidized to NO _x in the presence of dissolved oxygen, and phosphorus present in the liquid is taken up by activated sludge. The effluent mixed liquid from the aerobic tank is led to the final settling tank and subjected to solid-liquid separation, and the solid-liquid separated supernatant liquid is discharged outside the system as treated water, while a portion of the settled sludge is returned as sludge. The remaining sludge is sent to a complete anaerobic tank as surplus sludge, and the remainder is discharged outside the system as surplus sludge. However, in such a completely anaerobic-anaerobic-aerobic activated sludge method, phosphorus is removed only by the phosphorus that is discharged outside the system together with excess sludge, so it is difficult to remove phosphorus when the P/BOD of the wastewater is large or when the P/BOD of the wastewater is large. ΔWSS/ΔBOD (excess sludge/removal) for low-load operation
When the BOD) is small, it becomes difficult to discharge all of the inflowing phosphorus out of the system as phosphorus in excess sludge, resulting in the disadvantage that phosphorus leaks into the treated water. OBJECTS OF THE INVENTION The primary object of the present invention is to provide an effective and economical method for removing phosphorus and nitrogen from BOD-containing water. Structure of the Invention According to the present invention, a method for removing phosphorus and nitrogen from BOD-containing water is provided. The treated liquid after the anaerobic treatment is subjected to an aerobic treatment under a supply gas containing at least 50% by volume of oxygen, the treated liquid after the aerobic treatment is subjected to solid-liquid separation, and the treated liquid after the aerobic treatment is subjected to solid-liquid separation. The settled sludge obtained by solid-liquid separation exceeds 5 mg/NO _x
In cases where organic matter is added to the settled sludge, it is denitrified in an anaerobic state to reduce NO _x to 5
mg/ or less, and then the settled sludge is subjected to dephosphorization treatment. Specific explanation of the structure of the invention BOD-containing water, for example, organic wastewater flows into an anaerobic tank, and sludge is directly returned from the final settling tank and circulating nitrification liquid (aerobic treatment circulating liquid) from the end of the aerobic tank.
At this time, mainly organic matter (BOD) in the wastewater reacts with NO _x (nitrous acid and nitric acid) in the circulating nitrification solution, and the NO _x is reduced and denitrified. If desired, the dephosphorized sludge produced in the dephosphorization tank may be returned to the anaerobic tank as dephosphorized return sludge. In this specification, such anaerobic treatment in an anaerobic tank refers to treatment in an environment where dissolved oxygen (DO) is 0.5 mg/or less. The effluent mixture from the anaerobic tank then flows into the aerobic tank and is subjected to aerobic treatment under a feed gas containing at least 50% oxygen by volume to oxidize oxidizable substances such as ammonia and becomes NO _x such as nitrous acid and nitric acid. Moreover, the soluble phosphorus in the mixed liquid is taken up into the activated sludge. at least 50% by volume
The purpose of using a supply gas containing oxygen is to prevent the formation of an anaerobic environment within the aerobic tank, and under such an atmosphere, the amount of dissolved oxygen at the end of the aerobic tank is usually 5 mg/min or more. Furthermore, the aerobic tank is naturally a closed type. The mixed liquid discharged from the aerobic tank flows into the final settling tank where it is separated into solid and liquid, and the supernatant liquid is discharged outside the system as treated water. Usually, a gravity sedimentation type sedimentation basin can be advantageously used, but the method is not limited to this type of means, and any known solid-liquid separation means may be used. A portion of the settled sludge is directly returned to the anaerobic tank as return sludge, as described above. Dissolved oxygen in the final sedimentation tank supernatant is
It is preferable that the amount is 0.5mg/or more, and the amount of dissolved oxygen
If the concentration is less than 0.5mg/, phosphorus may leak into the treated water. A part or all of the remainder of the settled sludge from the final settling tank is sent to a dephosphorization tank and subjected to dephosphorization treatment, and the remainder is discharged outside the system as surplus sludge. At this time, the sludge flowing into the dephosphorization tank needs to have a total amount of NO _x of 5 mg/or less in order to perform efficient dephosphorization. In the present invention, when NO _x in settled sludge exceeds 5 mg/, this sludge is denitrified to reduce the NO _x to 5 mg/ or less, and then supplied to the dephosphorization tank. This denitrification treatment can be performed by supplying sludge to a sludge denitrification tank and placing it in an anaerobic state. By this method, it is possible to prevent the sludge from floating in the next dephosphorization tank and to rapidly release phosphorus from the sludge. The sludge denitrification tank may be either single-chamber or multi-chamber, open or closed, and the sludge may be stirred by either a gas circulation method or a mechanical stirring method. In order to promote the denitrification effect in a sludge denitrification tank, it is effective to inject organic matter, and many substances such as organic wastewater, methanol, acetic acid, etc. can be used as the organic matter. Dephosphorization treatment can be performed by a known method. For example, the dephosphorization can be carried out in an anaerobic environment as disclosed in Japanese Patent Publication No. 54-38823, and washing water is preferably introduced into the dephosphorization tank from the lower layer. The phosphorous-containing effluent wash water is then chemically treated to dephosphorize, and the sludge from the dephosphorization tank may optionally be returned to the anaerobic tank as dephosphorized return sludge, as described above, or A portion may be discharged outside the system. In addition, the dephosphorized washing water can be
It may also be supplied to an anaerobic tank. Examples Hereinafter, the present invention will be further explained by examples. The present inventors conducted a dephosphorization/denitrification test using initial settling overflow water from a certain sewage treatment plant. A flowchart of the experimental equipment used in this example is shown in FIG. Table 1 outlines the specifications of the main experimental equipment.

[Table] Anaerobic tank 1 is a closed tank and is partitioned into two compartments. The mixed solution was stirred by mechanical stirring. Further, the aerobic tank 2 is of a closed type and is partitioned into four compartments. Oxygen gas with a purity of over 99% was supplied to this aerobic tank, and aeration was performed by surface aeration. The DO at the end of aerobic tank 2 was 5 mg/or more, and the DO of the supernatant liquid in the final sedimentation tank 3 was 0.5 mg/or more.
Sludge denitrification tank 4 was installed from the beginning of the experiment,
It is a sealed tank and is divided into four compartments. This sludge denitrification tank contains inflow water (treated raw water) as organic matter.
A small amount of the same initial settling overflow water was injected. Table 2 shows the conditions used in this example, and Table 3 shows the conditions used in this example.
The table shows the treatment results. In the experiment, the dephosphorization tank 5, which had an effective volume of 5.1 m ³ , was operated with a volume of 1.0 m ³ by lowering the sludge blanket.

【table】

[Table] * Sludge amount standards

【table】

[Table] In the present invention, the aerobic treatment is at least 50% by volume.
This is done using a supply gas containing oxygen, and the DO at the end of the aerobic tank is set to 5 mg/min or more, so the mixed liquid that flows into the final sedimentation tank maintains an aerobic atmosphere for a long time, and therefore the mixing Reductive denitrification of NO _x accompanying the liquid is difficult to occur. As a result, sludge floating and fine SS suspension caused by nitrogen gas are extremely reduced, and the treated water has good SS.
That is, as shown in Table 3, the SS of the treated water is 4.
mg/, which was very good. In addition, in order to maintain a good SS of the treated water in this way, it is preferable to maintain the DO of the supernatant liquid in the final sedimentation tank at 0.5 mg/or more. Therefore, if the DO at the end of the aerobic tank is maintained at 5 mg/or more as described above, the DO of the supernatant liquid in the final sedimentation tank can generally be maintained at 0.5 mg/or more under normal operating conditions. It is. If the SS of the treated water is stably maintained at a low value, it will also lead to the BOD etc. of the treated water being stabilized at a low value, and especially in order to stabilize the T-P of the treated water at a low value. is important. _NOx is 5mg/in the sludge supplied to the dephosphorization tank
When the temperature exceeds 100%, the sludge rises to the surface in the dephosphorization tank due to the nitrogen gas generated by the reduction and denitrification of NO _x .
Also, the release of phosphorus from the sludge will be delayed. Therefore, when the amount of NO _x in the settled sludge from the final settling _tank exceeds 5 mg, the sludge is subjected to reduction and denitrification in the sludge denitrification tank before being supplied to the dephosphorization tank. This prevents the sludge from floating in the dephosphorization tank and promotes the release of phosphorus. Therefore, when the sludge after dephosphorization is returned as dephosphorization return sludge, the sludge is kept in the aerobic tank. This results in an active intake of phosphorus again. There was no sludge floating in the dephosphorization tank during the experimental period (18 days) in the example. Comparative Example The operations described in the Examples were repeated. However, here, the dephosphorization supply sludge 10 was directly supplied to the dephosphorization tank 5 without passing through the sludge denitrification tank 4. Table 4 shows the conditions used in this experiment, and Table 5 shows the treatment results.

[Table] * Sludge amount standards

【table】

[Table] The NO _x contained in the sludge supplied for dephosphorization was 6.3 mg/, and as a result of the delayed release of phosphorus from the sludge, the intake of phosphorus in the aerobic tank was insufficient. That is,
Treated water T-P concentration is 1.2mg/, T-P removal rate is
It was 67.6%.

[Brief explanation of drawings]

FIG. 1 is a flowchart of experimental equipment used in an example of the present invention. 1... Anaerobic tank, 2... Aerobic tank, 3... Final settling tank, 4... Sludge denitrification tank, 5... Dephosphorization tank, 6... Oxygen supply, 7... Inflow water, 8... Circulation Nitrification liquid, 9...
... Direct return sludge, 10 ... Dephosphorization supply sludge, 11 ...
... Surplus sludge, 12 ... Dephosphorization return sludge, 13 ... Washing water, 14 ... Organic matter supply, 15 ... Anaerobic tank effluent mixed liquid, 16 ... Aerobic tank effluent mixed liquid, 17 ... Treated water, 18 ...Sludge denitrification tank outflow sludge, 19...Dephosphorization tank outflow cleaning water.

Claims (1)

[Claims] 1 A method for removing phosphorus and nitrogen from BOD-containing water, which involves subjecting a mixture of BOD-containing water, activated sludge, and aerobic treatment circulation liquid to anaerobic treatment, and treating the treated liquid after the anaerobic treatment with at least 50% The sludge is subjected to aerobic treatment under a supply gas containing % by volume of oxygen, and the treated liquid after this aerobic treatment is subjected to solid-liquid separation, and the settled sludge obtained by the solid-liquid separation exceeds 5 mg/
In cases where NO _x is contained, organic matter is added to the settled sludge and denitrified in an anaerobic state to remove NO _x
5 mg/or less, and then subjecting this settled sludge to dephosphorization treatment.