JP2001025791A - Waste water treatment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve separation performance such as treated water quality by controlling a suspending solid material (MLSS) in accordance with water temp. in an aeration tank in a membrane separation activated sludge method. SOLUTION: In the method for purifying waste water by biologically treating water water in the aeration tank 2 and separating the suspending solid material (MLSS) by a membrane separation device 4, quantity of MLSS in the aeration tank 2 is controlled in accordance with the water temp. in the aeration tank to keep filtration performance of the membrane separating device 4 to a prescribed one or to be >=95% in BOD removing ratio in the purified water.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wastewater treatment method using a membrane separation activated sludge method, which is used for purification treatment of human waste, sewage, domestic wastewater and the like.

[0002]

2. Description of the Related Art In recent years, a membrane separation activated sludge method has been widely used in place of the traditional activated sludge method. As is well known, in the conventional activated sludge method (referred to as sedimentation separation activated sludge method), organic matter in sewage is sorbed and ingested by activated sludge floc in a tank, and the ingested organic matter is converted into microorganisms of activated sludge floc. Decompose by the oxidation catalysis of microorganisms and grow microorganisms. The solution in the tank is transferred to a sedimentation tank to separate and separate suspended solids. The supernatant is discharged and the solids separated and precipitated (activated sludge). ) Is returned to the tank to replenish the microorganisms. In contrast to the sedimentation activated sludge method, in the membrane separation activated sludge method, a liquid biologically treated in a tank is filtered and separated by a membrane separation device.

[0003] In the above-mentioned sedimentation-separation activated sludge method, when the concentration of the mixed liquid suspended solids (MLSS) in the tank is increased, the volume of the tank can be reduced.
When the value of is increased, the sedimentation velocity in the sedimentation tank becomes lower and the water area of the sedimentation tank needs to be increased. Therefore, the MLSS in the tank is usually set at 2000 to 6000 mg / liter as a standard. However, the membrane separation activated sludge method does not have such a restriction, and the MLSS in the tank is reduced by 200%.
It can be increased to about 00mg / liter,
Decrease the volume of the tank by the sedimentation separation activated sludge method.
Since the size can be reduced to about １０〜 to １ ／ and a sedimentation tank is not required, the installation space can be drastically reduced.

As is well known, when activated sludge is bulked in the sedimentation-separation activated sludge method, separation by natural sedimentation becomes impossible and activated sludge flows out, so that the seed activated sludge cannot be returned from the sedimentation tank to the tank. Processing instability has been further promoted. However, in the membrane separation activated sludge method, sludge is separated by membrane separation without sedimentation separation. For example, the BOD removal rate can be further increased. Thus, as an advantage of the membrane separation activated sludge method, a dramatic improvement in purified water quality and operability over the precipitation separation activated sludge method is generally highly evaluated. And MLS
S is 5000 to 20000 mg / depending on the volume of the tank.
It is almost constant within the liter range.

[0005]

However, according to the present inventors' examination results on the membrane separation activated sludge method, it was found that M
Under constant LSS conditions, it is difficult to maintain high quality purified water throughout the year. The reason is that in winter, the water temperature in the tank decreases, and the oxygen consumption rate per unit of MLSS decreases. As a result, the resolution of organic substances decreases, the BOD removal rate decreases, and the amount of organic substances accumulated without decomposition increases. At the same time, the anaerobic bacteria may grow and the liquid viscosity may increase, and in summer, the temperature of the water in the tank is high, so the dissolved oxygen amount cannot be secured sufficiently and the oxygen absorption of the activated sludge is performed satisfactorily. Without this, the BOD removal rate decreases, the amount of organic matter accumulated undegraded increases, and at the same time, the anaerobic bacteria grow and the viscosity of the solution may increase, resulting in the formation of a cake layer on the membrane surface. It is presumed that the film performance is lowered by the formation.

[0006] As described above, in the membrane separation activated sludge method, even if the MLSS in the separation tank is operated at a constant level to simplify the sludge management, the quality of the treated water is considerably higher as compared with the sedimentation separation activated sludge method. Although it can be improved, it is difficult to say that the membrane separation performance is rationally used, and there is still room for improving the purification performance such as the quality of treated water.

[0007] An object of the present invention is to provide a method for controlling the temperature of water in a tank based on the results of the above study on the membrane separation activated sludge method.
The purpose of the present invention is to improve the separation performance such as the quality of treated water by adjusting the LSS.

[0008]

According to the present invention, there is provided a wastewater treatment method, wherein wastewater is biologically treated in a tank, and suspended solids (MLSS) in the tank are separated by a membrane separator. In the method of purifying wastewater by separation treatment, the amount of MLSS in the tank is controlled so that the filtration performance of the membrane separation device is maintained at a predetermined performance or the BOD removal rate of purified water is 95% or more. MLSS at a water temperature of 30 ° C or higher, characterized in that it is adjusted according to the water temperature in the tank.
M at a water temperature of 25 ° C. or higher and lower than 30 ° C.
LSS amount is 120 to 160, MLSS amount at water temperature 20 ° C or higher and lower than 25 ° C is 160 to 200, MLSS amount at water temperature 15 ° C or higher and lower than 20 ° C is 200 to 280, water temperature 10
The MLSS amount at 15 ° C. or higher at 280 to 360 ° C. or higher,
The MLSS amount at a water temperature of 10 ° C. or less can be 360 to 420.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an example of a membrane separation wastewater treatment apparatus used in the present invention. In FIG. 1, reference numeral 1 denotes a raw water tank. Reference numeral 2 denotes a tank, and reference numeral 3 denotes a liquid feed pump for feeding raw water into the tank. Reference numeral 4 denotes a membrane separation device immersed and installed in the tank 2, and a microfiltration membrane, an ultrafiltration membrane, or the like is usually used as the membrane. The membrane separation device 4 includes a flat membrane type, a capital type, a tuber type,
A rotating disk type or the like can be used. Reference numeral 5 denotes a suction pump for reducing the pressure on the filtration side of the membrane separation device 4, and reference numeral 6 denotes a filtration water tank (treatment water tank). Reference numeral 7 denotes an air diffuser, and reference numeral 8 denotes a blower for sending air to the air diffuser 7. 9 is a sludge extraction pipe, and 10 is a sludge extraction pump.

In operation of this membrane separation wastewater treatment apparatus, the membrane surface of the membrane separation apparatus 4 in the tank 2 is washed by blowing air from the air diffuser 7. In addition, an organic substance is ingested into the MLSS floc floating in the liquid in the tank 2, and in the presence of oxygen absorbed in the floc by respiration, the organic substance is decomposed by using the microorganisms in the floc as an oxidation catalyst and the microorganisms are removed. Activated sludge increases due to the proliferation of However, if there is an imbalance between the oxygen supply from the air diffuser 7 and the oxygen consumption by the microorganisms in the flocks and the oxygen solubility of the liquid, the fluctuation of the dissolved oxygen amount (DO) cannot be avoided,
Therefore, the oxygen solubility of the solution in the tank 2 decreases due to the rise in the water temperature in the tank 2, or the ML per unit volume of the liquid is reduced.
When dissolved oxygen (DO) decreases due to an increase in SS and oxygen consumption becomes excessive and the dissolved oxygen (DO) decreases and becomes anaerobic, microorganisms in the activated sludge floc lose their activity or die and are one of the causes of bulking As a result, the organic matter absorbed in the floc is accumulated without being decomposed, and the floc is made highly viscous. As a result, the properties of the liquid in the tank are deteriorated.

[0011] The deterioration of the properties of the solution in the tank promotes the formation of a cake layer on the membrane surface of the membrane separation device 4. This leads to an increase in the transmembrane pressure difference under certain conditions, and inevitably decreases the separation efficiency and increases the frequency of membrane washing.

Thus, in the wastewater treatment method according to the present invention, the activated sludge (MLS
S) Change in oxygen consumption (MLSS increases as water temperature rises)
Due to the increase in oxygen consumption per unit amount) and the change in the amount of dissolved oxygen due to the change in the water temperature in the tank (the solubility decreases as the water temperature increases), the dissolved oxygen in the tank increases as the water temperature increases. DO) decreases and approaches an anaerobic state, and when the water temperature is high, the amount of activated sludge per unit volume of liquid in the tank is reduced, the oxygen consumption is reduced, and DO is kept sufficiently high. Considering that when the water temperature in the tank becomes extremely low, the DO becomes too large to cause an excess oxygen state, and when the water temperature is low, the amount of activated sludge per unit volume of the tank in the tank is increased. Oxygen can be effectively used as a whole for the decomposition of organic matter by activated sludge.

In the activated sludge treatment, as described above, the MLSS in the storage tank increases with time due to the growth of microorganisms, so that it becomes larger than the MLSS of the liquid supplied from the raw water tank. If the amount of MLSS extracted from the tank is V, the amount of MLSS sent from the raw water tank is v, and the flow rate under the same amount of the amount of wastewater and the amount of activated sludge withdrawn is f, the MLSS in the tank Is given by f (V−v), while P is the increase in MLSS due to proliferation,
The MLSS in the tank can be grasped by [f (V−v) −P], and the MLSS in the tank can be adjusted by adjusting the normal flow rate f.

In the wastewater treatment method according to the present invention, for example, a constant amount of filtration flow rate can be stably maintained under a substantially constant pressure difference between the membranes of the membrane separation device so as to maintain the separation performance of the membrane separation device at a predetermined performance. The MLSS of the solution in the tank is adjusted by adjusting the flow rate f according to the water temperature in the tank so as to ensure the quality of the filtrate solution of high quality (for example, a BOD removal rate of 95% or more). The adjustment can be performed by setting the MLSS amount at a water temperature of 30 ° C. or higher as 100 and the MLSS amount at a water temperature of 25 ° C. or higher and lower than 30 ° C. as 120 to 16%.
0, the MLSS amount at a water temperature of 20 ° C or higher and lower than 25 ° C is 16
MLSS at 0 to 200, water temperature 15 ° C or higher and lower than 20 ° C
The amount of MLSS at a water temperature of 10 ° C or higher and lower than 15 ° C is 280 to 360, and the ML at a water temperature of 10 ° C or lower.
It is preferable to set the SS amount to 400.

According to the present invention, it is possible to prevent the liquid in the tank from becoming highly viscous or deteriorating due to the accumulation of non-decomposed organic substances, the death of active microorganisms, or the growth of filamentous fungi resistant to anaerobic conditions. The formation of a cake layer on the membrane surface of the apparatus can be suppressed well, and high-quality purified water can be obtained in a stable separated state, as will be apparent from the examples described later.

In the membrane separation device used in the above embodiment, the filtration side of the membrane is depressurized by a suction pump in order to obtain a transmembrane pressure difference. However, the tank is located higher than the filtrate tank. Install to obtain the transmembrane pressure by the head pressure between the liquid level in the tank and the liquid level in the filtrate tank, and the top of the liquid level in the tank and the top of the filtrate discharge pipe from inside the membrane. It is also possible to obtain a transmembrane pressure difference by increasing the water head during the period.

Further, in the membrane separation wastewater treatment apparatus, as shown in FIG. 2, a membrane separation apparatus 40 is installed outside the tank 2,
The tank in the tank is fed under pressure to the membrane separation device 40 by the pressurized liquid sending pump 41, the filtrate is taken out to the filtrate tank 6, and the non-filtrate is returned to the tank 2. Can also be used. 2, the same reference numerals as those in FIG. 1 indicate the same components.

[0018]

[Example] [Example] A membrane separation wastewater treatment apparatus having the structure shown in Fig. 1 was used, and a polyolefin ultrafiltration membrane was used for a flat membrane of the membrane separation apparatus. 1
The average filtration flow rate is 0.5m ³ /
The suction pump is driven so as to set m ² · day, and the MLSS in the tank is reduced to 5,000 when the water temperature in the tank is 30 ° C. or more.
0-6,000mg / l, water temperature in the tank is 25 ℃
6,000 to 8,000 mg / liter for ~ 30 ° C, 8,000 ~ for water temperature in the tank of 20 to 25 ° C
10,000mg / L, water temperature in the tank is 15 ℃ ～
10,000 to 14,000 mg / liter for 20 ° C, 14,000 for water temperature in the tank of 10 to 15 ° C.
~ 18,000mg / l, water temperature in the tank 10 ℃
It was operated for one year while adjusting to 18,000 to 20,000 mg / liter. Water temperature in the tank during operation (℃), B per unit volume of the tank
OD volume loading ^{(Kg / m 3 · d)} , the suction pressure of the suction pump (kP
a), DO (mg / liter) in the tank, BOD of treated water
Table 1 shows the results of measurement of the removal rate (%) (all of them are average values, the same applies hereinafter).

[0019]

[Table 1]

[Comparative Example 1] The MLSS in the stripping tank was adjusted to a normal value (10,000 to 14,0) according to a conventional membrane separation activated sludge method.
00 mg / liter), and was the same as the example except that adjustment was not made particularly in relation to the water temperature in the tank.
During the operation, the water temperature in the tank (° C.) and the BOD volume load per unit volume of the tank (Kg / m ³ · d) were substantially common to the examples and operated in parallel with the examples. The suction pressure (kPa) of the suction pump during operation, the DO (mg / liter) in the tank, and the BOD removal rate (%) of the treated water were measured (all are average values). In December, continued operation was virtually impossible due to a significant increase in filtration resistance.

[Comparative Example 2] A sedimentation tank was provided in place of the membrane separation apparatus. The volume of the tank was the same as that of the embodiment, and the sedimentation separation activated sludge method was used.
1,600mg / L for water temperature of ℃ or more, 2,000-2,700mg / liter for water temperature in the tank of 25 ℃ -30 ℃
Liter, 2,7 for water temperature of 20 ℃ ～ 25 ℃
00-3,400mg / liter, water temperature in the tank 15
3,400-4,700mg / L for ℃ -20 ℃, 4,700〜10 ℃ for water temperature in the tank
The operation was carried out for one year while adjusting to 6,000 mg / liter and 6,000 to 8,000 mg / liter for a water temperature in the tank of less than 10 ° C. The DO (mg / liter) in the tank and the BOD removal rate (%) of the treated water during operation were measured (all were average values).

As is apparent from Table 1, in the membrane separation activated sludge method of Comparative Example 1, the DO in the tank became low in about 5 to September when the tank temperature was high and the suction pump sucked. As the pressure increases and the BOD removal rate of the treated water decreases, the active decomposition effect decreases due to lack of oxygen in the tank, and the viscosity of the tank in the tank increases due to accumulation of non-decomposed organic substances. It is clear that the separation performance is deteriorated due to the formation of the film surface cake layer. However, in the embodiment, the suction pressure is stable, and the quality of the treated water is high and stable. In addition, from the comparison between Comparative Example 2 and Example, according to the present invention, higher-quality purification treatment is possible than in the sedimentation-separation activated sludge method in which the MLSS in the tank is adjusted to prevent bulking. It is clear that.

[0023]

According to the wastewater treatment method of the present invention, the membrane separation activated sludge method is further improved in the separation performance by adjusting the MLSS in accordance with the water temperature in the tank so as to further improve the treated water quality. It has been experimentally confirmed that stabilization and quality improvement can be achieved. The operation is performed while adjusting the amount of MLSS in the tank in accordance with the water temperature in the tank. In addition, industrial wastewater treatment and the like can be performed with high purified water quality and stable operation under drastic reduction of installation space.

[Brief description of the drawings]

FIG. 1 is a drawing showing an example of a membrane separation wastewater treatment apparatus used in the present invention.

FIG. 2 is a drawing showing another example of a membrane separation wastewater treatment device used in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Raw water tank 2 Deposition tank 4 Membrane separation apparatus 40 Membrane separation apparatus 5 Suction pump 6 Treatment water tank 7 Air diffuser 8 Blower

Claims (3)

[Claims] 1. A method for purifying wastewater by biologically treating wastewater in a tank and separating floating solids (MLSS) in the tank by a membrane separator. A wastewater treatment method comprising adjusting the amount of MLSS in a tank according to the temperature of water in a tank so that the filtration performance of the apparatus is maintained at a predetermined level. 2. A method for purifying wastewater by biologically treating the wastewater in a tank and separating the suspended solids (MLSS) in the tank by a membrane separator to purify the wastewater. Wastewater treatment method, wherein the amount of MLSS in the tank is adjusted in accordance with the water temperature in the tank so that the BOD removal rate is 95% or more. 3. The amount of MLSS at a water temperature of 25 ° C. or higher and lower than 30 ° C. is set to 1 assuming that the MLSS amount at a water temperature of 30 ° C. or higher is 100.
MLS at 20-160, water temperature 20 ° C or higher and lower than 25 ° C
The S amount is 160 to 200, the MLSS amount at a water temperature of 15 ° C. or higher and lower than 20 ° C. is 200 to 280, and the water temperature is 10 ° C. or higher.
The wastewater treatment method according to claim 1 or 2, wherein the MLSS amount at a temperature lower than 10C is 280 to 360, and the MLSS amount at a water temperature of 10C or lower is 360 to 420.