JP4734160B2 - Septic tank - Google Patents

Description

  The present invention relates to a membrane separation type combined treatment septic tank in which an immersion sludge separation device is embedded in an activated sludge treatment tank. More specifically, the sludge in an activated sludge treatment tank that exceeds a predetermined water level is prevented or suppressed from being reduced in filtration capacity. The present invention relates to a membrane separation type combined treatment septic tank that is returned to a previous treatment tank through an apparatus for performing the process.

It is socially important to comprehensively purify domestic wastewater to prevent environmental pollution, and the application of the submerged membrane filtration method to the septic tank is also focused on development (for example, Japan Environmental Education Center "Research Report on Practical Use of Small Domestic Wastewater Treatment System Introducing Membrane Treatment Method", see 1992-1995).
Here, the submerged membrane filtration method is a method in which the activated sludge treatment liquid is filtered by a submerged membrane separator embedded in the activated sludge treatment tank of the septic tank, instead of the conventional sedimentation separation. While supplying oxygen, the flow accompanying the rise of bubbles is used for cross flow to suppress the growth of the filtration resistance layer on the film surface.

As pointed out in the above-mentioned report, in order to maintain the filtration capacity, the concentration of sludge in the activated sludge treatment tank requires an MLSS concentration of approximately 3000 mg / L or more (hereinafter referred to as the lower limit concentration). Before starting the operation of the submerged membrane separator, seed sludge is introduced so that the concentration is above the lower limit.
On the other hand, after the start of operation, even if the MLSS concentration exceeds approximately 15000 mg / L (hereinafter referred to as the upper limit concentration), the filtration capacity decreases (supervised by the Ministry of the Environment Septic Tank Countermeasures Office, “Nitrogen removal type / membrane separation type small The merger processing septic tank maintenance management guideline and commentary ”, Japan Environmental Education Center, issued in November 2001), sludge from the activated sludge treatment tank is returned to the pretreatment tank, or sludge provided separately It is pulled out to the storage tank and is kept below the upper limit concentration.

  However, the filtration capacity declined for some reason, and the sludge from the activated sludge treatment tank overflowed a large amount into the pretreatment tank. It is not uncommon for wastewater that has not been biologically treated to flow out rapidly as the further decline of the wastewater continues. Nevertheless, even when several years have passed since this method was put to practical use, no means for preventing or suppressing the concentration below the lower limit concentration due to overflow has been taken.

  In such a situation, the filtration capacity rarely recovers naturally, so conventionally, the membrane was washed with an aqueous solution of sodium hypochlorite to restore the filtration capacity, and seed sludge was re-introduced. Or it was necessary to restart the operation after replacing the entire amount of sludge. However, such a method is too burdensome to maintain. In addition, it is unknown about the existence of the literature regarding the above-mentioned prior art.

  Thus, the problem to be solved by the present invention is to provide a simple and economical means for suppressing a decrease in filtration flow rate caused by a decrease in MLSS concentration due to overflow, so that wastewater that has not been biologically treated does not flow out. .

  Although it has not yet been confirmed, according to the reports mentioned above, the basis of the lower limit concentration is the minimum concentration of activated sludge necessary for adsorbing filtration resistance substances in domestic wastewater, and the upper limit concentration The basis for this is a sudden rise in viscosity. In relation to the latter, if the sludge polymer secretion contains a filtration resistance substance (see, for example, Shinichiro Hamaya, et al., 38th Sewerage Research Conference Lecture, 7-94 (2001)), sludge The concentration of the filtration resistance substance increases with the concentration of. It is considered that the filtration flow rate is stabilized between the lower limit concentration and the upper limit concentration, which are in an appropriate range, because most of these filtration resistance components are adsorbed by the sludge.

However, after the start of operation, the reason why the conventional apparatus did not take any measures to prevent or suppress the concentration below the lower limit concentration is that the filtration flow rate actually decreases when the concentration falls below the lower limit concentration due to overflow. The fact that the decrease in the filtration flow rate is actually suppressed if it is maintained above the lower limit concentration, that is, whether the concentration of the filtration resistance substance is uniformly determined by the MLSS concentration is not necessarily clear. .
Therefore, the present inventors have examined the relationship between the filtration flow rate and the MLSS concentration again in detail from various viewpoints.

Regarding the lower limit concentration, activated sludge will adsorb BOD components in domestic wastewater containing filtration resistance substances in the process of extracellular enzyme reaction, and it is considered that there is a strong correlation with the reaction activity.
On the other hand, with regard to the upper limit concentration, while the extracellular enzyme reaction is active, the filtration resistance substance in the wastewater is actively adsorbed to the sludge, but when the sludge concentration increases, the sludge circulates uniformly due to the increase in viscosity. It is considered that not only does the supply of oxygen stagnate, the reaction activity decreases, and a free filtration resistance substance remains, but also a precipitate layer is formed. Therefore, if the sludge which formed the sedimentation layer secretes the filtration resistance substance, it is considered that the filtration resistance substance in the waste water and the secreted filtration resistance substance which are not adsorbed in this layer are gradually accumulated.

  If the extracellular enzyme reaction is active, nitrogen-containing components are oxidized to nitrate nitrogen (hereinafter referred to as NO3-N), so NO3-N relative to the total nitrogen concentration in the filtrate (hereinafter referred to as TN) The ratio should represent the activity of the extracellular enzyme reaction. Using this ratio to verify the above estimate of the lower limit concentration, in fact, the smaller this ratio, the lower the filtration flow rate. Conversely, when this ratio is close to 1, the MLSS concentration is less than 3000. However, the filtration flow rate was high, or even if it was less than 3000, the decrease in the filtration flow rate was relatively small.

  On the other hand, in order to verify the above estimation regarding the upper limit concentration, the relationship between the MLSS concentration, the NO3-N / TN ratio and the sludge concentration distribution was first examined. As a result, if the concentration of the upper layer MLSS circulating well exceeds a certain value that differs slightly depending on the tank structure and aeration volume, the NO3-N / TN ratio actually decreases, and the increase rate of the upper layer sludge concentration slows down. At the same time, the lower layer region with high concentration spreads toward the upper layer. As the MLSS concentration further increased, the upper layer sludge concentration reached a peak, and the proportion of the sediment layer that hardly circulated increased rapidly.

The filtration flow rate decreased with such a change in MLSS concentration. That is, there are few filtration resistance substances in the well-circulated layer with high NO3-N / TN ratio, and when the ratio of this layer is large, the filtration flow rate is large, but the ratio of the precipitation layer containing a lot of filtration resistance substances increases. At the same time, the filtration flow rate decreased.
Therefore, if the upper layer flows out of the sludge at or near the top due to overflow, not only the ratio of the layer that has settled increases, but also the filtration resistance material flows out of the precipitated layer that has been dispersed again due to a decrease in viscosity. The filtration flow rate decreases rapidly. As a result, the amount of overflow is further increased and the activated sludge is further reduced, so that the reduction of the filtration flow rate is accelerated.

However, if the sludge in the sediment layer containing a large amount of filtration resistance substance can be selectively discharged at the time of overflow, the ratio of the high active layer increases, so the decrease in the filtration flow rate is reduced, and the filtration flow rate is recovered. You can even expect that.
Thus, the problem of drainage of untreated biological wastewater due to the decrease in the filtration flow rate caused by the decrease in the MLSS concentration has been solved, and the present invention has been achieved.
That is, the present invention provides an activated sludge treatment tank equipped with a submerged membrane separator, and when the liquid level of the activated sludge treatment tank exceeds a predetermined water level, the lower end installed in the activated sludge treatment tank has the activated sludge treatment tank. Using a return device consisting of a trunk pipe near the bottom of the sludge treatment tank and a branch pipe branched from the trunk pipe at the water level, the sludge near the bottom is returned to the treatment tank upstream of the activated sludge treatment tank. It is comprised so that the liquid of the said processing tank of the front | former stage may be comprised so that it may transfer in the trunk | pipe of the said return apparatus with a transfer pump (Claim 1). Specifically, the outlet of the transfer pump is arranged to flow into the trunk pipe of the return device. In addition, this septic tank is suitable for the scale which carries out the merge processing of the domestic wastewater whose object number is 10 or less.

By the above means, in the case of overflow, the sludge containing a large amount of filtration resistance component is selectively returned to the previous tank, and the sludge that actively adsorbs the filtration resistance substance remains, so that the filtration flow rate is further reduced. Not only is suppressed, but often even filtration flow recovery is seen.
As a result, the problem of drainage of non-biologically treated wastewater due to a decrease in filtration flow rate due to overflow is solved. Is it possible to recover the filtration flow rate by washing the membrane with an aqueous solution of sodium hypochlorite and re-entering seed sludge? Alternatively, the burden on maintenance that has been performed in the past, in which the operation is resumed after replacing the entire amount of sludge, is remarkably reduced.

In the septic tank of the present invention, even when an overflow occurs, the MLSS concentration does not become lower than the lower limit concentration that causes a rapid decrease in the filtration flow rate. The filtration flow often recovers naturally. Therefore, the maintenance effect is particularly great.
In addition, the intermittent operation method not only consumes less energy, but also has the advantage of removing nitrogen components because sludge is anaerobic during stoppage, but also has the disadvantage that the filtration performance tends to decrease because the filtration time is shortened. is there. Therefore, an intermittent operation type membrane separation type combined treatment septic tank has not been put on the market yet. However, this disadvantage is greatly improved by the present invention.
Furthermore, the sludge circulation device of the present invention has a simple structure and is economical, and can be applied to most septic tanks.

Hereinafter, an embodiment of the present invention will be described with reference to FIG. However, this figure is for explaining the present invention, and does not limit the present invention.
The membrane separation type combined treatment septic tank 1 shown in FIG. 1 comprises a solid-liquid separation tank 2, an activated sludge treatment tank 3 and a disinfection tank 4, and the preceding treatment tank in this septic tank is the solid-liquid separation tank 2. .
Although there is no particular reason for limiting the scale of the septic tank to which the present invention is applied, generally, as the size of the septic tank is reduced, the water quality of the domestic effluent changes more severely, and the situation where the filtration flow rate suddenly decreases increases. The present invention exerts a remarkable effect on such a septic tank. Therefore, a particularly effective application target of the present invention is a septic tank of about 10 people or less.

  First, the wastewater treatment flow will be described. The domestic wastewater flowing into the solid-liquid separation tank 2 from the inlet 11 is transferred to the activated sludge treatment tank 3 separated from the solid-liquid separation tank 2 by the partition plate 5 after removing impurities by natural precipitation and the filter medium 9. It is transferred by the pump 7. The activated sludge treatment liquid filtered by the submerged membrane separator 6 is sent to the disinfection tank 4 by the filtrate pump 10 and discharged from the outlet 12 to the outside. The overflow of the activated sludge treatment tank 3 is returned to the solid-liquid separation tank 2 via the return device 8. The structure and function will be described in detail below.

As the transfer pump 7, a semi-quantitative pump that normally pushes out a certain amount of liquid stored in the pump chamber by air pressure is used. The entrance of the suction pipe 71 is at the lower limit position of the fluctuation range 16 of the water level represented by rough stippling, and the transfer automatically stops when the water level falls below the lower limit.
The outlet 72 of the transfer pump 7 is preferably located at a position where it does not interfere with operations such as periodic inspections, and the falling transfer liquid does not directly hit the membrane module 61 of the submerged membrane separation device 6. The transfer flow rate is normally set to 1.5 to 3 times the average inflow flow rate of waste water.
Although illustration is omitted, when the water level of the solid-liquid separation tank 2 exceeds the upper limit of the fluctuation range 16, it flows to the disinfection tank 4 without being biologically treated and is discharged to the outside.

  The activated sludge treatment tank 3 is provided with a submerged membrane separation device 6 comprising a membrane module 61 and a bubble guide tube 62. As the membrane module 61, any of known hollow fiber type and plate type can be used in the present invention. However, the membrane module composed of a number of tubular membranes developed by the present inventors is remarkably compact and economical. It is particularly preferable (see Japanese Patent Application Nos. 2000-363389 and 2000-363461). Further, since the tubular membrane itself has an air lift action, this module can be used even when it is considerably exposed from the activated sludge liquid. Therefore, the capacity of the activated sludge treatment tank 3 can be made relatively small. Therefore, this module is used in FIG.

As the tubular membrane, a known microfiltration membrane having a pore diameter of about 0.1 to 1 μm is used. For example, in the method described in Japanese Examined Patent Publication No. 56-35483, a tape-like film reinforced with a non-woven fabric is wound around a mandrel that defines an inner diameter in a spiral manner while superimposing the peripheral portions that are overlapped with each other in the longitudinal direction. A tubular membrane having an arbitrary hole diameter or inner diameter is continuously produced by sonic welding.
If the inner diameter is small, it is likely to be clogged with sludge. If the inner diameter is large, the membrane area per unit volume of the module is small, so the inner diameter suitable for the present invention is 8 to 15 mm, more preferably 10 to 13 mm.
Tubular membrane module is very lightweight and compact. For example, about 600 pipe membranes with an inner diameter of 11 mm and a thickness of 0.2 mm in a PVC cylinder with an inner diameter of about 30 cm and a length of about 57 cm so that the effective length is about 50 cm. When focused and fixed, the weight is about 3 kg and the effective membrane area is about 10 m ² that can accommodate at least 10 tanks.

In the bubble guide cylinder 62, there are an air diffuser 64 that sends bubbles almost uniformly to all the tubular membranes, and an eye for subdividing sludge masses that block the flow path of the activated sludge treatment liquid in the tubular membranes. A net 63 with an opening of about 6 mm is attached. Bubbles sent almost uniformly from the bubble guide cylinder 62 to the entire membrane module 61 are raised from the upper end of the membrane module after ascending with the activated sludge liquid. Therefore, during aeration, the activated sludge liquid circulates inside and outside the membrane module, and filtration is performed during this time.
As the filtration method, suction filtration, siphon filtration, and gravity filtration can all be used in the present invention. However, the gravity filtration method is preferable because the equipment is simple and less energy is required. In this example, the gravity filtration method is used. ing.

The filtrate line branches into two at a position 65 that determines the reference point of the filtration pressure, one line 67 is open to the atmosphere at the tip, and the other line 66 (drawn linearly in the drawing, actually The hose is used so that the submerged membrane separation device 6 can be easily attached and detached) is connected to the filtrate pump 10. As the filtrate pump 10, an air lift type pump is usually used.
The difference between the branch point 65 and the position of the activated sludge treatment liquid level is the filtration pressure. The liquid level of the activated sludge treatment liquid fluctuates in the range 17 shown by the dense stippling, and the filtration flow rate becomes maximum at the upper limit water level. The lower limit water level at which the filtration flow rate becomes 0 is lower than the branch point 65, as described above, the activated sludge treatment liquid is lifted to a position higher than the liquid level of the activated sludge treatment liquid by the air lift action of the tubular membrane. Because.

Air is supplied from one air pump 13 to the transfer pump 7, the air diffuser 64, and the filtrate pump 10, and the respective air flow rates are adjusted by valves 14 and 15. Of course, each air flow rate may be precisely controlled using a plurality of air pumps.
The overflow occurs when the upper limit water level of the activated sludge treatment tank 3 is exceeded, and is returned from the trunk pipe 82 of the return device 8 showing an example of the present invention to the vicinity of the drainage inlet 11 of the solid-liquid separation tank 2 through the branch pipe 81. . Here, since the overflowing sludge and the filtration resistance substance are precipitated and separated, the liquid transferred to the activated sludge treatment tank 3 hardly contains them.

  The condition for the occurrence of overflow depends on the inflow pattern of the waste water that changes every day, and thus cannot be specified accurately, but is generally when the maximum filtration flow rate is less than the average transfer flow rate. If the maximum filtration flow rate is further reduced to below the average inflow flow rate of the wastewater, the water level in the solid-liquid separation tank 2 exceeds the upper limit of the fluctuation range 16 indicated by rough sketch, and although not shown, the biological treatment is not performed. It flows into the disinfection tank 4 and is released to the outside world. However, as described above, since the average transfer flow rate is 1.5 to 3 times the average inflow flow rate of waste water, even if an overflow occurs, untreated waste water does not immediately flow out.

  For the main pipe 82 and the branch pipe 81, pipes made of JIS standard vinyl chloride are usually used. The inner diameter of the tank is about 25 mm for a 5-person tank and about 30 mm or more for a 10-person tank. In the case of sludge in which scum is likely to adhere to the inner surface of the trunk pipe 82, it may be thicker than the branch pipe 81, but it does not need to be thick enough to obstruct work such as periodic inspection, and is preferably about 100 mm or less. Further, if it is advantageous in manufacturing, the trunk pipe 82 may be molded together with the tank, and the branch pipe 81 may be attached later.

  About the upper end of the trunk pipe 82, since overflow flows through the branch pipe 81 without stagnation, it only needs to extend several cm from the position where the branch pipe 81 branches. On the other hand, the lower end of the trunk pipe 82 is as close to the bottom as possible so that the sludge that actively adsorbs the filtration resistance substance is not included as much as possible, and the sludge containing a large amount of the filtration resistance substance selectively returns to the solid-liquid separation tank 2. It is desirable. However, if it is too close, the sludge that has been deposited will be clogged and the fluidity will be lowered, so it is practically necessary to be 3 to 7 cm away from the bottom.

FIG. 2 shows the present invention for preventing clogging due to adhesion / growth of sludge and scum that is likely to occur when an overflow does not occur for a long period of time by sending the transfer liquid into a relatively thick trunk pipe 82 and flowing the sludge therein. It is another example of the return device.
The tip of the branch pipe 81 is placed at a position away from the suction pipe 71 of the transfer pump 7 so that the sludge and the filtration resistance substance that are overflowing are not transferred to the activated sludge treatment tank 3 again. However, also in this method, it is preferable to extend the tip of the branch pipe 81 to the vicinity of the inlet 11 as shown in FIG. 1 so that the overflowing sludge or the like does not accumulate on the filter medium 9.

  Although this method can easily prevent the main pipe 82 from being blocked by sludge or scum, since a part of the transferred liquid also returns to the solid-liquid separation tank 2, if a large amount of overflow continues for a long period of time, the solid-liquid separation tank 2 gradually increases. There is a risk that the concentration of the BOD component and the like will increase, and the concentration of the filtrate will also increase. However, such a situation hardly occurs in reality, but rather the effect on maintenance is much greater.

Membrane-separated combined treatment septic tanks not only for BOD components but also for nitrogen removal have various combinations of tank configuration, operation method, and processing flow, and some are directed to two tanks with different overflow functions. Yes (for example, the above-mentioned Japan Environmental Improvement Education Center "Research Report on Practical Use of Small Wastewater Treatment Equipment Introducing Membrane Treatment Method", Heisei 1992-1995, Akira Yabashi, et al., Monthly Septic tank, No. 267, 33, 1998, etc.)
The present invention is also applicable to such a septic tank, but at that time, the overflow is reduced so that the reduction or reduction of the MLSS concentration in the activated sludge treatment septic tank, which is the object of the present invention, is efficiently performed. It is also necessary to devise proper distribution to the two tanks.

The operation method of the septic tank is roughly divided into continuous operation and intermittent operation, and the present invention can be applied to both. Although continuous operation literally means a system that operates continuously for 24 hours, intermittent operation is an operation system that periodically repeats operation and stoppage, and operations other than the inflow of domestic wastewater are stopped during the stoppage. In the septic tank of FIG. 1, all operations other than the inflow of domestic wastewater are stopped by stopping the air pump 13, which is the only power source.
If the ratio of the stop time is too large, not only will the filtration time be shortened, but the membrane surface cleaning effect by aeration will decrease, or the filtration flow rate will decrease quickly regardless of the MLSS concentration. Therefore, also in the present invention, it is desirable that the ratio between the operation time and the stop time is approximately 0.7 or more. Also, if the stop time is too long, there is a high risk that the domestic wastewater will flow out without being biologically treated if a large amount of domestic wastewater flows in temporarily or suddenly, so within 60 minutes, preferably within 40 minutes. Should stop.

Compared to continuous operation, the filtration time is shorter, so there is less margin of filtration capacity for intermittent operation. When the filtration flow rate decreases for some reason not depending on the MLSS concentration, or when a large amount of sewage flows in a short time, it is untreated. The risk of drainage is increased.
For such an unexpected situation, if the water level of the solid-liquid separation tank 2 or the activated sludge treatment tank reaches the upper limit water level, the continuous operation is performed, and if the water level falls below the water level, the intermittent operation is resumed. The capacity is fully utilized and the risk of spillage of untreated wastewater is greatly reduced. It is easy to give such a function within the range of known techniques, and detailed description will not be necessary.

Next, the present invention will be described in more detail with reference examples in which the general household wastewater of the apartment house is treated in the septic tank of FIG.
Table 1 shows the main specifications of the septic tank 1 and membrane module 61 for five persons. The pore diameter of the tubular membrane is about 0.4 μm. The filtration method is gravity filtration, the operation method is continuous operation, and the amount of aeration is about 140 L / min.

The wastewater treatment amount per day in the reference example is 1 m ³ , and the septic tank has the pattern defined in the Japan Building Center “Septic tank performance evaluation method (additional commentary version)” Verl.05 (updated on October 19, 2001) 1 was allowed to flow. The transfer flow rate is 2.1 L / min, which corresponds to the design maximum daily throughput of 3 m ³ (however, the flow rate of the transfer pump 7 varies depending on the water level of the solid-liquid separation tank, and was about 1.5 L / min on average) It is. The return device 8 is not installed in this tank.

The amount of water discharged from the three houses was approximately 1 m ³ per day, and the transfer flow rate was set in the same way as in the reference example. The condition for the occurrence of overflow depends on the inflow pattern of the waste water that changes every day, and thus cannot be specified accurately, but is generally when the maximum filtration flow rate is less than the average transfer flow rate.
Both the trunk pipe 82 and the branch pipe 81 of the return device 8 are pipes having an inner diameter of about 50 mm, and the lower end of the trunk pipe 82 is at a position of about 5 cm from the bottom.
First, the test was started without installing the return device 8 in all three houses. Even when overflow occurred, wastewater that was not biologically treated did not flow out from the solid-liquid separation tank while the maximum filtration flow rate was higher than the average inflow rate of wastewater.
The meanings of the maximum filtration flow rate, MLSS concentration, and dissolved oxygen concentration (DO) in the graph showing the measurement results are as follows.

The filtration flux f [m / d] of the submerged membrane separator 6 is approximately equal to the filtration pressure P (the distance from the branching point 65 to the liquid level in FIG. 1 is converted to a unit of kPa) f = a × There is a relationship of (P + c), where a is a transmission coefficient [m / d / kPa] and c is a constant of 0.5 [kPa]. Therefore, using the permeation coefficient calculated by this formula from the measured values of flux and filtration pressure, the maximum filtration flow rate [L / min] is a × (10 ³ /24/60)×4.5×membrane area (= 10.9). Calculated. The MLSS concentration and DO are values measured with sludge collected approximately halfway between the liquid level and the submerged membrane separator 6.

(Reference Example 1)
This example verifies the relationship between the MLSS concentration and sludge concentration distribution and the filtration flow rate or filtration resistance substance.
Figures 3 and 4 show the changes in maximum filtration flow rate, MLSS concentration, DO, and NO3-N / TN ratio of the filtrate over about 200 days.
Prior to the start of operation, the sludge from the ordinary combined treatment septic tank was charged into the activated sludge treatment tank 3 so that the MLSS concentration was about 4000 mg / L.
On the 103rd day, the sludge in the activated sludge treatment tank 3 was extracted, and on the 202th day, the sludge was extracted and the membrane was washed with 0.6% sodium hypochlorite. In this example, no overflow occurred.

  First, from the viewpoint of the lower limit concentration, when the operation is started, the filtration flow rate has decreased for a while even though the amount of sludge seeding is sufficient. During this time, the NO3-N / TN ratio is low and the DO is high, so the sludge activity is low. After about 30 days, the NO3-N / TN ratio rose and the filtration flow stopped decreasing and recovery started. Therefore, it turns out that the filtration resistance substance in waste_water | drain is adsorb | sucked by activated sludge.

  Next, the change after the extraction of sludge on the 103rd day is examined. In this work, the sludge near the bottom of the septic tank was pulled out while stirring. Since the amount of sludge in the extracted upper layer is small, the NO3 / TN ratio decreases slightly, but the filtration flow rate suddenly decreases because the filtration resistance substance is released from the sedimentation layer. After that, it stabilized for a while, but when the MLSS concentration exceeded about 12000 mg / L, the decrease in the filtration flow rate began again. At the same time, the rate of increase in MLSS concentration slowed down, the proportion of precipitated layers containing a large amount of filtration resistance substances increased, and the NO3-N / TN ratio also decreased.

The concentration distribution of the sludge in the activated sludge treatment tank 3 naturally changes depending on the tank structure and the amount of aeration. For example, the sludge concentration distribution increases when the position of the submerged membrane separation device 6 is biased or the amount of aeration is small. . FIG. 5 shows a typical concentration distribution in the present example in which the submerged membrane separation device 6 is installed at substantially the center of the activated sludge treatment tank 3.
The water depth on the horizontal axis means the distance from the liquid surface, and the broken line is the water depth at the bottom of the septic tank. The shallowest measurement points are approximately in the middle from the submerged membrane separator 6 to the liquid level, and the deepest measurement points are three locations around the submerged membrane separator 6 that are about 5 cm away from the bottom of the septic tank. The other measurement points are three places around the submerged membrane separation apparatus 6 at a water depth approximately between these. The plot in the figure is the average value of MLSS concentration of sludge collected by suction at each water depth. The MLSS concentration in the upper layer described in (Means for Solving the Problems) almost corresponds to the MLSS concentration of sludge at the shallowest water depth.

The figure shows three curves with different concentrations in the upper layer, and all the curves show a large concentration distribution. However, as the concentration increases, the curve changes from downward to upward and the lower layer expands. ing. Further, since the concentrations of the upper layer and the lower layer are both approaching the peak state, it is presumed that the upper layer is well circulated, but the lower layer is in a precipitated state with poor fluidity.
FIG. 6 is a plot of the MLSS concentration of all the samples and the viscosity measured with a B-type viscometer. From this figure, it is easily estimated that the lower layer of sludge becomes difficult to circulate as the MLSS concentration increases. .

(Experimental example 1)
7 and 8 show changes in the maximum filtration flow rate, MLSS concentration, DO, and NO3-N / TN ratio of the filtrate for about one year in House A.
In House A, overflow from the partition plate 5 of the solid-liquid separation tank 2 and the activated sludge treatment tank 3 started from the 56th day, and the MLSS concentration, which was 4500 mg / L, dropped to 360 mg / L on the 64th day. The maximum filtration flow rate was also reduced to 0.5 L / min. Thereafter, membrane washing with sodium hypochlorite aqueous solution and replacement of all sludge were performed, but the increase in MLSS concentration and recovery of filtration flow rate were temporary, and on day 87, MLSS concentration was 170 mg / L, maximum filtration. The flow rate decreased again to 0.4 L / min. Therefore, it was washed again with an aqueous sodium hypochlorite solution, but a similar process was repeated thereafter.

When the return device 8 was installed on the 108th day indicated by the arrow in FIG. 8, the MLSS concentration started to rise from 450 mg / L. However, until about day 250, the NO3-N / TN ratio was low and the sludge activity remained low, so the filtration flow rate continued to decline, and untreated wastewater often flowed out. After that, the filtration flow rate recovered rapidly with activation, and the maximum filtration flow rate became 1.5L / min or more from around day 270, and no overflow occurred.
The BOD of the filtrate rarely exceeded 5 mg / L, regardless of the presence or absence of the return device, but was almost 3 mg / L or less.

(Experimental example 2)
As shown in FIG. 9 and FIG. 10, overflow began at about 35 days in House B. On day 42, the MLSS concentration decreased to 1500 mg / L and the maximum filtration flow rate decreased to 0.98 L / min. Therefore, when the entire amount of sludge was replaced on the 45th day and the membrane was washed with sodium hypochlorite aqueous solution and restarted, the maximum filtration flow rate was almost stable at about 2.5L / min, and the MLSS concentration increased. Continued.
On the 87th day, a return device was installed. Since the failure of the transfer pump and air pump occurred one after another from around the 100th day, the maximum filtration flow rate dropped to 1L / min and overflow occurred. The MLSS concentration at that time was approximately 10000 mg / L. However, despite the occurrence of such trouble, the MLSS concentration never drops below 5000 mg / L. After fixing the pump failure on the 120th day and replacing the air pump, the filtration flow rate gradually recovered. From the 180th day, overflow no longer occurred.

In addition, from the 120th day, intermittent operation was performed for 10 minutes from the start of 1 am to 6 pm when the waste water hardly flows, and for 50 minutes of stoppage. On the 205th day, the sludge was returned to the solid-liquid separation tank 2, and the MLSS concentration in the activated sludge treatment tank 3 was adjusted to about 5000 mg / L. On day 211, the precipitated sludge in the solid-liquid separation tank 2 was extracted. From the 247th day, the intermittent operation conditions were changed, and the period between 1 pm and 6 pm with a relatively large amount of drainage was set to 30 minutes for operation and 30 minutes for stop.
As mentioned above, it was not possible to actually confirm the overflow from the 180th day, but if you look closely at the figure, an overflow occurred around the 200th, 250th, 310th, and 340th days. Should have been. However, since the reduction of the filtration flow rate was suppressed by the effect of the return device, it was considered that the flow rate stopped in a short amount of time and could not be confirmed.
The BOD of the filtrate was the same as that of House A.

(Experimental example 3)
In house C, a return device was installed on day 114, but as shown in FIGS. 11 and 12, no overflow occurred until around day 120 when the MLSS concentration exceeded about 11000 mg / L. Thereafter, the occurrence of a small amount of overflow was repeated, but the further decrease in the filtration flow rate was suppressed by the effect of the return device, and the MLSS concentration increased again.
On day 168, a small amount of sludge was returned and the membrane was washed with an aqueous sodium hypochlorite solution, and the maximum filtration flow rate was stabilized at approximately 2.7 L / min.
On day 210, the precipitated sludge in the solid-liquid separation tank 2 was extracted. At that time, the filtration flow rate suddenly decreased due to the operation of stirring the precipitated sludge, and an overflow occurred around the 220th day. However, due to the effect of the return device, the maximum filtration flow rate was stopped at about 1 L / min in this case, and it recovered rapidly from around day 230.
In this mansion, the BOD of the filtrate never exceeded 3 mg / L.

Schematic of the membrane separation type merger processing septic tank showing an example of the implementation of the septic tank according to the present invention. Explanatory drawing which shows the other structure of the return apparatus with which a septic tank is equipped. The diagram which shows the test result of a reference example. The diagram which shows the test result of a reference example. The diagram which shows the typical density distribution in a reference example. The diagram which shows the relationship between the MLSS density | concentration in a reference example, and a viscosity. The diagram which shows the result of the evaluation experiment of this invention in the door-to-door residence A residence. The diagram which shows the result of the evaluation experiment of this invention in the door-to-door residence A residence. The diagram which shows the result of the evaluation experiment of this invention in the door-to-door residence B residence. The diagram which shows the result of the evaluation experiment of this invention in the door-to-door residence B residence. The diagram which shows the result of the evaluation experiment of this invention in the door-to-door residence C residence. The diagram which shows the result of the evaluation experiment of this invention in the door-to-door residence C residence.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Membrane separation type combined treatment septic tank 2 ... Solid-liquid separation tank (pretreatment tank)
3 ... Activated sludge treatment tank 4 ... Disinfection tank 6 ... Submerged membrane separator 7 ... Transfer pump 8 ... Return device 81 ... Branch pipe
82 ... Stem pipe 9 ... Filter material
10 ... Filtrate pump 13 ... Air pump

Claims (1)

In the activated sludge treatment tank provided with the submerged membrane separation device, when the liquid level of the activated sludge treatment tank exceeds a predetermined water level, the lower end installed in the activated sludge treatment tank is the bottom of the activated sludge treatment tank. Using a return device consisting of a trunk pipe in the vicinity and a branch pipe branched from the trunk pipe at the position of the water level, returning the sludge in the vicinity of the bottom to the treatment tank upstream from the activated sludge treatment tank ; and A septic tank characterized in that the liquid in the treatment tank is transferred into the trunk pipe of the return device .

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