JPS6154445B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a sludge discharge control device for a thickening tank in a wastewater treatment plant, which prevents sludge from floating in the thickening tank and improves the concentration of thickened sludge. It is desirable that sludge with good sedimentation properties be retained in a thickening tank for as long as possible to improve the thickened sludge concentration. On the other hand, in the case of sludge with poor settling properties, the sludge in the thickening tank floats to the surface, so it is necessary to pull out the sludge as quickly as possible. Conventionally, when extracting sludge from a thickening tank, the concentrated sludge concentration or the sludge interface is detected to control the amount of thickened sludge extracted. However, since it is not possible to determine the settling property of sludge, it has not been possible to deal with fluctuations in sludge settling property. In other words, it cannot be applied when sludge floats to the surface, and even when the sludge has good settling properties, it is impossible to predict when the sludge will resurface, so even if the concentration of thickened sludge increases further, it is necessary to remove it midway through the process. Ta. FIG. 1 shows an example of the relationship between the sludge residence time t and the thickened sludge concentration C in the thickening tank. Characteristics in Figure 1
Characteristic (a) shows an example of sludge with relatively good sludge settling properties (sludge that does not easily float to the surface), and characteristic (b) shows an example of sludge that has poor sludge settling properties (sludge that easily floats to the surface). In either case, the thickened sludge concentration C increases until a certain sludge retention time (indicated by _t1 and _t2 ). However, as the residence time becomes longer, the concentrated sludge concentration C decreases. This is due to the fact that sludge tends to float when it accumulates for a long time. For sludge with good settling properties, if the sludge residence time T can be set to _t2 , the concentrated sludge concentration C will be _C2 , as shown in (a). On the other hand, for sludge with poor settling properties, if the sludge residence time can be set to t ₁ , a concentration C ₁ can be obtained. However, the settling ability of sludge not only differs depending on the treatment plant, but also varies greatly over time. However, conventional methods cannot predict this sludge settling property. For this reason, in the method of drawing out thickened sludge so that the thickened sludge concentration C becomes constant, the target concentration must be set to a value lower than _C1 . If the target concentration
If the concentration is higher than C ₁ , the concentration of sludge (b) with poor settling properties will never be higher than C ₁ , so the thickened sludge will not be drawn out forever, and the sludge will accumulate in the thickening tank and become trapped in the separated liquid. It will overflow.
Therefore, if the target concentration is set to C ₁ , then in the case of sludge (a) with good settling properties, if the sludge retention time is chosen to be t ₂ , the sludge will drop midway even though the thickened sludge concentration C increases to C ₂ . The thickened sludge concentration will not increase. With the method of controlling the sludge interface at a constant level, the sludge interface can be measured for sludge that has good settling properties, but the sludge interface meter cannot be used for sludge that has poor settling properties and easily floats because the sludge interface is not formed. . Furthermore, even if only the sludge interface is detected, it is only when the sludge starts to float and the sludge interface is no longer formed that the sludge interface meter becomes unusable. Not applicable to sludge. Even for sludge with good settling properties, it is unclear when the sludge settling properties will deteriorate, so the set value of the sludge interface must be selected low, and as a result, the concentrated sludge concentration also becomes a low value. As described above, in the conventional method, only thickened sludge with a relatively low concentration of thickened sludge could be obtained. Therefore, the heating energy for the anaerobic digestion tank increases, or the amount of chemicals for the dehydrator increases. Moreover, when sludge floats to the surface, the amount of organic matter load, nitrogen and phosphorus load applied to the sewage treatment process increases, which increases the sewage treatment function and treatment cost. An object of the present invention is to provide a sludge drainage control device for a thickening tank that can prevent sludge from floating in the thickening tank and improve the concentration of thickened sludge by controlling the amount of thickened sludge drawn out according to the settling property of the sludge. . The feature of the present invention is that the sedimentation property of the sludge flowing into the thickening tank is determined by the gas concentration of specific components (carbon dioxide gas, methane gas, hydrogen sulfide gas, etc.) in the aeration exhaust gas, and this gas concentration is used as an index to determine the settling properties of the sludge in the thickening tank. time,
That is, the objective is to control the amount of thickened sludge drawn out. First, the basic idea of the present invention will be explained. Anaerobic digestion of sludge (especially acid fermentation) is one of the causes of worsening sludge settling properties. In other words, when the sludge that initially settles in the settling tank becomes anaerobic, the organic matter in the sediment is decomposed by the action of anaerobic digestive bacteria, producing carbon dioxide gas, organic acids, and the like. Most of the carbon dioxide gas is dissolved in the liquid, but some of it forms microbubbles and adheres to the sludge flocs. As a result, the apparent density of the sludge flocs decreases, resulting in poor settling properties.
If the amount of gas adhesion increases further, the sludge will float to the surface.
Therefore, the higher the carbon dioxide concentration in the sludge, the worse the sludge settling property becomes. FIG. 2 is a diagram obtained experimentally to determine the effect of carbon dioxide concentration in sludge on sludge floating time (time until sludge rises to the surface of the liquid) when performing a sludge batch sedimentation test. It can be seen from Figure 2 that the lower the carbon dioxide concentration in the sludge, the longer the sludge floating time. That is, the carbon dioxide concentration in sludge can be considered as one index of sludge settling (or flotation). Gases produced by the action of anaerobic digestive bacteria include, in addition to carbon dioxide gas, methane gas, hydrogen sulfide gas, hydrogen gas, and ammonia gas. In addition, in an anaerobic state, only trace amounts of nitrogen gas, nitrous oxide gas, etc. are produced by the action of denitrifying bacteria. Therefore, the concentration of these gases can also be considered as an indicator of sludge settling properties. Considering the detection accuracy of gas concentration, carbon dioxide gas, which is large in quantity, has the highest sensitivity and is suitable. Although it is difficult to directly measure the carbon dioxide concentration in sludge online, it can be calculated from the carbon dioxide concentration in the aeration exhaust gas discharged by aerating the sludge. The following relationship exists between the carbon dioxide concentration in sludge and the carbon dioxide concentration in exhaust gas. Cl=a・Qg・Cg (1) where Cl: carbon dioxide concentration in sludge, Cg: carbon dioxide concentration in exhaust gas, Qg: aeration air volume, and a: constant. Under conditions where the aeration air volume is constant, the following equation holds true. Cl=a′・Cg……(2) Here, a′: aQg. Therefore, if a' is determined in advance, the carbon dioxide concentration Cl in the sludge can be estimated by detecting the carbon dioxide concentration Cg in the exhaust gas. When the aeration air volume Qg changes, the aeration air volume Qg is detected in addition to the carbon dioxide concentration Cg in the exhaust gas.
The carbon dioxide concentration Cl in sludge can be calculated from equation (1). When the carbon dioxide concentration Cl in the sludge is high, the sludge has poor settling properties and floats to the surface in a short time. In this case, it is necessary to draw out the thickened sludge so that it does not remain in the thickening tank for a long time. Conversely, when the carbon dioxide concentration Cl in the sludge is low, the sludge has good settling properties and does not float for a long time. Sludge becomes thickened over time and the concentration of thickened sludge increases. Therefore, highly accurate thickened sludge can be obtained by controlling the amount of drawn-out thickened sludge so that the sludge stays in the thickening tank for as long as possible. As shown in Figure 1, even if the sludge settling property changes as shown in (a) and (b), the amount of thickened sludge withdrawn is controlled according to the sludge settling property, so in the case of characteristic (a), the thickened sludge The concentration can be close to C ₁ , while for property (b)
It can be produced close to C ₂ and high concentration sludge can be obtained. For example, if the carbon dioxide concentration Cl in the sludge is 100ml/, the sludge floating time will be approximately 50 hours as shown in Figure 2. Therefore, if the amount of thickened sludge to be drawn is controlled so that the sludge remains in the thickening tank for about 40 hours on the safe side, sludge floating can be prevented and highly concentrated thickened sludge can be obtained. The inventors used an experimental apparatus having 657 thickening tanks and 9.1 sludge aeration tanks, and conducted experiments using sludge flowing into the thickening tank of a sewage treatment plant. The experimental conditions and results are shown in the table below.

[Table] As is clear from the table, controlling the aeration process alone (RUN2) is effective against the conventional method without aeration (RUN1), but by manipulating the amount of thickened sludge extracted according to the present invention, it is even more effective. It was confirmed that the concentration of thickened sludge could be improved. An embodiment of the present invention will be described below with reference to FIG. The inflowing sewage first passes through a sedimentation tank 1 and an aeration tank 2 before being treated. On the other hand, the sludge in the initial settling tank 1 is discharged by the sludge pump 3 and supplied to the sludge aeration tank 5 through the sludge transport pipe 4. Gas is supplied to the sludge aeration tank 5 by an aeration blower 6, a gas pipe 7, and an aeration device 8, and the sludge is aerated. After aeration, the sludge is supplied to the thickening tank 9, where it is sedimented and concentrated. The thickened sludge is drawn out by the thickened sludge pump 10. The gas concentration of the aeration exhaust gas is measured by a carbon dioxide concentration meter 11. Examples of the carbon dioxide concentration meter include an infrared type, a process gas chromatography type, a membrane electrode type, and a semiconductor type, but an infrared type carbon dioxide gas analyzer is preferable. Measurement value signal Cg of carbon dioxide concentration meter 11
is guided to the gas concentration calculation device 12, and the carbon dioxide concentration Cl in the sludge is calculated based on equation (2). If sludge transport by the sludge pump 3 is continuous, equation (2) can be applied as is. On the other hand, in the intermittent case, the carbon dioxide concentration Cg in the exhaust gas fluctuates over time, so
It is sufficient to use the average value Cg within a certain period of time. The calculation formula is expressed, for example, by the following formula. =1/t _e −t _s ∫ ^te _ts Cg(t) dt ………(3) Here, t _s : start time for calculating the average value, t
_e : End time for calculating the average value. In this case, the carbon dioxide concentration Cl in the sludge is calculated using the following formula. Cl=a' (4) The calculated value Cl is converted into a signal for operating the thickened sludge pump 10 in the discharge control device 13.
This conversion method is shown in FIG. 4, for example. Cl
The calculated value signal b is converted into the sludge floating time T by the converter 101 using the relationship shown in FIG. 1, and becomes the signal c. In response to the signal c, the arithmetic unit 102 calculates the amount of concentrated sludge pulled out _Qput . The calculation formula is as follows. Q _put =k/c (5) Here, k is a constant and can be set by the operator. Equation (3) means that the larger the signal c (sludge floating time T), that is, the longer the sludge retention time, the smaller the concentrated sludge withdrawal amount _Qput . Q _pu
t becomes a signal d, and the timer circuit 103 sends a start signal. The timer circuit 103 sends the drive signal e to the drive circuit 10 for a predetermined period of time after receiving the activation signal d.
Send to 4. The drive circuit 104 operates the thickened sludge pump 10 for a predetermined period of time. This operating time is Q _pu
The larger _t is, the longer it becomes. In this way, the amount of concentrated sludge drawn out is controlled according to the carbon dioxide concentration Cl in the sludge. In this way, the amount of sludge drawn out from the thickening tank 7 is controlled, and since the sludge is concentrated for an allowable time until it floats, it is possible to increase the concentrated sludge concentration without including sludge in the separated liquid. I can do it. FIG. 5 shows an example in which the aeration air volume Qg changes. In this case, the gas flow meter 20 detects the aeration air volume Qg, and the gas concentration calculation device 1 calculates the carbon dioxide concentration Cl in the sludge using the carbon dioxide concentration Cg in the exhaust gas.
In step 2, calculation is performed based on equation (1). The following control is similar to the embodiment shown in FIG. The embodiment shown in FIG. 5 has the advantage of being able to handle the case where the aeration air volume is controlled and having high control accuracy. Next, since the sludge is aerated before concentration, the sludge floating time T becomes longer. For this reason, the amount of thickened sludge pulled out _Qput becomes small, and sludge may accumulate in the thickening tank, causing the sludge interface to rise too much. If the sludge interface rises too much, there is a risk that sludge will be mixed into the separated liquid. An embodiment for preventing this is shown in FIG. The embodiment shown in FIG. 6 is constructed by adding a sludge interface meter 21 to the embodiment shown in FIG. Here, 22 is a sludge interface detection end. The sludge interface in the thickening tank 9 is detected by the sludge interface meter 21, and a detection signal h is transmitted to the discharge control device 13'. FIG. 7 shows the processing flow of the control device 13'. The interface signal h is sent to the comparison circuit 105. Comparison circuit 1
In 05, the upper limit value h ₀ of the sludge interface is set by the setting device 106.
is given. When _hh0 , the comparator circuit 105 provides a start signal i to the timer circuit 107 and a stop signal l to the drive circuit 104 to stop it. On the other hand, the timer circuit 107
starts and sends the drive signal j for a certain period of time to the drive circuit 10.
Send to 8. The drive circuit 108 operates the thickened sludge pump 10 for a predetermined period of time to draw out the thickened sludge and lower the sludge interface. In this manner, when the sludge interface h is larger than the set value h ₀ , the thickened sludge pump 10 is operated to lower the sludge interface in priority to the thickened sludge drawing operation according to the embodiment shown in FIG. As a result, it is possible to prevent sludge from being mixed into the separated liquid from within the thickening tank. Here, the upper limit setting value h ₀ of the sludge interface is preferably about 1.0 m below the water surface. According to the present invention, the sedimentation property of sludge is detected, and an appropriate amount of thickened sludge is extracted from the thickening tank according to the sedimentation property. That is, for sludge that has poor settling properties and tends to float, the thickened sludge pump is operated so that the sludge does not remain in the thickening tank for a long time, thereby preventing the sludge from being mixed into the separated liquid. At this time, since the sludge in the thickening tank remains for an appropriate amount of time without floating, the thickened sludge concentration also approaches the maximum concentration. On the other hand, for sludge with good settling properties, the thickened sludge pump is operated so that the sludge stays in the thickening tank for as long as possible, so the thickened sludge concentration becomes high. In this way, in the present invention, the amount of concentrated sludge drawn out is controlled according to the sludge settling property, so that sludge floating in the thickening tank can be prevented and the concentrated sludge concentration can be maintained at a high level. As a result, it is possible to prevent a decline in the sewage treatment function and to significantly reduce the sludge treatment cost.

[Brief explanation of the drawing]

Figure 1 is a characteristic diagram showing the relationship between sludge retention time and thickened sludge concentration, Figure 2 is a characteristic diagram showing the relationship between carbon dioxide concentration in sludge and sludge floating time, Figures 3 and 5
6 and 6 are block diagrams showing one embodiment of the present invention, respectively, and FIGS. 4 and 7 are detailed block diagrams of the emission control device. 5...Sludge aeration tank, 6...Aeration blower, 1
1... Carbon dioxide concentration meter, 12... Arithmetic device, 13
...Control device, 9...Thickening tank, 10...Thickened sludge pump.

Claims (1)

[Claims] 1. A sedimentation tank that sediments and concentrates suspended solids from wastewater, a thickening tank that further sediments and thickens the sludge extracted from the sedimentation tank, a sludge aeration device that aerates the sludge supplied to the thickening tank, and the sludge aeration device. and a gas concentration meter that detects the gas concentration of a specific component contained in the aeration exhaust gas, and the amount of sludge drawn from the concentration tank is controlled according to the detected value of the gas concentration meter. Tank sludge control device.