JPS60114396A - Control method of do in oxidation ditch method - Google Patents

Abstract

PURPOSE:To maintain the DO value of sewage at a prescribed value by providing an outflow weir which fluctuates the water level in a discharging channel according to the inflow rate in the outflow part of a circulating water course and fluctuating the aeration rate by the change in the depth of the blade of an aerator on the basis of said fluctuation. CONSTITUTION:An outflow weir 3 is provided to fluctuate the water level in an aeration tank 1 according to an inflow rate in the case in which the sewage to be treated fluctuates in volume with lapse of time. More specifically, the outflow resistance of the weir 3 is set by an operation of moving vertically the notch width at the center of the slit plate 3a of the weir 3 or the plate 3a and outlet 11 by a screw mechanism 3b so that the immersion depth of the blade of the tank 3 attains the displacement in the specified range. The water level in the tank 1 is thus controlled and a specified fluctuation is given to the immersion depth of the blade 2a of the aerator 2. The aeration tank having the shape with which the aeration rate is changed by the blade 2a corresponding to the fluctuation is used.

Description

Detailed Description of the Invention The present invention provides a method for controlling Do (dissolved oxygen) in wastewater in an oxidation ditch method, in which a mechanical surface aerator is installed in an oblong circulation waterway to treat wastewater with activated sludge. It is related to.

The oxidation ditch method is an oblong circulation waterway (
An aeration tank) is installed, and a mechanical aerator (hereinafter referred to as
It is simply called an aerator. ) is a method for treating activated sludge by circulating sewage (herein referred to as a mixed solution of sewage and sludge in an aeration tank) while aerating it. The sewage must reach a certain level to be treated with an activated sludge treatment method such as the oxidation ditch method.

Required to maintain value. Therefore, conventionally, the Do value of sewage in the aeration tank is constantly or timely detected, the detected value is fed to the control device, and the rotation speed of the aerator is changed based on that value. Changing the immersion depth of the aerator blade by raising and lowering the outflow weir (hereinafter referred to as automatic control method).

), the amount of aeration was changed to maintain a predetermined Do value. The characteristics of this oxidation ditch method are:
Although it requires a relatively large site, it is easy to maintain and manage.
It can handle relatively large inflow load fluctuations and has advanced processing (N
, removal of P) is also possible, and therefore, in the middle,
It is suitable as a sewage treatment method for small cities (medium and small scale). However, on the other hand, small and medium-sized sewage facilities have a narrow facility area and a short time to reach the sewage treatment plant. The fluctuations are large, and the fluctuations in water quality (diagram ■) are also large. In this way, when sewage (in this specification refers to the water before being input into the aeration tank) flows into the aeration tank in a manner that fluctuates depending on the time, the sewage is maintained at a predetermined Do value. Therefore, it is required to change the amount of aeration, that is, the amount of oxygen supplied (oxygen dissolved M) in response to the above fluctuations.The adoption of the above automatic control method as a control means is difficult as the sewage treatment facility becomes smaller. The cost required for the automatic control equipment increases as a percentage of the construction cost of the entire facility, and the construction cost relative to the treatment capacity becomes relatively high. This places a large burden on automatic control equipment. still,
As an alternative to the automatic control described above, there is also a method of manually controlling the rotation speed, immersion depth, etc. based on successive measured values of Do, but in this case running costs are high and it is difficult to maintain a predetermined Do value. difficult.

The present invention has been made in view of the above-mentioned current situation, and it adopts manual operation and automatic control method in the oxidation ditch method, which treats wastewater with activated sludge by installing a mechanical surface aerator in an oblong circulation waterway. An outflow section is provided at the outflow section of the circulation channel that changes the water level in the channel according to the amount of inflow water, and the amount of aeration is varied by changing the immersion depth of the aerator blade based on the fluctuation of the water level. It is an object of the present invention to provide an inexpensive o control method that maintains the DO value of sewage at a predetermined value.

Hereinafter, an outline of the apparatus used to carry out the method according to the present invention will be explained, and then a DO control method in the oxidation ditch method according to the present invention will be specifically explained. Fig. 2 is a plan view of the entire apparatus for carrying out the oxidation ditch method according to the present invention, Fig. 3 is a front view of a slit-type outflow weir forming a part of the apparatus shown in Fig. 2, and Fig. 4 is a plan view of the entire apparatus for carrying out the oxidation ditch method according to the present invention. Similarly, FIG. 2 is a side view of an aerator forming part of the apparatus, and FIG. 5 is a detailed side view of the blade portion of the aerator, showing changes in the immersion depth of the blade. In the figure,
1 is an aeration tank, 2 is an aerator, 3 is an outlet, and 4 is a sewage inlet. In addition, 5 is a settling tank, 6 is a sludge return pump, and 7 is a sludge return port. The aeration tank 1 has an oval outer shape in a plan view, and has a central separation wall 1a along its longitudinal centerline.
The water is separated from the water to form a circulation waterway. This aerator 2 is a vertical surface aerator, and an aeration blade 2a is attached to the lower end of the rotating shaft of an electric motor. In addition, in the outflow section 3, a slit plate 3a, which is configured such that only a part of the center is cut out to provide outflow resistance and restrict the outflow of sewage, is inserted vertically into a notched recess 1b formed in a part of the aeration tank 1. Can be attached freely,
It is configured so that it can be moved up and down by a screw mechanism 3b. Therefore, the aerator 2 is installed at a position adjacent to one end of the central separation wall 1a in the semicircular curved part of the aeration tank 1, and the outflow part 3 is attached to a part of the outer wall of the aeration tank 1. ,
Similarly, a sewage inlet 4 and a sludge return lower are installed in the aeration tank 1. Note that the outflow section 3 may be a slit-type weir made of a slit plate having a triangular cutout depending on the amount of large fluctuation of the other flow, water quality, etc., as described above, or a conduit as shown in Fig. 6. The diameter of the water outlet 8 is made thinner, a bent part 9 is provided, and a water outlet 11 is provided which is movable up and down using a bellows 10 and a bellows mechanism 3b'. Cylindrical outflow 113 configured to limit outflow by adjusting height
It may be a'. Further, in the above, each outflow portion (slit plate 3a or water outlet 11) is connected to the screw mechanism 3b (3b
The reason why the aeration tank is configured to be able to move up and down freely is to enable adjustment so as to obtain an appropriate range or amount of fluctuation in the water level of the aeration tank corresponding to the initial or subsequent inflow amount.

Next, the method according to the present invention will be explained with reference to the above-mentioned apparatus and FIG. 7. The sewage to be treated is shown in line diagram A in Figure 7.
When the amount of inflow changes with time as shown in Figure B, the total amount of inflow to the aeration tank is as shown in diagram B, which includes the sludge returned from settling tank 5 to aeration tank 1 at a constant rate. Become. In the above state, by providing an outflow amount, the water level in the aeration tank (circulation waterway) is varied according to the inflow amount. In other words, the outflow amount from the aeration tank is as shown in diagram C, the water level in aeration tank 1 changes according to the inflow amount, and the immersion depth of the aerator blade is within a certain range as shown in the diagram. The outflow resistance for the outflow amount 3 is set so that the outflow amount 3 has a displacement of By controlling the water level of the aeration tank 1 (channel), the immersion depth of the blade 2a of the aerator 2 is varied at a constant level (5 to 23 am in the example). on the other hand,
The blade 2a is shaped so that when the immersion depth changes, the amount of sewage sprayed into the air (aeration amount) by the blade 2a changes accordingly. In other words, the required oxygen supply amount (aeration amount) for the blade 2a is approximately proportional to the increase or decrease in the immersion depth and the increase or decrease in the amount of sewage in the aeration tank 1 as shown in FIG. ). Since the oxygen supply capacity varies in accordance with the immersion depth D of the blade 2a, the amount of dissolved oxygen in the circulating wastewater varies approximately in proportion to the water level as shown in the diagram E.

Therefore, as the amount of sewage inflow changes, the BOD load of the sewage in the aeration tank fluctuates as shown in diagram F in Figure 1, and correspondingly, the amount of dissolved oxygen also fluctuates as shown in diagram E. Therefore, the Do value in the aeration tank can always be maintained within a predetermined appropriate range, as shown in diagram G. Furthermore, the 6th
H in the figure indicates the shaft output of the aerator.

As explained above, according to the present invention, the DO value of wastewater in the aeration tank can always be maintained within a predetermined appropriate range without using an expensive automatic control method in the oxidation ditch method. The oxidation ditch method can be implemented at a low cost, and economically contributes to promoting the development of sewage treatment facilities in small and medium-sized cities.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing changes in the amount of sewage inflow and water quality with time plotted on the horizontal axis, Fig. 2 is an overall plan view of the apparatus for carrying out the oxidation ditch method according to the present invention, and Fig. 3 is a front view of the slit-type outflow weir that forms part of the apparatus shown in Fig. 2, Fig. 4 is a side view of the aerator that similarly forms part of the apparatus shown in Fig. 2, and Fig. 5 is a side view of the aerator that forms part of the apparatus shown in Fig. 4. A side view showing changes in the immersion depth of the blades of the aerator, FIG. 6 is a cylindrical outflow amount showing another embodiment of the weir, and FIG. It is a diagram showing various conditions such as water volume, water level fluctuation, Do value, etc., with time plotted on the horizontal axis. A...Sewage inflow amount, B...Total aeration tank inflow amount, C...
・Aeration tank outflow amount, D... water level (blade immersion depth),
E... Oxygen dissolution amount, F... BOD load amount, G...
0 value, H...Aeration machine shaft output, ■...B of inflow sewage
OD value, 1... Aeration tank (circulation waterway), 2... Aerator, 3... Outflow amount, 4... Sewage inlet, 5... Sedimentation tank, 6... Sludge return pump , 7...Sludge return port, 8
... Conduit, 9 ... Bend part, 10 ... Bellows, 1
1... Water outlet. 0

Claims (1)

[Claims] In the oxidation ditch method, in which a mechanical aerator is installed in an oblong circulation waterway to treat wastewater with activated sludge, an outflow weir is installed at the outlet of the circulation waterway to change the water level in the waterway according to the amount of inflow. DO control in the oxidation ditch method, characterized in that the Do value of wastewater is maintained at a predetermined value by varying the amount of aeration by changing the immersion depth of the blade of the aerator based on the fluctuation of the water level. Method.