JPS6120352B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Field of Application of the Invention The present invention relates to a rainwater discharge control method for reducing the inflow of pollutants into public water surfaces. Here, to simplify the explanation, we will take as an example a rainwater pump installed at a rainwater pump station, a pumping station within a treatment plant, etc. of a combined sewer system that mixes rainwater and sewage and sends the water to a treatment plant. explain.

(2) Prior art In combined sewer systems, when there is a large amount of rainfall, the flow rate of sewage may exceed the capacity of the treatment plant. For this reason, at rainwater discharge pump stations and pump stations in treatment plants installed along the main sewer lines, only a certain amount of sewage is sent to the treatment facilities, and the rest is discharged directly into rivers, etc., reducing the load on the treatment facilities. Operation is required.

In conventional rainwater discharge, when the sewage flow rate reaches 2 to 3 times the flow rate on a clear day (this is called the dilution factor),
This has been done in the form of starting rainwater discharge.

Looking at the temporal changes in the concentration of pollutants during rainfall, we can see that the runoff rainwater at the beginning of rain (initial rainwater)
The water quality is extremely poor, with pollutant concentrations several to several tens of times higher than on clear days. This is called the first flushing effect, and it is believed that the cause of this is that pollutants that had accumulated on city streets and in sewage pipes were washed away by rain.

Although this first and flushing period is not that long, the amount of initial rainwater during this period is quite large, and in the conventional rainwater discharge method shown above, the initial rainwater is discharged into public water surfaces. The load of pollutants in water has increased, posing a major problem in terms of water quality conservation. In addition, with regard to this initial rainwater, we cannot expect much of a dilution effect, and even if the dilution factor is set high (for example, 5 to 6 times), the inflow load to the treatment plant will increase and the treatment efficiency will only decrease. This could end up having a negative effect.

On the other hand, after this first flushing period, the quality of the inflowing sewage water has been diluted by rainwater to the same level as the treated water when secondary treatment is performed at the sewage treatment plant.

Therefore, if initial rainwater can be sent to a treatment plant for treatment, the load of pollutants on public water surfaces can be significantly reduced.

Some attempts have been made to temporarily store rainwater in separate rainwater retention ponds and send it to treatment plants after the rain has stopped. In this case, it is difficult to store all the rainwater due to land shortages, increased construction costs, etc., and it is most likely to temporarily store only the initial rainwater.

From the above, it is extremely important to know the period of first flushing, that is, how long the initial rainwater of poor water quality will last. For this purpose, instruments that can measure the amount of pollutants in sewage online would be desirable, but such instruments (biochemical oxygen demand (BOD) meters, suspended solids (SS) concentration meters) have not yet been developed. Also, alternative instruments (total organic carbon (TOC) meter, turbidity meter), etc.
They were expensive, had low reliability, and required frequent maintenance inspections, making them impractical.

(3) Purpose of the Invention The present invention solves the above problems and provides a dissolved oxygen (DO) solution that is practical as a water quality meter and is inexpensive.
By using a concentration meter or a combination of a dissolved oxygen concentration meter and a flow meter to detect the first flushing period and outflow time of diluted rainwater, and controlling the start and stop of the rainwater pump, a decrease in treatment efficiency can be prevented. The purpose of the present invention is to provide a method that reduces the load on public water bodies without causing any problems.

(4) General description of the invention In order to achieve the above-mentioned object, the present invention provides dissolved oxygen concentration in sewage or dissolved oxygen concentration in sewage obtained by multiplying the dissolved oxygen concentration by the sewage flow rate per unit time (hereinafter referred to as When there is no need to make a distinction, dissolved oxygen (hereinafter referred to as dissolved oxygen) increases with rainwater runoff and returns to the same level as on a clear day once rainwater runoff stops.

This fact will be explained in detail as follows.

(1) There is little dissolved oxygen on sunny days.

(2) When rain begins, dissolved oxygen increases.

(3) When the first flushing period begins, dissolved oxygen decreases.

(4) After the first flushing period, dissolved oxygen increases rapidly.

(5) Dissolved oxygen also shows a peak at the peak of rainfall runoff.

(6) As rainfall runoff decreases, dissolved oxygen also decreases.

(7) When rainfall and runoff cease, dissolved oxygen returns to its clear-weather value.

This change appears in both the dissolved oxygen concentration and the amount of dissolved oxygen, but it is more noticeable in the latter. An example of this change is shown in FIG. It also shows the rainfall per unit time (bar graph) and runoff hydrograph (dashed line).

Since this dissolved oxygen increases only with rainfall, it is thought to clearly represent the state of runoff rainwater, the degree of mixing with wastewater, and the flow down.

As can be seen from the change in the amount of dissolved oxygen shown in FIG. 1, the amount of dissolved oxygen increases from point A to point B with the onset of rain. When the first flushing begins, the amount of dissolved oxygen decreases slightly from point B to point C. If the fast flushing is too much, it increases rapidly from point C to point D. When the rain runoff stops, it returns to point E, ie, the normal state. Therefore, initial rainwater can be considered to be sewage that flows in from the beginning of rainfall to the second increase in the amount of dissolved oxygen, that is, from point A to point C. If the curve of change in the amount of dissolved oxygen shows a smooth curve as shown in FIG. 1, point C can be determined as follows. The rate of increase in the amount of dissolved oxygen is calculated, and the point where the rate of increase increases for a certain period of time, then decreases for a certain period of time, and then starts to increase for the second time is point C. Point C can be found by counting this increase and decrease. However, due to the influence of noise and the like, the curve of change in the amount of dissolved oxygen may not be as smooth as shown in FIG. 1. In this case, the amount of dissolved oxygen reaches a point F or higher, which indicates an amount of dissolved oxygen equal to or higher than that of point B, or, if the influence of noise is significant, a point G or higher, which is set to avoid the influence. The time when
It may be used instead of point C. That is, rainwater is not discharged up to point F or point G, but the water is sent to a treatment plant or a rainwater retention pond by a sewage pump. Next, at point F or G, the rainwater pump is activated to discharge rainwater into a river or the like. Another method is that the rate of change in the amount of dissolved oxygen between points AC is small, and
Focusing on the fact that the difference between CDs is large, the point at which the magnitude of the differential value changes greatly can also be regarded as C. In this case, in order to remove the influence of noise, it may be possible to add integral operation and proportional operation.

The rainwater pump should be stopped at the point where the amount of dissolved oxygen approaches the value on a clear day (point E), that is, the point at which rainwater runoff stops, or when the flow rate becomes less than 2 to 3 times the flow rate on a clear day. Just stop.

By doing this, the initial rainwater is not discharged into public waters, making it possible to significantly reduce the load of runoff pollutants.

(5) Examples Hereinafter, the present invention will be explained in detail with reference to examples. FIG. 2 shows a system of the rainwater discharge control method according to the present invention, which includes a rainfall detection end 11, a control activation device 12, a dissolved oxygen concentration meter 13, a flow meter 14,
It is composed of a dissolved oxygen amount calculation device 15, a determination device 16, a rainwater pump control device 17, and a display device 18.

During fine weather, the starting device 12 does not operate and the control device is stopped. This is to prevent unnecessary discharge due to fluctuations in the amount of dissolved oxygen during clear weather, especially instrument noise. Rainfall detection end 11
is for detecting the presence or absence of rain, and if it is found that it is raining, it sends a signal 21 to the starting device 12 indicating that there is rain. In this case, when the amount of rainfall exceeds the amount that would require rainwater discharge, the signal 21
You can also send . for that purpose,
The detection end 11 may be used as an integrating rain gauge. When the starting device 12 receives a signal 21 indicating that there is rain,
A start signal 22 is sent to the dissolved oxygen concentration meter 13 and the flow meter 14. By this activation signal 22, the dissolved oxygen concentration value 23 in the sewage is output from the dissolved oxygen concentration meter 13.
A sewage flow rate 24 is sent from the flow meter 14 to a dissolved oxygen amount calculation device 15, and a dissolved oxygen amount 25 is calculated.
This dissolved oxygen amount 25 is sent to the determination device 16, and the first
Either detect point C, F, or G points shown in the figure (these are selected as appropriate depending on the situation at the implementation location), or calculate the degree of change from the previous point and change that has been input in advance. The time to send the start signal 26 of the rainwater pump is determined by a method such as detecting a sudden change in the degree of change by comparing it with the degree of change. This start signal 26 is the rainwater pump control device 1
7, this controller 17 activates the rainwater pump 19 and begins rainwater removal. Also,
Detect point E in Figure 1 or flow rate 22
becomes less than a predetermined flow rate, the determination device 16 sends a message to the rainwater pump control device 17.
A stop signal 27 is then sent to stop rainwater removal.

Further, by outputting the dissolved oxygen amount 25 to the display device 18, the function of the determining device 16 may be performed by a human being.

The above describes the case where the amount of dissolved oxygen is used as the determination index, but if the dissolved oxygen concentration 23 is used as the determination index as it is, the flowmeter 14 and the dissolved oxygen amount calculation device 15 are unnecessary, and the device is as shown in Fig. 3. It can be simplified as follows. In this case as well, the function of the determiner 16 may be performed by a human being.

If it takes a long time to start and stop the rainwater pump, measure the dissolved oxygen concentration meter 13 and flowmeter 14 for a distance calculated by multiplying the flow velocity in the pipe during rainy weather by the time required to start the pump. It can be installed at a point upstream from the site.

Further, in this method, the dissolved oxygen amount calculating device 15 and the determining device 16 may be configured by a computer. Furthermore,
While the dissolved oxygen concentration meter 13 and the flow meter 14 are kept working at all times, the output of the rainfall detection terminal 11 controls the start and stop of calculation in a computer that integrates the dissolved oxygen amount calculation device 15 and the determination device 16. You can do it like this.

(6) Summary As explained above, according to the present invention, by measuring dissolved oxygen concentration or dissolved oxygen concentration and flow rate, the outflow load to public water surface can be reduced without significantly reducing the treatment efficiency of sewage treatment plants. It is possible to realize rainwater discharge control that can significantly reduce water discharge. Furthermore, even if a rainwater retention pond is provided, it is sufficient to retain rainwater only during the first flushing period, and the required area is much smaller than the conventional design area.

This control method can be performed by a human, does not require very complicated judgment, and can be implemented at a relatively low cost.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing changes in the amount of dissolved oxygen in sewage flowing into a pump station during rainy weather;
FIG. 2 is a device diagram showing one embodiment of the present invention, and FIG.
The figure is an apparatus diagram showing a simplified modification of FIG. 2. 11 is a rain detection end, 12 is a control start switch,
13 is a dissolved oxygen concentration meter, 14 is a flow meter, 15 is a dissolved oxygen amount calculation device, 16 is a determination device, 17 is a rainwater pump control device, 18 is a display device, and 19 is a rainwater pump.

Claims (1)

[Claims] 1. In a sewage treatment system consisting of a sewage system and a sewage treatment means for treating and discharging sewage from the sewage system, the dissolved oxygen concentration or dissolved oxygen amount after the first crushing period from the start of rain falls below a predetermined value. A rainwater discharge control method characterized by discharging a portion of the sewage untreated when the amount of sewage exceeds the limit.