JPS5876180A - Method of controlling injection of chlorine in water- purifying plant - Google Patents

Abstract

PURPOSE:To hold the concentration of residual chlorine in purified water at a set point, by setting a reference ratio of injecting chlorine from the measured value of the quality of raw water, obtaining a ratio of injecting chlorine from the concentration of residual chlorine in the purified water, and feed-back controlling the amount of injecting chlorine. CONSTITUTION:In a water-purifying plant comprising a water intake well 1, a coagulating-mixing pool 2, a flocculating pool 3, a precipitating pool 4, a filtering pool 7, etc., oxidizable substance in raw water or the quality of the water relating to the oxidizable substance is measured by a water analyzer 16, a ratio Yf of consuming chlorine is calculated by an arithmetic unit 20, and a reference ratio CMV of injecting chlorine is set. On the other hand, the concentration of residual chlorine in purified water is measured by a densitometer 12, a ratio Yb of preinjecting chlorine is obtained from the set point of the concentration of residual chlorine, a factor for idling time corresponding to the flow amount of water, etc. by PID arithmetic unit 18, a ratio Ym of injecting chlorine is obtained by feeding back the ratio Yb to correct said reference ratio CMV of injecting chlorine with a multiplier 19, and an amount of injecting chlorine corresponding to the flow amount of water is controlled a ratio-calculating unit 11.

Description

[Detailed Description of the Invention] The present invention provides a water treatment plant in which chlorine is injected near a receiving well.
The present invention relates to a chlorine injection control method for controlling the amount of chlorine injection.

Generally, in water treatment plants, chlorine is injected for the purpose of sterilizing the raw water taken, or oxidizing and decomposing inorganic ions such as ammonia, manganese, iron ions, and organic pollutants.

FIG. 1 shows a conventional chlorine injection control system using a feedback injection control system.

In Figure 1, raw water is introduced into a landing well L, chlorine is injected from a chlorine injection machine T at a predetermined position (chlorine injection point 9) in the landing well, and then the raw water is led to a rapid stirring pond via a raw water flowmeter 10. . In the rapid stirring pond, flocculant, etc. added between the pond λ and the landing well L are mixed by a stirrer S (M is a drive motor), and the raw water containing the generated floc-like precipitate is fed to a flocculator 6. After the flocs are generated in the floc formation tank 3, the water containing the grown flocs flows into the settling tank which performs solid-liquid separation. Removal is carried out in a rapid filtration basin. The residual chlorine concentration (cpv) of the sample taken at the treated water sampling point /3 is determined by the residual chlorine concentration meter 12, and the deviation from the residual chlorine set value (CSV) is determined by the calculator /4 (
This deviation is inputted together with the information from the raw water flow meter 10, the pre-chlorine ratio, and the injection rate into the ratio calculator l/ to control the amount of chlorine injection into the chlorine injection device.

In Figure 1, rapid stirring tank ko, flocculation tank 3, and settling tank q are processes in which flocculant addition, floc formation, and solid-liquid separation are performed to remove suspended solids in water, respectively. Due to its nature, there is a residence time of at least 2 to 3 hours from the landing well to the settling tank outlet. Therefore, even if the raw water quality fluctuates and the chlorine consumption fluctuates, there is a time delay of at least 2 to 3 hours before the results appear.

On the other hand, the pre-chlorine injection rate is set by an operator manually measuring the chlorine demand and ammonia nitrogen concentration once a day. In addition, the chlorine injection rate can be corrected by visually determining the deviation between the residual chlorine concentration near the settling tank outlet and the target value, or by using a calculator, and then using a ratio calculator, etc., along with the amount of water intake from the flow meter 10. It is done by asking.

1. Chlorine consuming substances in raw water. If the concentration changes over time are small, it is also possible to keep the residual chlorine concentration constant using conventional methods. However, nowadays, as water intake sources such as rivers become increasingly polluted, there are temporal fluctuations in chlorine-consuming substances in many river waters. For example, water treatment plants in urban areas have sewage treatment plants and human waste treatment plants upstream, and these facilities discharge pollutants that have not been completely treated, and untreated laundry and kitchen wastewater from households. It is sometimes released into the river.

In addition, in rural areas, intensive livestock breeding has progressed, and wastewater containing large amounts of pollutants from pig farms and the like is sometimes discharged into rivers. In such a case, if the pre-chlorine injection rate is set by analyzing the deer raw water quality once a day, the residual chlorine concentration will naturally fluctuate greatly.

If the chlorine injection rate is higher than necessary, tap water will have a strong chlorine odor at the end of the water supply, and not only will chemical costs increase and become uneconomical, but the reaction between organic pollutants and excess chlorine will cause harmful substances such as trihalomethane. The amount of substance produced increases. On the other hand, if the chlorine injection rate is less than the required amount, sterilization and disinfection will not be completed completely, causing hygiene problems such as microorganisms remaining in the water and ammonia remaining undecomposed.

In this way, the conventional chlorine injection rate setting is based on the amount of raw water intake and
The chlorine consumption of raw deer water is analyzed 7 times a day by hand or based on the operator's experience and intuition, and the evaluation of treatment effectiveness is corrected by determining the difference between the residual chlorine concentration at the outlet of the settling tank and the target value. However, with this method, it is difficult to properly inject chlorine in response to fluctuations in raw water quality, and it is therefore not possible to maintain the residual chlorine concentration at the set value.

This invention was made to improve the drawbacks of the conventional method as described above, and it sets the pre-chlorine injection rate in a feedforward manner according to the concentration of chlorine-consuming substances contained in raw water, and P from the deviation between the residual chlorine concentration and the set value
It is an object of the present invention to provide a chlorine injection control method that can keep the residual chlorine concentration constant and stabilize the water purification process by correcting the chlorine injection rate in a feedback manner using calculations or the like.

In order to achieve the above objectives, we investigated the chlorine injection treatment for raw water quality and focused on the following points.

Substances that consume chlorine include metal ions such as reduced iron ions and manganese ions, inorganic ions such as ammonia nitrogen and nitrite nitrogen, and organic pollutants such as organic carbon compounds. It is currently impossible to continuously measure these water quality items using automatic instruments, as these water quality instruments have not been sufficiently developed. Furthermore, since the amount of chlorine consumed is determined by the total amount of these substances contained in the water, there is no need to analyze each individual substance, and there is no practical problem as long as the amount of chlorine consumed is determined.

Therefore, as a continuous automatic measuring instrument for chlorine consuming substances in raw water, we recommend using a chlorine demand meter or a water quality item measuring device that correlates with chlorine consumption, such as a potassium permanganate consumption meter, C
Install an OD meter, iodine consumption meter, redox potential meter, electrical conductivity meter, and UV meter.

Among these instruments, instruments appropriate to the actual situation of each water treatment plant will be selected to monitor the quality of raw water.

Chlorine consumption (Yf) and analysis value obtained with the above meter (X)
The relationship is determined in advance by manual analysis or plant experience as shown in equation (1).

Yf= a−X −1−b −−−−−−−−−−
-------1-----(1) In equation (1), coefficients a and b are constants determined for a specific water quality meter in each water purification plant. Furthermore, when the amount of chlorine consumed varies depending on the water temperature or the season, by setting the coefficients a and b as a function of the water temperature, etc., it is possible to obtain a chlorine consumption calculation formula according to the characteristics of each water purification plant. On the other hand, a treated water residual chlorine meter is installed downstream of the chlorine injection point, and the deviation from the residual chlorine concentration setting value is fed back by calculation from P, which takes into account waste time in the process, to correct the chlorine injection rate. It is added.

As described above, a feature of the present invention is that, in determining the chlorine injection rate, a feedforward ring based on the concentration of chlorine consuming substances in raw water or a water quality analysis value that correlates therewith is combined with a feedback ring based on the measurement of the residual chlorine concentration. That's what I did.

Hereinafter, one embodiment of the present invention will be explained based on FIG. In Figure 2, /A is a water quality analyzer that measures the water quality at point /S upstream from the chlorine injection point D.
The chlorine consumption rate (Yf) is calculated by the calculator 2.20. io is a raw water flow meter, and this value is input to the pre-chlorine injection proportional calculator// and the time delay element calculator/7. /2 is a residual chlorine concentration meter, and water sampling point 13 is a chlorine injection point? This is an arbitrary point on the downstream side and upstream of the filtration basin 7.

It is an arithmetic unit from P, and /9 is a multiplier.

Next, a chlorine injection control method in this configuration will be explained. Chlorine injection point? The intake raw water is sampled from a point /S on the upstream side, and water quality items correlated with chlorine consumption are analyzed using a water quality analyzer 76 at, for example, 70 minutes, 30 minutes, and 1 hour at the same frequency, and the analysis value ( X) is input to the arithmetic unit 20. The calculator 20 incorporates the analytical value (x) of the water quality index and the corresponding functional formula (1) of the chlorine consumption rate, and calculates the chlorine consumption (Yf) according to the actual situation of each water treatment plant. Ru. The coefficients a and b used in the arithmetic unit -0 are inputted by an operator or by autoregressive analysis of a computer, and can be changed as desired. Residual chlorine in the treated water after chlorine injection is measured using a residual chlorine concentration meter, and this residual chlorine concentration (cpv
), set concentration (C8V) and time delay element calculator/7
The waste time of the process according to the water intake flow rate is input from P to the calculator/1. The pre-chlorine injection rate (Yb) calculated from P is obtained from equation (2):f and equation (3).

EV ,, = OPV −csv −−−−−−−−
−−−−−−−−−m−−−−−(,7) Here, proportional gain (H), integral time (TI), differential time (TD)
For the control period, etc., values appropriate for process operation are set based on simulation or actual plant operation results, etc., depending on the characteristics of each water purification plant. Further, these gains can be inputted by a device such as a CRT and can be changed as desired.

The chlorine injection rate (Yb) obtained by feedback in this way is input to the calculator /q together with the chlorine injection rate (Yf) obtained by feedforward and the reference chlorine injection rate ((CMV). Arithmetic unit 19 In (
Calculate the actual chlorine injection rate (Ym) using the following formula.

Y, = CMV-4-WIF-Yf-1-WB@Y
b−−−−−−−−−−(41) Here, WF is the weighting coefficient of the feedforward ring, and VB is the weighting coefficient of the feedback ring, and these values can also be arbitrarily changed by the operator etc. is possible. The chlorine injection rate (Ym) calculated by the formula (R) is converted into the chlorine injection amount according to the water intake flow rate using a ratio calculator/l, and the appropriate amount is determined by controlling the valve opening of the chlorine injection machine g. The amount of chlorine injection can be maintained.

A weighting factor WF is used as an operational example of the control method of the present invention.

Since WB can be set arbitrarily using a CRT or the like, for example, if the water quality meter 16 breaks down, WF=O.

By modifying WB=/, (7), the impact on the plant can be minimized.On the other hand, untreated water from upstream dam discharge, localized heavy rain, or upstream sewage treatment plants, etc. When water quality suddenly changes due to overflow, etc., the feedforward weight is changed to WF
By setting = /, 0 and ignoring feedback, it is possible to quickly respond to sudden changes in water quality.

The above-mentioned chlorine injection control method can be similarly operated in the chlorine injection process in tertiary treatment of sewage treatment plants, human waste treatment plants, etc. in addition to water treatment plants. It can also be applied to processes such as manufacturing industrial water such as cooling water for industrial plants. On the other hand, a similar process using ozone instead of chlorine as an oxidizing disinfectant can also be carried out in a similar manner.

As described above, according to the present invention, the feedforward calculation and the feedback calculation are combined, and the weighting coefficients for both can be set from the outside, so that the chlorine injection rate can be set with good responsiveness to changes in water quality. can do. In addition, since the feedback of the treated water quality is performed by control rather than P, controllability is improved, so even if the raw water quality is measured once every 7 hours, the residual chlorine concentration can be stably maintained near the set value. .

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional chlorine injection control method, and FIG. 2 is a block diagram showing a chlorine injection control method according to an embodiment of the present invention. L... Landing well Co... Rapid stirring pond 3 @ Flock formation pond Da... Sedimentation pond!・・Mixer 6・・Floculator 7・・Rapid pond g・・Chlorine injection device 9・φ
Chlorine injection point 10...Water intake flow meter l/...Ratio calculator /-...Residual chlorine concentration meter/3 fists/Water sampling point l
lI...Calculator/S...Raw water sampling point 16...Water quality analyzer/7 Agent Shin Kuzuno -'

Claims (1)

[Claims] At water treatment plants where chlorine is injected into raw water before the injection of flocculants or flocculant aids, the chlorine consumption rate is measured using water quality meters that measure oxidizable substances in the intake raw water or water quality that correlates with oxidizable substances. The standard chlorine injection rate is determined before injection, and the pre-chlorine injection rate is determined from the deviation from the residual chlorine concentration set value by measuring the residual chlorine concentration in the treated water after chlorine injection and the time delay factor. A chlorine injection control method for a water purification plant, characterized in that a residual chlorine concentration is maintained at a set value by feeding back a previous chlorine injection rate and correcting a reference chlorine injection rate.