JP2002336871A - Method for controlling injection of flocculant in membrane filtration - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for controlling the injection amount of a flocculant in membrane filtration by which a membrane filtration operation can be performed stably regardless of whether the turbidity of raw water is low or high and the stable quality of the membrane-filtered water can be secured. SOLUTION: The injection amount of the flocculant is controlled according to the value of the chromaticity/turbidity ratio of raw water that is obtained by measuring the turbidity and the chromaticity of the raw water to be membrane-filtered and calculating the value. It is particularly preferable to control the injection amount of the flocculant in proportion to the value of the apparent chromaticity/turbidity ratio of the raw water.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling the injection of a flocculant into raw water having a chromaticity and turbidity, such as river water or lake water, when performing membrane filtration.

[0002]

2. Description of the Related Art A rapid sand filtration method has been used for many years in a water treatment plant that uses river water, lake water, or the like as raw water. In recent years, a membrane filtration method using a ceramic membrane or the like has become widespread. . In either method, PAC is added to the raw water
(Polyaluminum chloride) and other coagulants to add organic matter (color, turbidity, etc.) in the raw water into the flocculent floc and filter it, thereby improving the quality of the filtered water and the filtration pressure difference. Is restrained from rising.

In the membrane filtration method in particular, it is important to appropriately control the amount of coagulant to be injected. If the amount of coagulant is insufficient, organic substances (such as chromaticity and turbidity) are taken into the floc. Since the filtration is not performed sufficiently, the quality of the membrane filtration water decreases and the filtration pressure difference increases. Conversely, if the injection amount of the coagulant is too large, flocs are formed excessively, the membrane is rapidly closed, and the differential pressure of the membrane increases in a short time, so that processing becomes impossible. Moreover, since the water quality of river water and lake water fluctuates depending on the season and weather, controlling the injection amount of the coagulant according to the change in water quality is an important management item.

Therefore, conventionally, turbidity proportional injection control has been adopted in which the turbidity of a raw water sampled is measured by a turbidity meter and a coagulant in an amount proportional to the turbidity is injected. The automatic coagulant injection system employing this control method has long been used in combination with coagulation sedimentation as a pretreatment particularly in rapid sand filtration.

However, if the turbidity proportional injection control, which has been used in the pretreatment in the rapid sand filtration method, is directly applied to the pretreatment in the membrane filtration, the turbidity of the raw water may be low or the turbidity of the raw water may be affected by rainfall. When the degree rose, it turned out to be a problem. In other words, when the turbidity of the raw water is low, the amount of coagulant injected decreases, and depending on the water quality, the quality of the membrane filtration water deteriorates and the membrane is easily clogged. It has been found that during turbidity, a large amount of flocculant is injected, so that flocs are formed excessively and blockage of the membrane may occur.

[0006]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems, and performs a membrane filtration operation in a stable state with a small increase in the membrane filtration differential pressure even when the turbidity of raw water is low or high. The present invention has been made to provide a method for controlling the injection of a flocculant in membrane filtration that can ensure stable membrane filtration water quality.

[0007]

The method of controlling the injection of a flocculant in membrane filtration according to the present invention, which has been made to solve the above-mentioned problems, comprises the steps of: The control is based on the value of degree / turbidity. In particular, it is preferable to control the injection amount of the coagulant in proportion to the apparent chromaticity / turbidity value of the raw water.

As described above, in the present invention, the injection control of the coagulant is performed by using an unconventional variable such as the ratio of chromaticity to turbidity of raw water. As described above, chromaticity and turbidity are both related to the organic matter in raw water, whereas turbidity represents the number of suspended particles in raw water, whereas chromaticity is the absorbance at a wavelength of about 390 nm by an absorbance analysis method. It is a value obtained by measurement and represents the degree of yellowish to yellowish brown exhibited by soluble substances and colloidal substances contained in raw water. Therefore, the chromaticity component is much finer than the turbidity component.

Therefore, the chromaticity component is neglected in the conventional method in which the flocculant is added in proportion to the turbidity. If the amount of the flocculant injected is reduced during low turbidity, the chromaticity component is sufficiently taken into the floc. It is considered that the film is not clogged. In addition, coagulation requires a core substance, and in the presence of a coagulant, turbid particles also function as a nucleating material. Therefore, it is assumed that clogging of the film due to the chromaticity component occurs.

As described above, the present inventor believed that the injection amount of the flocculant should be controlled in consideration of not only turbidity but also chromaticity, and as a result of repeating experiments under various conditions,
The amount of coagulant injected into the raw water to be subjected to membrane filtration is determined by the chromaticity of the raw water /
It was concluded that the most stable membrane filtration was possible when controlled based on the turbidity value. Hereinafter, the present invention will be described in further detail with embodiments.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, reference numeral 1 denotes a raw water inflow tank into which river water or lake water flows, and a flocculant is added to raw water taken out of the raw water inflow tank 1 in a mixing tank 2 to form floc. It is the same as the conventional method that the organic matter is taken into the flocculated floc in the tank 3 and then the membrane filtration is performed by the membrane filtration device 4.

As the flocculant, PAC is usually used, but the present invention is not limited to this. The flocculant is added to raw water from a flocculant storage tank 5 by a flocculant injection pump 6. In this embodiment, a monolithic ceramic membrane having a pore diameter of 0.1 μm is used as the membrane filtration device 4, but another membrane such as a polymer membrane may be used.

In the present invention, the turbidity of raw water is measured by a turbidimeter 7 and the chromaticity of raw water is measured by a chromaticity meter 8. The measured values are input to a calculator 9 which controls the coagulant injection pump 6 based on the chromaticity / turbidity values of the raw water to control the coagulant injection amount into the raw water. The turbidimeter 7 may be the same as the conventional one, and the chromaticity meter 8 may be a device that measures the absorbance at a wavelength of about 390 nm as described above.

FIG. 2 is a graph showing the relationship between the chromaticity / turbidity value of the raw water used in the present invention and the coagulant injection amount. The apparent chromaticity shown on the horizontal axis means a value obtained by directly measuring the chromaticity of raw water. In addition, the chromaticity has a chromaticity, which means the chromaticity measured by collecting a centrifuged supernatant or a filtrate when the raw water is turbid. In the present invention, any value may be used as the chromaticity value, but the apparent chromaticity is used here.

In the graph of FIG. 2, the coagulant injection amount is proportional to the apparent chromaticity / turbidity value, and the coagulant is injected by the coagulant injection pump 6 accordingly. The chromaticity and turbidity components in the raw water are sufficiently taken into the flocculated flocs, and the flocculated flocs are separated by the membrane filtration device 4. By controlling the injection amount of the coagulant in this manner, the increase in the differential pressure of the membrane filtration device 4 is suppressed, and stable membrane filtration water quality can be secured for a long period of time.

The effects of the present invention described above are supported by the following experimental facts. The present inventor brought a plurality of membrane filtration devices of the same specification to a certain water treatment plant in Aichi Prefecture, and changed the amount of coagulant (PAC) injected into raw water for each membrane filtration device in parallel to perform membrane filtration. I drove. For example, the first unit has an injection rate of 10 mg / L, and the second unit has an injection rate of 20 mg / L.
mg / L, and the third unit had an injection rate of 30 mg / L. The experiment was carried out for about 4 months, during which the properties (color, turbidity) of the raw water changed significantly, but the raw water supplied to the plurality of membrane filtration devices was always common.

Under each condition, the rise of the transmembrane pressure is measured,
The differential pressure rise coefficient (kPa / day) to which the temperature was corrected was determined. Then, one of the plurality of membrane filtration devices having the smallest differential pressure increase coefficient was selected, and the coagulant injection rate was determined as the optimum coagulant injection rate most suitable for the raw water quality at that time.

FIG. 3 is a graph showing the relationship between the turbidity and the optimum coagulant injection rate in this experiment. In the conventional turbidity proportional injection control, the coagulant injection rate is increased in proportion to the turbidity.Therefore, a correlation should be established between the turbidity and the optimum coagulant injection rate. The opposite trend is to be observed when the turbidity is low and the coagulant injection rate should be increased. These experimental results also support the conventional problem that membrane clogging is likely to occur when the turbidity is low.

FIGS. 4 and 5 are graphs in which the true chromaticity and the apparent chromaticity are plotted on the horizontal axis, and the vertical axis is set to the optimum coagulant injection rate. Also in these graphs, there is no correlation between the true or apparent chromaticity and the optimal coagulant injection rate. That is, it can be seen that simply controlling the injection amount of the flocculant using the chromaticity as a scale has no effect.

However, as shown in FIG. 6, if the apparent chromaticity / turbidity value is plotted on the horizontal axis, a clear correlation appears between the value and the optimum coagulant injection rate. The graph of FIG. 2 described above is based on the results of this experiment. FIG. 7 is a graph in which the value of true chromaticity / turbidity is plotted on the horizontal axis, and although not as large as in FIG. 6, a correlation is observed between the optimum coagulant injection rate.

As described above, the present invention controls the amount of the coagulant to be injected into the raw water to be subjected to membrane filtration based on the chromaticity / turbidity value of the raw water, so that the properties of the raw water can be significantly changed. However, the coefficient of pressure rise of the membrane filtration device can always be kept at a minimum. This means that the membrane filtration is performed under optimum conditions, and it goes without saying that the quality of the membrane filtration water is kept stable even when the properties of the raw water greatly change. The proportionality constant for calculating the optimal coagulant injection rate based on the chromaticity / turbidity value is based on the type of tap water source such as river surface water, groundwater, reservoir water, lake water, spring water, and underground water. Because it varies depending on the characteristics of water quality,
It is not limited to the gradient of.

[0022]

EXAMPLES Example 1 (Low turbidity) When the turbidity of raw water is 3.2 and the apparent chromaticity is 8.3, according to the conventional turbidity proportional injection control method, the injection amount of the flocculant is, for example, It can be kept as low as 10 mg / L. On the other hand, according to the present invention, since the apparent chromaticity / turbidity = 2.59, the injection amount of the flocculant was set as large as 25 mg / L. As a result, in the conventional method, the differential pressure increase coefficient was as large as 20 kPa / day, and the membrane differential pressure was rapidly increased in a short time. However, in the present invention, the differential pressure increase coefficient was as extremely small as 0.70 kPa / day. .

Example 2 (High turbidity) When the turbidity of the raw water is 16.5 and the apparent chromaticity is 24.4, the amount of the coagulant injected is, for example, 30 mg according to the conventional turbidity proportional injection control method. / L. On the other hand, according to the present invention, since the apparent chromaticity / turbidity is 1.48, the injection amount of the coagulant is as small as 10 mg / L. As a result, the differential pressure increase coefficient was 1.40 kPa / day in the conventional method, but was extremely small as 0.10 kPa / day in the present invention.

Example 3 (During turbidity) When the turbidity of the raw water is 7.6 and the apparent chromaticity is 11.3,
According to the conventional turbidity proportional injection control method, the injection amount of the flocculant is, for example, 20 mg / L. On the other hand, according to the present invention, the apparent chromaticity / turbidity is 1.49, so the injection amount of the flocculant is 10 mg / L. As a result, the differential pressure increase coefficient was 0.70 kPa / day in the conventional method, but was 0.00 kPa / day in the present invention.

[0025]

As described above, according to the present invention, the turbidity and color of the raw water are controlled by controlling the injection of the flocculant using the unconventional variable of the chromaticity and the turbidity ratio of the raw water. Even when the degree changes greatly, stable membrane filtration can always be performed. Therefore, stable membrane filtration water quality can be secured while avoiding clogging and blockage of the membrane. Further, the amount of the coagulant used can be reduced as compared with the control by the conventional turbidity proportional injection control method, which can contribute to a reduction in running cost.

[Brief description of the drawings]

FIG. 1 is an apparatus configuration diagram showing an embodiment of the present invention.

FIG. 2 shows the coagulant injection amount and apparent chromaticity /
It is a graph which shows the relationship with the value of turbidity.

FIG. 3 is a graph showing a relationship between turbidity and an optimum coagulant injection rate in an experiment.

FIG. 4 is a graph showing the relationship between the true chromaticity and the optimal coagulant injection rate in an experiment.

FIG. 5 is a graph showing a relationship between an apparent chromaticity and an optimum coagulant injection rate in an experiment.

FIG. 6 is a graph showing a relationship between apparent chromaticity / turbidity and an optimum coagulant injection rate in an experiment.

FIG. 7 is a graph showing a relationship between true chromaticity / turbidity and an optimum coagulant injection rate in an experiment.

[Explanation of symbols]

1 Raw water inflow tank, 2 mixing tank, 3 floc forming tank, 4
Membrane filtration device, 5 flocculant storage tank, 6 flocculant injection pump, 7 turbidity meter, 8 chromaticity meter, 9 arithmetic unit

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4D006 GA02 KA01 KA64 KB13 KD08 KE12P KE12Q KE12R MA03 MA22 MB02 MC03 MC03X PA01 PB04 PB05 PB22 PB34 PC80 4D015 BA19 BA21 BB05 CA14 DA04 EA37

Claims (2)

[Claims] 1. A method for controlling the injection of a flocculant in membrane filtration, comprising controlling the amount of the flocculant to be injected into the raw water to be subjected to membrane filtration based on the chromaticity / turbidity value of the raw water. 2. The method according to claim 1, wherein the injection amount of the coagulant is controlled in proportion to the apparent chromaticity / turbidity value of the raw water.