JPS5929281B2 - Coagulant injection amount control device for water treatment plants - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a coagulant injection amount control device for a water purification plant, which allows turbidity to be flocculated in a mixing pond in a water purification plant.

FIG. 1 is a block diagram showing the purification process of raw water in a general water purification plant.

Raw water taken from a river 1 by a pump 2 is guided to a landing well 4 via a headrace 3.

In the receiving well 4, chlorine 11 is injected to perform sterilization and oxidation, and an alkaline agent 12 is injected to adjust the pH and alkali.In the mixing pond 5, a flocculant 13 is injected depending on the quality of the raw water flowing from the receiving well 4. Inject and stir rapidly.

At this stage, suspended solids (fine particles) contained in the raw water coagulate to form fine flocs.

In the floc formation pond 6, slow stirring is performed to grow fine flocs and form large flocs.

In the sedimentation tank 7, the grown flocs are precipitated and sent to the filtration tank 8.

The filtration basin 8 removes microscopic particles that could not be removed in the sedimentation basin 7, and supplies clean water to the water purification basin 9.

Clean water is stored in a water purification reservoir 9, sent to a distribution reservoir by a pump 10, and distributed to each customer.

As described above, in the purification process of a water purification plant, it is important to inject an appropriate amount of flocculant 13 into the mixing basin 5 in accordance with the quality of the raw water.

As is well known, aluminum sulfate or polyaluminum chloride is usually used as the flocculant 13.

The mechanism of action of this flocculant 13 will be explained with reference to FIG.

Suspended matter 21 is usually a negatively charged negative colloid,
Mutual aggregation does not occur due to repulsive forces between particles.

When the flocculant 13 is injected, it dissociates and becomes an aluminum colloid 20 which is a positive colloid.

This aluminum colloid 20 is adsorbed on the surface of the suspended solid 21, lowers the surface potential of the suspended solid 21, and forms a floc by the action of Brownian motion, Van der Waals attraction force, etc.

By the way, in order to inject such a flocculant, conventionally, the injection rate for a unit volume of raw water is determined from the raw water turbidity T and the raw water alkalinity A according to the following formula.

D=g(A,T) ・・・・・・α
) D: Coagulant injection rate g: Function A: Raw water alkalinity T: Raw water turbidity Note that the function g is determined by analyzing past operating data using a statistical method.

However, in actual river water, even if the raw water turbidity and alkalinity are the same, the water quality is completely different on sunny days and on rainy days, for example.

Therefore, even if the flocculant is injected and processed according to the injection rate calculation formula shown in equation (1), which is the same as in normal times, during so-called abnormal conditions such as rain or flooding, good results will not be obtained.

That is, whether the injection rate of the flocculant is larger or smaller than the appropriate value, the repulsive force between the suspended solids becomes stronger than the suction force, and the floc-forming effect is reduced.

As a result, the number of fine particles that do not form flocs and settle in the sedimentation tank increases.

Therefore, the load of E overcurrent increases, resulting in the phenomenon of clogging of the filter sand.

The reason why the above-mentioned unfavorable conditions occur is that the injection rate of the flocculant is determined only by the quantitative relationships of raw water, such as raw water turbidity and raw water alkalinity.

Misprocessing at water treatment plants causes a situation in which water cannot be distributed to customers, which poses a major problem from a public standpoint.

The present invention has been made in view of the above-mentioned problems, and its object is to provide a flocculant injection amount control device for a water purification plant that can effectively form flocs.

A feature of the present invention is that the total surface area of suspended solids contained in a unit volume of raw water is determined, and the injection rate of the flocculant is determined according to this surface area.

First, the basic concept of the present invention will be explained next.

The flocculant injected into the raw water is dissociated and adsorbed onto the surface of the suspended solids, forming flocs, as explained in FIG.

If the amount of coagulant injected is small and the amount of aluminum colloid adsorbed on the surface of the suspended solid is small, flocs will be difficult to grow.

Conversely, if the amount of coagulant injected is too large, the flocs once formed will dissociate.

In any case, the cause is that an appropriate amount of flocculant is not adsorbed on the surface of the suspended solid.

Therefore, in the present invention, the flocculant is injected in proportion to the total surface area of suspended solids contained in raw water per unit volume.

Next, one embodiment of the present invention will be described with reference to FIG.

In FIG. 3, the same numbers as in FIG. 1 indicate equivalents, 30 is a flow meter, 31 is a turbidity meter, 32 is a particle size measuring device,
These measuring instruments are provided on the upstream side of the landing well 4.

33 is an alkalinity meter, which is installed at the outlet of the landing well 4.

39 is an arithmetic circuit which calculates the flocculant injection rate from the raw water turbidity T1, the raw water suspended particle size d1, and the alkalinity A.

Note that the arithmetic circuit 39 has a function of determining the total surface area of suspended solids, as will be described later.

40 is a multiplication circuit which multiplies the raw water flow rate F by the flocculant injection rate to determine the flocculant injection amount.

A flocculant injection pump 38 injects the flocculant 13 according to the output value of the multiplier 40.

A diaphragm type pump is normally used as the pump 38, and the supply amount is adjusted by adjusting its stroke length.

34 is an alkalinity meter, and 35 is a pH meter, which are provided at the outlet of the floc formation pond 6.

42 is a pH-'A adjustment circuit, which determines the alkaline agent injection rate from the treated water alkalinity AL and the treated water pH, and adjusts the pH of the mixing basin 5.

37 is a diaphragm type alkaline agent injection pump, 36 is a chlorine injection pump, each with 12 alkaline agents and 11 chlorine agents.
is injected into landing well 4.

41 is a multiplication circuit that multiplies the alkaline agent injection rate and the raw water flow rate F to match.

Next, its operation will be explained.

The calculation circuit 39 calculates the total surface area of the suspended solids and calculates the flocculant injection rate.

It is difficult to count all the particles contained in a unit volume of raw water and to continuously determine their total surface area.

Therefore, the calculation circuit 39 calculates the surface area from the particle size distribution of the suspended solids and the turbidity of the raw water.

We will explain the concept behind the calculation to find this surface area.

Figure 4 shows the results of actual measurements of the relationship between the particle size distribution and weight of suspended solids in river water.

Ll is for sunny days, L2 is for rainy days, and raw water turbidity T
1 The raw water alkalinity A was the same.

If the total surface area of turbidity contained in a unit volume of raw water on a clear day is 81 and that on a rainy day is 82, the following equation is obtained.

however T ; raw water turbidity ρ ；Density of fine particles dl; Average particle size of suspended solids on clear skies d2; During rainy weather Ru.

From equations (2) and (3), when the density of suspended solids is constant, the total surface area S of suspended solids is the ratio of raw water turbidity T to average particle diameter d. The flocculant injection rate is determined from the T1 average particle diameter d1 and the alkalinity A.

The function f is determined by a beaker experiment such as a jar test or by analyzing past operating data.

Note that alkalinity is a major factor that determines the state of dissociation of the flocculant, so the term alkalinity A is included in equation (4).

This alkalinity only needs to be at least a predetermined value and does not need to be particularly controlled.

The multiplication circuit 40 multiplies the flocculant injection rate determined by the arithmetic circuit 39 by the raw water flow rate F to determine the flocculant injection amount, and controls the stroke length of the flocculant injection pump 38 in accordance with this value.

On the other hand, the pH adjuster 42 adjusts the injection amount of the alkaline agent 12 in order to improve the state of dissociation of the flocculant 13.

That is, the treated water alkalinity AL and the treated water pH are measured, and the alkaline agent injection rate for pH adjustment is determined.

For pH adjustment, a target pH value is usually determined (many places adopt a target pH value of 6.5 to 7.5).

), determine the alkaline agent injection rate so as to cancel out the deviation from the pH of the treated water.

The multiplication circuit 41 multiplies the alkali injection rate and the raw water flow rate F to obtain the alkali injection amount, and the alkali injection pump 3
Control 7.

The flocculant is injected as described above, and even if the particle size of the suspended solids in the raw water changes, the flocculant can be injected appropriately and can maintain good quality clean water. Obtainable.

In the above example, the suspended particle size is the average particle size based on the particle size distribution, but it goes without saying that the particle size of each fine particle may be measured off-line.

As explained above, according to the present invention, the injection rate of the flocculant is determined according to the total surface area of suspended solids in the raw water.
Facilitates good floc formation.

As a result, clean water of stable quality can be obtained.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the water treatment process at a water purification plant.
Figure 2 is a diagram showing the state of floc formation by suspended matter in raw water and aluminum colloid, Figure 3 is a configuration diagram showing an embodiment of the present invention, and Figure 4 is a characteristic diagram showing the particle size distribution of suspended matter. be. 1...River, 5...Mixing pond, 13...
...Flocculant, 30...Flowmeter, 31...
...Turbidity meter, 32...Particle size measuring device, 38...
. . . Coagulant injection pump, 39 . . . Arithmetic circuit, 40, 41 . . . Multiplication circuit.

Claims (1)

[Scope of Claims] 1. A mixing pond that is supplied with raw water and injected with a flocculant to form flocs of turbidity in the raw water, a turbidity meter that measures the turbidity of the raw water, and a turbidity meter that measures the turbidity of the raw water; A particle size measuring device that measures the particle size of suspended solids, and calculation for calculating the total surface area of suspended solids contained in a unit volume of raw water using the raw water turbidity and suspended solid particle size measured by the turbidity meter and particle size measuring device. A flocculant injection amount control device for a water purification plant, comprising a circuit and an injection device that determines the flocculant injection rate in proportion to the total surface area of suspended solids determined by the arithmetic circuit. 2. A patent characterized in that the arithmetic circuit calculates the total surface area of suspended solids contained in a unit volume of raw water based on the average particle diameter based on the particle size distribution of suspended solids contained in a unit volume of raw water and the raw water turbidity. A flocculant injection amount control device for a water purification plant according to claim 1.