JP7767864B2 - Wastewater treatment method and equipment - Google Patents

Description

The present invention relates to a wastewater treatment method and apparatus that performs coagulation and solid-liquid separation on wastewater, and in particular to a wastewater treatment method and apparatus that returns a portion of the coagulated sludge to a reaction tank and adds an alkaline agent such as slaked lime to this returned sludge.

High Density Sludge (HDS) processes, such as alkaline sludge processes, are known as methods for obtaining high-concentration sludge that is highly thick and has excellent dewaterability by subjecting wastewater to coagulation and solid-liquid separation. In the alkaline sludge process, an alkali such as slaked lime (Ca(OH) ₂ ) is added to a portion of the sludge that is separated and returned in the solid-liquid separation process that follows the coagulation process.

Figure 2 is a system diagram showing a typical system for a conventional high-density flocculation sludge treatment method. Raw water is adjusted to a pH of approximately 2.5 to 3.0 by the addition of a pH adjuster in raw water tank 1, and then introduced into reaction tank 2, where it is mixed with and reacted with modified sludge introduced from modification tank 8, described below. This reaction liquid is then sent to flocculation tank 3, where a polymer flocculant is added by polymer flocculant addition means 3a for flocculation treatment, and then undergoes solid-liquid separation in settling tank 4. The supernatant water from settling tank 4 is discharged outside the system as treated water. A portion of the separated sludge extracted from settling tank 4 by pump 5 is returned to modification tank 8 via sludge return line 7, and the remainder is discharged outside the system via discharge line 6.

In the reforming tank 8, the returned sludge is mixed with a dissolved/dispersed solution of slaked lime (a liquid in which part of the slaked lime is dissolved and the rest is dispersed) supplied by the slaked lime supplying means 9, which acts as an alkaline agent supplying means. This mixed solution (modified sludge) is supplied to the reaction tank 2. The reaction in this reaction tank 2 preferably occurs within a predetermined pH range (e.g., 6 to 11, particularly 6 to 8). The amount of slaked lime supplied to the reforming tank 8 is adjusted based on the pH detected by the pH meter 2a installed in the reaction tank 2 to maintain this pH range.

The sludge discharge line 6 and the sludge return line 7 are provided with open/close valves 11 and 12, respectively. By opening valve 11 and closing valve 12, the entire amount of sludge from pump 5 is discharged outside the system. By opening valve 12 and closing valve 11, the entire amount of sludge from pump 5 is returned to the reforming tank 8. The amount of sludge returned to the reforming tank 8 is controlled by switching the opening and closing of valves 11 and 12 using a control means (not shown).

Japanese Patent Application Publication No. 5-7879 Japanese Patent Application Laid-Open No. 2003-71469 JP 2013-208599 A

In the conventional example shown in Figure 2, when valve 11 is opened, valve 12 is closed, and the entire amount of sludge is discharged outside the system through discharge line 6, there is no sludge return to the reforming tank 8, so the supply of modified sludge (slaked lime-added sludge) from the reforming tank 8 to the reaction tank 2 is reduced or stopped, and the pH of the reaction tank 2 falls below the specified range. If valve 12 is then opened and valve 11 is closed to resume sludge return to the reforming tank 8, a large amount of high-pH modified sludge flows in from the reforming tank 8, causing the pH of the reaction tank 2 to rise rapidly. Such a sudden pH fluctuation can result in poor sludge modification and coagulation, leading to a deterioration in treated water quality and a high moisture content in the sludge discharged from the discharge line 6.

The objective of the present invention is to provide a wastewater treatment method and apparatus that stabilizes the flow rate of alkaline agent-added liquid flowing from the reforming tank to the reaction tank, thereby consistently maintaining good treated water quality and discharged sludge properties.

To solve the above problems, the wastewater treatment method and apparatus of the present invention are as follows:

[1] Adding alkaline agent-added return sludge to wastewater to cause a reaction, then subjecting it to coagulation treatment, and then separating the sludge from the treated water by solid-liquid separation;
A part of the separated sludge is used as return sludge, and this return sludge is introduced into a modification tank, and the alkaline agent is added to the wastewater, and then the returned sludge is added to the modification tank.
In a wastewater treatment method in which the remaining sludge is extracted and discharged as sludge,
A method for treating wastewater, comprising the step of supplying water to the reforming tank from a water supply line when the return of sludge to the reforming tank is stopped or when the amount of sludge returned falls below a predetermined amount.

[2] The wastewater treatment method of [1], in which water is supplied to the reforming tank through the water supply line when sludge return from the settling tank to the reforming tank is stopped.

[3] The wastewater treatment method of [2], wherein the amount of water supplied to the reforming tank is the same as the amount of sludge returned when sludge extraction is stopped.

[4] The wastewater treatment method according to any one of [1] to [3], wherein the water is industrial water.

[5] A reaction tank in which alkaline agent-added return sludge from the reforming tank is added to the wastewater;
a coagulation tank for adding a polymer coagulant to the liquid from the reaction tank;
a solid-liquid separation means for separating the flocculation treatment liquid to which the flocculant has been added into solid and liquid;
a modification tank in which an alkaline agent is added to a portion of the sludge separated by the solid-liquid separation means to produce the alkaline agent-added return sludge,
The wastewater treatment device is characterized in that a water supply line capable of supplying water to the reforming tank is provided.

[6] A valve is provided on a line for returning the sludge separated by the solid-liquid separation means to the reforming tank,
The wastewater treatment device according to [5], further comprising a water supply control means for supplying water from the water supply line to the reforming tank when the valve is closed.

With the wastewater treatment method and apparatus of the present invention, even when the amount of sludge returned from the solid-liquid separation means to the reforming tank decreases, water is still supplied to the reforming tank, allowing alkaline agent-added liquid to be introduced from the reforming tank into the reaction tank, maintaining the pH of the reaction tank within a specified range. As a result, coagulation is always performed well, improving the quality of the treated water and reducing the water content of the sludge.

FIG. 1 is a flow diagram for explaining a wastewater treatment method and device according to an embodiment. FIG. 1 is a flow chart for explaining a wastewater treatment method and device according to a conventional example.

Embodiments will now be described with reference to the drawings. Figure 1 is a flow diagram of a wastewater treatment facility to which a wastewater treatment method according to an embodiment of the present invention is applied. Note that parts that are the same as those in Figure 2 are assigned the same reference numerals.

In this embodiment, as in Figure 2, raw water is adjusted to a pH of approximately 2.5 to 3.0 by the addition of a pH adjuster in raw water tank 1, and then introduced into reaction tank 2, where it is mixed with and reacted with modified sludge introduced from modification tank 8. This reaction liquid is then sent to coagulation tank 3, where a polymer coagulant is added by polymer coagulant addition means 3a for coagulation treatment, and then undergoes solid-liquid separation in settling tank 4. The supernatant water from settling tank 4 is discharged outside the system as treated water. A portion of the separated sludge extracted from settling tank 4 by pump 5 is returned to modification tank 8 via sludge return line 7, and the remainder is discharged outside the system via discharge line 6.

In the reforming tank 8, a dissolved/dispersed solution of slaked lime supplied by the slaked lime supplying means 9, which serves as an alkaline agent supplying means, is mixed with the returned sludge. This mixture (modified sludge) is supplied to the reaction tank 2. The reaction in this reaction tank 2 preferably occurs within a predetermined pH range (e.g., 6 to 11, particularly 6 to 8). The amount of slaked lime supplied to the reforming tank 8 is adjusted based on the pH detected by the pH meter 2a installed in the reaction tank 2 to maintain this pH range.

The sludge discharge line 6 and the sludge return line 7 are provided with open/close valves 11 and 12, respectively. By opening valve 11 and closing valve 12, the entire amount of sludge from pump 5 is discharged outside the system. By opening valve 12 and closing valve 11, the entire amount of sludge from pump 5 is returned to the reforming tank 8. The amount of sludge returned to the reforming tank 8 is controlled by switching the opening and closing of valves 11 and 12 using a control means (not shown).

In this embodiment, a water supply line 14 is provided to supply water to the reforming tank 8, and a valve 13 is provided on the water supply line 14. The valve 13 may be an open/close valve or a flow control valve that can adjust the flow rate.

This valve 13 is opened by a control means (not shown) when valve 11 is open and valve 12 is closed. Also, when valve 12 is open and valve 11 is closed, valve 13 is closed.

Examples of water supplied from the water supply line 14 include industrial water, well water, city water, river water, and wastewater treatment plant runoff, with industrial water being particularly preferred.

When valve 12 is closed and valve 13 is opened to supply water to the reforming tank 8, it is preferable to set the amount of water supplied to be the same as the amount of sludge returned when valve 12 is open.

In this embodiment, when the return of sludge from the settling tank 4 to the reforming tank 8 is stopped, water is supplied to the reforming tank 8 via the water supply line 14, and alkaline slaked lime-containing liquid is introduced from the reforming tank 8 to the reaction tank 2. As a result, the pH in the reaction tank 2 is always maintained within a predetermined range (preferably 6 to 11, and particularly preferably 6 to 8), the coagulation reaction proceeds sufficiently, the treated water quality is improved, and the water content of the coagulated sludge discharged from the discharge line 6 is low.

Examples of raw water to be treated by the present invention include, but are not limited to, fluorine-containing wastewater, phosphorus-containing wastewater, and wastewater containing heavy metals such as iron, zinc, copper, chromium, nickel, lead, and aluminum.

In the above embodiment, slaked lime is added to the reforming tank 8 as an alkaline agent, but this is not limited to this. In the case of wastewater containing heavy metals, caustic soda or the like may also be used.

The above embodiment is one example of the present invention, and the present invention may be embodied in other forms. For example, valves 12 and 13 may be designed to be flow-adjustable, allowing the amount of sludge returned to the reforming tank 8 to be adjusted to an increased or decreased amount, and the amount of water supplied to the reforming tank 8 from the water supply line 14 may be increased so that the amount of sludge returned to the reforming tank 8 becomes less than the amount of sludge returned when valve 12 is fully open.

2 Reaction tank 3 Coagulation reaction tank 4 Sedimentation tank 8 Modification tank 9 Slaked lime adding means 14 Water supply line

Claims (5)

The alkaline agent-added return sludge is added to the wastewater to cause a reaction, followed by coagulation treatment, followed by solid-liquid separation treatment in a settling tank to separate the sludge from the treated water,
A part of the separated sludge is used as return sludge, and this return sludge is introduced into a modification tank , an alkaline agent is added, and the alkaline agent-added return sludge is added to the wastewater;
In a wastewater treatment method in which the remaining sludge is extracted and discharged as sludge,
A method for treating wastewater, comprising the step of supplying water to the reforming tank from a water supply line when the return of sludge to the reforming tank is stopped or when the amount of sludge returned falls below a predetermined amount. The wastewater treatment method of claim 1, wherein water is supplied to the reforming tank through the water supply line when sludge return from the settling tank to the reforming tank is stopped. The wastewater treatment method of claim 2, wherein the amount of water supplied to the reforming tank is the same as the amount of sludge returned when sludge extraction is stopped. 4. The method for treating wastewater according to claim 1, wherein the water supplied to the reforming tank is industrial water. a reaction tank in which the alkaline agent-added return sludge from the reforming tank is added to the wastewater;
a coagulation tank for adding a polymer coagulant to the liquid from the reaction tank;
a solid-liquid separation means for separating the flocculation treatment liquid to which the flocculant has been added into solid and liquid;
a modification tank in which an alkaline agent is added to a portion of the sludge separated by the solid-liquid separation means to produce the alkaline agent-added return sludge,
A wastewater treatment device provided with a water supply line capable of supplying water to the reforming tank,
a valve is provided in a line for returning the sludge separated by the solid-liquid separation means to the reforming tank;
a water supply control means for supplying water from the water supply line to the reforming tank when the valve is closed ;