JP7621872B2 - Water treatment method and water treatment device - Google Patents

Description

The present invention relates to a water treatment method and a water treatment device for reusing treated sewage water.

The technique of recovering water from treated sewage and using it as reclaimed water is widely known. When recovering water, reverse osmosis membranes are often used to remove ionic components.

For example, Patent Document 1 describes an apparatus that treats sewage or treated sewage water from a sewage treatment facility using a membrane module such as a reverse osmosis membrane to obtain reclaimed water.

Non-Patent Document 1 describes a method for producing reclaimed water from public sewage treatment water using reverse osmosis membranes.

As shown in the example of water quality of sewage treatment water in Table 2 of Non-Patent Document 1, the silica concentration in normal sewage treatment water is about 10 to 20 mg/L, which is the same level as environmental water in Japan, and the solubility of silica is about 120 mg/L at room temperature, so silica scale is almost not a problem when treating sewage treatment water with a reverse osmosis membrane. Therefore, reverse osmosis membranes are usually operated in the neutral pH range.

However, it has been found that in sewage treatment plants that receive large volumes of wastewater from certain large-scale factories, for example, the silica concentration in the treated sewage water can exceed 50 mg/L. When using reverse osmosis membranes to recover water from such wastewater, there is a problem in that a low water recovery rate must be set so that silica scale does not precipitate (for example, a concentration rate of 2.4 times or less when the silica concentration in the water being treated is 50 mg/L).

On the other hand, Patent Document 2 describes a reverse osmosis membrane treatment method in which water to be treated, which is industrial water, city water, or well water containing at least hardness components and silica, is treated using a reverse osmosis membrane to separate the water into permeate and concentrated water having a silica concentration equal to or higher than the solubility of silica, and in which the pH of the concentrated water is maintained at 6 or less during reverse osmosis treatment.

Patent No. 3305354 Patent No. 3187629

Kobelco Eco-Solutions Technical Report, 2010, Vol. 7, No. 1, pp. 16-22

As a result of the inventors' investigations, it was found that even if the silica concentration in the concentrated water is kept below the solubility of silica, when silica, aluminum, and sodium coexist in the treated sewage water, the silica, aluminum, and sodium form complex precipitates in the neutral to alkaline range, which can clog the reverse osmosis membrane.

Furthermore, as a result of the investigation, it was found that even if the aluminum in the sewage treatment water is at an extremely low concentration, for example, between 0.01 mg/L and 0.1 mg/L, when it coexists with silica and sodium, it forms precipitates that clog the reverse osmosis membrane.

The object of the present invention is to provide a water treatment method and water treatment device that can suppress the formation of composite precipitates of silica, aluminum, and sodium in reverse osmosis membrane treatment of sewage treatment water, even when silica, aluminum, and sodium coexist in the sewage treatment water, and that enables stable operation of the reverse osmosis membrane.

The present invention is a water treatment method for recovering and reusing effluent from sewage treatment, the effluent containing silica and containing 0.01 mg/L to 0.1 mg/L of aluminum and further containing sodium at an equimolar concentration to that of the aluminum, the water treatment method including a reverse osmosis membrane treatment step of treating the effluent with a reverse osmosis membrane to separate it into RO permeate and RO concentrate having a silica concentration of 50 mg/L or more, and in the reverse osmosis membrane treatment step, the pH of the RO concentrate is maintained at 5.5 or less while the reverse osmosis membrane treatment is performed.

In the water treatment method, it is preferable that the silica concentration of the RO concentrated water is less than 120 mg/L.

It is preferable that the water treatment method further includes an organic matter decomposition treatment step in which the RO permeate is subjected to an organic matter decomposition treatment.

In the water treatment method, it is preferable that the effluent from the sewage treatment is secondary sewage treatment water that has undergone solid-liquid separation treatment and biological treatment.

The water treatment method preferably further includes a turbidity treatment process for performing a turbidity treatment on the effluent from the sewage treatment prior to the reverse osmosis membrane treatment process.

The present invention is a water treatment device that recovers and reuses effluent from sewage treatment, the effluent containing silica and containing 0.01 mg/L or more and 0.1 mg/L or less of aluminum, and further containing sodium at an equimolar concentration with respect to the aluminum, the water treatment device comprising: a reverse osmosis membrane treatment means that treats the effluent with a reverse osmosis membrane to separate it into RO permeate and RO concentrated water having a silica concentration of 50 mg/L or more; and a pH adjustment means that maintains the pH of the RO concentrated water in the reverse osmosis membrane treatment means at 5.5 or less.

In the water treatment device, it is preferable that the silica concentration of the RO concentrated water is less than 120 mg/L.

It is preferable that the water treatment device further includes an organic matter decomposition treatment means for performing an organic matter decomposition treatment on the RO permeate.

In the water treatment device, it is preferable that the effluent from the sewage treatment is secondary treated sewage that has undergone solid-liquid separation and biological treatment.

It is preferable that the water treatment device further includes a turbidity treatment means for performing a turbidity treatment on the effluent from the sewage treatment upstream of the reverse osmosis membrane treatment means.

The present invention provides a water treatment method and water treatment device that can suppress the formation of composite precipitates of silica, aluminum, and sodium in reverse osmosis membrane treatment of sewage treatment water, even when silica, aluminum, and sodium coexist in the sewage treatment water, and enable stable operation of the reverse osmosis membrane.

1 is a schematic diagram illustrating an example of a water treatment device according to an embodiment of the present invention. FIG. 4 is a schematic configuration diagram showing another example of a water treatment device according to an embodiment of the present invention. 1 is a graph showing calculation results of solid precipitation for RO concentrated water having the water quality shown in Table 1. 1 is a graph showing the calculation results of solid precipitation for RO concentrated water when the water quality is shown in Table 1 and the _SiO2 concentration is 42 mg/L. 1 is a graph showing the change in the transmission coefficient retention rate (%) over time (days) in Example 1 and Comparative Example 1.

The following describes an embodiment of the present invention. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

An example of a water treatment device according to an embodiment of the present invention is outlined in FIG. 1, and its configuration will be described.

The water treatment device 1 is a water treatment device that recovers and reuses effluent from sewage treatment, and is equipped with a reverse osmosis membrane treatment device 16 as a reverse osmosis membrane treatment means that performs reverse osmosis membrane treatment of the effluent from sewage treatment to separate it into RO permeate and RO concentrated water with a silica concentration of 50 mg/L or more, and a pipe 38 that adds a pH adjuster as a pH adjustment means that maintains the pH of the RO concentrated water in the reverse osmosis membrane treatment device 16 at 5.5 or less. The effluent from sewage treatment to be treated contains silica, aluminum at 0.01 mg/L or more, and sodium at an equimolar concentration or more with aluminum.

The water treatment device 1 may include a sewage treatment tank 10 for storing effluent from sewage treatment, a turbidity membrane treatment device 12 as a turbidity treatment means for performing turbidity treatment on the effluent from sewage treatment, and a treated water tank 14 for storing treated water for reverse osmosis membrane treatment, which is the turbidity treatment water obtained by the turbidity treatment.

In the water treatment device 1 of FIG. 1, the outlet of the sewage treatment tank 10 and the discharge water inlet of the turbidity removal membrane treatment device 12 are connected by a pipe 24 via a pump 18. The turbidity removal treatment water outlet of the turbidity removal membrane treatment device 12 and the turbidity removal treatment water inlet of the treated water tank 14 are connected by a pipe 26. The treated water outlet of the treated water tank 14 and the treated water inlet of the reverse osmosis membrane treatment device 16 are connected by a pipe 28 via a pump 20. A pipe 30 is connected to the RO permeate outlet of the reverse osmosis membrane treatment device 16, and a pipe 32 is connected to the RO concentrated water outlet. The backwash water outlet of the treated water tank 14 and the middle of the pipe 26 are connected by a pipe 34 via a pump 22. A pipe 36 is connected to the backwash drainage outlet of the turbidity removal membrane treatment device 12. A pipe 38 for adding a pH adjuster is connected to the downstream side of the pump 20 in the pipe 28. A pH meter 42 may be installed downstream of the connection point of the pipe 38 in the pipe 28 as a treated water pH measurement means for measuring the pH of the treated water. A pH meter 44 may be installed in the pipe 32 as an RO concentrated water pH measurement means for measuring the pH of the RO concentrated water.

The water treatment method and operation of the water treatment device 1 according to this embodiment will be described.

The effluent from the sewage treatment is stored in the sewage treatment tank 10 as necessary, and then pumped by the pump 18 through the pipe 24 to the turbidity removal membrane treatment device 12. In the turbidity removal membrane treatment device 12, the effluent from the sewage treatment is subjected to turbidity removal treatment, and turbidity removed water (permeate) is obtained (turbidity removal treatment process). As the turbidity removal membrane, a microfiltration membrane (MF membrane), an ultrafiltration membrane (UF membrane), or the like is used.

The clarified water obtained by the clarification process is stored in the water tank 14 as necessary through pipe 26, and then pumped 20 through pipe 28 to the reverse osmosis membrane treatment device 16 as the water to be treated for reverse osmosis membrane treatment. In the reverse osmosis membrane treatment device 16, reverse osmosis membrane treatment is performed on the water to be treated, and the water is separated into RO permeate and RO concentrate with a silica concentration of 50 mg/L or more (reverse osmosis membrane treatment process). The RO permeate is discharged through pipe 30, and the RO concentrate is discharged through pipe 32.

When cleaning of the turbidity removal membrane is required, the turbidity removal treatment water is sent as backwash water from the treated water tank 14 to the secondary side of the turbidity removal membrane treatment device 12 by the pump 22 through the pipes 34 and 26. The backwash water is passed from the secondary side to the primary side of the turbidity removal membrane treatment device 12 to clean the turbidity removal membrane, and the backwash wastewater is discharged from the primary side of the turbidity removal membrane treatment device 12 through the pipe 36 (backwash process).

In the reverse osmosis membrane treatment process, the pH of the RO concentrated water is maintained at 5.5 or less while reverse osmosis membrane treatment is performed. As a result, even if silica, aluminum, and sodium coexist in the sewage treatment water during reverse osmosis membrane treatment, the formation of composite precipitates of silica, aluminum, and sodium can be suppressed, and the reverse osmosis membrane can be operated stably. Furthermore, the formation of precipitates of aluminum alone can also be suppressed, allowing the reverse osmosis membrane to be operated stably.

For example, Fig. 3 shows the calculation results of the amount of precipitates generated, calculated from the solubility of each component for RO concentrated water having the water quality shown in Table 1 below. In the graph of Fig. 3, the horizontal axis shows the pH of the RO concentrated water, and the vertical axis shows the amount (g) of precipitates of a compound of silica, aluminum, and _sodium ( _{NaAlSi2.3O6.6.4H2O} ), a compound of calcium and fluorine ( _Ca5F ( _PO4 ) ₃ ), and aluminum hydroxide (Al(OH) _{3 )} .

As can be seen from Figure 3, at neutral to alkaline pH, a complex of silica, aluminum, and sodium precipitates, and at pH levels above 5.5 and below 6.0, aluminum hydroxide precipitates, but at pH levels below 5.5, there is almost no precipitation. If the silica concentration in the RO concentrated water is below 50 mg/L and the aluminum concentration is below 0.01 mg/L, the effects of these factors are small. Note that precipitation of Ca-based precipitates can be suppressed by adding a dispersant to the water being treated.

Figure 4 shows the results of a similar calculation for RO concentrated water with the water quality shown in Table 1 above and a _SiO2 concentration of 42 mg/L. As can be seen from Figure 4, even if the _SiO2 concentration is below the saturation concentration (about 120 mg/L at room temperature), a small amount of composites of silica, aluminum, and sodium are precipitated near neutral.

The reverse osmosis membrane treatment should be carried out while keeping the pH of the RO concentrated water at 5.5 or less; the lower the pH, the less precipitate there will be, but if the pH is too low, a large amount of pH adjuster will be required and the reverse osmosis membrane may deteriorate, so it is preferable to carry out the reverse osmosis membrane treatment while keeping the pH of the RO concentrated water in the range of 4.0 to 5.5.

For example, the RO concentrate may be sampled in pipe 32, the pH of the RO concentrate may be measured, and a pH adjuster may be added to the water to be treated based on the measurement results so that the pH of the RO concentrate is maintained at 5.5 or less. Alternatively, the pH of the water to be treated may be measured in pipe 28 using a pH meter 42, and a pH adjuster may be added to the water to be treated based on the measurement results so that the pH of the RO concentrate is maintained at 5.5 or less, or the pH of the RO concentrate may be measured in pipe 32 using a pH meter 44, and a pH adjuster may be added to the water to be treated based on the measurement results so that the pH of the RO concentrate is maintained at 5.5 or less.

As a pH adjuster, an acid such as hydrochloric acid or sulfuric acid is usually used. As a pH adjuster, an alkali such as an aqueous solution of sodium hydroxide may also be used.

There are no particular limitations on the effluent from sewage treatment that is the subject of treatment, as long as it contains silica, 0.01 mg/L or more of aluminum, and sodium at an equimolar concentration to that of aluminum. Examples of effluent from sewage treatment include effluent from sewage treatment plants that receive and treat wastewater from semiconductor factories, etc.

The effluent from sewage treatment to be treated is, for example, biologically treated water, which is secondary sewage treatment water that has undergone solid-liquid separation treatment such as sedimentation, biological treatment such as activated sludge treatment, etc. It may also be biologically treated water that has undergone further solid-liquid separation treatment such as sedimentation in the subsequent stage of biological treatment.

The TOC of the sewage treatment effluent to be treated is, for example, in the range of 1 to 20 mg/L.

The silica concentration of the RO concentrated water may be 50 mg/L or more, and may be 50 mg/L or more but less than 120 mg/L. If the silica concentration of the RO concentrated water is less than 50 mg/L, the occurrence of precipitates is hardly a problem.

The RO permeate may be recovered and reused. When the RO permeate is reused as raw water for pure water production, it may be subjected to organic matter decomposition treatment (organic matter decomposition treatment process). By subjecting the RO permeate to organic matter decomposition treatment, the recovered water can be reused for applications requiring high water quality, such as raw water for pure water.

Figure 2 shows an example of the configuration of a water treatment device in which the target of treatment is biologically treated water, which is secondary sewage water that has undergone solid-liquid separation treatment and biological treatment, and organic matter decomposition treatment is performed on the RO permeate.

The water treatment device 3 shown in FIG. 2 further includes, in addition to the configuration of the water treatment device 1 in FIG. 1, a solid-liquid separation treatment device 50 as a solid-liquid separation treatment means for performing solid-liquid separation treatment on sewage, upstream of the turbidity removal membrane treatment device 12, and a biological treatment device 52 as a biological treatment means for performing biological treatment on the solid-liquid separation treatment water that has undergone solid-liquid separation treatment. In addition, downstream of the reverse osmosis membrane treatment device 16, an organic matter decomposition treatment device 54 is further included as an organic matter decomposition treatment means for performing organic matter decomposition treatment on the RO permeate.

In the water treatment device 3 of FIG. 2, a pipe 56 is connected to the inlet of the solid-liquid separation treatment device 50. The outlet of the solid-liquid separation treatment device 50 and the inlet of the biological treatment device 52 are connected by a pipe 58. The outlet of the biological treatment device 52 and the inlet of the sewage treatment tank 10 are connected by a pipe 60. The RO permeate outlet of the reverse osmosis membrane treatment device 16 and the inlet of the organic matter decomposition treatment device 54 are connected by a pipe 62, and the outlet of the organic matter decomposition treatment device 54 is connected to a pipe 64.

The sewage is sent to the solid-liquid separation treatment device 50 through the pipe 56. In the solid-liquid separation treatment device 50, the sewage is subjected to solid-liquid separation treatment, and solid-liquid separation treatment water is obtained (solid-liquid separation treatment process). The solid-liquid separation treatment water obtained by the solid-liquid separation treatment is sent to the biological treatment device 52 through the pipe 58. In the biological treatment device 52, the solid-liquid separation treatment water is subjected to biological treatment, and biological treatment water is obtained (biological treatment process). The biological treatment water obtained by the biological treatment is stored in the sewage treatment water tank 10 as necessary through the pipe 60, and then sent to the turbidity removal membrane treatment device 12 through the pipe 24 by the pump 18. Thereafter, turbidity removal treatment and reverse osmosis membrane treatment are performed in the same manner as in the water treatment device 1 of FIG. 1.

The RO permeate obtained by the reverse osmosis membrane process is sent to the organic matter decomposition treatment device 54 through the pipe 62. In the organic matter decomposition treatment device 54, the RO permeate is subjected to organic matter decomposition treatment to obtain organic matter decomposition treated water (organic matter decomposition treatment process). The organic matter decomposition treated water obtained by the organic matter decomposition treatment is discharged through the pipe 64 and is reused as raw water for pure water production, etc.

There are no particular limitations on the solid-liquid separation means (process) for performing solid-liquid separation on sewage, so long as it is capable of performing solid-liquid separation, and examples include a settling tank.

There are no particular limitations on the biological treatment means (process) for biologically treating the solid-liquid separation treated water, so long as it is capable of carrying out biological treatment. Examples include the standard activated sludge method and the rotating disk method.

The organic matter decomposition treatment means (step) for performing organic matter decomposition treatment on the RO permeate is not particularly limited as long as it can decompose organic matter, and examples of such means include ultraviolet irradiation treatment means (step), ozone treatment means (step), and ion exchange treatment means (step).

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

<Example 1 and Comparative Example 1>
The raw water used was secondary sewage treatment water (annual average silica concentration 50 mg/L, aluminum concentration 0.01 mg/L, sodium concentration 200 mg/L). This raw water was biologically treated water that had been subjected to solid-liquid separation treatment in a settling tank and then biological treatment using the standard activated sludge method. This raw water was treated in the water treatment device shown in Figure 1 with a turbidity removal membrane and then a reverse osmosis membrane, and the RO permeate was obtained as recovered water.

(Test conditions)
・Water volume at inlet of turbidity membrane: 460L/h
・Reverse osmosis membrane inlet water volume: 260L/h
・RO concentrated water amount: 120L/h
RO permeate volume: 140 L / h (recovery rate 54%, silica concentration in RO concentrated water 110 mg / L)
・Turbidity membrane: Asahi Kasei Corporation, UNV-3003
Reverse osmosis membrane: LFC3-LD-4040, manufactured by Nitto Denko Corporation

Water was passed through the RO concentrate for about one month in two cases: one in which acid (hydrochloric acid) was added as a pH adjuster before the reverse osmosis membrane so that the pH of the RO concentrate was 5.5 or less, and one in which water was passed through without adding acid (pH 7). Figure 5 shows the change in the permeability coefficient retention rate (%) over the number of days that have passed. Note that the permeability coefficient can be calculated as permeate flux (m/d)/effective pressure (MPa).

When water was passed through the filter at a neutral pH (pH 7), the permeability coefficient decreased by about 21%, whereas when the pH was made acidic (pH 5.5 or less), the permeability coefficient decreased by about 10%.

In this way, the method of the embodiment makes it possible to suppress the formation of composite precipitates of silica, aluminum, and sodium in reverse osmosis membrane treatment of sewage treatment water, even when silica, aluminum, and sodium coexist in the sewage treatment water, and makes it possible to stably operate the reverse osmosis membrane.

1, 3 Water treatment device, 10 Sewage treatment tank, 12 Turbidity removal membrane treatment device, 14 Water tank to be treated, 16 Reverse osmosis membrane treatment device, 18, 20, 22 Pump, 24, 26, 28, 30, 32, 34, 36, 38, 56, 58, 60, 62, 64 Piping, 42, 44 pH meter, 50 Solid-liquid separation treatment device, 52 Biological treatment device, 54 Organic matter decomposition treatment device.

Claims (10)

A water treatment method for recovering and reusing effluent from sewage treatment, comprising the steps of:
The effluent contains silica, aluminum in an amount of 0.01 mg/L or more and 0.1 mg/L or less, and sodium in an amount equal to or more than the molar concentration of the aluminum,
The reverse osmosis membrane treatment step includes subjecting the discharged water to a reverse osmosis membrane treatment to separate the discharged water into an RO permeate and an RO concentrated water having a silica concentration of 50 mg/L or more.
The water treatment method, wherein in the reverse osmosis membrane treatment step, the reverse osmosis membrane treatment is carried out while maintaining the pH of the RO concentrated water at 5.5 or less. The water treatment method according to claim 1,
The water treatment method according to claim 1, wherein the silica concentration of the RO concentrated water is less than 120 mg/L. The water treatment method according to claim 1 or 2,
The water treatment method further comprises an organic matter decomposition treatment step of performing an organic matter decomposition treatment on the RO permeate. The water treatment method according to any one of claims 1 to 3,
The water treatment method is characterized in that the effluent from the sewage treatment is secondary sewage treatment water that has been subjected to solid-liquid separation treatment and biological treatment. The water treatment method according to any one of claims 1 to 4,
The water treatment method further comprises a turbidity removal treatment step of removing turbidity from the sewage treatment effluent prior to the reverse osmosis membrane treatment step. A water treatment device that recovers and reuses effluent from sewage treatment,
The effluent contains silica, aluminum in an amount of 0.01 mg/L or more and 0.1 mg/L or less, and sodium in an amount equal to or more than the molar concentration of the aluminum,
a reverse osmosis membrane treatment means for treating the discharged water with a reverse osmosis membrane to separate the discharged water into an RO permeate and an RO concentrated water having a silica concentration of 50 mg/L or more;
a pH adjusting means for maintaining the pH of the RO concentrated water in the reverse osmosis membrane treatment means at 5.5 or less;
A water treatment device comprising: The water treatment device according to claim 6,
The water treatment device is characterized in that the silica concentration of the RO concentrated water is less than 120 mg/L. The water treatment device according to claim 6 or 7,
The water treatment device further comprises an organic matter decomposition treatment means for performing an organic matter decomposition treatment on the RO permeate. The water treatment device according to any one of claims 6 to 8,
1. A water treatment device, wherein the effluent from the sewage treatment is secondary treated sewage water that has been subjected to solid-liquid separation treatment and biological treatment. The water treatment device according to any one of claims 6 to 9,
The water treatment device further comprises a turbidity treatment means for performing a turbidity treatment on the effluent from the sewage treatment upstream of the reverse osmosis membrane treatment means.

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