JP7520764B2 - Water treatment method and apparatus - Google Patents

Description

The present invention relates to a water treatment method and a water treatment device that perform solid-liquid separation in water treatment by a membrane filtration method using an ultrafiltration membrane or a precision membrane. In particular, the present invention relates to a water treatment method for water purification that has a coagulation process and a membrane filtration process, and a water treatment device.

The main solid-liquid separation methods in water treatment include gravity settling, flotation, filtration and membrane separation.

Of these, the gravity sedimentation method separates suspended solids (SS) and other solids that tend to sink in the treated water by gravity using the density difference of the treated water without the addition of chemicals, or adds inorganic or polymeric coagulants to the treated water to perform coagulation treatment, and then separates the flocs that tend to sink by gravity.

The flotation separation method includes atmospheric flotation processing and pressurized flotation processing.
The atmospheric flotation treatment method is a method in which the water to be treated or the water to be treated is flocculated using an inorganic flocculant or a polymer flocculant, and then the flocs are separated by floating only with the buoyancy of the flocs.
In contrast, the pressurized flotation method is a method of flotation separation in which fine air particles are attached to the SS in the water to be treated or to the flocculated flocs after the flocculation treatment of the water to be treated, thereby increasing the separation speed compared to the normal pressure flotation method.

Filtration is a method in which the water to be treated is passed through a filter layer formed of filter sand or anthracite filter material, and the layer filters out the suspended solids and turbidity in the water. It is used in industrial wastewater treatment, secondary sewage treatment, and rapid filtration in water purification.

Furthermore, the membrane separation method is a method in which the water to be treated is permeated through an organic or inorganic membrane to obtain treated water. Since the treated water that permeates the membrane is very clear and solid-liquid separation is possible even with a high concentration of SS, the membrane separation method can be said to be an excellent solid-liquid separation method.
Generally, a solid-liquid separation method using a microfiltration membrane (hereinafter also referred to as an "MF membrane") or an ultrafiltration membrane (hereinafter also referred to as an "UF membrane") as a separation membrane is called a membrane separation method, while a term with the same meaning, particularly in the field of water purification treatment, is also called membrane filtration.
The water to be treated using the membrane filtration method includes river water and groundwater for manufacturing or industrial use, raw water from water supply such as river water and dam lake water, secondary treated water from sewage, and various industrial wastewater and treated water thereof.
Generally, membrane filtration devices that employ such membrane filtration methods include submerged membrane filtration devices and casing-stored type (hereinafter also referred to as "casing type") membrane filtration devices.

When applying low-pressure membrane filtration using MF or UF membranes in the field of water purification, the water to be treated may be directly filtered through the membrane. However, when using surface water such as river water as raw water for water supply, it often combines coagulation treatment using a coagulant as a pretreatment, since it contains soluble substances such as color components.

In this type of coagulation membrane filtration method, which combines coagulation treatment and membrane filtration (hereinafter also referred to as "coagulation membrane filtration method"), an inorganic coagulant or an inorganic coagulant and a polymer coagulant are added to the water to be treated, and the substances to be removed are captured in the inorganic coagulant or in the coagulated flocs formed from the inorganic coagulant and the polymer coagulant, and then further filtered through a membrane, thereby increasing the treatment effect of the substances to be removed.

As a conventional coagulation membrane filtration technology in water purification treatment, for example, there is a technology based on a coagulation membrane filtration treatment flow in which a mixing tank 22, a coagulation tank 23, and a membrane filtration tank 24 are adopted as shown in FIG.
The water purification process involves membrane filtering of raw water from a receiving well in a coagulation membrane filtration facility having a mixing tank 22, a coagulation tank 23, and a membrane filtration tank 24, and adding a disinfectant to the membrane-filtered water (hereinafter also referred to as "treated water") to produce tap water. The water purification process includes a coagulation process in which an inorganic coagulant is added in the mixing tank 22 to turn the turbid matter in the raw water into fine particles and coagulate the turbid matter, and a coagulation process in the next coagulation tank 23 to gather the fine particles by slow stirring to obtain coagulated flocs. The coagulated flocs are then subjected to membrane filtration to obtain membrane-filtered water.

As another conventional coagulation membrane filtration technique in water purification treatment, for example, a technique based on a submerged coagulation membrane filtration flow employing a mixing tank 25, a coagulation tank 26, and a submerged membrane filtration tank 27 as shown in FIG. 6 can be mentioned.
In this technology, raw water is received in a mixing tank 25, an inorganic coagulant is added, and the water is rapidly stirred, so that the turbidity of the raw water is converted into fine particles by the coagulation action of the inorganic coagulant. Then, by slowly stirring in the next coagulation tank 26, the fine particles gather to form large coagulated flocs, and the coagulated flocs are separated into solid and liquid in a submerged membrane module 28 installed inside a submerged membrane filtration tank 27. At this time, the membrane-filtered water (treated water) that has permeated the membrane is obtained from the submerged membrane module 28 by sucking with a suction pump 29 connected to the submerged membrane module 28. Meanwhile, a concentrated liquid containing solids separated from the solid and liquid is present inside the submerged membrane filtration tank 27. This concentrated liquid is appropriately discharged from the submerged membrane filtration tank 27 to the outside of the system via a concentrated liquid withdrawal pipe 30, and is concentrated or dehydrated.

Furthermore, as yet another conventional coagulation membrane filtration method in water purification treatment, for example, a technology based on a casing-type coagulation membrane filtration flow employing a mixing tank 31, a coagulation tank 32, and a casing-type membrane module 33 as shown in FIG. 7 can be mentioned.
In this technology, raw water is received in a mixing tank 31, an inorganic coagulant is added, and the water is rapidly stirred, causing the turbidity of the raw water to turn into fine particles due to the coagulation action of the inorganic coagulant. Then, by slowly stirring in the next coagulation tank 32, the fine particles gather together to form large coagulated flocs, which are then pumped by a membrane feed water pump 34 to one or more casing-type membrane modules 33 as membrane feed water containing the coagulated flocs. The membrane feed water pump 34 then pumps the water through the membrane from the casing-type membrane module 33 at the operating pressure of the membrane feed water pump 34, yielding membrane-filtered water (treated water).

As a more specific example of the prior art related to the above-mentioned water treatment methods and apparatus therefor, Patent Document 1 discloses a water treatment apparatus using a membrane module to remove metal ions from raw water, the water treatment apparatus including a contact retention tank for precipitating metal ions as solids, a membrane module for separating raw water from the contact retention tank, a cleaning wastewater tank for storing backpressure cleaning wastewater from the membrane module, and a return mechanism for sending at least a portion of the cleaning wastewater containing the metal ion solids from the cleaning wastewater tank to the contact retention tank (Patent Document 1).
The technology described in Patent Document 1 describes that, when the metal ions are manganese ions, by supplying wastewater from washing the membrane to a contact retention tank, the solids content in the raw water (the content of manganese dioxide formed by oxidizing manganese ions) is increased, thereby promoting a precipitation reaction from manganese ions to manganese dioxide in the contact retention tank, and thereby making it possible to reduce the manganese ion concentration.

Furthermore, for example, Patent Document 2 proposes a water treatment method and an apparatus thereof in which raw water containing manganese ions is subjected to membrane filtration using a submerged microfiltration/ultrafiltration membrane module, in which an oxidizing agent is added to the raw water, fine bubbles are generated from the membrane module during at least a part of the filtration process time, and normal bubbles are generated from the membrane module after completion of the filtration process to perform air washing.
In the technology described in Patent Document 2, the solid content containing manganese dioxide separated by the submerged membrane module does not settle to the bottom of the submerged tank due to the generation of fine bubbles in the submerged tank, and manganese ions in the raw water can be oxidized with an oxidizing agent while the solid content is suspended, thereby making it possible to remove manganese ions in the raw water as manganese dioxide.

Furthermore, Patent Document 3 proposes a method and apparatus for treating arsenic-containing water, which is characterized by generating flocs by oxidizing water containing arsenic and iron ions, and contacting the water to be treated with the flocs while the generated flocs are concentrated and maintained at a high concentration. It is described that this method improves the effect of removing ionic arsenic by incorporating molecular arsenic into the flocs at the stage of floc generation, or by adsorbing ionic arsenic to the surface of the flocs.
Furthermore, Patent Document 3 also discloses that flocs separated by a membrane filtration means in a downstream stage are returned to the oxidation tank by a solids circulation means, thereby maintaining the flocs in the oxidation tank at a high concentration.

JP 2015-188781 A JP 2012-86182 A JP 2003-126874 A

By the way, the membrane filtration method has an advantage that it can produce clear treated water compared to other solid-liquid separation methods, but on the other hand, membrane fouling progresses due to various causes, and it may become difficult to obtain a stable amount of treated water. The cause of such a decrease in the amount of treated water or an inability to secure a stable amount of water is deterioration or membrane fouling of the filtration membrane (hereinafter referred to as "membrane fouling").
Degradation of filtration membranes is an irreversible decrease in membrane performance caused by the alteration of the membrane itself, such as aging of the membrane or damage to the membrane caused by oxidizing agents, and the countermeasure for this is replacement of the filtration membrane.
On the other hand, membrane fouling of filtration membranes is not caused by the deterioration of the filtration membrane itself, but by the deterioration of membrane performance due to external factors, and the cause is the adhesion of inorganic scale and microbial slime to the membrane surface. If these are the causes, the membrane performance can be restored by chemical cleaning. For example, chemical cleaning with organic acids such as citric acid or chelating agents is effective for scale, and chemical cleaning with hypochlorite or surfactants is effective for slime.

Here, the main cause of membrane fouling occurring in the coagulation membrane filtration method among membrane fouling is the use of inorganic coagulants. If the inorganic coagulant is added to the raw water and sufficient stirring time is not available, or if the water is short-passed and sufficient mixing is not possible, the inorganic coagulant cannot be mixed uniformly, and furthermore, the hydrolysis of the constituent metal ions of the inorganic coagulant (for example, aluminum ions when the inorganic coagulant is polyaluminum chloride (PAC)) becomes insufficient, and all the metal ions do not become metal hydroxides, but remain as metal ions. These remaining metal ions precipitate on the membrane filtration surface and become hydroxides, causing membrane fouling. And as membrane fouling progresses, the membrane pressure difference applied to the filtration membrane increases, and the amount of membrane filtrate water decreases accordingly, so there remains a problem in terms of work efficiency.
The occurrence of membrane fouling in the coagulation membrane filtration method is particularly noticeable at low water temperatures. This is because the hydrolysis of the inorganic coagulant does not proceed sufficiently, so that metal ions resulting from the inorganic coagulant tend to remain, and these remaining metal ions precipitate and become hydroxides on the membrane filtration surface.

In order to eliminate membrane fouling caused by inorganic coagulants, it is effective to periodically chemically clean the filtration membrane. However, chemical cleaning leaves issues in terms of economic and operational efficiency, such as the cost of chemicals, the time required for the cleaning work, and the treatment of cleaning wastewater.

In this regard, the technologies described in Patent Documents 1 to 3 above only propose a means of removing harmful ions from raw water, and do not propose the avoidance of membrane fouling caused by inorganic coagulants in water treatment methods that employ coagulation membrane filtration.

Therefore, the objective of the present invention is to find a method capable of suppressing membrane fouling caused by inorganic coagulants in the coagulation membrane filtration method, as well as a water treatment method and apparatus therefor that employ this method.

That is, the present invention provides
a coagulation step of mixing the water to be treated with an inorganic coagulant;
a flocculation step in which flocculated flocs are formed in the water to be treated after the flocculation step;
A membrane filtration process for separating the treated water after the coagulation process into solid and liquid;
An extraction step of extracting at least a portion of the treated water containing solids that were not filtered through the membrane in the membrane filtration step;
A returning step of returning at least a portion of the treated water containing solids that have not been filtered through the membrane to the flocculation step;
The present invention relates to a water treatment method comprising the steps of:
Among these, a preferred embodiment is characterized in that the method further comprises a step of adding a polymer flocculant to at least a portion of the treated water containing solids that have not been filtered through the membrane.
More preferably, the formation of flocs in the flocculation step is carried out at a suspended solids (SS) concentration of 20 mg/L or more.
In a particularly preferred embodiment, the inorganic flocculant contains aluminum.
The present invention also provides
A flocculation means for mixing the water to be treated with an inorganic flocculant;
a flocculation means for forming flocs in the water to be treated after passing through the flocculation means;
A membrane filtration means for performing solid-liquid separation of the water to be treated that has passed through the coagulation means;
An extraction means for extracting at least a portion of the treated water containing solids that were not filtered by the membrane filtration means;
A return means for returning at least a portion of the treated water containing solids that have not been filtered through the membrane to the flocculation means;
The present invention also relates to a water treatment device comprising:
Of these, a preferred embodiment is characterized in that the present invention further comprises a means for adding a polymer flocculant to at least a portion of the treated water containing solids that have not been filtered through the membrane.
More preferably, the formation of flocs in the flocculation means is carried out at a suspended solids (SS) concentration of 20 mg/L or more.
In a particularly preferred embodiment, the inorganic flocculant contains aluminum.

The present invention can effectively suppress membrane fouling in membrane filtration, particularly in coagulation membrane filtration. As a result, stable membrane filtration can be achieved, and the frequency of chemical cleaning of the filtration membrane can be reduced.

FIG. 1 is a schematic diagram showing a flow of a coagulation membrane filtration treatment according to one embodiment of the present invention. FIG. 2 is a schematic diagram showing a flow of a coagulation membrane filtration treatment according to another embodiment of the present invention. FIG. 3 is a schematic diagram showing a submerged coagulation membrane filtration apparatus based on a submerged coagulation membrane filtration treatment flow, which is yet another embodiment of the present invention. FIG. 4 is a schematic diagram showing a casing type coagulation membrane filtration apparatus based on a casing type coagulation membrane filtration treatment flow, which is still another embodiment of the present invention. FIG. 5 is a schematic diagram showing a flow of a conventional coagulation membrane filtration process. FIG. 6 is a schematic diagram showing a submerged type coagulation membrane filtration apparatus based on a conventional submerged type coagulation membrane filtration treatment flow. FIG. 7 is a schematic diagram showing a casing type coagulation membrane filtration apparatus based on a conventional casing type coagulation membrane filtration treatment flow.

Hereinafter, as an example of an embodiment of the present invention, coagulation membrane filtration in water purification treatment will be described with reference to the examples shown in the drawings.
The present invention relates to coagulation membrane filtration including a coagulation step in a mixing tank, a coagulation step in a coagulation tank, or a combination of a coagulation step in a mixing tank and a coagulation step in a coagulation tank, prior to the membrane filtration step, regardless of whether it is coagulation membrane filtration for water purification treatment.

In addition, in the present specification, the term "concentrate" refers to the concentrate withdrawn from the submerged membrane filtration tank to the outside of the apparatus and returned to the coagulation tank, and is included in the above-mentioned "treated water containing solids that were not filtered through the membrane."
In addition, in the present specification, the term "membrane cleaning wastewater" refers to membrane cleaning wastewater that is drawn out of the apparatus from the casing-type membrane filtration tank and discharged during backflow cleaning with treated water, and that is returned to the coagulation tank, and is included in the above-mentioned "treated water containing solids that were not filtered through the membrane."
The process of extracting at least a portion of the treated water containing solids that were not filtered through the membrane in the membrane filtration process is defined as the extraction process, and the process of withdrawing the concentrated liquid and the process of discharging the membrane cleaning wastewater are included in the concept of this extraction process.
Extraction means and ejection means are likewise included within the concept of extraction means.

[First embodiment]
As a first embodiment of the present invention, there is shown a coagulation membrane filtration treatment flow having a mixing tank 1, a coagulation tank 2 and a membrane filtration tank 3, and having a means for returning a concentrated solution or membrane cleaning wastewater to the coagulation tank 2 (FIG. 1).
Examples of the membrane filtration tank 3 include a submerged type membrane filtration tank (shown in FIG. 3) and a casing type membrane module (shown in FIG. 4).
In the mixing tank 1, a coagulation process is carried out in which an inorganic coagulant is injected into the raw water to coagulate the turbid matter in the raw water. In the coagulation tank 2, the raw water after the coagulation process is subjected to a coagulation process in which coagulated flocs are formed. Furthermore, in the membrane filtration tank 3, the coagulated flocs obtained in the coagulation process are subjected to membrane filtration for solid-liquid separation. In this membrane filtration tank 3, treated water and a concentrate or membrane cleaning wastewater are obtained, and one of the features of the present invention is that at least a part of the concentrate or membrane cleaning wastewater is returned to the coagulation tank 2. This increases the SS concentration in the coagulation tank 2, and the turbid matter in the coagulated raw water, residual metal ions derived from the inorganic coagulant (e.g., aluminum ions when an aluminum-based inorganic coagulant is used) and hydrolysis products of metal ions derived from the inorganic coagulant (e.g., aluminum ions when an aluminum-based inorganic coagulant is used) are incorporated into the SS of the concentrate or membrane cleaning water, and these are coagulated to form coagulated flocs with good membrane filterability.
In particular, since aluminum is an amphoteric metal, when an aluminum-based inorganic coagulant is added to the water to be treated and the water is filtered through a coagulation membrane, the residual aluminum concentration of the treated water reaches a minimum in the narrow pH range of coagulation pH 6 to 7. On the other hand, iron-based inorganic coagulants other than aluminum-based inorganic coagulants, such as ferric chloride, reach a minimum in the residual iron concentration of the treated water in a wide pH range of coagulation pH 6 or higher. In other words, aluminum-based inorganic coagulants increase the concentration of metal ions and aluminum ions remaining in the treated water much more than iron-based inorganic coagulants due to fluctuations in the coagulation pH.

The return of the concentrated solution or the membrane cleaning water to the coagulation tank 2 is preferably such that the concentrated solution or the membrane cleaning water is returned to the coagulation tank 2 so as to achieve a preset SS concentration in the coagulation tank 2. Such an SS concentration in the coagulation tank 2 is, for example, 20 mg/L or more, preferably 20 to 300 mg/L, and more preferably 20 to 100 mg/L. If the SS concentration in the coagulation tank 2 is 20 mg/L or more, the coagulation property in the coagulation tank 2 is increased, and the membrane filtration property is improved. Also, if the SS concentration in the coagulation tank 2 is 300 mg/L or less, the coagulation property in the coagulation tank 2 is increased without being affected by the turbidity of the raw water, and the membrane filtration property is improved.
By returning the concentrated liquid or membrane cleaning wastewater to the coagulation tank 2, the SS concentration in the coagulation tank 2 becomes higher than the turbidity of the raw water and the concentration of generated sludge derived from inorganic coagulants, so the SS concentration in the coagulation tank 2 is dominated by the SS concentration in the concentrated liquid or membrane cleaning wastewater. Therefore, the SS concentration in the coagulation tank 2 can be set arbitrarily by the SS concentration in the concentrated liquid or membrane cleaning wastewater and the return flow rate thereof.

The concentrated liquid or the membrane cleaning wastewater may be returned to the coagulation tank 2 in its entirety or in part. Furthermore, the extracted concentrated liquid may be returned as is, or the discharged membrane cleaning wastewater may be returned as is, or these may be returned to the coagulation tank 2 after being concentrated by gravity or the like to increase the SS concentration.

In the return step, for example, in the case of a submerged coagulation membrane filtration process, the concentrated liquid from the submerged membrane filtration tank is returned to the coagulation tank 2. In the case of a casing type coagulation membrane filtration process, the membrane cleaning wastewater from the casing type membrane module is returned to the coagulation tank 2 in the coagulation step.
As already mentioned above, by returning the concentrated liquid or the membrane cleaning wastewater to the coagulation tank 2 of the coagulation step, the SS concentration in the coagulation tank is increased compared to when they are not returned, and the turbidity in the coagulated raw water, residual metal ions derived from the inorganic coagulant (e.g., aluminum ions when an aluminum-based inorganic coagulant is used), and hydrolysis products of metal ions derived from the inorganic coagulant (e.g., aluminum ions when an aluminum-based inorganic coagulant is used) are taken into the SS sludge of the concentrated liquid, and they are coagulated to form coagulated flocs with good membrane filterability. As a result, the coagulated flocs are sufficiently coarsened, the membrane filterability of the coagulated flocs can be improved, and the turbidity of the raw water can be reduced. In particular, the coagulation membrane filtration effect can be improved for low turbidity raw water with a turbidity of 10 degrees or less, or even turbidity of 2 degrees or less, low temperature raw water with a water temperature of 10°C or less, and low alkalinity raw water with an M-alkalinity of less than 10 mg/L.
On the other hand, the treated water obtained by the coagulation membrane filtration has a quality suitable for use as tap water, and therefore is subjected to sterilization, disinfection, and the like before being supplied as tap water.

In addition, in the case of a submerged coagulation membrane filtration apparatus (hereinafter also referred to as "submerged type") using a submerged membrane module as a membrane module in the membrane filtration apparatus based on the flow chart in Fig. 1, solid-liquid separation into membrane filtered water and concentrated liquid is performed by the membrane module in the membrane filtration tank. The concentrated liquid is extracted from the membrane filtration apparatus and concentrated and dehydrated.
In the case of a casing-type coagulation membrane filtration apparatus (hereinafter also referred to as "casing type") using a casing-type membrane module as the membrane module, while membrane filtered water is obtained by membrane filtration, turbid matters captured on the membrane surface are discharged outside the system as washing wastewater (hereinafter also referred to as "membrane washing wastewater") in the backwash washing performed by interrupting the membrane filtration. This membrane washing wastewater is concentrated and dehydrated.
In addition, the membranes are periodically cleaned, and the concentrated liquid and membrane cleaning wastewater discharged from cleaning the membranes are separated into sludge and supernatant water in the wastewater tank. The sludge settles and thickens by gravity in the thickening tank and is then fed to the dehydrator. The supernatant water from the thickening tank is returned to the receiving well together with the supernatant water from the wastewater tank.
In addition, when the flocs formed in the coagulation tank 2 are small or difficult to form by using only an inorganic coagulant, a polymeric coagulant for water supply can be added to the coagulation tank 2 to increase the size of the flocs and promote membrane filtration.

[Second embodiment]
Next, as a second embodiment of the present invention, a coagulation membrane filtration process flow will be described in which a step of adding a polymer coagulant to the return pipe 7 when returning the concentrated solution or membrane cleaning wastewater extracted from the membrane filtration tank 6 to the coagulation tank 5 via the return pipe 7 is further added (FIG. 2).
The flow in FIG. 2 includes a coagulation step in which an inorganic coagulant is injected into raw water in a mixing tank 4 to coagulate turbid matters in the raw water, a coagulation step in which a polymer coagulant is added to at least a part of the concentrated liquid or membrane washing wastewater in a membrane filtration tank 6 and the resulting solution is returned to a coagulation tank 5 for the coagulation step, and coagulated flocs are formed in the presence of the raw water after the coagulation step and the polymer coagulant, and the coagulated flocs obtained in the coagulation step are separated into solid and liquid by membrane filtration.

As explained in the first embodiment, the amount of the concentrated liquid or the membrane cleaning wastewater returned to the coagulation tank 5 can be adjusted so that the SS concentration in the coagulation tank 5 reaches a preset value, or the SS concentration in the coagulation tank 5 can be monitored and the amount of the concentrated liquid or the membrane cleaning wastewater returned can be adjusted so that the SS concentration reaches a preset value.
Then, based on the SS concentration of the concentrated liquid or membrane cleaning wastewater to be returned to the coagulation tank 5, the amount of polymer coagulant to be added is determined based on a preset polymer coagulant addition rate, and the determined weight amount of polymer coagulant is added to the concentrated liquid or membrane cleaning wastewater to be returned, and the concentrated liquid or membrane cleaning wastewater containing the polymer coagulant is returned to the coagulation tank 5.

In raw water with low turbidity of 10 degrees or less, as seen in recent river water, coagulation reaction may not occur easily even if inorganic coagulant or polymer coagulant is injected at an optimal injection rate. In contrast, the water purification method according to the embodiment of the present invention makes it possible to stably and continuously obtain a high coagulation membrane filtration effect while maximizing the effect of adding the coagulant.
In particular, in winter when the temperature of the raw water is low, the coagulation reaction does not proceed easily, and flocs with poor coagulation properties are formed, which is more likely to cause membrane fouling and reduce the efficiency of the membrane filtration treatment. Therefore, in the conventional method of injecting inorganic coagulants, the weight of the polymer coagulant is based on the flow rate of the raw water, and good coagulation properties may not be obtained in response to fluctuations in the turbidity of the raw water or fluctuations in the amount of the inorganic coagulant injected. Furthermore, since the polymer coagulant solution has a high viscosity, it is difficult to sufficiently mix it with the raw water when it is added to a coagulation tank with weak stirring power at a slow stirring speed as in the conventional method, and good coagulated flocs cannot be generated. Therefore, it is not possible to sufficiently remove the turbidity from the raw water and the hydrolysis products including hydroxides from the inorganic coagulant.
In addition, in winter, the coagulation reaction does not proceed smoothly due to the low turbidity, low water temperature and low alkalinity of the raw water, so there is a tendency for the inorganic coagulant to be added in excess. When an inorganic coagulant is added in excess, hydrolysis of the inorganic coagulant does not occur sufficiently, and the metal ions that make up the inorganic coagulant remain, which causes membrane fouling in membrane filtration.
In contrast, in this embodiment of the present invention, the concentrated liquid or membrane washing wastewater is returned to the coagulation tank 5 so that the SS concentration in the coagulation tank 5 is set to a preset value. Therefore, even if an inorganic coagulant is not added in excess by using a polymer coagulant in combination, coagulated flocs with good coagulation properties can be generated and coagulation membrane filtration performance can be maintained. At low water temperatures or low turbidity raw water, the injection rate of the inorganic coagulant can be reduced by reducing the amount of excess inorganic coagulant. Furthermore, the amount of sludge generated due to the inorganic coagulant can be reduced, reducing the load on the wastewater treatment.

In addition, by determining the amount of polymer coagulant to be added based on the SS weight of the concentrated liquid or membrane cleaning wastewater, it is possible to always add the polymer coagulant in an optimal amount regardless of the properties of the raw water and the state of water purification treatment of the raw water, compared to the conventional method of setting the injection rate of the polymer coagulant based on the flow rate of the raw water. This makes it possible to optimize the amount of polymer coagulant used for efficient treatment while stably and continuously obtaining a high coagulation membrane filtration effect.
The addition rate of the polymer flocculant to the concentrated solution or the membrane cleaning wastewater is not limited to the following numerical range, but can be 0.001 to 3.0 wt% relative to the SS weight of the concentrated solution or the membrane cleaning wastewater, preferably 0.01 to 2.0 wt% relative to the SS, and more preferably 0.02 to 0.15 wt% relative to the SS.
If the addition rate of the polymer flocculant is 0.001 wt% or more relative to SS, the flocculation effect is enhanced, and the flocculation membrane filtration effect is also enhanced. If the addition rate of the polymer flocculant is less than 3.0 wt% relative to SS, the polymer flocculant is appropriate for the sludge of the concentrated liquid or membrane cleaning wastewater, low-viscosity flocs are generated, and the dispersibility of the low-viscosity flocs is good, so that turbidity from the raw water, etc. can be captured, and the flocculation membrane filtration effect is obtained.
The concentration of the polymer flocculant can be, for example, 0.1 to 0.3 wt%. Since the polymer flocculant solution can be stored at a high concentration, the deterioration of the polymer flocculant can be delayed, and the storage stability of the polymer flocculant solution can be improved. The polymer flocculant dissolving device, injection device, and storage tank for the dissolving solution can also be made compact.

[Third embodiment]
Next, as a third embodiment of the present invention, there is shown a submerged coagulation membrane filtration apparatus based on the submerged coagulation membrane filtration treatment flow of the present invention, which has a mixing tank 8, a coagulation tank 9, a submerged membrane filtration tank 10, and a return pipe 11 for the concentrated liquid, and in which a polymer coagulant is added to this return pipe 11 (FIG. 3).
In the third embodiment of the present invention, raw water for water supply is received in a mixing tank 8, an inorganic flocculant is added from an inorganic flocculant injection facility, and the inside of the mixing tank 8 is rapidly stirred by a rapid stirrer. Then, the next coagulation tank 9 is slowly stirred by a slow stirrer to form large flocs, and the raw water for water supply containing the flocs is subjected to solid-liquid separation in a submerged membrane module 12 installed inside a submerged membrane filtration tank 10.
Then, by sucking with a suction pump 13 connected to the submerged membrane module 12, treated water (membrane filtered water) that has permeated the membrane is obtained from the submerged membrane module 12. Meanwhile, inside the submerged membrane filtration tank 10, a concentrated liquid containing solids resulting from solid-liquid separation remains in the tank.

3, cleaning air can be introduced from the bottom of the submerged membrane module 12, so that the filtration surface of the submerged membrane module 12 can be constantly cleaned with air. When the suction pressure of the suction pump 13 increases, backwash water, which is treated water, can be backwashed by the backwash pump 14 from the suction pump suction pipe. If the suction pump pressure cannot be restored by the treated water alone, an oxidizing agent such as an organic acid or sodium hypochlorite can be dissolved in the treated water, and the treated water can be used for backwashing to clean and remove clogging substances inside the submerged membrane module 12.
As the coagulation membrane filtration continues, the SS concentration of the concentrated liquid inside the submerged membrane filtration tank 10 increases, and when the membrane module 12 is backwashed, backwashing wastewater containing only treated water without any cleaning chemicals is discharged into the submerged membrane filtration tank 10, and this backwashing wastewater further increases the SS concentration of the concentrated liquid. Therefore, the concentrated liquid is appropriately discharged from the submerged membrane filtration tank 10 to the outside of the system through a concentrated liquid withdrawal piping.

When a polymer flocculant is used in combination, a return pump is provided in the return pipe 11 branched off from the concentrated liquid withdrawal pipe 15, and a polymer flocculant is added to the concentrated liquid from a polymer flocculant addition device in the middle of the return pipe 11 or at the return pump suction part (not shown) before the concentrated liquid is returned to the coagulation tank 9.
Alternatively, although not shown, a return pipe may be provided at the bottom of the submerged membrane filtration tank 10 and a return pump may be provided midway along the return pipe to return the concentrated liquid to the coagulation tank 9.
The return means is composed of a return pump for returning the concentrated liquid and a return piping 11, etc., and may be equipped with an SS concentration detection means for detecting the SS concentration of the concentrated liquid and a concentrated liquid flow rate detection means for detecting the return flow rate of the concentrated liquid.
As the SS concentration detection means, a commercially available sludge concentration meter such as a near-infrared light type sludge concentration meter, a laser light type sludge concentration meter, or a microwave concentrated liquid concentration meter can be used.
As the concentrated liquid flow rate detection means, a commercially available electromagnetic flow meter, ultrasonic flow meter, etc. can be used. For return, a commercially available pump can be used. Among them, a pump capable of quantitatively transferring concentrated liquid containing high concentration SS, such as a gear pump or a rotary positive displacement type uniaxial eccentric screw pump (mono pump), may be used, and the set flow rate may be adjusted by controlling the rotation speed.

Although not shown, the extracted concentrated liquid may be further concentrated and the concentrated liquid may be returned to the coagulation tank 9. The concentration method may be gravity concentration using a separately installed gravity concentration tank, or mechanical concentration using a centrifugal concentrator or the like. Also, a portion of the concentrated liquid may be taken from the concentration equipment and used as the concentrated liquid of the present invention.

When adding a polymer flocculant to the concentrated liquid returned to the coagulation tank 9, the polymer flocculant can be added by first dissolving it in a solvent to form a polymer flocculant aqueous solution. By adding the polymer flocculant aqueous solution to the concentrated liquid, the highly viscous polymer flocculant aqueous solution is diluted with the concentrated liquid, which improves the dispersibility of the polymer flocculant compared to when the polymer flocculant is added directly to the coagulation tank 9, shortening the slow stirring time, which is the coagulation reaction time, and accelerating the coagulation reaction to make it easier to generate coagulated flocs. In addition, the coagulation of the coagulated flocs is improved, and membrane filtration is improved compared to when the polymer flocculant is added directly to the coagulation tank 9 as in the past.

It is also preferable to carry out the process with rapid mixing in the mixing tank 8 and with slow mixing in the coagulation tank 9. By combining rapid mixing and slow mixing in this way, the incorporation of metal ions resulting from the remaining inorganic coagulant into the coagulated flocs is further promoted. The desired speed (speed gradient) of rapid mixing is preferably 150 l/s or more in terms of G value, and more preferably 250 l/s. The desired speed (speed gradient) of slow mixing is preferably 75 l/s or less in terms of G value.

In addition, in FIG. 3, air can be introduced from the bottom of the submerged membrane module 12 to constantly wash the membrane surface of the submerged membrane module 12 with air. When the suction pressure of the suction pump 13 increases, the backwash pump 14 can backwash the treated water from the suction pump suction pipe. If the suction pump pressure does not recover by backwashing with the treated water alone, it is also possible to remove the clogging substances inside the submerged membrane module 12 by chemical washing by dissolving an organic acid or an oxidizing agent such as sodium hypochlorite in the treated water and backwashing with the washing water.

[Fourth embodiment]
Furthermore, as a fourth embodiment of the present invention, there is mentioned a casing-type coagulation membrane filtration apparatus based on the casing-type coagulation membrane filtration treatment flow of the present invention, which has a mixing tank 16, a coagulation tank 17, a casing-type membrane module 18, and a return pipe 19 for membrane cleaning wastewater, and in which a polymer coagulant is added to this return pipe 19 (FIG. 4).
In the fourth embodiment of the present invention, the membrane supply water after flocculation is pumped by a membrane supply water pump 20 to a plurality of casing-type membrane modules 18, and is subjected to membrane filtration in the casing-type membrane modules 18 to obtain membrane filtered water.

As membrane fouling progresses, the pressure difference in the membrane module 18 increases and the amount of water filtered through the membrane decreases, so in order to suppress this, the membrane module 18 is washed with cleaning air, or the treated water is used as backwash water and backwashed with the backwash pump 21. However, if the pressure difference in the membrane module 18 does not decrease even by backwashing, it is preferable to perform chemical washing using chemical washing water in which treated water washing chemicals are dissolved and a backwash pump.
The membrane cleaning wastewater after backflow cleaning using only treated water without adding cleaning chemicals contains SS that has been separated into solid and liquid in the membrane module 18, so at least a portion of the membrane cleaning wastewater is returned to the coagulation tank 17 via a return pipe 19 branched off from the membrane cleaning wastewater piping.
In addition, when a polymer coagulant is used in combination, a return pump is provided in the return pipe 19 branched off from the middle of the membrane cleaning wastewater pipe, and a polymer coagulant is added from a polymer coagulant adding means to the middle of the membrane cleaning wastewater return pipe 19 or to the return pump suction part, and the membrane cleaning wastewater is returned to the coagulation tank 17.

In addition, when the suction pressure of the suction pump increases, cleaning air can be introduced from the bottom of the casing-type membrane module 18 to appropriately clean the filtration surface of the casing-type membrane module 18 with air. In addition, backwashing can be performed with treated water, which is backwash water, from the suction pump suction piping by the backwash pump 21. If the suction pump pressure cannot be restored by backwashing with treated water alone, it is also possible to remove clogging substances inside the casing-type membrane module 18 by chemical cleaning by dissolving an organic acid or an oxidizing agent such as sodium hypochlorite in the treated water and backwashing with the treated water.
The membrane cleaning wastewater generated by backwashing with treated water is discharged outside the system and is concentrated and dehydrated.
Furthermore, wastewater from chemical cleaning using chemicals is discharged outside the system, where it is treated to be harmless before being released.

By returning the concentrated solution from the submerged membrane filtration tank or the membrane cleaning wastewater from the casing-type membrane module to the coagulation tank as in the above-mentioned embodiment of the present invention, membrane fouling caused by inorganic coagulants can be effectively prevented or suppressed, and further the following effects can be expected.
・The amount of inorganic coagulant used can be reduced, resulting in savings in chemical costs. The reduction in the amount of inorganic coagulant used is particularly noticeable during periods of low water temperatures.
・The frequency of chemical cleaning of the membrane or the amount of cleaning chemicals used can be reduced.
・The frequency of chemical cleaning is reduced, and deterioration of the membrane caused by chemicals is suppressed, reducing membrane replacement costs.
・It becomes possible to ensure a stable amount of membrane filtered water.
・Reduces the amount of wastewater from chemical cleaning, including chemicals, and the effort required to treat it.
- It is advantageous for safety and health as there are fewer opportunities to handle chemicals.
- Improved membrane operation management allows for the elimination of non-routine tasks such as cleaning, thereby reducing operation time.
- The membrane concentrated liquid can be highly concentrated, which can improve the efficiency of subsequent concentration and dehydration.

Furthermore, by adding the polymer flocculant not directly to the coagulation tank but to the concentrated solution of the submerged membrane filtration tank or the concentrated solution or membrane washing wastewater extracted from the casing type membrane module during return to the coagulation tank and then returning the solution to each coagulation tank, the following effects can be expected.
- Membrane fouling substances derived from inorganic coagulants are incorporated into the coagulated flocs, reducing the frequency of chemical cleaning.
- The amount of water treated through coagulation membrane filtration is stabilized, making operation management easier and reducing work hours.

The inorganic flocculants used in the present invention are those used in coagulation and precipitation treatments, and examples of such flocculants include aluminum-based inorganic flocculants containing aluminum, such as aluminum sulfate (aluminum sulfate) or polyaluminum chloride (PAC), and iron-based inorganic flocculants containing iron, such as polyferric sulfate, ferrous sulfate, and ferric chloride. In the present invention, it is preferable to use an aluminum-based flocculant as the inorganic flocculant.
The injection rate of the inorganic coagulant is 5 to 100 mg/L based on the amount of raw water to be treated.

In addition, the polymer flocculant used in the present invention is preferably a polymer flocculant for water supply in water purification treatment or wastewater treatment at a water purification treatment plant, while in water treatment other than water purification treatment, it is preferably a polymer flocculant for water supply or a polymer flocculant other than a polymer flocculant for water supply.
As the polymer flocculant other than the water supply polymer flocculant, a commercially available product may be used, but is not particularly limited. As such a polymer flocculant, for example, any one or more selected from the group consisting of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, ammonium polyacrylate, polymethacrylic acid, sodium polymethacrylate, potassium polymethacrylate, and ammonium polymethacrylate can be used. Among these, sodium polyacrylate is particularly preferred.
Also usable is a polyacrylamide-based polymer flocculant, which is a copolymer of polyacrylamide and poly(meth)acrylate, and is available commercially as an anionic polymer flocculant.
In general, the injection rate of polymer flocculants for water purification is 0.01 to 1 mg/L based on the amount of water to be treated. The injection rate of polymer flocculants used in various industrial wastewater treatments other than water purification is preferably 0.1 to 5 mg/L because the injection rates of SS and inorganic flocculants in the water to be treated are high.
Polymer flocculants are powder or liquid products, and are dissolved in water such as tap water to a concentration of 0.01 to 0.3% by weight. The dissolved and prepared polymer flocculant for water supply is added to the raw water in accordance with the injection amount.

The water treatment method or water treatment device of the present invention has at least the following excellent features as compared with the conventional techniques, for example, the technique described in Patent Document 1.
The technology described in Patent Document 1 uses manganese dioxide as a catalyst and an oxidizing agent to remove manganese ions from raw water, but the present invention does not require the use of a catalyst such as manganese dioxide or an oxidizing agent. This reduces the need to handle chemicals, which is advantageous in terms of safety and hygiene.

Moreover, the water treatment method or water treatment device of the present invention has at least the following excellent features as compared with the technique described in Patent Document 2.
The technology described in Patent Document 2 does not disclose a means for returning the concentrated solution in the immersion tank of the membrane module to a means in a stage preceding the immersion tank of the membrane module. Therefore, in the technology described in Patent Document 2, there is a risk that the raw water containing manganese ions will precipitate on the membrane surface, causing membrane fouling, due to insufficient contact between the raw water containing manganese ions and the solid matter containing manganese dioxide separated by solid-liquid separation in the immersion tank of the membrane module.
In contrast, in the present invention, since the return means is provided, membrane fouling can be effectively suppressed.
In the technology described in Patent Document 2, manganese ions are removed from raw water in the presence of an oxidizing agent in combination with manganese dioxide as a catalyst, whereas the present invention does not require the use of a catalyst such as manganese dioxide or an oxidizing agent, which reduces the need to handle chemicals and is therefore advantageous in terms of safety and hygiene.

Moreover, the water treatment method or water treatment device of the present invention has at least the following excellent features as compared with the technology described in Patent Document 3.
The technology described in Patent Document 3 uses an oxidation tank or air oxidation, whereas the present invention does not require the use of such means, and therefore the working efficiency of the present invention is better.
The technology described in Patent Document 3 requires a means for oxidizing iron ions, which is another target for removal, in order to remove arsenic ions, whereas the present invention does not require a means for oxidizing such a target for removal. Therefore, the present invention has better work efficiency. In addition, the present invention differs from the technology described in Patent Document 1 in that an aluminum-based inorganic flocculant is preferably used as the inorganic flocculant in the present invention.
The technology described in Patent Document 3 is specific to removing arsenic and iron ions, whereas the present invention is directed to removing all metal ions resulting from inorganic coagulants. Therefore, the present invention is more versatile in suppressing membrane fouling in the coagulation membrane filtration method.

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

[Example 1]
A submerged coagulation membrane filtration apparatus based on the flow shown in FIG. 3 was used, except that no polymer coagulant was added.
River water with a pH of 7.1, a turbidity of 3 degrees, and a water temperature of 20 to 25°C was used as raw water (water to be treated). In a mixing tank 8, the raw water and 20 mg/L of polyaluminum chloride (hereinafter, PAC) (product name: Taipac; manufactured by Taimei Chemical Industry Co., Ltd.) were mixed and stirred at a G value of 200 l/s for 5 minutes to coagulate, and then in a coagulation tank 9, the raw water was mixed and stirred at a G value of 50 l/s for 10 minutes to coagulate. Thereafter, the raw water was transferred to a submerged membrane filtration tank 10 to perform membrane filtration. Furthermore, the concentrated solution of the submerged membrane filtration tank 10 was returned from the submerged membrane filtration tank 10 to the coagulation tank 9, so that the SS concentration in the coagulation tank 9 was adjusted to be within the range of 20 to 300 mg/L.
On the other hand, when the concentrated liquid was subjected to membrane filtration without being returned to the coagulation tank 9, the SS concentration in the coagulation tank 9 was 5 mg/L.
The membrane-filtered water was obtained by suction using a suction pump 13 .
The pump operating pressure of the suction pump 13 was used as an index of membrane fouling.
In addition, the residual aluminum concentration in the coagulation tank outlet water (treated water leaving coagulation tank 9) was determined by filtering the coagulation tank outlet water through an MF membrane having a pore size of 0.45 μm and measuring the aluminum in the filtrate by ICP atomic emission spectrometry (JIS K0102, 2016 Industrial Wastewater Testing Method 58.4).
The specifications and test conditions of the submerged coagulation membrane filtration apparatus used in Example 1 are shown in Table 1 below.
Table 2 below shows the test results of the submerged membrane filtration.

As shown in the results of Table 2 above, when the concentrated solution in the submerged membrane filtration tank 10 was not returned to the coagulation tank 9, the SS concentration in the coagulation tank 9 was 5 mg/L, and the pump operating pressure reached 9.8 kPa four months after the start of the test (Test No. 1). The residual aluminum concentration in the water at the outlet of the coagulation tank at this time was 1.03 mg/L.
On the other hand, when the concentrated liquid in the submerged membrane filtration tank 10 was returned to the coagulation tank 9 and the SS concentration in the coagulation tank 9 was within the range of 20 to 300 mg/L, the pump operating pressure was suppressed within the range of 7.4 to 8.5 kPa six months after the start of the test (Test Nos. 2 to 6).
In addition, the residual aluminum concentration in the coagulation tank outlet water at this time was 0.29 to 0.60 mg/L, and by returning the concentrated liquid in the submerged membrane filtration tank 10 to the coagulation tank 9, the residual aluminum concentration in the coagulation tank outlet water was significantly reduced.
When the SS concentration in the coagulation tank was set to 50 to 200 mg/L, the pump operating pressure reached 7.4 to 7.5 kPa six months after the start of the test, which indicates that more stable coagulation membrane filtration was possible (Test Nos. 3 to 5).
The residual aluminum concentration in the water at the outlet of the coagulation tank at this time was 0.32 to 0.43 mg/L.
Moreover, the turbidity of the treated water was 0.02 degrees or less under all test conditions and test numbers.

[Example 2]
Using a submerged coagulation membrane filtration apparatus based on the flow shown in FIG. 3, 20 mg/L of inorganic coagulant PAC was added to the same raw water as in Example 1 in a mixing tank 8 and stirred for 5 minutes, and a polymer coagulant for water supply (EvaGrose WA-521; anionic, manufactured by Suing Co., Ltd.) was added at a rate of 0.01 to 0.2% relative to the SS of the concentrated solution of membrane filtration. The raw water was then transferred to a submerged membrane filtration tank 10 and membrane filtration was carried out. Furthermore, the concentrated solution at that time was returned to the coagulation tank 9 and stirred for 10 minutes in the coagulation tank 9, after which a coagulation membrane filtration test was carried out.
The SS concentration in the coagulation tank 9 was adjusted and maintained in the range of 20 to 300 mg/L by returning the concentrated liquid to which a polymer coagulant for water supply had been added to the coagulation tank 9.
The results of the submerged membrane filtration test are shown in Table 3 below.

As shown in Table 3, when the SS concentration in the coagulation tank 9 was adjusted to 50-200 mg/L and a water supply polymer flocculant was added at a rate of 0.02-0.15% of the SS weight of the concentrated solution for membrane filtration to perform coagulation membrane filtration, the pump operating pressure was suppressed to 5.8-6.6 kPa six months after the start of the test, and stable coagulation membrane filtration was possible (Test Nos. 5, 6, 9-12, 14-16). The residual aluminum concentration in the coagulation tank outlet water at this time was 0.30 mg/L or less. This shows that the residual aluminum concentration in the coagulation tank outlet water was greatly reduced by extracting the concentrated solution from the submerged membrane filtration tank 10 and returning the concentrated solution to which the water supply polymer flocculant was added to the coagulation tank 9. And due to the reduction in the residual aluminum concentration in the coagulation tank outlet water, the pump operating pressure also reduced six months after the start of the test.
In contrast, in Test Nos. 3 to 5 of Example 1 above, the pump operating pressure 6 months after the start of the test was 7.4 to 7.5 kPa, and the residual aluminum concentration in the coagulation tank outlet water was 0.14 to 0.30 mg/L, indicating that adding a polymer coagulant for water supply to the concentrated liquid and returning it makes it possible to keep the residual aluminum concentration in the coagulation tank outlet water low, and as a result, makes it possible to lower the pump operating pressure.
On the other hand, when the SS concentration in the coagulation tank 9 was adjusted to 20 mg/L and the addition rate of the polymer coagulant for water supply was 0.1 to 0.5% relative to SS, the pump operating pressure increased to 7.1 to 7.8 kPa six months after the start of the test (Test Nos. 1 to 3).
The residual aluminum concentration in the water at the outlet of the coagulation tank at this time was 0.32 to 0.48 mg/L.
Furthermore, when the SS concentration in the coagulation tank was adjusted to 300 mg/L and the addition rate of the polymer coagulant for water supply was 0.02 to 0.07% relative to SS, the pump operating pressure was somewhat high at 6.9 to 7.4 kPa six months after the start of the test (Test Nos. 17 to 19).
At this time, the residual aluminum concentration in the water at the outlet of the coagulation tank was 0.23 to 0.28 mg/L.
Moreover, the turbidity of the treated water was 0.02 degrees or less under all test conditions and test numbers.

1, 4, 8, 16, 22, 25, 31: Mixing tank 2, 5, 9, 17, 23, 26, 32: Coagulation tank
3, 6, 24: Membrane filtration tank
7, 11, 19: Return piping
10, 27: Submerged membrane filtration tank
12, 28: Submerged membrane module
13, 29: Suction pump
14, 21: Backwash pump 15, 30: Concentrate extraction pipe 18, 33: Casing type membrane module 20, 34: Membrane supply water pump

Claims (6)

a coagulation step of mixing the water to be treated with an inorganic coagulant;
a flocculation step in which flocculated flocs are formed in the water to be treated after the flocculation step;
A membrane filtration process for separating the treated water after the coagulation process into solid and liquid;
An extraction step of extracting at least a portion of the treated water containing solids that were not filtered through the membrane in the membrane filtration step;
A returning step of returning at least a portion of the treated water containing solids that have not been filtered through the membrane to the flocculation step;
A step of detecting a concentration of suspended solids (SS) in the treated water containing solids that were not filtered through the membrane in the membrane filtration step;
A process of returning the treated water containing solids that were not filtered through the membrane in the membrane filtration process to the coagulation process so that the treated water has a predetermined suspended solids (SS) concentration in the coagulation process based on the suspended solids (SS) concentration detected in the process;
A water treatment method comprising the steps of : The method further comprises the step of adding a polymer flocculant to at least a portion of the treated water containing solids that have not been filtered through the membrane;
The water treatment method according to claim 1, characterized in that the step of adding the polymer coagulant includes determining the amount of polymer coagulant to be added based on a preset polymer coagulant addition rate on the basis of the suspended solids (SS) concentration of the treated water containing solids that were not membrane filtered in the membrane filtration step and that is returned to the coagulation step, and adding the determined weight amount of polymer coagulant to the treated water containing solids that were not membrane filtered in the membrane filtration step and that is returned to the coagulation step. 3. The water treatment method according to claim 1, wherein the treated water containing the solids not filtered through the membrane is returned to the coagulation step so that the formation of coagulated flocs in the coagulation step has a suspended solids (SS) concentration of 20 mg/L or more. The water treatment method according to claim 2, characterized in that the amount of polymer flocculant added, determined based on the weight of suspended solids (SS) in the treated water containing solids that have not been filtered through the membrane, is 0.001 to 3.0 wt % relative to the weight of suspended solids (SS) in the treated water containing solids that have not been filtered through the membrane . A flocculation means for mixing the water to be treated with an inorganic flocculant;
a flocculation means for forming flocs in the water to be treated after passing through the flocculation means;
A membrane filtration means for performing solid-liquid separation of the water to be treated that has passed through the coagulation means;
An extraction means for extracting at least a portion of the treated water containing solids that were not filtered by the membrane filtration means;
A return means for returning at least a portion of the treated water containing solids that have not been filtered through the membrane to the flocculation means;
A suspended solids (SS) concentration detection means for detecting a suspended solids concentration in the treated water containing solids that have not been filtered through the membrane;
A means for returning treated water including solids that were not filtered by the membrane filtration means so that the concentration of suspended solids in the flocculation means is preset based on the detected concentration of suspended solids;
A water treatment device comprising : The method further comprises adding a polymer flocculant to at least a portion of the treated water containing solids that have not been filtered through the membrane;
The water treatment device described in claim 5, characterized in that the means for adding the polymer coagulant determines the amount of polymer coagulant to be added based on a predetermined polymer coagulant addition rate, using the suspended solids (SS) concentration of the treated water containing solids that have not been membrane filtered by the membrane filtration means and returned to the coagulation means as a standard, and adds the determined weight amount of polymer coagulant to the treated water containing solids that have not been membrane filtered by the membrane filtration means and returned to the coagulation means.

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