JP7562499B2 - Method for treating wastewater generated during water purification and device for treating wastewater generated during water purification - Google Patents

Description

The present invention relates to a method for treating wastewater generated during water purification and a device for treating wastewater generated during water purification, and in particular to a method for treating wastewater generated during water purification, which treats cleaning wastewater generated during the cleaning of water purification equipment and returns it to the previous stage of the water purification process, and a device for treating wastewater generated during water purification.

The conventional treatment of washing wastewater generated in a rapid filter basin of the water purification process described in Non-Patent Document 1 is explained with reference to Figures 18 and 19.

Figures 18 and 19 are flow sheets showing conventional water purification processes and the treatment of cleaning wastewater and drinking water sludge generated in the water purification process. The difference between the two is whether or not the supernatant water from the sludge basin is sent to the wastewater basin.

In the conventional rapid filtration water purification process at water purification plants, raw water first enters the receiving well 150, where an inorganic coagulant is added in the coagulation mixing basin 152 (chemical mixing basin), and then coagulated flocs are formed in the flocculation basin. The coagulated flocs are separated into solids and liquids in the next settling basin 156 to obtain coagulated sedimentation treated water. The coagulated sedimentation treated water is sent to the rapid filtration basin 158, where fine turbidity that could not be removed in the settling basin 156 is filtered in a filtration tank to obtain filtered water. The filtered water is disinfected to obtain tap water. Meanwhile, the drinking water sludge extracted from the settling basin 156 is sent to the sludge discharge basin 160, where it is concentrated in the thickening tank 152, and the concentrated sludge is dewatered in the dewatering equipment 174 (dewatering machine).

The rapid sand filter 158 needs to have its filter layer washed periodically, and when this is done, a large amount of washing wastewater is temporarily generated from the rapid sand filter 158. This washing wastewater from the rapid sand filter 158 passes through the wastewater tank 166, and is either left entirely untreated, or, as shown in Non-Patent Document 1, is subjected to the floating treatment shown in Figures 18 and 19, and the treated water is returned to the receiving well 150 as return water.

The return water is transferred to a receiving well 150, which is located in the first stage of the water purification process, and mixed with the raw water in the water purification process. According to the invention in Non-Patent Document 1, by performing a flotation process on the washing wastewater from the rapid sand filter basin 158 prior to return, it is possible to remove some of the SS and turbidity in the washing wastewater.

Patent Document 1 discloses a method for treating filter basin cleaning wastewater in a water purification facility, which is characterized by temporarily storing the cleaning wastewater from a filter basin in a drainage basin, filtering the cleaning wastewater from the drainage basin with a membrane filtration device, storing the membrane filtered water in a cleaning water tank as cleaning water, supplying the cleaning water from the cleaning water tank to the filter basin to clean the filter basin, and returning the cleaning wastewater to the drainage basin for circulating and reusing it.

According to the invention of Patent Document 1, the filter basin is cleaned with membrane filtered water, which results in a higher cleaning effect for the filter basin than when using treated water that has been normally sand filtered.

Patent Document 2 discloses a method for producing pure water, which is characterized by having: (1) a flocculation step in which an inorganic flocculant is added to raw water and returned sludge containing a polymer flocculant is added to carry out a flocculation reaction; (2) a solid-liquid separation step in which the flocculated flocs produced in the flocculation step are separated into solid and liquid; (3) a desalination step in which treated water obtained from the solid-liquid separation step is passed through a reverse osmosis membrane device and/or an ion exchange device to desalt the treated water; (4) a sludge return step in which a portion of the flocculated sludge discharged from the solid-liquid separation step is returned to the flocculation step; and (5) a step in which a polymer flocculant is added to the flocculated sludge returned to the flocculation step.

According to the invention described in Patent Document 2, the diameter of the flocs formed during the coagulation process is increased, and as a result, the SS concentration in the treated water obtained from the solid-liquid separation process can be reduced.

Patent No. 3859402 Japanese Patent Application Publication No. 11-104696

Japan Water Works Association Publishing, Waterworks Facility Design Guidelines (2012), published July 2012, 5.22 Wastewater Treatment Facilities, pp. 378-381

According to the treatment of washing wastewater generated in a rapid filtration basin in the water purification process in Non-Patent Document 1, although the SS and turbidity in the washing wastewater can be removed to a certain extent by the flotation treatment of the washing wastewater, the turbidity for which the flotation treatment is effective is turbidity that is likely to have air bubbles attached thereto and is likely to float. That is, the turbidity components in the wastewater generated in the water purification process, such as clay components derived from the raw water of the water supply and aluminum hydroxide, which is a hydrolyzate of an inorganic flocculant, are hydrophilic and have a density of 1 g/ ^cm3 or more and are heavier than water, so that it is difficult to remove the turbidity components in the wastewater generated in the water purification process by the flotation treatment.

In addition, during the cleaning process of rapid filtration basins, activated carbon adsorption basins, and membrane filtration equipment, a large amount of cleaning wastewater is temporarily generated and flows intermittently into the drainage pond. The impact of the inflow stirs up the sludge that has accumulated in the drainage area, and the water quality of the supernatant water in the drainage pond, such as the solids content and turbidity, deteriorates. In addition, if the frequency of inflow of cleaning wastewater is high, sufficient retention time cannot be secured in the drainage pond for solid-liquid separation, and the water quality of the supernatant water in the drainage pond, such as the solids content and turbidity, deteriorates. This deterioration in water quality affects the water quality of the return water returned to the receiving well.

According to the invention of Patent Document 1, the cleaning effect of the filter basin is improved compared to using treated water that has been subjected to normal sand filtration, but since no treatment is performed on the cleaning wastewater that is returned to the receiving well, the problem of the water quality of the return water returned to the receiving well is not resolved.

The pure water production method disclosed in Patent Document 2 discloses that the flocculated sludge generated in the settling tank is recycled during the process, but there is no mention whatsoever of the treatment of wastewater generated by this pure water production method.

The object of the present invention, which has been made in consideration of the above problems, is to provide a wastewater treatment method and wastewater treatment device that can stably improve the water quality of the returned water after treatment in a wastewater treatment process that treats wastewater generated during water purification and returns it to the previous stage of the water purification process.

The inventors conducted extensive research to achieve the above-mentioned objective, and discovered that by stirring a wastewater basin into which the washing wastewater from the washing of rapid filtration basins and other equipment in water purification processes and the liquid portion (such as the supernatant water from the sludge basin) generated during the treatment of drinking water sludge flow in, the liquid quality in the wastewater basin, i.e., the water quality of the water to be treated, can be homogenized, and by subjecting this homogenized water to coagulation and sedimentation treatment, turbidity from the raw water and aluminum hydroxide, which is a hydrolyzate of an inorganic coagulant, etc. can be separated, resulting in return water with improved water quality and greater stability than in the past, which led to the completion of the present invention.

That is, it was found that the above-mentioned object is achieved by a method for treating wastewater generated in a water purification process, comprising a first transfer step of transferring the washing wastewater generated in the water purification process to a drainage tank, a second transfer step of transferring the drinking water sludge generated in the water purification process to a drainage tank, a third transfer step of transferring the supernatant water of the drainage tank, which is the supernatant water of the drainage tank, to the drainage tank, a fourth transfer step of transferring the supernatant water of a thickening tank for thickening the drainage tank sludge generated in the drainage tank to the drainage tank, a fifth transfer step of transferring the dehydrated filtrate generated in a dehydration equipment for dehydrating the concentrated sludge generated in the thickening tank to the drainage tank, a stirring step of stirring the inside of the drainage tank to obtain water to be treated, a coagulation and sedimentation treatment step of performing a coagulation and sedimentation treatment on the water to be treated to obtain coagulation and sedimentation treated water and coagulation and sedimentation sludge, and a return step of returning the coagulation and sedimentation treated water to the previous step of the water purification process.

A preferred embodiment of the method for treating wastewater generated in a water purification process according to the present invention is as follows.
(1) Between the coagulation sedimentation treatment step and the return step, there is a biological treatment step in which the coagulation sedimentation treated water obtained in the coagulation sedimentation treatment step is biologically treated to obtain biologically treated water, and in the return step, the biologically treated water obtained in the biological treatment is returned to the previous step of the water purification treatment instead of the coagulation sedimentation treated water. As a result, organic matter, iron, manganese, and algae remaining in the coagulation sedimentation treated water obtained in the coagulation sedimentation treatment step are further removed by the biological treatment, and the water quality of the return water returned to the previous step of the water purification treatment is further improved.
(2) The method includes a separated sludge obtaining step of obtaining separated sludge by adding a polymer flocculant to at least a part of the coagulated sedimentation sludge obtained in the coagulated sedimentation treatment step, and a separated sludge returning step of returning the obtained separated sludge to the coagulated sedimentation treatment step. In this way, by returning the separated sludge to which the polymer flocculant has been added to the coagulated sedimentation treatment step, the settling property of the coagulated flocs formed in the coagulated sedimentation treatment step is improved, and as a result, the water quality of the returned water returned to the previous step of the water purification treatment is further improved.

The above object is also achieved by providing a first transfer means for transferring the washing wastewater generated in the water purification process to a drainage tank, a second transfer means for transferring the drinking water sludge generated in the water purification process to a drainage tank, a third transfer means for transferring the supernatant water of the drainage tank, which is the supernatant water of the drainage tank, to the drainage tank, a fourth transfer means for transferring the supernatant water of a thickening tank for thickening the drainage tank sludge generated in the drainage tank to the drainage tank, a fifth transfer means for transferring the dehydrated filtrate generated in a dehydration facility for dehydrating the concentrated sludge generated in the thickening tank to the drainage tank, a stirring means for stirring the inside of the drainage tank to obtain the water to be treated, and a condensing means for mixing the water to be treated. This can also be achieved by a wastewater treatment device generated in a water purification process, which is characterized by having a coagulation and sedimentation treatment means for performing a collection and sedimentation treatment to obtain coagulation and sedimentation treated water and coagulation and sedimentation sludge, a return means for returning the coagulation and sedimentation treated water to the upstream equipment of the water purification process, an extraction means for extracting the coagulation and sedimentation sludge from the coagulation and sedimentation treatment means, a polymer coagulant addition means for adding a polymer coagulant to at least a portion of the coagulation and sedimentation sludge extracted by the extraction means to obtain separated sludge, and a separated sludge return means for returning the separated sludge to the coagulation and sedimentation treatment means.

According to the present invention, the cleaning of the water purification equipment temporarily generates a large amount of cleaning wastewater, which leads to insufficient solid-liquid separation in the wastewater pond, which causes fluctuations and deterioration in the water quality of the return water. However, by stirring the wastewater pond into which the cleaning wastewater and the liquid portion (such as the supernatant water from the sludge pond) generated during the treatment of the drinking water sludge flow, the quality of the resulting treated water is homogenized. Then, by subjecting the treated water whose water quality has been homogenized in this way to coagulation and sedimentation treatment, it is possible to remove turbidity derived from the raw water supply more effectively than with conventional flotation treatment, and this, combined with the effect of stabilizing the water quality of the treated water, makes it possible to steadily improve the water quality of the return water.

1 is a flow chart for explaining an example of a method for treating wastewater generated in the water purification process of the present invention. 4 is a flowchart for explaining another example of a method for treating wastewater generated in a water purification process according to the present invention. FIG. 1 is a schematic diagram showing a typical example of a treatment flow of a method for treating wastewater generated in a water purification process according to the present invention. FIG. 13 is a schematic diagram showing an example of a case where air agitation is selected for agitation in the agitation step. FIG. 13 is a schematic diagram showing an example of a combination of a draft tube and air agitation in the agitation step. FIG. 1A is a cross-sectional view of the discharge part of a drainage pond, showing an example of a case in which pump circulation mixing is used for mixing in the mixing process, and FIG. 1B is a vertical cross-sectional view of the drainage pond. FIG. 11 is a cross-sectional view of the discharge portion of a drainage pond showing another example in which pump circulation mixing is used for mixing in the mixing process. FIG. 2 is a schematic diagram showing an example of a coagulation-sedimentation treatment system used in the coagulation-sedimentation treatment step. FIG. 2 is a schematic diagram showing another example of a coagulation-sedimentation treatment system used in the coagulation-sedimentation treatment step. FIG. 1 is a schematic diagram showing a treatment flow of a method for treating wastewater generated in a water purification process of the present invention to which biological treatment is added. FIG. 1 is a schematic diagram showing an example of a biological treatment system used in a biological treatment step. FIG. 2 is a schematic diagram showing an example of a treatment flow of a method for treating wastewater generated in a water purification process according to the present invention, in which separated sludge is returned. This is a modified example of the coagulation and sedimentation treatment system of FIG. This is a modified example of the coagulation and sedimentation treatment system of FIG. FIG. 1 is a schematic diagram showing a reference example of a treatment flow of a method for treating wastewater generated in a water purification process, in which separated sludge is returned. 1 is a schematic diagram showing a treatment device 10 for wastewater generated in a water purification process according to the present invention. FIG. 2 is a schematic diagram showing a reference example of a treatment device for wastewater generated in a water purification process, which returns separated sludge. 1 is a flow sheet showing a conventional water purification process and the treatment of washing wastewater and drinking water sludge generated in the water purification process. In this flow sheet, only washing wastewater is transferred to a wastewater pond. This is a flow sheet showing a conventional water purification process and the treatment of washing wastewater and drinking water sludge generated in the water purification process. In this flow sheet, in addition to washing wastewater, the supernatant water from the sludge basin and the like are also transferred to the wastewater basin.

<Method of treating wastewater generated during water purification process>
FIG. 1 is a flow chart for explaining an example of a method for treating wastewater generated in the water purification process of the present invention, and FIG. 3 is a schematic diagram showing a representative example of a treatment flow by the method for treating wastewater generated in the water purification process of the present invention. The present invention has a first transfer step (S100) of transferring washing wastewater to a drainage pond, a second transfer step (S101) of transferring drinking water sludge to a drainage pond, a third transfer step (S102) of transferring supernatant water from the drainage pond to the drainage pond, a fourth transfer step (S103) of transferring supernatant water from the thickening tank to the drainage pond, a fifth transfer step (S104) of transferring dehydrated filtrate produced in the dewatering equipment to the drainage pond, a stirring step (S105) of stirring the inside of the drainage pond to obtain water to be treated, a coagulation sedimentation treatment step (S110) of performing coagulation sedimentation treatment on the water to be treated to obtain coagulation sedimentation treated water and coagulation sedimentation sludge, and a return step (S130) of returning the coagulation sedimentation treated water to the previous step of the water purification treatment.

As shown in Figures 18 and 19, which explain the conventional technology, water purification is a process in which raw water is stored in a receiving well 150, processed through a flocculation mixing basin 152 and a rapid filtration basin 158 to obtain filtered water, which is then disinfected to obtain tap water. In this specification, the upstream process of water purification refers to the process before the flocculation mixing basin 152 where inorganic flocculants are added.

Washing wastewater is generated by washing the filter tank of the rapid sand filter basin 158, and drinking water sludge is discharged from the settling basin 156.
(Drainage)
In the present invention, the wastewater generated in the water purification process is cleaning wastewater and drinking water sludge, which is coagulation-sedimentation treated sludge generated in the water purification process.
(Water to be treated)
In the present invention, the treated water to be treated includes wash wastewater generated during water purification treatment, supernatant water from a sludge basin which receives drinking water sludge generated in the wastewater treatment process of conventional water purification plants, supernatant water from a thickening tank which thickens (by gravity) the sludge basin sludge generated in the sludge basin, and dehydrated filtrate from a dehydration facility which dehydrates the concentrated sludge generated in the thickening tank.

The cleaning wastewater includes backwash wastewater from rapid filtration basins in the water purification process, backwash wastewater from activated carbon adsorption basins, and if the water purification process includes a biofilm filtration process, the backwash wastewater from that process, as well as the cleaning wastewater from the biofilm filtration added in this invention.

When the water purification process uses a rapid filtration method and a membrane filtration method in parallel, the concentrated liquid discharged from a submerged membrane filtration device or the membrane cleaning wastewater from a casing-type (hereinafter also referred to as "casing type") membrane filtration device that does not contain membrane cleaning chemicals is also included in the cleaning wastewater of the present invention.
(Discharge water)
In the present invention, the water to be treated in the wastewater treatment is subjected to a coagulation-sedimentation treatment step (S110), and in addition to returning the obtained coagulation-sedimentation-treated water to the previous step of the water purification treatment, at least a part of the coagulation-sedimentation-treated water can be discharged into a public water body such as a river. By subjecting the water to the coagulation-sedimentation treatment step (S110), the water to be treated is highly purified, which contributes to maintaining and improving the water environment of the river, etc.

The method for treating wastewater generated during the water purification process of the present invention will be explained below with reference to Figures 1 and 3.

[First Transfer Step (S100)]
In this step, the washing wastewater generated in the water purification process is transferred to the wastewater reservoir 11 (this is the first transfer step (S100)).

[Second Transfer Step (S101)]
In this step, the drinking water sludge generated in the water purification process is transferred to the wastewater basin 13. The definition of drinking water sludge has already been described, and for example, as shown in Figures 18 and 19, it is the coagulated sedimentation sludge discharged from the sedimentation basin 156 in the water purification process (this is the second transfer step (S101)).

[Third Transfer Step (S102)]
In this process, the supernatant water of the sludge basin 13 is transferred to the wastewater basin 11. Solid-liquid separation is carried out in the sludge basin 13, and the supernatant water is transferred to the wastewater basin 11, while the wastewater basin sludge, which is the settling portion, is transferred to the thickening tank 15 (the above is the third transfer process (S102)).

[Fourth Transfer Step (S103)]
In this process, the supernatant water of the thickening tank 15, which thickens the wastewater tank sludge produced in the wastewater tank 13, is transferred to the wastewater tank 11. In the thickening tank 15, solid-liquid separation is performed into the thickened sludge and the supernatant water of the thickening tank, and the supernatant water of the thickening tank is transferred to the wastewater tank 11 and the thickened sludge to the dewatering equipment 17 (fourth transfer process (S103)).

[Fifth Transfer Step (S104)]
In this step, the dewatering filtrate produced in the dewatering equipment 17, which dewaters the concentrated sludge produced in the concentration tank 15, is transferred to the wastewater reservoir 11. In the dewatering equipment 17, the concentrated sludge is dewatered to obtain a dewatered cake and a dewatered filtrate. The dewatered filtrate is transferred to the wastewater reservoir 11 (fifth transfer step (S104)).

[Stirring step (S105)]
In this process, the inside of the drainage pond 11 is stirred to obtain the water to be treated. The stirring means provided in the drainage pond 11 uniformly stirs and mixes the water to be treated in the drainage pond 11. The stirring method inside the drainage pond 11 is selected from mechanical stirring using a stirring blade, air stirring, and pump circulation stirring, and a commercially available stirring device can be used.

Mechanical mixing, air mixing, and pump circulation mixing may be combined.

The mixing time may be continuous, or it may be intermittent only during the time period when the downstream coagulation and sedimentation treatment process is in operation. In addition, since the washing wastewater that temporarily flows in in large volumes is a large volume of water, the water to be treated in the wastewater pond 11 can be mixed with that water flow alone, so the mixing device can be stopped while the washing wastewater is flowing in.

While mechanical mixing is an effective mixing method for small-scale facilities, air mixing or pump circulation mixing is preferred for mixing the wastewater pond 11 of the present invention in terms of the size of the mixing device and mixing power. By providing a draft tube that generates an upward flow inside the wastewater pond 11, the treated water in the wastewater pond 11 can be mixed more effectively and uniformly than air mixing or pump circulation mixing alone, reducing power costs, so using a draft tube in combination is more preferred. The draft tube can be installed in the center of the wastewater pond 11, or multiple draft tubes can be installed on the cross section of the wastewater pond 11.

When the drainage pond 11 is stirred only by air agitation without using a draft tube, it is better to install air piping on the entire bottom of the drainage pond so that the water to be treated in the drainage pond can be mixed by a swirling flow, as shown in Figure 4, rather than installing air piping on the entire bottom of the drainage pond. Figure 4 is a schematic diagram showing an example of a case where air agitation is selected for agitation in the agitation process (S105).

According to the air agitation in Figure 4, a mixture of the washing wastewater, the supernatant water of the sludge tank, the supernatant water of the thickening tank, and the dehydrated filtrate flows into the drainage tank 11 from the inlet of the washing wastewater, either simultaneously or separately, and agitation air is sent from the bottom of the drainage tank to uniformly agitate the inside of the drainage tank by a swirling flow. After the washing wastewater, etc. flows in, the homogenized treated water passes through the treated water piping and is transported to the next coagulation and sedimentation process.

Also, as shown in FIG. 4, pressurized air is introduced into the drainage pond 11 from an air diffusion pipe installed at the bottom of the drainage pond, and as the air rises, the water to be treated in the drainage pond 11 is mixed and stirred uniformly by a swirling flow. In this case, no sludge is discharged from the sludge discharge pipe at the bottom of the drainage pond 11.

Next, an example of a combination of draft tube and air agitation will be described with reference to Figure 5.

With the combination of draft tube and air agitation shown in Figure 5, pressurized air is introduced from the bottom of the draft tube installed in the wastewater tank 11, and as the air rises inside the draft tube, the water to be treated also rises, generating a swirling flow of the water to be treated inside the wastewater tank 11, and the water to be treated is mixed and agitated uniformly.

Furthermore, an example of a case where the mixing is pump circulation mixing will be described with reference to FIG. 6. FIG. 6(A) is a cross-sectional view of the discharge part of a drainage pond showing an example of a case where pump circulation mixing is used for mixing in the mixing process, and FIG. 6(B) is a vertical cross-sectional view of the drainage pond. In FIG. 6, the drainage pond has a rectangular shape when viewed from above.

As shown in FIG. 6(A), the pump discharge section in cross section can be placed at the corners of the drainage basin 11 and in the center of the side. In either case, when the treated water is discharged from the pump, a swirling flow is generated along the side wall, and the inside of the drainage basin 11 is mixed and stirred uniformly.

As shown in FIG. 6 (B), the water to be treated is sucked in from near the bottom of the drainage pond and discharged from the discharge section at the top of the drainage pond by a circulation pump, and the inside of the drainage pond 11 is mixed and stirred uniformly by the circulation pump.

Furthermore, another example of the case where the mixing is pump circulation mixing will be described with reference to FIG. 7. FIG. 7 is a cross-sectional view of the discharge part of the drainage pond, showing another example of the case where pump circulation mixing is used for mixing in the mixing process. In FIG. 7, the drainage pond has a circular shape when viewed from above. Since there are no changes compared to FIG. 6 other than the shape of the drainage pond, for the longitudinal cross-sectional view, refer to FIG. 6(B) and a detailed description will be omitted.

As shown in FIG. 7, the pump discharge section is disposed in the tangential direction of the drainage pond 11. When the circulating pump discharges the treated water in the tangential direction of the drainage pond 11, a swirling flow is generated along the side wall, and the inside of the drainage pond is mixed and stirred uniformly.

In addition to the washing wastewater, the wastewater tank 11 is supplied with the following: the supernatant water from the wastewater tank 13, which receives the drinking water sludge discharged from the settling tank 156 of the water purification facility; the supernatant water from the thickening tank 15, which receives and thickens the sludge from the wastewater tank 13; and the dehydrated filtrate from the dehydration facility 17, which receives and dehydrates the concentrated sludge concentrated in the thickening tank 15. Therefore, in this process, the mixture of the washing wastewater, the supernatant water from the wastewater tank, the thickening tank, and the dehydrated filtrate is stirred to obtain water to be treated that has an equal volume and quality (the above is the stirring process (S105)).

[Aggregation and precipitation treatment step (S110)]
In this step, the water to be treated is subjected to coagulation-sedimentation treatment to obtain coagulation-sedimentation treated water and coagulation-sedimentation sludge.

Figure 8 is a schematic diagram showing an example of a coagulation sedimentation treatment system used in the coagulation sedimentation treatment process. As shown in the figure, the coagulation sedimentation treatment for wastewater treatment at private facilities consists of a mixing tank, a coagulation tank, and a settling tank, and the settling tank is generally an upflow coagulation sedimentation tank.

In the mixing tank, an inorganic coagulant is added to the water being treated, and an acid such as sulfuric acid or an alkaline agent such as caustic soda is added as a pH adjuster to adjust the coagulation pH, and the SS and other substances in the water being treated are turned into microflocs by the coagulation action of the inorganic coagulant.

In the coagulation tank, a polymeric coagulant is added to produce coagulated flocs that are easy to separate into solids and liquids in the upward-flow coagulation sedimentation tank. The water to be treated, which contains coagulated flocs, is introduced into the center well of the upward-flow coagulation sedimentation tank, and the water to be treated flows upward at the bottom of the center well, causing solid-liquid separation of the coagulation-sedimentation treated water and the coagulated flocs. The coagulated flocs settle and thicken at the bottom of the sedimentation tank, and are withdrawn from the bottom of the tank as coagulated sedimentation sludge for dehydration and other treatments.

As inorganic coagulants, commercially available products such as aluminum sulfate, polyaluminum chloride (PAC), polyferric sulfate (polyferric iron), ferric chloride, or a mixture of these can be used. In addition, since the use of these inorganic coagulants lowers the pH of the water being treated, commercially available caustic soda or the like is used as an alkaline agent to adjust the pH to the appropriate coagulation pH.

The polymer flocculant used in the present invention is a polymer flocculant for water supply use in water purification treatment and wastewater treatment at water purification plants.

As the polymer flocculant for water supply, a commercially available product is used, but there is no particular limitation. 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. Of these, sodium polyacrylate is particularly preferred.

Polyacrylamide-based polymer flocculants can also be used, which are copolymers of polyacrylamide and poly(meth)acrylate, and commercially available waterworks polymer flocculants can be used as anionic polymer flocculants.

Generally, 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.

Polymer flocculants are available in powder or liquid form, and are dissolved in tap water or other dissolving water to a concentration of 0.01-0.3% by weight. The dissolved and prepared polymer flocculant for water supply is then added to the water to be treated in accordance with the injection amount.

Furthermore, an example of a coagulation sedimentation treatment system that is more space-saving than the coagulation sedimentation treatment system having the upflow coagulation sedimentation tank of FIG. 8 is described with reference to FIG. 9. FIG. 9 is a schematic diagram showing another example of a coagulation sedimentation treatment system used in the coagulation sedimentation treatment process. In the example of FIG. 9, a high-speed granulation sedimentation device is used as the coagulation sedimentation tank. In the coagulation sedimentation treatment system of the example of FIG. 9, an inorganic coagulant and a pH adjuster for adjusting the coagulation pH are added to the water to be treated in the mixing tank. A polymer coagulant is added midway through the pipe for the water to be treated, and a strong granular material with good settling properties is generated from the coagulated flocs at the bottom of the high-speed granulation sedimentation device, so that it is easily separated into solid and liquid and a blanket layer is formed. The water to be treated passes through the blanket layer and is discharged as treated water. The coagulation sedimentation sludge is discharged from the blanket layer (the above is the coagulation sedimentation treatment process (S110)).

[Returning process (S130)]
In this process, the coagulated and sedimented water is returned to the upstream process of the water purification process. The upstream process of the water purification process to which the water is returned may be any process prior to the coagulation mixing basin 152 shown in Figures 18 and 19, but it is common to return the water to the receiving well 150 that stores raw water for the water supply (return process (S130)).

As described above, the method for treating wastewater generated during water purification according to the present invention involves first receiving the washing wastewater generated in the water purification process, the supernatant water from the gravity thickening tank (thickening tank supernatant water) generated in the wastewater treatment process, the dehydration filtrate during dehydration, and the supernatant water from the sludge tank (sludge tank supernatant water) in the wastewater tank 11 to form a mixed liquid, and then stirring the inside of the wastewater tank 11 to obtain the water to be treated. This homogenizes the turbidity, SS, etc. of the water to be treated, and the water is transferred at a constant flow rate from the wastewater tank 11 to the coagulation and sedimentation treatment process (S110), where the turbidity, SS, etc. are removed, and the water is returned as return water to the receiving well 150 of the water purification process.

The difference between the method of treating wastewater generated in the water purification process according to the present invention shown in FIG. 3 and the treatment of washing wastewater generated in the rapid sand filter of the conventional water purification process shown in FIG. 19 is as follows.
(i) In Figure 3, the purpose of the drainage pond is to equalize the water volume and homogenize the water quality, and the only function is to adjust the flow rate, etc. The water is stirred to homogenize the water quality. Since solid-liquid separation is not the purpose, the drainage pond sludge is not discharged from the drainage pond.
(ii) In Fig. 3, solid-liquid separation is not performed in the drainage pond, so no drainage pond sludge is generated and therefore the drainage pond sludge is not transferred to the sludge pond. The water to be treated, including the washing wastewater, is not subjected to solid-liquid separation but is equalized and homogenized in the drainage pond and then subjected to coagulation and sedimentation treatment.

The reasons for (i) and (ii) are as follows:

In other words, the drainage pond is used to adjust the flow rate and water quality. During the cleaning process in rapid sand filtration ponds and other facilities, a large amount of cleaning wastewater is generated temporarily and intermittently flows into the drainage pond, so it is not possible to ensure sufficient retention time for solid-liquid separation in the drainage pond. As a result, solid separation is not sufficient in the drainage pond. Because solid-liquid separation is not sufficient, the quality of the supernatant water from the drainage pond is the same as the quality of the cleaning wastewater that flows into it.

In addition, because solid-liquid separation in the drainage pond is insufficient, the sludge does not thicken easily and the sludge concentration of the drainage pond sludge does not increase. If drainage pond sludge with a low sludge concentration is transferred to the wastewater pond, solid-liquid separation and sludge thickening will deteriorate even in the drainage pond where the drainage pond sludge and drinking water sludge flow in.

(iii) The flotation process in Figure 19 is a coagulation and sedimentation process.

The reason is that the quality of treated water from the coagulation and flotation process is inferior to that from the coagulation and sedimentation process, and clear treated water cannot be obtained, and the froth generated by the coagulation and flotation process contains air bubbles, making it difficult to concentrate and dehydrate.

Although not shown in Figure 19, in order to improve the thickening properties of water supply sludge, the thickened sludge in the thickening tank may be further thickened and dewatered by flocculation treatment. A polymer flocculant is used as a flocculation aid in the flocculation treatment. In addition, the dehydrated filtrate (dehydrated separated liquid) containing the polymer flocculant added as pre-dehydration treatment may be returned to the front stage of the secondary thickening tank to make effective use of the residual polymer flocculant.

The coagulation sedimentation treatment of the present invention differs from the coagulation sedimentation treatment of the present invention in that the coagulation treatment is performed using a dewatered filtrate (dewatered separated liquid) containing a polymer coagulant that remains as a pretreatment for dehydration of the concentrated sludge in the concentration tank, or a coagulant. The treatment target of the coagulation sedimentation treatment step (S110) of the present invention is a mixture of the washed wastewater that is not separated into solid and liquid in the wastewater tank, the supernatant water of the sludge tank, the supernatant water of the concentration tank, and the dewatered filtrate.

In addition, adding only polymer flocculants is insufficient to clarify the supernatant water from the secondary thickening tank. The combined use of inorganic and polymer flocculants is essential for clarification.

In the method for treating wastewater generated by the water purification process according to the present invention, no sludge is extracted from the wastewater pond 11 during normal operation, so the purpose of the wastewater pond 11 is not solid-liquid separation, but equalization of the water volume and homogenization of the water quality.

Furthermore, since no solid-liquid separation occurs in the drainage pond 11, no drainage pond sludge is generated and the drainage pond sludge is not transported to the sludge pond. The washed wastewater is not separated into solid and liquid, but is equalized and homogenized in the drainage pond together with the supernatant water from the sludge pond, the supernatant water from the thickening tank, and the dewatered filtrate, and is then sent to the coagulation and sedimentation treatment process (S110).

The drainage pond 11 is equipped with an agitation device to equalize the water volume, homogenize the water quality, and prevent sludge accumulation. By agitating the inside of the drainage pond 11, the quality of the water being treated is homogenized in a short period of time.

Any method or commercially available device can be used as the agitation device, and it can be air agitation, mechanical agitation using an agitator blade, or pump agitation using a pump circulation. Air agitation is preferable because it is simple and prevents anaerobic conditions inside the drainage pond 11.

In addition to the above-mentioned mixing devices, the drainage pond 11 can also be mixed with cleaning wastewater. The water to be treated in the drainage pond 11 is mixed by the water flow of cleaning wastewater that intermittently flows into the drainage pond 11 in large quantities and by the impact of the water flow when it flows in. Multiple cleaning wastewater inlets can be provided upward from the bottom of the drainage pond 11 or downward along the wall of the drainage pond 11.

Agitation of the drainage pond 11 by the washing wastewater can be combined with any one of air agitation, mechanical agitation, and pump agitation.

The coagulation and sedimentation sludge generated by the solid-liquid separation in the coagulation and sedimentation treatment process (S110) of the present invention is transported together with the drinking water sludge to the wastewater tank 10 and treated together with the drinking water sludge. The treated water from the coagulation and sedimentation treatment process (S110) may be used as return water or may be discharged into public waters.

The coagulation and sedimentation process (S110) is composed of a mixing tank, a coagulation tank, and an upward-flow coagulation and sedimentation tank in an upward-flow coagulation and sedimentation device (see Figure 8). In a high-speed granulation and sedimentation device, it is composed of a mixing tank and a high-speed granulation and sedimentation device (see Figure 9).

An inorganic coagulant and a pH adjuster (acid, alkali, etc.) are added to the mixing tank to achieve the appropriate coagulation pH. Alkaline agents include caustic soda, and acid agents include sulfuric acid.

The inorganic coagulant injection rate is 100 to 500 mg/L depending on the turbidity and SS concentration.
A polymer flocculant is added to the coagulation tank of the upflow coagulation sedimentation device to produce coagulated flocs.

In the case of the upflow coagulation and settling device in Figure 8, the polymer coagulant injection rate is 0.5 to 2 mg/L per raw water volume. In the case of the high-speed granulation and settling device in Figure 9, the polymer coagulant injection rate is 1 to 5 mg/L per raw water volume.

The method for treating wastewater generated during water purification according to the present invention, in particular, by subjecting the washing wastewater, the supernatant water from the sludge tank, the supernatant water from the thickening tank, and the dewatered filtrate to coagulation and sedimentation treatment together as the water to be treated, in addition to clarifying the water quality of all return water returned from the wastewater treatment process to the receiving well 150, the following effects can be obtained.
(1) Even if the quality of the supernatant water and the dehydration filtrate from the thickening tank 15 and the dehydration equipment 17 deteriorates due to deterioration of thickening and sedimentation properties during low water temperatures in winter, clear return water can be ensured by subjecting them to coagulation and sedimentation treatment.
(2) By increasing the water surface load of the coagulation sedimentation treatment of the present invention, the amount of water to be treated can be increased, and the water to be treated can be transported from the drainage pond 11 to the coagulation sedimentation treatment in a short period of time, thereby ensuring the retention time in the drainage pond 11 and stabilizing the amount and quality of the water to be treated in the drainage pond 11, thereby contributing to the effectiveness of the coagulation sedimentation treatment.
(3) In the coagulation and sedimentation treatment, it is essential to use both inorganic and polymeric coagulants. However, by combining this with powdered activated carbon, etc., it is possible to improve the water quality, including the turbidity of the water returned to the receiving well, as well as the amount of algae, picoplankton, color, and organic matter such as TOC.

In addition, in the method for treating wastewater generated in the water purification process according to the representative example shown in FIG. 3, the coagulation-sedimentation-treated water obtained in the coagulation-sedimentation treatment process (S110) is returned to the receiving well 150 as return water in the return process (S130). However, as shown in FIG. 1, a biological treatment process (S120) may be optionally included between the coagulation-sedimentation treatment process (S110) and the return process (S130).

Figure 10 is a schematic diagram showing the processing flow of the method for treating wastewater generated in the water purification process of the present invention to which biological treatment has been added. With reference to this figure, the method for treating wastewater generated in the water purification process of the present invention to which a biological treatment process (S120) has been added will be described. Note that since there are no changes other than the biological treatment process (S120), explanations of the processes other than the biological treatment process (S120) will be omitted here.

[Biological treatment step (S120)]
In this step, the coagulation-sedimentation treated water obtained in the coagulation-sedimentation treatment step (S110) is biologically treated to obtain biologically treated water.

The aerobic biological treatment equipment used in this process is preferably an equipment that employs a biological treatment method that does not require a solid-liquid separation device, since the organic matter concentration in the water to be treated is low. For example, there is the contact oxidation method in which a contact material is placed in the biological treatment tank, the carrier flow treatment method in which carriers with attached microorganisms are suspended, and the biological membrane filtration method, but the biological membrane filtration method is preferable because it combines biological treatment and filtration functions and produces clear treated water.

Figure 11 is a schematic diagram showing an example of a biological treatment system used in the biological treatment process, which shows a biological treatment system using a biological membrane filtration method.

The biofilm filtration equipment used in the biofilm filtration method is composed of a packed bed filled with filler, a support layer made of gravel etc. to support the packed bed, an aeration pipe that diffuses air into the bottom of the equipment to keep the packed bed aerobic, and a backwash pump for backwashing the packed bed. Raw water flows in from the top of the equipment, and aerobic microorganisms living on the surface of the packing material in the packed bed decompose organic matter and ammonia nitrogen in the raw water and oxidize iron ions and manganese ions, and hydroxides of iron etc., as well as SS and turbidity of the raw water are removed from the treated water by the filtering action of the packed bed.

The packed bed is backwashed using treated water from the treatment tank, and the cleaning wastewater is discharged from the top of the device. Backwashing is performed at set biofilm filtration times or according to a preset water level in the water layer above the packed bed. The cleaning wastewater is treated in a wastewater treatment facility during water purification.

The filler can be anthracite (effective diameter 0.7-4.0 mm, uniformity coefficient 1.4 or less, specific gravity 1.4-1.6), granular activated carbon (effective diameter 0.7-4.0 mm, uniformity coefficient 1.5 or less, specific gravity 1.7 or less), filter sand, etc. The raw material for the granular activated carbon can be coal or coconut shells. In addition, used activated carbon from an activated carbon adsorption pond, dried used activated carbon, or activated regenerated carbon can be used.

According to the biological treatment system using the biofilm filtration method of Figure 11, the coagulation-sedimentation treated water is passed through a packed bed formed of a packing material whose surface is covered with microorganisms, and the microorganisms living on the surface of the packing material decompose and remove organic matter and ammonia nitrogen. In addition, the turbidity and SS of the coagulation-sedimentation treated water can be removed by the filtering action of the packing material, and clear biofilm filtration treated water is obtained. Even if the biofilm filtration treated water is returned to the receiving well as return water, it does not affect the load on the water purification process.

The resulting biologically treated water is then sent to the return process (S130) (the end of the biological treatment process (S120)).

According to the wastewater treatment method of the present invention, which includes biological treatment, the coagulation-sedimentation-treated water from the coagulation-sedimentation treatment process (S110) is subjected to biological treatment to remove BOD, organic matter such as algae, turbidity, SS, soluble manganese ions, and soluble iron ions remaining in the coagulation-sedimentation-treated water. As a result, the amount of chlorine used to remove BOD, organic matter such as algae, soluble manganese ions, and soluble iron ions in the water purification process can be reduced, which not only reduces chemical costs but also suppresses disinfection by-products such as trihalomethanes. In addition, the amount of inorganic coagulant used to remove turbidity and SS and the amount of alkaline agent caused by inorganic coagulant can be reduced. In addition, since clear return water is transferred to the receiving well 150, water quality fluctuations such as turbidity of the raw water in the receiving well 150 are mitigated, allowing for stable water purification and reducing the amount of maintenance work.

Furthermore, in the method for treating wastewater generated in the water purification process according to the representative example of Figure 3, the entire amount of coagulation-sedimentation sludge generated in the coagulation-sedimentation treatment step (S110) was treated in the treatment flow for drinking water sludge (Figure 3, first and second coagulation-sedimentation sludge), but a portion of it may be separated, a polymer coagulant may be added, and the separated sludge may be returned to the coagulation-sedimentation treatment step (S110).

Figure 2 is a flow chart for explaining another example of a method for treating wastewater generated in the water purification process of the present invention, and Figure 12 is a schematic diagram showing an example of a processing flow of a method for treating wastewater generated in the water purification process of the present invention, in which separated sludge is returned.

As shown in FIG. 2, in the present invention, after the coagulation treatment step (S110), a flow is inserted that passes through a separated sludge acquisition step (S200) and a separated sludge return step (S210) and then proceeds to the coagulation treatment step (S110), separate from the flow that goes to the return step (S130). As the coagulation sedimentation treatment system used in this flow, for example, the coagulation sedimentation treatment system shown in FIG. 13 and FIG. 14 can be used.

Figure 13 shows a modified example of the coagulation sedimentation treatment system having the upward flow coagulation sedimentation tank of Figure 8. In the coagulation sedimentation treatment system of Figure 13, a pipe is provided to return the coagulation sedimentation sludge extracted from the bottom of the upward flow coagulation sedimentation tank between the mixing tank and the coagulation tank, and this pipe is provided with an addition point for a polymer coagulant.

Figure 14 shows a modified example of the coagulation and sedimentation treatment system having the high-speed granulation and sedimentation device of Figure 9. In the coagulation and sedimentation treatment system of Figure 14, a pipe for discharging coagulation and sedimentation sludge provided in the blanket layer of the high-speed granulation and sedimentation device branches off, and a return pipe is provided with the branch connected to a pipe going from the mixing tank to the bottom of the high-speed granulation and sedimentation device, and a polymer coagulant addition point is provided in the return pipe.

The separated sludge acquisition process (S200) and the separated sludge return process (S210) are described below.

[Separated sludge obtaining step (S200)]
In this step, a polymer flocculant is added to at least a portion of the flocculating and settling sludge obtained in the flocculating and settling treatment step (S110) to obtain separated sludge.

When using a coagulation and sedimentation treatment system having an upflow coagulation and sedimentation tank as shown in FIG. 13, a polymer coagulant is added to at least a portion of the coagulation and sedimentation sludge generated in the coagulation and sedimentation treatment step (S110) to obtain separated sludge.

When using a coagulation and sedimentation treatment system with the high-speed granulation and sedimentation device shown in Figure 14, the separated sludge, to which a polymer coagulant has been added at least in part from the sludge extracted from the high-speed granulation and sedimentation device, is mixed with the treated water in which the SS and other substances have been coagulated by adding an inorganic coagulant and an acid-alkali agent to adjust the pH to the appropriate level.

The polymer flocculant is added in an upflow coagulation and settling tank or a high-speed granulation and settling device, or in the suction or discharge section of the return pump for the separated sludge, or in a line mixer or mixing tank installed in the return piping. The mixing tank installed in the return piping can be agitated by mechanical agitation or by the water flow of the separated sludge.

The residence time of the separated sludge in the mixing tank installed in the return pipe varies depending on the mixing method, mixing strength, return flow rate and its concentration, but can be, for example, 0.2 to 3 minutes. If the residence time is 0.2 minutes or more, the polymer flocculant and the separated sludge are mixed sufficiently. On the other hand, if the residence time is 3 minutes or less, the possibility of the separated sludge flocs accumulating in the mixing tank installed in the return mud pipe and the possibility of the flocculation properties decreasing can be reduced (the separated sludge acquisition process (S200) is completed).

[Separated sludge return process (S210)]
In this step, the separated sludge obtained is returned to the coagulation and sedimentation treatment step (S110).

When using a coagulation sedimentation treatment system with an upflow coagulation sedimentation tank as shown in Figure 13, the separated sludge with added polymer coagulant is returned to the coagulation tank, where coagulated flocs are formed, and solid-liquid separation is performed in the upflow coagulation sedimentation tank. By adding an inorganic coagulant to the water to be treated and an acid-alkali agent to adjust the pH to the appropriate coagulation, the water to be treated in which SS and other substances have been coagulated is mixed with the separated sludge with added polymer coagulant in the coagulation section, forming coagulated flocs with good settling properties, improving solid-liquid separation performance.

When using a coagulation sedimentation treatment system with the high-speed granulation sedimentation device of Figure 14, separated sludge mixed with the treated water in which the SS and other substances in the treated water have been coagulated by adding an inorganic coagulant and an acid-alkali agent to adjust the coagulation pH to an appropriate level is passed through the bottom of the high-speed granulation sedimentation device in an upward flow. By mixing the treated water and separated sludge to which a polymer coagulant has been added in the coagulation section, coagulated flocs with good settling properties are formed, improving solid-liquid separation performance.

In this process, a sludge flow meter and a sludge concentration meter are installed in the return piping to monitor the return flow rate and sludge concentration, which allows the amount of separated sludge returned in the coagulation and sedimentation treatment process (S110) to be controlled, enabling stable mechanical solid-liquid separation.

To detect the sludge concentration, commercially available sludge concentration meters such as near-infrared sludge concentration meters, laser light sludge concentration meters, and microwave sludge concentration meters can be used. To detect the sludge flow rate, commercially available electromagnetic flow meters and ultrasonic flow meters can be used. The pump that returns the sludge can be a commercially available product, and the flow rate can be adjusted to a set value by controlling the rotation speed before being returned (this concludes the separated sludge return process (S210)).

In the above-mentioned separated sludge obtaining process (S200), the addition rate of the polymer flocculant is preferably 0.05 to 5.0% by weight of the separated sludge, i.e., SS, and more preferably 0.1 to 1.0% by weight of SS.

Conventionally, when a polymer flocculant is added to the water to be treated, the polymer flocculant injection rate is calculated based on the weight of the polymer flocculant (mg/L) relative to the amount of water to be treated. However, when a polymer flocculant is added to separated sludge as in the present invention, it is preferable to use the "wt/wt% vs. SS" index used in dewatering in sludge treatment as the polymer flocculant addition rate. In the present invention, the polymer flocculant is used to flocculate the turbidity and SS of the water to be treated and the sludge particles of the separated sludge, and the addition rate of the polymer flocculant should be determined based on the weight of solids such as SS. The addition rate of the polymer flocculant in mg/L relative to the amount of water to be treated does not take into account the amount of SS or solids in the sludge on which the polymer flocculant acts.

In the coagulation and sedimentation treatment process (S110), the SS concentration of the treated water, including the SS in the separated sludge, changes depending on the return conditions of the separated sludge, so the amount of polymer coagulant injected cannot be properly managed or controlled using the amount of treated water or the polymer coagulant injection rate (unit: mg/L), which is the weight of polymer coagulant relative to the amount of treated water.

As in the present invention, by returning the separated sludge to which a polymer flocculant has been added based on the sludge weight of the separated sludge and mixing the treated water and the separated sludge in the coagulation and sedimentation treatment step (S110), it is possible to always add the optimal amount of polymer flocculant regardless of the properties of the treated water and the wastewater treatment status of the treated water, compared to the conventional method of setting the injection rate of the polymer flocculant based on the flow rate of the treated water. This makes it possible to optimize the amount of polymer flocculant used for efficient treatment while consistently achieving a high flocculation effect.

If an optimal amount of polymer flocculant is added to the separated sludge and returned to the coagulation and sedimentation treatment step (S110), there is no need to add additional polymer flocculant to the coagulation step. Even if additional polymer flocculant is added to the coagulation and sedimentation treatment step (S110), it may actually result in a decrease in processability, since it takes time to disperse and homogenize the solution in the coagulation step due to the viscosity of the polymer flocculant solution.

The SS in the coagulation tank, which is located before the upflow coagulation sedimentation tank, is composed of SS from the treated water, SS derived from the inorganic coagulant, which is mainly hydroxides of iron and aluminum, and SS derived from the returned separated sludge. The SS concentration in the coagulation tank is the total concentration of these, which is 20 to 2000 mg/L.

In the high-speed granulation sedimentation device, the water to be treated and separated sludge are mixed in the pipe before the bottom inlet of the high-speed granulation sedimentation device, so the SS after mixing is made up of SS from the water to be treated, SS derived from the inorganic coagulant, which is mainly hydroxides of iron and aluminum, and SS derived from the returned separated sludge, and the SS concentration after mixing is the total concentration of these, which is 20 to 2000 mg/L.

Whether an upflow coagulation sedimentation tank or a high-speed granulation sedimentation device is used, the SS concentration in the coagulation tank of the upflow coagulation sedimentation tank and the SS concentration in the piping before the bottom inlet of the high-speed granulation sedimentation device are 1.5 to 20 times the SS concentration in the treated water.

The water to be treated and separated sludge to which a polymer flocculant has been added flow into the coagulation tank of the coagulation and sedimentation treatment process (S110), and the sludge concentration and SS concentration in the coagulation tank are preferably 100 to 10,000 mg/L, and the SS concentration is more preferably 500 to 2,000 mg/L.

In addition, if the SS of the water to be treated exceeds 300 mg/L, the ratio of SS in the coagulation tank to SS of the water to be treated is preferably 1.5 to 10, and more preferably 2 to 5.

In addition, because flocs with good settling properties are produced, the flocculation-sedimentation sludge discharged from the flocculation-sedimentation treatment process (S110) of the present invention (FIG. 12, first flocculation-sedimentation sludge and second flocculation-sedimentation sludge) has improved settling and thickening properties even when it is transferred to the wastewater tank 13 or thickening tank 15.

Indicators of sludge concentration include SS (suspended solids) and TS (total solids).

In the present invention, SS and TS are defined as follows.
SS:K 0102:2019 Industrial wastewater test method 14.1 Suspended solids TS:K 0102:2019 Industrial wastewater test method 14.2 Total evaporation residue Furthermore, in the method for treating wastewater generated in the water purification process according to the representative example of FIG. 3, the washing wastewater, the supernatant water from the sludge tank, the supernatant water from the thickening tank, and the dewatered filtrate are transferred to the wastewater tank 11, and the mixed liquid obtained by stirring these is used as the water to be treated. However, as a reference example, a case in which only the washing wastewater is transferred to the wastewater tank 11 is described below.

Figure 15 is a schematic diagram showing a reference example of a processing flow for a method of treating wastewater generated during water purification, in which separated sludge is returned.

According to the process flow in FIG. 15, in the transfer step (S100), only the washing wastewater is transferred to the wastewater pond 11 and becomes the water to be treated.

The wastewater to be treated by the present invention is the washing wastewater, the supernatant water from the thickening tank, the dewatering filtrate, and the supernatant water from the sludge tank. Among these, the wastewater volume and the SS load are washing wastewater >> the supernatant water from the thickening tank > the supernatant water from the sludge tank >> the dewatering filtrate. Therefore, by treating washing wastewater with a large wastewater volume and SS load, as shown in Figure 15, even if the supernatant water from the thickening tank, the dewatering filtrate, and the supernatant water from the sludge tank are returned to the receiving well untreated, the impact is smaller than the wastewater volume and SS load of the washing wastewater. However, when the raw water is highly turbid, or when the raw water is low turbidity in winter or the water temperature is low, the settling property and concentration of the water supply sludge discharged from the water purification process deteriorates. In such a case, as shown by the dashed line in the figure, the supernatant water from the thickening tank is transferred to the drainage tank and coagulated and settled together with the washing wastewater, thereby suppressing the SS load and turbidity load of the raw water supply.

The method of treating wastewater generated during water purification in the reference example of Figure 15 involves treating only the washing wastewater generated in the water purification process in a coagulation and sedimentation treatment process, and returning the coagulation and sedimentation treated water from which turbidity, SS, etc. have been removed from the washing wastewater as return water to the receiving well 150 of the water purification process.

By subjecting the washing wastewater, the supernatant water from the thickening tank, the dewatering filtrate, and the supernatant water from the sludge tank to coagulation and sedimentation treatment, the turbidity and SS of the return water can be reduced, which reduces the load on the water purification process. However, since a large amount of washing wastewater is discharged at one time and has a high turbidity and SS concentration, in order to reduce the turbidity and SS of the return water, it is important to first remove the turbidity and SS by coagulation and sedimentation treatment of the washing wastewater.
<Treatment equipment for wastewater generated during water purification>
FIG. 16 is a schematic diagram showing a representative example of an apparatus for treating wastewater generated in the water purification process of the present invention. As shown, the apparatus 10 for treating wastewater generated in the water purification process of the present invention has a first transfer means 12, a second transfer means 14, a third transfer means 16, a fourth transfer means 18, a fifth transfer means 20, an agitation means 22, a coagulation and sedimentation treatment means 24, a return means 32, an extraction means 30, a polymer coagulant addition means 26, and a separated sludge return means 28.

The first transfer means 12 is a means for transferring the cleaning wastewater generated during the water purification process to the wastewater reservoir 11.

The second transfer means 14 is a means for transferring the drinking water sludge generated during the water purification process to the wastewater tank 13.

The third transfer means 16 is a means for transferring the supernatant water of the sludge pond to the drainage pond 11.

The fourth transfer means 18 is a means for transferring the supernatant water from the thickening tank 15, which thickens the wastewater tank sludge produced in the wastewater tank 13, to the wastewater tank 11.

The fifth transfer means 20 is a means for transferring the dehydrated filtrate produced in the dehydration equipment 17, which dehydrates the concentrated sludge produced in the concentration tank 15, to the wastewater tank 11.

The purpose of the drainage pond 11 is not to separate solids and liquids, but to equalize the water volume and water quality, so as described above, it has agitation means 22, and during normal operation, the drainage pond sludge is not removed.

The agitation means 22 is a means for agitating the inside of the drainage pond 11 to obtain treated water. In FIG. 16, the agitation means 22 is an air agitation device that sends agitation air to the drainage pond 11, but is not limited to this. For example, an agitation device such as mechanical agitation or pump circulation agitation can also be used. Specific examples are as described in FIG. 4, FIG. 5, FIG. 6, and FIG. 7, and their description will be omitted here.

The coagulation and sedimentation treatment means 24 is a means for carrying out coagulation and sedimentation treatment on the water to be treated, and obtaining coagulation and sedimentation treated water and coagulation and sedimentation sludge. As described in the section <Method for treating wastewater generated in water purification treatment>, a coagulation and sedimentation treatment system having an upflow coagulation and sedimentation tank (see FIG. 8) or a coagulation and sedimentation treatment system having a high-speed granulation and sedimentation device as a coagulation and sedimentation tank (see FIG. 9) can be appropriately used, but is not limited to these.

The return means 32 is a means for returning the coagulation-sedimentation treated water to the upstream equipment of the water purification process. For example, it may be a pipe for returning the coagulation-sedimentation treated water obtained by the coagulation-sedimentation treatment means 24 to the upstream process of the water purification process, such as the receiving well 150, or a pump for transporting the liquid in the pipe (not shown).

Note that the upstream equipment in the water purification process refers to the equipment preceding the coagulation mixing basin 152 (see Figures 18 and 19) where inorganic coagulants are added in the water purification process.

The extraction means 30 is a means for extracting the coagulated sedimentation sludge from the coagulation sedimentation treatment means 24, and can be, for example, a return pipe with both ends attached to the coagulation sedimentation tank, with a return pump as power to extract the coagulated sedimentation sludge from one end and circulate the coagulated sedimentation sludge extracted from the other end back to the coagulation sedimentation tank.

The polymer flocculant adding means 26 is a means for obtaining separated sludge by adding a polymer flocculant to at least a portion of the coagulated and settled sludge extracted by the extraction means 30. The polymer flocculant adding means 26 is, for example, a line mixer or a mixing tank provided at the suction section or discharge section of the return pump or in the middle of the return piping. The mixing tank provided in the middle of the return piping can be mechanically mixed or mixed by the water flow of the returned sludge.

The separated sludge return means 28 is a means for returning the separated sludge to the coagulation and sedimentation treatment means 24. The separated sludge return means 28 is, for example, a return pipe that circulates the coagulation and sedimentation sludge extracted from one end to the coagulation and sedimentation tank from the other end. In other words, when a return pipe is used, the return pipe is both the extraction means 30 and the separated sludge return means 28.

According to the wastewater treatment device 10 of the present invention, when a large amount of cleaning wastewater is temporarily generated due to cleaning of the water purification equipment, solid-liquid separation in the wastewater pond is insufficient, which causes fluctuations and deterioration in the water quality of the returned water. However, by transferring the cleaning wastewater to the wastewater pond where it is stirred, the quality and amount of the water to be treated in the wastewater pond are homogenized. Then, by subjecting the water to be treated whose water quality has been homogenized in this way to coagulation and sedimentation treatment, it is possible to remove turbidity derived from the raw water of the water supply more effectively than with conventional flotation treatment, and this, combined with the effect of stabilizing the water quality of the water to be treated, allows the quality of the coagulation and sedimentation treated water to be returned to be stably improved.

Furthermore, the washing wastewater, the supernatant water from the sludge tank, the supernatant water from the thickening tank and the dewatered filtrate are stirred and homogenized by the stirring means 22 inside the wastewater tank 11 to become the water to be treated, and this water to be treated is subjected to coagulation and sedimentation treatment, which further removes SS and turbidity components from the return water, thereby reducing the SS load and turbidity load in the water purification process.

In addition, a polymer flocculant is added to at least a portion of the coagulated sedimentation sludge extracted from the coagulation sedimentation means 24 to form separated sludge, and this separated sludge is returned to the coagulation sedimentation means 24, improving the settling properties of the coagulated flocs formed in the coagulation sedimentation process. As a result, the quality of the return water returned to the upstream equipment of the water purification process, such as the receiving well 150, is further improved.

The coagulation and sedimentation sludge (first coagulation and sedimentation sludge and second coagulation and sedimentation sludge) obtained by the coagulation and sedimentation treatment means 24 is sent to the drinking water sludge treatment flow.

As above, the treatment device for wastewater generated by the water purification process of the present invention has been described with reference to FIG. 16. However, as a reference example not according to the present invention, there is a case where only the wash wastewater is transferred to the wastewater reservoir 11. In this case, only the wash wastewater is stirred and mixed by the stirring means 22 inside the wastewater reservoir 11 to become the water to be treated, and this water to be treated is supplied to the coagulation sedimentation treatment means 24 to obtain coagulation sedimentation treated water (return water). In this way, by selectively supplying the wash wastewater with a large wastewater volume or SS load to the coagulation sedimentation treatment among the wastewater to be treated, the wastewater volume and SS can be efficiently reduced from the return water returned to the upstream equipment of the water purification process.

Furthermore, a treatment device (reference example) for wastewater generated in a water purification process is described with reference to FIG. 17, which removes the flow of extracting a portion of the coagulation-sedimentation sludge that has been subjected to solid-liquid separation in the coagulation-sedimentation treatment means 24 from the treatment device for wastewater generated in the water purification process of the present invention shown in FIG. 16, adding a polymer coagulant to obtain separated sludge, and returning this separated sludge to the coagulation-sedimentation treatment means 24.

Figure 17 is a schematic diagram showing a reference example of a treatment device for wastewater generated during water purification treatment that does not conform to the present invention and returns separated sludge.

As shown in the figure, the wastewater treatment device 200 generated in the water purification process of this reference example does not have the extraction means 30, polymer flocculant addition means 26, or separated sludge return means 28, as compared to the case of FIG. 16.

However, even in such a case, the treated water, the quality and quantity of which have been homogenized, is supplied to the coagulation and sedimentation treatment means 24, so that the quality of the return water (coagulation treatment sedimentation water) that is supplied to the first stage of the water purification process after the coagulation and sedimentation treatment can be stably improved.

The present invention will be explained in more detail below with reference to the following examples.

1. Reference Example A water treatment test was conducted using a wastewater treatment device (see FIG. 9 ) equipped with a mixing tank and a high-speed granulation and settling device, using sand filtration washing wastewater (pH 6.8, turbidity 60 degrees, SS 80 mg/L, M-alkalinity 30 mg/L) discharged from a rapid filtration basin of a rapid filtration facility in a water purification facility that uses river water as the raw water for the water supply.

Table 1 shows the specifications and test conditions for the mixing tank and high-speed granulation and precipitation equipment. For small-scale testing, a complete mixing type mixing tank was used, which allows easy setting of test conditions.

Polyaluminum chloride (PAC) (10% purity as alumina oxide) was added as an inorganic coagulant to the raw water at an injection rate of 150 mg/L, and sodium hydroxide was added appropriately to adjust the pH to 6.5-7.0 in the mixing tank. After that, a polymer coagulant (anionic, Evergloss WA521, manufactured by Suing Co., Ltd.) was added to a portion of the coagulated and settled sludge extracted from the high-speed granulation and settling device to form separated sludge. The separated sludge and raw water containing PAC were supplied to the bottom of the high-speed granulation and settling device via the raw water piping, and coagulated and settled at a water surface load of 100-200 mm/min. The polymer coagulant addition rate to the returned separated sludge was 0.38-2.7 wt% relative to the SS, based on the SS weight of the separated sludge.

For comparison, a test was also conducted in which no polymer flocculant was added to the separated sludge, and the polymer flocculant was injected into the raw water piping connected to the bottom of the high-speed granulation settling device. The polymer flocculant injection rate in this case was 0 to 3.0 mg/L. The results are shown in Table 2.

Table 2 also shows the test results when a polymer flocculant was added to the raw water piping of the high-speed granulation and settling device without returning the separated sludge.

When coagulation and sedimentation was performed using PAC alone without adding polymer coagulants and without returning separated sludge to which polymer coagulants had been added, with a water surface load of 100 mm/min, the SS concentration of the treated water was 42 to 44 mg/L (No. 1-1 to 1-2).

When polymer coagulant was added to the raw water without adding PAC and without returning the separated sludge to which polymer coagulant had been added, and coagulation and sedimentation treatment was performed at a water surface load of 100 mm/min, the SS concentration in the treated water was 33 to 35 mg/L (No. 1-3 to 1-4).

When the separated sludge to which polymer coagulant had been added was not returned and a polymer coagulant was added to the raw water coagulated by PAC and coagulation and sedimentation treatment was performed at a water surface load of 100 mm/min, the SS concentration of the treated water was 10 to 15 mg/L (No. 1-5 to No. 1-7).

When separated sludge to which polymer flocculant was added at a rate of 0.2-0.8 wt% relative to SS was returned and subjected to coagulation and sedimentation treatment with an SS concentration of 90 mg/L in the coagulation section and a water surface load of 100 mm/min, the SS concentration of the treated water was 2.5-3.4 mg/L (No. 1-8 to No. 1-11). At a water surface load of 150 mm/min, the SS concentration of the treated water was 4.3-4.8 mg/L (No. 1-12 to No. 1-14). At a water surface load of 200 mm/min, the SS concentration of the treated water was 4.9-5.3 mg/L (No. 1-15 to No. 1-17).

If untreated washing wastewater is returned as return water without going through the coagulation and sedimentation treatment process of the present invention, the washing wastewater with a turbidity of 60 degrees and SS of 80 mg/L will be mixed with raw water in the water purification process.

When raw water with SS 80 mg/L was coagulated with PAC 150 mg/L, and the separated sludge to which polymer coagulant was added at 2.0% relative to SS was returned and subjected to coagulation and sedimentation treatment at a water surface load of 100 mm/min, the SS of the treated water was significantly reduced to 2.8 mg/L (No. 1-10).

2. Comparative Example 1
A water treatment test was carried out on the raw water of the reference example using a wastewater treatment device equipped with a mixing tank, a coagulation tank, and a pressurized flotation treatment device. Table 3 shows the specifications and test conditions of the complete mixing type mixing tank and the pressurized flotation treatment device.

PAC was added to the raw water at an injection rate of 150 mg/L, and the pH was adjusted to 6.5-7.0 in the mixing tank with sodium hydroxide. The same polymer flocculant as in the reference example was then added to the coagulation tank and the water was allowed to sit for 10 minutes under the slow stirring conditions described above to obtain coagulated water. Pressurized water was added to this coagulated water at a pressurized water ratio of 50% to conduct a coagulation pressurized flotation treatment test. A pressurized flotation treatment was also conducted in the same way, without coagulation using PAC or polymer flocculant before the pressurized flotation treatment.

Pressurized water was prepared by using treated water No. 1-10 in Table 2 and saturating the treated water with pressurized air at 0.3 MPa. Pressurized water was prepared by mixing 50% of the raw water volume with the raw water at the inlet of the flotation device. The results are shown in Table 4.

As in the Reference Example, 150 mg/L of PAC was added to the mixing tank, and 3.3 wt% SS (3.0 mg/L based on the raw water volume) was added to the froth obtained by the coagulation and pressure flotation treatment, based on the SS weight of the froth, and the froth was returned so that the SS concentration in the coagulation tank was 90 mg/L, and the coagulation and pressure flotation treatment was performed in the same manner. The SS concentration of the treated water from the coagulation and pressure flotation treatment was 22 mg/L, which was significantly lower than the pressure flotation treatment without chemical injection, but the SS concentration of the treated water could not be reduced compared to the SS concentration of 2.5 mg/L of No. 1-11 in Table 2 of the Reference Example. The coagulation and sedimentation treatment of the Reference Example produced clearer treated water than the coagulation and pressure flotation treatment of Comparative Example 1.

3. Example 1
Four types of wastewater, namely, sand filtration washing wastewater, sludge pond supernatant water, thickening tank supernatant water and dewatered filtrate, were mixed at the respective wastewater volume ratios (see Table 5) to prepare raw water. Tests were conducted using the test equipment and test conditions in Table 1, as in the Reference Example, with a PAC (purity 10%) injection rate of 200 mg/L.

The water quality of the above four types of wastewater is shown in Table 5.

The raw water had a pH of 6.8, turbidity of 63, SS of 82 mg/L, M-alkalinity of 31 mg/L, and BOD of 28 mg/L. The turbidity, SS, and M-alkalinity were calculated as follows:

SS; 80×1+100×0.05+100×0.05+75×0.01=90.75÷1.11=82mg/L
M-alkalinity; 30 x 1 + 35 x 0.05 + 40 x 0.05 + 35 x 0.01 = 34 ÷ 1.11 = 31 mg/L
Turbidity: 60 x 1 + 85 x 0.05 + 90 x 0.05 + 80 x 0.01 = 69.6 ÷ 1.11 = 63 degrees. The polymer flocculant addition rate is based on the SS weight of the sludge and is 0.3 to 0. The results are shown in Table 6.

When the polymer flocculant was added directly to the raw water piping instead of returning the separated sludge to which the polymer flocculant had been added, and coagulation and sedimentation treatment was performed at a water surface load of 100 mm/min, the SS concentration of the treated water was 13 to 18 mg/L (No. 2-1 to No. 2-3).

The separated sludge to which polymer flocculant was added at a rate of 0.2-0.8 wt% relative to SS was returned, and the SS concentration in the flocculation section was 200 mg/L, and the SS concentration in the treated water was 2.8-3.8 mg/L at a water surface load of 100 mm/min (No. 2-4-No. 2-7). At a water surface load of 150 mm/min, the SS concentration in the treated water was 4.5-4.8 mg/L (No. 2-8-No. 2-10). At a water surface load of 200 mm/min, the SS concentration in the treated water was 6.4-6.8 mg/L (No. 2-11-No. 2-13).

If the four types of mixed wastewater, i.e. untreated sand filtration washing wastewater, sludge tank supernatant water, thickening tank supernatant water, and dewatered filtrate, were returned as return water without going through the coagulation and sedimentation treatment process of the present invention, the mixed wastewater with SS of 82 mg/L would be mixed with raw water in the water purification process. However, by carrying out the coagulation and sedimentation treatment, the SS load in the water purification process is reduced compared to returning the raw water untreated.

When a polymer flocculant was added directly to the raw water after PAC coagulation at an injection rate of 2.0 mg/L and coagulation and sedimentation treatment was performed at a water surface load of 100 mm/min, the treated water SS concentration was 16 mg/L, the turbidity was 17 degrees, and the BOD was 35 mg/L (No. 2-2).

The separated sludge to which polymer flocculant was added at a rate of 0.4 wt% relative to SS was returned, and the SS concentration in the flocculation section was 200 mg/L, the SS concentration in the treated water at a water surface load of 200 mm/min was 6.6 mg/L, the turbidity was 6.8 degrees, and the BOD was 24 mg/L (No. 2-12).

4. Example 2
Tests were conducted on the biofilm filtration devices and test conditions shown in Table 7 for the multiple coagulation-sedimentation treated waters of Example 1 shown below.

The raw water for the biofilm filtration was the coagulation sedimentation treated water (hereinafter referred to as raw water A) of test number 2-2 in Table 6, in which no polymer coagulant was added to the separated sludge, and the water quality was SS concentration 18 mg/L, turbidity 17 degrees, BOD 35 mg/L, soluble manganese concentration 2.3 mg/L, and ammonia nitrogen 1.2 mg/L.

The quality of the coagulation sedimentation treated water (hereinafter referred to as raw water B) of test number 2-12 in Table 8, which was returned after adding a polymer coagulant, was SS concentration 6.6 mg/L, turbidity 6.8, BOD 24 mg/L, soluble manganese concentration 2.3 mg/L, and ammonia nitrogen 1.2 mg/L. The quality of the biofilm treated water (biologically treated water) obtained by subjecting raw water A and raw water B to biological treatment is shown in Table 8 below.

Ten days after the start of biofilm filtration treatment, the biofilm-treated water from both raw water A and raw water B had insufficient soluble manganese concentration and ammoniacal nitrogen removal. This is thought to be the acclimation stage.

20 days after the start of biofilm filtration treatment, the quality of the biofilm-treated water from raw water A was SS concentration 2.3 mg/L, turbidity 3.0, BOD 5 mg/L or less, soluble manganese concentration 1.5 mg/L, and ammonia nitrogen 0.8 mg/L. Because the polymer coagulant was added directly to the raw water after coagulation in the coagulation sedimentation treatment, excess polymer coagulant remained in the raw water for the biofilm filtration treatment, which is the coagulation sedimentation treated water, and it is believed that there was contamination by the polymer coagulant in the packed bed of the biofilm filtration.

On the other hand, 20 days after the start of the biofilm filtration process, the quality of the biofilm-treated water from raw water B was SS concentration 1.5 mg/L, turbidity 1.0, BOD 5 mg/L, soluble manganese concentration 0.2 mg/L, and ammonia nitrogen 0.1 mg/L. By adding separated sludge to which polymer coagulant had been added in advance to the raw water piping, excess polymer coagulant was eliminated, and there was no residual polymer coagulant in the raw water for the biofilm filtration process, allowing for good biofilm filtration.

By using biological membrane filtration, it was possible to further remove the SS and turbidity from the return water, and also to remove soluble manganese and ammoniacal nitrogen, which consume chlorine in the water purification process.

Claims (4)

A first transfer step of transferring cleaning wastewater generated by the water purification treatment to a wastewater pond;
A second transfer step of transferring the drinking water sludge generated by the water purification process to a wastewater basin;
A third transfer step of transferring supernatant water of the sludge basin to the wastewater basin;
A fourth transfer step of transferring supernatant water from a thickening tank that thickens the wastewater basin sludge generated in the wastewater basin to the wastewater basin;
A fifth transfer step of transferring a dehydrated filtrate generated in a dehydration facility that dehydrates the concentrated sludge generated in the concentration tank to the drainage pond;
A stirring step of stirring the inside of the drainage pond to obtain water to be treated;
a coagulation-sedimentation treatment step of subjecting the water to coagulation-sedimentation treatment to obtain coagulation-sedimentation treated water and coagulation-sedimentation sludge;
A returning step of returning the coagulation-sedimentation treated water to a previous step of the water purification treatment;
A method for treating wastewater generated during water purification, comprising: Between the coagulation sedimentation treatment step and the return step, a biological treatment step is provided in which the coagulation sedimentation treated water obtained in the coagulation sedimentation treatment step is biologically treated to obtain biologically treated water,
2. The method for treating wastewater generated in a water purification process according to claim 1, characterized in that, in the return process, biologically treated water obtained in the biological treatment is returned to a previous process of the water purification process instead of the coagulation and sedimentation treated water. a separated sludge obtaining step of adding a polymer flocculant to at least a part of the flocculated sludge obtained in the flocculating sedimentation treatment step to obtain separated sludge;
a separated sludge returning step of returning the separated sludge to the coagulation sedimentation treatment step;
3. The method for treating wastewater generated in the water purification process according to claim 1 or 2, further comprising the steps of: A first transfer means for transferring washing wastewater generated in the water purification process to a wastewater reservoir;
A second transfer means for transferring the drinking water sludge generated by the water purification process to the wastewater basin;
A third transfer means for transferring the supernatant water of the sludge basin to the wastewater basin;
A fourth transfer means for transferring supernatant water of a thickening tank for thickening the wastewater basin sludge generated in the wastewater basin to the wastewater basin;
A fifth transfer means for transferring a dehydrated filtrate generated in a dehydration facility for dehydrating the concentrated sludge generated in the concentration tank to the wastewater reservoir;
A stirring means for stirring the inside of the drainage pond to obtain water to be treated;
a coagulation-sedimentation treatment means for performing a coagulation-sedimentation treatment on the water to be treated to obtain coagulation-sedimentation treated water and coagulation-sedimentation sludge;
A return means for returning the coagulation-sedimentation treated water to a upstream facility of the water purification treatment;
an extracting means for extracting the coagulating and settling sludge from the coagulating and settling treatment means;
a polymer flocculant adding means for adding a polymer flocculant to at least a portion of the coagulated and settled sludge extracted by the extraction means to obtain separated sludge;
A separated sludge returning means for returning the separated sludge to the coagulation sedimentation treatment means;
A treatment device for wastewater generated in a water purification process, comprising:

Images