Water treatment method and water treatment apparatus

Abstract

To provide a water treatment method capable of performing advanced treatment while reducing power consumption.SOLUTION: A water treatment method includes a biological treatment process in which organic matter in organic wastewater is decomposed and ammoniacal nitrogen is treated with nitrogen by an oxygen permeable membrane 11 installed in a sedimentation tank 10, and a sludge collection process in which sludge that has settled at the bottom of sedimentation tank 10 is collected.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a water treatment method and a water treatment apparatus.

Background Art

[0002] Wastewater discharged from homes, factories, etc. is mainly treated by the activated sludge method. Organic substances contained in the wastewater are oxidized and decomposed in a biological reaction tank. Ammonia nitrogen contained in the wastewater is generally nitrified in a nitrification tank in the latter stage of the biological reaction tank after the adsorption and decomposition of organic substances in the former stage of the biological reaction tank.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the case of advanced treatment for nitrification of ammonia nitrogen (i.e., a treatment for converting ammonia nitrogen to nitrate nitrogen by the action of nitrifying bacteria) and denitrification (i.e., a treatment for reducing nitrate nitrogen to nitrogen gas by the action of denitrifying bacteria), in the conventional technology, the required capacity of the reaction tank and the amount of supplied air are about twice those in the case of performing only BOD (biochemical oxygen demand) removal. Therefore, due to the restrictions on the site and the increase in power consumption, the spread of advanced treatment has been delayed. Especially recently, reduction of power consumption has been strongly demanded from the perspective of carbon neutrality, and customers are demanding to perform advanced treatment while simultaneously reducing power consumption.

[0005] Therefore, an object of the present invention is to provide a water treatment method and a water treatment apparatus capable of performing advanced treatment while reducing power consumption.

Means for Solving the Problems

[0006] In one aspect, there is provided a water treatment method including a biological treatment step of decomposing organic substances in organic wastewater and nitrifying ammonia nitrogen by means of an oxygen permeable membrane installed in a sedimentation tank having at least a sludge collection section, and a sludge collection step of collecting sludge sedimented at the bottom of the sedimentation tank.

[0007] In one aspect, the sedimentation tank corresponds to a primary sedimentation tank into which the organic wastewater flows. A stirrer is installed in the primary sedimentation tank, and at least a part of the returned sludge is returned to the primary sedimentation tank to cause the primary sedimentation tank to function as an anoxic tank in which activated sludge is present. In one aspect, the amount of the sludge returned to the primary sedimentation tank is 30% to 100% of the amount of the inflowing organic wastewater.

[0008] In one aspect, there is provided a water treatment apparatus including an oxygen permeable membrane having a biological membrane attached to the surface thereof for decomposing organic substances in organic wastewater and nitrifying ammonia nitrogen, the oxygen permeable membrane being installed in a sedimentation tank having at least a sludge collection section, and a sludge scraping machine installed at the bottom of the sedimentation tank.

[0009] In one aspect, the sedimentation tank corresponds to a primary sedimentation tank into which the organic wastewater flows. The water treatment apparatus includes a stirrer installed in the primary sedimentation tank, and at least a part of the returned sludge is returned to the primary sedimentation tank to cause the primary sedimentation tank to function as an anoxic tank in which activated sludge is present.

Effects of the Invention

[0010] By installing an oxygen permeable membrane in the sedimentation tank, advanced treatment can be performed while reducing power consumption.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a water treatment apparatus. As shown in FIG. 1, the water treatment apparatus 1 includes a primary sedimentation tank facility 2 that decomposes organic substances in organic wastewater and nitrifies ammonia nitrogen, a biological reaction tank facility 3 that biologically treats the treated water treated in the primary sedimentation tank facility 2, and a final sedimentation tank facility 4 that finally treats the treated water treated in the biological reaction tank facility 3. The water treatment apparatus 1 including these plurality of facilities 2, 3, and 4 may be referred to as a water treatment system.

[0013] The primary sedimentation tank facility 2 includes a primary sedimentation tank 10 that stores the inflowing organic wastewater, an oxygen permeable membrane 11 installed in the primary sedimentation tank 10, a blower 12 that is connected to the oxygen permeable membrane 11 and supplies air (or pure oxygen) to the oxygen permeable membrane 11, and a reciprocating sludge scraper 15 disposed at the bottom of the primary sedimentation tank 10. The oxygen permeable membrane is a membrane that allows oxygen in the gas existing inside the membrane to permeate through the membrane in a molecular state and supply it to the outside liquid. In recent years, it has been adopted in water treatment as MABR (Membrane Aerated Bio-Reactor). The primary sedimentation tank facility 2 including the oxygen permeable membrane 11 may be referred to as a primary sedimentation tank with a biological treatment function, in other words.

[0014] In the embodiment shown in FIG. 1, the water treatment apparatus 1 includes a plurality of oxygen permeable membranes 11, but may include at least one oxygen permeable membrane 11. As an example of the organic wastewater applicable to the water treatment apparatus 1, wastewater (domestic wastewater, general sewage, etc.) containing biodegradable organic components and Kjeldahl nitrogen can be mentioned.

[0015] When the primary sedimentation tank 10 is divided into an inflow side area where organic wastewater flows in and an outflow side area where organic wastewater flows out, the oxygen permeable membrane 11 is arranged in the outflow side area of the primary sedimentation tank 10. The oxygen permeable membrane 11 has excellent oxygen transfer efficiency and has the property of preferentially growing nitrifying bacteria on the surface of its gas separation membrane. By installing the oxygen permeable membrane 11 in the primary sedimentation tank 10, it is possible to improve the treatment capacity of organic matter in the organic wastewater and at the same time achieve energy savings.

[0016] Figure 2 is a diagram showing a part of the oxygen permeable membrane. The oxygen permeable membrane (more specifically, the biofilm-retaining membrane oxygen supply device) 11 is a device that can attach a biofilm to a bundle of hollow fiber membranes (that is, gas separation membranes) that selectively permeate oxygen and remove substances such as organic matter and nitrogen. The oxygen permeable membrane 11 has stable and high advanced treatment (nitrification and denitrification) performance. More specifically, since the oxygen permeable membrane 11 can retain a biofilm on the surface of the gas separation membrane, a stable biological reaction can be carried out.

[0017] The oxygen permeable membrane 11 enables the movement of air (or pure oxygen) supplied from the blower 12 (see Figure 1) to the gas separation membrane due to the oxygen concentration gradient. Therefore, the power consumption of the blower 12 can be suppressed. As a result, the oxygen permeable membrane 11 can have a smaller air supply pressure than aeration by a general membrane diffuser device. As a result, overall energy savings of the water treatment device 1 (30% - 50% compared to the conventional technology) can be achieved.

[0018] In the conventional technology, circulation of the nitrification liquid is required for denitrification, and it is necessary to install a circulation pump for circulating the nitrification liquid. As a result, power is also required to operate the circulation pump. The oxygen permeable membrane 11 (more specifically, the biofilm growing on the surface of the oxygen permeable membrane 11) can grow a large number of nitrifying bacteria near the surface of the gas separation membrane and a large number of denitrifying bacteria at a position away from the gas separation membrane.

[0019] When the oxygen permeable membrane 11 is used in the primary sedimentation tank 10, the biofilm attached to the surface of the gas separation membrane can perform nitrification at a position close to the surface of the gas separation membrane and denitrification at a position away from the surface of the gas separation membrane. Thus, the oxygen permeable membrane 11 enables efficient advanced treatment, does not require circulation of the nitrified liquid, and eliminates the need for a circulation pump.

[0020] Most existing sewage treatment plants were designed and constructed mainly for BOD removal due to historical circumstances. The conversion of existing sewage treatment plants to advanced treatment facilities that achieve nitrification and denitrification is not easy in terms of space and power consumption. On the other hand, in existing treatment facilities, there are many plants that operate with the primary sedimentation tank partially suspended, and there are situations where the primary sedimentation tank is not fully utilized. This is because, for the reasons listed below, in order to reduce the removal rate of organic matter in the primary sedimentation tank, not all of the existing sedimentation tanks are used, but rather the sedimentation tank is partially used. Reason 1: There is a margin in the designed surface area load of the primary sedimentation tank. Reason 2: The actual inflow water volume does not reach the designed water volume. Reason 3: Organic matter is required when performing biological denitrification in the reaction tank.

[0021] According to the present embodiment, by modifying a part of the primary sedimentation tank 10 that can be operated in an existing treatment facility and installing the oxygen permeable membrane 11 in the existing primary sedimentation tank 10, advanced treatment can be performed while taking advantage of the function of the primary sedimentation tank 10. Note that the oxygen permeable membrane 11 is applicable not only to the existing primary sedimentation tank 10 but also to a newly installed primary sedimentation tank 10.

[0022] Patent Document 1 discloses a configuration in which an oxygen permeable membrane is installed in an anaerobic treatment reaction tank, and after treating the wastewater, the treated treated water is allowed to flow into an aerobic treatment tank. However, Patent Document 1 does not disclose a configuration in which the oxygen permeable membrane 11 is installed in the primary sedimentation tank 10 as in the water treatment apparatus 1. Therefore, the water treatment apparatus 1 according to the present embodiment has a configuration different from the configuration disclosed in Patent Document 1.

[0023] In large city sewage treatment plants, due to historical reasons, combined sewage is often treated. During rainy days, the inflow water (sewage) exceeding the maximum daily flow rate on sunny days: Qsh is only sedimented in the primary sedimentation tank and then discharged. Therefore, the water quality of the discharged water during rainy days deteriorates. Thus, improvement of the treated water quality of combined sewage has been desired. Even in existing standard activated sludge process treatment plants, depending on the water quality conditions of the discharge destination, the allowable value of ammonia nitrogen may be low. In this case, nitrification of the treated water is required. In the treatment of a small amount of sewage (tens of m 3 / day or less, etc.), when the sedimented sewage is discharged, since the sewage has not undergone biological treatment, the sewage will be discharged without removing pollutants (BOD and ammonia nitrogen), leading to environmental deterioration.

[0024] By installing the oxygen permeable membrane 11 in the primary sedimentation tank 10, in a combined sewage treatment plant, it is possible to improve the water quality of the discharged sewage during rainy days, improve the water quality of the sedimented treated water, and remove a certain amount of BOD and ammonia nitrogen.

[0025] Returning to FIG. 1, the oxygen permeable membrane 11 disposed in the primary sedimentation tank 10 biologically treats the organic sewage separated by solid-liquid separation in the primary sedimentation tank 10 to promote advanced treatment. The process of highly treating the organic matter in the organic sewage by the oxygen permeable membrane 11 is called the biological treatment process.

[0026] FIG. 3(a) and FIG. 3(b) are enlarged views of the primary sedimentation tank facilities. The solid-liquid separated sludge settles to the bottom of the primary sedimentation tank 10. The bottom of the primary sedimentation tank 10 is inclined. The sludge scraping machine 15 is configured to collect the sedimented and separated sludge into the sludge pit 10a disposed in the inflow side area of the primary sedimentation tank 10 by its reciprocating motion. The process of collecting the sludge that has settled to the bottom of the primary sedimentation tank 10 is called the sludge collection process.

[0027] The sloping bottom and the sludge pit 10a constitute the sludge collection section of the primary sedimentation tank 10. The sludge collected in the sludge pit 10a is sent as primary sedimentation sludge to the sludge treatment facility 40. A withdrawal pipe (not shown) for withdrawing the sludge collected in the sludge pit 10a and a pump (not shown) for transporting the sludge through the withdrawal pipe to the sludge treatment facility 40 constitute the sludge discharge section of the primary sedimentation tank 10.

[0028] The treated water partially nitrified and denitrified by the oxygen permeable membrane 11 flows into the biological reaction tank facility 3 through the overflow weir 10b arranged in the outflow side area of the primary sedimentation tank 10. The biological reaction tank facility 3 is a facility for further advanced treatment of the treated water. Before flowing into the biological reaction tank facility 3, by partially nitrifying and denitrifying the organic wastewater in the primary sedimentation tank facility 2, the load of biological treatment in the biological reaction tank facility 3 can be reduced.

[0029] The biological reaction tank facility 3 includes a biological reaction tank 20 for storing the treated water treated by the primary sedimentation tank facility 2. The biological reaction tank 20 is provided with a partition wall 20a that divides its interior into an anoxic tank 26 and an aerobic tank 27.

[0030] The anoxic tank 26 is arranged in the inflow side area of the biological reaction tank 20, and the aerobic tank 27 is arranged in the outflow side area of the biological reaction tank 20. The treated water treated by the primary sedimentation tank facility 2 first flows into the anoxic tank 26 of the biological reaction tank 20. The biological reaction tank facility 3 is provided with a stirrer 23 arranged in the anoxic tank 26. The treated water flowing into the anoxic tank 26 flows into the aerobic tank 27 through the lower part of the partition wall 20a while being stirred by the stirrer 23. The biological reaction tank facility 3 may further be provided with an oxygen permeable membrane 21 arranged in the anoxic tank 26 of the biological reaction tank 20 as required.

[0031] The biological reaction tank facility 3 includes an aeration device 25 disposed at the lower part of the aerobic tank 27 and a blower 22 that supplies gas to the aeration device 25. The aeration device 25 is configured to supply an oxygen-containing gas into the aerobic tank 27 by the operation of the blower 22. The treated water aerated in the aerobic tank 27 flows into the final sedimentation tank facility 4.

[0032] Figures 4(a) and 4(b) are enlarged views of the final sedimentation tank facility. The final sedimentation tank facility 4 includes a final sedimentation tank 30 and a reciprocating sludge scraping machine 35 disposed at the bottom of the final sedimentation tank 30.

[0033] The bottom of the final sedimentation tank 30 is inclined, similar to the primary sedimentation tank 10. The sludge scraping machine 35 is configured to collect the sedimented and separated sludge into a sludge pit 30a disposed in the inflow side area of the final sedimentation tank 30 by its reciprocating operation. The process of collecting the sludge sedimented at the bottom of the final sedimentation tank 30 is called the sludge collection process.

[0034] A part of the sludge collected in the sludge pit 30a is sent to the sludge treatment facility 40 as excess sludge. The final sedimentation tank 30 may include a sludge collection section for collecting sludge and a sludge discharge section for discharging sludge, similar to the primary sedimentation tank 10. If the sludge separated in the primary sedimentation tank facility 2 is drawn out from the primary sedimentation tank 10, it becomes unnecessary to draw out the sludge from the final sedimentation tank 30.

[0035] As shown in FIG. 1, the final sedimentation tank facility 4 may include an oxygen permeable membrane 31 installed in the final sedimentation tank 30 and a blower 32 connected to the oxygen permeable membrane 31 and supplying air (or pure oxygen) to the oxygen permeable membrane 31 for performing advanced treatment of the treated water. These oxygen permeable membrane 31 and blower 32 are arranged as required. The final sedimentation tank facility 4 equipped with the oxygen permeable membrane 31 may be called a final sedimentation tank with biological treatment function, in other words.

[0036] The final sedimentation tank facility 4 is a facility for solid-liquid separation of the treated water treated in the biological reaction tank facility 3. Although not shown, the final sedimentation tank facility 4 may include a chain disposed in the final sedimentation tank 30, a flight plate driven by the chain and scraping the sludge in the final sedimentation tank 30, and a driving device for driving the chain.

[0037] The treated water solid-liquid separated by the final sedimentation tank facility 4 is sent to the subsequent facilities (such as chlorine mixing facilities) of the water treatment device 1 through the overflow weir 30b disposed in the outflow side area of the final sedimentation tank 30. Most of the sludge withdrawn from the final sedimentation tank 30 (excluding excess sludge) is returned to the biological reaction tank facility 3 as return sludge.

[0038] In one embodiment, part or all of the return sludge may be returned to the primary sedimentation tank facility 2. It is desirable that the amount of return sludge to the primary sedimentation tank 10 is 30% to 100% of the organic drainage volume flowing into the water treatment device 1. By returning such an amount of sludge to the primary sedimentation tank facility 2, the function of the oxygen permeable membrane 11 can be fully exerted.

[0039] When the primary sedimentation tank 10 is used as an anaerobic tank with the presence of activated sludge by receiving the return sludge, in principle, solid-liquid separation is not performed. In this case, the primary sedimentation tank facility 2 is provided with a stirrer 13 (see FIGS. 1 and 3). The return sludge becomes a mixed liquid with the raw water (organic drainage) in the primary sedimentation tank 10 as activated sludge, and the mixed liquid overflows the overflow weir 10b and flows into the biological reaction tank 20. Even in this case, the sludge settling at the bottom of the primary sedimentation tank 10 is periodically collected in the sludge pit 10a by the sludge scraping machine 15 and sent to the sludge treatment facility 40 periodically.

[0040] In the above-described embodiment, each of the stirrers 13 and 23 is a submersible stirrer, but the type of the stirrer is not limited to the submersible stirrer. In one embodiment, each of the stirrers 13 and 23 may be a vertical stirrer. From the viewpoints of the installation space and the horizontal fluidization of the sludge in the tank, it is preferable that each of the stirrers 13 and 23 is a submersible stirrer.

[0041] The above-described embodiments are described for the purpose of enabling those having ordinary knowledge in the technical field to which the present invention pertains to implement the present invention. Various modifications of the above embodiments can be naturally made by those skilled in the art, and the technical idea of the present invention can also be applied to other embodiments. Therefore, the present invention is not limited to the described embodiments, but is construed in the broadest scope in accordance with the technical idea defined by the claims.

Explanation of Reference Numerals

[0042] 1 Water treatment apparatus 2 Primary sedimentation tank facility 3 Biological reaction tank facility 4 Final sedimentation tank facility 10 Primary sedimentation tank 10a Sludge pit 10b Overflow weir 11 Oxygen permeable membrane 12 Blower 13 Agitator 15 Sludge scraper 20 Biological reaction tank 20a Partition wall 21 Oxygen permeable membrane 22 Blower 23 Agitator 25 Aeration device 26 Anaerobic tank 27 Aerobic tank 30 Final sedimentation tank 30a Sludge pit 30b Overflow weir 31 Oxygen permeable membrane 32 Blower 35 Sludge scraper 40 Sludge treatment facility

Claims

1. A water treatment method, comprising: a biological treatment step of decomposing organic substances in organic wastewater and nitrifying ammonia nitrogen by an oxygen permeable membrane installed in a sedimentation tank having at least a sludge collection section; a sludge collection step of collecting sludge sedimented at the bottom of the sedimentation tank; the sedimentation tank corresponds to a primary sedimentation tank into which the organic wastewater flows; A water treatment method, wherein a stirrer is installed in the primary sedimentation tank, and at least a part of the returned sludge is returned to the primary sedimentation tank to cause the primary sedimentation tank to function as an anaerobic tank in which activated sludge exists.

2. The water treatment method according to claim 1, wherein the amount of the returned sludge to the primary sedimentation tank is 30% to 100% of the inflowing organic wastewater volume.

3. A water treatment apparatus, comprising: an oxygen permeable membrane having a biofilm attached to its surface for decomposing organic substances in organic wastewater and nitrifying ammonia nitrogen, the oxygen permeable membrane being installed in a sedimentation tank having at least a sludge collection section; a sludge scraping machine installed at the bottom of the sedimentation tank; the sedimentation tank corresponds to a primary sedimentation tank into which the organic wastewater flows; the water treatment apparatus includes a stirrer installed in the primary sedimentation tank, and at least a part of the returned sludge is returned to the primary sedimentation tank to cause the primary sedimentation tank to function as an anaerobic tank in which activated sludge exists.

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