Water treatment method and water treatment device

Abstract

One aspect of the present disclosure relates to a water treatment method characterized by comprising: a half-batch treatment step including an inflow step for channeling water to be treated into a reaction tank, a biological treatment step for biologically treating the water to be treated within the reaction tank with biological sludge, a settling step for allowing the biological sludge to settle within the reaction tank, and a discharge step for discharging the biologically treated water within the reaction tank; a first detection step for detecting the sludge interface position or the sludge concentration within the reaction tank during the settling step; and a calculation step for calculating the settling speed of the biological sludge on the basis of the sludge interface position or the sludge concentration detected in the first detection step.

Description

Water treatment method and water treatment apparatus

[0001] This disclosure relates to water treatment methods and water treatment equipment technologies.

[0002] Traditionally, biological wastewater treatment has employed the activated sludge method, which utilizes microbial aggregates called flocs (aerobic biological sludge). However, in the activated sludge method, when separating the flocs (aerobic biological sludge) from the treated water in the sedimentation tank, the slow settling rate of the flocs sometimes necessitates a very large surface area in the sedimentation tank. Furthermore, the treatment rate of the activated sludge method depends on the sludge concentration in the biological treatment tank. While increasing the sludge concentration can increase the treatment rate, this can lead to problems such as bulking in the sedimentation tank, which can cause solid-liquid separation failures and make it impossible to maintain treatment.

[0003] On the other hand, anaerobic biological treatment typically utilizes granules, which are dense aggregates of microorganisms. Granules have a very fast settling rate, and because the microorganisms are densely aggregated, it is possible to increase the sludge concentration in the biological treatment tank, enabling high-speed wastewater treatment. However, anaerobic biological treatment has drawbacks compared to aerobic treatment (activated sludge method), such as being limited to certain types of wastewater that can be treated and requiring the treatment water temperature to be maintained at around 30-35°C. Furthermore, anaerobic biological treatment alone may result in poor water quality, and if the treated water is to be discharged into rivers or other bodies of water, it may be necessary to carry out aerobic treatment such as the activated sludge method separately.

[0004] In recent years, it has become clear that by treating wastewater under special conditions using a semi-batch treatment system that intermittently infuses wastewater into a reaction tank, it is possible to form granulated biological sludge with good settling properties not only from anaerobic biological sludge but also from aerobic biological sludge (see, for example, Patent Documents 1 to 4). The granulated biological sludge has, for example, an average particle size of 0.2 mm or more and a settling velocity of 5 m / h or more. In semi-batch biological treatment, it is common to repeatedly perform the following steps in a single reaction tank: (1) inflow of wastewater, (2) biological treatment of wastewater with biological sludge, (3) settling of the biological sludge, and (4) discharge of treated water.

[0005] Furthermore, Patent Document 5 discloses a biological treatment method that repeatedly performs the following steps: (1) inflow of wastewater and discharge of treated water, (2) biological treatment of wastewater with biological sludge, and (3) sedimentation of the biological sludge. This makes it possible to obtain biological sludge with high sedimentation properties, such as granulated biological sludge.

[0006] International Publication No. 2004 / 024638, Japanese Patent Publication No. 2008-212878, Japanese Patent Publication No. 4975541, Japanese Patent Publication No. 4804888, Japanese Patent Publication No. 2016-77931

[0007] Incidentally, calculating the settling rate of biological sludge in the reaction tank while operating a semi-batch processing system is important for understanding the properties of the biological sludge in the reaction tank. Previously, understanding the properties of biological sludge required complicated procedures, skilled operators, or expensive analytical equipment.

[0008] The purpose of this disclosure is to provide a water treatment method and a water treatment apparatus that can calculate the settling rate of biological sludge in a reaction tank while operating a semi-batch treatment apparatus.

[0009] One aspect of the present disclosure is a water treatment method characterized by comprising: an inflow step of introducing water to be treated into a reaction tank; a biological treatment step of biologically treating the water to be treated in the reaction tank with biological sludge; a sedimentation step of allowing the biological sludge in the reaction tank to settle; and a discharge step of discharging the biologically treated water from the reaction tank; a first detection step of detecting the sludge interface position or sludge concentration in the reaction tank during the sedimentation step; and a calculation step of calculating the sedimentation rate of the biological sludge based on the sludge interface position or sludge concentration detected in the first detection step.

[0010] Furthermore, it is preferable that the water treatment method includes a sludge property determination step in which the properties of the biological sludge are determined based on the settling velocity of the biological sludge calculated in the calculation step.

[0011] Furthermore, the water treatment method preferably includes a second detection step for detecting the sludge concentration in the reaction tank during the biological treatment step, and in the sludge property determination step, it is preferable to determine the properties of the biological sludge based on the settling velocity of the biological sludge and the sludge concentration detected in the second detection step.

[0012] Furthermore, one aspect of the present disclosure is a water treatment apparatus comprising a reaction tank, an inflow step of introducing water to be treated into the reaction tank, a biological treatment step of biologically treating the water to be treated in the reaction tank with biological sludge, a sedimentation step of allowing the biological sludge in the reaction tank to settle, and a discharge step of discharging the biologically treated water from the reaction tank, a first detection means for detecting the sludge interface position or sludge concentration in the reaction tank during the sedimentation step, and a calculation means for calculating the sedimentation rate of the biological sludge based on the sludge interface position or sludge concentration detected by the first detection means.

[0013] Furthermore, it is preferable that the water treatment apparatus has a sludge property determination means for determining the properties of the biological sludge based on the settling velocity of the biological sludge calculated by the calculation means.

[0014] Furthermore, the water treatment apparatus preferably includes a second detection means for detecting the sludge concentration in the reaction tank during the biological treatment process, and the sludge property determination means preferably determines the properties of the biological sludge based on the settling velocity of the biological sludge and the sludge concentration detected by the second detection means.

[0015] According to this disclosure, it is possible to provide a water treatment method and a water treatment apparatus that can calculate the settling velocity of biological sludge in a reaction tank while operating a semi-batch treatment apparatus.

[0016] This is a schematic diagram showing an example of the water treatment apparatus according to this embodiment. This is a schematic diagram showing another example of the water treatment apparatus according to this embodiment. Exponential function (V = V ０ e （－ｋ・Ｘ） These are the settling characteristic curves of granular sludge and activated sludge based on the formula. This figure shows the change in sludge concentration in the reaction tank during the settling process in Examples 1 and 2.

[0017] Embodiments of this disclosure are described below. These embodiments are examples of implementing this disclosure, and this disclosure is not limited to these embodiments.

[0018] <Water Treatment Method and Water Treatment Apparatus> Figure 1 is a schematic diagram showing an example of a water treatment apparatus according to this embodiment. The water treatment apparatus 1 comprises a semi-batch treatment apparatus 10, a sludge concentration meter 12, and a calculation device 14. The semi-batch treatment apparatus 10 comprises a reaction tank 16. A wastewater supply pipe 18 is connected to the wastewater inlet of the reaction tank 16. A pump 20 and a valve 22 are installed in the wastewater supply pipe 18. In addition, a treated water pipe 26 is connected to the treated water outlet of the reaction tank 16. A valve 28 is provided in the treated water pipe 26. An aeration device 32 connected to an aeration blower 30 is installed in the lower part of the reaction tank 16.

[0019] The sludge concentration meter 12 is installed at a predetermined height inside the reaction tank 16. The installation position of the sludge concentration meter 12 is, for example, in the range from the water surface to 2 m below the water surface, preferably in the range from the water surface to 1 m below the water surface. The sludge concentration meter 12 is connected to the calculation device 14, and is configured to transmit the sludge concentration detected by the sludge concentration meter 12 to the calculation device 14.

[0020] The arithmetic unit 14 is composed of, for example, a microcomputer consisting of a CPU that executes a predetermined program, ROM and RAM that store programs and calculation results, and electronic circuits. As will be described later, the arithmetic unit 14 functions as a calculation means for calculating the settling rate of biological sludge based on the sludge concentration in the reaction tank 16 detected by the sludge concentration meter 12. Furthermore, as will be described later, the arithmetic unit 14 functions as a sludge property understanding means for understanding the properties of biological sludge based on the calculated settling rate of biological sludge, etc.

[0021] The following describes an example of the operation of the water treatment device 1.

[0022] <(1) Inflow Process> The pump 20 is operated and the valve 22 is opened to allow a predetermined amount of water to be treated to flow into the reaction tank 16 from the wastewater supply pipe 18. Examples of water to be treated include wastewater containing organic matter, and specifically include wastewater from food processing plants, chemical plants, semiconductor plants, machinery plants, sewage, human waste, and other organic wastewater containing biodegradable organic matter.

[0023] <(2) Biological Treatment Process> After stopping the pump 20 and closing the valve 22, the aeration blower 30 is activated, and oxygen-containing gas such as air supplied from the aeration blower 30 is supplied to the reaction tank 16 through the aeration device 32. As a result, the water to be treated is biologically treated by biological sludge in the reaction tank 16. The biological reaction is not limited to an aerobic reaction; it is also possible to perform an anaerobic reaction by stirring without supplying air, etc., or a combination of aerobic and anaerobic reactions is possible. An anaerobic state refers to a state in which dissolved oxygen is not present, but oxygen derived from nitrite or nitrate is present. For example, as shown in Figure 2, a stirring device consisting of a motor 34, a stirring blade 36, a shaft connecting the motor 34 and the stirring blade 36, etc. can be installed in the reaction tank 16, and stirring can be performed by stopping the aeration blower 30 and using the stirring device. The stirring device is not limited to the above configuration.

[0024] Furthermore, in the biological treatment process, it is desirable to detect the sludge concentration in the reaction tank 16 using a sludge concentration meter 12. The calculation device 14 receives and stores the sludge concentration detected by the sludge concentration meter 12. The detection of sludge concentration by the sludge concentration meter 12 may be performed multiple times at predetermined time intervals. The calculation device 14 may then, for example, calculate the average value from the multiple detected sludge concentrations and store that average sludge concentration.

[0025] <(3) Settlement Process> The aeration blower 30 is stopped and left to stand for a predetermined time to allow the biological sludge in the reaction tank 16 to settle.

[0026] Furthermore, during the sedimentation process, the sludge concentration in the reaction tank 16 is detected by the sludge concentration meter 12. The calculation device 14 then calculates the sedimentation rate of the biological sludge based on the change in sludge concentration detected by the sludge concentration meter 12. The specific calculation method is described below.

[0027] For example, at the start of the sedimentation process, the sludge concentration detected by the sludge concentration meter 12 is about the same as the sludge concentration during the biological treatment process. However, as the biological sludge begins to settle, it becomes concentrated, and the sludge concentration detected by the sludge concentration meter 12 increases. Furthermore, as the biological sludge continues to settle, the sludge interface of the biological sludge falls below the position of the sludge concentration meter 12, causing the sludge concentration detected by the sludge concentration meter 12 to decrease rapidly. Therefore, the time from the start of the sedimentation process until the sludge concentration detected by the sludge concentration meter 12 decreases rapidly can be estimated as the time from the start of the sedimentation process until the sludge interface of the biological sludge falls below the position of the sludge concentration meter 12. Thus, the value obtained by dividing the distance from the water surface to the sludge concentration meter 12 by the above time can be estimated as the sedimentation velocity of the biological sludge. The calculation device 14 receives the sludge concentration in the reaction tank 16, which is sequentially detected by the sludge concentration meter 12, and measures the time it takes for the sludge concentration detected by the sludge concentration meter 12 to fall below a predetermined concentration. The predetermined concentration is preferably set to 1 / 4 of the sludge concentration initially detected by the sludge concentration meter 12, and more preferably to 1 / 10. The calculation device 14 then calculates the settling velocity of the biological sludge from the measured time and the predetermined distance from the water surface to the sludge concentration meter 12.

[0028] The calculation device 14 may further determine the properties of the biological sludge based on the calculated settling velocity of the biological sludge (hereinafter sometimes referred to as the settling velocity of the biological sludge (A)) and the sludge concentration during the biological treatment process (the average sludge concentration may also be used). This will be explained in detail below.

[0029] The relationship between the settling velocity of biological sludge in the settling process and the sludge concentration in the biological treatment process is expressed by the following exponential function: V = V ０ e （－ｋ・Ｘ） V: Settlement velocity of biological sludge in the settling process (m / h) X: Sludge concentration k, V in the biological treatment process ０ :constant

[0030] Figure 3 shows the exponential function equation (V = V ０ e （－ｋ・Ｘ）The settling characteristic curves of granular sludge and activated sludge based on the above are shown. As shown in Figure 3, the settling velocity of granular sludge is faster than that of activated sludge containing a large amount of flocs, but it tends to slow down if the sludge concentration is high, similar to the settling velocity of activated sludge. Therefore, to understand the properties of biological sludge, it is preferable to consider not only the settling velocity but also the sludge concentration.

[0031] The calculation unit 14 stores information regarding the relationship between sludge concentration and sedimentation velocity related to the sedimentation characteristics of granular sludge (an exponential function representing the sedimentation characteristics of granular sludge). From the measured sludge concentration during the biological treatment process and the pre-stored sedimentation characteristics of granular sludge, the sedimentation velocity (A) is calculated to determine the properties of the granular sludge at the current sludge concentration.

[0032] The calculation unit 14 compares the calculated biological sludge settling velocity (Vm) with the range of the settling velocity (Vg) calculated from the settling characteristics and sludge concentration of the granular sludge to determine the properties of the biological sludge. Specifically, it compares the calculated biological sludge settling velocity (Vm) with the range of the biological sludge settling velocity (Vg) calculated to determine the granular properties. If (Vm) is greater than or equal to (Vg), it can be determined that the biological sludge has good settling properties and is granulated, and therefore the condition of the biological sludge is judged to be good. On the other hand, if Vm is less than Vg, it can be determined that the biological sludge has poor settling properties and is not sufficiently granulated, and therefore the condition of the biological sludge is judged to be poor.

[0033] By recording the relationship between (Vg) and (Vm) over time, it is possible to understand the trend of deterioration or improvement in sludge properties from the trend. If there is a deterioration trend, it is possible to adjust the operating conditions of the semi-batch treatment device to improve the sludge properties, and if there is an improvement trend, it is possible to determine that it is preferable to maintain the current operating conditions.

[0034] Further, by previously storing in the arithmetic unit 14 the sedimentation characteristics corresponding to the SVI of the granular sludge, it is also possible to output a predicted value of the SVI of the granular sludge from the measured Vm and the calculated Vg. By doing so, it is not necessary to perform SVI measurement daily in operation management, and it can contribute to labor saving in operation management.

[0035] In the present embodiment, for example, when the sludge concentration is operated within a certain range or when the calculated sedimentation rate (Vg) of the biological sludge is in a sedimentation rate range that cannot be achieved by normal activated sludge, the properties of the biological sludge may be grasped based only on the calculated sedimentation rate (Vm) of the biological sludge. Specifically, when the calculated sedimentation rate (Vm) of the biological sludge is equal to or greater than a predetermined value, the state of the biological sludge is determined to be good, and when it is less than the predetermined value, the state of the biological sludge is determined to be bad.

[0036] <(4) Discharge step> In accordance with the previously determined end time of the sedimentation step, the valve 28 is opened, and the supernatant water obtained in the sedimentation step is discharged as treated water from the reaction tank 16 to the treated water pipe 26.

[0037] The treatment performed in the order of (1) inflow step, (2) biological treatment step, (3) sedimentation step, and (4) discharge step is taken as one cycle, and this is repeated to treat the water to be treated.

[0038] In the present embodiment, it is not limited to a form in which the inflow step and the discharge step are performed separately, and a form in which the discharge step is performed while the inflow step is being performed may also be used. That is, the treatment performed in the order of (1) inflow step / discharge step, (2) biological treatment step, and (3) sedimentation step may be taken as one cycle, and this may be repeated to treat the water to be treated.

[0039] Taking the water treatment apparatus 1 shown in FIG. 1 as an example, while operating the pump 20 and opening the valve 22 to allow the water to be treated to flow into the reaction tank 16 from the drainage supply pipe 18, the valve 28 is opened to discharge the treated water in the reaction tank 16 to the treated water pipe 26 ((1) inflow process / discharge process). After a predetermined time has elapsed, the valves 22 and 28 are closed, and the aeration blower 30 is operated to supply an oxygen-containing gas such as air supplied from the aeration blower 30 to the reaction tank 16 through the aeration device 32, and the water to be treated is biologically treated with biological sludge ((2) biological treatment process). Next, the operation of the aeration blower 30 is stopped and the reaction tank 16 is left in a static state for a predetermined time to settle the biological sludge in the reaction tank 16 ((3) sedimentation process). The detection of the sludge concentration in the reaction tank 16, the calculation of the sedimentation rate of the biological sludge, and the grasping of the properties of the biological sludge are as described above.

[0040] Examples of the sludge concentration meter 12 include conventionally known methods such as a transmission light type, a scattered light type, a microwave type, and an ultrasonic type. Further, as a detector for detecting the sludge concentration in the reaction tank 16, an instrument having a correlation with the sludge concentration such as a viscometer or a turbidimeter may be used. That is, the sludge concentration in the reaction tank 16 may be estimated from the detection value detected by an instrument having a correlation with the sludge concentration such as a viscometer or a turbidimeter.

[0041] In the water treatment apparatus 1 shown in FIG. 1, a sludge concentration meter 12 is used as a detector (first detection means) for detecting the sludge concentration in the reaction tank 16 during the sedimentation process, but a sludge interface meter for detecting the sludge interface position in the reaction tank 16 during the sedimentation process may also be used. Examples of the sludge interface meter include conventionally known methods such as an ultrasonic type, a turbidity detection type, and a transmission light type. The arithmetic unit 14 calculates the sedimentation rate (m / h) of the biological sludge from the sludge interface position in the reaction tank 16 detected by the sludge interface meter after a predetermined time has elapsed since the start of the sedimentation process.

[0042] According to this embodiment, while operating the water treatment apparatus, the sedimentation rate of biological sludge can be calculated, and based on the calculation result, the properties of the biological sludge can be grasped. Therefore, it becomes possible to change the operating conditions of the water treatment apparatus according to the change in the properties of the biological sludge. For example, when it is determined that the state of the biological sludge is good from the calculated sedimentation rate of the biological sludge, examples include increasing the volume load or sludge load of the reaction tank to improve the water treatment efficiency.

[0043] In this embodiment, when it is determined that the state of the biological sludge is good, it can be inferred that self-granulated biological sludge (so-called granular sludge) is formed in the reaction tank 16. Granular sludge is, for example, sludge with an average particle size of 0.2 mm or more or an SVI5, which is a sedimentation index, of 80 mL / g or less. SVI is a sedimentation index of biological sludge and is obtained by the following method. First, 1 L of sludge is put into a 1-L graduated cylinder, gently stirred so that the sludge concentration becomes as uniform as possible, and then the sludge interface when left standing for 5 minutes is measured. Then, the volume ratio (%) of the sludge in the graduated cylinder is calculated. Next, the MLSS (mg / L) of the sludge is measured. These are applied to the following formula to calculate SVI5. When calculating SVI30, the 5-minute standing still may be changed to 30-minute standing still. SVI5 (mL / g) = volume ratio of sludge occupied × 10,000 / MLSS

[0044] The volume load of the reaction tank 16 is preferably in the range of 0.15 kg BOD / m ３ / day to 1.00 kg BOD / m ３ / day, and more preferably in the range of 0.30 kg BOD / m ３ / day to 0.60 kg BOD / m ３ / day. By setting the volume load of the reaction tank 16 within the above range, it becomes possible to form better granules.

[0045] The sludge load in the reaction tank 16 is preferably in the range of 0.05 kg BOD / kgMLSS / day to 0.30 kg BOD / kgMLSS / day, and more preferably in the range of 0.10 kg BOD / kgMLSS / day to 0.20 kg BOD / kgMLSS / day. By setting the sludge load in the reaction tank 16 within the above range, it becomes possible to form better granules.

[0046] Under aerobic conditions, the dissolved oxygen (DO) in the reaction vessel 16 is preferably 0.5 mg / L or more, and particularly preferably 1 mg / L or more.

[0047] In order to promote the granulation of biological sludge, Fe is added to the water to be treated in the reaction tank 16 or to the water to be treated before it is introduced into the reaction tank 16. ２＋ Fe ３＋ Ca ２＋ Mg ２＋ Ions that form hydroxides, including the above, may be added. The addition of these ions can promote granule nucleation.

[0048] The present disclosure will be described in more detail below with reference to examples, but the present disclosure is not limited to the following examples.

[0049] Effective volume: 1.4 m³ ３ A water flow test was conducted using a semi-batch treatment apparatus equipped with a reaction tank. The wastewater used in the test was sewage that had undergone primary treatment by sedimentation at a sewage treatment plant.

[0050] The reaction tank operation cycle was carried out as follows: (1) Inflow / Outflow process: Over 90 minutes, wastewater was introduced into the reaction tank, and the supernatant water was discharged as treated water. (2) Biological treatment process: For 460 minutes, air was supplied from an aeration device installed at the bottom of the reaction tank to perform biological treatment of the wastewater. (3) Settlement process: The supply of air from the aeration device was stopped, and the tank was left to stand for 8 minutes to allow the biological sludge in the reaction tank to settle. The above operation cycles (1) to (3) were repeated as one cycle.

[0051] <Calculation of the settling velocity of biological sludge> In Example 1, biological sludge with SVI5: 50 mL / g, SVI30: 37 mL / g, and average particle size: 650 μm was added to the reaction tank so that the MLSS in the reaction tank was 4000 mg / L, and the above operating cycle was performed. In Example 2, biological sludge with SVI5: 260 mL / g, SVI30: 120 mL / g, and average particle size: 180 μm was added to the reaction tank so that the MLSS in the reaction tank was 2500 mg / L, and the above operating cycle was performed. In both Examples 1 and 2, the sludge concentration in the reaction tank during the settling process was sequentially detected using a sludge concentration meter installed 1 m below the water surface in the reaction tank.

[0052] Figure 4 shows the changes in sludge concentration in the reaction tank during the sedimentation process in Examples 1 and 2. The horizontal axis of Figure 4 represents the elapsed time of the sedimentation process, and the vertical axis represents the sludge concentration in the reaction tank detected by a sludge concentration meter installed 1 m below the water surface of the reaction tank. In Example 1, the sludge concentration in the reaction tank gradually increased from the start of the sedimentation process, reaching 6350 mg / L 75 seconds after the start of the sedimentation process. Subsequently, the sludge concentration in the reaction tank decreased rapidly, falling to 800 mg / L 2 minutes after the start of the sedimentation process, which was less than 1 / 4 of the sludge concentration immediately after the start of the sedimentation process. The sludge concentration meter was located 1 m below the water surface of the reaction tank, and it took 2 minutes for the sludge concentration to decrease to 1 / 4 of the sludge concentration immediately after the start of the sedimentation process, so the sedimentation velocity of the biological sludge was calculated to be 30 m / hr. In Example 2, the sludge concentration in the reaction tank gradually increased from the start of the sedimentation process, then decreased sharply to 0 mg / L 10 minutes after the start of the sedimentation process. From this result, the sedimentation velocity of the biological sludge was calculated to be 6 m / hr. As mentioned above, if the sedimentation velocity of the biological sludge can be calculated, it is also possible to understand the properties of the biological sludge.

[0053] 1 Water treatment device, 10 Half-batch treatment device, 12 Sludge concentration meter, 14 Calculation unit, 16 Reaction tank, 18 Wastewater supply piping, 20 Pump, 22, 28 Valves, 26 Treated water piping, 30 Aeration blower, 32 Aeration device, 34 Motor, 36 Agitator blade.

Claims

1. A water treatment method characterized by comprising: an inflow step of introducing water to be treated into a reaction tank; a biological treatment step of biologically treating the water to be treated in the reaction tank with biological sludge; a sedimentation step of allowing the biological sludge in the reaction tank to settle; and a discharge step of discharging the biologically treated water from the reaction tank; a first detection step of detecting the sludge interface position or sludge concentration in the reaction tank during the sedimentation step; and a calculation step of calculating the sedimentation rate of the biological sludge based on the sludge interface position or sludge concentration detected in the first detection step.

2. The water treatment method according to claim 1, further comprising a sludge property determination step for determining the properties of the biological sludge based on the settling velocity of the biological sludge calculated by the calculation step.

3. The water treatment method according to claim 2, further comprising a second detection step for detecting the sludge concentration in the reaction tank during the biological treatment step, wherein the sludge property determination step determines the properties of the biological sludge based on the sedimentation rate of the biological sludge and the sludge concentration detected in the second detection step.

4. A water treatment apparatus comprising: a reaction tank, an inflow step of introducing water to be treated into the reaction tank; a biological treatment step of biologically treating the water to be treated in the reaction tank with biological sludge; a sedimentation step of allowing the biological sludge in the reaction tank to settle; and a discharge step of discharging the biologically treated water from the reaction tank; a first detection means for detecting the sludge interface position or sludge concentration in the reaction tank during the sedimentation step; and a calculation means for calculating the sedimentation rate of the biological sludge based on the sludge interface position or sludge concentration detected by the first detection means.

5. The water treatment apparatus according to claim 4, further comprising a sludge property determination means for determining the properties of the biological sludge based on the settling velocity of the biological sludge calculated by the calculation means.

6. The water treatment apparatus according to claim 5, further comprising a second detection means for detecting the sludge concentration in the reaction tank during the biological treatment process, wherein the sludge property determination means determines the properties of the biological sludge based on the settling velocity of the biological sludge and the sludge concentration detected by the second detection means.

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