Aerobic biofilm treatment method and device

Abstract

In a method and apparatus for supplying raw water to an aeration tank (2) and performing aerobic biological treatment on a target substance to be removed from the raw water by means of a biofilm retaining carrier (C) or particles filled in the aeration tank, a relationship between the oxygen consumption rate of a unit biofilm retaining carrier or particle and a corresponding target dissolved oxygen concentration value and / or a corresponding set aeration intensity value is pre-set, and the target dissolved oxygen concentration value and / or the set aeration intensity value are adjusted according to the relationship in response to a change in a measured value of the oxygen consumption rate of the unit carrier or particle, and the aeration device is controlled so that the dissolved oxygen concentration reaches the target value or becomes the set aeration intensity setting value.

Description

Technical Field

[0001] The present invention relates to a method and apparatus for biofilm treatment of wastewater containing biooxidizable pollutants using self-granulated particles, fluidized bed carriers, fixed bed carriers, and the like, and particularly to controlling aeration intensity. In the present invention, wastewater outside the biofilm undergoing microbial treatment is referred to as bulk water. Background Art

[0002] As a method for treating wastewater containing pollutants that can be biologically oxidized, in addition to the activated sludge method using floating sludge, there are also biofilm methods such as the self-granulation particle method, the fluidized bed carrier method, and the fixed bed carrier method, in which microorganisms are treated in a form of aggregated proliferation called a biofilm.

[0003] In the activated sludge process using the aforementioned floating sludge, microorganisms are maintained in a dispersed state in the reaction tank in the form of microbial flocs. By removing the microorganisms that increase with wastewater treatment as excess sludge, the microbial population in the reaction tank is kept constant, thereby maintaining the oxygen consumption generated by the microorganisms' autolysis process at a fixed level. Therefore, the amount of oxygen required in this process increases or decreases proportionally with the raw water load. The required oxygen consumption can be determined by adding an offset to this oxygen consumption, which is fixed during the microbial autolysis process. In this process, microorganisms are typically maintained in the form of aggregates of microorganisms approximately 1 mm in diameter, called flocs, ensuring sufficient contact area between the microorganisms and the bulk water phase. Therefore, the permeability and diffusivity of oxygen and pollutants within the flocs are not the limiting factors in the main treatment performance of pollutant removal. Therefore, the aeration air volume to be supplied to the system is considered to be proportional to the oxygen consumption. Patent Document 1 describes the use of an instrument to measure the pollutant load and control the aeration air volume based on this measurement.

[0004] In the activated sludge process using suspended sludge and the biofilm process (self-granulation granule process, fluidized bed carrier process, fixed bed carrier process, etc.), as a method of simply adjusting the oxygen supply in proportion to the raw water load, the so-called DO control system is widely used to control the air volume to maintain a constant dissolved oxygen concentration (hereinafter referred to as DO) in the liquid.

[0005] Patent document 2 states that in the self-granulation particle method and the fluidized bed carrier method, when the BOD volumetric load is less than the specified value, the fluidization of the microbial carrier is used as the judgment basis, and when the BOD volumetric load is greater than the specified value, the oxygen demand of the wastewater is used as the judgment basis to control the aeration amount of the wastewater.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2001-353496.

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 63-256185.

[0010] In biofilm treatment methods such as the granulation granulation method, fluidized bed carrier method, and fixed bed carrier method, it is strictly difficult to appropriately adjust the oxygen supply based solely on the inflow load (which is calculated by multiplying the raw water flow rate per unit time and the raw water pollutant concentration), or the tank load (which is calculated by dividing the inflow load by the reaction tank volume). The reasons for this are as follows.

[0011] In methods utilizing microbial films, there's no mechanism to maintain a constant amount of microorganisms held in the biofilm form within the reaction tank. Consequently, the retained microbial population fluctuates over time, and thus the oxygen consumption generated by the microorganisms' autolysis process also fluctuates. Therefore, in biofilm methods, the oxygen supply to the system must be determined taking into account not only the fluctuations in oxygen consumption proportional to the raw water load, but also the fluctuations in oxygen consumption associated with changes in the retained microorganism population.

[0012] Due to these factors, the amount of oxygen required to oxidize organic matter in raw water using biofilm treatment methods varies with load fluctuations. Changes in the amount of biofilm maintained within the treatment equipment also lead to changes in the required oxygen supply. Furthermore, biofilm methods typically form microbial films with a thickness of 3mm or more, resulting in a smaller contact area between a single microorganism and the bulk water than in planktonic methods. Therefore, when supplying oxygen to microorganisms within the biofilm, oxygen diffusion across the interface between the bulk water and the biofilm becomes the primary rate-limiting factor in oxygen supply.

[0013] The diffusion rate of oxygen within the biofilm depends on the DO level of the bulk water, so the DO level must be adjusted to adjust the oxygen supply. Furthermore, from the perspective of the aeration system, even with the same oxygen supply, the required aeration air volume varies depending on the DO level. It is well known that the required aeration volume increases with higher DO levels, while the required aeration volume decreases with lower DO levels.

[0014] Therefore, as the load increases, the amount of oxygen required to oxidize organic matter in the raw water increases. The required oxygen supply is determined by taking into account the oxygen consumption caused by the autolysis process, which varies with the amount of microorganisms retained as a biofilm. Adjustments to increase the DO of the bulk water in response to the increase in the required oxygen supply are made, and it is also necessary to increase the aeration air volume to achieve the target DO.

[0015] Conversely, when the load is reduced, the amount of oxygen required to oxidize organic matter in the raw water decreases. The required oxygen supply is determined by taking into account the oxygen consumption caused by the autolysis process, which varies with the number of microorganisms maintained as a biofilm. This reduction in the required oxygen supply allows the DO of the bulk water to be maintained low, allowing the target DO to be achieved by reducing the aeration rate.

[0016] For this reason, when the aeration air volume is not adjusted or controlled according to the load, in order to maintain a high bulk water DO and oxygen supply even under high load, it is necessary to operate at a constant air volume with excessive aeration air volume.

[0017] Under constant air volume operation, which maintains the required high DO at high loads, the air volume is not suppressed to correspond with the reduction in oxygen consumption when the load decreases, resulting in wasted energy. When DO control is performed with a high DO target value set, assuming oxygen supply during high loads, the biofilm treatment device can also reduce the DO level to maintain the reduced load. Therefore, lowering the target DO level for DO control allows for limiting the aeration air volume. However, since conventional DO control does not suppress the air volume in response to the reduction in the DO target value, this also results in wasted energy.

[0018] For these reasons, wasteful energy consumption is particularly pronounced under conditions of large load fluctuations. However, even in situations where this wasteful energy consumption occurs, conventional techniques have struggled to adjust the air volume in response to load fluctuations to match the target DO level without degrading the treated water quality. When operators appropriately adjust the air volume, even low loads are assumed to be somewhat high. Consequently, even low loads often require excessive DO level settings and aeration to provide a certain margin for oxygen supply exceeding requirements. Consequently, energy is often wasted.

[0019] When adjusting aeration intensity using the typical raw water load, which serves as an indicator of raw water load, it is necessary to measure the concentration of the target substance to be removed in the raw water. In this case, one possible solution is to install a total organic carbon (TOC) meter, ammonia sensor, or absorbance meter in the raw water tank as an online measurement device for organic matter load. However, the initial cost of the measurement equipment is high, and stable measurement is difficult due to the high concentration of solids in organic species. Furthermore, analytical accuracy cannot be achieved depending on the composition of the organic matter. This makes installation difficult in practice, and load monitoring through automated measurement is not possible. Consequently, determining the appropriate aeration intensity is often difficult.

[0020] Furthermore, when using a total organic carbon meter, ammonium ion sensor, or absorbance meter to measure the concentration of target organic substances in raw water, maintenance tasks such as sensor cleaning and calibration require significant effort and time. In particular, the complex mechanisms of total organic carbon meters increase the probability of failure. Consequently, maintaining a stable control system using a total organic carbon meter can be difficult from an operational management perspective, leading to concerns about malfunctions. Summary of the Invention

[0021] Problems to be solved by the invention

[0022] An object of the present invention is to provide a method and apparatus for appropriately controlling aeration in wastewater treatment using an aerobic biofilm.

[0023] Means for solving problems

[0024] The aerobic biofilm treatment method of the present invention is a method of supplying raw water to an aeration tank, using an aeration device for aeration, and performing aerobic biological treatment on the removal target substance in the raw water by using biofilm-retaining carriers or particles filled in the aeration tank. The method is characterized in that the relationship between the oxygen consumption rate of the carrier or particle per unit and the corresponding DO target value and / or the corresponding aeration intensity setting value is pre-set, and the DO target value and / or the aeration intensity setting value are adjusted according to the relationship in response to changes in the measured value of the oxygen consumption rate per unit carrier or particle, and the aeration device is controlled so that the DO reaches the target value or becomes the set aeration intensity setting value.

[0025] The aerobic biofilm treatment device of the present invention is an aerobic biological treatment device having an aeration tank for supplying raw water, an aeration device for aerating the aeration tank, carriers or particles with biofilm filled in the aeration tank, and a controller for controlling the aeration device. It is characterized in that the aerobic biofilm treatment device has: a mechanism for presetting the relationship between the oxygen consumption rate of the carrier or particle per unit and the corresponding DO target value and / or the corresponding aeration intensity setting value; and a mechanism for adjusting the DO target value and / or the aeration intensity setting value according to the relationship in response to changes in the measured value of the oxygen consumption rate per unit carrier or particle, and the controller controls the aeration device so that the DO reaches the target value or becomes the set aeration intensity setting value.

[0026] According to one aspect of the present invention, the oxygen consumption rate of the unit carrier or particle is any one of the oxygen consumption rate per unit filling volume of the carrier, the oxygen consumption rate per unit total surface area of the carrier group, the oxygen consumption rate per unit filling volume of the particle, and the oxygen consumption rate per unit total surface area of the particle group.

[0027] According to one aspect of the present invention, the oxygen consumption rate of the unit carrier or particle is calculated based on the measured value of the aeration air volume, the measured value of the oxygen concentration in the gas phase discharged from the aeration tank or the measured value of the DO of the aeration tank, the experimental value or calculated value of the oxygen dissolution efficiency of the aeration tank, and the measured value or calculated value of the filling volume or surface area of the carrier.

[0028] According to one aspect of the present invention, the aeration intensity is controlled by controlling the aeration air volume, the aeration stop time or the aeration suppression time.

[0029] According to one aspect of the present invention, the relationship is set using any one of experimental results, actual operational performance, and a mechanism model that takes into account the diffusivity of oxygen in a biofilm.

[0030] Effects of the Invention

[0031] In the present invention, the oxygen consumption rate of the aeration tank is used instead of the raw water load, and aeration control can be performed without using a sensor such as a total organic carbon meter to measure the raw water concentration. This reduces maintenance manpower and costs, and allows for appropriate aeration control even when a total organic carbon meter cannot be used due to on-site facilities or the type of raw water.

[0032] In addition, according to one aspect of the present invention, the oxygen consumption rate per unit carrier or particle is used to estimate the sufficient oxygen supply required for the properties of the carrier or particle in the aeration tank that change over time, and the target value of DO or the set value of the aeration intensity itself is changed for control, thereby enabling more appropriate aeration control. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a diagram showing the structure of a biological treatment apparatus to which the present invention is applied.

[0034] Figure 2 It is a structural diagram of a biological treatment apparatus of a comparative example.

[0035] Figure 3 It is a graph showing the results of Examples and Comparative Examples.

[0036] Figure 4 It is a graph showing the results of Examples and Comparative Examples.

[0037] Figure 5 It is a graph showing the results of Examples and Comparative Examples.

[0038] Figure 6 It is a graph showing the total organic carbon load of raw water. DETAILED DESCRIPTION

[0039] Figure 1 This is a diagram showing the structure of a biological treatment apparatus to which the present invention is applied.

[0040] Treated wastewater (raw water) is introduced into an aeration tank 2 via a pipe 1. Aeration tank 2 is filled with a biofilm-supporting carrier C. Aeration pipes 3a, 3b, and 3c are installed at the bottom of aeration tank 2. Air is supplied from a blower 4 through a pipe 5 and branch pipes 5a, 5b, and 5c for aeration. Aeration tank 2 is provided with a top cover 2r.

[0041] The water aerobically biologically treated by the biofilm passes through the mesh 2a and is taken out from the pipe 6 as treated water.

[0042] This biological treatment apparatus includes, as measuring mechanisms, an exhaust gas meter 7 for measuring the oxygen concentration in the gas phase above the aeration tank 2 and below the top cover 2r, a DO meter 8 for measuring the DO within the aeration tank 2, and an air volume meter 9 for measuring the amount of air supplied from the blower 4 to the diffusion pipes 3a to 3c.

[0043] In the present invention, the raw water load is not used as a management index, but aeration control is performed based on the oxygen consumption rate of the unit carrier or particles in the aeration tank.

[0044] Generally, when the raw water load is large, the oxygen consumption rate of the aeration tank is large, and when the raw water load is small, the oxygen consumption rate is small. This oxygen consumption rate includes oxygen consumption caused by the autolysis process of microorganisms.

[0045] On the other hand, in the biofilm method, a biofilm typically forms a film thickness of over 3 mm, maintaining a smaller contact area between each microorganism and the bulk water compared to the microbial floatation method, which typically forms flocs around 1 mm. Therefore, when supplying oxygen to the microorganisms within the biofilm, oxygen diffusion at the interface between the bulk water and the biofilm becomes the primary rate-limiting factor in oxygen supply. Furthermore, it is known that the contact area between the biofilm and the bulk water is a major factor in the diffusion rate of oxygen into the biofilm. A larger contact area increases the oxygen diffusion rate, while a narrower contact area decreases the oxygen diffusion rate. Furthermore, it is known that the diffusion rate of oxygen within the biofilm depends on the DO level of the bulk water, necessitating adjustment of the DO level to adjust the oxygen supply. Furthermore, from the perspective of the aeration system, even with the same oxygen supply, the required aeration air volume varies depending on the DO level. It is well known that the required aeration volume increases when the DO level is high, while it decreases when the DO level is low. Therefore, conventionally, in treatment apparatuses utilizing biofilms, the DO level of the bulk water is often maintained high to generate sufficient oxygen diffusion to meet the required oxygen consumption. As a result, excessive aeration is often performed, often resulting in energy waste.

[0046] Research into the root cause of the energy waste associated with maintaining high DO levels in aeration revealed that one of the causes is a change in the contact area between the microbial membrane and the bulk water, which changes proportionally with the retention amount of the carriers and particles filled in the aeration tank, and a change in the oxygen diffusion efficiency.

[0047] For example, when an aeration tank filled with fluidized bed carriers is operated for a long time, there is a possibility that the carriers are cut and reduced in particle size, and flow out of the tank in the form of SS through the gaps in the screen. The carrier filling rate in the tank is reduced, and the contact area between the biofilm surface and the bulk water is reduced, thereby reducing the treatment performance.

[0048] Furthermore, when using an expanded bed aeration tank utilizing sedimentation carriers, regular backwashing is required to remove excess sludge and SS between the carriers. This occurs due to carrier wear caused by collisions and shear forces between the carriers, gradually reducing the carrier filling rate. This lowers the carrier filling rate within the tank, reducing the contact area between the biofilm surface and the bulk water, which in turn reduces the contact area between the biofilm and the bulk water that facilitates oxygen diffusion into the biofilm. This reduces the oxygen transfer rate and reduces treatment performance.

[0049] In biological treatment tanks using self-granulating particles, the number of individuals and particle size of the self-granulating particles change over time, the amount of biofilm in the aeration tank increases or decreases, and thus the contact area between the biofilm and the bulk water increases or decreases, and the diffusivity of oxygen into the biofilm changes. Therefore, even if the organic matter load is the same, the aeration air volume required for wastewater treatment will change.

[0050] For these reasons, while load management using oxygen consumption rate has the advantage of being able to monitor oxygen demand, including oxygen consumption caused by changes in microbial biomass, the oxygen transfer rate from the bulk water to the biofilm is affected by changes in the carrier amount and contact area. Therefore, even if the oxygen consumption rate remains the same, changes in the properties of the carriers and particles can cause changes in the appropriate DO level and aeration air volume, making it difficult to manage the aeration volume that meets oxygen demand without incurring energy losses. For these reasons, in aeration control using oxygen consumption rate, as in aeration control using raw water load, it is preferable to use the oxygen consumption rate per unit carrier or particle as an indicator for aeration control that can take into account the effects of increases and decreases in the carrier and particle amounts. Because the contact area between the bulk water and the biofilm per unit carrier or particle amount can be assumed to be fixed, there is no need to consider changes in the oxygen transfer rate caused by changes in contact area.

[0051] Therefore, in the present invention, aeration is performed using the oxygen consumption rate per unit carrier or particle as a management index, rather than just the oxygen consumption rate.

[0052] Examples of the oxygen consumption rate per unit carrier or particle include an oxygen consumption rate per unit filling volume of the carrier or particle or an oxygen consumption rate per unit total surface area of the carrier group or particle.

[0053] <Raw water load>

[0054] Calculate the raw water load using the following formula.

[0055] Load = Q·Concentration

[0056] Load: Raw water load [kg / d].

[0057] Q: Raw water flow [m 3 / d].

[0058] Concentration: Raw water concentration [kg / m 3 ].

[0059] Examples of the raw water concentration include total organic carbon, ammonia nitrogen, and the total organic carbon / N concentration estimated from UV absorbance.

[0060] <Carrier volume load>

[0061] The carrier volume load is calculated according to the following formula.

[0062] load 载体容积 = load / V 载体

[0063] load 载体容积 :Carrier volume load [kg / (m 3 ·d)].

[0064] V 载体 : Carrier filling volume in aeration tank [m 3 ].

[0065] <Carrier surface area loading>

[0066] The carrier surface area loading was calculated according to the following formula.

[0067] load 载体表面积 = load / S carrier

[0068] load 载体表面积 :Carrier surface area load [kg / (m 2 ·d)].

[0069] S 载体 : Total surface area of the carrier group in the aeration tank [m 2 ].

[0070] While the raw water load in an aeration tank can fluctuate rapidly over time, on a minute-by-minute basis, the properties of the carriers (the carrier filling volume within the aeration tank or the total surface area of the carrier group within the aeration tank) change relatively slowly over time, on a daily to monthly basis. Therefore, it is desirable to frequently update the calculated raw water load value. Alternatively, the carrier filling volume within the aeration tank or the total surface area of the carrier group within the aeration tank can be updated by regularly sampling and analyzing the carriers (e.g., approximately once every 1 to 3 months).

[0071] [Calculation method of oxygen consumption rate]

[0072] <Case 1: Estimating oxygen consumption rate using an air flow meter and exhaust gas meter>

[0073] By measuring the aeration air volume and the oxygen concentration in the exhaust gas, the oxygen consumption rate qO2 can be directly calculated using the following formula.

[0074] Mathematical formula 1

[0075]

[0076] Mathematical formula 2

[0077]

[0078] OTE: Oxygen transfer efficiency [-].

[0079] Z0: mole fraction of oxygen in the blown air [-].

[0080] Z: mole fraction of oxygen in the exhaust gas [-].

[0081] qO2: oxygen consumption rate [kg / d].

[0082] Gv: Aeration air flow rate converted to standard state [Nm 3 / d].

[0083] v m : Specific volume of oxygen [Nm 3 / kg].

[0084] <Case 2: Calculating oxygen consumption rate using a DO meter and aeration air volume>

[0085] Measure aeration air volume and DO, and indirectly calculate oxygen consumption rate qO2.

[0086] (i) Calculate the oxygen solubility index φ required for estimating the oxygen consumption rate according to the following formula (preparation before installing the control device).

[0087] Mathematical formula 3

[0088]

[0089] Mathematical formula 4

[0090]

[0091] OTE: Oxygen transfer efficiency [-].

[0092] Z0: mole fraction of oxygen in the blown air [-].

[0093] Z: mole fraction of oxygen in the exhaust gas [-].

[0094] φ: oxygen solubility index [m].

[0095] v m : Specific volume of oxygen [Nm 3 / kg].

[0096] h: water depth of the air diffuser [m].

[0097] Cs: Saturated dissolved oxygen concentration [kg / m 3 ].

[0098] C: Concentration of dissolved oxygen in the mixed solution [kg / m 3 ].

[0099] (ii) Continuously measure (while the device is operating) the change in oxygen consumption rate over time.

[0100] The oxygen consumption rate qO2 is continuously estimated using the following formula based on the continuous measurement data of the DO meter and the aeration air volume and the oxygen solubility index φ calculated in advance.

[0101] Mathematical formula 5

[0102]

[0103] qO2: oxygen consumption rate [kg / d].

[0104] Gv: Aeration air flow rate converted to standard state [Nm 3 / h].

[0105] h: water depth of the air diffuser [m].

[0106] Cs: Saturated dissolved oxygen concentration [kg / m 3 ].

[0107] C: Concentration of dissolved oxygen in the mixed solution [kg / m 3 ].

[0108] φ: oxygen solubility index [m].

[0109] In the present invention, the relationship between the oxygen consumption rate of the unit carrier or particle of the aeration tank and the corresponding DO target value or the corresponding aeration intensity setting value is pre-set, and the corresponding DO target value or the aeration intensity setting value is adjusted according to the relationship in response to changes in the measured value of the oxygen consumption rate.

[0110] Then, the aeration device is controlled so that the DO reaches a target value or a set aeration intensity setting value.

[0111] [Relationship between oxygen consumption rate and DO target value and / or aeration intensity setting value]

[0112] The relationship between the oxygen consumption rate and the DO target value and / or the aeration intensity setting value is set using preliminary experimental result data, actual operation performance data, simulation results of a mechanism model that takes into account the diffusivity of oxygen in the biofilm, and the like.

[0113] This relationship may be any of a standard curve (approximate function), a control table, and the like.

[0114] [Biofilm structure model for preparing standard curves or control tables]

[0115] As a method for discovering the relationship between the biofilm load in raw water and the DO target value or the aeration intensity set value, a kinetic model (hereinafter sometimes referred to as a biofilm mechanism model) can be used to estimate the reduction of pollutants and the increase and decrease of the amount of activated sludge bacteria in the biofilm when the biofilm comes into contact with the bulk water phase containing pollutants and oxygen in a flowing state. This kinetic model also needs to consider the simultaneous occurrence of bacterial growth, pollutant consumption, and oxygen consumption within the biofilm, the diffusion of dissolved oxygen from the bulk water phase into the biofilm, and the dissolution of oxygen in the bulk water due to aeration. In addition, the growth and shrinkage of the biofilm are caused by the increase and decrease in the volume of the bacterial population accompanying the growth and death of the bacteria, the attachment of bacteria from the bulk water, and the detachment of bacteria from the bulk water. When using a kinetic model in biofilm utilization treatment, these phenomena need to be mathematically modeled. Since this phenomenon originally occurs in three-dimensional space, the model formula becomes complicated. However, by expressing the growth and shrinkage of the biofilm with a one-dimensional model formula that only considers changes in the thickness direction, it can be simulated relatively easily. As mathematical models for simulating wastewater treatment using activated sludge, for example, a series of mathematical models proposed by a task force of the International Water Association can be effectively used (Reference 1). As an example of a mathematical model for biofilm, (Reference 2) can be used.

[0116] Reference 1: M Henze; IWA.Task Group on Mathematical Modeling for Design and Operaton of Biological Wastewater Treatment; et al.

[0117] Reference 2: Boltz, JP, Johnson, BR, Daigger, GT, Sandino, J., (2009a). "Modeling Integrated Fixed-Film Activated Sludge and Moving Bed BiofilmReactor Systems I: Mathematical Treatment and Model Development". WaterEnvironment Research, 81(6), 555-575.

[0118] By utilizing mathematical models, for example, it is possible to construct a mathematical model of the fluidized bed carrier. Typically, these mathematical models are often expressed in the form of simultaneous ordinary differential equations, allowing the dynamic behavior of the target process to be simulated using numerical integration software designed for these equations. For example, it is possible to predict the treated water quality based on the DO of the bulk water phase, which varies depending on the specific device configuration, load assumptions, and aeration intensity.

[0119] By utilizing mathematical models, it is possible to predict, for example, the total organic carbon concentration in treated water when treating at various aeration intensities under various load conditions. Based on the simulation results, the minimum DO target value and aeration intensity adjustment required to prevent treatment degradation are determined. A table summarizing the simulation results is created and effectively used as a control table in the control system of this patent.

[0120] [Control of aeration intensity]

[0121] Aeration intensity can be controlled by, for example, changing the aeration air volume (air flow rate), the aeration stop time per a specified period, or the aeration suppression time (the duration of weak aeration). The aeration stop time refers to the time during which aeration is stopped for a specified period in so-called intermittent aeration. The aeration suppression time refers to the time during which weak aeration is used, during alternating strong and weak aeration operations.

[0122] The aeration air volume, aeration stop time, and aeration suppression time are controlled continuously or in stages according to the raw water load.

[0123] <Oxygen diffusivity index>

[0124] When using biofilm treatment that uses biofilms attached to self-granulated microbial particles, fluidized beds, or fixed bed carriers to remove pollutants, the surface area of the flowing liquid phase in contact with microorganisms is less than that of the floatation method, and oxygen and pollutants need to diffuse and penetrate into the interior of the biofilm (in the thickness direction) for biological decomposition of pollutants. The speed of this diffusion and penetration process is slower than the proliferation rate of microorganisms and the oxygen consumption rate. The diffusion and penetration process is one of the main factors that determine the treatment performance.

[0125] The surface area of the biofilm in contact with the bulk water influences the diffusion and permeation process. When the surface area is small, even at the same DO of the bulk water, the total amount of oxygen diffusing into the biofilm is relatively reduced, leading to reduced treatment performance and deterioration in treated water quality. Conversely, when the surface area is increased, even at the same DO of the bulk water, the total amount of oxygen diffusing into the biofilm is relatively increased, improving treatment capacity and tending to improve treated water quality. Furthermore, even at low DO, sufficient treatment performance can be achieved, reducing aeration volume and the power required for aeration.

[0126] In the case of an apparatus utilizing self-granulating microbial granules, as the granules grow larger with long-term use, the specific surface area in contact with bulk water per unit volume of the self-granulating microbial granules decreases, and the surface area in contact with bulk water per unit volume of the apparatus decreases.

[0127] In systems utilizing carriers, as the amount of sludge held by the carriers increases with long-term use, the void spaces within the carriers become clogged with the microbial membrane itself and biologically inactive solid components such as scale, reducing the contact area between the bulk water and the biofilm. As a result, the specific surface area in contact with the bulk water per unit volume of the carriers decreases, and the surface area in contact with the bulk water per unit volume of the aeration tank decreases.

[0128] In particular, when using biofilms attached to fixed supports, there's a tendency for excessive biofilm to remain in the spaces between the supports over extended periods. In this case, as the amount of biofilm retained increases, the volume of the bulk water phase decreases. Furthermore, as this situation progresses, the spaces between the supports become clogged with microbial film, creating spaces into which the bulk water phase cannot flow. As a result, the contact area between the bulk water phase and the biofilm gradually decreases, and the permeability of oxygen and pollutants into the biofilm tends to decrease over time.

[0129] [Biological treatment other than fluidized bed]

[0130] exist Figure 1 In the above, the biological treatment using a fluidized bed carrier is described, but the present invention can also be implemented by the same method when a fixed bed carrier or particles are used.

[0131] In this embodiment, the case where wastewater containing organic matter is treated by aerobic biofilm treatment accompanied by aeration is described. In addition, the present invention can also be implemented in the same way when performing biological treatment such as biological nitrification and denitrification treatment using a biofilm, which includes an aerobic treatment step using a biofilm in an aeration tank.

[0132] [Sensor maintenance]

[0133] Air volume measurement is typically performed using a micro differential pressure gauge combined with an orifice, a micro differential pressure gauge combined with a pitot tube, or a hot-wire air flow meter. Micro differential pressure gauges and hot-wire air flow meters used in industrial equipment offer high measurement stability. Since they measure atmospheric air, the risk of accuracy loss due to sensor contamination is lower compared to measuring wastewater containing pollutants. Therefore, typically, only annual maintenance is required to ensure stable measurement.

[0134] For measuring the oxygen concentration in the exhaust gas discharged from the aeration tank, the measuring unit does not come into direct contact with the wastewater, which is highly contaminated by the sensor. Therefore, regular cleaning operations are not required. Typically, regular calibration using atmospheric oxygen concentration is performed once a day. With regular maintenance once a year, stable measurements can be achieved.

[0135] In recent years, fluorescent DO meters have been used as concentration meters for dissolved oxygen in aeration tanks. In principle, these meters have less variation in measured values due to contamination. Typically, their accuracy can be maintained through monthly regular cleaning and calibration using air oxygen concentration.

[0136] Example

[0137] [Experimental Example 1: Method for estimating oxygen consumption rate using air flow meter and exhaust gas meter]

[0138] (1) Figure 1 In the biological treatment device, the oxygen consumption rate qO2 is estimated based on the online data of aeration air volume and oxygen concentration in exhaust gas.

[0139] (2) Evaluation of the calibration frequency of the exhaust gas meter 7 (exhaust gas oxygen concentration meter) required for continuous measurement

[0140] Once a day, a standard gas of known concentration of O₂ is passed through the exhaust meter 7 to confirm the meter's reading. When the difference between the standard gas concentration and the meter's reading is 0.2% or greater, the meter 7 is calibrated. The required calibration frequency is once every seven days. Furthermore, this calibration operation can be automated.

[0141] [Experimental Example 2: Method for estimating oxygen consumption rate using a DO meter and an air flow meter]

[0142] (1) Figure 1In the biological treatment device, the oxygen concentration of the gas phase at the top of the aeration tank 2 is collected by an exhaust meter, the aeration air volume of the reaction tank, the oxygen concentration in the exhaust gas, and the DO in the reaction tank are measured, and the oxygen solubility index φ is calculated based on the measurement results.

[0143] (2) Based on the oxygen solubility index φ measured in advance, the oxygen consumption rate qO2 is continuously estimated according to the online measurement data of the DO meter and the aeration air volume.

[0144] (3) Evaluation of the calibration frequency of the DO meter required for continuous measurement

[0145] The DO in the reaction tank was measured once a day using a portable DO meter calibrated with saturated water. The difference between the reading and the reading on the DO meter 8 was checked. If the difference reached ±0.5 mg / L or more, the DO meter 8 was calibrated. The required calibration frequency was once every 15 days.

[0146] [Experimental Example 3: Calculating the raw water load based on the total organic carbon meter and flow rate]

[0147] For use Figure 2 The maintenance frequency of the biological treatment apparatus shown was checked when aeration control based on the raw water load using the measured value of the total organic carbon concentration of the raw water was performed.

[0148] exist Figure 2 In the biological treatment device, the wastewater to be treated (raw water) is introduced into an aeration tank 11 through a pipe 10. The aeration tank 11 is filled with a carrier C supporting a biofilm. An air diffusion pipe 13 is provided at the bottom of the aeration tank 11, and air is supplied from a blower 14 through a pipe 15 to perform aeration.

[0149] The water aerobically biologically treated by the biofilm passes through the screen 12 and is taken out from the pipe 16 as treated water.

[0150] This biological treatment apparatus includes, as measuring mechanisms, a flowmeter 17 and a total organic carbon meter 18 for measuring the flow rate and total organic carbon concentration of the raw water flowing through the pipe 10; a DO meter 19 for measuring the DO within the aeration tank 11; and an air volume meter 20 for measuring the amount of air supplied from the blower 14 to the diffusion pipe 13. These detected values are input into a controller 21. The controller 21 controls the motor speed of the blower 14 to control the aeration intensity.

[0151] The flow rate of raw water was measured by a flow meter 17, and the total organic carbon concentration of the raw water was measured by a total organic carbon meter 18 to calculate the total organic carbon load.

[0152] Raw water is sampled daily and filtered through a 0.45 μm filter. The total organic carbon concentration in the filtrate is measured using a total organic carbon meter in the analysis room. The meter is calibrated if the difference between the readings of the total organic carbon meter 18 and those in the analysis room exceeds 5%. Calibration is required every three days.

[0153] Table 1

[0154] Calibration frequency required to maintain accuracy Exhaust meter of Experimental Example 1 7 days (can be automated) DO meter of Experimental Example 2 15th Total organic carbon in Experimental Example 3 3rd

[0155] As shown in Table 1, it was confirmed that the instrument calibration labor and time were less in Experimental Examples 1 and 2 in which the oxygen consumption rate in the aeration tank was directly or indirectly calculated than in Experimental Example 3 in which the total organic carbon in the raw water was measured using the total organic carbon meter 18.

[0156] [Example 1]

[0157] <Creating a control sheet>

[0158] Using the biofilm mechanism model, the effects of the oxygen consumption rate per unit filling volume of the carrier and the corresponding DO target value and weak aeration time on the treated water quality are simulated. The relationship between the DO target value and the weak aeration time setting value, which can achieve the minimum aeration air volume that can maintain the target treated water quality based on the oxygen consumption rate per unit filling volume of the carrier, is calculated in advance, and the control table shown in Table 2 is prepared.

[0159] Table 2

[0160]

[0161] The oxygen consumption rate per unit filling volume of the carrier was calculated by the measurement and estimation in Experimental Example 2, and the target DO value and the weak aeration time setting value were adjusted every 2 hours based on this value.

[0162] In this control table, for example, the total organic carbon carrier volumetric loading (kg C / (m 3 ·d) (hereinafter, units may be omitted) is 0.5 or more and less than 2.5, the target DO value is 3.1 mg / L; 2.5 or more and less than 2.7, the target DO value is 3.8 mg / L; 2.7 or more and less than 3.0, the target DO value is 3.9 mg / L; 3.0 or more and less than 3.2, the target DO value is 4.4 mg / L; 3.2 or more, the target DO value is 4.8 mg / L, and each is set to an appropriate value.

[0163] When the total organic carbon carrier volume load is 0.5 or more and less than 0.8, the weak aeration time setting value is set to 110 minutes every 2 hours, when it is 0.8 or more and less than 1.2, it is 90 minutes every 2 hours, when it is 1.2 or more and less than 1.5, it is 80 minutes every 2 hours, when it is 1.5 or more and less than 2.0, it is 60 minutes every 2 hours, when it is 2.0 or more and less than 2.5, it is 40 minutes every 2 hours, and when it is 2.0 or more and less than 2.5, it is 20 minutes every 2 hours. The total organic carbon carrier volume load is 2.7 (kg C / (m 3 d)) or above, the weak aeration time setting value is set to zero (ie, intermittent aeration is not performed).

[0164] The total organic carbon load is Figure 6 The raw water with the changes shown is discharged as the treatment object.

[0165] According to the 2-hour moving average of the carrier volume load, the DO target value and the weak aeration time within the 2-hour cycle are adjusted once every 2 hours according to the control table in Table 2, and the motor speed of the blower is controlled so that the weak aeration time is set to the specified low air volume (3m 3 / (bottom area m 2 ·hr)) and the set DO target value is reached during the time other than weak aeration.

[0166] The time variation of the weak aeration time is shown in Figure 3 , the time variation of DO is shown in Figure 4 In addition, the time-varying changes in the power consumption of the blower are shown in Figure 5 .

[0167] [Comparative Example 1]

[0168] The same procedure as in Example 1 was followed except that the target DO value was set to a fixed value of 3.5 mg / L and the weak aeration time was maintained at a fixed value of 10 minutes / 2 hours. Figures 3-5 .

[0169] <Inspection>

[0170] In Example 1, the DO target value and weak aeration time were adjusted according to the oxygen consumption rate per carrier, resulting in lower power consumption for the blower than in Comparative Example 1. Specifically, while Comparative Example 1 consumed approximately 1150 kWh / day, Example 1 consumed approximately 950 kWh / day, a reduction of approximately 17%.

[0171] In addition, there was almost no difference in the treated water quality between Example 1 and Comparative Example 1.

[0172] Although the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the present invention.

[0173] This application is based on Japanese Patent Application No. 2020-063032 filed on March 31, 2020, the entire contents of which are incorporated herein by reference.

[0174] Description of Reference Signs

[0175] 2, 11: aeration tank; 2a, 12: screen; 3a, 3b, 3c: air diffusion pipe; 4, 14: blower; 7: exhaust meter; 8, 19: DO meter; 9, 20: air volume meter.

Claims

1. An aerobic biofilm treatment method, wherein raw water is supplied to an aeration tank, aeration is performed using an aeration device, and a removal target substance in the raw water is aerobic biologically treated by biofilm-retaining carriers or particles filled in the aeration tank, characterized in that: The relationship between the oxygen consumption rate of the carrier or particle and the corresponding dissolved oxygen concentration target value and / or the corresponding aeration intensity setting value is preset as a unit. According to the change in the measured value of the oxygen consumption rate per unit carrier or particle, the target dissolved oxygen concentration value and / or the aeration intensity setting value are adjusted according to the relationship in response to the change. The aeration device is controlled so that the dissolved oxygen concentration reaches the adjusted dissolved oxygen concentration target value or the aeration intensity reaches the adjusted aeration intensity set value.

2. The aerobic biofilm treatment method according to claim 1, wherein: The oxygen consumption rate per unit carrier or particle is any one of the oxygen consumption rate per unit filling volume of the carrier, the oxygen consumption rate per unit total surface area of the carrier group, the oxygen consumption rate per unit filling volume of the particle, and the oxygen consumption rate per unit total surface area of the particle group.

3. The aerobic biofilm treatment method according to claim 1, wherein The oxygen consumption rate of the unit carrier or particle is calculated based on the measured value of the aeration air volume, the measured value of the oxygen concentration in the gas phase discharged from the aeration tank or the measured value of the dissolved oxygen concentration in the aeration tank, the experimental value or calculated value of the oxygen dissolution efficiency of the aeration tank, and the measured value or calculated value of the filling volume or surface area of the carrier.

4. The aerobic biofilm treatment method according to claim 2, wherein The oxygen consumption rate of the unit carrier or particle is calculated based on the measured value of the aeration air volume, the measured value of the oxygen concentration in the gas phase discharged from the aeration tank or the measured value of the dissolved oxygen concentration in the aeration tank, the experimental value or calculated value of the oxygen dissolution efficiency of the aeration tank, and the measured value or calculated value of the filling volume or surface area of the carrier.

5. The aerobic biofilm treatment method according to any one of claims 1 to 4, wherein Aeration intensity is controlled by controlling aeration air volume, aeration stop time or aeration suppression time.

6. The aerobic biofilm treatment method according to any one of claims 1 to 4, wherein The relationship is set using any one of experimental results, actual operational performance, and a mechanism model that takes into account the diffusivity of oxygen in the biofilm.

7. The aerobic biofilm treatment method according to claim 5, wherein: The relationship is set using any one of experimental results, actual operational performance, and a mechanism model that takes into account the diffusivity of oxygen in the biofilm.

8. An aerobic biofilm treatment device comprising an aeration tank for supplying raw water, an aeration device for aerating the aeration tank, carriers or particles with biofilm filled in the aeration tank, and a controller for controlling the aeration device, characterized in that: The aerobic biofilm treatment device comprises: A mechanism for presetting a relationship between the oxygen consumption rate per unit of the carrier or particle and a corresponding target value of dissolved oxygen concentration and / or a corresponding set value of aeration intensity; as well as means for adjusting the target dissolved oxygen concentration value and / or the set aeration intensity value according to the relationship in accordance with the fluctuation in the measured value of the oxygen consumption rate per unit carrier or particle; The controller controls the aeration device so that the dissolved oxygen concentration reaches the adjusted dissolved oxygen concentration target value or the aeration intensity reaches the adjusted aeration intensity set value.

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