JP7435196B2 - Aerobic biofilm treatment method and device - Google Patents

Description

The present invention relates to a method and apparatus for biofilm treatment of wastewater containing biologically oxidizable pollutants using self-granulating granules, fluidized bed carriers, fixed bed carriers, etc., and particularly relates to control of aeration intensity. In the present invention, wastewater existing outside the microorganisms that undergo microbial treatment is referred to as bulk water.

Treatment methods for wastewater containing pollutants that can be biologically oxidized include the activated sludge method using suspended sludge, the self-granulation granule method, the fluidized bed carrier method, and the fixed bed carrier method, which allow microorganisms to form biofilms. The biofilm method, which treats bacteria in a state where they accumulate and proliferate, is used.

In the former activated sludge method, which uses suspended sludge, there is sufficient contact area between microorganisms and the bulk aqueous phase inside and outside of aggregates of microorganisms, typically around 1 mm in diameter, called microbial flocs. The permeability and diffusivity of oxygen and pollutants are not the main rate-limiting factors for pollutant removal rate. Patent Document 1 describes that the load of pollutants is measured with a meter and the aeration air volume is controlled in proportion to this.

In activated sludge methods using suspended sludge, and biofilm methods such as self-granulation granule methods, fluidized bed carrier methods, and fixed bed carrier methods, as a method to easily adjust the oxygen supply amount in proportion to the load of raw water, BACKGROUND ART A so-called DO control system that performs air volume control to maintain a constant dissolved oxygen concentration (hereinafter referred to as DO) in a liquid is widely used.

Regarding the self-granulation granule method and the fluidized bed carrier method, Patent Document 2 states that when the BOD volume load is smaller than a predetermined value, fluidization of the microbial carrier is used as a criterion; A wastewater treatment method and apparatus are described in which the amount of aeration of wastewater is controlled based on the oxygen demand of the wastewater.

Japanese Patent Application Publication No. 2001-353496 Japanese Unexamined Patent Publication No. 63-256185

In treatment methods that utilize biofilms, such as the self-granulation granule method, fluidized bed carrier method, and fixed bed carrier method, the flow rate per unit time of raw water, which is a general indicator of the amount of inflowing pollutants, is Strictly speaking, it is difficult to appropriately adjust the oxygen supply amount based only on the raw water load, which is calculated by multiplying the raw water by the pollutant concentration, or the tank load, which is calculated by dividing the raw water load by the reaction tank volume. be. The reasons for this are as follows.

(i) Even if the raw water load is the same and the amount of oxygen required to oxidize the organic matter in the raw water is the same, in the method using a biofilm, the microorganisms retained in the reaction tank in the form of a biofilm As the amount changes over time, the amount of oxygen consumed due to the autolysis process of the microorganism itself changes. Therefore, the amount of oxygen supplied to the device must be determined by taking this factor into consideration.

(ii) Treatment methods using biofilms require oxygen to diffuse into the biofilm where microorganisms are accumulated. The main factors that affect the diffusivity of oxygen into biofilms are the contact area between biofilm and bulk water and the level of DO in bulk water, but in the self-granulation granule method, As the granule loading and granule size change, the contact area between microorganisms and bulk water changes.

(iii) In the fluidized bed carrier method, the contact area between the biofilm and bulk water changes due to changes in the amount of microorganisms attached inside and outside the carrier. In particular, if the carrier has a structure with voids and biofilm grows in the voids, the amount of biofilm adhering to the carrier will increase, and if all the voids are blocked, contact between bulk water and the biofilm will increase. The area is significantly reduced.

(iv) Such changes in the contact area between bulk water and biofilms have a significant effect on oxygen diffusivity into biofilms. For example, if the contact area between bulk water and biofilm decreases, it is necessary to increase the DO of bulk water even if the same amount of oxygen is supplied into the biofilm, increasing the dissolved oxygen concentration of bulk water. This requires a larger flow rate of air blowing. Furthermore, when the load of raw water increases, the amount of oxygen consumed increases. Therefore, in order to supply the necessary oxygen into the biofilm through a diffusion phenomenon, it is necessary to increase the dissolved oxygen concentration in the bulk water.

In this way, the amount of oxygen required to oxidize raw water organic matter changes according to load fluctuations, and the amount of oxygen that needs to be supplied changes depending on the amount of biofilm retained in the treatment equipment. In the case of the biofilm method, which relies on diffusion phenomena, it is necessary to adjust the DO of the bulk water according to the amount of oxygen that should be supplied to the biofilm, and the amount of aeration air to maintain the DO of the bulk water must also be adjusted. There is a need.

In addition, when the aeration air volume is not controlled, operation is often performed with the DO of bulk water kept excessively high assuming a high load.

When operating at a constant air volume without aeration control in order to maintain an excessively high DO, aeration is constantly performed at an air volume that matches the maximum load, resulting in wasted energy consumption.

Furthermore, when performing general air volume control with a constant DO, it is necessary to set the DO with a view to ensuring a sufficient amount of oxygen diffusion into the biofilm during high loads. Therefore, during low load, the DO is maintained at a DO level higher than that required to maintain the required oxygen supply amount. As a result, the amount of aeration air required to maintain the DO is greater than the required amount, resulting in wasted energy consumption.

However, even when the raw water load and operation items of the reaction tank are constant, or even when appropriate operation control is performed according to the raw water load, the quality of the treated water may fluctuate. This tends to occur particularly when the biological treatment equipment is operated for a long period of time.

The reason for this is that in treatment equipment that uses spherical biofilms such as self-granulating microbial granules or biofilms attached to fluidized bed or fixed bed carriers, the amount of biofilm retained increases over time. changes in the amount of oxygen consumed due to the self-degradation process of the microorganisms themselves, and decreases the rate of oxygen diffusion into the biofilm due to a decrease in the contact area between the biofilm and bulk water, which in turn reduces the rate of oxygen diffusion into the biofilm. It is possible that processing performance deteriorates over time.

For this reason, even though aeration control was carried out by setting the aeration intensity according to load fluctuations, when the quality of treated water deviates from a predetermined value due to continuous operation, the amount of biofilm retained in the aeration tank decreases. It is conceivable that the aeration conditions may be changed based on the judgment that the treatment performance has decreased due to the increase.

Specifically, when the quality of treated water is not reduced to the target value and the water quality deteriorates, it is determined that the oxygen diffusivity has decreased due to an increase in the amount of biofilm retained, and the amount of oxygen supplied is insufficient. Change the control logic to an aeration condition control logic with stronger aeration intensity than the control logic of .

However, there is a problem in that when aeration conditions with high aeration intensity are applied, the required aeration power increases. Furthermore, if an aeration system that can perform aeration stronger than a predetermined level has not been constructed in the first place, it is not possible to increase the aeration intensity and restore the quality of the treated water.

The present invention provides a method and apparatus for improving treatment performance when the amount of biofilm retained increases due to long-term continuous operation of aerobic biofilm treatment using self-granulating granules or biofouling carriers. The purpose is to

The aerobic biofilm treatment method of the present invention is a method for aerobic biofilm treatment of substances to be removed in raw water using biofilm retention carriers or granules filled in the aeration tank in an aeration tank to which raw water is supplied. , when any one or more of the following is satisfied, the biofilm retention amount reduction treatment of the carrier or the partial disassembly treatment of the granules is performed.
i) The calculated value of the oxygen diffusivity index is below a predetermined lower limit.
ii) The average particle size of the granules exceeds a predetermined upper limit.
iii) The amount (weight) of biofilm attached per carrier exceeds a predetermined upper limit.
iv) A specified water quality item of treated water exceeds a specified target value in a situation where the aeration air volume per raw water load exceeds a specified upper limit.
v) A predetermined period of time has elapsed since the start of operation or the previous processing.

The aerobic biofilm treatment device of the present invention comprises an aeration tank to which raw water is supplied, a biofilm holding carrier or granules filled in the aeration tank, and an aeration device for aerating the aeration tank. The membrane treatment device has a means for reducing the biofilm retention amount of the carrier or a means for partially disassembling the granules, which is activated when any one or more of the following is satisfied.
i) The calculated value of the oxygen diffusivity index is below a predetermined lower limit.
ii) The average particle size of the granules exceeds a predetermined upper limit.
iii) The amount (weight) of biofilm attached per carrier exceeds a predetermined upper limit.
iv) A specified water quality item of treated water exceeds a specified target value in a situation where the aeration air volume per raw water load exceeds a specified upper limit.
v) A predetermined period of time has elapsed since the start of operation or the previous processing.

In one aspect of the present invention, the carrier biofilm retention amount reduction or granule partial disassembly treatment is performed by strong stirring using a rotating stirring blade or backwashing, strong aeration, high flow rate circulation, and water flow in the tank to a crushing pump. Perform one or more of the following.

In one aspect of the present invention, the aeration tank is filled with a biofilm-retaining carrier, and the reduction in the amount of biofilm retained on the carrier is achieved by placing a new carrier and/or a biofilm-attached carrier that has been subjected to biofilm removal treatment in the aeration tank. This is done by adding it to the carrier or replacing it with the carrier in the aeration tank.

According to the present invention, when a control item deviates from a predetermined value, the self-granulating microbial granules are partially disassembled, the biofilm attached to the carrier is detached, or the biofilm retained between the carriers is detached and removed. By doing so, it is possible to improve the oxygen diffusion performance of the self-granulating microorganism granules or the carrier, and improve the energy saving performance of the motive power involved in aeration.

In particular, when biofilm treatment using a carrier is operated for a long time, a dense biofilm with low treatment capacity tends to accumulate inside the carrier, which deteriorates oxygen diffusivity into the carrier and tends to reduce treatment performance. However, by detecting the biofilm deposited inside the carrier and removing the biofilm, it is possible to restore oxygen diffusivity into the carrier and improve treatment performance.

1 is a configuration diagram of a biological treatment device to which the present invention is applied. 1 is a configuration diagram of a biological treatment device to which the present invention is applied. 1 is a configuration diagram of a biological treatment device to which the present invention is applied. FIG. 2 is a configuration diagram of a biological treatment device. FIG. 2 is a configuration diagram of a biological treatment device.

In the present invention, when control items deviate from predetermined values, it is determined that the amount of biofilm retained has increased excessively, especially in the case of self-granulating microbial granules, the granules have become enlarged, and the carrier Either reducing the amount of biofilm retained (for example, detaching the biofilm from the carrier) or partially disassembling the granules.

Specifically, when any one or more of the following i) to v) occurs, the amount of biofilm retained on the carrier is reduced, such as by detaching the biofilm from the carrier, or the granules are partially disassembled.
i) When the calculated value of the oxygen diffusivity index is below a predetermined lower limit value The oxygen diffusivity index is determined by the amount of biofilm retained per carrier filling volume (mg/m ³ ) and the contact area of the biofilm with bulk water. However, since it is difficult to directly measure these indicators with an actual machine, they are estimated by simulation calculations based on the raw water load per carrier filling volume and changes in treated water quality with respect to aeration conditions.
ii) When the average particle size of the granules exceeds a predetermined upper limit, the average particle size of the granules can be determined by visual observation or optical observation by sampling the tank water from a completely mixed aeration tank with a Vandoorne water sampler, etc. Confirm by measuring the average particle size.
iii) When the amount (weight) of biofilm attached per carrier exceeds the predetermined upper limit, the weight of biofilm attached to the carrier during stable operation is measured in advance, and the value obtained by adding the predetermined width to this value is calculated as " ``predetermined upper limit value''.

The weight of the attached biofilm can be measured by drying a fixed amount of the bioattached carrier sampled with a Vandoorne water sampler and measuring the weight difference between it and a new carrier, or by burning the carrier and measuring the weight difference between the bioattached carrier and the new carrier. Calculated by measuring (VSS).
iv) When the specified water quality item of the treated water exceeds the specified target value when the aeration air volume per raw water load exceeds the predetermined upper limit. , setting the upper limit of the aeration air volume per raw water load from the perspective of energy conservation, or deciding because the treated water quality cannot be maintained even with the maximum aeration air volume for the equipment even though the raw water load is within the design maximum value. This is the setting value.
v) When a predetermined time has elapsed since the start of operation or the previous treatment The elapsed time of operation until the biofilm becomes thickened is estimated experimentally or by simulation, and is defined as the "predetermined time".

Note that even when no abnormality is detected in the management items ii) to iv), the oxygen diffusivity index may decrease. Therefore, management using i) is more preferable.

As a means of detaching the biofilm or partially disassembling the granules, there are methods of forcibly reducing the biofilm by applying shearing force as shown in (1) to (4) below, and methods of forcibly reducing the biofilm by applying shearing force as shown in (1) to (4) below. It is preferable to add the adhering carrier or to replace it with the carrier in the tank.
(1) Strongly stir using rotating stirring blades or backwashing.
(2) Strong aeration.
(3) High flow rate circulation.
(4) Pass the tank water to the crushing pump inside or outside the tank.

In addition, the biofilm generated by detachment of the biofilm from the carrier or partial disassembly of the granules is discharged as SS to the outside of the tank along with the treated water, and subjected to solid-liquid separation treatment (coagulation sedimentation, coagulation pressure flotation, coagulation filtration, etc.). Afterwards, it is discharged from the system.

The biofilm stripping means and partial disassembly means may be temporarily installed when stripping or partial disassembly is performed, or may be installed as permanent equipment.

Hereinafter, the configuration of an aerobic biological treatment apparatus equipped with a means for removing attached biofilm from a fluidized bed carrier or a means for partially dismantling self-granulating granules will be described with reference to the drawings.

In the biological treatment apparatus shown in FIG. 1, wastewater to be treated (raw water) is introduced into an aeration tank 2 through a pipe 1. The aeration tank 2 is filled with a carrier C carrying granules or biofilm. An aeration pipe 3 is installed at the bottom of the aeration tank 2, and air is supplied from a blower 4 through a pipe 5 to perform aeration.

The water that has been aerobically treated by the biofilm passes through the screen 6 and is taken out from the pipe 7 as treated water.

In this biological treatment device, the granules and carriers in the aeration tank 2 are pulled out with a suction pump 11 and introduced into the stirring water tank 12, and strongly agitated with the stirrer 13 to partially disassemble the granules or adhere to the carriers as a biofilm detachment means. After removing the biofilm, it is returned to the aeration tank 2 through the piping 14.

The degree of partial disassembly of the granules or peeling of the biofilm attached to the carrier is adjusted by the following (a), (b), (c), etc.
(a) Adjust the discharge amount of the suction pump 11 to adjust the residence time in the stirring water tank 12.
(b) Adjust the rotational speed of the stirrer 13 to adjust the intensity of biofilm disassembly/peeling.
(c) Adjust the above two together.

Note that instead of providing the stirring water tank 12 and the stirrer 13, an underwater crushing pump may be installed instead.

In FIG. 1, the water from the suction pump 11 is directly supplied to the stirring water tank 12, but as shown in FIG. Alternatively, only carriers and granules that have a large amount of biofilm retention and a high sedimentation rate may be selectively supplied to the stirring water tank 12 to perform partial disassembly of the granules or biofilm peeling treatment from the carrier. Carriers and granules that have a small amount of biofilm retained and a low sedimentation rate are returned to the aeration tank 2 through the pipe 16.

FIG. 3 shows a biological treatment device that uses strong aeration to partially dismantle granules or peel off biofilms attached to carriers.

In the biological treatment apparatus shown in FIG. 3, wastewater to be treated (raw water) is introduced into an aeration tank 2 through a pipe 1. The aeration tank 2 is filled with a carrier C carrying granules or biofilm. An aeration pipe 3 is installed at the bottom of the aeration tank 2, and air is supplied from a blower 4 through a pipe 5 to perform aeration. Air can also be supplied to the pipe 5 from a blower 17 for strong aeration.

The water that has been aerobically treated by the biofilm passes through the screen 6 and is taken out from the pipe 7 as treated water.

This biological treatment equipment is provided with a DO meter 19 that measures the DO concentration in the aeration tank 2, and an air flow meter 20 that measures the amount of air supplied from the blower 4 to the piping 5, and these detected values are controlled. The signal is input to the device 21. The aeration intensity is controlled by controlling the blower 4 by the controller 21.

When partially dismantling the granules or peeling off the biofilm attached to the carrier, the blower 17 is operated to provide strong aeration.

<Oxygen diffusivity index>
In the case of biofilm treatment that uses self-granulating microbial granules or biofilm attached to a fluidized bed or fixed bed carrier to remove pollutants, compared to the floating method, the microorganisms come into contact with the fluidized liquid phase. To biodegrade pollutants, it is necessary for oxygen and pollutants to diffuse into the biofilm (in the thickness direction), and the speed of this diffusion and osmosis process depends on the growth rate and oxygen consumption of microorganisms. The diffusion-osmosis process is one of the main factors determining treatment performance because it is slow compared to the speed.

The surface area of biofilm contact with bulk water is a factor that influences the diffusion-osmosis process. When the surface area becomes narrower, even if the DO of the bulk water is the same, the total amount of oxygen diffused into the biofilm will be relatively reduced, the treatment performance will decrease, and the quality of the treated water will tend to deteriorate. Conversely, as the surface area increases, even if the DO of the bulk water is the same, the total amount of oxygen diffused into the biofilm will relatively increase, the treatment capacity will increase, and the quality of the treated water will tend to improve. In addition, sufficient treatment performance can be achieved even at low DO, and the amount of aeration and electric power involved in aeration can be reduced.

In the case of equipment that uses self-granulating microbial granules, if the granules enlarge due to long-term operation, the specific surface area of the self-granulating microbial granules in contact with bulk water per filling volume decreases, and the surface area in contact with bulk water is reduced.

In the case of a device that uses a carrier, especially when using a carrier with a structure that has voids inside the carrier, if the amount of biofilm retained by the carrier increases due to long-term operation, the void space inside the carrier may become a biofilm. The contact area between bulk water and biofilm decreases due to blockage by non-bioactive solids such as itself and scale components. As a result, the specific surface area in contact with bulk water per carrier volume is reduced, and the surface area in contact with bulk water per aeration tank volume is reduced. In addition, regardless of the presence or absence of voids inside the carrier, if it is operated for a long period of time, the biofilm density within the biofilm will increase, or the biofilm will deteriorate due to the accumulation of non-biologically active solids such as scale components. The increase in solids density reduces the rate of oxygen diffusion.

In particular, in the case of treatments that utilize biofilms attached to fixed carriers, if the operation period is extended over a long period of time, there is a tendency for excessive biofilms to be retained in the spaces between the carriers. In such a situation, the capacity of the bulk aqueous phase decreases relatively as the biofilm retention increases. Furthermore, if this condition progresses further, the spaces between the carriers become blocked by biofilms, creating spaces into which bulk water 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 to the biofilm tends to decrease over time.

<Example of control logic construction>
As an aeration control method used in the present invention, a case will be described in which general DO control is performed at high loads, and so-called intermittent aeration is combined, in which weak aeration and strong aeration are alternately repeated at low loads. In the intermittent aeration of this example, a weak aeration process in which the aeration volume is suppressed at a minimum constant air volume for a predetermined time period, and a strong aeration process in which DO control is performed for the remaining time are repeated every fixed time cycle. In the explanation of intermittent aeration in this case, the total process time of the control cycle consisting of a weak aeration process and a strong aeration process is called cycle time, and the process time of the weak aeration process is called the weak aeration process time, and the process of the strong aeration process is called the cycle time. The time is referred to as the strong aeration process time.

Create in advance the correlation between the raw water load and oxygen consumption rate of the reaction tank and the appropriate values of the DO target value and aeration intensity setting value (in this example, the setting value of the weak aeration process time) using multiple oxygen diffusivity indicators. . The following Table 1, in which the correlation between raw water load, DO target value, and weak aeration process time is arranged in a control table, will be explained as an example.

In Table 1, five control tables (correlation tables) are shown in order from the top. Each table has been created assuming different conditions for oxygen diffusivity in biofilms, with the first table being created under conditions with the highest oxygen diffusivity, the oxygen diffusivity decreasing sequentially, and the fifth table being created under conditions with the highest oxygen diffusivity. The table assumes the worst oxygen diffusivity. Each table represents the relationship between TOC carrier volume load, DO target value, and weak aeration process time setting value.

For example, in the third table from the top, the TOC load per carrier filling volume [kgC/(m ³ ·d), hereinafter the unit may be omitted. ] is 0.1 or more and less than 0.6, the target value of DO in the strong aeration process is set to 3.1 mg/L, and when the TOC load is 0.1 or more and less than 0.2, the weak aeration process is set. If the TOC load is 0.2 or more and less than 0.3, the weak aeration process time is 90 minutes every 2 hours. If the TOC load is 0.3 or more and less than 0.4, the weak aeration process is If the TOC load is 0.4 or more and less than 0.5, the weak aeration process time is 60 minutes every 2 hours. If the TOC load is 0.5 or more and less than 0.6, the weak aeration process is The appropriate time is set to 40 minutes every 2 hours, and if the TOC load is 0.6 or more and less than 0.7, the DO target value for the strong aeration process is set to 3.8 mg/L, and the weak aeration process time is set as an appropriate value. for 20 minutes every 2 hours. If the TOC load is 0.7 or more, do not perform intermittent aeration (weak aeration time is 0 minutes). If the TOC load is 0.7 or more and less than 0.9, reduce the DO target value to 3. 9. When the TOC load is 0.9 or more and less than 1.0, the DO target value is set to 4.4, and when the TOC load is 1.0 or more, the DO target value is set to 4.8. The same applies to other tables.

When the quality of the treated water (for example, TOC concentration) is good over a predetermined period of time, the table moves up one level, and when the quality of the treated water is poor over a certain period of time, the table moves down one level. For example, if the quality of the treated water deteriorates despite continued appropriate aeration control using the standard control table (third table from the top), it is assumed that the oxygen diffusivity has deteriorated, and the level is lowered. Switch to aeration control using the side control table (fourth table from the top). On the other hand, when the quality of the treated water becomes excessively good, it is assumed that stable treatment can be achieved even if the aeration is weakened, and the aeration control is switched to using the control table one level above (the second table from the top). By applying this control procedure, a control table that assumes oxygen diffusivity according to the state of the actual biofilm will be automatically selected, and as a result, the oxygen diffusivity index will be estimated.

Although it is difficult to directly measure the oxygen diffusivity index with an actual machine, the above operation makes it possible to estimate it from normal operation data.

Now, if the water quality of the treated water is higher than the TOC upper limit value of the control target when controlling by selecting the control table (the bottom table in Table 1) that assumes a situation where the treatment performance has decreased, then However, since no control table has been set for increasing the aeration intensity, performance is restored by partially disassembling the granules or by removing the biofilm attached to the carrier.

Even if the treatment performance is slightly degraded (for example, the state shown in the fourth control table from the top), partial dismantling of the granules or removal of biofilm attached to the carrier may be necessary, taking into consideration the energy efficiency and cost of strengthening aeration. Peeling may be performed to recover performance.

<Control management indicators>
A method of calculating the raw water carrier load when the raw water load is used as a management index will be explained next using FIG. 4.

[How to calculate raw water load from TOC meter and flow meter]
The biological treatment apparatus shown in FIG. 4 performs aeration control based on the raw water load using the measured value of the TOC concentration of the raw water.

In the biological treatment apparatus shown in FIG. 4, wastewater to be treated (raw water) is introduced into an aeration tank 2 through a pipe 1. The aeration tank 2 is filled with a carrier C carrying a biofilm. An aeration pipe 3 is installed at the bottom of the aeration tank 2, and air is supplied from a blower 4 through a pipe 5 to perform aeration.

The water that has been aerobically treated by the biofilm passes through the screen 6 and is taken out from the pipe 7 as treated water.

In this biological treatment device, as measurement means, a flow meter 22 and a TOC meter 23 that measure the flow rate and TOC concentration of raw water flowing through the piping 1, a DO meter 19 that measures the DO concentration in the aeration tank 2, and a An airflow meter 20 is provided to measure the amount of air supplied to the air diffuser 3 , and these detected values are input to the controller 21 . The aeration intensity is controlled by controlling the blower 4 by the controller 21.

By measuring the raw water flow rate with the flow meter 22 and measuring the TOC concentration of the raw water with the TOC meter 23, the TOC load is calculated as the raw water load.

<Raw water load>
The raw water load is calculated using the following formula.

Load=Q・Conc
Load: Raw water load [kg/d]
Q: Raw water flow rate [m ³ /d]
Conc: Raw water concentration [kg/m ³ ]
The raw water concentration is not limited to TOC, but other indicators may be used depending on the purpose of treatment as long as it is the concentration of a substance to be oxidized by microorganisms. Typically, concentrations of specific chemical substances such as COD _Cr , _CODMn , nitrite nitrogen, ammonia nitrogen, organic amines, etc. can be utilized.

<Carrier volume load>
The carrier volume load is calculated by the following formula.

Load _{Carrier Vol} = Load/V _Carrier
Load _CarrierVol : Carrier volume load [kg/( ^m3・d)]
V _Carrier : carrier filling volume in the aeration tank [m ³ ]

<Surface area load of carrier>
The carrier surface area loading is calculated by the following formula.

Load _CarrierSurf = Load/S _Carrier
Load _CarrierSurf : Carrier surface area load [kg/( ^m2・d)]
S _Carrier : Total surface area of the carrier group in the aeration tank [m ² ]

[Case 1: Method of calculating oxygen consumption rate from air flow meter and exhaust gas meter]
The aeration air volume and the oxygen concentration in the exhaust gas are measured, and the oxygen consumption rate qO ₂ is directly calculated using the following equation.

OTE: Oxygen transfer efficiency [-]
Z ₀ : Oxygen molar fraction in blown air [-]
Z: Oxygen mole fraction in exhaust gas [-]
qO ₂ : Oxygen consumption rate [kg/d]
Gν: Aeration air blowing flow rate converted to standard conditions [Nm ³ /d]
ν _m : Specific volume of oxygen [Nm ³ /kg]

[Case 2: Method of calculating oxygen consumption rate from DO meter and aeration air volume]
The aeration air volume and DO are measured to indirectly estimate the oxygen consumption rate _qO2 .
(i) (Preparation before installing the control device) Calculate the oxygen solubility index φ necessary for estimating the oxygen consumption rate using the following formula.

OTE: Oxygen transfer efficiency [-]
Z ₀ : Oxygen molar fraction in blown air [-]
Z: Oxygen mole fraction in exhaust gas [-]
φ: Oxygen solubility index [m]
ν _m : Specific volume of oxygen [Nm ³ /kg]
h: Water depth of the air diffuser [m]
Cs: Saturated dissolved oxygen concentration [kg/m ³ ]
C: Dissolved oxygen concentration in the mixed liquid [kg/m ³ ]

(ii) (During equipment operation) Continuously measure changes in oxygen consumption rate over time.

The oxygen consumption rate qO ₂ is continuously estimated from the DO meter, continuous measurement data of the aeration air volume, and the oxygen solubility index φ determined in advance using the following equation.

qO ₂ : Oxygen consumption rate [kg/d]
Gν: Aeration air blowing flow rate converted to standard conditions [Nm ³ /d]
h: Water depth of the air diffuser [m]
Cs: Saturated dissolved oxygen concentration [kg/m ³ ]
C: Dissolved oxygen concentration in the mixed liquid [kg/m ³ ]
φ: Oxygen solubility index [m]

[Correlation between raw water load or oxygen consumption rate and DO target value or aeration intensity setting value]
The correlation between the raw water load or oxygen consumption rate and the DO target value or aeration intensity setting value can be determined using preliminary experiment result data, actual operating performance data, simulation results of a mechanistic model that takes into account the diffusivity of oxygen in biofilms, etc. is set using

Note that in an aeration tank, the raw water load or oxygen consumption rate may change rapidly over time on a minute-by-minute basis; ) changes relatively slowly over time, from days to months. Therefore, it is preferable to update the calculated raw water load or oxygen consumption rate frequently. In addition, the carrier filling volume in the aeration tank or the total surface area of the carrier group in the aeration tank is determined by sampling and analyzing the carriers periodically (for example, once every 1 to 3 months). , the total surface area data of the carrier group may be updated.

[Correlation between raw water load or oxygen consumption rate and appropriate value of DO target value and/or aeration intensity setting value]
In one aspect of the aeration control method used in combination with the present invention, the raw water load or oxygen consumption rate per unit volume or unit surface area of the carrier or granules in the aeration tank, and the DO target value and/or weak aeration time for this are determined. A plurality of correlations with the appropriate value are set in advance according to differences in oxygen diffusivity, and responses are taken based on the correlations assuming the characteristic oxygen diffusivity according to fluctuations in the measured value of the oxygen consumption rate. Set appropriate values for the DO target value or other aeration intensity settings.

The combination of the raw water load or oxygen consumption rate and the DO target value and/or weak aeration time setting value should be determined based on preliminary experiment result data, actual operating performance data, or a mechanism that takes into account the diffusivity of oxygen in biofilms. It is set using model simulation results.

To implement this correlation in the control system, implement it with a functional formula that describes the correlation between the raw water load or oxygen consumption rate and the DO target value and/or the appropriate value of the weak aeration time, or use a control table, etc. Any of the following methods of expression may be used.

[Biofilm mechanism model for creating control table]
One method for constructing a control table is to measure the decrease in pollutants and the increase or decrease in the amount of activated sludge bacteria in the biofilm when the biofilm comes into contact with the bulk water phase in a fluid state containing pollutants and oxygen. A dynamic model to estimate (hereinafter sometimes referred to as a biofilm mechanism model) can be used. Such a dynamic model assumes a situation in which bacterial growth, contaminant consumption, and oxygen consumption occur simultaneously within a biofilm, and a situation in which dissolved oxygen in the bulk water phase diffuses into the biofilm and oxygen increases in bulk water volume due to aeration. It is necessary to consider the phenomenon of dissolution when constructing the system. In addition, the increase or shrinkage of the biofilm occurs due to an increase or decrease in the volume of a group of bacterial cells due to proliferation and death of bacterial cells, adhesion of bacterial cells from bulk water, or detachment of bacterial cells to bulk water. When using a dynamic model for biofilm treatment, it is necessary to mathematically model these phenomena. Since such a phenomenon originally occurs in three-dimensional space, modeling is complicated, but it is possible to express the increase and shrinkage of biofilm using a one-dimensional model that takes into account changes only in the thickness direction. Simulations can be performed relatively easily. As a mathematical model for simulating wastewater treatment using activated sludge, for example, a series of mathematical models proposed by the International Water Association's Task Group can be used (Report 1 below). As an example of a mathematical model targeting biofilms, the following paper 2 has been reported.

1. M Henze; IWA. Task Group on Mathematical Modeling for Design and Operaton of Biological Wastewater Treatment; et al
2. Boltz, JP, Johnson, BR, Daigger, GT, Sandino, J.,(2009a). “Modeling Integrated Fixed-Film Activated Sludge and Moving Bed Biofilm Reactor Systems I: Mathematical Treatment and Model Development”. Water Environment Research, 81( 6), 555-575

By using the mathematical model as described in the previous section, for example, a mathematical model of a fluidized bed carrier can be constructed. Generally, such mathematical models are often described in the form of simultaneous ordinary differential equations, and the dynamic behavior of the same process can be simulated using numerical integration software for simultaneous ordinary differential equations. For example, it is possible to predict the quality of treated water according to the DO conditions of the bulk water phase, which vary depending on the specific equipment configuration, load assumption, and aeration intensity.

By using the mathematical model described in the previous section, it is possible to calculate, for example, the TOC of treated water when the treatment is carried out under various oxygen permeability conditions in various biofilms, under various loading conditions, and at various aeration intensities. concentration can be predicted. A table in which simulation results are organized can be created and used to create a control table used in the control system of the present invention.

[Control of aeration intensity]
The aeration intensity can be controlled, for example, by the aeration air volume or the air supply volume to the blower, the DO target value of DO control, the weak aeration process time in intermittent aeration operation, or a combination thereof.

The aeration intensity is controlled continuously or stepwise depending on the raw water load or oxygen consumption rate.

[Weak aeration process air volume, minimum carrier flow aeration air volume, maximum aeration stop time, maximum weak aeration time]
In the present invention, the constant air volume in the weak aeration process is referred to as the weak aeration process air volume. This air volume is the air volume required to maintain minimum agitation of the liquid phase in the treatment tank in the weak aeration process and to maintain contact between the biofilm and the bulk water. When aeration is completely stopped in the weak aeration process, stirring by aeration disappears, so a mechanical stirring function separate from aeration is required. In this example, it is assumed that even in the weak aeration process, minimal aeration is performed and stirring is performed by aeration. The minimum carrier fluidization aeration air volume is particularly important in equipment that uses fluidized bed carriers, by ensuring the fluidity of the entire carrier during the strong aeration process, preventing the carrier from accumulating at the bottom of the aeration tank, and reducing the biofilm and bulk that decreases with accumulation. This is the minimum amount of aeration air required to suppress the decrease in the contact area with water, as well as the problem of sludge decay and hydrogen sulfide odor caused by accumulation at the bottom of the carrier, and is usually a weak aeration. It will be more than the process air volume. In the strong aeration process, DO control is performed, but the control is performed with the constraint that the air volume is always equal to or greater than the minimum carrier flow aeration air volume.

In the explanation of intermittent aeration in this case, the total process time of the control cycle consisting of the weak aeration process and the strong aeration process is referred to as cycle time, and the process time of the weak aeration process is called the weak aeration process time, and the process time of the strong process is called the cycle time. This is called strong aeration process time. The weak aeration step time and the DO target value in the strong aeration step are controlled continuously or stepwise depending on the raw water load. The strong aeration process time is automatically determined as the cycle time minus the weak aeration process time. Further, the longest time when adjusting the weak aeration process time is referred to as the longest weak aeration process time. Therefore, the maximum weak aeration process time is shorter than the cycle time.

In a fluidized bed carrier device that uses an intermittent aeration method that repeats aeration and aeration stop, the minimum amount of carrier fluid aeration air volume is not secured during the aeration stop or weak aeration operation process, so carriers accumulate on the bottom of the device during this process. do. Remobilization of the accumulated carriers is achieved by limiting the time of this process to a certain period of time and ensuring an air volume greater than the minimum carrier flow aeration air volume (referred to as the air volume in the strong aeration process in the present invention) during the remaining cycle time. This aims to suppress the problem of sludge putrefaction and the generation of hydrogen sulfide odor that occur due to long-term accumulation at the bottom of the carrier.

It is preferable that the minimum carrier flow aeration air volume or the maximum aeration stop time be determined based on preliminary experiment result data, actual operation data in an actual machine, and the like. In this case, when the raw water load is high, intermittent aeration operation that repeats weak aeration and strong aeration is not performed, but continuous aeration is performed to maximize the capacity of the aeration equipment. When the raw water load decreases, a lower DO target value is set according to the control table to suppress the aeration air volume, but when the aeration air volume reaches the minimum carrier flow aeration air volume, the aeration method is switched to intermittent aeration operation. The operation of switching from continuous aeration operation to intermittent aeration operation can be performed by directly measuring the air flow rate and using the minimum carrier flow aeration air flow rate as a criterion, but by monitoring any of the indicators (a) to (d) below and determining the index value. By pre-evaluating the relationship between airflow and airflow, the aeration airflow is estimated based on the index, and continuous aeration is applied when aeration airflow ≧ minimum carrier flow aeration airflow, and continuous aeration is applied when aeration airflow < minimum carrier flow aeration airflow. It is also possible to control intermittent aeration.
(a) The measured value of the raw water load is below the predetermined value (b) The measured value of the oxygen consumption rate of the aeration tank is below the predetermined value (c) The DO target value controlled according to the load under continuous aeration is below the predetermined value (d ) The set value of the aeration intensity (including aeration air volume), which is controlled according to the load under continuous aeration, is below the specified value.

The raw water load in (a) above is preferably any one of an inflow load, a tank load, a carrier volume load, and a carrier surface area load.

[Biological treatment other than fluidized bed]
In FIG. 1, biological treatment using a fluidized bed carrier has been described, but the present invention can be carried out in a similar manner when using a fixed bed carrier or granules.

In this embodiment, it has been explained that it is used when wastewater containing organic matter is treated by aerobic biofilm treatment accompanied by aeration. The present invention can also be carried out using the same method when performing biological treatment including an aerobic treatment step using biofilm.

[Example 1]
When operating the fluidized bed carrier aerobic biological treatment equipment shown in Figure 1, the control table shown in Table 1 is switched depending on the quality of the treated water while appropriately controlling aeration at any time according to the raw water load. controlled. The five types of control tables based on differences in oxygen diffusivity are designated from the top to the bottom by control table names "Excellent,""Good,""Standard,""SlightlyWorsened," and "Worst." The criterion for switching to a control table with high oxygen diffusivity is the treated water TOC of less than 5 mg/L, and the treated water quality in this situation is referred to as "below target". The criterion for switching to a control table with low oxygen diffusivity is that the treated water TOC is greater than the upper limit of 10 mg/L, and the treated water quality in this situation is referred to as "deteriorated". When the treated water TOC is 5 mg/L or more and 10 mg/L or less, the control table is not changed, and the treated water quality in this situation is called "good." Confirmation of treated water quality by manual analysis and decision to change the control table were conducted once a day. At the start of operation, operation control was started using the fourth "slightly lower" control table from the top.

90 days after the start of operation, the treated water TOC was 11 mg/L under aeration control using the "slightly decreased" control table, and the treated water quality was in a "deteriorating" state, so the control table was changed to "decreased" and the next After 91 days, the TOC of the treated water recovered to 7 mg/L, which was within the target TOC range and became "good", so aeration control using the "decrease" control table was continued. However, after 180 days, the treated water quality became 11 mg/L even under aeration control with the control table set to "Decrease", resulting in a "Deterioration" condition. Since the aeration control according to the "decrease" control table deviated from the treatment target range, it was determined that the oxygen diffusivity had fallen beyond the allowable range, and temporary strong aeration (air volume 20 m ³ / (reaction tank The biofilm on the surface of the carrier was peeled off for 12 hours. The detached biofilm was discharged from the aeration tank 2 together with the treated water.

After the stripping process, aeration control was continued using the "decrease" control table. The next day, 181 days later, the treated water TOC improved to 3 mg/L and was now "below the target", so the control table was changed to "slightly decreased", and 182 days later, the treated water TOC was still 3 mg/L and was "below the target". Since it was confirmed that the control table was ``standard,'' the control table was changed to ``standard.'' After 183 days, it was confirmed that the TOC of the treated water was 6 mg/L, which was ``good''. Continued. As a result, the control table was switched from "decreased" to "standard" by the biofilm peeling process, and it was confirmed that the oxygen diffusivity of the microbial carrier was restored by the biofilm peeling process of the present invention.

2 Aeration tank 3, 3a, 3b, 3c Aeration pipe 4, 17 Blower 12 Stirring water tank 13 Stirrer 15 Sorting device

Claims (4)

In a method for aerobic biofilm treatment of substances to be removed in raw water using a biofilm retention carrier filled in the aeration tank in an aeration tank to which raw water is supplied,
An aerobic biofilm treatment method, characterized in that a treatment for reducing the amount of biofilm retained on a carrier is performed when any one or more of the following is satisfied.
i) The calculated value of the oxygen diffusivity index (value calculated by simulation calculation from changes in treated water quality with respect to raw water load per carrier filling volume and aeration conditions) is below a predetermined lower limit value .
iv) A specified water quality item of treated water exceeds a specified target value in a situation where the aeration air volume per raw water load exceeds a specified upper limit . The biofilm retention amount reduction treatment of the carrier is performed by one or more of any one or more of strong agitation using a rotating stirring blade or backwashing, strong aeration, high-flow circulation, and passing water in the tank to a crushing pump. The aerobic biofilm treatment method according to claim 1, characterized in that: The aeration tank is filled with a biofilm-retaining carrier, and the amount of biofilm retained in the carrier can be reduced by adding a new carrier and/or a biofilm-attached carrier that has been subjected to biofilm removal treatment to the aeration tank, or by adding aeration to the aeration tank. 2. The aerobic biofilm treatment method according to claim 1, wherein the method is carried out by replacing the carrier in the tank. An aerobic biofilm treatment device having an aeration tank to which raw water is supplied, a biofilm holding carrier filled in the aeration tank, and an aeration device for aerating the aeration tank,
An aerobic biofilm treatment device characterized by having a means for reducing the amount of biofilm retained on a carrier, which is activated when any one or more of the following is satisfied.
i) The calculated value of the oxygen diffusivity index (value calculated by simulation calculation from changes in treated water quality with respect to raw water load per carrier filling volume and aeration conditions) is below a predetermined lower limit value .
iv) A predetermined water quality item of the treated water exceeds a predetermined target value when the aeration air volume per raw water load is a predetermined upper limit value .

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