Method of obtaining cod of treated water from activated sludge

Abstract

To provide a method to calculate COD of treated water from measured data such as raw water and treated water based on an activated sludge operation status.SOLUTION: COD of treated water is calculated by formula (1), by using a readily degradable BOD value and a slow degradable BOD value of treated water obtained in a process of sampling the activated sludge mixed liquid that has finished treatment in an aeration tank, aerating it with a constant overall mass transfer coefficient (KLa) under a constant temperature condition, and measuring change data of dissolved oxygen concentration (DO) from the start of aeration to a specified aeration time, and calculated values of concentration of virtual raw water BODts (virtual raw water BODts) at a sampling point. Formula (1): COD of treated water=k1*readily degradable treated water BOD+k2*slow degradable BOD of treated water+k3*virtual raw water BODts.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for obtaining COD of treated water in wastewater treatment using activated sludge or the like that utilizes aerobic microorganisms. [Background technology]

[0002] Treatment methods such as the activated sludge process, which uses aerobic microorganisms, are the most common treatment methods for wastewater containing organic pollutants. In the activated sludge process, BOD management of treated water is extremely important for operational management, but since measuring BOD using the official method takes a long time, it is practically unsuitable for operating activated sludge. For this reason, it is common to focus on COD, which requires a short measurement time, and measure the COD of the treated water to estimate the BOD of the treated water and the treatment state of the activated sludge.

[0003] However, if wastewater (hereinafter, raw water) flowing into activated sludge contains not only easily degradable pollutants but also slowly degradable and non-degradable components, when the raw water is treated with activated sludge, some of the easily degradable and slowly degradable pollutants are treated by the activated sludge and the BOD of the treated water is reduced, but the water-soluble non-degradable pollutants cannot be treated and remain as they are in the treated water. Therefore, when there is a load fluctuation in the raw water, even if there is a certain correlation between the raw water COD and the raw water BOD on a raw water basis, when the treated water BOD is well treated and becomes a small value on a treated water basis, the COD corresponding to the treated water BOD also becomes smaller in proportion to this, but the COD due to the water-soluble non-degradable pollutants remains as it is in the treated water and fluctuates with the raw water fluctuation, so the correlation between the treated water COD and the treated water BOD is significantly reduced.

[0004] The value of treated water COD is also necessary from the viewpoint of legal regulations, and generally, the COD of raw water and treated water for activated sludge is measured by an independent automatic COD measuring device or by manual COD measurement based on the official method. The COD of organic polluted wastewater is generally treated with activated sludge, but if the correlation between the treated water COD and treated water BOD decreases, it becomes difficult to estimate the treated water BOD value from the measured value of treated water COD, and it becomes difficult to judge whether the purification action of activated sludge is working normally, resulting in problems in the operation and management of activated sludge.

[0005] Because the measurement principles for COD and BOD are different, and the removal effects in activated sludge are also different, no calculation method for relating the COD and BOD of treated water has been found in existing patent disclosures. For example, Patent Document 1 describes converting the COD value obtained from a UV meter to a BOD value, but this is the COD and BOD on a raw water basis, not the conversion of COD and BOD on a treated water basis.

[0006] Patent Document 2 by the present applicant discloses a method in which an activated sludge mixed liquid after treatment in an aeration tank is sampled, aerated at a constant temperature and constant overall mass transfer coefficient KLa, and calculated from data on changes in dissolved oxygen concentration from the start of aeration until a specified aeration time, the readily degradable BOD and slowly degradable BOD values ​​in the activated sludge mixed liquid. However, the relationship between these values ​​and the COD of the treated water is not mentioned. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent Publication No. 2000-107796 [Patent Document 2] Patent application 2022-70753 [Patent Document 3] Patent Publication No. 2001-235462 [Patent Document 4] Patent Publication No. 2006-84240 DISCLOSURE OF THEINVENTION [Problem to be solved by the invention]

[0008] In view of the above problems, the present invention provides a method for obtaining treated water COD by partially utilizing the BOD measurement method disclosed in Reference 2 by the present applicant. [Means for solving the problem]

[0009] In view of the above problems, the present invention provides the following: A method for obtaining treated water COD in a biological treatment device having an aeration tank that mainly uses planktonic aerobic microorganisms for treatment, comprising: A method for obtaining treated water COD, characterized in that the activated sludge mixed liquid after treatment in the aeration tank is sampled, aerated at a constant overall mass transfer coefficient (KLa) under constant temperature conditions, and the change in dissolved oxygen concentration (hereinafter referred to as DO) from the start of aeration to a specified aeration time (hereinafter referred to as measurement time) is measured.The method then uses the treated water's readily degradable BOD value and slowly degradable BOD value, as well as the calculated virtual raw water BODts concentration at the sampling point (hereinafter referred to as virtual raw water BODts), to obtain the treated water COD according to equation (1). Treated water COD = k1 x easily degradable BOD of treated water + k2 x slowly degradable BOD of treated water + k3 × Virtual raw water BODts (1) where k1, k2, and k3 are coefficients for converting each BOD value into a COD value, and are determined according to the substrate composition of readily degradable BOD, slowly degradable BOD, and water-soluble pollutants in the treated water, respectively. In formula (1), the readily biodegradable BOD of the treated water is the value obtained by multiplying the area enclosed by the C1_hf(t) value change curve (curve 2) according to formula (2), which represents the change in DO value during aeration, and the measured DO value change curve (curve 1), by KLa. C1_hf(t)=DOhf1-(DOhf1-C0)exp(-KLa·t)···(2) formula The slowly degradable BOD of the treated water is the value obtained by multiplying the area enclosed by the C2_hf(t) value change curve (curve 3) calculated by equation (3) and the measured DO value change curve (curve 1) by KLa, minus the easily degradable BOD of the treated water. C2_hf(t)=DOhf2-(DOhf2-C0)exp(-KLa·t)···(3) formula The hypothetical raw water BODts is the BODts value at the sampling point of the raw water that flowed into the aeration tank before the sampling, and is an estimated value calculated based on the residence time distribution of the raw water and returned sludge from the point where they flow into the aeration tank to the time they reach the sampling point. Here, the BODts value for raw water means the amount of oxygen consumed during the period from when the DO value of a sampled activated sludge mixed liquor reaches DOhf under conditions of constant temperature and constant aeration intensity, until the DO value returns to DOhf, when raw water is added to the activated sludge mixed liquor, converted into the amount of oxygen consumed by the raw water based on the amount of raw water added and the amount of activated sludge mixed liquor in the measuring device. In equations (2) and (3), C0 is the initial DO value of aeration. DOhf1, DOhf2, and DOhf are the DO values ​​at which the oxygen consumption rate and the oxygen supply rate by aeration are balanced when there is easily degradable BOD, slowly degradable BOD, or no BOD remaining in the sampled activated sludge mixed liquor, respectively. Effect of the Invention

[0010] This invention makes it possible to correlate the COD and BOD of the treated water from activated sludge. This makes it possible to calculate and output the COD of the treated water based on the operational management status of the activated sludge, and conversely, to estimate the treatment status of the activated sludge based on the measured COD value of the treated water. Furthermore, it will be possible to appropriately comply with COD regulations regarding industrial wastewater, thereby contributing to the improvement of activated sludge operation and management technology. It will also serve as useful learning data for AI when building an AI-based activated sludge operation management support system. [Brief description of the drawings]

[0011] [Figure 1] This is a graph showing the relationship between the change in DO and the easily degradable BOD and slowly degradable BOD over the duration of aeration. [Diagram 2]1 is a graph showing the change in the readily degradable BOD and slowly degradable BOD values ​​in treated water over time, using the measurement method described in the embodiment. [Diagram 3] FIG. 2 is a graph showing the change in raw water BODts value. [Figure 4] 1 is a graph showing the measured values ​​of raw water BODts, raw water flow rate, and returned sludge volume at each measurement time (horizontal axis). [Diagram 5] FIG. 5 is a diagram showing a hypothetical raw water BODts calculated using the measurement data of FIGS. 3 and 4. [Figure 6] FIG. 1 is a graph showing treated water COD calculated by formula (1). [Figure 7] FIG. 1 is a graph comparing the measured and calculated COD values ​​of treated water. [Figure 8] FIG. 1 is a diagram showing a method for calculating slowly degradable BOD when the KLa measurement operation and the raw water BODts measurement operation are performed after the measurement of readily degradable BOD and during the measurement of slowly degradable BOD. BEST MODE FOR CARRYING OUT THEINVENTION

[0012] The removal of organic pollutants from wastewater using activated sludge is a mechanism by which the aerobic microorganisms that make up the activated sludge ingest and decompose the pollutant components in the wastewater. It is practical to consider the organic pollutants by classifying them into easily degradable components, slowly degradable components, and difficult to decompose components.

[0013] The characteristics of each component from the viewpoint of the removal action of activated sludge are as follows. Easily decomposable components are quickly taken up by microorganisms and stored as nutrients within the microbial body, which then use the nutrients to carry out biological activity and grow. The portion that cannot be taken up by the microorganisms flows out as treated water BOD and is also detected as treated water COD. The insoluble components of the slowly degradable components are first trapped in the flocs, where some are hydrolyzed and then ingested by microorganisms in the same way as the easily degradable components. Once trapped in the flocs, they are removed from the wastewater, and the hydrolyzed portion is converted to BOD, while the remainder is discharged from the system as excess sludge as an inactive substance. The water-soluble components of the slowly degradable components are ingested and decomposed by organisms at a slow reaction rate, but the portion that cannot be decomposed is discharged as treated water. Some of the discharged components are measured as BOD and COD using the official methods, while the remainder is not measured as BOD but as COD. The insoluble components of the difficult-to-decompose components are trapped in the flocs and discharged as excess sludge, and are removed from the wastewater. Water-soluble components cannot be decomposed by microorganisms and are discharged as wastewater. However, since these components cannot be measured by the official BOD method, they are measured as the total treated water COD. From the above, it can be concluded that the factors that affect the COD of treated water are the COD due to untreated easily decomposable components in the treated water, the COD due to untreated slowly decomposable pollutant components, and the COD due to water-soluble persistent components.

[0014] The concept is as above, but in actual activated sludge treatment, there is no clear division between the components, and many pollutant components exhibit intermediate behavior. Below, we will show a practical and effective method of classification.

[0015] Referring to FIG. 1, the activated sludge mixed liquid that has finished being treated in the aeration tank is sampled in the aeration container of the measuring device, and the activated sludge mixed liquid is aerated at a constant overall mass transfer coefficient KLa that is greater than the oxygen consumption rate of the activated sludge mixed liquid in the endogenous respiration state while keeping the liquid temperature in the aeration tank constant. The DO change value C changes as expressed by equation (a) with elapsed time being t. dC / dt=KLa(Cs-C)-Rr...(a) formula Here, Cs is the saturated dissolved oxygen concentration, and Rr is the oxygen consumption rate by sludge.

[0016] When aeration continues and the untreated BOD components in the activated sludge mixed liquor disappear, Rr becomes the oxygen consumption rate due to endogenous respiration of the sludge, so Rr can be treated as a constant value. If the initial DO value at the start of aeration is taken as C0 (hereafter referred to as C0), the change in DO, C_hf(t), will be the change expressed by equation (b), where DOhf is the DO value at which the oxygen consumption rate of the sludge and the oxygen consumption rate due to aeration are ultimately balanced. C_hf(t)=DOhf-(DOhf-C0)exp(-KLa·t)...(b) formula

[0017] If untreated easily decomposable BOD components remain in the activated sludge mixed liquor at the time of sampling, the activated sludge mixed liquor will ingest the residual BOD components and consume additional oxygen to incorporate them into the microorganisms as nutrients, so Rr will be a value that changes with t. In this case, equation (a) cannot be integrated, and the measured DO change curve will change due to the oxygen consumption rate of Rr, and will end at the DOhf value when the ingestion and decomposition of the residual BOD components is completed (this DO change curve is called curve 1 (Figure 1)).

[0018] If there are no untreated easily decomposable BOD components remaining in the activated sludge mixed liquid at the time of sampling, Rr is the oxygen consumption rate due to endogenous respiration of the sludge alone and can be treated as being almost constant. The DO change curve (curve 2) in this case is expressed by equation (2) starting from C0, and ultimately becomes constant at DOhf1, where the oxygen consumption rate of the sludge and the oxygen consumption rate due to aeration are balanced. C1(t)=DOhf1-(DOhf1-C0)exp(-KLa·t)···(2) formula

[0019] The area S1 enclosed by curves 1 and 2 multiplied by the overall mass transfer coefficient KLa, which represents the strength of aeration, is the amount of oxygen consumed when the activated sludge microorganisms ingest the residual BOD components and incorporate them into their bodies as nutrients. This value corresponds to the readily degradable BOD in equation (1).

[0020] In curve 1, while the same substrate in the easily decomposable BOD components is being decomposed, the reaction occurs at a nearly constant oxygen consumption rate, so the DO value (DOhfn) remains nearly constant and balances with the oxygen supply rate through aeration, and the rate of DO increase slows. When the decomposition ends and the process moves to the decomposition of the next substrate, the decomposition of the next substrate has a smaller oxygen consumption rate, so there is a step-like change where the DO value becomes nearly constant at a slightly higher value than DOhfn. It is characterized by a temporary increase in the rate of DO increase at this transition point, so the point at which this change in the rate of DO increase stops is the end of the decomposition of the easily decomposable BOD components, and the DO value at this point is DOhf1 in Figure 1.

[0021] If the activated sludge mixed liquid, from which the untreated easily decomposable BOD components have been removed, is further aerated, DOhf will rise more slowly and little by little. The rate of increase differs depending on the MLSS and the nutritional state of the sludge, so it cannot be generalized, but it is generally about 0.001 to 0.01 mg / l / min. This stage is represented by the change in DO during the process in which water-soluble slowly degradable pollutants are gradually decomposed and the process in which slowly degradable pollutants incorporated into the activated sludge mixture are hydrolyzed and then absorbed into the sludge.

[0022] If there are no untreated easily or slowly decomposable BOD components remaining in the activated sludge mixed liquid at the time of sampling, the DO change will be expressed by equation (3), starting from C0 and remaining almost constant at the DO value (DOhf2) where the oxygen consumption rate by the sludge's endogenous respiration and the oxygen supply rate by aeration are balanced (this DO change curve will be called curve 3). C2(t)=DOhf2-(DOhf2-C0)exp(-KLa·t)···(3) formula

[0023] If there are residual easily degradable BOD components or slowly degradable BOD components at the time of sampling, the measured DO change will transition at a DO value slightly lower than that of curve 3. When there are no more easily degradable BOD components remaining, it will reach DOhf1, and when there are no more slowly degradable BOD components remaining, it will end at the same DO value (DOhf2) as curve 3. The value multiplied by the (area 1 + area 2) enclosed by this measured DO change curve and curve 3, times the overall mass transfer coefficient KLa, which is the aeration strength, represents the total BOD. The value obtained by subtracting the easily degradable BOD from the total BOD is equivalent to multiplying the area S2 surrounded by curve 2 before DOhf1 and measured curves 1 and 3 after DOhf1 by KLa. This is the amount of oxygen consumed when the slowly degradable BOD components are ingested by the activated sludge microorganisms and incorporated into the microbial bodies as nutrients, and corresponds to the slowly degradable BOD in equation (1).

[0024] Since the oxygen consumption rate due to the ingestion and decomposition of slowly decomposable BOD components and the oxygen consumption rate due to endogenous respiration of sludge are small, there will be a large error if the end point of the reaction is determined based on the oxygen consumption rate. Therefore, it is practical to determine DOhf2 based on the calculated slowly decomposable BOD and the measured treated water BOD, and to end the measurement when the change in DO reaches DOhf2.

[0025] Of the treated water COD, the COD due to persistent components cannot be removed by activated sludge, so the entire amount comes from the persistent components contained in the raw water. The amount of persistent components contained in raw water is generally proportional to the pollutant load of the raw water, so this relationship is also used in the calculation in the present invention. The amount of water-soluble pollutants in raw water can be evaluated using indicators such as COD, BOD, TOC, and UV absorption value of the raw water, but since it is necessary to prepare separate measuring devices for each, in the present invention, the oxygen consumption when microorganisms take in the readily degradable BOD in the raw water as nutrients in their bodies (hereinafter referred to as raw water BODts) is used. Since raw water BODts is a value that has a strong correlation with raw water BOD, it can be used as an indicator for evaluating the amount of water-soluble pollutants in raw water, and since it can be obtained in a series of operations to obtain readily degradable BOD and slowly degradable BOD as described below, no separate measuring device is required.

[0026] The method for measuring raw water BODts is described in the applicant's references 3 and 4. The gist of these is as follows. After the sampled activated sludge mixed liquor reaches a DO value (DOhf) that is balanced between the oxygen consumption rate of the activated sludge mixed liquor with no residual BOD and the oxygen supply rate by aeration under conditions of constant temperature and constant aeration intensity, raw water is added to the activated sludge mixed liquor, and the amount of oxygen consumed until the DO value returns to DOhf is converted into the oxygen consumption of raw water from the amount of raw water added and the amount of activated sludge mixed liquor in the measuring device, and the BODts value of the raw water is obtained. The procedure for measuring the BODts of raw water is the same as that of the method of the present invention until it reaches DOhf, and can be carried out as a series of procedures following the measurement of the slowly decomposable BOD.

[0027] After raw water flows into the top of the aeration tank, there is a time delay in the residence time distribution due to the flow of raw water and returned sludge in the aeration tank until it reaches the sampling position where treatment ends. Therefore, it is necessary to calculate the virtual raw water BODts at the sampling position at the time the treated water is sampled, based on the raw water BODts, raw water flow rate, and returned sludge volume before the treated water is sampled.

[0028] It is extremely difficult to accurately calculate the residence time distribution in an aeration tank. For this reason, one method is to use a tracer to experimentally determine the residence time distribution for several cases of raw water flow rates, and then use the above data to interpolate or extrapolate for other raw water flow rates. However, when the operating conditions are wide-ranging, determining the residence time distribution for various cases through tracer experiments would be an enormous amount of work and would be unrealistic. Therefore, it is common to obtain the residence time distribution from a numerical calculation that models the flow conditions in the aeration tank as shown below. There are several methods for modeling, but one example is to model the aeration tank as a complete mixing tank series based on the shape of the aeration tank normally used in activated sludge, and obtain the residence time distribution. The method for calculating the hypothetical raw water BODts at the point where the treated water was sampled using this method is shown below.

[0029] From the time when the activated sludge mixed liquid after treatment is sampled to a time approximately three times before the average residence time in the aeration tank, the raw water inflow volume and the returned sludge volume (hereinafter referred to as the flow rate) are subdivided into m portions for each time period, and the delta response equation (4) of the subdivided flow rate is calculated.

number

[0030]

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[0031] In addition, since the value of RS_BODtsi for the return sludge is the same as the virtual raw water BODts of the treated water at the time of subdivision, it can be calculated using equation (5) using the raw water flow rate Fi, BODtsi, and the return sludge flow rate FRSi and RS_BODtsi from even earlier than the time of subdivision. By storing this value, it can be used as RS_BODtsi at the time of subdivision. When starting to calculate the virtual raw water BODts in equation (5), the value of RS_BODtsi is unknown, so the calculation is started with an initial value of RS_BODtsi = 0. If the calculation is repeated up to a time approximately three times the average residence time in the aeration tank, the influence of the initial value will be largely eliminated, and after the time approximately three times the average residence time, the accurate, refined RS_BODtsi will be obtained. The results obtained here are accurate enough for practical use, as will be shown in the examples below.

[0032] Using these measured values, the treated water COD is calculated as shown in Equation (1). Treated water COD = k1 × Readily biodegradable BOD of treated water + k2 × Slowly biodegradable BOD of treated water + k3 × Virtual raw water BODts (Equation (1)) Here, k1, k2, and k3 are coefficients for converting each BOD value to a COD value.

[0033] k1 is the proportionality coefficient at which readily biodegradable BOD is measured as COD, and its value varies depending on the substrate of readily biodegradable BOD. Since the readily biodegradable BOD in the treated water is derived from the substrate composition of the readily biodegradable BOD in the raw water and is the result of the same activated sludge treatment, even if it is treated as a constant value, the error is small.

[0034] k2 is the proportionality coefficient at which slowly biodegradable BOD is measured as COD, and its value varies depending on the substrate of slowly biodegradable BOD. Since the slowly biodegradable BOD in the treated water is derived from the substrate composition of the slowly biodegradable BOD in the raw water and is the result of the same activated sludge treatment, even if it is treated as a constant value, the error is small. The substrate indicating readily biodegradable BOD is different from the substrate indicating slowly biodegradable BOD, and the conversion coefficient k1 for converting readily biodegradable BOD to COD is different from the conversion coefficient k2 for converting slowly biodegradable BOD to COD. Generally, k1 < k2. In this sense, it is meaningful to separate readily biodegradable BOD and slowly biodegradable BOD.

[0035] k3 is the coefficient for converting water-soluble refractory pollutants to COD. The water-soluble refractory pollutants in the treated water are evaluated by virtual raw water BODts, and k3 is expressed as the coefficient for converting virtual raw water BODts to COD. Since the content rate of water-soluble refractory pollutants in the raw water can be treated as not changing even with load fluctuations, even if k3 is treated as a constant value, the error is small.

[0036] The overall mass transfer coefficient KLa, which indicates the strength of aeration, can be calculated using equation (6) as follows. The sampled activated sludge mixed liquid is aerated under conditions of constant temperature and constant aeration strength until no BOD remains. When the change in DO becomes almost constant, the aeration is stopped and DO is temporarily lowered to C1 (Fig. 8) where the difference with DOhf in equation (6) is a value that allows KLa to be calculated with sufficient accuracy. When aeration is then resumed, the change in DO (DO(t)) will be the value calculated using equation (6). KLa is changed and the value where C_hf(t) matches DO(t) is taken as KLa. This operation can be performed after the measurement of the slowly degradable BOD as a series of operations to determine the readily degradable BOD and slowly degradable BOD. C_hf(t)=DOhf-(DOhf-C1)exp(-KLa·t)...Equation (6)

[0037] Furthermore, it is also possible to perform the raw water BODts measurement operation and the KLa measurement operation during the measurement of the slowly degradable BOD after the measurement of the readily degradable BOD. Figure 8 shows an example of this, where the broken line (A) indicates the KLa measurement operation and the broken line (B) indicates the BODts measurement operation process. In this case, since the DO change is already sufficiently small after the measurement of the readily degradable BOD, the calculation of the slowly degradable BOD can be performed as if the KLa measurement operation and the raw water BODts measurement operation were not performed, and the error is negligible and does not cause any practical problems. Specifically, in the figure, the DO change during the KLa measurement is replaced with a straight line L1 connecting the DO values ​​(SP1, EP1) at the start and end of the measurement, and the DO change during the raw water BODts measurement is replaced with a straight line L2 connecting the DO values ​​(SP2, EP2) at the start and end of the measurement, and the calculation is performed. EXAMPLES

[0038] An example of obtaining COD using the results of treating water BOD measurement according to the present invention will be described below. After the treatment in the aeration tank (volume 1000m3) of the standard activated sludge equipment was completed, the activated sludge mixed liquid was sampled in the aeration tank of the measuring equipment, and aerated at a constant temperature and constant KLa in the aeration tank, and the treated water's readily degradable BOD, slowly degradable BOD, and raw water's BODts were measured approximately every 4 hours using the above-mentioned method based on the change in DO. A measurement and analysis device that can automatically perform repeated measurements (product name: "TS Analyzer" manufactured by Ogawa Environmental Research Institute Co., Ltd.) was used for the measurements.

[0039] Figures 2, 3, and 4 show the changes in the measured values ​​(vertical axis) of the easily degradable BOD and slowly degradable BOD of the treated water (Figure 2), the BODts of the raw water (Figure 3), and the raw water flow rate and returned sludge volume (Figure 4) at each measurement time (horizontal axis). Figure 5 is a graph showing the hypothetical raw water BODts calculated by the method of obtaining residence time distribution by tank row modeling described above, using the measurement data of Figures 3 and 4 as a basis, dividing the measured values ​​at each point proportionally to create data divided every 30 minutes. Note that N in the complete mixing tank row model was set to 3.

[0040] Furthermore, FIG. 6 shows the treated water COD values ​​for each hour calculated using equation (1) above with k1 = 0.5, k2 = 1.0, and k3 = 0.15. In addition, k3 was calculated by operating a laboratory-level activated sludge test machine under constant raw water load conditions, measuring the raw water BODts at that time, and calculating the value k3 = treated water COD / raw water BODts from the measured treated water COD value when the treated water BOD was almost 0 mg / l. The values ​​of k1 and k2 can be roughly estimated from the COD and BOD values ​​of each substrate in the raw water. However, since the remaining substrates in the treated water differ from those in the raw water, they were determined by trial and error, referring to the rough values, so that the value calculated using equation (1) would match as closely as possible with the COD values ​​measured at several points. Fig. 7 is a graph comparing the calculated treated water COD trend with the actually measured treated water COD value. The calculated and measured values ​​matched well within the practical range, proving that the treated water COD can be obtained by the calculation method of the present invention. [Industrial Applicability]

[0041] The present invention is not limited to activated sludge, but can be applied to any device that partially uses planktonic aerobic microorganisms, and can be applied to biological denitrification devices, carrier activated sludge used in combination with suspended sludge, fixed bed devices, etc. It can also be applied to obtaining TOD (total oxygen consumption) and TOC (total organic carbon), which have different measurement principles from BOD.

Claims

[Claim 1] A method for obtaining treated water COD in a biological treatment device having an aeration tank that mainly uses planktonic aerobic microorganisms for treatment, comprising: A method for acquiring treated water COD, comprising the steps of: sampling an activated sludge mixed liquid after treatment in an aeration tank; aerating the sample at a constant temperature and constant overall mass transfer coefficient (KLa); and measuring data on changes in dissolved oxygen concentration (DO) from the start of aeration to a specified aeration time; using the readily biodegradable BOD value and slowly biodegradable BOD value of the treated water; and a calculated value of the hypothetical raw water BODts concentration at the sampling point (hereinafter, hypothetical raw water BODts), the treated water COD is calculated according to formula (1). Treated water COD = k1 x easily degradable BOD of treated water + k2 x slowly degradable BOD of treated water + k3 × Virtual raw water BODts ... (1) where k1, k2, and k3 are coefficients for converting each BOD value into a COD value, and are determined according to the substrate composition of readily degradable BOD, slowly degradable BOD, and water-soluble pollutants in the treated water, respectively. In formula (1), the readily biodegradable BOD of the treated water is the value obtained by multiplying the area enclosed by the C1_hf(t) value change curve (curve 2) according to formula (2), which represents the change in DO value during aeration, and the actually measured DO value change curve (curve 1), by KLa. C1_hf(t)=DOhf1-(DOhf1-C0)exp(-KLa・t)...Equation (2) The slowly degradable BOD of the treated water is the value obtained by multiplying the area enclosed by the C2_hf(t) value change curve (curve 3) calculated by equation (3) and the measured DO value change curve (curve 1) by KLa, minus the easily degradable BOD of the treated water. C2_hf(t)=DOhf2-(DOhf2-C0)exp(-KLa・t)...Equation (3) The hypothetical raw water BODts is the BODts value at the sampling point of the raw water that flowed into the aeration tank before the sampling, and is an estimated value calculated based on the residence time distribution of the raw water and returned sludge from the point where they flow into the aeration tank to the time they reach the sampling point. Here, the BODts value for raw water means the amount of oxygen consumed during the period from when the DO value of a sampled activated sludge mixed liquor reaches DOhf under conditions of constant temperature and constant aeration intensity, until the DO value returns to DOhf, when raw water is added to the activated sludge mixed liquor, converted into the amount of oxygen consumed by the raw water based on the amount of raw water added and the amount of activated sludge mixed liquor in the measuring device. In equations (2) and (3), C0 is the initial DO value of aeration. DOhf1, DOhf2, and DOhf are the DO values ​​at which the oxygen consumption rate and the oxygen supply rate by aeration are balanced when there is easily degradable BOD, slowly degradable BOD, or no BOD remaining in the sampled activated sludge mixed liquor, respectively.

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