JPH08117792A - Nitration liquid circulation volume control and denitrifiable circulation ratio-setting in activated sludge circulation variation method - Google Patents

Abstract

PURPOSE: To make it possible to estimate oxygen consumption time and set a circulation ratio within the limits of being free from denitrification obstruction. CONSTITUTION: Various measurement values are input in a circulation volume control part 11. The measurement values are the following five: the first value is one measured using a DO meter 12 based on the dissolved oxygen DO of an inflow water; the second value is one measured using a flow meter based on the flow of the inflow water; the third value is one estimated by measuring the non-oxygen state of the first section of an aerobic tank (nitration tank) 42 using a respiration speedometer (Rr meter) 14; the fourth value is one measured using the DO meter 15 based on the dissolved oxygen of the fourth section of the nitration tank 42, and the fifth value is one measured using a flow meter 16 based on the circulating flow of a nitration liquid in the fourth section of the nitration tank. If these readings are entered into the circulation volume control part 11, the various measurement values are controlled and a pump 45 for circulating the nitration liquid (circulating water) is controlled based on the control value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling the amount of nitrification liquid circulation and a method for setting a denitrifying circulation ratio, which are optimum for stabilizing the denitrification reaction in a modified activated sludge circulation method for nitrogen removal. Regarding

[0002]

2. Description of the Related Art Generally, most of nitrogen components in sewage are ammoniacal nitrogen and organic nitrogen, which are finally oxidized to nitric acid nitrogen by nitrifying bacteria under aerobic conditions in the presence of oxygen. . This reaction formula is as follows when the oxidation of ammonia nitrogen is considered.

NH ₄ ⁺ + 2O ₂ → NO ₃ ⁻ + H ₂ O + 2H ⁺ On the other hand, the denitrification reaction is in the absence of oxygen under anoxic conditions (in a broad sense, it can be said that it is an anaerobic state, but at present, bound oxygen such as nitric acid also does not exist). The absolute anaerobic state is called anaerobic and is distinguished from the anoxic state.) By the action of denitrifying bacteria, it is reduced to nitrogen gas and processed. This reaction is represented by the following equation.

2NO ₃ ⁻ + 5H ₂ → N ₂ + 4H ₂ O + 2OH ^{− The} above reaction requires hydrogen, which is generally supplied by decomposition of organic matter in sewage, though it depends on the treatment process. . As described above, biological nitrogen removal consists of two steps of nitrification and denitrification, and various processes can be considered because the denitrification reaction requires organic matter. Among them, there is a circulation type nitrification denitrification method as a method implemented in sewage treatment. In this method, only agitation is performed without performing aeration in order to make the front stage of the biological reaction tank anoxic, and the subsequent rear stage is aerated, and the nitrate nitrogen generated in the rear stage is circulated to the front stage, and the inflow water is circulated. It is characterized in that denitrification is performed using the organic substance. Depending on the number of times this denitrification and nitrification is repeated, it is divided into a single-stage type and a two-stage type.

FIG. 4 is a system configuration diagram showing a single-stage circulation method. 41 is an anoxic tank (denitrification tank) into which inflowing water (raw water) enters, 42 is an aerobic tank (nitrification tank), and 43 is the last. It is a sedimentation pond. The aerobic tank 42 is provided with a blower 44 for sending air. The nitrification liquid (circulation water) in the fourth tank of the aerobic tank 42 is returned to the first tank of the anoxic tank 41 by the pump 45. Further, in the first tank of the oxygen-free tank 41, the sludge from the final settling tank 43 is returned by the pump 46 through the return sludge path 47. Reference numeral 48 is a surplus sludge discharge pump.

FIG. 5 is a system block diagram showing the two-stage circulation method. The oxygen-free tanks 41 and the aerobic tanks 42 are divided into two tanks each, and the oxygen-free tanks 41a and 41b and the aerobic tanks 42a and 42 are shown.
b and oxygen-free 41c, 41d and aerobic tank 42c, 42d
The nitrification solution in the aerobic tank 42b is returned to the anoxic tank 41a by the pump 45a, and the nitrification solution in the aerobic tank 42d is pumped by the pump 4a.
The two-stage circulation method is adopted so as to return to the oxygen-free tank 41c at 5b. At this time, the stepwise inflow is supplied to the oxygen-free tank 41c through the inflow path 49. Other reference numerals indicate the same parts as in FIG.

In both of the single-stage type and the two-stage type, the operation management points are (a) optimum nitric acid circulation amount, (b) sludge retention time (S) necessary for growth of nitrifying bacteria.
RT), (c) Dissolved oxygen concentration in the nitrification tank and its distribution are considered.

[0008]

Assuming that each reaction of nitrification and denitrification occurs in 100% in the above-mentioned single-stage system, the nitrogen removal rate E is determined by the circulation ratio α of the nitrification liquid and the sludge return ratio β (both inflow water amount). Ratio) and is represented by the following equation.

E = (α + β + γ) / (1 + α + β) (1) Here, γ is the ratio of nitrogen taken into the activated sludge and generally has a value of 0.2 to 0.3. As can be said from the above formula (1), if the circulation ratio is increased, the removal rate increases, but if the circulation ratio is increased too much, the amount of dissolved oxygen brought in from the nitrification tank increases and the anoxic state in the denitrification tank can be maintained. The denitrification rate will decrease. The retention of the anaerobic state is affected not only by the circulation amount but also by the inflow load amount, the activated sludge amount and the water temperature. Especially in a treatment plant where rainwater flows in, the dissolved oxygen concentration of the inflow water tends to rise during rainfall.

Further, this tendency changes at the beginning and after the rainfall, and is influenced by the number of days of the interval of independent rainfall, so that it is difficult to stably maintain the anoxic state. In this way, it is necessary to change the circulation amount according to the situation of the inflow water and the reaction tank, which change from moment to moment, but since there are no measurement items as indicators or sensors, effective measures are not currently taken. is there.

The present invention has been made in view of the above circumstances, and makes it possible to estimate the oxygen consumption time and
The circulation ratio can be set within a range that does not cause denitrification inhibition, and the denitrification circulation ratio can be estimated even when rainwater is mixed in. The purpose is to provide a method.

[0012]

Means and Actions for Solving the Problems
In order to achieve the above-mentioned object, the first invention is that the raw water is introduced into a denitrification tank consisting of a plurality of stages for denitrification, and then, the raw water is introduced into a nitrification tank of a plurality of stages for nitrification, and then the nitrification tank Pump the nitrification solution from the final tank back to the initial tank of the denitrification tank, and separate outflow water from the final tank of the nitrification tank in the settling tank to discharge the supernatant water as treated water. In the modified method of activated sludge circulation modified to, while measuring the dissolved oxygen and the flow rate of the raw water flowing into the denitrification tank, to measure the respiratory rate and dissolved oxygen of the nitrification solution in the nitrification tank,
In addition, after the measured values of the flow rate of the nitrification liquid sucked up by the pump are controlled by the circulation amount control unit, the suction amount of the pump is varied by the control value.

In a second aspect of the present invention, two sets of the denitrification tank and the nitrification tank are provided, both sets are connected in cascade, and a circulation amount control unit is provided for each set to control the measured values of both sets for each circulation amount. After being controlled by the unit, each pump is variably controlled, and raw water is taken into the denitrification tanks of the both sets.

The third aspect of the invention is to estimate the oxygen consumption time in the denitrification tank using the measured values of the dissolved oxygen concentration at the raw water and the circulation nitrification solution intake port of the nitrification tank and the respiratory rate meter, and It is characterized by setting a circulation ratio.

Α <(− B + √ (B ² −4 · A · C)) / 2 · A where α is the circulation ratio, β is the sludge return ratio, and γ is the ratio of nitrogen A = Q · K _N · X・ C _0r B ＝ (S1 ・ C _Nin・ Q ・ Rr ＋ K _N・ X (S2 ・ Q ・ C _0r −Rr ・ V ＋ Q
・ C _0in ) C ＝ (S2-1) S1 ・ C _Nin・ Q ・ Rr−S2 ・ K _N・ X (Rr ・ V−Q ・ C
_0in ) Q is the inflow water amount, K _N is the denitrification rate, X is the sludge concentration, C _0r is the dissolved oxygen in the final nitrification tank, C _Nin is the total inflow nitrogen concentration, Rr is the measured value of the Rr meter, and V is the denitrification tank volume. , S1 = 1-γ and S2 = 1 + β.

The fourth aspect of the invention is characterized in that the circulation ratio α is set by the following equation using the denitrification possible time Tn.

Tn = {Rr · V−Q (α · C _0r + C _0in )} / Q ·
Rr (1 + α + β) However, Q is the inflow of water, C _0r is the dissolved oxygen in the final nitrification tank, and C is
_Nin is the total inflow nitrogen concentration, Rr is the measured value of the Rr meter, V is the denitrification tank volume, and β is the sludge return ratio.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. 1 is a control system configuration diagram of a single-stage circulation method showing a first embodiment of the present invention, and the same portions as those in FIG. 4 are designated by the same reference numerals. In FIG. 1, 11 is a circulation amount control unit, and various measurement values are input to this circulation amount control unit 11. The following five values are measured. The first is the dissolved oxygen DO of the inflow water D
The value measured by the O meter 12, the second is the flow rate of the inflow water by the flow meter 1
The value measured in No. 3 is the third, the anoxic condition of the first tank of the aerobic tank (nitrification tank) 42 is estimated by measuring with the respiration rate meter (Rr meter) 14, and the fourth is the four tanks of the nitrification tank 42. The dissolved oxygen in the eye is a value measured by the DO meter 15, and the fifth is a value measured by the flow meter 16 for the flow rate of circulating the nitrification solution in the fourth tank of the nitrification tank 42. When these measured values are input to the circulation amount control unit 11, the measured values are controlled here, and the pump 45 that circulates the nitrification liquid (circulating water) is controlled according to the control values.

In the control system configured as described above, the Rr meter 14 is used to predict the time until the dissolved oxygen DO flowing into the denitrification tank 41 is consumed and the oxygen-free state is reached. The quality of denitrification is determined based on this predicted time. That is, the shorter the time, the longer the denitrification time in the denitrification tank 41 can be secured, and therefore the nitrogen removal efficiency becomes higher.

However, the shortening of the above time is performed by lowering the dissolved oxygen DO at the outlet of the nitrification tank 42 or by using the pump 45.
Must be controlled to reduce the amount of circulating water, resulting in a decrease in nitrification efficiency and a decrease in denitrification amount. As a result, the nitrogen removal rate will decrease. Therefore, Rr total 14
Using the equation below to estimate the actual denitrification time, it is possible to perform operation control that maximizes the ratio of circulating water (circulation ratio) under given conditions to increase the nitrogen removal rate. Explain.

First, the inflow water amount is Q, the inflow water DO is C _0in , the aerobic final tank DO is C _0r , the circulation ratio is α, the sludge return ratio is β, and the DO becomes “0” in the denitrification tank 41. If the time required is T _c , the DO balance equation is as follows.

Q (α · C _0r + C _0in ) = Q (1 + α + β) Rr · T _c (2) From the above equation (2), the time T _c is given by the following equation.

T _c = (α · C _0r + C _0in ) / Rr (1 + α + β) (3) Next, when the volume of the denitrification _tank is V, the total residence time Tt is given by the following equation.

T _t = V / Q (1 + α + β) (4) Residence time T _t minus T _c is defined as denitrification available time T
_{Assuming n} , T _n is given by the following expression (5).

[0025]

[Equation 1]

Therefore, with respect to the measured value of Rr, the circulation ratio α can be determined so that the denitrification reaction is completed within the time T _n . On the other hand, _assuming that the total nitrogen concentration in the inflow water is C _Nin and the total nitrogen concentration in the aerobic final tank is C _Nr and the nitrification and denitrification reactions are completed, the relationship between the nitrogen removal rate and the circulation ratio is as follows (6).
It becomes an expression.

[0027]

[Equation 2]

When the equation (6) is modified, the equation (7) is obtained.

C _Nr = (1−γ) C _Nin / (1 + α + β) (7) When the nitrification rate is perfect, all C _Nr becomes nitrate nitrogen. When the concentration when flowing into the denitrification tank is C _Nr 'and the returned sludge is taken into consideration, the following equation (8) is obtained.

[0030]

(Equation 3)

When the denitrification rate per unit sludge is K _N and the sludge concentration is X, in order to complete the denitrification,
The denitrification available time Tn should satisfy the following equation.

T _n > C _Nr '/ K _N · X (9) Equations (5) and (8) are substituted into equation (9), and (1+
Organizing by multiplying by α + β) ² gives the following equation (10).

Q ・ K _N・ X ・ C _0r・ α ² + (S1 ・ C _Nin・ Q ・ Rr + K _N・ X (S2 ・ Q ・ C _0r −Rr ・ V + Q ・ C _0in )) α + (S2-1 ) S1 ・ C _Nin・ Q ・ Rr−S2 ・ K _N・ X (Rr ・ V−Q ・ C _0in ) <0 …… (10) However, S1 = 1−γ: S2 = 1 + β where A = Q・ K _N・ X ・ C _0r B = (S1 ・ C _Nin・ Q ・ Rr ＋ K _N・ X (S2 ・ Q ・ C _0r −Rr ・ V + Q
・ C _0in ) C ＝ (S2-1) S1 ・ C _Nin・ Q ・ Rr−S2 ・ K _N・ X (Rr ・ V−Q ・ C
_0in ), the following equation (11) is obtained.

[0034]

[Equation 4]

When the denitrification is completed, the maximum circulation ratio is determined by the above equation (11). However, the denitrification rate is affected by the concentrations of nitrate nitrogen and BOD except pH and water temperature, but in practice it is regarded as a zero-order reaction, so the values measured in advance are corrected with the pH and water temperature at that time. To use.

2 and 3 are control system configuration diagrams showing second and third embodiments of the present invention. The second and third embodiments are for a two-stage circulation method. Although it is necessary to add a part of the inflow water to the latter stage (step inflow), the ratio to the total inflow is called the step ratio, and the case where this ratio is changed frequently is shown in the second embodiment. This is shown in the third embodiment. Since the operation can be achieved by repeating the single-stage operation of FIG. 1, a detailed description thereof will be omitted. However, when the step ratio is changed as in the second embodiment, the value of Rr is the same as that of the front and rear stages. Since the correlation is not established in the above, it is necessary to provide one Rr meter 14 and 14 'for each of the front and rear stages. In the second and third embodiments, the same parts as those in the first embodiment are designated by the same reference numerals and dashes.

[0037]

As described above, according to the present invention,
By using the Rr meter, it is possible to estimate the oxygen consumption time flowing into the denitrification tank and also to estimate the actual denitrification time in the denitrification tank, so the maximum circulation ratio can be set within the range that does not cause denitrification inhibition. . Further, it is possible to predict the effect of bringing in DO when rainwater is mixed, and to estimate the maximum value of the denitrifying circulation ratio. Furthermore, since the denitrification tank can be efficiently operated by the circulation method for removing nitrogen, the volume of the reaction tank can be reduced.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a control system showing a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a control system according to a second embodiment.

FIG. 3 is a configuration diagram of a control system according to a third embodiment.

FIG. 4 is a control system configuration diagram showing a conventional single-stage circulation method.

FIG. 5 is a control system configuration diagram showing a conventional two-stage circulation method.

[Explanation of symbols]

 11, 11 '... Circulation amount control unit 12, 15, 15' ... DO meter 13, 13 ', 16, 16' ... Flow meter 14, 14 '... Rr meter 41 ... Anoxic tank (denitrification tank) 42 ... Aerobic Tank (nitrification tank) 43 ... Final settling tank 44 ... Blower 45, 46, 48 ... Pump 47 ... Return sludge path

Claims (4)

[Claims] 1. The raw water is introduced into a denitrification tank consisting of a plurality of stages for denitrification, and then is introduced into a nitrification tank consisting of a plurality of stages for nitrification, and then the nitrification solution is sucked up by a pump from the final tank of the nitrification tank. Activated sludge circulation modified treatment method in which the water discharged from the final tank of the nitrification tank is solid-liquid separated in the settling tank and the supernatant water is discharged as treated water. In, while measuring the dissolved oxygen and flow rate of the raw water flowing into the denitrification tank, each measurement of the respiratory rate and dissolved oxygen of the nitrification solution in the nitrification tank, and the measurement of the flow rate of the nitrification solution sucked up by the pump A method for controlling the amount of circulation of nitrification liquid in an activated sludge circulation modified method, comprising controlling the value by a circulation amount control unit, and then varying the suction amount of the pump by the control value. 2. A combination of the denitrification tank and the nitrification tank is provided in two sets, both sets are connected in cascade, and a circulation amount control unit is provided in each set, and the measured value of both sets is controlled by each circulation amount control unit. After that, the respective pumps are variably controlled so that the raw water is taken into the denitrification tanks of the two sets, and the nitrification solution circulation amount control method in the activated sludge circulation modification method according to claim 1, characterized in that. 3. The oxygen consumption time in the denitrification tank is estimated using the measured value of the dissolved oxygen concentration at the raw water and the circulation nitrification solution intake of the nitrification tank and the measured value of the respiration rate meter, and the circulation ratio is calculated by the following equation. The method for setting a denitrifying circulation ratio in the modified activated sludge circulation method according to claim 1, wherein the method is set. α <(− B + √ (B ² −4 · A · C)) / 2 · A where α is the circulation ratio, β is the sludge return ratio, and γ is the nitrogen ratio A = Q ・ K _N・ X ・ C _0r B = (S1 ・ C _Nin・ Q ・ Rr ＋ K _N・ X (S2 ・ Q ・ C _0r −Rr ・ V ＋ Q
・ C _0in ) C ＝ (S2-1) S1 ・ C _Nin・ Q ・ Rr−S2 ・ K _N・ X (Rr ・ V−Q ・ C
_0in ) Q is the inflow water amount, K _N is the denitrification rate, X is the sludge concentration, C _0r is the dissolved oxygen in the final nitrification tank, C _Nin is the total inflow nitrogen concentration, Rr is the measured value of the Rr meter, and V is the denitrification tank volume. , S1 = 1-γ and S2 = 1 + β. 4. The denitrifying circulation ratio setting method in the activated sludge circulation modification method according to claim 3, wherein the circulation ratio α is set by the following equation using the denitrification possible time Tn. Tn = {Rr ・ V-Q (α ・ C _0r + C _0in )} / Q ・ Rr (1 + α + β) where Q is the inflow water amount, C _0r is the dissolved oxygen in the final nitrification tank, and C is
_Nin is the total inflow nitrogen concentration, Rr is the measured value of the Rr meter, V is the denitrification tank volume, and β is the sludge return ratio.