JPS59132999A - Controlling method for process of biological denitrification - Google Patents

Abstract

PURPOSE:To properly operate circulation, by adjusting the flow amount of water to be treated, nitrogen content in the water to be treated being estimated from said flow amount and the amount of a liquid to be circulated through a denitrification tank in response to the predetermined allowable value of nitrogen content in the water to be treated. CONSTITUTION:The flow amount Q1 of inflow water obtd. from a flow meter 22 is inputted to a differentiating circuit 31 to obtain the variable speed q1 of Q1. Said variable speed q1 is inputted to a deciding circuit 32 to select a correlation coefficient K1 corresponding to the positive or negative of q1. The correlation coefficient K1 and the flow amount Q1 of inflow water are inputted to a calculating circuit 33 to estimate the concentration N'1 of inflow nitrogen. Said estimated value N'1 of the concentration of inflow introgen is inputted to another calculating circuit 34, to calculate a circulation ratio r' to be operated from allowable nitrogen content Na in water to be treated on the basis of 6. In addition, the circulation ratio r' is integrated with the flow amount Q1 of inflow water by a calculating circuit 35, to obtain the flow amount Qr of water to be returned to a denitrification tank 1. Hence, nitrogen content in the water to be treated is adjusted to the allowable nitrogen content Na.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a biological denitrification process for a nitrification solution circulation system, and particularly to a method for controlling a nitrification solution circulation liquid that is returned to a denitrification tank.

[Prior art]

In recent years, eutrophication has become a major social problem in closed water bodies. For this reason, wastewater regulations based on concentration standards for each closed water body are required for each business establishment, and this is beginning to be applied to sewage treatment plants as well.

Nitrogen in wastewater is considered to be a contributing factor to eutrophication and is a target substance for removal at sewage treatment plants. In particular, in sewage treatment plants that discharge water into closed bodies of water, it is an urgent issue to maintain wastewater concentrations below the permissible level.

The biological membrane nitrogen method, which utilizes the biological nitrogen cycle, is attracting attention as the most promising denitrification method for sewage treatment plants. This method generally consists of two microbial reaction tanks, a copperization tank and a denitrification tank, and a settling tank. Generally, nitrogen flowing into a sewage treatment plant is in the form of NH3-N and organic nitrogen.

In the nitrification tank, N is converted from organic nitrogen by oxidizing bacteria.
H3N and original NH, 3-N are converted to N03-N by nitrifying bacteria.
oxidized to

NH”4+20□→N 01i +2 H”+ N20
(1) In the denitrification tank, NO3N generated in the nitrification tank is reduced to N2 gas by denitrification bacteria, and the denitrification rate is completed.

2 NOi +5 (N2) → N2 +20H-+
4 N20 (2) In the sedimentation tank, microorganisms such as nitrifying bacteria, denitrifying bacteria, and oxidizing bacteria are separated and recovered.

In this biological denitrification method, H+
It is necessary to inject an alkaline agent to neutralize the nitrogen, and a hydrogen donor to replenish N2 in the denitrification tank to promote microbial reactions.

The nitrifying solution circulation system shown in Figure 1 is attracting attention as a biological method of dechlorination that is effective in reducing the newly required alkaline agents and hydrogen donors. This system consists of a denitrification @l in the first stage, a nitrification tank 2 in the latter stage, and a settling tank 3 in the final stage. 13 to the denitrification tank 1. As a result, the OH- produced in the denitrification tank 1 can be used as an alkali source in the nitrification tank 2, and the organic matter in the inflow water 11 can be used as a hydrogen source in the denitrification tank 1, making it possible to reduce the amount of alkaline agents and hydrogen donors. . On the other hand, the nitrification liquid circulation method is characterized in that complete denitrification is not performed because the nitrification liquid 15 corresponding to the inflow water 11 does not pass through the denitrification tank 1, and there is a theoretical denitrification rate η defined by the circulation ratio γ. have.

η=100・γ/(1+γ) ... (3) Here, the circulation ratio γ is the circulating fluid flow rate Q+, the return sludge flow rate Qh
, the inflow water flow rate Qt is expressed by the following equation.

γ−(Q, j1Q b ) / Q +
(4) Generally, in sewage treatment plants that treat urban sewage, it is known that the inflow water flow rate QI varies greatly on a daily basis, which acts as a process disturbance. Therefore, in the nitrification soil circulation system where the denitrification rate is controlled by the circulation ratio γ, it is important to appropriately manipulate the circulating fluid in response to changes in the inflow water.

Conventionally, it has been known that a constant circulation ratio control system should be used to control the circulating fluid in proportion to the inflow water.

This is because if this method is adopted, a constant denitrification rate υ can be obtained as shown in equation (3). However, in sewage treatment plants, it is a problem to satisfy the permissible concentration of wastewater, and even if the denitrification rate is reduced, there are cases where this permissible concentration cannot always be achieved. Figure 2 shows the daily variation pattern of urban sewage, and both the inflow water flow (fit Q, +) and the nitrogen concentration N vary greatly. In this figure, N and QI are the daily average values of nitrogen concentration N and flow rate QtO. Under these inflow conditions, the general value N+ = 30 mg
-N/L, when the circulating fluid is operated using r=2, the treated water nitrogen concentration N0 changes following the inflow nitrogen concentration N, as shown in FIG. If the permissible nitrogen concentration is set to 10 mg-N/l in consideration of recent regulatory trends, it can be seen that the permissible value is exceeded for a considerable period of time, deteriorating the quality of treated water. As described above, although the circulating fluid control using the constant circulation ratio type can obtain a constant denitrification rate, the nitrogen concentration of the treated water changes greatly depending on the nitrogen concentration of the inflow water. In order to maintain the treated water nitrogen concentration below the permissible value at all times, the circulation ratio should be increased so that the permissible value is obtained at the maximum inflow type nest concentration. However, increasing the circulation ratio leads to an increase in power costs, and also increases the amount of hydrogen donor consumed because a large amount of nitrogen present in the bath, which consumes the hydrogen donor, is returned to the denitrification tank. Furthermore, during times when the inflow nitrogen level is low, as is often the case in the oil tank, circulating fluid operations are performed more than necessary, leading to excessive waste of power and hydrogen donors. Furthermore, there is a practical problem in that a device for online measurement of valley glaze nitrogen levels has not been developed, making it difficult to operate the circulating fluid.

[Purpose of the invention]

One aspect of the present invention is to estimate the nitrogen concentration in the water to be treated without online measurement, thereby enabling suitable operation of the circulating fluid. We are here to provide you with a method.

[Summary of the invention]

The main point of the present invention is that the circulating fluid to the denitrification tank is adjusted according to the flow rate of the water to be treated, the nitrogen concentration in the water that is estimated empirically, and the permissible concentration value in the treated water chamber system. Adjust the amount.

[Embodiments of the invention]

In the nitrification liquid circulation system, if there is sufficient hydrogen donor in the denitrification tank, fcNOs is brought in by the circulating liquid and frozen.
N is denitrified first. Furthermore, if the nitrification tank is properly managed, NHsN can easily be completely nitrified. Under these conditions, the nitrogen flowing out from the denitrification tank is NH3N and organic nitrogen, and the organic nitrogen is converted into NHs in the nitrification tank.
Since it becomes NOs N through N, it can be treated as NH3N.

Therefore, NH3-N and organic nitrogen in the influent water can be collectively expressed as influent water nitridation. The current inflow nitrogen concentration NI is expressed in the denitrification tank flow, as well as in the circulating fluid and return sludge, which contain almost no NH3N and organic nitrogen, and becomes the nitrogen concentration Nd expressed by the following equation.

Nd=NI/ (1+γ)...(5)
This nitrogen concentration Nd flows into the nitrification tank with a delay corresponding to the residence time in the denitrification tank, and NO3, N, which corresponds to Na, flows into the nitrification tank with a delay corresponding to the residence time in the nitrification tank. flows out from. Since the form of nitrogen hardly changes in the settling tank, the treated water crab plant N0 is Nn, that is, N
K shows almost the same value as d. Therefore, by controlling the inlet of the denitrification tank to the allowable nitrogen concentration N, the treated water can be made nitrogen. can always maintain N. This operation is done by maintaining the circulation ratio γ as shown in the following equation! ll achieved.

γ-Nt/N, 1
(6) However, since the circulation ratio γ depends on the inflow nitrogen concentration N, it is impossible to predict it in advance due to the problems in developing the measuring instrument mentioned above.

The Ministry of Invention and others found that depending on the type of water to be treated, there is a correlation between the flow rate and the hydrogen concentration based on the similarity of the species. Figure 4 shows the variation ratio between the flow rate Q of urban sewage flowing into the final treatment plant and the nitrogen concentration NI. From this figure, it can be seen that the flow rate Q and the nitrogen concentration N1 have a substantially linear relationship, and that the correlation coefficient is different when the flow rate increases and when it decreases. That is, the nitrogen concentration N+
and the flow rate Q can be expressed by the following equation.

N + = to +-Qt...
(7) Here, is a correlation coefficient, and takes an appropriate Iiii depending on the increase/decrease state of the flow rate Q.

The present invention adjusts the amount of circulating fluid based on the calculated value obtained by equation (7), and will be explained in detail with reference to an embodiment shown in FIG.

In FIG. 5, 1 is a denitrification tank, 2 is a nitrification tank, 3 is a settling tank, 6 is an agitator drive device, 11 is inflow water, 12 is a circulating liquid,
13 is return sludge, 14 is denitrification liquid, 15 is nitrification liquid, 16 is treated water, 17 is excess sludge, 18 is hydrogen donor, 19 is alkaline agent, 20 is air, and the circulating fluid regulating device 5 is as follows. controlled as follows. First, the inflow water flow cap QI obtained from the flowmeter 22 is input manually to the differentiating circuit 31, and the rate of change q of Qt is determined. Differential calculations can be performed at any period, or may be calculated using the average difference between calculation periods. The rate of change q1 is input to the determination circuit 32, and a correlation coefficient corresponding to the positive or negative of Qt is selected. The correlation coefficient and the inflow water flow rate Q are input manually to the arithmetic circuit 33, and the inflow nitrogen concentration N'+ is estimated based on equation (7). The correlation coefficient used in the estimation is a value specific to the treatment plant.If the inflow water flow rate Q and the nitrogen concentration Nt have a uniform correlation regardless of the increase or decrease in Q, then k , can be estimated by keeping , constant.

Inflow nitrogen concentration estimation W: Ili N ', is a (manually input to the calculation circuit 34 and preset allowable nitrogen concentration N in the treated water 16), and calculates the circulation ratio γI to be operated based on (6). Furthermore, in the arithmetic circuit 35, the circulation ratio γ′
and the inflow water flow rate Q, and calculate the allowable nitrogen concentration N.

In order to achieve this, the amount of reflux Q1 to the denitrification tank 1 is determined. The reflux flow rate Q is the total of the circulating fluid L volume Q+ and the return sludge flow rate Qh. Therefore, the arithmetic circuit 36 subtracts the return sludge flow rate Qk obtained from the flowmeter 24 from the return flow rate Q, and
, (!i12 Calculate the operation amount Q+.

In the regulating circuit 37, the circulating fluid Q'j obtained by the flow meter 23
The circulating fluid regulating device 5 is controlled so that the operating flow rate QJ becomes the operating flow rate QJ. The regulating device 5 is constituted by a pump or a valve, and the regulating circuit 37 is, for example, a PI controller.

In this way, by controlling the circulating fluid flow rate based on the inflow conditions, it becomes possible to perform denitrification operations that comply with drainage regulations.

Note that the present invention is not limited to a system in which a denitrification tank, a nitrification tank, and a sedimentation tank are arranged in series. It can also be applied to systems that have a contact biofilm inside a nitrogen tank or nitrification tank.

〔Effect of the invention〕

According to the present invention, it is possible to control the amount of circulating fluid corresponding to the allowable nitrogen temperature of the treated water, and the L
Excessive circulation and high flow rates can be prevented during times when the sea level is high. Therefore, circulating power costs and hydrogen donors can be kept to the necessary minimum.

Also, is there a compound in the inflow water? It is possible to provide a one-time (goods) system that can track the # degree without online measurement, is easy to maintain, and has a high <i-axis property.

[Brief explanation of drawings]

Figure 1 is a thread diagram of the biological dechlorination process using the nitrifying solution circulation method, Figure 2 is a diagram of the fluctuations in inflow flow rate and nitrogen concentration in an urban sewage treatment plant, and Figure 3 is the circulation ratio. −
Time-series change diagram of oxygen concentration in treated water during foot operation, No. 4
The figure is a correlation diagram between inflow water flow rate and nitrogen concentration, and FIG. 5 is a control system diagram of an embodiment of the present invention. 1... Denitrification tank, 2... Nitrification tank, 3... Sedimentation tank, 5
...Circulating fluid regulating device, 11...Inflow water, 12...
Circulating fluid, 13... Returned sludge, 16... Treated water, 22
, 23.24...Flowmeter, 31...Differential circuit, 32
... Judgment circuit, 33°34, . 35.36...Arithmetic circuit, 37...Adjustment circuit. Agent Patent Attorney Akio Takahashi No. 10 Tt'nn Mae (L) 3 En Mori

Claims (1)

[Claims] 1. A denitrification tank that is supplied with wastewater and a hydrogen donor and reduces nitrate or nitrite nitrogen to nitrogen gas by denitrifying bacteria under anaerobic conditions; It is equipped with a nitrification tank that oxidizes nitrogen to nitrate or nitrite nitrogen by nitrifying bacteria under aerobic conditions, and a sedimentation tank that sediments and separates the denitrifying bacteria and nitrifying bacteria, In the biological nitriding process in which circulating fluid and sludge returned from the AiJ sedimentation tank are returned to the denitrification tank, a means for detecting the flow rate of the wastewater is provided, and the flow rate is estimated based on this flow rate measurement value. Circulation #A'FW to the denitrification tank according to the reflux amount determined from the nitrogen concentration in the wastewater and the allowable nitrogen concentration of the treated water of the preset process,
1. A method for controlling a biological elimination process, characterized by comprising a means for adjusting the flow rate.