JPH021555B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention can be used to treat treated water in water treatment facilities such as water purification plants.
This invention relates to an alkaline agent injection control method for maintaining pH at a set value. Conventionally, the quality of treated water has been maintained by injecting alkaline agents only by adjusting the alkalinity level, which is easy to predict. (Alkalinity here refers to molecules or ions that can release hydroxide ions,
HCO ₃ ^- +H ₂ O→H ₂ CO ₃ +OH ^- , CO ₃ ^2- +2H ₂ O→
In the case of H ₂ CO ₃ + 2OH ^- , the alkalinity is [HCO ₃ ^- ]
+2 [CO ₃ ^2- ] + [OH ^- ] + [NH ₃ ] +..., and when natural water is neutral, the alkalinity is [HCO ₃ ^- ] + [OH ^- ]
It is. ) This is because the PH of treated water is affected by multiple factors such as raw water quality and each chemical injection rate, making it difficult to predict. A conventional device of this type is shown in FIG. In Figure 1, 1 is a landing well for adjusting the water level of raw water taken from a river, etc., 2 is a mixing pond for mixing raw water and chemicals, 3 is a floc formation pond for flocculation, and 4 is a floc formed 5 is a sedimentation basin for sedimentation and separation of the particles, and 5 is a filter basin for removing fine particles. In addition, 8 is a water intake flow meter, 9 is a pre-chlorination machine,
10 is an alkali injection machine, 11 is a flocculant injection machine,
13 is a PH meter, 14 is a deviation calculator between the set value PH and the actual measured value, and 15 is a ratio calculator for calculating the alkaline agent injection rate with respect to the raw water flow rate. Next, the operation will be explained. The raw water taken into the washing plant is disinfected by pre-chlorination injected into the receiving well 1, and furthermore, a flocculant 11 and, if necessary, an alkaline agent are injected from an alkaline agent injector to remove suspended solids. After that, mixing pond 2, floc formation pond 3
After that, it is sent to settling tank 4, where most of the suspended solids are removed.
The quality of treated water is sampled near the settling tank outlet 12 and measured using an online PH meter 13. Actual PH
The deviation between (PV) and the set PH value is calculated (EV) by the calculator 14, and the alkali injection rate is increased or decreased by the calculator 16 according to this EV, thereby determining the alkali injection rate. The alkali injection rate set here is set by adjusting the valve opening degree of the alkali injection machine 10, which is converted into an injection amount according to the water intake flow rate by the ratio calculator 15. The conventional alkali injection method is configured as described above, but since there is a time delay of about 2 to 3 hours between the alkali injection machine 10 and the treated water sampling point 12, the deviation (EV) of the treated water PH Only the PH could not be maintained at the set value. In addition, the relationship between alkali injection rate and PH cannot be uniquely determined because it is affected by the free carbonate concentration in water, but since this is approximated, the predicted PH value and the actual measured value may be different from each other. There were some shortcomings. This invention was made to eliminate the drawbacks of the conventional methods as described above, and by introducing consumption coefficients of alkalinity components in water, that is, bicarbonate ions and hydroxide ions, to predict the pH of the treated water. Treated water by calculating the free carbonate of
The purpose of this study is to provide an alkaline agent injection control method that can maintain a stable PH value of treated water by accurately determining PH and feeding back the measured PH using PID calculation. In other words, this invention can be used in water treatment facilities such as water treatment plants that inject one or more of chlorine, flocculants, and alkaline agents into raw water.
The free carbonate concentration contained in the treated water is estimated from the measured values of PH, alkalinity, and water temperature and the injection rate of one or more of chlorine, flocculant, and alkali, and the estimated free carbonate concentration and treated water alkali are calculated. The alkali agent standard injection rate is determined as an approximate value of the alkaline agent injection rate by determining the treated water PH from the calculated value, and the feedforward injection rate is determined as a fraction thereof. Finely correct the forward injection rate and further adjust the PH of the treated water.
Feedback the actual measured values using PID control,
This is an alkali agent injection control method for a water purification plant that maintains an appropriate alkali agent injection amount by correcting the alkali agent reference injection rate with a feedback injection rate. Alkaline agents such as caustic soda are used primarily to replenish the alkaline content necessary for the flocculation reaction. When flocculants such as PAC and sulfuric acid band are injected into water, they consume dissolved alkalinity components (bicarbonate ions, hydroxide ions) and form flocs. Therefore, when the alkalinity of raw water is low or when a large amount of flocculant is injected, it is necessary to replenish the alkalinity by injecting caustic soda or the like.
There are two water quality indicators that affect the quality of flocculation: PH and alkalinity.Of these, alkalinity is an indicator necessary for the reaction with the flocculant, and PH is an indicator that affects the condition of the flocs produced. be. As shown in equation (1), alkalinity increases and decreases linearly with the chemical injection rate. ALK _t = ALK ₀ +a ₁ ×NMV−a ₂ ×PMV−a ₃ ×BMV…(1)
In the formula: ALK _t : Calculated value of treated water alkalinity,
NMV: Caustic soda injection rate (20% caustic soda), PMV: PAC injection rate, BMV: Pre-chlorine injection rate, ALK ₀ : Raw water alkalinity, a ₁ : Constant (=
0.25), a ₂ : constant (=0.15), a ₃ : constant (=1.41)
However, PH is an index obtained by multiplying the natural logarithm of the hydrogen ion concentration by minus 1, and is influenced by free carbonate in water, so the PH value cannot be predicted directly from the chemical injection rate. free carbonic acid dissolved in water,
It is well known that alkalinity and PH have a relationship expressed by the following equation (2). PH = log (ALK) + Log (0.88/Kc) - log (CO ₂ )...(2) Here, the dissociation constant (Kc) of carbonic acid is a constant determined by temperature, and the relationship between temperature and Kc can be expressed as Shown in the table (General Water Chemistry (Part 1) translated by W. Smith, J. J. Morgan, Abe and Hantani, Kyoritsu Shuppan).

[Table] Next, the reaction between the flocculant and the alkalinity components (bicarbonate ion, hydroxide ion) in water is described. Al ³⁺ +3HCO ₃ ^- =Al(CH) ₃ C+3CO ₂ ...(3) Al ³⁺ +3OH ^- =Al(OH) ₃ ...(4) When bicarbonate ions and flocculant react in this way, free carbonic acid is generated. do. A similar reaction also occurs with chlorine injection. Cl ₂ +HCO ₃ ^- = HOCl + Cl ^- + CO ₂ ...(5) Cl ₂ +OH ^- = HOCl + Cl ^- ...(6) Therefore, if you know the percentage of alkalinity components HCO ₃ ^- and OH ^- consumed, you can The free carbonate concentration is calculated, and the pH is determined using equation (2). That is, PH _t log(ALK ₀ +a ₁ ×NMV−a ₂ ×PMV−a ₃ ×BMV
) +log((0.88/Kc)−log(CO _2p +a ₄ ×(
a ₂₁ × PMV + a ₃₁ × BMV))…(7) (In the formula: PH _t : Calculated PH value of treated water, CO _2p : Raw water free carbonate, aD: Conversion constant = 0.88, a ₂₁ : HCO consumed during PAC injection ₃ ^- coefficient (0.15), a ₃₁ HCO consumed during pre-chlorine injection ₃ ^- coefficient (1.41)) Figure 2 shows the results of measuring the carbon dioxide concentration distribution in the settling tank at an actual facility. -o in the diagram
− is the average value of actual measurements, and the PAC injection rate is 15~
It is 30ppm. As can be seen from Figure 2, almost no free carbon dioxide is released into the atmosphere in the settling tank. In other words, in order to calculate the free carbonate in treated water, it is not necessary to consider the emission rate to the atmosphere, and the consumption ratio of alkalinity components at the time of chemical injection (a ₂₁ ,
a ₃₁ ). Based on the above considerations, an embodiment of the present invention will be explained based on FIG. 3. 17 is a raw water alkalinity meter, 18 is a raw water PH meter, and 19 is a raw water temperature meter. 20 is a calculator that calculates the free carbonate of the raw water according to the formula (2) shown earlier; 21 is a calculator that calculates the alkalinity of the treated water according to the formula (1) shown earlier; 2
2 is an arithmetic unit that calculates the pH of the treated water using the formula 7 shown above. 23 is a ratio calculator for making sure that both alkalinity and PH of the quality of the treated water satisfy set values. 25 is a PID calculator; 24
is an adder for the reference injection rate, feedback injection rate, and feedback injection rate. Next, the operation of the present invention will be explained. The alkalinity, PH, and water temperature of the raw water quality are measured by online measuring devices 17, 18, and 19, respectively, and then input into the calculator 20, and the free carbon dioxide (CO ₂ ) concentration of the raw water is calculated using equation (2). be done. Next at 21
The treated water alkalinity (ALK _t ) is calculated using equation (1). In step 22, the PH of the treated water is calculated according to (7). Here, the coefficients a ₂₁ and a ₃₁ vary depending on the raw water free carbonate concentration or the caustic soda injection rate. this
The values of a ₂₁ and a ₃₁ are changed by an operator using a CRT or the like, or the coefficients are changed by automatic correction by a computer. As an online estimation method for a ₂₁ and a ₃₁ , a ₂₁
An approximation is made in which the value of a ₃₁ is constant. Let k be the time at which the caustic soda reference injection rate (NVS) is determined by feed forward control.
Then, at time (k-1), equation (8) is transformed to obtain equation (11). [H ⁺ ] [HCO ₃ ^- ] / [H ₂ CO ₃ ] = K...(8) Converting the above formula to common units and rearranging it, (CO ₂ ) _t = 10 ^- PH×(ALK) _t / K'...(9) That is, at time (k-1), C(k-1) + Z ₁ × α ₂ (k-1) + Z ₂ × α ₃ (k-1)
= 10 ^- PH ^(k-1) /K′×α ₁ (k-1)…(10) From this formula (10), α ₂ (k-1)×Z ₁ + α ₃ (k-1)×Z ₂ = 10 ^- PH ^(k-1) ×
α ₁ (k-1)/K'-C(k-1)=b(k-1)...(1
1) However, C(k-1)=(CO ₂ ) ₀ (k-1) α ₁ (k-1)=ALK ₀ (k-1)+a ₁ ×NVS(k-1)-
a ₂ ×PVS (k-1) − a ₃ ×BVS (k-1) Z ₁ = a ₄ × a ₂₁ , Z ₂ = a ₄ × a ₃₁ , α ₂ (k-1) = PVS
(k-1): PAC reference injection rate, α ₃ (k-1) = BVS
(k-1): Pre-chlorine reference injection rate. Therefore, formula (11) is calculated and rearranged for times (k-1) and (k-2), resulting in formula (12). α ₂ (k-1), α ₃ (k-1) α ₂ (k-2), α ₃ (k-2) [Z ₁ Z ₂ ]=b(k-1) b(k-2)... (12) Solve the linear equation from equation (12) above to find Z ₁ and Z ₂ ,
A ₂₁ and a ₃₁ are determined to set the caustic soda reference injection rate at time k. The actual measurement results for _a21 in actual facilities are 0.05 to 0.12.
It was moderately hot. Next, the 23 arithmetic units will be explained. As is generally known, flocculation requires pH and alkalinity above a certain value. That is, PH _nio PH _t …(13) ALK _nio ALK _t …(14) Here, PH _nio and ALK _nio are the minimum values of PH and alkalinity, respectively, and are based on the intake raw water of the water treatment plant, the type of flocculant, and the settling tank. Although it varies depending on the characteristics, generally PH _nio is 6.0 to 6.3 and ALK _nio is 10 to 15 (ppm)
It is considered to be a degree. Therefore, PH _t that satisfies equation (13)
The caustic soda injection rate that gives ALK _t that satisfies equation (14) is found using equation (1). The comparison calculator 23 compares the two values and outputs the larger value. On the other hand, at 25, PHt from the sedimentation tank treated water PH meter 13.
Actual measured values are input and used as feedback control.
There are 25 computing units that perform PID calculations. In other words, the caustic soda injection rate (NVB) based on feedback is as follows: NVB=K _p (EV+1/TI∫EV+TDdEV/dt) = K _p {EV(k)+1/TI _K = K _p {EV(k)+1/TI _K 〓 ^K=0 (EV(k)×TC)+TD(EV(k-1)−EV(k)
/TC)}...(15) Here, EV(k)=NSV−NPV(k), TI: Integral time, TD: Differential time, TC: Control period, Kp:
It is a proportional gain. The calculator 24 calculates a reference injection rate (NVS), a feed forward injection rate (NVF),
Add the feedback injection rate (NVB) to determine the caustic soda injection rate (NMV). That is, NMV=NVS+WF×NVF+WB×NVB…(16) Here, WF: Feedback weighting factor, WB: Feedback weighting factor, WF+
WB=1. The standard injection rate is an injection rate that gradually updates its value, for example, once every two hours, taking into account predicted fluctuations in raw water quality, etc., whereas the feed forward injection rate is an injection rate that updates the value, for example, once every two hours, taking into account predictions of fluctuations in raw water quality, etc. The injection rate is measured every 15 minutes and adjusted accordingly, and the control cycle for both is different. The reason why the reference injection rate and the feedforward injection rate were separated is that the feedforward injection rate and the feedback injection rate without a reference injection rate cannot be controlled to the extent that they are sensitive to system noise in a short period of time. This method is not suitable for water treatment processes because the chemical injection rate fluctuates rapidly and there is a risk of hunting against the water quality set value. Therefore, approximate feedforward control is set using the standard injection rate, and minute changes in water quality, etc. are successively corrected using the feedforward injection rate. The relationship between the reference injection rate and the feed forward injection rate will be explained in detail with reference to FIGS. 4 and 5. Figures 4 a to b show calculation values of injection rate when water quality changes are small, FF injection rate, standard injection rate,
(Reference + FF) Injection rates are shown respectively, and in Figure 4 a.
X ₁ and X ₂ indicate temporary noise caused by sudden changes in water quality meter readings. If the change in water quality is small, the fourth
The FF injection rate in Figure b has a pattern similar to the calculated injection rate in Figure A, and at first glance it appears that control by the FF injection rate can be performed successfully. However, the alkalinity value required for the water treatment process at a water treatment plant is not strict, such as having to match the target value, for example 20 mgl. It is of a nature that it is thought that there will be no major impact on the situation. Therefore, considering practicality, it is not necessarily necessary to change the alkali injection rate in small steps as shown in FIG. 4b. Conversely, in the case of such a process, it is actually difficult to correctly maintain the intended water quality value (alkalinity 20 mgl) due to dead time in the response system. Here, if the timing of the process and the chemical injection rate are out of sync, fluctuations in the alkalinity of the treated water may be amplified, and in this case, this may adversely affect the feedback for monitoring the alkalinity of the treated water. This may become a disturbance to the control and cause a hunting phenomenon. In this way, in the case of this process, there is a risk that oversensitive FF control will have a negative effect on process stabilization. On the other hand, the standard injection rate controlled over a long period changes as shown in Figure 4c, compared to the change in Figure 4a, and the injection rate changes slowly, which at first glance may seem like control is not being performed well. It looks like. However, from the experience of actual plant control operation, it has been found that when the change in water quality is small, the control shown in FIG. 4 c is practically preferable to that shown in FIG. 4 b. In addition to the above-mentioned problem of wasted time in the case of water treatment processes at water treatment plants, this is due to mixing and
This is because the alkalinity of the treated water in the actual process is smoothed out compared to the calculated value due to diffusion and the like. In other words, when there is a small change in water quality, the FF injection rate alone does not necessarily stabilize the process. On the other hand, when the water quality of raw water changes significantly as shown in Figure 5, and the raw water quality is changing in the direction of deterioration, the flocculant injection rate and pre-chlorine injection rate that consume alkalinity also increase, so the alkalinity increases. The agent injection rate needs to be increased by more than the decrease in raw water alkalinity. In such cases, the alkalinity of the treated water should be
Assume that the injection rate of alkaline agent is 20 mgl as shown in Figure 5a. The FF injection rate for short-cycle control is shown in Figure b.
The reference injection rate performed under long-period control is as shown in c of the same figure. When the amount of change in water quality is large and fast,
The FF injection rate follows closely as shown in b, and there is almost no shortage of alkaline agent injection rate. On the other hand, if the standard injection rate is changed only once an hour, the alkaline agent injection rate may be considerably insufficient as shown in c, and the alkalinity is at the lower limit of 10 to 15 mgl. It is also possible that the situation will decrease even further. In other words, when water quality changes are rapid and large,
Standard injection rates alone may not be enough to respond quickly to changes in water quality. As mentioned above, when there is a small change in water quality, using only the FF injection rate may pick up noise from water quality meters (temporary changes in water quality), resulting in unnecessary and frequent changes in the control amount. There is a risk that the process may become unstable, and conversely, when there is a large change in water quality, the standard injection rate alone cannot keep up with the sudden change in water quality, resulting in a situation where the alkali injection rate is insufficient (or excessive). It will continue for a long time. In the present invention, the drawbacks of both the standard injection rate and the FF injection rate are improved by combining them. In other words, in contrast to Fig. 4 (a) when the water quality change is small, the injection rate (reference + FF) becomes as shown in Fig. 4 (d), and the pattern in Fig. 4 (b) is smoothed.
The process is more stable than with FF injection rate alone. Figure 4 d is smoothed using the FF injection rate (NVF) when calculating the alkali injection rate using equation (16).
This is because the weighting factor (WF) of is smaller than 1 (usually about 0.5). In addition, when applying the (standard + FF) injection rate to the case shown in Fig. 5 a, where water quality changes are large and fast, the result is as shown in Fig. 5 d, and the followability is better than that shown in Fig. 5 c, and the standard injection rate The controllability is clearly improved compared to the case where the injection rate (NMV) obtained by equation 16 is calculated by the calculation unit 15, and is converted into the amount of water injected according to the water intake flow rate. The injection amount is adjusted by controlling the valve opening. The alkali injection control method described so far can be applied not only to water purification plants but also to other water treatment facilities or sludge treatment processes. Also,
In this example, PAC is used as the flocculant and caustic soda is used as the alkali agent, but other chemicals can be used in the same manner. As described above, according to the present invention, when estimating the PH of the treated water, the ratio of free carbonate in the raw water and the alkalinity component consumed by chemical injection is taken into consideration, so the PH of the treated water can be calculated with high accuracy. Can be done. In addition, this method takes into account both the alkalinity and pH required for flocculation, so the flocculation process can be operated safely. Furthermore, the control method incorporates feedback control of the quality of treated water using PID calculations, and combines this with feedback control that takes into account weighting coefficients when calculating the alkali injection rate, making it possible to quickly respond to process fluctuations. The alkali injection rate can be changed by By controlling the alkali injection rate in this manner, the pH and alkalinity of the treated water can be maintained at the set values.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of a conventional alkali injection control system, Fig. 2 is a diagram showing the carbon dioxide concentration distribution in a settling tank of an actual facility, and Fig. 3 is a diagram showing an alkali injection control system according to an embodiment of the present invention. Figures 4 and 5
The figure is a diagram showing the relationship between the reference injection rate and the feed forward injection rate. 1...Water landing well, 2...Mixing pond, 3...Flot formation pond, 4...Sedimentation pond, 5...Rapid overflow pond, 6...
... Stirrer, 7 ... Flocculator, 8 ... Water intake flow meter, 9 ... Pre-chlorine injection machine, 10 ... Alkali agent injection rate, 11 ... Coagulant injection machine, 12 ... Treated water sampling point, 13 ... ...Processed water PH meter, 14...
Arithmetic unit, 15... Ratio computing unit, 16... Arithmetic unit,
17...Raw water alkalinity meter, 18...Raw water PH meter,
19...Raw water temperature gauge, 20...Calculator, 21...
Arithmetic unit, 22...Arithmetic unit, 23...Comparison arithmetic unit,
24...adder, 25...PID calculator, and the same reference numerals in the drawings indicate the same or corresponding parts.

Claims (1)

[Claims] 1. At water treatment plants and other water treatment facilities that perform water treatment by injecting one or more of chlorine, flocculants, and alkaline agents into raw water, the measured values of the PH, alkalinity, and water temperature of the raw water, as well as the chlorine and flocculant agents. The free carbonate concentration contained in the treated water is estimated from the injection rate of one or more types of alkali agents, and the treated water
By determining the PH, the alkali agent standard injection rate is determined as an approximate value of the alkaline agent injection rate, and the feed forward injection rate is determined as a fraction thereof, and the alkali agent standard injection rate is set to be shorter than this alkaline agent standard injection rate. By making slight corrections using the feedback injection rate that is updated periodically, and further feeding back the measured water PH value of the treated water using PID control, the alkali agent reference injection rate is corrected using the feedback injection rate. A method for controlling alkaline agent injection in a water purification plant, characterized by maintaining an appropriate amount of alkaline agent injection.