ANC CONTROL
Flow rate required in
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Lime-feeder
The effluent from the lime feeder will increase the alkalinity of the raw water during the coagulation process. This process will help the original carbonate system build a buffer zone to neutralize the acidity resulting from the addition of aluminum sulfate (Instead of increasing the pH, the hydrogen ion from aluminum sulfate will react with the hydroxide from lime). This will improve the quality of the treated effluent water. Our team wanted to find the relationship between the flow rate of the lime feeder, pH, and the alkalinity in raw water.
There are two major criteria in the experiment: the dosage of lime and the upflow velocity. We need to find the proper combination of dosage and flow rate to maintain the effluent pH of lime feeder at the lime saturation pH of 12.4. Our assumption is that with a continuous flow system, the dissolved lime would be washed out and provide an effluent pH of 12. Lime particles suspended in the bed would also slowly dissolve. With the proper combination of lime dosage and flow rate this function could be kept constant until all the lime comes out with effluent water, and provide us with a constant effluent pH for a relatively long time (12 hours for routine operation).
Since the effluent pH should be relatively constant under the assumption previously discussed, the way we can change the lime concentration in raw water is to change the flow rate of the lime feeder. A constant effluent pH also makes it more convenient for the operator to use the apparatus because to adjust pH and alkalinity in raw water, all he would have to do is to change the flow rate of lime feeder. The flow rate should be in a certain range which could maintain plant pH between 6.5 to 7.5, which is the range where coagulation is the most efficient. <Water Quality and Treatment by Letter 1999>
Our next step is to analyze the relationship between flow rate and change in pH and alkalinity in the Marcala and Cuatro Comunidades plants based on the data from Honduras report.
MARCALA | RAW WATER | TREATED WATER |
|---|---|---|
pH (UN) | 6.87 - 7.26 | 6.33 - 6.56 |
Alkalinity (mg/L CaCO3) | 16.3 | 11.2 |
CUATRO COMUNIDADES | RAW WATER | TREATED WATER |
pH (UN) | 6.34 - 7.00 | 6.80 - 6.85 |
Alkalinity (mg/L CaCO3) | 7.65 | 4.59 |
To get the proper lime dosage it is necessary to know the initial total carbonate (CT) in the system. e.g CT, from the initial Alkalinity and pH of Marcala and Cuatro Comunidades. We can find the CT based on equation 1, and because we assume this is a closed system, the CT will not change during the process, so that the relationship between Alkalinity and pH can be also measured with equation 1:
equation 1:
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The lime feeded by lime feeder will help to increase the Alkalinity in raw water during coagulation process, help the original carbonate system to build the buffer to neutralize the Acidity from adding Aluminum Sulfate, thus improve the quality of treated effluent water, our team want to find the relationship between flow rate of lime feeder and the pH and Alkalinity in raw water. There are several ways to change the concentration of lime in raw water through lime feeder, such as the dosage of lime, the operation time, lime feeder velocities, lime feeder effluent pH etc. Technically, the effluent pH of lime feeder should be kept at around 12, that is the lime saturate pH, our assupmtion is that with a continously flow comes from a distribution of raw water into lime feeder the previously dissolved lime would be washed out and provide the effluent pH at around 12, it follows more and more settled lime become dissolvable in water, and with a proper combination of lime dosage and flow rate this function could be kept until all the settled lime dissolve in water and come out with effluent water, thus provide us the constant effluent pH for a relatively long time, we hope it can achieve 12 hours for routine opertation. \\ Since the effluent pH should be constant based on our assumption, the way we can change the lime concentration in raw water is depend on the flow rate of lime feeder, it also create convenience for the operator to use the apparetus changing pH and AlK in raw water. The flow rate should be in certain range which could provide the pH environment in the tank between 6.5 to 7.5, it is the range coagulation will be most efficient. <Water Quality and Treatment by Letter 1999> \\ Our next step is to build the model between flow rate of lime feeder and the change in pH, Alkalinity in plant Marcala and Cuatro Comunidades based on the data from Honduras report spanish version translated by our team member Ximena. || MARCALA || RAW WATER || TREATED WATER || | pH (UN) | 6.87 - 7.26 | 6.33 - 6.56 | | Alkalinity (mg/L CaCO3) | 16.3 | 11.2 | || CUATRO COMUNIDADES || RAW WATER || TREATED WATER || | pH (UN) | 6.34 - 7.00 | 6.80 - 6.85 | | Alkalinity (mg/L CaCO3) | 7.65 | 4.59 | \\ To get the proper lime doseage it is necessary to know the initial total carbonate in the system. e.g CT, from the initial Alkalinity and pH of Marcala and Cuatro Comunidades we can find the CT based on equation 1: And because we assume this is a closed system so the CT will not change during the process, so the relationship between Alkalinity and pH can be also measured with equation 1: \\ equation 1 {latex} \large $$ ANC = C_T (\alpha _1 + 2\alpha _2 ) + OH^ - - H^ + $$ {latex}\\ In the next step, considering the flow rate of the plant will also change, the team would like to use the ratio between the lime feeder flow rate and the plant flow rate to get a more practical function {latex} |
In the next step, considering the flow rate of the plant will also change, the team would like to use the ratio between the lime feeder flow rate and the plant flow rate to get a more practical function
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$$Ratio = {Q_{feed}}/{Q_{plant}}$$
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When this flow ratio changes, lime concentration will also change, and we can acquire the relationship between this flow ratio and the concentration of hydroxide ion in raw water from this mass balance equation,
equation 2:
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}\\ When this flow ratio changes, lime concentration will also change, we can acquire the relationship between this flow ratio and the concentration of hydroxide ion in raw waterFrom mass balance equation, equation 2: Mass balance equation {latex} \large $$ OH_{Balance}^ - = {\textstyle{{[OH^ - ]_{added} Q_{feed} + [OH^ - ](Q_{Plant} - Q_{feed} )} \over {Q_{feed} + Q_{Plant} }}} $$ {latex}\\ \\ . The relationship between OH\- concentration and alkalinity can be caculated with equation 4, the premise is that we have to know the equivalent of proton created by adding aluminum sulfate, which can be measured from the difference of initial alkalinity and final alkalinity from our data. \\ Based on all these datas and equations now we can make the model showing the change of pH and alkalinity as the funcion of flow ratio between lime feeder and plant. The model which predict these relationships of Marcala and Cuatro Comunidades are shown below: \\ \\ \\ Both of the two plants have almost the same increasing pattern of pH and alkalinity after we increase the flow rate ratio, and we can also notice the zero point of both plants represent when there is no lime feeded in water, and the pH and alkalinity are only affected by aluminum sulfate. What the model can show us is that the change of pH is affacted by the buffer intensity of bicarbonate, since the buffer intensity is simply the differential of alkalinity over pH, we can generate the buffer graph to show this relationship more clearly, the buffer graph are shown below: \\ The different buffer intensity in Marcala and Cuatro Comunidades is simply because they have different initial alkalinity. As the initial pH starts from 6, it just locate in the zone which the buffer intensity is greatest(the highest point is when pH equals to pK1 of carbonate system), as the pH continously increase the buffer intensity becomes weak and finally it can not control the pH increasement anymore, the pH starts increasing exponentially which would easily cause the coagulation mechanism faliure and undesirable effluent water quality, just as what the two models predict. \\ \\ \\ should be able to increase the pH of water to the degree we want, while neutralizing the effect of the alum that will be shortly added to it. By calculating the alkalinity under the orginial pH we can find the total carbonate e.g.CT in the system, and then we can find the relationship between the target ANC and the desired pH we want(see figure 1). The total lime dose required in tank is the sum of the amount using to neutralize the alum and to raise the raw water to the desired pH(see equation 2). \\ \\ \\ After we get the required lime dose demand in raw water, we want to find the lime feeder flow rate to satisfy this dosage, under the assumption that the effluent pH of lime feeder keeps at 12.4, and the plant flow rate keeps at 50 L/min, we get the required flow in limefeeder. Based on equation 3, we can go further to get the ratio between flow rate in lime feeder and the plant with the pH requirement in raw water, this relationship shows in figure 2. +(How did you obtain your initial estimate for initial alkalinity? Is this different for different plants?)+ !OH and ANC.jpg!<equation 1> !Creq.jpg!<equation 2> \\ !anc pH.jpg! Figure 1: The target ANC with the pH change. \\ \\ \\ !ratio pH.jpg! Figure 2: The ratio of required lime flow rate and plant flow rate with the required pH in raw water. \\ \\ h3. Relationship between the ratio of flow rates and pH Under actual plant conditions, even if we try to keep the flow rate of water in the lime feeder constant. The plant flow rate will change in certain degree in daily operation, so the dosage of lime required in the plant will change as per the amount of water entering the plant. So the calculations must be based on the variable 'ratio' i.e. the fraction of plant flow rate entering the lime feeder. The effect of this ratio on the ANC and pH are illustrated in the [MathCad file|https://confluence.cornell.edu/pages/viewpageattachments.action?pageId=113934807&sortBy=date&highlight=ANC+mathcad.docx]. It was assumed that the effluent pH of limefeeder is constant at 12.4, and the ratio of the flow rate between lime feeder and plant changes from 10^(-5) to 0.01. Other values in the calculations were kept the same as in the previous calculation. A hardness of 0.02g/L acts as a good buffer for the system (see figure 3). +(Explain more how hardness is related to alkalinity.)+ From figure 1 it is observed that when the flow rate changes, the change of pH is not so dramatic but the case is much different if the hardness decreases. For instance, for the raw water of CUATRO COMUNIDADES for which the inital ANC is 7.65mg/L, ,and we assume the same initial pH(6.5), using the same method gives a much steeper slope (see figure 4). +(Can you show a comparison plot of the two?)+ \\ NOTE: For the present calculations, it has been assumed that water will remain neutral even after the addition of alum so the final ANC is affetced only by the initial ANC (caused by the hardness of raw water) and the hydroxide provided by the lime feeder. But in reality, the alum addition will contribute to a further decrease in the pH of water and so one of our future tasks is to re-calculate the effluent pH required to negate the effect of the alum dose. \\ +(In the future add in the effect that the alum dose will have)+ \\ The equation to calculate balance OH ions in plant and the final ANC are also shown below: \\ \\ !flow rate ratio.jpg|width=369,height=97! balance OH in plant <equation 3> \\ !ANCadded.jpg! final ANC of plant <equation 4> \\ \\ !ANC 20.jpg! Figure 3: Ratio of lime feeder flow rate to plant flow rate versus final ANC and pH (Alkalinity is 20mg/L, good buffer.) \\ \\ \\ !ANC 7.65.jpg! Figure 4: Ratio of lime feeder flow rate to plant flow rate versus final ANC and pH (Raw water in Cuatro Comunidades, Alkalinity is 7.65mg/L) |
The relationship between OH- concentration and alkalinity can be calculated with equation 3:
equation 3
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$$
ANC_{final} = ANC_{initial(Carbonate)} + OH_{feed(Lime)}^ - - H_{feed(Alum)}^ +
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The premise of using the above equation is that we have to know the equivalent of proton created by adding aluminum sulfate, which can be measured from the difference of initial alkalinity and final alkalinity from our data.
Based on this data and these equations we can make a model showing the change of pH and alkalinity as the function of flow ratio between lime feeder and plant. The model which follow these relationships in Marcala and Cuatro Comunidades are shown in figure 1:
Figure 1: The relationship between flow ratio, ANC and pH of Cuatro Comunidades (figure 1.a) and Marcala (figure 1.b)
The zero point of both plants represent when there is no lime fed into the water, and the pH and alkalinity are only affected by alum. After we increase the flow ratio from 0 to 0.01, we can see from figure 1 that both of the plants - Cuatro Comunidades(figure 1.a) and Marcala(figure 1.b) have almost the same increasing pattern of pH and alkalinity. The increase of alkalinity has a linear relationship with an increase of flow ratio: with a higher flow ratio the lime concentration in the raw water will increase and cause the alkalinity to increase (equation 3). The change of pH is more complicated than alkalinity due to the existence of bicarbonate acting as the buffer. In order to find how the buffer affects pH we can make the model of buffer intensity in the system, as seen in figure 2:
Figure 2: Buffer intensity in Marcala and Cuatro Comunidades
The buffer intensities in Marcala and Cuatro Comunidades are different because of different initial alkalinities. Buffer intensity is greatest at a pH of 6.3. Here the pH equals to pK1 of carbonate systems. As pH increases from 6.3 the buffer intensity becomes weaker and the pH of the system will increase much faster, which could easily cause coagulation mechanism faliure and an undesirable effluent water quality.