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ANC CONTROL

Flow rate required in Lime-feeder

The lime feeded by effluent from the lime feeder will help to increase the Alkalinity in alkalinity of the raw water during the coagulation process, . This process will help the original carbonate system build a buffer zone 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 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 several ways to change the concentration of lime in raw water through lime feeder, such as two major criteria in the experiment: the dosage of lime , the operation time, lime feeder velocities, lime feeder effluent pH etc. Technically, and the upflow velocity. We need to find the proper combination of dosage and flow rate to maintain the effluent pH of lime feeder should be kept at around 12, that is the lime saturate pH, our assupmtion saturation pH of 12.4. Our assumption is that with a continously flow comes from a distribution of raw water into lime feeder the previously dissolved lime would be consumed, it result in more and more settled lime become dissolvable in water, and 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 the effluent pH could be kept constantrate 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 constant based on our assumptionbe relatively constant under the assumption previously discussed, the way we can change the lime concentration in raw water is depend on to change the flow rate of the lime feeder, it also create convenience . A constant effluent pH also makes it more convenient for the operator to use the apparetus changing pH and AlK 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 certain in a certain range which could provide the pH environment in the tank could  maintain plant pH between 6.5 to 7.5,  it which is the range coagulation will be where coagulation is the 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 possible analyze the relationship between flow rate and change in pH , Alkalinity in plant and alkalinity in the Marcala and Cuatro Comunidades based Comunidades plants based on the data from Honduras report spanish version translated by our team member Ximena.

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The lime dos\age we want 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).

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(How did you obtain your initial estimate for initial alkalinity? Is this different for different plants?)

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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.

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?)

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report.

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:

\large
$$
ANC = C_T (\alpha _1 + 2\alpha _2 ) + OH^ - - H^ +
$$

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

\large
$$Ratio = {Q_{feed}}/{Q_{plant}}$$

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:

\large
$$
OH_{Balance}^ - = {\textstyle{{[OH^ - ]_{added} Q_{feed} + [OH^ - ](Q_{Plant} - Q_{feed} )} \over {Q_{feed} + Q_{Plant} }}}
$$

The relationship between OH- concentration and alkalinity can be calculated with equation 3:
equation 3

\large
$$
ANC_{final} = ANC_{initial(Carbonate)} + OH_{feed(Lime)}^ - - H_{feed(Alum)}^ +
$$

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: 
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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: 
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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.

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Figure 3: Ratio of lime feeder flow rate to plant flow rate versus final ANC and pH (Alkalinity is 20mg/L, good buffer.)

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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)