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ANC CONTROL

Flow rate required in 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.

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: 
Image Added

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: 
Image Added

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

The relationship between the ratio of flow rates and pH

The lime feeder water flow rate required for the experiment was calculated with Mathcad. The analysis was made according with the dosage of lime, the operation time, water velocities, water pH and other variables, most of them were taken from the previous team report.
But in actual condition, even if we made the flow rate in lime feeder keeps constant, but the flow rate of plant can still change in a considerable degree in operation, so we have to figure out what will this affact on pH and ANC. We assume the effluent pH of limefeeder is constant at 12.4, and the flow rate between lime feeder and plant changes from 10^(-5) to 0.01, we want to find the change of ANC and pH of raw water in the plant sensitivity to this ratio between flow rate(see figure 1.).

Right now we haven't taken into consideration the change of pH after addition of Alum. So the final ANC is affetced by the initial ANC caused by the hardness of raw water and the hydrogen provided by the limefeeder, and we assume the additional alum will consume certain part of the hydrogen that could contribute to the ANC decrease.

The eqation to caculate balance OH in plant and the final ANC are also shown below:

Image Removed balance OH in plant
Image Removed final ANC of plant                               

Image Removed

Figure 1: Graph - Ratio of lime feeder flow rate to plant flow rate versus final ANC and pH

On this stage, the team also continued to search for other information including the cost of distilled water, lime properties (solubility with temperature), and Honduras' water regulation.