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h2. ANC CONTROL


h3. Flow rate required in Lime-feeder

The limeeffluent fedfrom bythe lime feeder will help to increase the alkalinity inof the raw water during the coagulation process. This process, will help the original carbonate system to build thea buffer zone to neutralize the acidity resulting from the addition addingof aluminum sulfate (Instead of increasing the pH, the hydrogen protonion from aluminum sulfate will react with the hydroxide from lime),. andThis thus,will improve the quality of the treated effluent water. Our team wantswanted 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 lime feeder velocitiesupflow velocity. We need to find the proper combination between theof dosage and flow rate to maintain the effluent pH of lime feeder kept at aroundthe a lime saturation pH of 12.4. Our assupmtionassumption is that with a continuouslycontinuous flow system, the previously dissolved lime would be washed out and provide thean effluent pH atof around12. 12,Lime itparticles followssuspended morein and more previous settled lime dissolve in water. With athe 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 comecomes out with effluent water, and provide us thewith a constant effluent pH for a relatively long time, we hope it can achieve  (12 hours for routine opertationoperation).
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Since the effluent pH&nbsp;should be&nbsp;relatively constant under ourthe assumption previously discussed, the way we can change the lime concentration in raw water is to change the flow rate of the lime feeder. TheA constant effluent pH also makes it createsmore convenienceconvenient for the operator to&nbsp;use the apparatus adjusting&nbsp; because to adjust pH and AlKalkalinity in raw water by simply changing, all he would have to do is to change the flow rate of lime feeder.&nbsp;The flow rate should be in&nbsp;a certain range which could&nbsp; maintain plant pH between 6.5 to 7.5,&nbsp;it which is the range where coagulation &nbsp;will beis the most efficient.&nbsp; <Water Quality and Treatment by Letter 1999>
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Our next step is to buildanalyze the relationship model&nbsp;between&nbsp;flow rate&nbsp;of lime feeder and the change in pH, and alkalinity in&nbsp;plant the Marcala and Cuatro Comunidades&nbsp; plants based on&nbsp;the data&nbsp;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 |
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To&nbsp;get the&nbsp;proper lime doseagedosage 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&nbsp;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 that the&nbsp;relationship between Alkalinity and&nbsp;pH can be also measured with equation 1:
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equation 1:
{latex}
\large
$$
ANC = C_T (\alpha _1 + 2\alpha _2 ) + OH^ - - H^ +
$$
{latex}
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In the next step,&nbsp;considering the flow rate of the plant will also change,&nbsp;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}
\large
$$Ratio = {Q_{feed}}/{Q_{plant}}$$
{latex}
When this flow ratio changes, lime concentration will also change, and we can acquire the relationship between this&nbsp;flow ratio and the concentration of&nbsp;hydroxide ion in raw water. from Fromthis mass balance equation,
equation 2:
{latex}
\large
$$
OH_{Balance}^ - = {\textstyle{{[OH^ - ]_{added} Q_{feed} + [OH^ - ](Q_{Plant} - Q_{feed} )} \over {Q_{feed} + Q_{Plant} }}}
$$
{latex}
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The relationship between OH\- concentration and alkalinity&nbsp;can be caculatedcalculated with equation 3:
equation 3
{latex}
\large
$$
ANC_{final} = ANC_{initial(Carbonate)} + OH_{feed(Lime)}^ - - H_{feed(Alum)}^ +
$$
{latex}
The premise of&nbsp;using the above equation is&nbsp;that we have to know the&nbsp;equivalent of proton&nbsp;created by adding aluminum sulfate, which can be measured from the difference of initial alkalinity and final alkalinity from our data.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
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Based on all this &nbsp;data and&nbsp; these equations&nbsp; we can&nbsp;make a&nbsp;model showing the change&nbsp;of pH and alkalinity as the funcionfunction of flow ratio between&nbsp;lime feeder and plant. The model which predictfollow these relationships&nbsp;of in Marcala and Cuatro Comunidades are shown in figure 1:&nbsp;
!Marcala and Cuatro.png!\\
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Figure 1: The&nbsp;relationship between flow ratio, ANC and pH of Cuatro Comunidades (figure 1.a)&nbsp;and Marcala (figure 1.b)
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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 aluminum sulfatealum. After we increase the flow ratio from 0 to 0.01, we can see from figure 1 that both of the twoplants plants:- Cuatro Comunidades(figure 1.a) and Marcala(figure 1.b)&nbsp;have almost the same increasing pattern of pH and alkalinity. The increase of alkalinity has a linear relationship with thean increseincrease 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 existexistence of bicarbonate acting as the buffer. In order to find how does the buffer affectaffects on pH we can make the model of buffer intensity in the system, seeas graphseen in figure 2:&nbsp;
\\  !Buffer intensity.png!\\
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Figure 2: Buffer intensity in Marcala and Cuatro Comunidades

The different buffer intensityintensities in Marcala and Cuatro Comunidades isare different because of different initial alkalinityalkalinities. As the&nbsp;initial pH&nbsp;starts from 6, it just locate in the zone which the buffer intensity is greatest(the highest&nbsp;point is whenBuffer intensity is greatest at a pH of 6.3. Here the pH equals to&nbsp;pK1&nbsp;of&nbsp; carbonate system), as the pH&nbsp;continously increase systems. As pH increases from 6.3 the buffer intensity becomes weakweaker and the pH begin toof the system will increase much more faster, which wouldcould easily cause the coagulation mechanism faliure and an undesirable effluent water quality, just as what the two models predict.
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