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Flow rate required in Lime-feeder
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 cancel the impact 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 feederFor our experiment, the lime feeder water flow rate required was calculated in a MathCad file.
The variables required in the analysis, 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, water velocities, water pH etc. were taken from the values in the previous team's report. Hence the raw water was assumed to have a hardness (hardness or alkalnity?) of 0.02gm/L and an initial pH of 6.5. 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 consumed, it result in more and more settled lime become dissolvable in water, and with a proper combination of lime dosage and flow rate the effluent pH could be kept constant.
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 between 6.5 to 7.5, it is the range coagulation could be most efficient.
The lime dosage 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).
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.
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