Alum Dosing Theory
Alum, or aluminum hydroxide, is added to raw water in order to coagulate the particles suspended in the water. Before alum is added, the particles have a slight negative surface charge and therefore repel one another. Dosing the water with alum neutralizes the charge on these particles so they are more likely to stick together. The particles are allowed to grow in a flocculator, where the dosed water is thoroughly mixed in order to disperse the alum and promote the flocculation of the particles. These conglomerates are referred to as flocs.
Following the flocculation, the water enters a sedimentation tank, where the flocs begin to settle. This settling creates a "blanket" of flocs, which serves to trap more flocs and provide effective filtration as it grows. The floc blanket is an integral part of the AguaClara technology and enables the system to achieve much lower effluent turbidity.
Underdosing
An underdosed system occurs when the alum dosing is lower relative to the dosing conditions that are considered "ideal." In this situation, the flocs that are formed will be made up of a larger portion of suspended particles, which in our system are clay particles, and a smaller portion of the flocs will be composed of the alum. This results in a smaller floc and, consequently, a more dense floc blanket.
Overdosing
An overdosed system is accomplished by providing an alum dose that is at a higher concentration than what is considered to be the "ideal" dose. In this case, the flocs that are formed will have a higher alum-to-suspended-particle ratio. The flocs are therefore larger and more "fluffy" and the resulting floc blanket is less dense.
Overview of Methods
In these experiments the alum dosage supplied to the flocculation system was varied in order to study how properties of flocs and the floc blanket affect the effluent turbidity produced by the tube settler. The experimental set-up is identical to the one used in Spring 2009, and from our results we hoped to analyze velocity gradient thresholds and possibly investigate how changing influent water chemistry affects the setter's efficiency.
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Using the Spring 2009 team's process controller methods, we subjected an ideal geometry to non-ideal conditions. Specifically we altered the alum dose to see how different alum doses affected the effluent turbidity. Though the Spring 2009 team had success with a 9.5 mm diameter tube, due to what we think was ineffective air bubble traps in the flocculator, or the addition of a flow accumulator to the method, we experienced failure with this geometry. We achieved an acceptable effluent turbidity (less than 1 NTU) with a 15.1 mm diameter tube that had a length of 30.5 mm. With the good experimental results, we then subjected this tube settler to varying alum dosage to investigate the affect of dosing on tube settler performance. At each alum dosage, the tube settler was tested at a variety of capture velocities and at two different floc blanket levels: the lower level is when the height of the floc blanket falls below the bottom of the tube settler, the high floc blanket level is when the floc blanket height is above that of the bottom of the tube settler.
Alum Dosing Theory
Experiment 1: Alum Dose = 45 mg/L
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