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h2. {toggle-cloak:id=Abstract} Abstract
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h2. {toggle-cloak:id=Introduction} Introduction
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Coagulation is the process of chemically destabilizing solutions whereby electrolytes are added into solution so as to reduce the charge on the colloidal particles, thus facilitating their close approach and aggregation. In water treatment processes, the primary electrolyte introduced into solution as the primary coagulant is aluminum sulfate (Al2(SO4)3), also known as Alum. Flocculation is the subsequent process whereby particles collide and form large aggregates, or flocs. Flocculation involves producing collisions between suspended particles that are strong enough to overcome the repulsive potential barrier between particles, but that will not simultaneously break up the flocs already in suspension. Improving the effectiveness of coagulation and flocculation will greatly enhance the efficiency of small scale water treatment systems, which will drive down costs and make the technology more feasible and attractive to communities.
One of the important contributors to the formation of flocs is the chemical coagulant aluminum sulfate (Alum). When dissolved in water, alum dissociates and the multivalent aluminum ions (Al3+) form various species that have an average positive charge. Since most suspended colloidal matter in surface waters have an overall negatively charged surface, the dissolved Al species will adsorb onto the surfaces of these particles and reduce its net surface charge, thus lowering the barrier for floc formation. The appropriate amount of alum required to facilitate total particle removal via the sedimentation of flocs is dictated by a couple key parameters: The concentration of clay particles and other natural organic matter (specifically the chemical properties of these contaminants and their reactivity with the dissolved Al species) will effect how much alum is needed. The alkalinity of the water will also effect how well the coagulant will perform.
In the current laboratory flocculator experimental setup, there are several parameters that control the flocculator process. Our main objective in conducting research with the lab flocculator is to be able to isolate each of the parameters in order to understand their specific roles in the flocculation process and to optimize overall flocculator efficiency. While the need to fully understand the mechanism by which alum coagulates particles was not immediately pressing, it was important for our research to determine the most effective alum dose for various levels of turbidity in order to temporarily remove alum dosing as a variable in our later experiments. In order to isolate and temporarily remove alum as a variable, a relationship between alum dose and influent turbidity must be determined. For a given influent turbidity, there is, theoretically, an optimal amount of alum that should be added to the influent in order to form the greatest amount of flocs and thus producing the lowest effluent turbidity. This optimal alum dose should, therefore, change with different influent turbidities. The objective of our first experiment was to determine the optimal alum dose for various influent turbidities.
While finding this relationship is integral to our later experiments, the empirical values would not necessarily apply to vertical flocculators used in water treatment plants designed for Honduras. First, differences in flocculator design, and thus intensity and quality of mixing, could greatly affect the appropriate alum dose for various turbidities. Moreover, the levels of contaminants in the waters in Honduras will have very different dissolved matter in it that will react to alum much differently than in our experiments. However, the purpose of our investigating optimal alum dose is not to give operators in Honduras a alum dosing guide, but to develop a control method in our experiments that will render alum dosing a temporary non-variable. Future experiments may need to be performed to determine whether the effects of alum can vary with different mixing intensities or other variables.
After determining the optimal alum dose for different influent turbidities, we proceed to run experiments with other variables, such as velocity gradient (G) and Gtheta, an indicator of the cumulative extent of mixing experienced after one pass through a reactor.
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h2. {toggle-cloak:id=Methods} Methods
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Enter text hereThe [flocculator setup|Flocculator Setup] was used in this experiment.
In order to isolate alum dose as the only variable in our experiment, we must maintain mixing at a constant. If we maintain both flow rate and flocculator length (hence residence time) constant, we can keep mixing constant. The length of the tube was inherited from the summer team, as our team did not replace the tube that had already been attached to the set up. The calculation for the length of the tube was done by measuring the outer diameter of the cylinder around which the tube was wrapped (plus adjustments for the thickness of the tube itself) and multiplying that by ¿ to give the circumference and then multiplying the circumference by the number of times the tube wrapped completely around the cylinder. The affective diameter was 42 cm and the tube wrapped around the cylinder approximately 15.5 times, giving a total length of approximately 20.45 meters.
Given that the flocculator length is fixed, we must select and hold constant a flow rate that is will cause adequate amount of mixing but that will not break up flocs. From observations of past experiments and reports done by past lab floc teams, a Gbar (average velocity gradient) of roughly 40 sec-1 was selected as the Gbar that will be used in every run of this particular alum dose experiment. The equations relating Gbar to the flow rate Q for laminar pipe flow are as displayed below. Note that the transition between laminar flow to turbulent flow in long smooth pipes occurs at a Reynolds number of roughly Re = 2300. This translates to a flow rate of roughly 9.69 mL/sec through the 0.42m diameter pipes used in this experimental setup. Since the pumps are unable to pump fluid through the pipes at those speeds, it is valid to assume that the flow in the pipes will be laminar for our experiments. The flow rate at which all our experiments will be run is Q = 1.33 mL/sec which produces a Gbar = 42 sec-1.
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h2. {toggle-cloak:id=Results} Results
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h2. {toggle-cloak:id=Discussion} Discussion
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