Incrementing Turbidity, G, and Gtheta

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The Lab Flocculator research team has an important focus on investigating the relationship between fluid shear (G) and its effect on overall flocculator mixing (Gtheta). Floc formation and breakup rates depend greatly on the Gtheta parameter, which is intrinsically related to the fluid shear present inside a reactor. The semester research goals of the Lab Flocculator research team was to measure the relationship between G and Gtheta by performing experiments using a coiled-tube flocculator experimental apparatus. The team decided to first perform experiments to determine the appropriate alum dose to achieve optimal floc formation for the experimental apparatus. Experimental data showed that a 25 mg/L concentration of alum produced the best results for a range of influent turbidity between 50-150 NTU and a humic acid concentration of 35 mg/L with minimal marginal improvements thereafter; thus, subsequent experiments were conducted with a constant 25 mg/L alum dose. Throughout the semester, the team was faced with numerous obstacles originating from the physical apparatus and the various software used by the team. Sedimentation of clay inside feed tubing and the flocculator inhibited various experimental controls including influent turbidity, flow rates, and also data collection. Measures were taken to overcome this issue, but sedimentation should ideally only occur in the settling column during the appropriate operational state. The flocculator team has overcome a large learning curve of needing to understand the experimental setup, Process Controller, and the MathCAD data processor tools. Much data was collected of different combinations of important variables such as flow rate, influent turbidity, and flocculator length. Currently, only a qualitative analysis of the data has been performed and the team is satisfied with the data set obtained. During coming semester, the team will focus on developing an analytical model to better analyze the data as well as to present it in intelligible figures and charts in preparation for publication.

The goal of finding a relationship between G and G¿ is not simple; it requires many experiments and a lot of data analyses. It is an important relationship to determine because it is hypothesized that with the increase in the length of the flocculator, the max G required to continue to build flocs decreases. Keeping the flocculator at a constant G throughout the tube may be compromising the floc strength and growth, as the flocs will build throughout the flocculator and then break up from the shear forces at the end of the flocculator. Floc growth depends on G, alum concentration, particle concentration, and the residence time of the flocculator. As particles floc together and the floc become larger, the strength of the floc becomes weaker with volume, so it is more prone to break up at the end of the flocculator. By reducing the shear forces on the flocs at the end of the flocculator, the flocs will remain intact, keeping the effluent turbidity low.
The effectiveness of alum dosage, flocculator residence time, and G is measured by the size of flocs after flocculation. Since the equipment needed to observe and measure the sizes of flocs formed after each experiment does not exist, the rate at which flocs settle under quiescent conditions must be analyzed along with the final settled turbidity. Since the experiments were run under identical conditions, the mechanisms creating the flocs are identical from run to run; therefore, the composition of the flocs (their shape and density) should be relatively uniform. This fact allows us to directly relate the settling rate of the flocs to the size of the flocs, while neglecting factors such as electrostatic interactions and porosity (which would affect drag coefficients and densities). This principle is the basis of the laboratory flocculator research.

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