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Figure 2. Process flow diagram of experimental setup.

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The graphs show that as plant flow decreases, the effluent turbidity also decreases, until we reach 50 mL/min. At 25 mL/min, we observed that the effluent turbidity is unstable, but not lower than that of 50 mL/min.

From Figure 3, it was determined that running the plant at 50 mL/min resulted in the lowest effluent steady-state turbidity. The empirical optimal plant flow is much lower than the 100 mL/min design flow. This is most likely due to design flaws in the sedimentation tank. We thought that the parallel lamellas design would equalize the flow paths, and thus the flow rates, in each lamella, but this was not the case. We still observed faster flow rates through some of the lamellas, resulting in a settling time that was shorter than that required in the design. Thus, when plant flow rate was decreased, resulting in a longer settling time, the flocs had a more adequate amount of time to settle out, and effluent turbidity decreased. In addition, at flow rates less than 50 mL/min the degree of mixing was insufficient to produce sizable flocs; at greater flow rates, especially above 100 mL/min, the maximum shear attained in the flocculator exceeds the shear limit of the flocs, leading to floc break-up. Both these cases lead to small flocs exiting the sedimentation tank without settling, thus increasing turbidity.

Figure 4 below shows the total degree of mixing and the degree of mixing contributed by the 180° bends only, against plant flow. It also shows the maximum shear attained at the 180° bends and the average shear throughout the plant, againt plant flow. All values are calculated using a slightly modified version of the flocculator design program.

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From the data acquired and our direct observations, we conclude that the performance of the Demo Plant improves with lower plant flow. The performance limit of the plant is approximately 50 mL/min, below which there is no decrease in effluent turbidity. We hypothesize that above this flow limit, the shear is too high and therefore causes floc break up; below this limit the flocs don't fully form. With small floc size in both extremes, proper floc settling in the Sedimentation Tank does not occur, yielding a higher output turbidity.

In addition, it is possible that the uneven flow in the Sedimentation Tank provides significantly less settling time than predicted. This effect is magnified at higher flow rates and thus further increases effluent turbidity.

Finally, the experimental calculations indicate that the frictional shear in the vertical sections of the flocculator contributes significantly to flocculation, and cannot be neglected at this small scaleThe empirical optimal plant flow is much lower than the 100 mL/min design flow. This is most likely due to design flaws in the sedimentation tank. We thought that the parallel lamellas design would equalize the flow paths, and thus the flow rates, in each lamella, but this was not the case. We still observed faster flow rates through some of the lamellas, resulting in a settling time that was shorter than that required in the design. Thus, when plant flow rate was decreased, resulting in a longer settling time, the flocs had a more adequate amount of time to settle out, and effluent turbidity decreased.