Analysis of the effect of alum dose and flocculator length on flocculation

Procedure

For our next experiment, we chose to investigate the optimal alum dosage for two different influent turbidities (100 NTU, and 500 NTU) and three different flocculator lengths (2796 cm, 5592 cm, and 8388 cm). The plant flow rate was held constant at 5 mL/s. For all our experiments, we used the same Flocculator Residual Turbidity Analyzer (FReTA) setup that was developed previously (Spring 2009). FReTA consisted of five parts: an alum stock bucket, a kaolin clay stock bucket, a raw water reservoir, a coiled tube (insert diameter) serving as the flocculator, and the residual turbidty analyzer with a settling column. The raw water turbidity was controlled using a feedback loop mechanism; clay from the stock bucket was metered in automatically if the turbidity became too low. Peristaltic pumps were used to provide the flow rate and meter in the alum solution. All flow rates and chemical dosages were calculated, monitored and controlled using the Process Controller sotware (Weber-Shirk 2008). For detailed information on FReTA setup and the Process Controller figuration, please see Ian Tse's MS thesis.Thesis

In order to investigate alum dosage at a given turbidity and length, we planned to run two experiments. The first experiment would vary the alum dosage over a large range in larger intervals while holding the plant flow rate (shear), influent turbidity, and residence time constant. Then we planned to run a second experiment varying the alum dose across the best performing interval from the first experiment with small increments to determine the optimal dose.
For a flocculator length of 2796 cm, we began by testing water with an influent turbidity of 100 NTU with a plant flow rate of 5 mL/s. The alum dosage was varied from 20-60 mg/L in increments of 5 mg/L for each run. During each run, the influent raw water combined with the correct alum dosage was allowed to run through the plant until two residence times had passed, insuring a steady-state effluent floc distribution. Then the pumps gradually ramped down, and a valve sealed off the settling column from the rest of the flocculator.

Results and Discussion

To find the ideal alum dosage for a certain influent turbidity, there are several qualities that must be considered. First, the resulting floc size should be large enough so that by the time it reaches the sedimentation tank, it will weigh enough to sink and settle out. Second, the resulting NTU must be small indicating an efficient flocculation/sedimentation process and clean water, and third, the alum dosage should preferably be low to save cost.

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TABLE 2: _Shows the Lowest turbidity reached by specific alum dosage ranging 30 mg/L~55 mg/L during an influent turbidity of approximately 500 NTU.