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Effect of the alum dose on flocculation:
The next figure (figure 10) we are plotting the residual turbidity as a function of alum dose for water with a different influent turbidity in the short flocculator (2796 cm). We can observe a similar behavior between the plots. At the beginning, the residual turbidity is decreasing when we increase alum dose and then we reach a limit where the residual turbidity is almost constant when we increase alum dose. These results mean that we could optimize the cost by using the minimum alum dose that gives the minimum effluent turbidity.
We can also observe on this figure that at a given alum dose, the mean residual turbidity is different for each influent turbidity. The highest residual turbidity is found for 500 NTU then 25 NTU and 5 NTU. The lowest residual turbidity is found for an influent water of 100 NTU. This could be explained by the fact that two phenomena are happening at the same time. For the low turbidity, 5 NTU and 25 NTU, as I explained earlier, it is longer to produce large flocs and the residence time in the 2796 cm long flocculator might be too short so the residual turbidity is relatively high. For the high influent turbidity, 500 NTU, we reach a steady state in the flocculator but as we are starting with a really high turbidity, the residual turbidity is high too. Logically, the optimum result should be found for a medium influent turbidity and it is found for 100 NTU.

Figure 10: _Plot of the residual turbidity as a function of alum dose at different influent turbidity with a 2796 cm long flocculator _