Physicochemical Characterization, Chlorination and ... - IJGHC

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Physicochemical...Andzi et al.All the results are deposited in the Table 8. To illustrate these results, Figures 2, 3 and 4 presents,respectively, the appearance time of the plop, the residual turbidity and the yields of the reduction ofthe turbidity according to the dose of coagulant.ParametersTable-8: Jar Test ResultsRawWaterWaters of River Djoué (Sample)Water treated by sulfate of Aluminum and LimeNumber of Becher 0 1 2 3 4 5 6Dose of Sulfate of aluminum(mg/L)0 5 10 15 20 25 25Dose of lime (mg/L) 0 5 5 10 10 10 10pH 7,5 6,87 6,76 7,04 6,62 6,57 6,57T°C 20 20 20 20 20 20 20Turbidity 15 10 6,28 5,01 1,12 2,07 2,07Color 7,5 8 2,5 1,25 0 0 0Time of appearance of plop (mn,s)Yield of eliminat° of Turbidity(%)3’ 2’17’’2’ 1’20’’ 2’25’' 2’25’33,33 58,13 15 92,53 86,20 86,20It appears (Figure-2) that the appearance time of the plop evolves in two phases. In the first phase,the appearance time of plop decreases with the increase of the coagulant dose. In the second phase itsdecrease with the increase of coagulant dose. The minimal time of appearance of the plop is obtainedafter 80 seconds (1min 20s) in pH = 6.62. The doses of sulfate of aluminum and lime are respectively20 mg/L and 10 mg/L.Figure-2: Time of Appearance of the Plop according to the Dose of Coagulant (mg/L)625 IJGHC; June – August 2013, Vol.2, No.3, 618-630.
Physicochemical...Andzi et al.It is necessary to note that this appearance time is included in the fast phase of our Jar test essays.These results are shape to the works of 61,62 , who showed that this phase is most important of theflocculation. In this pH value (pH = 6.62), the mechanisms of adsorption are favored, because in thisdomain aluminum hydroxide (Al(OH) 3 ) is the dominant reagent 63 . More, the optimal dose of thereduction of turbidity is 20 mg/L (Figure-3).Figure-3: Residual Turbidity according to the Dose of Coagulant (mg/L).Figure 3 illustrates the evolution of the residual turbidity according to the dose of coagulant. Itappears that this evolution follows the same tendencies as the appearance time of plop. We canobserve (Table 8) that the optimal value of elimination of the turbidity (20 mg/L) is obtained inpH = 6,62. These results approximate to those of 64-67 , which show that the reduction of organiccompounds during the flocculation makes globally in pH value included between 5 and 7, accordingto the nature of the coagulant.Indeed, the addition of sulfate of aluminum in the water lowers le pH value. The use of lime asadditive drives to the formation of aluminum hydroxide. Because lime increases the pH value andfavors the coagulation mechanism by this reaction.3Al 3+ + 3H 2 O Al(OH) 3 + 3H + (7)Indeed, contrary to the use of the only sulfate of aluminum, the use of lime favors the formation of theplops and accelerates their rapidity of decantation. The CaCO 3 formed during its reaction with thewater, in trend to form agglomerates of dense crystals, which decant in strong speed 68-70 . The reactionof the lime on raw water is extremely slow in the absence of germs of crystallization (Crystals ofCaCO 3 precipitated). On the other hand, in the presence of germs, the reaction of precipitationbecomes rapid. The presence of organic colloids being susceptible to obstruct the crystallization. Thisis the raison we use the combination lime and sulfate of aluminum to eliminate better colloids 71 .The evolution of the yields on the elimination of the turbidity (Figure-4) shows two phases. At first,the yields increases according to the coagulant dose until reach one maxima, then, its begins decreaseafter maxima in spite of the increase of the coagulant dose.626 IJGHC; June – August 2013, Vol.2, No.3, 618-630.
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