3.2. Experimental Results:
3.2.1. Effect of pH value:
At different contact times (10-120min), pH ranges (3–10) were used to determine the effect of pH in the clarification process at a clay suspension of 4000 ppm and a fungal concentration of 0.4 g/L. Figure 5 shows that turbidity removal increases with increasing pH up to pH 6.7 and then decreases. Results show that the minimum values of residual turbidity are obtained at pH 6.7, followed by pH 5 and 8, while the least removal of turbidity is obtained at pH 3 and pH 10. The percentage reduction in residual turbidity increased from 59.5% at pH 5 to 60% at pH 6.7 after 120 minutes, which was due to the increase in electrostatic charges on the surface of Mushroom (+ ve) and clay (-ve) as pH increased up to pH 6.7. The percentage of residual turbidity decreased to 57.4% at pH 10. This is because when pH value was risen then the particles tended to acquire more negative charge. This leads to a zero or negative values for Zeta potential. Then the particles in suspension will tend to repel each other and there will be no tendency for the particles to come together [27, 28]. Thus, neutral pH is recommended in the clarification process rather than the alkaline or highly acidic medium. Thus, the following experiments of the study are carried out at a pH value of 6.7.
Mushroom has a high content of amine groups, which provide a cationic charge on the fungal surface at acidic and natural pH. The electrostatic attraction between the cationic charge of Mushroom and the suspended solids weakens the colloidal suspension, promoting the formation of bulky, rapidly settling flocs, which then flocculate. The positive charges of the long-chain polymer of mushroom can efficiently coagulate the negative charges of both the natural particles and the colloidal materials by one or more of the following mechanisms: Sorption, neutralization, and hydrophobic flocculation [29].
3.2.2. Effect of flocculent dose
The effect of flocculent dosage on residual and percent reduction in turbidity was studied at a clay content of 4000 ppm and pH of 6.7 with different Mushroom concentrations (0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 g/L). Figure 6a shows that the best residual turbidity of 11.2 NTU is obtained at a flocculent dose of 0.4 g/L after 120 minutes, while the worst results are obtained using a flocculent dose of 0.2 g/L under the same conditions. The flocculent doses of 0.6, 0.8 and 1.0 g/L gave very similar results. In general, Mushroom is rapidly hydrolyzed in water to form a series of cationic species that can be absorbed by negatively charged suspended particles and neutralize their charge. This is one of the mechanisms by which particles can lose their ability to distribute in solution; which promotes flocculation [30]. Excessive dosages can interfere with turbidity reduction, so careful control of coagulant dosage should be performed in water treatment plants.
Figure 6b shows that a flocculent dose of 0.4 g/L achieves a maximum percent reduction in turbidity of 96.5%, while a flocculent dose of 0.2 g/L achieves a lower percent reduction in turbidity of 93.4%. This means that this concentration of flocculent was not sufficient to cause the deposition of all suspended solids in a reasonable time [31].
3.2.3. Effect of clay concentration:
Effect of clay concentration in absence of flocculent:
Clay suspensions with concentrations between 1000 and 6000 ppm were used to determine the effect of clay concentration on residual turbidity at different settling times (Fig. 7a). This figure shows lower residual turbidity for clay suspensions at 1000 ppm, while higher residual turbidity is observed for suspensions at 6000 ppm. The residual turbidity of clay concentrations 1000, 2000, 3000, 4000, 5000 and 6000 ppm were 16.2, 19.1, 20.1, 24.7, 25.0 and 32.0 NTU, respectively. Therefore, the growth of flocs is promoted with higher clay concentration [29, 16]. Figure 6b shows the percentage reduction of turbidity after 120 minutes for different concentrations of clay suspensions. The maximum percentage of reduction reached 96.5% for the suspension with clay concentration of 4000 ppm, followed by the suspensions with 5000 and 6000 ppm clay concentration. The lowest percentage of reduction was 82.8% for clay suspension with 2000 ppm.
Effect of clay concentration using flocculent (0.4g/l):
Figure 8a shows the change of residual turbidity with settling time under the conditions of adding 0.4 g/L of Mushroom flocculent in the different clay concentrations at pH 6.7. The residual turbidity drops steeply for the clay concentrations 5000 and 6000 ppm after 60 minutes of settling time, after which the change in residual turbidity is very small. The results show that the lowest residual turbidity of 11.1 NTU is observed for the 1000 ppm clay suspension, while the highest value of 22 NTU is observed at 6000 ppm. Therefore, the effect of flocculent is more pronounced at lower suspension concentrations. The slope of the straight line appears to become smaller (lower turbidity reduction) as the clay suspension concentration increases, which is due to the lower influence of agglomerated particles in clearing other suspended particles. Figure 8b shows that the maximum percent reduction in turbidity is 96.5%
3.2.4. Effect of ratio of flocculent dose to clay suspension concentration:
Table 2 shows that a maximum percent reduction in turbidity (96.5%) was achieved when the ratio of flocculent (Mushroom) to clay in suspension was 1:10. Overdosing of the flocculent results in a slight decrease in turbidity removal efficiency, which is due to charge reversal or steric stabilization; where the flocculent produce strong repulsion between particles in the suspension. In addition, overdosing destroys the electrostatic bridge between clay particles and Mushroom, which leads to an increase in residual turbidity [32].
Table 2
Percentage reduction in turbidity with different ratios of Clay/Mushroom for clay concentration 4000 ppm
Flocculent dose, g/l | Ratio (Mushroom/Clay) | Percentage reduction in turbidity % |
0.2 | 1:20 | 83.4 |
0.4 | 1:10 | 96.5 |
0.6 | 1:6.6 | 94.5 |
0.8 | 1:5 | 94.4 |
1.0 | 1:4 | 94.3 |
1.0 | 1:3.3 | 94.1 |
3.2.5. Statistical Modeling
The response surface model of the residual turbidity and percentage reduction in turbidity:
The experimental values for residual turbidity and percent reduction in turbidity responses at the design points and for all three noncoding independent variables are shown in Table 3. The experimental data were analyzed and the following regression equations were obtained with respect to the coded factors for the expected values of the initial responses for both models developed (at 4000 and 5000 ppm clay concentration).
For 4000 ppm clay concentration the model is:
Residual Turbidity = + 18.99 + 2.7A – 2.1B + 0.62AB – 2.41A2 + 0.22B2
%Reduction in turbidity = + 94.08–0.83A + 0.67B -0.2AB + 0.73A2 − 0.067B2
For 5000 ppm clay concentration the model is:
Residual Turbidity = + 17.78 + 0.56A – 0.98B + 0.29AB − 9.12A2 – 0.61B2
%Reduction in turbidity = + 95.47–0.15A + 0.25B -0.073AB + 2.35A2 + 0.15B2
Figures 9 and 10 show that the expected values for the responses to turbidity removal (residual turbidity and percent turbidity reduction) are significantly close to the actual values for both models developed at a clay concentration of 4000 (Fig. 9a, b) and 5000 ppm (Fig. 10a, b); confirming the consistency of the models created to establish a relationship between the independent process variables (mushroom dose and time) and the responses.
Table 3
Experimental design for turbidity removal process
Run | Mushroom dose (g/l) | Time (min) | Residual Turbidity (NTU) | %Reduction in Turbidity |
at 4000 ppm clay | at 5000 ppm clay | at 4000 ppm clay | at 5000 ppm clay | at 4000 ppm clay | at 5000 ppm clay |
1 | 0.3 | 0.3 | 60 | 21.1 | 9.51 | 93.4 | 97.59 |
2 | 0.2 | 0.2 | 60 | 16.9 | 17.6 | 94.7 | 95.5 |
3 | 0.4 | 0.3 | 90 | 19.25 | 9.4 | 94 | 97.6 |
4 | 0.4 | 0.3 | 120 | 17.9 | 7.5 | 94.4 | 98.1 |
5 | 0.2 | 0.2 | 90 | 14 | 17.6 | 95.6 | 95.5 |
6 | 0.2 | 0.2 | 120 | 11.2 | 16.9 | 96.5 | 95.7 |
7 | 0.3 | 0.1 | 90 | 18.9 | 8.09 | 94.1 | 98 |
8 | 0.3 | 0.1 | 120 | 17.4 | 5.9 | 94.6 | 98.5 |
9 | 0.4 | 0.1 | 60 | 21.12 | 9.09 | 93.4 | 97.7 |
The statistical significance of the models for residual turbidity and percent reduction in turbidity was evaluated using ANOVA (Table 4). The data in the table show that the F values of the expected models at a clay concentration of 4000 ppm are 223.06 and 272.62 for residual turbidity and percent reduction in turbidity, respectively. The Model F-value of 223.06 and 272.62 imply that there is only a 0.05% and 0.03% chance, respectively that an F-value this large could occur due to noise. This indicates that both models are important. In addition, the F-values of the models for 5000 ppm clay concentration were 80.62 and 88.92 for residual turbidity and percent reduction in turbidity, respectively. The F-value of 80.62 and 88.92 imply that there is only a 0.21% and 0.19% chance, respectively that an F-value this large could occur due to noise. Values of "Prob > F" less than 0.0500 indicate that model terms are significant. The R-squared values for the four model equations (0.997, 0.999 and 0.993) indicate that the fit of the quadratic models is high.
Table 4
ANOVA for the predicted quadratic models of the turbidity removal responses
Source | Sum of squares | Mean square | F value | p-value prob > F | Standard Deviation | Squared-R |
At 4000 ppm clay model | Residual Turbidity | 83.32 | 16.66 | 223.06 | 0.0005 | 0.27 | 0.997 |
% Reduction | 8.08 | 1.62 | 272.62 | 0.0003 | 0.077 | 0.999 |
At 5000 ppm clay model | Residual Turbidity | 175.04 | 35.01 | 80.62 | 0.0021 | 0.66 | 0.993 |
% Reduction | 11.61 | 2.32 | 88.92 | 0.0019 | 0.16 | 0.993 |
3.2.6. Effect of process parameters:
Effect of coagulant dose:
Coagulant dosage is one of the most important factors considered in determining the optimal performance state of coagulants in coagulation and flocculation. Insufficient or excessive dosages can result in poor flocculation performance. For this reason, it is essential to determine the optimal dose to reduce dosing costs and sludge formation and to achieve optimal treatment performance [33, 34]. In this work, the optimal dose of Mushroom (0.4 g/L) was the lowest dose required and it resulted in the maximum turbidity removal. The best performance of the process was at pH 6–7, and the coagulation efficiency of mushroom remained almost constant in the dose range of 0.6-1 g/L. Figure 11 shows the reaction surface curves for both developed models (4000 and 5000 ppm clay concentration). The response surface curves were constructed to show the effects of the independent variables (Mushroom dose and time) on the dependent variables (residual turbidity and percent turbidity reduction) for the four models developed.
3.2.7. Optimization of the operating parameters:
The optimal operating parameters for the turbidity removal process were determined to maximize the percent reduction in turbidity based on the appropriately developed mathematical models. The optimal values for the 4000 and 5000 ppm clay concentration models were as follows: Mushroom dose, 0.2 and 0.1 g/L for 4000 and 5000 ppm clay concentration, respectively, at 120 minutes contact time. The target residual turbidity values achieved under these conditions were 11.38 and 6.21 NTU for 4000 and 5000 ppm clay concentration, respectively, with maximum percent turbidity reduction of 96.4 and 98.4%, respectively [35, 36].