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-2.70 to -9.22

a 6-8 independent contact angle measurements were conducted at different experimental times at each SRT.

a 6-8 independent contact angle measurements were conducted at different experimental times at each SRT.

small amount of soluble chemical oxygen demand (COD) and inorganic salts with a lower ionic strength (1.5 x 10-4 mol/l).15 The presence of small amount of soluble COD28 and inorganic salts29 has only limited influence on the surface tension of water.

On the other hand, surface tensions of sludge flocs were significantly different with respect to SRTs. A lower surface tension was associated with sludge flocs at the higher SRTs (16 and 20 days), as compared to that at the lower SRTs (4 and 9 days) (Analysis of Variances [ANOVA], p < 0.05). It appeared that a transit range of SRT (about 12 days) existed for a significant change in surface tension of sludge surfaces. This was the same for the interfacial tension between sludge surfaces and treated effluent, as shown in Table 19.1. These results suggest that the surface tension of sludge flocs and the interfacial tension between sludge surfaces and effluent can be biologically manipulated through the control of physiological status at a microbial community level.

A plot of ESS against the AGflocculation is shown in Figure 19.1. There is a strong positive correlation between the AGflocculation values and the effluent suspended solids (Spearman's coefficient, rs = 0.85, p < 0.01). A higher ESS is associated with a higher level of AGflocculation, which is close to 0, particularly for sludge at lower SRTs. This is consistent with the prediction of the surface thermodynamic approach in that dispersed cells and fine flocs will aggregate to form settleable large flocs driven by the decrease in interfacial free energy (more negative). The large variation of the ESS for a given value of AGflocculation, particularly for the higher level of AGflocculation, indicates that electrostatic interactions might also be involved in governing bioflocculation except for hydrophobic interactions.

The results from this study provide firsthand information on the surface tension of sludge flocs and its relationship to bioflocculation in a well-controlled laboratory activated sludge system. A higher SRT produces a sludge surface with a lower surface tension. A strong positive correlation was found between the interfacial free energy of bioflocculation and the level of ESS, indicating the importance of surface

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