The optimum DAF pH will be dependent on several things. Most important is the application which you have not stated. Some applications such as oil & food application need a lower pH. Another major factor is that some coagulants will only work optimally within a certain pH range.

The process of air flotation itself is not impacted by pH. However dissolved air flotation processes normally incorporate the use of a coagulant to improve performance. As bimr suggests the coagulent and the coagulation process will have optimum pH ranges for operation. So depending on the coagulant in use if any a pH of 10.5 may or may not be okay.

The high TDS itself should not affect the DAF process substantially although the viscosity and density of the water is different to what freshh water might be be. This might have a very minor impact on bubble formation and particle rise rates, but probably not enough to be a problem.

Without knowing the chemistry of the water being treated you should also check the compatibility and performance of the coagulant in the high TDS conditions.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"