Hi Hung,  in response to ...

Hi Hung,  in response to your first question, you do need to calculate the volume of your peak flow, not just the peak flow rate itself.  this means the integration of the above average flow vs time as this is then the volume of your equalization basin (plus allow additional for non average days as well as possible maintenance shut downs on the treatment plant).

Regarding the contaminants, others have responded better than I can.