pH Control:

•  To adjust the pH in the treatment process to make wastewater pH neutral.
  For acidic wastes (low pH): NaOH, Na2CO3 , CaCO3or Ca(OH)2.
•  For alkali wastes (high pH): H2SO4 , HCl.
• Chemical coagulation and Flocculation:
•  Coagulation refers to collecting the minute solid particles dispersed in a liquid
  into a larger mass.
•  Chemical coagulants like Al2 (SO4)3 {also called alum} or Fe2 (SO4)3
  are added
  to wastewater to improve the attraction among fine particles so that they come
  together and form larger particles called flocs.
•  A chemical flocculent (usually a polyelectrolyte) enhances the flocculation
  process by bringing together particles to form larger flocs , which settle out
  more quickly.
•  Flocculation is aided by gentle mixing which causes the particles to collide.