Dear Inamul, First, what is ...

Dear Inamul, First, what is your source of raw water ? Is it seawter or Brackish water ? Furthermore, in most cases water coming from RO should not be used as drinking water directly as concentration of some minerals are too low, especially magnesium and calcium (there are recommendations of the WHO). In most cases, you should proceed for a remineralisation, which will also correct your pH. There are different solutions, most often using calcium based rock (dolomite or calcite). I would not recommend using caustic soda. Best regards.