Is it possible to treat waste water for the drinking purpose ??

yes but difficult to convince the customers, and u need stricy disinfection procedure 

It is possible to treat waste water for the drinking purpose is using RO system after the STP while treating water

OUI BIEN SURE MAIS APRES TROIS TYPES DE TRAITEMENT ÉPURATION. ULTRA _FILTRATION ET NANO _FILTRATION  et enfin injection dans les nappes a partir des bassins d'infiltration ce qu'on le nommer : alimentation artificielle des nappes

In my opinion, water treatment like any other processes have input, process and output.  So firstly, it depends on the input quality, how "bad" is the waste water?  Domestic waste water would generally be easier than Industrial waste water, and even the industrial type can have a very broad spectrum.

Process-wise, existing treatment technologies have their capabilities & limitations.  Also, how clean you would want the water to be also depend a lot on how much you are willing to spend. 

The most crucial is output quality.  What parameters do we need to consider?  I believe the WHO standard should only be for treating raw water from natural sources.  More parameters should be controlled when treating polluted sources.  Even domestic wastewater may carry harmful viruses that can cause an epidemic.  So the control parameters must be set, and must be able to accurately read/analyze them in order to determine whether the processes (such as disinfection) were sufficient.  The list of parameters need not be endless though, but careful study is needed here.

By the way... what do astronauts drink?  But as mentioned, serving it to the masses would be a completely different matter... :)

yes it is posiible in water treatment plant