Desalination seawater & RO for sewage treatment?

Question

Which is cheaper in term of energy cost? Seawater desalination or waste water treatment by RO?

And which one is more environmentally friendly?

Note:

With a reference plz

Answer 1

Not sure what you mean by wastewater treatment by RO.  I believe you're referring to basically just cleaning up and reusing water that's been used in households and possibly sewer water.  If so, that's cheaper than desalination because those systems are already in place at all the water utilities throughout the country.  

Desalination is becoming cheaper and cheaper though as more and more areas are experiencing drought.

Answer 2

RO