Water treatment?

Question

What is thee different between Multiple flash distillation of sea-water treatment for drinkable usage?

And RO in sewage treatment for advance treatment? In term of cost and energy? Advantages and disadvantages.

Note:

Because I am doing a project about sewage treatment…in my country they don’t treat sewage by RO. I wrote about the possibility of applying RO in my country, but I've been asked by the professor to look at it in term of the cost, environment/ green living, and energy usage

Answer 1

Used to serve in the Navy. We had distillation plants that produced feed water for boilers and potable water. The production was supervised by one guy round the clock and it was hot work. Water use was carefully controlled. Now the ships use reverse osmosis and there are no more water restrictions.

Distillation of sea water is very energy intensive. Disadvantages are many including significant increase in the temperature and salinity of water near the distillation plant causing changes in the local environment. Apparently the water from shore installations tastes pretty bad too. Includes anything that evaporates at temperatures and pressures used. In the ship we couldn't distill water within 12 miles of the shore for fear of contamination. Drove through an oil slick once. Disaster.

Distillation requires evaporation of water. Salts and most other minerals do not evaporate. The process is made more efficient by operating at a partial vacuum thus allowing evaporation at lower temperatures.

I don't know much about Reverse Osmosis except that it is certainly more energy efficient than distillation. I'd be looking at how the quality of water at the input affects quality at the output. It is possible to buy small RO devices for recreational sailboats so that says a lot about energy efficiency.

Answer 2

Wignet Maker is partly correct.  I was a marine pipefitter that could trouble shoot, clean, repair  the naval evaporators that the sailors can't repair or fix for over 30 years.

Basically the salt water is boiled and steam cooled to drink.  If your country has a lot of sun, my suggestion would be to use a solar distilling unit.