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I see lot of drinkable water wasted in household and industrial activities, water usually gets contaminated with detergents(and other household materials),chemical effluents released from factories . Why we don't just boil off the contaminated water to get the steam and again condense it to get pure water safe for drinking?

I get this idea from the fact that when we boil out salted water we get all the salt as precipitate thus distilling the water.I don't know much about distillation but I think this process can be economically feasable when done on a large scale.

By this method of treatment,I am considering countries that cannot afford water treatment plants on a large scale and with problem of water scarcity,as this would be quick way to get drinkable water.

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    $\begingroup$ All that wasted water does go to a wastewater treatment plant. Evaporating water on a large scale requires a large amount if energy and some mechanism to scrape off the precipitates $\endgroup$
    – Avyansh Katiyar
    Dec 14, 2018 at 11:59
  • $\begingroup$ @AvnishKabaj I mean to say that by evaporating water we can again use it for drinking thereby saving water.It might be a efficient method for saving water when you live in countries with water scarcity(african countries,latin american countries and espescially India) $\endgroup$
    – pranjal verma
    Dec 14, 2018 at 12:07
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    $\begingroup$ You are under-estimating the energy required to boil the water. The specific heat capacity of water is 4.2 J/g per degree C. Try a little calculation to work out how much energy is required to evaporate 1kg of water in the way you suggest. $\endgroup$
    – Waylander
    Dec 14, 2018 at 12:49

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Boiling a water mixture doesn't always produce pure water.

Boiling a solution of salt and water (or evaporating the water) will separate the salt from the water, but those are compounds with very different properties.

In the making of distilled alcohols (Ethanol), the vapor phase has more alcohol than water. Repeated distillations can increase the concentration of the alcohol. But, the leftover undistilled solution is not pure water. Ethanol and water are very "sticky" to each other.

Ethanol is just an example. Household 'grey' water (waste water that doesn't include human waste) will have a whole mess of compounds. Some you can isolate through boiling, some, like ethanol, will 'come along for the ride'. Wastewater treatment is complicated. Look at the system that had to be engineered for the International Space Station. Actually, a version of this system has been tested for use in the types of communities you are describing.

Purifying grey water isn't a simple process. There are, however, plenty of things that can be done with grey water. Using grey water to flush toilets is perfectly fine. With some simple amount of treatment, grey water can be perfectly suitable for watering crops.

So, since it's cheaper and easier to repurpose grey water, and harder to purify it back to drinking-water, reuse is more efficient than trying to completely treat it at home.

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We do, and so does the planet

The thing about water is that it is a renewable resource. The weather systems of the Earth already recycle water by evaporating it and them returning it to us as rain. So many countries don't need to take extra measures to have enough potable water as their lakes and rivers already provide enough.

Those countries that don't have enough fresh water already use various means to purify non-drinkable sources (though the most abundant source isn't contaminated water but sea water which is far more abundant than the flows of used water). Also, distillation is nothing like the most efficient way to produce pure water as it is very, very energy intensive. The current favourite tech (widely used in countries with inadequate rainfall but access to the sea) is reverse osmosis.

So distillation is perfectly viable for creating drinkable water, but would be a very poor choice for poor countries as it is very much more expensive than the alternatives.

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