Distillation uses a heat source to heat the water and vaporize it. Water has a lower boiling point than salts and other contaminants present in the water. Hence, the vapour contains only water, which is condensed using a condenser, and converted back to its liquid form. The salts and contaminants, whose boiling points are too high to evaporate, are left behind as sediments. Often, the condensate is re-distilled to ensure that all sediments are left behind and the water is free of all contaminants.
Advantages: Distillation is a well-known process that has been practised for ages that generates pure water. In fact, many alcoholic beverages (brandy, whiskey) are distilled with apparatus that is very similar to the water distillation setup.
Disadvantages: Distillation involves a heat source, and requires time to first vaporize the water then condense it (and then re-distil it if required). Thus, high maintenance expenses and time requirements are two major limitations of distillation systems. Moreover, distillation is virtually ineffective in removing volatile chemicals that have lower boiling points than water.
Reverse Osmosis (RO) uses a semi-permeable membrane to allow only water to pass through the membrane, while blocking all other contaminants that are larger than the water molecule. RO requires a driving force for the water to pass through the membrane. This can be achieved by the pressure of the incoming water through the pipes, or by using a simple pump. Thus, the energy requirement of an RO system is miniscule. RO is capable of removing a wide variety of contaminants from water – bacteria, viruses, chemicals, pharmaceutical contaminants, arsenic, cyanide, and even radioactive substances. Some people have been concerned about the loss of essential minerals following RO treatment. Recent studies, however, have suggested that the requirement of essential minerals by the human body is fulfilled primarily by food sources, and there isn’t any need to rely on drinking water for these essential minerals.
Advantages: RO can handle all contaminants that are larger than the water molecule. It can remove volatile compounds unlike distillation. RO consumes very little energy, and has high productivity (takes much less time to generate purified water).
Disadvantages: RO creates a large proportion of reject water – the efficiency of a typical RO system is 25%. However, the reject water can be used for non-potable purposes such as washing, flushing toilets etc. to avoid the reject water from simply being wasted.
Thus, both distillation and RO produce pure water. The only difference are the maintenance expenses and the time required to produce pure water. These are definitely the main reasons why RO turns out to be the more preferred technology for water purification, especially in domestic households.