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What is reverse osmosis?

oliverar

Member
Joined
5 Jun 2010
Messages
190
Location
Canterbury, UK
I think it is designed to make pure water? But why would you want it, is it so you have water to use for your aquarium with out having to treat it?
 
Hi all,
What we describe as "water" is almost always a dilute solution of several salts rather than being pure "H2O". In the case of a lot of the water supply in the south of the UK the main salt present is calcium carbonate, dissolved from the limestone aquifers where much of our drinking water is sourced. Water originating in reservoirs and rivers will often have other salts present, N. P. K. from sewage works and agricultural run-off, NaCl from road salting, herbicide traces etc. Gasses may also be dissolved (O2 and CO2), as well as the substances added by the water companies (chlorine, chloramine, sodium bi-carbonate, phosphate compounds for the control of "plumbosolvency") and possibly metals (Pb, Zn, Cu) and/or organic carbon compounds (foe example plasticisers) from domestic and municipal pipework.

All of R.O (Reverse Osmosis), distillation and de-ionisation work by removing these substances from the water supply. Basically they leave a "clean canvas", where you know exactly what you have added to the water, and therefore what it contains. If you keep black-water fish from extremely soft water, you may use your R.O. water with a minimum of treatment, otherwise you need to re-mineralise it with salts to give it some GH and KH buffering. Pure water is in many ways a strange substance, it has no buffering, so pH is a meaningless measurement, and it is an electrical insulator.

Reverse osmosis (RO) is a filtration method that removes molecules and ions from solutions by applying pressure to the tap water on one side of a selective membrane. The result is that the salts ("solute") are retained on the pressurized side of the membrane, and the H2O ("solvent") is allowed to pass to the other side. The selective or semi-permeable membrane works because the large molecules or ions don't fit through the pores (holes), but the pores allow the small water molecules (the solvent) to pass.

It is "Reverse osmosis", because the pressure forces the solute through the membrane, if the pressure was equal on either side of the membrane, and one side contained pure H2O and the other a solution, the water molecules would actually pass from the H2O into the solution along a concentration gradient, this is the process of "osmosis".

cheers Darrel
 
Nah, they just asked for reverse osmosis, not reverse reverse osmosis ;)
 
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