Reverse osmosis per its construction removes both harmful contaminants present in the water, as well as some desirable minerals. Modern studies on this matter have been quite shallow, citing lack of funding and interest in such study, as re-mineralization on the treatment plants today is done to prevent pipeline corrosion without going into human health aspect. They do, however link to older, more thorough studies that at one hand show some relation between long-term health effects and consumption of water low on calcium and magnesium, on the other confess that none of these older studies comply to modern standards of research 
A reverse osmosis filter is the do-it-all of water purification. The process is the only one that addresses both harmful microorganisms and pollutants at the same time. It works by forcing water under pressure through a membrane made of thin film composite, with a inner matrix of dense polymers. The result leaves purified water on one side of the membrane, and contaminants on the other side. The technology is reliable, but expensive and relatively cumbersome, and requires electricity to work. It is therefore a sound choice for use in fixed positions or by those who can afford to tow a small trailer with a small electrical generator around, but anyone on the move or without access to electricity needs to use other methods.
Pretreatment is important when working with reverse osmosis and nanofiltration membranes due to the nature of their spiral-wound design. The material is engineered in such a fashion as to allow only one-way flow through the system. As such, the spiral-wound design does not allow for backpulsing with water or air agitation to scour its surface and remove solids. Since accumulated material cannot be removed from the membrane surface systems, they are highly susceptible to fouling (loss of production capacity). Therefore, pretreatment is a necessity for any reverse osmosis or nanofiltration system. Pretreatment in sea water reverse osmosis systems has four major components:
Filter Speed While there are reverse osmosis filters that can filter water as you need it, most of them take some time to refill. If you are replacing your regular tap water with purified water, look for a unit that can filter 50 or more gallons a day. If you're just using it for drinking water, you can opt for a unit with a slower refill rate and a smaller tank.
The addition of inorganic coagulants such as aluminum sulfate (or alum) or iron (III) salts such as iron(III) chloride cause several simultaneous chemical and physical interactions on and among the particles. Within seconds, negative charges on the particles are neutralized by inorganic coagulants. Also within seconds, metal hydroxide precipitates of the iron and aluminium ions begin to form. These precipitates combine into larger particles under natural processes such as Brownian motion and through induced mixing which is sometimes referred to as flocculation. Amorphous metal hydroxides are known as "floc". Large, amorphous aluminum and iron (III) hydroxides adsorb and enmesh particles in suspension and facilitate the removal of particles by subsequent processes of sedimentation and filtration.:8.2–8.3