First of all, water quality varies greatly from well water to surface water to municipal water. Well water may have things like iron, sulfur, manganese +and tannin which almost always have to be removed, especially in the case of a whole-house RO system. Those contaminants must always be removed before the reverse osmosis process. Let’s not forget that reverse osmosis removes the largest spectrum of contaminants at the most economical cost of any water treatment process. Essentially, a whole house reverse osmosis system will remove 98 to 99% of most contaminants including Total Dissolved Solids (TDS), sodium, chloride, sulfate, nitrate, arsenic and a plethora of other chemical and organic contaminants.
One of the key ingredients to a whole house reverse osmosis system is proper per-treatment, which includes removing the iron, sulfur, manganese, tannin and other nuisance elements. So, ahead of an RO system, it is essential that filtration or oxidation of these contaminants are accomplished. If the water is hard, then it needs to be softened or (what I prefer) use an anti-scalant to prevent hard water build-up on the membranes. Anti-scalant systems are gaining popularity because no water is wasted and no salt is needed.