The performance of reverse osmosis system is, to a large degree, controlled by the composition of the feed water. Feed water analysis is important to RO system design, for example, one may avoid calcium carbonate precipitation by adding acid and/or antiscalant or prevent silica scaling by decreasing the water recovery rate. 

The constituents of feed water:
Cation: Calcium (Ca), Magnesium (Mg), Potassium (K), Sodium (Na), Stronium (Sr), Barium (Ba), Aluminum (Al), Manganese (Mn), Iron (Fe), and Ammonium (NH4). 
Anion: Bicarbonate (HCO3), Carbonate (CO3), Chloride (Cl), Sulfate (SO4), Fluoride (F), Phosphate (PO4), Nitrate (NO3), and Silica (SiO2).
Others: pH, Conductivity, Total Dissolved Solids (TDS), Silt Density Index (SDI), Total Organic Carbon (TOC), Turbidity and Temperature.