n one word: analysis. Every source of water is different, and you never know what's in your water until you have it analyzed. The water analysis, LSI, SDI, or CFI values are used to determine the precise pretreatment requirements for a particular  RO System. Since water supplies vary considerably from one location to another, each pretreatment requirement will be different. On average, most Reverse Osmosis Systems need an antiscalant injector or water softener to prevent damage to the membrane.