A full and precise feed water analysis must be provided before a reverse osmosis system is designed. The water analysis report is meant to show the form and the concentration of all chemicals in the feed water. The chemicals consist of dissolved ions, silica, colloids, and organic (TOC).
- Typical dissolved anions
- Typical dissolved cations
o Iron(Fe2+ or Fe3+)
o Zinc (Zn2+)
Specific groupings of anions and cations create sparingly soluble salts in the water and scaling of a reverse osmosis membrane may occur when the salts are concentrated within the reverse osmosis element beyond their solubility limit. Typical sparingly soluble salts and their solubility product limit are shown in the list above.
In conventional reverse osmosis systems, the most common soluble salts encountered are CaSO4, CaCO3 and silica. Other salts creating a potential scaling problem are CaF2, BaSO4 and SrSO4, though they are much less prevalent. Other ions causing problems are described below.
Alkalinity consists of negative ions which include bicarbonate, carbonate and hydroxide. Most of the alkalinity in naturally occurring water sources is in the form of bicarbonate (HCO3-).
Nitrates are very soluble in water and thus will not precipitate in a reverse osmosis system. Nitrates are a health concern because, when ingested by humans, they are converted to nitrites which interfere with the hemoglobin. This can cause serious problems especially for a fetus and a child. For this reason, it is desirable to maintain a nitrate concentration below 40 mg/L in drinking water. Typical nitrate removal by a reverse osmosis system is in the range of 90 ~ 96%.
Iron and manganese are present in water, which forms insoluble hydroxides. The soluble iron (Fe2+) can come from either a well water or the rust of pump, piping and tanks, especially if acid is injected upstream of the equipment.
Aluminum is usually not noticeably present in naturally occurring water sources. Aluminum will form very insoluble hydroxide [Al(OH)3] at the normal operating pH range of 5.3 to 8.5 in a reverse osmosis system. A concentration of aluminum greater than 0.01mg/l in the dialysis water is a health concern for kidney dialysis patients. In this regard, iron [FeCl3 or Fe2(SO4)3] may be preferred as a coagulant.
Silt density index (SDI), also known as the Fouling Index (FI), is a good guide line to determine the colloidal fouling potential of the reverse osmosis feed water. The source of colloids in the feed water for a reverse osmosis system varies and generally includes clay, bacteria, iron corrosion and colloidal silica products. Pre-treatment chemicals used in a clarifier can also cause colloidal fouling if not removed in the clarifier or through proper media filtration.
Turbidity is another guideline as an indicator for the rate of reverse osmosis membrane fouling. Turbidometers (also called nephelometers) measure the scattering of light caused by various suspended solids in the water sample. Water samples having turbidity reading greater than 1 will tend to foul the membranes. These readings are typically given in nephelometric turbidity units (NTU). Like the SDI test, turbidity is only an indicator of fouling potential. High turbidity does not necessarily mean that the suspended solid is going to deposit on the reverse osmosis membrane.