What is the purpose of using antiscalants prior to RO membrane filtration?

The purpose of using antiscalants prior to RO membrane filtration is to prevent or minimize the formation of scale on the RO membrane surface. Scale is the precipitation and deposition of sparingly soluble salts, such as calcium carbonate (CaCO3), calcium sulfate (CaSO4), and silica (SiO2), onto the membrane surface. Scale formation reduces permeate flux, increases pressure drop, and shortens membrane life. Antiscalants are chemicals that inhibit scale formation by interfering with the nucleation and growth of scale crystals. They work through several mechanisms, including threshold inhibition, crystal distortion, and dispersion. Threshold inhibition means that the antiscalant prevents scale formation even at concentrations well below the stoichiometric amount required to react with the scale-forming ions. Crystal distortion means that the antiscalant alters the crystal structure of the scale, making it less adherent to the membrane surface. Dispersion means that the antiscalant prevents the aggregation of small scale crystals into larger particles. By preventing or minimizing scale formation, antiscalants help to maintain stable RO system performance, reduce the frequency of membrane cleaning, and extend membrane life. The type and dosage of antiscalant used depend on the water chemistry and the specific types of scale that are likely to form.