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Reverse Osmosis

Reverse osmosis (RO) is the most economical method of removing 95% to 99% of all contaminants. The pore structure of RO membranes is much tighter than that of UF membranes. RO membranes are capable of rejecting practically all particles, bacteria and organics > 200 Dalton molecular weight (including pyrogens) at a rate close to 99%. Natural osmosis occurs when solutions with two different concentrations are separated by a semi-permeable membrane. Osmotic pressure drives water through the membrane; the water dilutes the more concentrated solution; and the end result is an equilibrium.

Benefits & Limitations
  • Effectively removes all types of contaminants to some extent (particles, pyrogens, microorganisms, colloids and dissolved inorganics), and is therefore useful as a first purification step.
  • Requires minimal maintenance.
  • Operation parameters (pressure, temperature, flow rate, ionic rejection) are easy to monitor.
  • LImited flow rates per surface unit require either large membrane surfaces or an intermediate storage device to satisfy user demand.
  • Requires good pretreatment to avoid rapid membrane damage by water contaminants: scaling (CaCO3 deposits on the surface), fouling (deposits of organics or colloids on the surface) or piercing (RO membrane cut by hard particulates).

Reverse Osmosis
In water purification systems, hydraulic pressure is applied to the concentrated solution to counteract the osmotic pressure. Pure water is driven from the concentrated solution at a flow rate proportional to applied pressure and collected downstream of the membrane.

Because RO membranes are very restrictive, they yield slow flow rates per surface unit. Storage tanks are required to produce an adequate volume in a reasonable amount of time.

RO also involves an ionic exclusion process. Only solvent (i.e., water molecules) is allowed to pass through the semi-permeable RO membrane, while virtually all ions and dissolved molecules are retained (including salts and organic molecules such as sugars). The semi-permeable membrane rejects salts (ions) by a charge phenomenon action: the greater the charge, the greater the rejection. Therefore, the membrane rejects nearly all (> 99%) strongly ionized polyvalent ions but only 95% of the weakly ionized monovalent ions like sodium. Salt rejection increases significantly with applied pressure up to 5 bar.

Different feed water may require different types of RO membranes. Membranes are manufactured from cellulose acetate or thin-film composites of polyamide on a polysulfone substrate.

If the system is properly designed for the feed water conditions and the intended use of the product water, RO is the most economical and efficient method for purifying tap water. RO is also the optimum pretreatment for reagent-grade water polishing systems.


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