Drinking-water quality is an issue of concern for human health in developing and developed countries world-wide. WHO produces international norms on water quality and human health in the form of guidelines that are used as the basis for regulation and standard setting, in developing and developed countries world-wide.
The reverse osmosis process includes pretreatment, trans-membrane transport whereby water is transported across the membrane under pressure and salts are excluded, and often post treatment prior to distribution. Suspended solids are removed by filtration, pH adjustments protect the membrane and control the precipitation of salts.
Partial desalination or breaches in membranes may have potential health implications due to trace elements and organic compounds, including oil and refined petroleum products, that might be introduced because of pollution of seawater. In addition, seawater sources may contain hazards not encountered in freshwater systems. These include diverse harmful algal events of micro and macro algae and of cyanobacterial, certain free living bacteria (including Vibriospecies such as V. parahaemolyticus and V. choerae) and some chemicals such as boron and bromide which are more abundant in seawater.
Reverse osmosis systems reverse the natural process that is driven by osmotic pressure of solvent transport across a semi-permeable membrane from a region of lower solute concentration into one of higher solute concentration so as to equalize the free energies. In RO external pressure is applied to the high solute (concentrated) water to cause solvent (water) to migrate through the membrane pores leaving the solute (salts and other non permeates) in the more concentrated brine.
The membrane provides a form of "hyperfiltration" by restricting passage of many substances. Some membranes will reject 99% of all ionic solids and commonly have molecular weight cut off as low as 50 to 100 Daltons. The mechanisms of salts removal by RO membranes are not fully understood, and some salts (e.g. borate, arsenite) are not removed with high efficiency. Some believe the pure water preferentially passes through the membrane, while others believe that surface charges on the membrane polymer affect the polarity of the water. Increased pressure increases the rate of permeation, however fouling would also increase.
Common membranes are polymeric materials such as cellulose triacetate or polyamides and polysulfones. Selection factors for membranes include pH stability, working life, mechanical strength, pressurization capacity and selectivity for solutes. Membranes are located in a module and they can be configured as hollow fiber, spiral, plate, and tubular. Each has its own characteristics that affect selection in particular cases. Spiral wound configurations generally have more favorable operating characteristics of performance relative to cost and they are most commonly used. Operating pressures are in the range of 250 - 1000 psi (17 to 68 atm).
Membranes are typically layered or thin film composites. The surface contact layer (rejection layer) is adhered to a porous support, which can be produced from the same material as the surface. Thin film membranes can be made by polymerization of the rejection layer to the surface of the porous support. Membrane thicknesses are on the order of 0.05 mm.
A filtration process that removes dissolved salts and metallic ions from water by forcing it through a semipermeable membrane. This process is also highly effective in removing microbes from water. Mechanical pressure is applied to an impure solution to force pure water through a semi-permeable membrane.
What impurities will reverse osmosis remove?
Reverse osmosis (RO) has become a common method for the treatment of household drinking water supplies. Effectiveness of RO units depends on initial levels of contamination and water pressure. RO treatment may be used to reduce the levels of:
1. Naturally occurring substances that cause water supplies to be unhealthy or unappealing (foul tastes, smells or colors).
2. Substances that have contaminated the water supply resulting in possible adverse health effects or decreased desirability.
RO systems are typically used to reduce the levels of total dissolved solids and suspended matter. The principal uses of reverse osmosis are for the reduction of high levels of nitrate, sulfate, sodium and total dissolved solids.
RO units with carbon filters may also reduce the level of some SOCs (soluble organic compounds) like pesticides, dioxins and VOCs (volatile organic compounds like chloroform and petrochemicals). An RO unit alone may not be the best solution for these types of contaminants, but installing a properly design-ed RO unit to reduce the levels of other contaminants may provide a reduction in SOCs and VOCs.
How reverse osmosis works
Reverse osmosis is sometimes referred to as ultrafiltration because it involves the movement of water through a membrane as shown in Figure 1. The membrane has microscopic openings that allow water molecules, but not larger compounds, to pass through. Some RO membranes also have an electrical charge that helps in rejecting some chemicals at the membrane surface. Proper maintenance is essential to retain effectiveness over time. Some units are equipped with automatic membrane flushing systems to clean the membrane.
What types of equipment make up an RO System?
A typical home reverse osmosis treatment system is shown in Figure 2. The system is normally located beneath the kitchen sink since it is used to treat water for drinking and cooking purposes. RO systems consist of the pre-filter, RO membrane unit, a pressurized storage tank for the treated water, a post-filter and a separate delivery tap for the treated water supply.