This water purification process has gained in popularity over the last 20 years since it was first used commercially in 1968. It is used extensively for purifying sea water and uses membrane (a thin film or skin through which water molecules can pass) technology.
The easiest way to explain reverse osmosis is by firstly explaining osmosis.
Osmosis is the movement of a low concentration solution through a semi permeable membrane into a high concentration solution such as sea water or contaminated water.
In reverse osmosis, the idea is to use the membrane to act like an extremely fine filter to create drinking water from contaminated water. Pressure is applied to the contaminated water, reversing the osmotic process and forcing water molecules through the membrane.
Reverse osmosis membranes don’t allow particles or molecules larger than 0.0005 microns to pass through to the other side of the membrane. Essentially only water (H20) passes through while other contaminants, bacteria, viruses, chemicals and other dissolved substances are flushed to drain.
As a size comparison against 0.0005 microns; a human hair is 100 microns, the smallest particle visible to the human eye is 50 microns, the smallest bacteria is 0.2 microns and the smallest virus is 0.002 microns.
Reverse Osmosis membranes are manufactured with different pore sizes and support structures and from different materials. Membrane pore sizes can vary from 0.0001 microns to 5 microns depending upon the type and purpose.
Sea water or Desal membranes are constructed to operate at very high pressures up to 7,000 kpa with small pores that reject in excess of 99% of salt at loading greater than 32,000 mg/l.
Brackish/Salt contaminated water membranes operate at high pressures around 1,500 kpa and have larger pores to reject in excess of 99% of salt at loadings greater than 2,000 mg/l.
Lower pressure/low energy membranes are designed to operate between 400 and 1500 kpa with more open larger pores designed to reject 99% of salt at loading around 500 mg/l for ground water and municipal supplies.
There are two main types of reverse osmosis membranes commonly used in home reverse osmosis filter systems:
- Thin Film Composite (TFC)
- Cellulose Triacetate (CTA)
TFC membranes have considerably higher rejection rates; and filter out more contaminants than CTA membranes. However they are more susceptible to degradation by chlorine and other oxidants and need to be protected from them by pre-filters.
These membranes are made by forming a thin, dense contaminant rejecting surface film on top of a porous substructure. The materials of construction and the manufacturing process for these two layers can be different and altered for the desired combination of pure water produced versus contaminants rejected. The pure water production and contaminant rejection characteristics are predominantly determined by the thin surface layer which thickness ranges from 0.01 to 0.1 micrometres.
Several types of TFC membranes have been developed including aromatic polyamide, alkyl-aryl poly urea/polyimide and polyfurane cyanurate. Polyimide membranes are highly susceptible to degradation by oxidants such as chlorine and chloramine. These must be removed to prevent damage and destruction of the membrane.
These membranes were developed along with the first reverse osmosis systems in the late 1950’s. They are composed of a thin dense surface layer (0.2 to 0.5 micrometres) and a thick porous sub-structure. Contaminant rejection is undertaken by the thin dense layer with the sub-structure providing structural support. They are relatively inexpensive to manufacture and hence are cheaper to buy than TFC membranes.
CTA membranes also have a low rejection of organic contaminants, low pH tolerance but a high tolerance to oxidants such as chlorine.
The chemical 1,4 dioxane is used to create the membrane porosity features. This chemical causes cancer with some traces of it left after manufacturer requiring considerable flushing before use.
The removal of inorganic contaminants by reverse osmosis membranes is complex and is dependent upon the interactions and mixture of irons in the feed water. Ionic contaminants are more readily rejected than neutral ones and polyvalent ions are rejected to a greater extent the monovalent ions. If the polyvalent ion is strongly hydrated, rejection is even higher.
As electrical neutrality must be preserved, ions diffuse across the membrane as a cation-anion pair. As a consequence, rejection of a particular ion depends on the rejection of its counter ion. An example of this is with sodium; sodium sulfate has a higher rejection than sodium chloride because the divalent sulfate ion is rejected to a greater extent than the monovalent chloride ion.
pH variations also affect the rejection characteristics of the membrane depending upon membrane composition and ion type. For example, fluoride rejection increases from 45% to over 90% as pH increases from 5.2 and 7.2 whereas nitrate rejection decreases slightly as pH increases from 5.2 to 7.0.
A large number of councils are now using chloramine to treat drinking water supplies instead of chlorine. This chloramine contains ammonium ions which are poorly removed by activated carbon causing dramatically reduced rejection rates and degradation of the membrane. The variability of local water conditions and sources also varies the performance of the membrane although a Total Dissolved Solids (TDS) monitor will show the current performance of the unit. Specific ion rejection performance can however only be determined by selected testing. As a general guide reverse osmosis membranes are more effective in rejecting ions or organic solubles with molecular weights greater than 200 however carbon filters before and after the membrane can greatly affect the contaminants rejected or absorbed thereby affecting the overall performance of the system. The larger the pre-carbon filter ie 12” in the Aqua Safe ASRO4 unit the greater the chloramine and contaminant removal.
While membranes are successful at removing bacteria and viruses the systems can be contaminated from the product water side colonizing the tape tubes and storage tank. Regular disinfection (every 6-12 months) is necessary to maintain the water quality. This can generally be done by the system owners at very low expense or carried out by service technicians.