Reverse Osmosis Technology

Reverse Osmosis is a sophisticated water purification process that was first successfully demonstrated on the UCLA campus by Professor Sydney Loeb in 1960. To understand reverse osmosis it is logical to first describe “osmosis”. Osmosis is a vital function performed by living cells and therefore this is the context in which it is explained.  Through osmosis cells are nourished by allowing passage of small molecules like oxygen, carbon dioxide, glucose and amino acids while larger molecules like starch and protein are not allowed. 

Osmosis is the passage of water from a dilute solution through a semi-permeable membrane, such as a living cell, to a more concentrated solution.  Osmosis occurs as these different concentrations attempt to equalize. Osmosis will stop only when osmonic pressure is equal on each side of the membrane. Osmonic pressure is defined as the pressure differential required to stop osmosis. For example, the osmonic pressure of typical tap water is 10 psi. If tap water and pure water were separated by a semi-permeable membrane the tap water would have to be pressurized to 10 psi to eliminate osmosis from occurring. The osmonic pressure of sea water is 376 psi. This water would require being pressurized to 376 psi to eliminate osmosis from occurring.

Simply stated osmosis is the flow of fluid from an area of dilute concentration through a semi-permeable membrane to an area of heavier concentration. To “reverse” this process requires that water with high Total Dissolved Solids content flow through a semi-permeable membrane to an area of low Total Dissolved Solids (TDS). Water with high TDS has significant amounts of dissolved mineral such as sodium, sulfate, chloride, fluoride and even pharmaceutical residual. Water with low TDS is more pure. In order to actually “reverse” the process of osmosis we must not only overcome but significantly exceed osmonic pressure. To desalinate sea water using reverse osmosis the osmonic pressure of 376 psi must be significantly exceeded. To make substantial amounts of permeate water from sea water the incoming water must be pressurized to 800 psi. Today this is commonly done in arid parts of the world.

Semi-permeable membranes employed presently have made great gains in efficiency since the inception of reverse osmosis. Professor Loeb used a membrane made of cellulose acetate. Today thin film composite membranes made of an ultra-thin active layer of polyamide polymer coated on a much thicker polysulfone polymer support layer are used. These membranes are considered to be high flux. Flux is defined as the amount of water in gallons that can be passed through one square foot of membrane and is represented by gallons per square foot per day. This results in units capable of delivering increased permeate to concentrate ratios. The permeate to concentrate ratio is an expression of the volume of purified water (permeate) which can be produced per gallon of waste concentrate. The waste concentrate is the stream of water that is directed across the face of the membrane which continually cleans the membrane of accumulated contaminants and prevents membrane flux erosion.

Most reverse osmosis systems are designed to provide adequate supply of permeate water. During design it is important to not only consider pressure required to overcome osmonic pressure but also feed water temperature must be factored. All membranes base their performance data using 77 F water temperature. Warmer water will increase the permeate while decreasing the TDS rejection. Colder water will decrease the permeate while increasing the TDS rejection. Feed water at 50 F will only produce 63% of the amount of permeate the same membrane will produce with 77 F feed water.

Reverse osmosis systems come in all different sizes and configurations. A typical “point of use” residential system can be installed under the kitchen sink and requires no electricity. There are several municipal systems in the U.S. and abroad that desalinate sea water delivering millions of gallons of potable water daily. These systems require massive equipment, teams of technicians and substantial energy demands. Regardless of the size, pertinent information such as feed water temperature, pressure, TDS and pre-treatment filtration need to be weighed carefully to select an appropriate well designed system.

John Crean
www.kleenwater.com
Pavel Water Filtration
WQA Certified Water Specialist