Drinking water Via Reverse Osmosis (R.O)
   
Develop nearly 40 years ago for industrial uses and for big seawater desalting plants, the reverse osmosis (R.O) process represents another large scale treatment that has been scaled down to modify and improve small volumes of water for home drinking and cooking purposes. Normally, in industrial sized applications, very high pressure (200 to over 800 psi) is employed to accomplish the separation of dissolved solids from the product water. Advances in RO membrane engineering have overcome the need for extra high pressure when only two to ten gallons per day are needed in the average home setting for drinking/cooking requirements.

The phenomena of osmosis was originally discovered by a French scientist in 1748 who observed that water would diffuse spontaneously through a pig bladder membrane into a parallel chamber of alcohol. This interesting concept, osmosis and its counterpart, reverse osmosis, for the next 200 years was not much more than a laboratory topic because natural membranes were scarce and unreliable. In the mid-1950s, the work of Dr. S. Sourirajan at UCLA and others advanced the RO technology to the point where artificial membranes could be manufactured. During this era, considerable work was done for the U.S. Office of Saline Water into methods of desalination with serious research emphasis on reverse osmosis.
   
The movement of water from soils into plant rots in an example of osmosis at work in nature. When a semi-permeable membrane, like a living cell wall, separates two solutions having different solids concentrations, the pure water will flow from the least concentrated solution through the membrane and into the solution containing the higher solids concentration. The flow will stop when the osmotic pressure on both sides of the membrane equalize. This process occurs continuously in the organs of living things as an example of how water is transported in nature.
Osmosis
 
Reverse Osmosis

Reverse osmosis is just what its little implies; it is the reverse of naturally occurring osmosis. By applying artificial pressure to the water solution of higher solids concentration, the flow of liquid is reversed.

The membrane will, under these conditions, only allow the water molecule to pass through retaining the highly concentrated salts solids on the opposite side of the semi-permeable membrane.
 
During mid 1970s, RO became a practical drinking water process as new "low pressure" systems were designed to function on line pressure of 35 psi or higher. For the most plant, drinking water RO systems are installed on a separate single tap. The apparatus is usually located under the sink in homes and clinical offices. The standard components, as illustrated, consist of a prefilter, RO module membrane cylinder, drain for (brine) reject water, a small storage tank and a post carbon filter.
 
 
Typical RO system with Pressurized storage tank
 
Because only small daily quantities are needed for this purpose, the RO devices produce an average of two to 12 U.S. gallons of low-mineral (low TDS) water in a 24 hour time span. To increase daily output of these RO systems, an increase in line pressure will directly increase the amount of product water delivered. For this reason, most RO suppliers now offer systems with a small booster pumps. When pressure is increased 50%, an average drinking water RO (with thin film composite membrane) will produce 60% more product water. At the same time, this membrane will maintain the low TDS quality product water at the higher pressure level.

 
 
Crossflow filtration (TFC membrane)
 
 

Operating home RO systems at their best level calls for careful pretreatment and the need to have a safe, potable water supply. Much of the difficulty in getting good performance from an RO is attributed to inadequate pretreatment. Often, these small devices are expected to do more than designed to handle. In other words, the water supply may need disinfection plus filtering before being feed to an RO system. High hardness (over three grains/gallon) waters should always be presoftened before entering an RO processor as calcuim deposits will make the membrane less permeable and shorten membrane life.

The more sensitive nature of the RO process calls for an after-sale testing servicing program to assure homeowners of efficient performance. Commonly included in service contracts are change of the prefilter element as preventative maintenance; replacement of membrane based on rejection ability; annual change of post-carbon filter; and sanitizing the entire system (storage tank too) yearly.