Reverse Osmosis

What is it?

A separation process in which pressure is used to force a solvent though a membrane that retains the solute on one side and allows the pure solvent to pass to the other, is called reverse osmosis. This process is just the reverse of the normal osmosis process. The membrane that is used in reverse osmosis has a dense barrier layer in the polymer most of the separation occurs. In most of the cases, the membrane is so designed that only water passes through its dense layers while it prevents the passage of solutes, like salt ions.

Reverse osmosis is very effective in removing impurities from water. The impurities that are removed by reverse osmosis are total dissolved solids, asbestos, lead, turbidity and other toxic heavy metals like radium and other dissolved organics. Not only this, the process also removes chlorinated pesticides.

This process is the most efficient and economical method of purifying water if the system has been properly designed for the feed water conditions and the intended use of the product water. The optimum pretreatment for reagent-grade water polishing systems is also Reverse osmosis.

The Process

Reverse Osmosis drinking water systems follow this process:

• Pre-filtration
  
  This is the first step and the one that removes the largest particles and other impurities in preparation for the next process. The pre-filter is usually made up of carbon or activated carbon-based filter. This filter needs to be replaced at regular intervals in order to keep the reverse osmosis drinking water in good working water.

• Reverse Osmosis Membrane
  
  During this process, the membrane removes nearly all the impurities and contaminants with the help of a very fine membrane through which the pre-filtered water is passed. Being so fine, the membrane must be passed through under pressure to ensure that the purified water makes it to the other side. Impurities and contaminants are flushed away as wastewater.

• Holding Tank
  
  The holding tank is the place where the purified water is held until it is used for drinking. This is an important feature of the purifying system because this process is rather slow.

Uses of Reverse Osmosis

Drinking water purification

Household drinking water systems in the United States include a reverse osmosis step. These systems include four or five stages that are:

• A filter to trap particles such as rust and calcium carbonate
• A second optional filter that includes smaller pores
• A carbon filter that has been activated to trap organic chemicals, and chlorine
  A reverse osmosis filter
• A second carbon that is optional
• An ultra violet lamp that is used for disinfections of any microbes that may escape by filtering

Waste and Water Purification

Reverse osmosis water purifiers are used to purify rainwater collected from storm drains. This water is later used for landscape irrigation and industrial cooling.

In industries too, reverse osmosis is used to remove minerals from boiler water at power plants. The water in industries are boiled and condensed many times over to make it as pure as possible to ensure that it does not leave deposits on the machinery or cause corrosion.

Dialysis

Reverse osmosis is also the technique used in dialysis. People who have kidney failure use a dialysis machine, because it mimics the function of the kidneys.

Food Industry

This process is used extensively in the dairy industry for the production of whey protein powders and to concentrate milk in order to reduce shipping costs. Reverse osmosis is also used in the wine industry for many practices including juice and wine concentration and taint removal.

Car Washing

Since reverse osmosis is low in mineral content it is used in car washes during the final vehicle rinse in order to prevent water spotting on the vehicle. This process also enables the car wash operators to reduce the demands on the vehicle drying equipment.

Maple Syrup Production

A few maple syrup producers began using reverse osmosis to remove sap before being further boiled down to syrup. Using reverse osmosis enables approximately 75-80% of the water to be removed from the sap. This reduces energy consumption and exposure of the syrup to high temperature.

Hydrogen Production

In the small scale production of hydrogen, reverse osmosis is sometimes used to prevent the formation of minerals on the surface of the electrodes as also to remove chlorine and organics from drinking water.

Reef Aquarium Keeping

Since ordinary tap water often contains excessive chloramines, copper, nitrogen, phosphates, chlorines and other chemicals, reverse osmosis is used for their artificial mixture of seawater. A combination of deionization and reverse osmosis, is most popular among reef aquarium keepers and is preferred above other water purification processes.