What is RO Membrane, Reverse Osmosis Explained

RO Membrane is a very fine sheet of filtering material with very fine holes or pores, the membrane filter material is called a semi permeable membrane.. Reverse Osmosis purifies water by filtering the water through the very fine holes or pores in the RO membrane. The pores on these membranes are so small that it will allow the water molecules to pass through easily but will stop the passage of larger molecules of salts and organic chemicals to pass through. Nanofiltration membrane pore size are slightly larger as explained below.

Reverse Osmosis Pore Size

The RO pore size diagram below (not to scale) will help in understanding the relative pore sizes of Membranes and the sizes of the common impurities.

The pore size of RO membrane is 0.0005microns which is smaller than the size of the Sodium Chloride Molecule which is 0.0007 micron and will not let it through. Nor will it let through germs and viruses, and organic molecules which are very much larger than the RO pore.

So an RO membrane will purify water from almost all chemicals. What can pass through the RO membrane is water and molecules smaller than the water molecule, like CO2 gas. Smaller molecules than water is generally not harmful to health. The RO Water is distilled water (almost) and very pure. Bottled water manufacturers usually use Reverse Osmosis Water Filters to purify their water.

The Nanofiltration pore size at 0.0008 microns, is only slightly larger than the RO pore (0.0005 microns) and also only very slightly larger than the size of the common salt (NaCl) molecule. So it allows monovalent common salt Sodium Chloride to pass through but will not allow divalent Calcium salts like Calcium Carbonate of dia 0.0009 microns to pass through. Nanofiltration and Reverse Osmosis RO membranes remove Mercury, Lead, Heavy Metals, Arsenic, all germs and viruses from water and makes Hard Water Soft.

The diagrams below from Koch Membranes helps us to better understand the concept of Nanofiltration and Reverse Osmosis.

Nanofiltration Membrane allows only pure water, monovalent salts and lower molecular weight divalent salts to pass through

RO membrane only allows pure water to pass through it. Everything else like monovalent and multivalent salts, bacteria and viruses, dirt, etc. are completly blocked

Membrane Manufacturers

The leading manufacturers of Membranes in the world are CSM Woongjin, FilmTech, Hydranautics, Koch, Osmonics, Toray, etc. All of them manufacture different types of membranes like Microfiltration, Ultrafiltration, Nanofiltration and Reverse Osmosis. Most of their membranes are  Thin Film Composite (TFC) Siral Wound type.

Nanofiltration

Nanofiltration membrane has a slightly larger pore size than Reverse Osmosis Membran. Thus monovalent salts like sodium chloride (common table salt) molecules passes through the membrane but large divalent salts of Calcium, Magnesium and other metals like iron, heavy metals, etc. are all blocked.

This is an ideal solution where there are problems with hard water and water having excess iron and metal salts including heavy metals. The advantage of Nanofiltration is that it requires much lower pressure than RO to drive the water through the membranes. This means that a Nanofilter membrane plant will consume only about half the electriciy of an RO plant. Also the initial cost of a Nanofilteration plant will be less than that of an RO plant since the pressure in a Nanofiltration plants are about half or less, than that of an RO plant.

Hard waters normally do not contain excess monovalent salts like sodium chloride. Hence Nanofiltration plants are the ideal solution for obtaining soft water for the whole house. At the bottom of this page we describe how to size plants for whole house purposes.

Undersink RO Water Purifier

The picture on the right shows a typical point of use undersink RO water filter purifier insalled under a kitchen sink. RO is highly recommended for people using well water and especially in areas susceptible to toxic chemicals like Arsenic, Heavy metals and Nitrates (near farms and agricultural areas).

Reverse Osmosis is the opposite of Osmosis. We know that water flows from a dilute or pure solution to a more concentrated solution when separated by a membrane - like the membrane covering an egg just below the egg shell. We also know that a pressure is built up in the concentrated side and the water column there would rise and this is called the osmotic pressure. Plants and trees depend on osmotic pressure to drive up water from the roots to their leaves and fruits. Osmotic pressure is responsible to get the water from the ground, right up a high coconut tree, to fill its tender coconuts with the tasty juice.

Reverse Osmosis uses pressure which is more than the osmotic pressure, to drive pure water from the concentrated side to the dilute or permeate side of the membrane. A membrane is the semi-permeable barrier across the concentrated and dilute solutions. In industrial use this semi-permeable membrane is called as just a membrane, and is made of special synthetic plastics sheets. RO is the process used in desalination, the process of making drinking water from sea water.

The same technology is now available for your home use. The advantage of RO or Reverse Osmosis units is that it will remove not only all the germs including the smallest viruses, but will also remove harmful chemicals like Arsenic, heavy metals, Nitrates, Salts, etc. In fact bottled water manufacturers use RO to purify the water before they bottle it.

Reverse Osmosis Diagram

The RO flow diagram of a typical undersink RO filter purifier is shown here. The reverse osmosis water filter diagram shows several filtration stages including an Activated Carbon filtration stage which are there as pre-treatment before the RO membrane to protect it. These pre-treatment filters remove suspended particles and the activated carbon filter removes Chlorine, which can damage the RO membrane.

The disadvantages of RO are that it requires high water pressure and that some water is wasted in the reject stream. If there is not enough pressure in your pipeline then a small electric motor is required to pump the water to a high pressure. Secondly, it wastes some water in its reject stream which carries all the concentrated impurities that was there in the water originally. This means that when it purifies say 1 liter of water, it drains away about half a liter of water in which all the impurities are concentrated. This waste water is known as the reject. This reject quantity of water goes up as the water becomes saltier or has more TDS. For seawater the recovered permeate or product water is only about 30% and the wasted reject water is nearly 70%. Normal home water supplies will get about 70% product or permeate pure water and the waste reject will be 30%. It may be possible to make use of the reject water, like for cleaning or watering plants.

Ultrafiltration purifier

Ultrafiltration is useful because it will filter away all micro organisms, including viruses, the smallest of microbes. UF does not require as high a pressure as required by RO. Since UF will remove all suspend matter including all microorganisms and viruses, it is an attractive solution for purifying all the water coming into the house. Note that Ultrafiltration will not remove dissolved substances. Thus it is a good for use with public supply water, well, pond, river and rain harvested waters, when you are sure that the dissolved solids (TDS) is within the limits and that it contains no toxic impurities. f you are not sure about the dissolved solids in the water, then it is best to opt for an RO unit.

If the water has no problems with dissolved solids, then an Ultrafiltration purifier unit could be the best solution for water purification needs. Ultrafiltration purifiers do not require a high pressure like an RO membrane purifier and thus requires less energy and is more economical. All bacteria and viruses are blocked by Ultrafiltration purifiers. Ultrafiltration purifiers combined with an Activated Carbon filter and a UV sterilizer could be the next best solution after RO for drinking water, especially when dissolved solids are not a problem.

Sizing of NanoFiltration & Reverse Osmosis Units

Nanfiltration or RO units are usually classified as having a flow of so many Liters Per Day or LPD. To calculate the amount of water required per day, you could use an average of 150 liters required per day per person. So for example, a family of 6 will need a 6x150=900 liters per day.

You would not require RO purified water to flush toilets and for cleaning floors, etc., so you should have a separate tank and separate pipeline for the toilet flushes and other uses not requiring RO water. The reject water from the RO could be used to fill this cleaning and flushing water tank in areas where water is in short supply.

The quantity of reject water from a Nanofiltration plant is not as much as the RO plant. The Nanofilteration reject water is best disposed off far from you intake water source, because it will have very high content of Hardness.

For cleaning, initially you could use the RO reject water and a final rinse with the RO permeate or pure water. This is because the reject water of the RO contains a lot of salts, and when the water dries, these salts would leave a whitish colored stain marks on the cleaned surface. You avoid this by a final flush with the permeate pure water.