Water is life. Without water life is not possible. For sustaining a good health, pure drinking water is a sine quo non. Simple filtration candles or filtration materials such as sand, pebbles, charcoal, etc., can remove only the suspended materials from dirty water and filtered water carried over with it both the essential as well as the toxic minerals along with microbial germs in the dissolved form. On the other hand, water without the essential minerals, viz., sodium, potassium, calcium, magnesium, etc., is tasteless. It is mentioned in the Rig Veda that the drinking water should be Sheetham, which means cold, Sushihi (clean) and Sivam which means it should have nutritive values with requisite minerals, Istham which means transparent and Vimalam Lahu Shadgunam, which means it should have a balanced acid-base limit. With the development of technology, the strategy of drinking water purification changed and the concept of use of aqua guard came in with the use of ultrafiltration technique killing the microbial germs through e-boiling or use of ultraviolet lamp. But the problem of separation of the toxic minerals such as heavy metals, arsenic, lead, fluoride, etc., still remained an issue. Finally the technology of reverse osmosis (RO) using a semipermeable membrane in the filter candle has successfully been applied in the thrust area drinking water purification for removal of all the dissolved minerals and microbes from it.
It may be mentioned in this context that the National Green Tribunal (NGT) has however recently prohibited the use of RO (reverse osmosis) filters in Delhi having total dissolved solids (TDS) in water less than 500 mg per litre and directed the RO filter manufacturers to ensure that more than 60% water is recovered setting deadlines to the Central Pollution Control Board and Ministry of Environment to implement it by December 31, 2019. The reason is that the RO system removes all the dissolved minerals including the essential ones and discard 80% of water which is processed that leads to huge wastage. Against this order, the RO filter manufacturers represented by the Water Quality India Association filed a petition in the Supreme Court as there is heavy metals in the groundwater in many of the cities. However, the Supreme Court on November 22 last declined to interfere with the NGT order. Rather, the court asked the association to approach the Centre and directed the Government to consider the representation before passing a notification.
Now, what actually is RO? If a concentrated solution of certain salt(s) is separated from a dilute solution of the same type by a semi-permeable membrane that selectively allows the movement of particles, the solvent molecules from the dilute solution will flow from the dilute solution to the concentrated solution and this movement, a colligative property of a solution, continues until it reaches an equilibrium. This process of flow of solvent molecules due to the difference in concentration is known as osmosis. The driving force for the movement of the solvent is the reduction in free energy of the system when the difference in solvent concentration on either side of a membrane is reduced, generating osmotic pressure due to the solvent moving into the more concentrated solution. This natural flow of solvent from the dilute solution towards the concentrated solution can be made reversed overcoming the osmotic pressure by applying an external pressure, which is reverse osmosis (RO) and the pressure applied is known as reverse osmotic pressure. This device is used in water purification process that uses a partially permeable membrane to remove larger particles, ions, unwanted molecules from drinking water to give highly pure water. However, there is heavy wastage of water in RO filtration as it gives only 20-30% pure water. It is used in both industrial processes and the production of potable water.
The RO membrane used is a thin film composite and is made up basically of two different materials, viz., cellulose acetate or aromatic polyamide and do not have definable pores in the way that film used in ultrafiltration. There are only spaces between the fibres making up the film which can take up water because of the different groupings which form the surface. Cellulose acetate is resistant to chlorine but is affected by bacteria whereas polyamide is resistant to bacteria but is affected by chlorine. Further, pH is a major factor and at high pH cellulose acetate is hydrolysed to cellulose and the changes are inevitable and can be reduced by choosing the condition of operation and finite life of 3-5 years could be expected. The pressure exerted in RO on the side of membrane having the impure water is generally 2-17 bar for ordinary water and 4042 bar for sea water to overcome the osmotic pressure. The latest development in the sphere of RO filtration membrane is the use of nanomaterial, viz., graphene oxide. This is now considered as the most promising and ultimate membrane material for it is stronger, thinner, chemically more resistant than polyamide or cellulose acetate and has excellent hydrophilicity, high water permeability, attractive ionic/molecular separation properties with high surface area, mechanical durability, atomic thickness and nano-sized pores to separate particles larger than 0.1 nm.
The subject of presence of essential minerals and RO water filtration created controversy and disagreement among water and health professionals. This is said to be a weak argument to suggest for making up the deficiency through use of mineralized water. The WHO however clarified that majority of healthy minerals that are needed for human body are from food or dietary supplementary sources only and not from drinking water. Because plants convert the inorganic minerals from soil to useful organic minerals which in our stomach get attached to a specific protein molecule (chelation) in order to be absorbed and then it gains access to the needed tissue sites. Once a plant mineral is digested within the body, it is utilized as a co-enzyme for composing body fluids forming blood and bone cells and the maintaining of healthy nerve transmission. The essential minerals can be accessed from milk and milk products, green leafy vegetables and other vegetables such as pea, broccoli, cabbage, bean, sea food, whole grains, etc.
Of course, as stated above, water without the essential minerals is completely tasteless. Thus, the main disadvantage of RO filter is that there is heavy wastage of water and gives tasteless pure water removing 99.9% contaminants. From that perspective the Supreme Court has righteously given the judgment not to interfere the prohibitory order of NGT. In that context, is it advisable to go for avoiding RO water filter? The answer is definitely ‘no’ because RO filter gives highly pure water. The only solution for this is that the RO filter manufacturers should go for adopting latest affordable technology to ensure at least 60% recovery of water and use re-mineralizer cartridge or TDS modulator that remineralize with essential minerals in the filtered water using GO nano RO membrane. <Dr Amrit Goswamj>
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