TYPES OF WATER FILTERS
There are several types of water filters in the market today. According to filtering action, filters can be classified into mechanical filters, adsorption filters, and chemical filters.
Mechanical Filters
The “screening action” of mechanical filters is based on the size differential between the contaminants and the pores of the filter. Pores of the filter must be smaller than the contaminants to be eliminated, and the rest are allowed to pass through. Filter pores can range from 0.0001 microns to 100 microns. (1 micron = 0.001 millimeter). On the basis of pore size, mechanical filters can be further classified into Particulate Filters (1 to 100 microns), Micro Filters (0.1 to 3.5 microns), Ultra Filters (0.006 to 0.25 microns), Nano Filters (0.0008 to 0.01 microns) and Reverse Osmosis (0.0001 to 0.002 microns).
Particulate Filters screen out relatively larger particulate contaminants such as suspended fine sands, dusts, rusts, organic matters, decaying materials, etc. They allow all other contaminants to pass through. An example of Particulate Filter is the multi-media sand filter consisting of layers of sand of sizes varying from coarse, to medium, to fine, and extra fine. These are contained in a cylindrical tank where the feed water is introduced to the coarse sand first and then passed on to the next finer size sand, until finally to the extra fine sand.
Particulate Filters are cheap and easy to maintain. They can be regenerated by way of backwashing
Micro Filters are designed to remove all particles and microorganisms greater than their pore size. They are generally used as pre filters to remove almost all suspended solids, and protect other more expensive filter elements downstream from clogging and fouling. However, they do not remove dissolved inorganic and organic chemicals, bacteria, viruses and colloids. Though cheap and they require minimal maintenance, they can not be regenerated and are therefore relatively more expensive over the long term.
Ultra Filters use tough, thin selectively permeable membrane with infinitesimally fine pores that act as a molecular sieve. They retain most, but not necessarily all, large molecules above their rated size including colloids, microorganisms, and viruses, but not smaller molecules like water and ionized contaminants. Ultra filters are available in several ranges and they produce very high quality water for least amount of energy. Since they can be regenerated, Ultra Filters are therefore more economical over the long term.
Reverse osmosis (RO) uses a semi-permeable membrane that has a pore structure much tighter than Ultra Filters. RO membranes are capable of rejecting practically all particles, bacteria, viruses, heavy metals, ions (like salts, whether strongly ionized or weakly ionized), colloids, and dissolved molecules (like sugars). The water produced has very low Total Dissolved Solids (TDS).
In normal osmosis, when two water solutions of different concentrations are separated by a semi-permeable membrane, the natural tendency is for the solvent to migrate through the semi-permeable membrane from the less concentrated solution to the more concentrated one, until the degree of concentration on both sides equalize thus equilibrium is achieved. It is believed that a pressure called Osmotic Pressure drives the water through the membrane until the concentrations equalize.
In the reverse osmosis (RO), as employed in water purification, hydraulic pressure is applied to the concentrated solution to overcome the osmotic pressure. Pure water is forced from the concentrated solution through the membrane and collected downstream of the membrane.
Because RO membranes are very restrictive, the rate of production is very slow, thus requiring storage tanks for the accumulation of purified water in volumes.
Overall, reverse osmosis as a mode of purifying water is very effective against practically all kinds of contaminants, requiring minimal maintenance. Relatively high upfront cost, but very economical over the long term. (As a note, combination RO and Activated Carbon Filters presents the most effective filtration process against the broadest range of water impurities and contaminants.