What is Water Filtration and Water Filter Purifiers
Modern day water filters and Purifiers came into being about 150 years ago. A piece of cloth, most probably, was what people used before this time for filtration. They would sieve the river, pond, well or rain water through a piece of cloth. Once the cloth gets dirty or clogged, they would rinse the cloth clean and reuse. Then most probably their drinking water supply would have been boiled.
Modern day filtration still uses the same principle, but the piece of cloth has given way to more sophisticated materials and the boiling water part has been simplified by killing the germs in water by more convenient means. We call it eBoiling!! (eBoiling means to treat water with ultra Violet rays or UV which also kills germs like ordinary boiling)
Water Filter Pore Size
Filtration still depends on sieving or passing the water through fine pores or holes, only now the pore sizes have become very much smaller. The size of the pores or holes in the cloth is very large when compared to modern day filter media. While the pore size of the cloth would be measured in fractions of a millimeter (1/1,000 or 0.001 Meter) a modern day water filter pore size is measured in Microns represented as μm (1 by a million=1/1,000,000 or 0.000001 Meter. Note: 1 million=10 lacks in India). The smaller size of the pores means that it is more difficult for the water to pass through them, hence more pressure. In the case of gravity driven filters, the pressure is small, so the water is filtered almost drop by drop and hence much more time is required at lower pressures. The smaller pores also get blocked more easily.
Filtration Stages
The modern day domestic water purifiers combine different types of filtration methods into one unit. We will first discuss about the different steps or stages in a modern day water purifier.
- A rough, pre or sediment filter for removal of large particles, like mud. The pore size of the sediment filter is usually about 20 microns. If the water contains a lot of mud or other larger particles, then this cartridge element becomes blocked quickly. It can be removed, cleaned by jetting with water and reused. It is like the cloth being reused after rinsing by the earlier generation.
- A finer filter with pore size varying anywhere from 5 microns down to 1 micron or even less. These finer pore size filters can capture germs or microbes depending on its size and the filter pore size. The smaller the pore size, the more efficient it will be in capturing the microbes or germs. The disadvantage of having a very small size pore is that higher water pressure will be required to drive the water through. If the pressure is not sufficient, the water coming out of the filter will be very slow. In gravity filtration the finer pore size will mean that the water will trickle through the pores much more slowly and it will take a much longer time to fill. Since these multi-stage filters normally have a germ killing devise like Ultra Violet light (UV) or other germ killing methods at a later stage, any germs passing through the finer filter will be killed later. So a compromise is made usually by having a 5 micron second step filter cartridge.
- An Activated Carbon water filter Block Cartridge element commonly referred to as an activated charcoal water filter (See Activated Carbon below), which adsorbs chemicals which cause smell and odour in the water. The activated carbon also adsorbs toxic organic chemicals like pesticides. It is very important to understand that there are many qualities of activated carbons and some may not be very efficient. Also the activated carbon has limited capacity to hold harmful chemicals, so make sure it is changed at the recommended intervals.
- A germs killing devise, usually UV (Ultra Violet) light irradiating lamps, the so called e-boiling (See Ultraviolet UV below). Other methods are by passing through special media which release chlorine, or through medium like nano-silver particles.
The filtration steps or stages 1 to 3 described above can be as separate filtration units one after the other or it is possible to combine all steps or stages into one filter cartridge by having the different media one under the other, so that the water flows through each media.
As the filter removes the particles from the water, their pores are getting blocked and water pressure is being lost. This means over time the flow out of the filter becomes slower and slower until it almost stops. The time it takes for the filter to become blocked depends on how much dirt there is in the water. Most of the filters in the market have replaceable filter elements called cartridges, and it is a simple procedure to change the cartridges. Once the cartridges are replaced, the flow will be restored.
Activated Carbon Water Filter
Activated Carbon water filters are made from charcoal or coal. Special grades of charcoal or coal are activated by special process using steam or chemicals. Activated Carbon is an extremely porous material that attracts and holds on its surface harmful chemicals by a process known as adsorption. It removes some toxic organic compounds like pesticides and heavy metal organic compounds. Many toxic organic compounds, such as chlorinated and non-chlorinated solvents, petroleum products and trihalomethanes are also adsorbed. Coconut shell charcoal can be activated and is the preferred carbon for use in drinking water filtration because of its larger optimum size pores or macroporous structure. Most popular forms of activated carbon used in drinking water filters are granular activated carbon (GAC), extruded solid carbon block (CB) and powdered activated carbon (PAC). Activated Carbon in the form of solid Carbon Block is generally considered the most efficient for water treatment.
The efficiency of filtration with activated carbon depends on two important factors – the quantity of carbon in the filter and the speed of the flow of water over the carbon. When there is more of the activated carbon in the filter the better it is because the water will flow over more area of carbon and also the cartridge will last longer. The slower the flow of the water over the carbon, the more time the contaminants in the water will be in contact with the activated carbon, and it gives more time for absorption to take place.
Lower water temperatures improve the performance of activated carbon filters. It should be noted that activated carbon filters have a limited lifetime. Its surface will become saturated with adsorbed pollutants and it cannot adsorb any more. At this stage the activated carbon will start rejecting some of the previously adsorbed pollutants into the water flowing over it. This is called ‘breakthrough’. When this happens, the concentration of pollutants in the treated water will be higher than in the raw water being treated. The activated carbon cartridge must be replaced well before this happens.
Most of the activated carbon filter elements are sold saying that they are good for so many liters of water. This quantity is based on a previously fixed standard quantity of contaminants in the water. Your water may have more contaminants and so it is important to know the quality of your incoming water.
Bacteria can grow on the surface of the activated carbon, especially when not being used for a long time. Some manufacturers counter this by impregnating the activated carbon with Silver particles. Silver ion is a bactericide and will be helpful in preventing this bacterial growth. If there is a UV or other disinfecting means after the activated carbon filter, then bacterial growth will not be of any consequence.
Ultraviolet - UV water sterilisers
UV water sterilisers are designed specifically to kill harmful microorganisms such as bacteria and viruses in water supplies. With a UV sterilizer you will have the peace of mind that the water is properly disinfected prior to drinking. UV does not introduce any harmful chemicals to your water. UV rays penetrate the cells of bacteria and viruses destroying their ability to reproduce. Without this ability, these organisms cannot multiply and eventually die. It is a simple but very effective process, with the system generally destroying 99.99% of harmful microbes. The normal UV unit for your Point of Use drinking water supply in the kitchen will use about the same amount of energy as a 40 watt light bulb. It is possible to connect the UV to an inverter or UPS, so that it can be operated even when there is no electricity.
Membrane based Water Filters
Membranes have very fine pores and the pore size can range from 1000 to 0.0001 micro-meter. A micrometer is one-millionth of a metre and it is usually represented by the symbol 'μm'. Membranes are classified into four main types: Microfiltration (MF), Ultrafiltration (UF), Nanofiltration (NF) and Reverse Osmosis (RO). The pore size on micro filtration membranes ranges from 0.1–0.5 μm, Ultrafiltration has a pore size range of 0.01-0.1μm, Nanofiltration has a pore size range of 0.01-0.001μm. and Reverse Osmosis has a pore size range of 0.001–0.0001μm
There are available in the market Ultrafiltration (UF) Membrane based water purifiers which purify water by filtering out all dirt, viruses, germs and their eggs. This is because the UF membranes are such fine filters with pore size (filtering hole size) of 0.01 micron. The size of the smallest virus, the very smallest of germs, is more than 0.02micron in size and they are blocked by membranes whose pore size is less than 0.01 micron.
Bath Water Shower Head Filter
Municipalities use chlorine to kill bacteria in the water. High chlorine levels are designed to kill organisms it contacts, so Chlorine is not good for any living organism, including humans . Unfortunately, when we shower our skin absorbs chlorine from the bathing water. It is estimated that more chlorine can be absorbed through our skin during a shower bath than by drinking six glasses of tap water which has not passed through an activated carbon filter. Hot showers cause the pores of the skin to dilate, allowing chlorine to be absorbed. Also chlorine is in the water vapor which we breath into our lungs while having a shower. What it does for our hair and hair-lines (bald head) can only be guessed. So it would be recommended to have a shower head shower water filter with an activated carbon filter in it.
Whole House Water Filter
A house can have two ways of filtering their water.
Point of Entry – POE filtration is an whole house water filter, when you purify all of the water coming into your house.
Point of Use or POU filtration is when you filter only your drinking water at the kitchen tap, for example.
Obviously the Point of Entry (POE) filter unit has to be much bigger to meet the requirement of the entire flow of water into the house.
Reverse Osmosis Water Filter
The difficult part of water purification is to get rid of the dissolved chemicals in water. The removal of suspended solids and microorganisms is fairly straight forward – filter first and then disinfect.
Underground water obtained from wells, bore wells, etc. can contain chemicals which can be very dangerous to health even if present in the water in very minute quantities. Some of these harmful chemicals are compounds of Arsenic, Hexavalent Chromium or Chromium-6, Nitrates, Lead, Mercury, etc. To remove the dissolved matter from water the most efficient way is by a special purification process known as Reverse Osmosis or RO. We have a dedicated page on how does reverse osmosis work.
How to Sterilize Water or How to Disinfect Water in an emergency
Sometimes there arises crisis situations like an epidemic of Cholera, Typhoid or such other water borne diseases in the area around you. In such cases it is better that you disinfect or sterilize your whole house water supply. The best way to disinfect water is by pouring Sodium Hypochlorite Solution (Clorox) directly into your main water tank. Special Water Purification Tablets like Katadyn Micropur which are based on Silver Salts are available. Similar tablets based on iodine are also available. These water purification tablets are ideal for camping and such outdoor activity.
The best way to Sterilise water is with Clorox Bleach or Sodium Hypochlorite Solution. Half a litre of 12% Sodium Hypochlorite Solution or 1 litre of Clorox (5% chlorine) in a 1000 liter water tank will be more than enough for such crisis situations. For drinking purpose this water can pass through your regular kitchen water purifier with activated carbon to remove the high chlorine taste of the water.