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Most people assume that RO water purifiers are, by default, a better class of water purifiers than UV ones. While this may be true in technological terms, it does not mean that RO water purifiers should be the automatic choice for every home. And these days, a third class of purifiers, namely UF, has gained prominence and added to the confusion of buyers. This article lists the factors to evaluate to choose the right water purifier for your needs.
A RO purifier offers a higher level of filtration than UF or UV, but whether this enhanced filtration capability is really necessary depends on the quality of drinking water supplied to your home, and that alone must be the criteria to decide the type of purifier to go for.
Before we discuss the types of water purifiers available and what they can (and cannot) filter, let’s discuss the common types of impurities found in drinking water.
Common Types of Impurities in Drinking Water
There are various sources of drinking water supply, such as lakes and rivers, bore wells, harvested rain water, etc. These days, even seawater can be purified for drinking purposes by using industrial scale RO plants. Depending on the source of water, the quality of water supplied may differ in terms of hardness and presence of pollutants, chemicals and harmful micro-organisms such as viruses and bacteria.
Some of the common types of impurities in water are:
|Type of impurity||Source of contamination||Commonly found in||Effect on drinking water|
|Undissolved solids such as sand and mud||Loose soil or sand mixing with flowing water||River water or piped water where pipes are damaged||Muddy or turbid appearance|
|Dissolved inorganic salts like Sodium & Potassium||Salt deposits from surface||Bore wells and seawater||Salty or brackish taste|
|Dissolved inorganic compounds like Calcium & Magnesium||Water flowing over rocky terrain||Bore wells and river water||Hardness and scale formation|
|Organic compounds||Absorbed from plant life growing inside water body||Lakes and ponds||Foul smell or odour|
|Decontaminants like Chlorine||Added by municipal water suppliers to kill microorganisms||Municipal piped water supply||Bitter taste|
|Microorganisms such as viruses & bacteria||Biological contamination of water source||Piped water where pipes are damaged||Biological contamination|
Generally, water supplied from lakes and rivers as well as harvested rain water will have a lower concentration of dissolved solids, while water drawn from bore wells is likely to have a higher concentration of dissolved solids and other harmful chemicals such as lead, arsenic, etc. Water supplied through pipes or stored in tanks may be prone to biological contamination if the pipes are old and corroded or the tanks are not cleaned regularly.
Hard Water vs Soft Water
Depending on the concentration of dissolved solids in water, it is classified as soft or hard water. The degree of hardness is expressed in terms of Total Dissolved Solids (TDS) and is measured in units of Parts Per Million (PPM) or Milligrams per Litre (mg/L).
1 ppm = 1 mg/L
Soft water (for example, mineral water) will have a low TDS value (typically under 150-300 ppm), while hard or polluted water will have a high TDS value (over 500 ppm). You can check the TDS level of your water supply using a TDS meter as shown below.
Types of Water Purifiers
1. Reverse Osmosis (RO) Purifiers
To understand how RO purifiers work, let us understand Osmosis first.
Osmosis, as we learnt in school, is the flow of water molecules across a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration. The solute concentration here refers to the TDS level of the water. The semi-permeable membrane has tiny holes (about 0.0001 microns wide) that are large enough for water molecules to pass through but too small for the dissolved impurities to move across. As a result, water moves across the membrane from the region of lower TDS level to the region of higher TDS level as shown below.
RO attempts to do the opposite of Osmosis, that is, push water molecules from the region of higher TDS level to the region of lower TDS level (hence the name Reverse Osmosis). It does this by pumping water into the RO chamber at high pressure, causing water molecules to be pushed across the membrane to the other side while leaving the dissolved solids and other impurities behind. These impurities along with a part of the inlet water are then discharged through another outlet as shown below.
As a result of this filtration, the drinking water generated by RO purifiers will have a much lower TDS content compared to the inlet water. RO purifiers are therefore best suited for use where the tap water TDS is very high, i.e. it is hard.
The downside of this technology is that it needs a pump to generate pressure to push water across the membrane, and therefore a RO purifier cannot operate without electricity. Also, since part of the inlet water is discharged along with the dissolved solids, this is a wasteful method of filtering water. Often a RO purifier will discharge 2-3 litres of water per litre of pure water generated.
2. Ultrafiltration (UF)
Ultrafiltration also uses a semi-permeable membrane to purify water, similar to RO. However, here the membrane has much larger holes (about 0.01 microns wide) as compared to a RO membrane. As a result, while UF can remove undissolved solids and even microorganisms such as viruses and bacteria from the water, it cannot remove dissolved solids and reduce the TDS level. UF purifiers are therefore suitable for use where the TDS of the tap water is low i.e. it is not hard.
On the positive side, since the UF membrane has much larger holes, water can pass through it on its own using the force of gravity. Therefore a UF filter can work even without electricity. And additionally, there is zero wastage of water since there is no water held back by the membrane.
3. Ultraviolet (UV) Purification
A UV purifier subjects water to ultraviolet radiation which causes microorganisms such as viruses and bacteria to be killed or “inactivated”. It however cannot remove any dissolved or undissolved solids or chemicals. As a result, commercially sold UV purifier units come with external sediment pre-filters to remove undissolved solids as well as an activated carbon unit to remove Chlorine and some dissolved solids from the water. UV purifiers, like UF, are suitable for use where the TDS of the tap water is low. But if the water is too muddy, a UF purifier is preferable.
If the biological contamination of water is high, a combination of UF+UV may be considered.
4. Tap/Faucet Mounted or Storage Filters
These are simple sediment or sediment-cum-activated carbon filters which can remove undissolved solids such as sand, mud, etc., some chemicals and some microorganisms. The smallest models are the size of your fist and can be directly fitted on taps and can provide thousands of litres of purified drinking water before they need to be replaced. Slightly larger models come with built-in storage tanks where water can be filled and stored for use. These are suitable for use where the water TDS level is low and there is nil or minimal biological and sediment contamination of water.
Which Water Purifier Should I Choose?
As stated earlier, a RO purifier is necessary only if the water supply in your home has a high level of TDS. As per the Bureau of Indian Standards (BIS) Drinking Water Specification (IS 10500) the maximum TDS limit for safe drinking water is 500 ppm. But realistically speaking, 500 ppm is on the higher side and it is recommended to go for a RO purifier if the TDS level exceeds 300 ppm.
But below 300 ppm, a RO purifier is not necessary at all and can in fact be detrimental to your health as it will reduce the TDS to abnormally low and unhealthy levels. In this case, your choice of purifier should be determined by other factors such as turbidity of water and/or presence of biological contamination.
The below table explains clearly which water purifier is suitable for different types of contaminations:
|TDS content||Is water muddy?||Is water biologically contaminated?||Which purifier to use?|
|Low (below 300 ppm)||No||No||Tap/faucet or storage filter|
|Low (below 300 ppm)||No||Yes||UV|
|Low (below 300 ppm)||Yes||No||UF|
|Low (below 300 ppm)||Yes||Yes||UF or UF+UV|
|High (over 300 ppm)||No||No||RO|
|High (over 300 ppm)||No||Yes||RO+UV|
|High (over 300 ppm)||Yes||No||RO+UF or RO+UF+UV|
|High (over 300 ppm)||Yes||Yes||RO+UF+UV|
Components of a Commercial RO Water Purifier
A commercial RO water purifier has several additional components apart from the RO membrane, for improved filtration and performance. These include:
- Sediment Pre-filter: This is added at the inlet of the system to remove sediments from water as well as chemicals like Chlorine added by municipality into the water supply. It also helps to enhance the life of the RO/UF membranes. If the purifier doesn’t have this, it can be added externally as well.
- Pressure Enhancing Pump: If the incoming water flow from the tap does not have an adequately high water pressure, the RO filter will not work optimally. In such cases, a pump is added to boost the inlet water pressure before the RO stage.
- Activated Carbon Pre-filter: This removes some dissolved solids and chemicals from the water.
- RO/UF membranes: Commercial RO purifiers have a UF membrane in parallel to the RO membrane. The UF membrane delivers the same degree of filtration as RO except that it does not remove the dissolved solids. The inlet water is distributed between the two and the final mixture is adjusted using a TDS controller to control the output TDS of the system.
- TDS controller: As stated above, this mechanism allows adjusting the output TDS of the system. It does this by adjusting the ratio of RO-purified water (which has reduced TDS) and UF-purified water (which has normal TDS) in the final mix. A practical benefit of this is that waste water can be reduced by increasing the output TDS if the inlet water TDS is already low.
- UV Filter: Some RO water purifiers add a UV purification stage after the RO/UF stage to eliminate any leftover microogranisms.
- Carbon Post-filter: This is added to remove unpleasant odors from water, and enhance its taste.
Reducing and Reusing RO Waste Water
As mentioned before, RO purifiers generate 2-3 litres of waste water for every litre of purified water. For a resource-starved country like India, this level of wastage is almost criminal! However, the good news is that this waste water generation can be reduced and the rest reused reduced by simply following the below steps:
- Ensure that the output TDS is at least 150 ppm. If lower than that, increase it using the TDS controller knob. An excessively low TDS value not only generates excessive waste water, it’s also bad for health! Increasing the TDS will make the drinking water healthier and also reduce wastage.
- Store the waste water and reuse it. The waste water generated by RO purifiers is good enough to be safely reused for many domestic purposes such as watering plants, mopping floors and even for washing clothes as shown below (picture courtesy Apartment Adda). Waste water can be stored in containers and reused during the day.