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Many people assume that RO-based water purifiers are, generally speaking, a better class of water purifiers than UV-based ones. While this may be true in technological terms, it does not mean that a RO-based water purifier should be an automatic choice for every home.
A RO purifier offers a higher level of purification, but whether this enhanced purification capability is required depends on the quality of drinking water supplied to your home.
This article explains what factors to evaluate before taking a decision on whether to go in for a RO-based water purifier or stick to a UV-based one.
Let’s start with the basics. RO stands for Reverse Osmosis. But to understand how RO works, it is essential to 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 (see below image).
The semi-permeable membrane above has holes, or gaps, that are large enough for water molecules to pass through, but too small for the solid molecules to move across.
In case of drinking water, solute refers to the dissolved salts and minerals such as Calcium and Magnesium (referred to as Total Dissolved Salts or TDS) that are present in unfiltered water.
Depending on the concentration of these dissolved salts (calculated in units of Parts Per Million or PPM, or Milligrams per Litre or mg/L), water is classified as soft water or hard water. Soft water (for example, mineral water) has a lower TDS value, while hard or polluted water will have a higher TDS value.
1 ppm = 1 mg/L
RO attempts to do the opposite of Osmosis, that is, move water molecules from a region of higher solute concentration to a region of lower solute concentration (hence the name Reverse Osmosis). It does this by using the pressure of the incoming water to push it across the membrane, while leaving the dissolved solids and other impurities behind. The remaining water and solids are discharged through another outlet (see below image).
There are various sources of drinking water supply, such as lakes and rivers, bore wells, water tankers, harvested rain water, etc. Depending on the source of water, the quality of water supplied may differ in terms of TDS levels, pollutants, chemicals as well as presence of harmful micro-organisms such as viruses and bacteria.
Generally, water supplied from lakes and rivers, as well as harvested rain water, will have a lower concentration of TDS, whereas water drawn from bore wells or supplied by tankers is likely to have a higher concentration of TDS and other harmful chemicals such as lead, arsenic, etc. Tanks constructed to store water can also cause contamination if not cleaned regularly and properly.
The concentration of chemicals and micro-organisms in water can also vary depending on the quality and cleanliness of the water delivery infrastructure, such as treatment plants, pipelines, intermediate storage tanks, etc. Old and/or corroded pipes are a major source of water contamination. And if a water pipe is placed close to a sewage pipe and leakage takes place, the contamination can be far more severe.
Components of a RO-based Water Purifier
A RO-based water purifier uses the Reverse Osmosis technique explained earlier to remove dissolved salts, impurities and chemicals from the water. The basic component of such a purifier is the RO membrane which performs the RO process.
Apart from this, a commercial RO water purifier may have some additional components for improved filtration and performance. These include:
- Ultrafiltration (UF) membrane: Before the RO stage, a UF membrane can be used to remove larger solids from the water, thereby reducing the load on the RO membrane and enhancing its life.
- 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 before the RO filtration unit to boost the inlet water pressure.
- UV Filter: Some RO water purifiers add a UV-based purification stage after the initial RO-based filtration to further purify the water by controlled exposure to UV radiation. This is because while RO is good at removing impurities from water, it may not be capable of removing micro-organisms completely. The UV stage kills any living organisms left over after the RO stage, thereby delivering completely pure water.
- Sediment Pre-filter: This is added to remove sediments from water, as well as chemicals like Chlorine added by water suppliers. Using this can enhance the life of the RO/UF membranes. If not built into the purifier, this can be added externally as shown below.
- Carbon Post-filter: This is added to remove unpleasant odors from water, and enhance its taste.
When to use RO Water Purifiers
It should be obvious by now that a RO-based water purifier is really useful only if the water supply in your home has high levels of TDS and impurities. Therefore, before choosing which water purifier to buy, you should check the TDS level of your water supply using a TDS meter, as shown below.
Based on the TDS level of your water supply, you can decide to go for a UV or RO purifier as explained:
- If the TDS is less than 150 ppm, a UV purifier is enough to ensure good quality water.
- If the TDS is between 150-300 ppm, a UV purifier is still quite adequate, although a RO purifier will enhance the taste of water.
- If the TDS exceeds 300 ppm, then it is recommended to install a RO-based water purifier although not mandatory.
- If the TDS exceeds 500 ppm, which is the upper limit set by the Bureau of Indian Standards (BIS) for safe drinking water, then it is strongly recommended to get a RO-based purifier.
Click here to see the BIS Drinking Water Specification (IS 10500).
Most water purifier manufacturers (e.g. Kent, Eureka Forbes, Zero B, etc.) offer free check-up of the water TDS level, so you can avail of that. Also remember to check the water pressure at your house. If the inlet water pressure is below the stipulated level for the purifier, you may need to install a Pressure Enhancing Pump as explained above.
Important Specifications of a RO Water Purifier
There are some important specifications to keep in mind when purchasing a RO water purifier. These are:
- Number of purifying stages: Based on your needs, you can either go for a RO-only purifier, a RO+UV purifier, or a RO+UF+UV purifier, with additional pre- or post-filtration stages as necessary.
- Storage Tank capacity: RO filtration is a slow process, which is why most RO purifiers provide a built-in storage tank. A tank size of 7 to 9 litres is common and should be adequate.
- Flow Rate and Duty Cycle: If heavy usage is likely, it is necessary to check the flow rate, i.e. the rate at which the purifier can deliver filtered water. It is also important to know the maximum amount of water that can be purified in a day, called the duty cycle. These are indicated in units of liters per hour/day.
Lastly, as with any product, please remember to check the product reviews, the quality of after-sales service and availability of spares before narrowing down on any specific model.
Pros and Cons of RO Technology
- Clean & effective: Delivers far more effective purification for water with high levels of TDS and impurities, and does so without the use of chemicals.
- Removes all impurities: Apart from dissolved solids, it can also remove pesticides, metals and chemicals which contaminate water.
- Multi-stage purification: Multiple stages can be chosen based on need and can effectively filter out both living and non-living impurities, remove odors, enhance taste, etc.
- Energy efficient: RO filtration does not require electricity (although other components such as the UV lamp or pump require it).
- Excessive water wastage: For every liter of water that is purified, about 2-3 liters of water are discharged as “waste”. The good news is that this water can be used for mopping the floor, watering the plants, washing utensils and the car, etc.
- Can remove essential minerals: Natural drinking water contains some essential minerals like Calcium, Magnesium and Potassium which are good for the body. When using a RO purifier, do ensure that the output TDS level does not dip below 30-40 ppm so that some minerals are retained. This can be done using a TDS controller which comes built into most models.
- Expensive: Being a relatively new technology, and because of the multiple filtration stages, RO water purifiers are currently quite expensive. A good RO water purifier can cost 2 to 4 times as much as a good UV water purifier.