When RO salesmen or technicians come to your house to give you a demo about their product one of the first things they will do is check the TDS value of your water supply. But, what is TDS ?
TDS stands for ‘Total Dissolved Solids’. Which in turn means, it gives you an idea of hard your water supply is. The unit used in TDS meters is PPM (Parts per million). So a TDS value of 100 means it has 100 parts of solids dissolved in 1 million parts of water.
Now, PPM values when converted to mg/l format (milligrams per litre) remain roughly the same. So 100 PPM is roughly 100 mg/l. This information is important since all the standards mentioned in the WHO (World Health Organization) guidelines or in the BIS (Bureau of Indian Standards) documentation is given in mg/l.
Now you must be wondering, what is the WHO or BIS say about the ideal TDS of water ?
WHO : TDS below 1000 is okay for human consumption !
BIS : TDS below 500 is okay !
In case you think that the above data is wrong, check out the following links:
http://www.who.int/water_sanitation_health/dwq/chemicals/tds.pdf
http://bis.org.in/sf/fad/FAD25(2047)C.pdf
So, should you throw out your RO systems, or get a refund ? No, not quite yet.
The problem with TDS value is that it is only an indicator of how many minerals are dissolved. Not which minerals are dissolved. Just like a thermometer can tell you that you have a fever. But fever due to what ? That requires a blood test. To know what exactly is dissolved in you water requires a water report, typically done in a laboratory. So the TDS value in your home could be 300. Which is safe according to the WHO or BIS, but what if all that 300 TDS is mercury. Then what ?
So an RO system provides a middle solution. It brings the TDS value down to below 100. At these values, it does not matter much what is dissolved in the water as your kidneys will do the rest. Typically, a TDS of 70-90 is good value, as it provides safety and good taste. Very low TDS (<40) is typically not recommended as the human body also requires some minerals, especially in the summer season, when one tends to sweat a lot.
Questions Regarding Reverse Osmosis Systems 
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