In this instalment we will examine TDS (Total Dissolved Solids) and how it impacts the water in your home. Many readers wrote to me about this so it is useful to be armed with some information.
TDS as a concept applies to fresh water, and saline sea water is excluded from the definitions. Fresh water from a river will dissolve the minerals from the soil or ground over which it flows. It can be contaminated by pesticide runoff from farms and grey and black water discharge from homes.
When routed through metallic pipes, fresh water picks up more solids which then dissolve into the water. Similarly ground water passes through several strata of soil and can dissolve the minerals along the way. The sum of these dissolved solids in water are collectively referred to as TDS and is expressed in parts per million (ppm). Besides this, water can contain other particles or pollutants like silt, clay, debris which do not dissolve and are also larger . The most common constituents of TDS are calcium, phosphates, nitrates, sodium, potassium , chloride and iron.
TDS and drinking water quality
The Bureau of Indian Standards (BIS) fixes the upper limit of TDS in drinking water at 500 ppm. Crucially the standard also mentions that in case no alternative source of drinking water is available, then this upper limit can be relaxed to 2,000 ppm.
This TDS limit is for normal fresh water and in special cases where toxic contaminants like arsenic or fluoride are present in the water source, other special BIS standards will be applicable.
Some of the dissolved minerals in water are useful for human health and therefore very low or zero TDS is also not desirable. When the TDS is reduced severely it also affects the Ph of the water, which BIS specifies should be in the range 6.5-8.5. Water with very low TDS will taste insipid and while there is no official lower limit, TDS of at least 80 ppm is the accepted minimum level.
Excessive TDS particularly with salts of calcium and magnesium leads to hardness of water which then causes scaling in household devices that use water.
TDS management
TDS meters are easily available today for less than a thousand rupees and to start with one can measure the TDS of both the ground water as well as the water supplied by corporation/municipality. You can also get the water samples tested for TDS and potability at any government approved lab. If the measured TDS is above the BIS limit of 500 ppm, you need a long term strategy to address the issue.
Of the common water purification systems available in the market, only reverse osmosis (RO) systems can remove the TDS in the input water. However RO systems waste over 50 per cent of the source water. The purified water has zero TDS which is unhealthy and all RO systems need an additional step where minerals or TDS are added back before the water is fit for drinking. Compounding the problem is the fact that the rejected waste water has very high levels of TDS making it unfit for most applications.
Rainwater is naturally distilled water and has extremely low TDS. If you have correctly implemented the rain-water harvesting method in your home, the rainwater that charges your ground water will keep the TDS level in check. Rainwater harvesting is free, helps manage TDS and also leads us closer to water self-sufficiency. In the next part of this series we will discuss other simple water purification methods including sun charging, and using native herbs.
