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Natural waters contain both dissolved solids and suspended solids. Dissolved solids pass through a 0.45-µm (micrometer) membrane filter, while suspended solids are retained by the filter. TDS can be expressed in mg/l (milligrams per litre) or ppm (parts per million). Mg/l is the weight of the dissolved material in one litre of water, while ppm is the weight of dissolved material in 1 million equal weights of solution (that is, milligrams per kilogram). By multiplying TDS in ppm with water density, TDS in mg/l is obtained. As density is the mass of any substance per unit volume at a designted standard temperature such as 20ºC, water temperature has to be also taken into account for a more precise determination of TDS expressed in ppm.

Most workers dealing with fresh water express TDS in mg/l, while oceanographers dealing with saline waters use salinity instead of TDS and express it in ppm. As density of fresh water is nearly one, there will not be much difference in TDS expressed in mg/l or ppm. When TDS is under 7,000 mg/l, no correction need be made as the difference in TDS expressed in either way is within the experimental error. But when TDS is more, correction has to be necessirily made. Thus TDS of 35,000 ppm of sea water with a density of 1.028 will be 35,980 mg/l.

The international reference points for standard setting and drinking-water safety given by most public health agencies in the world including the World Health Organization (WHO), European Union (EU) and U.S. Environmental Protection Agency (EPA) do not include any health-based guideline for TDS, implying that human health is not affected by drinking high-TDS water. On grounds of palatability, several agencies including the Bureau of Indian Standards (BIS) and the Indian Council of Medical Research (ICMR) have presecribed 500 mg/l as the desirable limit of TDS for drinking water. The upper limit of TDS in the absence of alternate source is fixed at 2000 mg/l by the BIS and 3000 mg/l by the ICMR. Although many people find water with high TDS to be not palatable, the same is equally true with that very low TDS. Those accustomed to drinking highly mineralised waters find even water with a TDS of 500 mg/l to be tasteless. More than the actual TDS of drinking water, a change in the water TDS from high to low and vice versa is not desirable as it can even cause gastric disturbances.

In popular usage, ‘high-TDS water’ is used as a synonym to ‘hard water’. In the scientific usage, high-TDS water means the sum of all its dissolved constituents including sodium, potassium, calcium, magnesium, carbonate, bicarbonate, chloride, sulphate etc., while hardness of water is taken as the sum of calcium and magnesium expressed as mg/l CaCO3. Based on the hardness value, water is said to be soft when it is less than 60, moderately hard when between 61 and 120, hard when between 121-180 and very hard when over 180. In view of the distinctly different scientific meanings attached to the terms – ‘high-TDS water’ and ‘hard water’, it is best not to use them as synonyms and thereby avoid needless confusion.

Plants grow best with rainwater or low-TDS water rather than with high-TDS water. It is not economically viable to treat high-TDS water into low-TDS water for irrigation. Like oceanographers, agricultural scientists also use salinity as a synonym to TDS. If high-TDS water alone is available for irrigation, hard water should be preferred over soft water. Waters with high alkalinity and high ratio of sodium to calcium should be avoided as they enhance soil pH, reduce soil tilth and permeability and impair plant growth. Water with high ratio of calcium to sodium is desirable because calcium flocculates the soil colloids and tends to maintain good soil structure and permeability.

It is however not desirable to spray high-calcium water on leaves of plants as precipitation of calcium carbonate blocks the pores, causing loss of leaf. If drip irrigation is practiced with such waters, the dripper heads have to be cleaned regularly to avoid their blockage by calcium carbonate. The same is true even with domestic pipelines and appliances.

Industrial tolerances for TDS differ widely, but few industrial processes will permit more than 1000 mg/l. In contrast to irrigation, soft water is preferred over hard water in many industrial processes. Although water treatment is rarely practiced for waters used at household and farm levels, water treatment to convert hard water into soft water through ion exchange and high-TDS water into low-TDS water through reverse osmosis are essential in many industrial processes.

Dr. R. Jagadiswara Rao
Former Professor of Geology
Sri Venkateswara University
Tirupati, AP 517502

Dr. R. Jagadiswara Rao, Professor of Geology Retired, Sri Venkateswara University, Tirupati, AP 517502, India


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