This is the sum of the dissloved compounds-hardness salts. treatment chemicals e.c.t. in the water. Its importance has been recognised over the years as giving an indication as to whether the pool is becoming saturated with chemicals in one form or another, and hence the length of time the water has been in the pool. TDS is measured electronically as the conductivity of the water to which a factor is applied (usually about 0.7) to convert it to TDS in mg/l.
A popular recommendation for the maximum TDS in a pool is 1000 mg/l above the feed (mains) water. So if the feed water has a TDS of 400 mg/l, the maximum for the pool should be 1400 mg/l. TDS can only be reduced by dilution and this must be carried out as a matter of priority if the water reaches the absolute maximum of 3000 mg/l, as at this level the water can be :
• salty to taste
• conductive – to produce corrosive conditions
• dull in appearance
The Total Dissolved Solids content of pool and spa water is a measure of the total quantity of solid material dissolved in it. In mains water this comprises of hardness and other natural salts and the level will depend on the source of supply, but is within the region of 50 – 500 mg/l.The TDS level will gradually increase in a pool due to evaporation and concentration of hardness salts, impurities introduced by the natural elements, wind and rain, and the chemicals added to the water as part of the treatment process – chlorides and sulphates for example. 46 Pool & Spa Water Treatment and Analysis 10/2013 The real value of TDS measurement is that it can indicate whether too many chemicals have been added as a result of heavy bather load or lack of dilution and the water is becoming ”stale”.
It should be monitored by comparison between the pool and the mains feed water to the pool. TDS should ideally not be allowed to rise more than 1000 mg/l above the feed water, up to a maximum of 3000 mg/l Should it become necessary to reduce the TDS level, this is carried out by replacing some of the water in the pool with fresh water. In some pools a satisfactory TDS level can be maintained by regular backwashing of the filters. Measurement is by electronic meter which is really taking a conductivity reading of the water and applying an internal factor to display the TDS in mg
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Source: Lovibond Handbook