THE ROLE OF TOTAL DISSOLVED SALTS (TDS) IN ASSESSMENT OF WATER QUALITY
The only information obtained from a TDS meter or measurement is, as the name implies the following:
· TDS is a measure of the Total Dissolved Salts in the water and sometimes also incorrectly stated to measure the Total Dissolved Solids.
· The correct scientific definition of the TDS measurement is in fact the determination of the Total Dissolved Salts.
· The important information obtained from this analytical measurement is very plain and simple, how much mineral salt is dissolved in the water under investigation.
· No other information can be obtained or deduced from the simple TDS analytical test.
Taking this into consideration it is now important to know the following:
ð What amount, what concentration or what level of mineral salts, normally found in water, is the correct or safe amount?ð Is there a maximum level of mineral salts in water, which when exceeded, makes the water unfit for humans consumption?
ð Is there a minimum level of mineral salts in water which when absent makes the water unfit for human consumption?
These basic questions were answered by the World Health Organization (WHO).
The World Health Organization have seriously been doing research on what can be considered or regarded as good quality drinking water since 1946. The study was in fact launched as a result of problems encountered by the allied forces, stationed in numerous areas during the second world war, where suitable water for general potable use, was in short supply or simply not available. The same study is today still in progress and over the years adapted and refined to include all general data made available from all over Europe and the USA. The results of this study directly produced what is today commonly known as the “WHO drinking or potable water standards of 1993”, which provides minimum and maximum limits of mineral salts normally found in water.
This study highlighted three very important points to take into consideration, when determining the quality of any water source, to be used as drinking water:
1. A minimum of 20 analytical parameters or constituents present in the water must be measured before a safe assessment or a secure deduction can be made about the quality of the water source under investigation. (Only the uninformed fool, will make an assessment about water quality, by examining or looking at the TDS measurement alone and ignore the rest of the analytical parameters of interest).
2. The WHO stipulated the minimum analytical requirements for secure quality assessment to be the following:
Table-1: Analytical data required to assess water quality.
ð Total Dissolved Salts TDS @ 105 °Cð pH
ð Turbidity
ð Colour
ð Total Alkalinity
ð Calcium
ð Magnesium
ð Iron Total
ð Total Heavy Metal Content / ð Sodium
ð Chloride
ð Sulphate
ð Nitrate
ð Nitrite
ð Total Suspended Solids
ð Dissolved Organic Carbon
ð Total Microbial Count (Colonies per ml)
ð Total Coliforms (Colonies per 100 ml)
ð Total Faecal Coliforms (Colonies per 100 ml)
ð Total Macro-biological Count (Org per 100 ml)
Please take note that TDS is but one of the 20 analytical parameters required to gauge the quality of a water source.
3. Both minimum and maximum limits stipulated in the “WHO drinking or potable water standards of 1993” must be considered during interpretation of the information.
Since the analytical TDS test is discussed in this report and used to correlate the quality of a water source, the World Health Organisation obtained the following conclusive and relevant data:
Table-2: Potable or Drinking water – TDS limits.
If TDS is below 20 mg/l / The mineral content of the water source is too low and should be increased via standard water treatment procedures.If TDS is between 20 and 450 mg/l / The mineral salt content appear to be ideal and final water quality will depend individual mineral spread.
If TDS is between 450 and 1000 / The mineral salt content is high and important to examine individual mineral spread in greater detail
If TDS is greater than 1000 mg/l / The mineral salt content is too high and should be reduced to conform to the recommended standard
A more detailed description of how TDS as an analytical parameter, influence the water quality is given below Table-3:
Table-3: TDS and related parameters linked to corresponding water quality.
TDS (mg/l) / DRINKING / Food Preparation / Bathing / LaundryHealth
/ Aesthetic< 20 / Long-term slow loss of minerals / Tastes Good / Loss of nutritional content / Rinsing Problems / Rinsing Problems
20 < TDS < 450 / No Problems / Tastes Good / No Problems / No Problems / No Problems
450 < TDS < 1000 / Insignificant effect on sensitive groups / Reasonable / Insignificant effect on sensitive groups / No Problems / No Problems
1000 < TDS < 2400 / Salt overload in sensitive groups / Distinctive unpleasant salty taste / Salt overload in sensitive groups / Lathering Problems / Lathering Problems
2400 < TDS < 3400 / Health risk to all individuals / Tastes extremely salty / Health risk to all individuals / Impaired soap lathering / Impaired detergent lathering
TDS > 3400 / Dehydration / Tastes extremely salty & bitter / Dehydration / Negates the use of soap / Negates the use of detergent
Note that with TDS data available, no further deductions about water quality can be made, other than those described above.