What can we find in water and how can we check its quality?

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Water quality refers to its chemical, physical, biological, and radiological characteristics. When we talk about water, we are talking only about water intended for human consumption.

The substances that can be found in water can be divided into three categories:
1. Suspended solids
2. Chemicals (including radioactive ones).
3. Micro-organisms

The chemicals can be categorized into:
1. Disinfecting agents
2. Organic compounds (that is carbon compounds)
3. Salts & Ions
4. Heavy metals
5. Dissolved gases

The problematic (or non-problematic) substances that can be found in water are therefore classified into 7 categories:
1. Turbidity & suspended solids such as mud, stones, soil, asbestos fibers, rust, suspended particles etc.
2. Disinfecting agents such as chlorine, etc.
3. Organic compounds such as polycyclic aromatic hydrocarbons, trihalomethanes (THMs), pesticides, volatile compounds that impart an unpleasant taste and odour in water (VOCs, phenols) etc.
4. Microorganisms such as e-coli, enterococci, coliforms, viruses, etc.
5. Salts & Ions such as water hardness (calcium and magnesium salts), chloride, nitrate, nitrite, sodium, sulphate, ammonium, boron, etc.
6. Metals-heavy metals such as iron, manganese, chromium, cadmium, lead, arsenic, mercury, nickel, aluminium, etc.
7. Dissolved gases such as hydrogen sulphide, carbon dioxide, dissolved oxygen, etc.

The Council Directive 98/83/EC of the European Union, sets limits for each chemical, microbiological and organoleptic parameter (odour, colour, taste and turbidity) relating to water intended for human consumption. There are, of course, also substances for which there are no limits (for example, hardness of water, which we analyse in another article).

Water quality indicators

In order of course to be fully aware of the quality of a water, we will need to carry out a chemical analysis of all the elements (monitoring) listed in the 98/83/EC directive, employing an accredited chemical laboratory. Accreditation shall ensure the reliability of the results obtained during laboratory analysis.

However, there are two (2) indicators that capture a general picture of water quality (in terms of chemical substances), which are measurable using specific instruments called Conductivity & TDS meters. These two (2) indicators are:

1) Conductivity: This is the total ions (anions and cations) present in the water and measured in μSiemens per centimeter (μS/cm). In deionized water, the conductivity value is below 50 μS/cm (remember that deionized water is a poor conductor of electricity). Water exceeding 2,500 μS/cm is considered unfit for human consumption.

(2) Total dissolved solids (TDS): This the total of dissolved solids, including minerals, salts or metals, dissolved in a given volume of water, expressed in mg units per liter of water (mg/L), also referred to as parts per million (ppm). TDS is measured using TDS meters, by reference to conductivity using as a basis a factor of 0.5 or 0.7. It is recommended that the measurement be made using the 0.5 factor (if this is possible by the measuring instrument). The measurement of TDS (as well as of conductivity) is influenced by water temperature. In the United States, there is a (indicative) limit for TDS which is 500 mg/L (ppm). Some TDS are beneficial to health (for example, calcium, magnesium, etc.), while others are harmful if their values are out of range (for example heavy metals, nitrites, nitrates, etc.).
In addition to the two (2) indicators above, the hydrogen ion (pH) concentration expressed in pH units should also be measured. The measurement is made using pH meters and the limits for water to be considered suitable for human consumption should be between 6.5 and 9.5.
TDS and pH relate not only to the quality of water intended for human consumption but also to its taste.
Typical TDS values in water types range (approx.):
• from 0-20 ppm in deionized water
• around 130 to 300 ppm for water in large urban cities
• at 130 to 300 ppm in bottled water
• and up to 5000 ppm in mains waters across Greece.
• Values from 5.000 and above are found in brackish drilling waters,
• ending at even 25,000 to 35,000 ppm for sea water.
Even if water is considered fully suitable for human consumption on the basis of chemical analysis, if it exceeds 300 ppm, it may not be potable due to its taste. On the basis of an investigation by the manufacturer through focus groups and research conducted at times worldwide by similar companies in the industry, the following conclusion was reached:

Potable water is acceptable in terms of flavour (although taste limits are subjective) when TDS are between 100 and 250ppm (approximately) and pH between 6.5 and 9.5.

Panos Nanopoulos

Panos Nanopoulos


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