Water is omnipresent on Earth as it covers 72% of its surface (97% salt water and 3% fresh water). Water is the living environment for many species and is necessary for all life to thrive and survive. It is also a major part of human economy, for it can be used as a solvent, washing agent, cooling agent, or raw material for industrial purposes.

Water is liquid at ambient temperature and pressure, colorless, odorless, tasteless, and neither volatile nor toxic. Water molecules are composed of one oxygen atom and two hydrogen atoms (H2O). Despite its apparent simplicity, water possesses exceptional physical and chemical properties that explain why it is involved in all major industrial processes.

In actual fact, pure water does not exist. Different types of water can be found: bottled water, spring water, mineral water, natural mineral water, urban waste water (UWW), industrial waste water (IWW), distilled water, demineralized water, permuted water, and milli-Q® water.

Each type of water is a complex solution containing, in addition to water molecules:

  • Ionic mineral species less than 1 nm in size. We generally distinguish metal ions (Ca2+, Mg2+, Na+, Fe2+) from halide ions (F-, Cl-), sulfate ions (SO42-), nitrogeneous substances (NO3-, NO2-, NH4+), phosphoric substances (orthophosphates like H2PO4-, HPO42- and PO43- ) and carbonaceous substances (HCO3-);
  • Oxidizable organic matter (O2, CO2, proteins, lipids, carbohydrates, carbonaceous or nitrogenous organic matter) and inorganic oxidizable matter;
  • Settleable suspended matter (sand particles, organic debris) or non-settleable (colloidal particles, limes, microscopic organisms). “total suspended solids” (TSS).
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