The most important values to be evaluated are: the determination of pH, salinity, hardness, the research of undesirable factors and of toxic substances.
- As regards the pH of water, it is generally neutral or slightly acidic. Too high or too low values could be dangerous: too acidic waters (especially if there is aggressive carbonic acid) or alkaline waters (especially in case of ammonia) put into metal pipes can be aggressive and solubilize some components that affect the palatability of water (iron, manganese, zinc) or involve a veritable toxic danger (lead, cadmium).
- Salinity expresses the total amount of salt in the water. The presence of salt is essential for drinking water, since the human body absolutely needs a certain amount of it in the water. The absence of salts alters the activities of osmotic exchange that occur in gastrointestinal cells and causes nutritional deficiencies (bone and dental alterations). Excessive amounts of mineral salts is also damaging, because it both makes the water too hard and can alter its organoleptic properties. The recommended maximum value of salinity (determined at 180° C) is 1500 mg per liter.
- Water's hardness is determined by the concentration of alkaline earth salts, mainly calcium and magnesium. There is a "temporary" hardness, caused mainly by the presence of bicarbonate minerals and minimally by phosphates and silicates, and a "permanent hardness", caused by the presence of chlorides, sulphates and nitrates. Temporary hardness disappears after prolonged boiling. Permanent hardness doesn’t disappear with boiling. Water hardness is expressed in degrees of hardness. There are three systems: French, German and English degrees.
The most widely used unit is the French one. Waters are divided into:
- Soft water, if its hardness is less than 14 French degrees;
- Moderately hard water, in the range of 14 and 28 French degrees;
- Hard water: over 28 French degrees.
The recommended values are 15-50 French degrees.
Hard water has significant drawbacks: it often has abnormal organoleptic properties; it is not suitable for cooking, as it cooks legumes badly and alters tastes, especially of tea and coffee; it requires a considerable waste of soapy substances when doing laundry and forms deposits in pipes, tanks and boilers.
Water's hardness is closely linked to soil composition and to possible causes of pollution.
It is also important to determine the presence of ammonia, nitrites and nitrates, as they indicate a degradation of nitrogenous organic substances in the water.
Ammonia and nitrites can be chemically and biochemically oxidized. Their presence, when they don't derive directly from industrial waste, is a sign of an ongoing degradation of organic materials. Nitrates, which are stable, reveal a remote pollution caused by agricultural waste.
In deep waters, the presence of phosphates and hydrogen sulfide is not a good sign. Phosphates are present in liquid discharges of domestic origin, while hydrogen sulfide derives from putrefactive degradation of organic substances.
- As regards undesirable factors, they are elements and substances that do not enter into the composition of good drinking water and which can alter its quality, making it unpalatable or unsuitable for household and industrial purposes, without, however, reaching important levels of toxicity.
Among these elements there are iron and manganese, which cause abnormal colours, turbidity and precipitates, copper and zinc that can cause astringent tastes, abnormal colors, opalescence, mineral oils, surfactants, phenols (industrial contaminants) and halogenated organic solvents.
- Toxic substances include elements and compounds having their own toxicity, whose presence in the waters is due to pollution of different origin: seepage of liquid discharges, solubilization of metals of pipes etc. They include arsenic, cadmium, cyanides, chromium, nickel, lead, antimony, selenium, pesticides and polycyclic aromatic hydrocarbons.
Microbiological control aims to ensure that no water is or may become a vehicle of transmission of pathogenic microorganisms. The most common diseases caused by microbiological pollution of water are: typhoid, paratyphoid fever, dysentery, cholera. It can also occur that water is the vehicle of viruses (enteroviruses, hepatitis A, polio virus), protozoa and pinworm eggs (tapeworms, roundworms).
For this reason, microbiological indexes are used, such as the determination of total viable count (at 22 and 36° C) and the indexes of fecal contamination.
- Total Viable Count gives an idea about the quality of water. The count at 22°C informs us about the quantity of microorganisms in the water; the count at 36° may indicate the presence of bacteria which can live on warm-blooded animals, in conditions of symbiosis or parasitism.
- A more specific index is fecal pollution, consisting in the research of fecal and total coliforms.
Fecal coliforms are only of fecal origin, the most common member being Escherichia coli.
Total coliforms are mostly of the same origin but, in suitable conditions, they can multiply on non-animal substrates such as plants and soil.
The limit (absence in 100 ml of water) allows wide margins of safety.
Other signs of fecal contamination are enterococci (fecal streptococci) and sulfite-reducing clostridia (spores).
- Enterococci have less resistance in water than coliforms; their presence indicates a recent pollution.
- The spores of sulfite-reducing clostridia can withstand many months; their presence, if there are no other signs of fecal pollution, indicates a remote pollution.
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