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Water is life – an old statement that no one can disagree with. However, a lot of attention should 
be paid not only to the amount of water available, and its distribution, but also to its quality. Water is around and inside of the human beings; it is in the atmosphere, on the surface of Earth, under the earth, it has different states (from hard to liquid) and the first association that people get when asked to describe water in simple words, is that it has no color, no taste, no smell and that it is liquid. In the modern society water is taken for granted, not any single day can be imagined without water: it is running out of the tab, it is sold in the bottles, it is used in absolutely all industrial processes, in agriculture, and one of the sad and alarming consequences of that is that water is treated badly, contaminated by very harmful substances, wasted in excessive amounts and there are not enough purifying facilities.
Many people know that water is hard or soft, without giving a thought what it is and why it may be different. Hardness is in fact a property of water, which is determined by the presence of calcium and magnesium in it. In fact, it is a quality characteristic and it plays a huge role in assessment of the quality of water supplied for households, industry, technical needs. These elements, calcium and magnesium are not the only ones that are contained in the water, but particularly these two values influence the hardness of water the most. They are known as important for the organisms and from this side, certain amounts of calcium and magnesium in the water used for consumption are even beneficial. Teeth, bones, cells in the body – these parts in the human body do need auxiliary elements, like calcium and magnesium, for firmness and healthy functioning and thus, water becomes a good source of them. From the other side, the International Health Organization defines the optimal amounts of calcium and magnesium which should be in the water in mg per liter, and the limits should not be exceeded to avoid intense accumulation of, for example, calcium, especially for people with health problems. Excessive amounts of salts in the water may lead to the stones formation or cause problems with the joints. Thus, achievement of the most optimal level of mineralization is an important task.
Water quality is continuously monitored at the water treatment plants and the level of hardness is regulated depending on the application. Water sources usually contain different types of water: surface and underground waters, lakes, rivers, precipitation. Depending on which amounts of natural rocks (limestone, gypsum or dolomite) were in contact with the water, the amount of calcium and magnesium may vary and thus, the value of hardness is never the same and may fluctuate depending on the area, weather, season and even amount of precipitation.
Tab water, for example, which is used for the household needs, should not be too hard, since it may lead to the problems with household appliances and pipes. At the first glance it is not possible to detect whether the water is hard or soft, but after boiling or even using it over some time period, for example, in the kitchen or bathroom, it becomes obvious: scale (a kind of crystallization) on the surfaces, or thin film on the surface of water in the kettle. This is an indication of the so-called carbonate hardness (also known as temporary). Calcium and magnesium bicarbonates under the influence of high temperatures turn into a hard, insoluble sediment – the majority of people are familiar with this process. The total hardness of water consists of both, carbonate and non-carbonate hardness and is measured in mol/m3 or mmol/l.
Not only for the human body the optimal level of hardness is of vital importance, but also for all the appliances that are in contact with water, for example, dishwashers, kettles, washing machines, pipes, boilers, heaters. Not in vain in many commercials various means are offered in order to protect the washings machines against the limescale on the working parts and to prolong its service life. Though excessive amounts of calcium and magnesium are very harmful, it does not mean that the softer the water is, the better. Tab water with very low values may also cause damage, like corrosion, to the pipes, that is why maintenance of the balance is so important.
To know the level of hardness of water is important in order to adjust the operation of the devices, to select the correct amount of lime scale cleaning agents and the system of filtration and purification in the best way. To get the detailed water analysis is possible in the laboratory, though in a daily life many people try to adjust by buying additional mediums that help to fight against scale. Application of a TDS meter is a professional approach that may be very helpful, but it provides information about all the solids dissolved in the water, so special training is recommended to use the device in the correct way.
Knowing the fact that in different countries the water hardness is measured in different units may significantly help to read the user manuals correctly and to adjust the household appliances produced abroad in the correct way. Thus, for example, the German carbonate hardness °DH should be converted
It should be noted, that the values do not correspond 1 to 1 and thus application of the calculator is required to read the values correctly.
| Germany | Gr. Britain | France | USA | Russia | |
| °DH | 1 | 1,25 | 1,78 | 17,84 | 0,357 |
| °Clark | 0,80 | 1 | 1,43 | 14,3 | 0,285 |
| °F | 0,56 | 0,7 | 1 | 10 | 0,20 |
| Ppm | 0,056 | 0,070 | 0,1 | 1 | 0,02 |
| °Ж | 2,80 | 3,51 | 5,00 | 50,04 | 1 |
The taste and appearance of cold and hot beverages can be influenced by water hardness. Flavoured beverages usually contain organic components that can react with the ions causing the water hardness. According to today's definition, the organic compounds in the beverages are no longer characterised by the fact that they are of organic origin, i.e. that they are produced by living organisms. The organic compounds also include technically produced carbon compounds that do not occur in nature. However, the carbonic acid HCO3 and its salts contained in many cold drinks are classified as inorganic compounds.
Surely everyone has heard that tea connoisseurs prefer soft water for infusing their tea for the reasons of taste. The leaves of the real tea, from which green and black teas are made, contain about 4,000 different organic compounds. A cup of black or green tea therefore contains a large number of organic compounds even without the addition of other substances such as sugar or milk. These include, for example, caffeine and tannin, but also flavonides, amino acids and vitamins B, C and E. In both coffee and tea, through the higher number of the substances dissolved in hard water more flavour-forming acids can be neutralized, so that the flavour of the drink changes. Coffee or tea therefore taste different, despite the same type and otherwise the same preparation, if water with a significantly different degree of hardness is used.
The water hardness of the added water also influences the taste of fruit nectars, fruit juice drinks and lemonades. The ingredients of the water, the concentration of which is indicated by the degree of hardness, primarily cause the weakening of the acids added to the drink and thus also the ratio of acidity to sweetness. To ensure that there are no major fluctuations in the taste or mineral content of a beverage, the water used is regularly checked with a water hardness meter.
In the case of beer, the water hardness determines not only the taste, but also the ingredients and 
the course of the brewing process. Here, it is not only the total hardness that matters, but also the individual hardness constituents. Calcium ions intensify the activity of the starch-splitting enzymes in the mashing process, while magnesium ions in higher concentrations cause a bitter taste. The carbonate hardness, which is also given in degrees of German hardness, only takes into account the calcium and magnesium ions for which a corresponding number of dissolved hydrogen carbonate ions are present in the brewing water. The hydrogen carbonate ions cause an increase in the pH value and can thus hinder the digestion of the starch. In addition, when the carbonate hardness is too high, this can lead to the undesirable bitterness in the beer. In order to optimise the brewing process and the desired taste, not only the total hardness and the carbonate hardness of the brewing water are determined with the water hardness meter, but also the residual alkalinity and the pH value.
Wastewater from households, public institutions and enterprises is usually treated and clarified in sewage treatment plants in several stages. The purified water should be free of harmful substances when it is returned to the natural water cycle. So that the chemical and biological purification processes can be optimized, the wastewater is tested in the treatment plant for the content of certain substances.
The parameters measured include water hardness, pH and alkalinity. The pH value, which is buffered against acids by high alkalinity, plays an important role above all for the microorganisms in aeration basins and digestion towers, but also in the natural waters into which the treated wastewater is discharged. The alkalinity of the wastewater, which can also be referred to as the carbonate hardness, is therefore determined at various measuring points in the wastewater treatment plant with a water hardness meter.
Many wastewater treatment plants use digesters to thicken and stabilize the sewage sludge and to recover energy from the methane produced. In the digesters, the sludge is constantly mixed under anaerobic conditions at a temperature of about 35 to 40 °C by stirring or pumping. Different species of bacteria decompose many, but not all, of the organic components of the sewage sludge under suitable conditions.
The development of the different bacterial strains in the digestion tower is influenced not only by the food supply, but also by the environmental conditions. Therefore, during the digestion process, both the temperatures are monitored and the pH and alkalinity of the sewage sludge are measured. The pH value to be aimed at for the digestion process is pH 6.7 to pH 7.5. If the pH value deviates from this, complete sludge digestion no longer takes place because some of the bacteria required for this die off.
The pH value does not drop due to the acids produced during digestion if the sewage sludge has sufficient buffer capacity for acids. By enriching the sewage sludge with calcium in the form of quicklime, hydrated lime, chalk or dolomite, the acid capacity can be increased and the pH – stabilized. To ensure that these additives can be dosed as required even if the composition of the wastewater varies, the alkalinity must be checked regularly with the water hardness meter or other suitable measuring technology in addition to the pH value.