For the separation of suspension, the same technological devices are used as for separation of suspension resulting from the coagulation of surface water. For the oxidation of iron, atmospheric oxygen and chlorine (at smaller treatment plants sodium hypochlorite) are utilized. For the oxidation of manganese, which is more resistant than iron, potassium permanganate is used almost without exception.

To saturate water with air oxygen, mostly two types of aeration are used: packed stripping aeration towers and/or bubble plate devices (aeration of water through the perforated bottom). The former procedure is quite effective from an chemical engineering point of view.

When designed properly, it enables to achieve thermodynamical equilibrium in reasonable residence times. However, this process may fail when higher concentrations of manganese and especially iron are present in the raw underground water. Ferric hydroxide can clog the package of the aeration tower. In this case, usage of bubble plate devices is reasonable, even though less effective.

The same aeration devices are used for the removal of gases from underground water, e.g.: radon, hydrogen sulfide, or aggressive carbon dioxide. Volatile bad soluble non-polar organic contaminants (chlorinated hydrocarbons, BTX aromatics) can be removed in this way, too.

The phenomenon of corrosion is also another important factor. The stabilization of water ususally includes the removal of aggressive carbon dioxide and the increase of alkalinity (concentration of bicarbonates) in water. When only removal of CO2 is required, it can be done by mechanical processes as discussed above, or by chemical reaction with calcium hydroxide and/or lime stone, dolomite or thermally treated dolomite.

The latter processes also increase the concentration of bicarbonates in water. If a bigger concentration of bicarbonates in water is needed to insure its stability, "water hardening" by means of dosing of gaseous carbon dioxide and calcium hydroxide is applied.

As for ammonia in underground water, two procedures are most frequently used -break-point chlorination and biological nitrification. The latter process is preferred, since the former requires high doses of chlorine (hypochlorite) and subsequent dechlorination.

The biological process consists of aeration of raw water and biological nitrification in filters. As a filtration medium, sand covered with manganese oxides is utilized. The manganese oxides layer serves as a rough support for nitritying bacteria. The bacteria are firmly attached on the surface of sand grains and are not removed from the filter during its backwashing.

Nitrites can also be removed from underground water by biological nitrification. Like all over the world, nitrates in sources of underground waters cause a problem that has not been solved so far. Water treatment technologies to decrease concentration of nitrates in drinking water are expensive, complex, and sometimes not reliable enough in practice (particularly biological methods).

Ion exchange technologies produce concentrated exhausted regeneration solutions (brines) that may cause serious environmental problems. The solution of the "nitrate" problem lies in resource and water management as well as the policy in the aquifer region, rather than in water treatment process. Disinfection is the important and usually the last technological step. Like in most countries of the world, chlorine or sodium hypochlorite are most frequently used as disinfecting agents.

Chlorine is preferred in larger treatment plants, hypochlorite in small waterworks (dosing of hypochlorite is easy, but the technological-grade solution contains only 14% of chlorine, which is disadvantageous for greater consumption in larger plants).

Ozonization is generally installed in greater treatment plants or in areas with microbiologically worse raw water quality. This technological step has a long tradition in our country as the first ozonization unit on our territory was put into the operation as early as in 1916. Chlorine dioxide and UV-irradiation have been introduced in a full scale during the last decade.

Information & Contact

Dr. Frantisek Kozisek
National Institute of Public Health
Srobarova 48
100 42 Praha 10