Ozone Drinking Water
Ozone has been used in drinking water plants since 1906 where the first industrial ozonation plant was built in Nice, France. Ozone as a viable process train is gaining more importance as the sources of pristine raw water are dwindling. Source waters from aquifers have to be replaced with surface waters. Without exception, these surface waters are contaminated with a varying degree of naturally-occurring dissolved organics, synthetic chemicals, and often naturally-occurring metals – sulfides, nitrites, and arsenic.
The most popular application of ozone in water treatment today is found in drinking water applications. The largest ozonation systems have been built for municipalities treating surface waters. In certain instances, ground water as a raw water source has also been ozonated. Applications with ozone which recycle the water are swimming pools and cooling towers. The challenge is to minimize in a recycle environment the amount of makeup water and continuous disinfection.
In water treatment applications with ozone which require ozone gas concentrations between 7 and 15 percent, oxygen is used as feed gas. Also, ozone dosages of up to 100 ppm are common. The industries utilizing high concentrations and large ozone dosages can be found in the industrial wastewater treatment arena, and also in the treatment of landfill leachate. Ozone as a bleaching agent has been accepted in the pulp and paper industry.
The U.S. Food and Drug Administration has accepted ozone as being safe; and it is applied as an anti-microbiological agent for the treatment, storage, and processing of foods. How powerful ozone exhibits disinfection properties can be seen by comparing the 3-log inactivation efficiency of giardia cysts with chlorine. The EPA has established guidelines for the CT values. Assuming a pH of the raw water of 7 and a water temperature of 15oC, a free chlorine residual of 1 ppm would require a CT value of 75. Treating the same water with ozone at 1 ppm would require a CT value of .95. The cost to build these structures for a large retention time will make a significant contribution to the overall capital outlay of the water treatment plant. There is also some serious concern whether chlorine at all will be able to disinfect cryptosporidium. Cryptosporidium will not be a disinfection barrier for ozone.
Oxidation of metals such as iron and manganese is quick and efficient over a wide pH range. Iron in the ferrous state will be rapidly oxidized by ozone to the ferric state whereby gelatinous ferric hydroxide will precipitate. Soluble manganate occurs in the divalent form and will be oxidized by ozone to the tetravalent form also very rapidly. Tetravalent manganese hydrolyzes, and it will produce insoluble manganese oxydehydroxide which can be precipitated over a filter bed.
A word of caution: Over-ozonation will oxidize the tetravalent manganese to soluble permanganate which will give the water a purple color. The permanganate, however, can be reduced to the tetravalent manganese by running the water through a carbon filter.
Nitrite ions can be oxidized with ozone also very rapidly, forming nitrate ions which are stable and soluble.
Hydrogen sulfide which can give the water a foul odor (rotten egg) can quickly be oxidized by ozone in various stages, the last stage being the sulfate ion. Sulfates are innocuous and, in many cases, can be filtered. Another important oxidation with ozone is for arsenic. However, it is important that the raw water contains magnesium or calcium cations. In the presence of these ions, ozone will produce insoluble magnesium and/or calcium ortho-arsenates which will precipitate.
In general, ozonation can be accompanied by and made part of the process through microflocculation. This is the phenomenon by which organic material, when partially oxidized, becomes more polar. Since the raw water always contains polyvalent cations, like calcium, magnesium, iron, aluminum, and manganese, these organic polar groupings combine with the polyvalent cations. Complex materials are being formed which are insoluble and can readily be filtered.
Turbidity will also be affected by ozonation. By definition, turbidity forms through the suspension of microscopic particles or colloidal particles which can be organic or inorganic in nature. These particles often have the same electrical charge – namely being positive. Ozone has a negative charge, and upon reaction, the particles are neutralized and will precipitate.
In summary, when the ozonation process train is properly engineered and designed, water quality can be achieved which, without exception, cannot be matched with any type of chemical treatment.