Disinfection of Drinking Water

	Chlorine	ClO2	Ozone	UV
Disinfection capacity	medium	strong	strongest	medium
Depot effect	hours	days	minutes	none
pH dependence	extreme	none	medium	none
By-products	THM, AOX	Chlorite	possibly Bromate	possibly Nitrite
Investment	low - high	medium	medium - high	medium
Maintenance	medium	medium	low	low

Chlorination

For chlorination, chlorine gas, sodium or calcium hypochlorite is added to the water. The quantity required depends on the chlorine demand of the water and the disinfection requirements. For drinking water there must be a minimum residual concentration of 0.1% mg/l chlorine. A reaction period of at least 20 minutes is also required for effective disinfection. The efficiency of the chlorination is extremely dependent on the pH value of the water. Particularly in organically contaminated water odor and taste of the water can be impaired and undesirable by-products such as chlorinated hydrocarbon can be generated. Avoiding bacterial aftergrowth by Chlorine excess is often overestimated, as due to the inevitable Chlorine loss in the supply network, the sufficient Chlorine concentration is quickly depleted.

Chlorine Dioxide

Chlorine dioxide is a means of disinfection that thanks to its multiple advantages replaces chlorine in more and more applications. Its effect is stronger and - above all - independent of the pH value of the water. Due to its chemical qualities it does not produce chlorine by-products. The longer half-life period helps create a better depot effect in the treated water. For drinking water a minimum residual concentration of 0.05 mg/l chlorine dioxide at a reaction time of at least 15 minutes should be observed. It is remarkable that chlorine dioxide - contrary to chlorine, degrades biofilm in pipelines and containers and thus renders growth of legionella impossible.

Ozonation

Ozone is the strongest means of oxidation and disinfection that can be used for water treatment. Its major advantage is that no undesirable by-products are generated and ozone itself decomposes to oxygen. Its disadvantage is a short half-life period and weak solubility in water. In drinking water disinfection, it is mainly used for disinfection if any type of additional oxidation e.g. for decolorization, deferrization, demanganization or for the degradation of organic substances such as e.g. humic acids is required. In Germany, Ozone may be used for oxidation purposes only.

Ultraviolet Disinfection

For ultraviolet disinfection, the water to be disinfected is exposed to short wave ultraviolet light. This is an effective germicide that does not affect the water quality. UV disinfection does not provide depot action in the pipe network, however, UV radiation outmatches chemical disinfectants by far in combating parasites such as Cryptosporidium or Giardia.

Silver Ion Sterilization

For silver ion sterilization, silver ions are added to the water in a concentration of 0.05 to 0.1 mg/l. Silver ions have germicidal effect (oligodynamic). However, the correct coherence is not known. A residence time of several hours is required. Nowadays, the application area for this process has been reduced mainly to drinking water disinfection on ships or for essential supply with drinking water in disaster areas.

Sterile Filtration

High costs mean that sterile filtration is largely confined to medical or pharmaceutical applications. This process uses ultra filtration membranes with pore sizes of < 0.5 µm, that must be disinfected at regular intervals. Simple ultra filters are also used in small drinking water filters for domestic use. However, there is always the risk of germs growing through the filters. Silver ion sterilization does not provide long-term protection against germ growth either. 

Slow Sand Filtration

Slow sand filtration with filter speeds of ca. 0.1 m/h achieves a significant reduction of the microbial count. Due to the large filter surface required, the use of slow sand filtration in drinking water treatment decreases constantly. For problematic raw waters, additional disinfection must take place in many cases.