Disinfection of Potable Water

Disinfection of Potable Water

A steadily growing world population is facing a resource that is becoming constantly scarcer: hygienicly clean potable water. This scarce resource may soon become a luxury item. However, there exists a large range of procedures to disinfect and render water potable. ProMinent is competent with virtually all availables potable water treatment technologies.

 

disinfection of potable water

Often, raw water containing bacteriological impurities, e.g. surface water or bank-filtered water, is used in potable water recovery processes. Disinfection is an absolute necessity here. The selection of a suitable method for potable water disinfection must be adjusted to local requirements in order to afford highest microbiological safety and lowest risk of undesirable disinfection by-products. Moreover, local regulations permitting only specific disinfectants for use in potable water must be observed.

Overview of Potable Water Disinfection Processes

 

 

Chlorine

ClO2

Ozone

UV

Disinfection capacity

medium

strong

strongest

medium

Depot effect

hours

days

minutes

none

pH value 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

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 potable 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 potable 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 potable water disinfection on ships or for essential supply with potable 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 potable 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 potable water treatment decreases constantly. For problematic raw waters, additional disinfection must take place in many cases.

 

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.

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