Disinfection

The main goal of disinfection is to destroy - or at the very least inactivate - pathogenic micro-organisms likely to adversely affect the health of water consumers.


The term "sterilization" is sometimes used to refer to disinfection in the water treatment process, but this is incorrect.


This is because drinking water still contains some germs (non pathogens) which are harmless to consumers, whilst strictly speaking sterile water does not contain any germs at all.


Disinfection - which is usually the last link in the chain in the drinking water system - must allow for the provision and maintenance of bacteriostactic residue in the water distribution system in order to prevent the regrowth of micro-organisms and the resulting deterioration in water quality.


Even today contamination from micro-organisms is still one of the main causes for the non compliance of certain water supplies around the world.

The different processes used to disinfect water destined for human consumption can be divided into two categories - physical and chemical processes.

Physical processes



Physical processes are some of the oldest water treatment processes around, e.g. distillation and pasteurization, which are only used for specific applications with low flow rates due to the costs involved.


The physical disinfection processes used for water treatment on a large scale are as follows :


  • U.V. disinfection.

  • membrane filtration (ultra-filtration, nano-filtration, reverse osmosis) which, although they are not disinfection processes, retain 5 log or more germs

Chemical processes

Zoom



Chemical processes are used far more widely than physical processes and rely on the effect of oxidizing reagents.



Ozone was the first oxidizing germicidal agent to be used in the treatment of drinking water at the end of the 19th century (1886).

However for economic reasons, chlorine gas and its oxygen derivatives (hydroxyl ion and chlorine dioxide) were used and then more widely developed at the beginning of the 20th century.

Chloramination, when ammonia and chlorine are used together to disinfect the water, is also one of the chemical disinfection processes to be found in drinking water systems.