Filtration and/or coagulation-flocculation systems are extremely effective for removing all types of particles including pathological organisms, however this is not their main function. Disinfection is the elimination or inactivation of pathological and other microorganism using chemical or physical procedures. Nowadays, water standards require a 99.9% inactivation of giardia cysts and viruses. Technically there are two stages of disinfection:
Primary Stage: At this stage, the desired level of microorganism removal is achieved.
Secondary Stage: This stage is used to set and maintain residuals of the disinfectant in order to keep the water free of microorganisms.
Numerous techniques can be applied for disinfection. Historically, chlorine compounds have been quite popular due to is high efficacy, ease of use and low cost. The following are some of the most common disinfectant agents:
Chlorine: Chlorine is an extremely reactive element so it always comes in compounds such as sodium hypochlorite or calcium hypochlorite. These chemicals react with water to form hypochlorous acid, which is the true disinfectant agent. All chlorine applications should be properly designed in order to reduce excessive amounts of residuals, which can be toxic.
Chlorine Dioxide: Chlorine dioxide presents the advantage of being less ph sensitive than typical chlorine. Chlorine dioxide has been commonly used as a bleaching agent in pulp and paper industry, however is not very popular for water and wastewater treatment.
Ultraviolet Interactions: Of all physical means of disinfection, UV radiation is becoming quite accepted in the water treatment industry due to its high efficiency and low maintenance. UV radiation can be produced by the use of antimony vapor or mercury vapor lamps.