Potable water is collected from natural resources like canals, rivers, lakes or aquefiers, processed to remove contaminants in drinking water before being stored and delivered.
Disinfection of drinking water is necessary to remove any microbial contaminants like bacteria, viruses, protozoa, spores, etc. Failure to inactivate microbial contaminants from potable water can lead to Cryptosporidium outbreaks, Giardia outbreaks, cases like recent Nigeria Fowlerii infections, leading to propagation of waterborne diseases.
Drinking water disinfection processes can be divided into
Chemical disinfection (e.g. Chlorine, Ozone)
Non-Chemical disinfection viz: Ultraviolet disinfection of drinking water
Chemical Disinfection of Potable water
Oxidizing chemicals like chlorine and Ozone oxidize the cells of microorganisms and kill them. While chlorine disinfection of potable water has been used for almost a century, if suffers from several limitations.
Limitations of Chlorination of Drinking Water
Chlorine Resistant Microorganisms: Chlorine resistant microorganisms like Crypto, Guardia and many viruses remain largely unaffected by chlorination.
Even increasing chlorine dose requirements: As a part of natural evolution microorganisms mutate into new chlorine resistant strains that require even increasing chlorine dose levels for inactivation.
Disinfection By Products: Chlorine Disinfection Byproducts are formed when chlorine reacts with naturally occurring organic substances and halogen ions in water to form compounds like TriHaloMethanes (THMs), Chlorite, halonitromethanes, haloacetonitriles, haloamides, iodo-acids, Chloramines, etc. Chloramine is found to form N-NitroSodiMethylAmine (NDMA) which is linked to cancers. The type and amount of Disinfection By Products in drinking water depends on several factors including the type of disinfectant chemical used, its dose, time, temperature, pH, water chemistry, etc.
Limitations of Ozonation of Drinking Water
Though Ozone is a much stronger oxidant than chlorine, ozone disinfection of drinking water suffers the same limitations of disinfection byproducts. Ozone is known to form ozone disinfection byproducts like ketones, carboxylic acids, aldehydes & bromate. Many of the disinfection byproducts are potent carcinogens.
Ultraviolet Disinfection of Drinking Water
Germicidal ultraviolet (UV) radiation in UV-C band is proven to damage the DNA of microorganisms like bacteria, viruses, spores, cysts, oocysts, yeasts, fungi, algae, etc. UV disinfection of potable water is rapidly becoming the preferred disinfection method because of several key advantages.
Advantages of UV disinfection include
- No microorganism is known to be immune to sufficient & properly delivered UV dose. Properly implemented ultraviolet disinfection of potable water is proven to be effective against bacteria, viruses, spores, cysts, oocysts, yeasts, fungi, algae, etc.
- No disinfection byproducts
- Non chemical hence inherently safe
- Easy to measure and control online
- Online disinfection without requiring mixing tanks or holding periods
- Availability of UV Dose – response curves for most commonly occurring pathogens
- No risk of overdosing – generally the higher the UV Dose the better
- No need to deliver or manage hazardous chemicals
- Very economical
- No corrosion
- Does not depend on water chemistry, water pH, etc.
Limitations of UV disinfection
- UV disinfection depends on “optical clarity” of the water to UVC wavelength, measured by UVT (UV transmittance) at 254nm. The higher the UVT of water, the more effective the UV disinfection
- No residual effect
UV Disinfection of Potable Water: Process Flow
While there are variations, the most common process for ultraviolet disinfection of potable water is
Micron filtration -> UV disinfection
UV Dose for drinking water disinfection
UV Dose for drinking water disinfection
UV dose delivered is the most important factor affecting efficacy of UV disinfection.
Depending on the main pathogen of concern in the water source, some applications may use UV dose of 300J/m2 (30mJ/cm2).
USEPA and majority of EU authorities recommend minimum UV Dose>400J/m2 (i.e. 40mJ/cm2) at the end of recommended UV lamp life.
Some microorganisms (e.g. viruses) require much higher UV dose to be effective.
Typically UV dose if selected based on the main pathogen/s of concern in the water source.
- Whole house Potable water disinfection
- Mine site Potable water disinfection
- Applications by market …
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