Potable Water Disinfection
As we all know, potable water – i.e. drinking water, comes from natural sources. For example, most of our drinking water comes from rainwater collected in dams, canals, rivers, lakes, aquifers, or rainwater tanks. Rainwater runoffs from many sources pick up contamination of different types along the way. Waterborne bacteria, viruses, cysts, spores and similar microorganisms are indeed responsible for many contagious diseases. For example, Cryptosporidium outbreaks, Giardia infections, or cases like recent Naegleria fowleri infections can be traced back to microorganisms in water.
As a result, we must adequately treat water to make it safe to drink.
Potable water treatment includes multiple steps to remove impurities.
Disinfection is a key step to make water safe to drink.
Disinfection is the process of inactivating microorganisms like bacteria, viruses, protozoa, spores, cysts and the like to make water safe for potable uses.
We can divide drinking water disinfection processes can into
- Chemical disinfection (e.g. Chlorine, Ozone) and
- Non-Chemical disinfection viz: Ultraviolet disinfection of drinking water
Chemical Disinfection of Potable water
Chlorine and Ozone are strongly oxidizing chemicals. These chemicals oxidize the cells of microorganisms and kill them. In fact, Chlorine is quite popular for water disinfection for almost a century. It looks cheap, oxidizes bacteria, has a residual effect and has been around for a long time, after all.
Is chlorine disinfection perfect? Nope!
Limitations of Chlorine Disinfection of Drinking Water
Chlorine has many limitations:
Chlorine Resistant Microorganisms
Firstly, some microorganisms simply can’t be inactivated by Chlorine. It took a few cases of Cryptosporidium outbreaks and Giardia infections before this limitation of chlorination was widely accepted . Yes, that’s right: some microorganisms like Crypto, Guardia and many viruses remain can’t be controlled by chlorination.
This is a major and serious limitation of chlorination. After all, what use is a disinfectant if it can’t protect us from waterborne diseases?
Disinfection By Products
Secondly, Chlorine is essentially an oxidizing chemical. As a result, it tries to oxidize whatever substances it comes in contact with. This can includes microorganisms as well as many types of organic and inorganic impurities in water.
When Chlorine reacts with the many types of impurities in water, it forms a Disinfection By Products or DBPs. Disinfection byproducts can be grouped into chemicals, e.g: TriHaloMethanes (THMs), Chlorite, halonitromethanes, haloacetonitriles, haloamides, iodo-acids, Chloramines, etc. Regardless, many scientific studies have linked disinfection byproducts to a wide range of adverse health effects.
Change of pH, Taste, Smell, Overdose, OH&S Risks
Chlorine changes the pH of water. Even a small amount of overdose causes change in taste and smell of water. Accidental overdosing with chlorine can be dangerous.
Handling Chlorine may create additional OH&S risks. To make matters worse, chlorine promotes corrosion. As a result, the equipment, structures that come in long term contact with chlorinated water need to be replaced every few years.
If we calculate lifetime cost of Chlorine disinfection, it works out to be quite expensive!
Limitations of Ozone Disinfection of Drinking Water
Ozone is a much stronger oxidant than chlorine. Ozone disinfection of drinking water has similar limitations to Chlorine.
Ultraviolet Disinfection of Drinking Water
Light with wavelength between 200-280nm is called UVC radiation. UV-C radiation damages 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 of Potable Water
- Effective against all known microorganisms. When used correctly, sufficient UV dose protects from all known bacteria, viruses, spores, cysts, oocysts, yeasts, fungi, algae, etc.
- No generate disinfection byproducts in most situations
- Non chemical. So, inherently safe
- Easy to monitor and control in real time
- Online disinfection. No need for mixing tanks or holding periods
- Easy to target log reduction in specific microorganisms, because UV Dose – response curves are available for most common pathogens.
- There is no risk of UV overdosing – generally the higher the UV Dose the better
- Very economical
- No corrosion
- UV Disinfection 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
- UV has no residual effect. As a result, in many situations a small Chlorine residual is recommended
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 delivered is the most important factor affecting efficacy of UV disinfection.
For potable water disinfection, Australian Drinking Water Guidelines, US EPA UVDGM 2006, and most of EU regulators recommend minimum UV Dose>40mJ/cm2 (i.e. 400J/m2) at the end of recommended UV lamp life.
As mentioned in US EPA (UVDGM 2006), some microorganisms (e.g. viruses) require much higher UV dose to achieve 4 log reduction.
In conclusion, UV disinfection offers reliable protection against waterborne diseases, when used correctly in a multi-barrier treatment. Filtering water and improving UVT are important. Some applications require residual chlorine.
If you have any questions related to potable water disinfection, or if you would like to size a UV unit for potable water disinfection, please call (02) 9896 1165 or contact us