1. Bacteria and other micro-organisms common in water disinfection
Bacteria, viruses and protozoa are micro-organisms with a relatively simple structure that are present in our environment in many forms and sizes. From their origin in ground and surface water, they easily enter storage tanks and the main distribution network. Size is obviously an important factor in filtration, and these microorganisms are distributed as follows:
- Protozoa are single-cell organisms ranging in size from about 1 to 50 μm and are typically responsible for gastro-intestinal diseases.
- Bacteria, also single-cell organisms, are in the size range of 0.1 to 10 μm.
- Viruses are the smallest of waterborne agents. Typically ranging in size from about 0.02 to 0.2 μm, they are too small to be seen with a light microscope.
A number of common water contaminants, also known as substrate, are used by bacteria for proliferation and growth:
- Organic compounds based on Carbon (C) and Nitrogen (N)
- Minor elements such as compounds of Phosphorus (P), Potassium (K), Sulphur (S) and Magnesium (Mg)
- Metal ions such as Iron (Fe), Nickel (Ni) and Cobalt (Co).
Generally, the most problematic pollution is “organic compounds”, however water stagnation, scale in pipes and high ambient temperatures will also accelerate the formation of biofilm and facilitate bacteria proliferation and growth.
2. Water disinfection. How is it different to sterilization?
Water disinfection is the term used to describe the killing or de-activation of micro-organisms present in water.
Water disinfection is different to water sterilization in the way that sterilization is characterised by the removal/destruction of all micro-organisms present in water. On the contrary, a small number of germs can remain in water after disinfection, however they would present no risk to human health.
Disinfection effectiveness is sometimes expressed in logarithmic units representing the percentage of residual living organisms after treatment:
3. Minimum conditions required for a satisfactory water disinfection
A comprehensive sediment filtration is mandatory before any disinfection is carried out. TSS (total suspended solids) must be 1mg/L or less, as bacteria and viruses are often attached to these small particles.
Another important consideration is that suspended solids usually obstruct the effect of disinfectants, for example interrupting light transmission in the case of UV disinfection.
In regards to organics, dissolved or in suspension this matter must be reduced as much as possible before disinfection, otherwise water will have a higher disinfectant demand and dosing will need to be higher. Organics in water also represent a risk for public health in a way that, when oxidised by Chlorine, Ozone or any other oxidising agent, can generate toxic by-products such as THMs.
4. Membrane filtration before water disinfection
Membrane filtration is characterised by a very fine pore exclusion (RO and NF mechanisms are actually more complex but the result is the same). RO (reverse osmosis), NF (nanofiltration) and UF (ultrafiltration) differ in the pore size being RO the method with a higher retention.
RO and NF present a complete blockage to bacteria, virus and microorganisms, however the need of very high pressure to operate and the technical complexity compared to UF makes both methods more suitable for desalination and softening.
Ultrafiltration is characterised by the use of membranes with a nominal pore diameter in the range of 0.02 to 0.04 μm what makes them very effective with protozoa (LOG removal >6) and bacteria (LOG removal >4) although results are not so good with viruses, which are considerably smaller (LOG removal > 1).
5. Available water disinfection methods
When it comes to water disinfection, there are two main technologies that, in most occasions and depending on the application, will be combined to obtain the best results::
- Chemical oxidation: oxidants such as Chlorine, Chloramines or Ozone affect the DNA of bacteria and viruses inhibiting all reproduction.
- Ultraviolet light: UV light emitted with a wavelength of 254 nm effectively fractures the DNA of microorganisms with a very powerful germicidal effect.
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5.1. Chemical Oxidation as water disinfection method
Typical chemical oxidants used in water treatment are Chlorine, Hypochlorites, Ozone and Chloramines. Oxidation deactivates microorganisms through several mechanisms: damages the cell wall, alters the cell protoplasm and inhibits the cell capacity to feed itself.
One fo the major advantages of using chemical oxidants as water disinfectants is that in many cases they produce an effective residual. Maintaining a residual means an extra capacity that persist in water and keeps the oxidation potencial to prevent regrowth or attack of new microorganisms. This characteristic is particularly important in long water distribution networks where disinfection might be required again downstream the point of dosing.
The main drawback of chemical oxidants is that oxidation doesn’t only affect undesired micro-organisms, but also to other inorganic and organic compounds which oxidise producing what are known as disinfection by-products.
A typical inorganic reactions is the oxidation of dissolved Iron (Fe2+), which after precipitates as Fe3+ leaving a reddish colour in the water along with ferric particles. The same happens with Manganese leaving a brown precipitate.
Special care must be taken with Ozone, as its high oxidation capacity may create Bromates (BrO3-) that are regarded as dangerous for human health at levels as low as 10 μg/L (ppb).
Organic disinfection by-products represent a higher risks than inorganic by-products as some of them, like THMs (trihalomethanes), are particularly damaging for the human health.
An effective way to prevent organic by-products is using UF and absorption (activated carbon) to reduce dissolved and suspended organic compounds as much as possible before Chlorine or any other oxidan is dosed.
5.2. Ultraviolet UV-C radiation as water disinfection method
UV radiation applied as part of a disinfection treatment is recognised as the only method (other than filtration) that doesn’t create damaging by-products and is effective against all micro-organisms including protozoan cyst.
On the electromagnetic spectrum, UV light lies between X-ray and visible light. The typical wavelengths of UV light are from 100 to 400 nanometers (nm).
The UV spectrum is composed of UV-A, UV-B, UV-C, and vacuum UV. Although the entire UV wavelength range is capable of disinfection, the most effective wavelength for UV disinfection in water is 254nm wich is part of the UV-C range.
The main draw-back of UV is that it is not possible to check the delivered dose by measuring any residual, so it is essential for the reactor to be equipped with an UV intensity sensor and any type of flow control that guarantees a minimum dose of 40 mJ/cm2 .
Any lamp failure must be immediately noticed and flagged to stop the process or activate a stand-by lamp.
With no residual effect, except in the case of short and well maintained distribution networks, the UV disinfection will have to be combined with another method that provides the residual.
6. What means dosing and contact time for chemical disinfectants and UV?
The efficiency of chemical and UV disinfection methods depends greatly on the quality of the source water, pH, temperature, turbidity and organic content in the water. However the most important parameters are always dosing (or concentration) and contact time.
Disinfection requires a certain dosing, or amount of disinfecting agent, combined with a minimum contact time to react and deactivate micro-organisms. Both values will depend on the microorganism that needs to be destroyed or deactivated, the type of disinfectant, the water temperature and the PH.
Dosing is measured in mg/L for chemical agents such as Chlorine or Ozone or in mJ/cm2 for UV radiation.
7. Is Silver-Ion a reliable water disinfectant?
Some systems and water treatment manufacturers use silver-ion as active ingredient in their water disinfection tablets and units.
According to the Australian Drinking Water Guidelines:
“Silver is a weak biocide/bacteriostat that has been used occasionally for disinfection, particularly in point-of-use devices. However, there is no reliable evidence that these products worked effectively to kill micro-organisms. A long exposure time of several hours to days is required for any biocidal effect to be observed” .
For this reason we recommend any of the previously described alternative disinfection methods.
8. Risks associated with Ozone
Ozone has the highest oxidation potential of all chemicals and is widely used for disinfection of ultra-pure water in hospitals, However it is not recommended in small-scale systems for two reasons:
- Adequate dosing and control is very complex
- Any trace of Br can be oxidised to Bromates (BrO3-) which are regarded as dangerous for human health at levels as low as 10 μg/L
9. What water disinfection method is best for commercial and industrial applications?
It must be noted that there is no such thing as a “best water disinfection method” but a number to methods that can be combined to obtain suitable results for each industry and application.
At Watercore, we always recommend the use if a “multiple barrier” approach, relying on the use of ultrafiltration backed by UV radiation or chemical oxidation depending of each circumstance.
Learn more about Watercore water disinfection products