Introduction to water disinfection
Disinfection of water or wastewater is the removal, destruction or inactivation of pathogenic microorganisms.
Given the risk of illness caused by infection from the faecal oral route, disinfection is arguably the most important step in water treatment.
Disinfection is not to be confused with sterilisation. Sterilisation makes the water completely free of living microorganisms (good, bad and indifferent). Disinfection merely aims to make the water non-infective rather than completely sterile.
Methods of Disinfection
There are a wide range of water treatment technologies that can be used to disinfect water.
- Chlorine gas (Cl2)
- Sodium hypochlorite solution (NaOCl)
- Calcium hypochlorite powder or tablets (Ca(OCl)2)
- Chloramination (NH2Cl, NHCl2)
- Chlorine dioxide (ClO2)
- Other Halogens
- Bromine (Br2)
- Iodine (I2)
- Ozone (O3)
- Hydrogen peroxide (H2O2)
- Silver (Ag)
- Advanced Oxidation Processes (AOPs)
- Rolling boil sterilisation
- Distillation (MSF, MED, etc.)
- Ultrafiltration (UF)
- Nanofiltration (NF)
- Ceramic filters
- Ultraviolet (UV)
- Solar disinfection
Primary and Residual Disinfection
Primary disinfection relates to the main disinfection process intended to assure the disinfection of a water. While most water treatment works employ multiple barriers, the primary disinfection step is considered the principal point of disinfection. The primary disinfection step is therefore often closely monitored, and may have safeguards and interlocks to ensure that water cannot pass this step untreated.
Where chlorine is used as the primary disinfectant, a combination of concentration and time is required to give the desired level of disinfection for any given micro-organism. The time can be assured by using a contact tank with a known minimum residence time. Concentration can then be assured by testing the chlorine concentration at the outlet of the contact tank – often using double or triple validated chlorine monitors.
Other Points of Disinfection at the Treatment Plant
Primary disinfection is often the final stage of treatment, prior to water leaving the treatment plant and going into supply (or effluent to reuse/discharge). Water may also be disinfected at source. This is true of most desalination systems where the sea water is pre-chlorinated to stop growth within the treatment works. Pre-chlorination or pre-ozonation is also common practice at conventional water treatment works, where it often takes place at the head of the works (inlet). In some cases boreholes and wells may also be chlorinated.
Other treatment processes may also disinfect as a by-product. For example thermal desalination boils the water, and membrane processes such as Ultra Filtration (UF), which may be employed primarily for other reasons can remove pathogens by size exclusion.
Once water leaves the treatment works and goes into supply, there is a risk that it may become contaminated and re-infected. Maintaining a residual concentration of disinfectant in the water serves two main purposes. Firstly it will inhibit the multiplication of surviving or newly introduced pathogens. Secondly, it acts as an easy way to measure the quality of the water. It has traditionally taken days to determine the presence of viable pathogens in a water sample. The chlorine residual, however, can be measured very rapidly. Since a large population of pathogens would consume a trace residual of chlorine, the presence of chlorine in the water indicates the lack of such contamination.
Unfortunately not all disinfectants leave a sufficiently long lasting residual. Ozone for example reacts completely within minutes, hence leaving no residual. None of the physical, thermal or photochemical methods mentioned above leave a residual. This is one of the reasons for chlorine’s popularity as a residual disinfectant. In networks with very high residence times where even chlorine’s decay is too rapid, chloramination can be employed to give longer residuals.