Disinfection is the process of killing or inactivating disease-causing organisms, called pathogens, which are in water.
These organisms may include:
- protozoa
- bacteria
- viruses
Different methods can be used to destroy or inactivate pathogens, the most common methods being chlorination and ultraviolet light. The effectiveness of your disinfection process is measured by your water quality monitoring program.
Disinfecting with Chlorine
Chlorination is a relatively simple and inexpensive way to disinfect drinking water and is the most commonly used. It has the ability to kill off many different types of pathogens and provides a barrier to microbial survival in the storage and distribution systems if contaminants are accidentally introduced.
Disinfectants can be effective for killing viruses and bacteria but are not as effective for protozoa when used at concentrations safe for water treatment.
The amount of chlorine for disinfection of water will vary according to these and other factors:
- the volume of water
- the quality of the water
- the amount of pre-treatment
Checking Chlorine Levels
In order for chlorine to be most effective, a certain amount must be in the system at all times.
Two factors are critical for ensuring the destruction of harmful microbes:
- concentration of chlorine
- length of contact time with organisms
Several factors affect the speed of the disinfection process:
- chlorine concentration
- cleanliness of the water
- pH level of the water
- water temperature
Chlorination becomes less effective as the pH level rises and as the water temperature goes down. Cleaner water reduces chlorine demand.
The amount of chlorine that must remain in a system after the initial disinfection application will vary with a number of factors:
- quality of the water
- volume of storage
- amount of debris in the watermains
- size of the distribution system
The chlorine levels in the system are highest near the source of the chlorine and decrease toward the far ends of the system. To make sure the chlorine levels are sufficient in all parts of the system, samples need to be taken at various points, including points where the level is likely the lowest.
Disinfecting with Ultraviolet Light
Ultraviolet (UV) light is a non-chemical way to disinfect water. A special UV light bulb gives off a wavelength of light that can kill off some pathogens entirely and prevent others from later causing illness.
Typically, the UV bulb is inserted into a quartz glass sleeve, which is located in a housing (usually shaped like a wider section of pipe). Water flows at a controlled rate between the outside of the quartz sleeve and the interior wall of the housing. The UV light is transmitted through the quartz sleeve to the water.
Typical UV Light System for Water Disinfection.
Water clarity (or transmissivity) affects the disinfection process. Turbid water or water that is high in colour will need to be adequately pre-filtered for the UV light to be effective.