Thompson Rivers University
Thompson Rivers University

Water Treatment

Water treatment refers to the process of improving water quality by removing undesirable physical, chemical, or biological matter.

There are specific water quality objectives to be met, whether for health reasons and/or aesthetic reasons or as a condition for an operating permit. More complex treatment can also condition the water for factors such as pH levels.

Two major types of water treatment for small water systems are:

  • Filtration; removes matter from water by use of filter media, e.g. screens.
  • Disinfection; kills or inactivates biological agents such as bacteria and protozoa.

Small Water System Flow Chart: Treatment


”Source to tap” protection of water quality means putting up barriers to stop contaminants from becoming a threat at any point, from the source itself (i.e., watershed management or wellhead protection,) to the end user.

Water treatment methods range from simple filtering solutions such as screening for large debris to solutions that use technology such as ultraviolet light. Deciding how to proceed can sometimes be complicated. When in doubt, get the advice of a drinking-water safety professional.

The following are common treatment options for water at intake.

Coarse Screening

In surface sources of water, an intake screen is used to prevent larger objects, such as tree branches and rocks, from entering the system. Of course, screening is not a method for removing harmful organisms from the water.


Correct Installation of an Intake Screen

In wells, a well screen is used to help prevent pebbles and coarse sand from being drawn into the pump.


Filtration is a process for removing smaller particles of debris, either for health or aesthetic reasons. Filtration may reduce turbidity, which can mask pathogens and reduce the effectiveness of the disinfection process.

There are several methods of filtration, including sand filters and cartridge filters. Each technology has advantages and disadvantages.


Cartridge Filter and Casing Assembly


Typical Cartridge Filter Casing Installed

Testing of your source water, otherwise known as piloting, may be required to determine which technology is appropriate for your situation.

You may be able to purchase a package plant that is a self-contained, complete system. Contact a drinking water safety professional for further information and advice about filtration.


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

Chemical Feed Pumps


Chemical Feed Pump Installation

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.

Public Notification and Reporting

Keeping People Informed

As an owner or operator of a small water system, you are required to report on aspects of your water system and, in the case of a public health risk, provide appropriate public notices.

In addition to possible legal requirements for reporting, there are many good reasons for a water operator to let others know about routine business related to the system operation.

You are also required to have an Emergency Response Plan in place. Providing information may be necessary in an urgent situation or an emergency.


The operator may also be required to file an annual report detailing his or her activities for the year and the general state of the system, as well as periodic reports related to the monitoring of the system.

Annual Reports

Annual reports give the operator a chance to advise users about:

  • the general operation of the system
  • events that have occurred that affect the system
  • progress made on upgrades
  • the financial situation
  • the need for any future upgrades that may require large expenditures

Monitoring Reports

Monthly or quarterly reports are documents that may include information such as summaries of disinfection monitoring, bacteriological testing results, and water usage. These reports can help keep users informed about routine operations and may be helpful in promoting water conservation initiatives.

More information about reporting, see documentation.

Emergency/Urgent Conditions

Despite the best intentions, unexpected problems may occur. Water systems are often complicated and can have many components. Source waters can change in quality. Mechanical systems can break down. Unforeseen circumstances, even disasters, can occur. When problems arise that may affect the quality or quantity of the water, it is important that the users be advised.

The type of notification should reflect the severity of the concern.

Critical Situations

Rapid response during a critical situation depends on effective communications. Your top priority will be to alert all the users on your system as soon as possible, especially if there is any possible risk to health.

For a small system you can organize a telephone tree to alert everyone quickly. (To set up a tree, divide the task of phoning the water users among several people.) Also have a backup method for reaching people who can’t be contacted by phone.

Public notification of water quality problems can take different forms. The content and wording of the advisory need to be appropriate for:

  • the type of problem encountered
  • the duration of problem
  • corrective measures that will be needed
  • actions that users should take to protect themselves

Water Quality Advisory

When the risk to water users is very low, a water quality advisory can be used to advise users that a potential threat to water quality is a possibility.

“Boil Water” Notice

When there is an actual threat to health from a microbiological source, a “Boil Water” notice is issued. Boiling water for several minutes will eliminate the threat from microorganisms.

Although boiling water is not the most efficient way to disinfect water, it is used in emergency situations. Boiling water does not remove chemical contaminants.

“Do Not Drink” Notice

The “Do Not Drink” notice should be used when the health threat cannot be eliminated by boiling the water. For example, a toxic chemical spill near a water intake may be reason to issue this type of notification.


Public Alert to Unsafe Water

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