A centralized water treatment system treats water in a central location that is typically near the water source. Treated water is then delivered to the customers via a dedicated distribution network. It may or may not have conventional treatment which can include a combined process of coagulation, flocculation, sedimentation (or clarification), filtration, and disinfection.
A centralized small water treatment system can treat large volumes of water at high rates to accommodate all residential, business, and industrial uses. This approach is well developed and, if designed properly, can effectively remove raw water turbidity, along with pathogens (bacteria, viruses, and protozoa). For small water systems with a surface water source, it is still the preferred method of treatment.
Centralized treatment can also reduce metals (e.g. iron, manganese, etc.) and chemical contaminants such as nitrates, arsenic and uranium from groundwater sources.
Primary and secondary disinfection with chlorine is very cost effective.
Centralized Water Treatment Systems for a Surface Source
In urban areas, centralized water treatment systems have been the common practice used to treat and deliver safe water (water that meets the water quality standards of Health Canada’s Drinking Water Guidelines). Usually, the treatment process for a surface water source is a combination of some or all of the following:
Coagulation is considered a pre-treatment step in the water treatment process. Chemicals are added (e.g. aluminum sulfate) to encourage suspended particles to cling together.
Flocculation is a water treatment process where solids form larger clusters, or flocs, which can then be removed from water. This process is assisted by the addition of chemicals in the coagulation step.
Sedimentation (or Clarification) - The mixture of water and flocs is next pumped into a settling tank that allows the flocs, which are heavier than water, to settle to the bottom. Once this sedimentation occurs, the water can be pumped to the next section of processing.
Filtration - There are various means of achieving physical filtration of unsettled flocs, contaminants and pathogens. Filtration systems can make use of several different materials to achieve the desired result. For example, slow sand filtration uses various layers of sand and gravel to filter water. Membrane technology and biologically active carbon filtration are other means of physically removing contaminants and pathogens from water.
Disinfection - All surface water and groundwater at risk of containing pathogens (GARP) in BC must be disinfected. Disinfection inactivates a microorganism by destroying the cell wall or interfering with its metabolic processes. This can be achieved by adding chemicals (e.g., chlorine or ozone), using heat, or ultraviolet (UV) light radiation. Destroying pathogens in the raw untreated water can prevent them from entering the distribution system and re-growing. Without disinfection, the risk of waterborne disease increases.
To reduce the initial infrastructure investment for treatment facilities, in many cases a centralized treatment system can be constructed as a package plant, where treatment units are preassembled in a factory, skid mounted, and transported to the site. The package plant normally uses the following three stages to achieve water potability:
- Filtration: slow sand filtration, membrane filtration (e.g. microfiltration/ultrafiltration/nanofiltration/reverse osmosis), approved bag filters, and/or approved cartridge filters (1-micron nominal and 1-micron absolute).
- Water Softening and Adsorption: Softening (cation exchange process), iron and manganese removal (greensand), arsenic removal (adsorption onto a granular ferric oxide bed/activated alumina), and/or Total Organic Carbon (TOC) reduction.
- Disinfection: ultraviolet radiation, chlorination, and/or ozone. A 5-micron prefilter and a 1-micron absolute filter are required upstream from the ultraviolet light radiation.
Alternatives to Centralized Treatment
Small Water Systems may use Decentralized Water Treatment Systems, such as point-of-entry (POE) or point-of-use (POU). There are advantages and disadvantages to both treatment options, which you can find outlined on our page Centralized vs Decentralized Water Treatment Systems.
Small Water System Guidebook, BC Ministry of Health, February 2017
Protocol for Centralised Drinking Water Systems in First Nations Communities, Indigenous Services Canada, April 2010 - Contains standards for the design, construction, operation, maintenance, and monitoring of centralised drinking water systems in Indigenous communities.
Water Quality - Reports and Publications, Guidelines for Canadian Drinking Water Quality, Health Canada - A collection of reports and publications published by Health Canada on Water Quality.
Drinking Water Protection Act: Drinking Water Protection Regulation, Province of BC, Reg. 200/2003
Health Canada’s Drinking Water Guidelines
Disinfecting Drinking Water, HealthLinkBC, File 49b, July 2021