B.C.'s Drinking Water Protection Regulation gives small water systems—defined as a system that serves up to 500 individuals during any 24-hour period—the option to use centralized or de-centralized water treatment systems. The advantages and disadvantages of each system are discussed in the section “Centralized vs Decentralized Water Treatment Systems”. Here we discuss centralized water treatment systems.
A centralized water treatment system, also known as conventional treatment, is a combined process of coagulation, flocculation, sedimentation (or clarification), filtration, and disinfection. It treats water in a central location and then distributes the treated water via dedicated distribution networks.
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 can effectively removes practically any range of raw water turbidity, along with harmful pathogens, including bacterial, virus, and protozoa. For smaller water systems for a surface source, it is still the preferred method of treatment. A well protected ground water source, without any treatments for metals (iron, manganese, arsenic). Primary and secondary disinfection with chlorine is most cost effective.
In urban areas, centralized water treatment systems have been the common practice to treat and deliver safe water that meets the drinking water qualities required by Health Canada under the Guidelines for Canadian Drinking Water Quality. Usually, the applied treatment process for a centralized system for a surface source is a combination of some or all of the following:
- Coagulation is a first pre-treatment step, in which chemicals are added (e.g., aluminum sulfate) to the water to promote particles clumping together.
- Flocculation involves gently mixing the water to encourage the clumps created by coagulation to form larger clumps called “flocs”.
- Sedimentation (or Clarification): Once the mixture of water and flocs enter into a settling tank (clarifier), the flocs settle to the bottom of the tank as sedimentation.
- Filtration: Water passes through a substance, such as sand or a membrane, that helps remove unsettled flocs, particles, contaminants and pathogens.
- Disinfection: Water is disinfected to kill any pathogens that may be present in the water supply and to prevent them from re-growing in the distribution system. Without disinfection, the risks from 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 flirtation, membrane filtration (e.g., micro-filtration/ ultrafiltration/ nanofiltration/ reverse osmosis), approved bag filters, and/or approved cartridge filters (1-micron nominal and 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 TOC reduction
- Disinfection: Ultraviolet irradiation, chlorination, and/or ozone. A 5-micron filter is required upstream from the ultraviolet irradiation.