Centralized Water Treatment Systems

Introduction

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.

In urban areas, a centralized 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, it is still the preferred method of treatment.

Glossary of Terms
absorption

The physical process occurring when liquids, gases, or suspended matter adhere to the surfaces of, or in the pores of, an absorbent material. Absorption occurs without chemical reaction.

cation

A positively charged ion in an electrolyte solution, attracted to the cathode under the influence of a difference in electrical potential. Sodium ion (Na+) is a cation.

chlorination

The application of chlorine to water, generally for the purpose of disinfection, but frequently for accomplishing other biological or chemical results (aiding coagulation and controlling tastes and odours).

clarification

Any process or combination of processes of which the main purpose is to reduce the concentration of suspended matter in a liquid.

clarifier

A large circular or rectangular tank or basin in which water is held for a period of time during which the heavier suspended solids settle to the bottom. Clarifiers are also called settling basins and sedimentation basins.

conventional filtration

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.

disinfection

The process designed to kill or inactivate most microorganisms in water, including essentially all pathogenic (disease-causing) bacteria. There are several ways to disinfect, with chlorination being the most frequently used in water treatment.

floc

Clumps of bacteria and particulate impurities that have come together and formed a cluster. Found in flocculation tanks and settling or sedimentation basins.

flocculation

The gathering together of fine particles after coagulation to form larger particles by a process of gentle mixing.

protozoa

Single-celled organisms with a more complex physiology than viruses and bacteria; average diameter of 1/100 mm.

raw water

Water in its natural state, prior to any treatment. Usually the water entering the first treatment process of a water treatment plant.

turbidity

The cloudy appearance of water caused by the presence of suspended and colloidal matter. In the waterworks field, a turbidity measurement is used to indicate the clarity of water. Technically, turbidity is an optical property of the water based on the amount of light reflected by suspended particles.

ultrafiltration

A membrane filter process used for the removal of some organic compounds in an aqueous (watery) solution.  Removes particles with pore sizes which range from 10-2 to 10-6 millimeters.

 

ultraviolet (UV)

In water treatment, a specific wavelength of light produced by a device used for disinfection.

virus

A very simple life form that only multiplies inside the living cells of a host; average diameter of 1/10,000 mm.

Overview of Centralized Water Treatment Systems

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 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.
Package Plants

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, virtually ready to operate. 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)
  • Conditioning: softening (cation exchange process), iron and manganese removal (greensand), arsenic removal (absorption onto a granular ferric oxide bed/ activated alumina), and/or TOC reduction
  • Disinfection: Ultraviolet irradiation, chlorination, and/or zonation
Figure 1: A Centralized Water Treatment System​​​​​
Figure 1: A Centralized Water Treatment System​​​​​
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