Centralized vs Decentralized Water Treatment Systems

Introduction

A critical decision to make when planning a water treatment system is whether it will be a centralized or decentralized system.

Under BC's Drinking Water Protection Regulation, small water systems have the option to use centralized or decentralized systems. However, to have a sustainable water treatment system suitable to your situation, it is important to assess both alternatives, based on the need to produce acceptable finished water quality at the most attractive overall cost. In general, the choice depends on source water quality, water demand, treatment targets, capital and operating costs, operator capability, and long-term operating and maintenance costs.

As each water source is unique, the selection of a system to meet particular treatment objectives should be evaluated on a case-by-case basis, taking these factors, among others, into account. Water suppliers are encouraged to consult with the local Environmental Health Officer and the Regional Public Health Engineer to determine an appropriate treatment process.

Glossary of Terms
bacteria

Microscopic living organisms that usually consist of a single cell. Most bacteria use organic matter for their food and produce waste products as a result of their life processes.

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.

finished water

Water that has passed through a water treatment plant; all the treatment processes are completed or "finished." This water is ready to be delivered to consumers.

potable water

Water that does not contain objectionable pollution, contamination, minerals, or infective agents and is considered satisfactory for drinking.

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.

reservoir

A pond, lake, basin, or other structure (natural or artificial) that stores, regulates, or controls water.

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.

virus

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

watermains

Large distribution pipes that run along road major roadways.

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.

Benefits and Costs of a Centralized Water Treatment System

A centralized water treatment approach, also known as conventional treatment, uses a combined process of coagulation, flocculation, sedimentation, filtration, and disinfection. It treats water in a central location and then distributes 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 remove practically any range of raw water turbidity along with harmful pathogens, including bacteria, virus, and protozoa.

However, the capital cost and operating and maintenance costs for a centralized system can be significant. It consists of water source development, construction of significant infrastructures (e.g., the treatment facility, reservoir, and water distribution main), implementation of automated monitor and control systems, and on-site operators.

Smaller communities can reduce costs by using a “package plant”, where treatment units are preassembled in a factory, skid mounted, transported to the site, and virtually ready to operate. However, even then a centralized treatment system may be still financially out of reach for some underdeveloped communities.

Image 1: Centralized Water Treatment System
Image 1: Centralized Water Treatment System
Benefits and Costs of a Decentralized Water Treatment System

Where a centralized community treatment system is unavailable or unaffordable, a decentralized system – point-of-entry (POE) or point-of-use (POU) – installed at the individual home or business can be used to achieve potable water. POEs treat the raw water before it enters the property or home, while POUs are installed to treat water where needed, such as at kitchen and bathroom taps.

In underdeveloped areas, where there are significant deficiencies in financial resources and technical supports, POEs/POUs may be the only treatment option. Although POEs/POUs are inexpensive relative to centralized systems, because they defer large initial capital investments and reduce operating and maintenance costs, they are limited by their treatment capacity and capability for dealing with multiple contaminates. They also require full community buy-in and monitoring.

Image 2: Point-of-Entry and Point-of-Use Water Treatment Systems
Image 2: Point-of-Entry and Point-of-Use Water Treatment Systems
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