Designing Treatment for Small Water Systems


Treatment is an important, central component to designing and constructing a small water systems. Common objectives and potential hazards of a small water treatment system need to be taken into account, including the objectives of the system, the design and construction of the structure itself, equipment, power sources, and treatment processes.

As no one type of treatment system is effective in treating all contaminants, a multi-barrier approach, which typically includes two or more forms of treatments, is usually required to adequately address all risks.

Glossary of Terms
small water systems

Water systems serving fewer than 500 people in a 24-hour period, as defined by the Government of BC

AWWA – American Water Works Association

The American Water Works Association is an international, nonprofit, scientific and educational society dedicated to providing total water solutions assuring the effective management of water. Founded in 1881, the Association is the largest organization of water supply professionals in the world, with 50,000 members representing the full spectrum of the water community: public water and wastewater systems, environmental advocates, scientists, academicians, and others who hold a genuine interest in water.


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


The introduction into water of micro-organisms, chemicals, toxic substances, wastes, or wastewater in a concentration that makes the water unfit for its  intended use.

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.


A waterborne intestinal parasite that causes a disease called cryptosporidiosis in infected humans. Symptoms of the disease include diarrhea, cramps, and weight loss. Cryptosporidium contamination is found in most surface waters and some groundwaters. Commonly referred to as "crypto."

direct filtration

A method of treating water that consists of the addition of coagulant chemicals, flash mixing, coagulation, minimal flocculation, and filtration. The flocculation facilities may be omitted, but the physical-chemical reactions will occur to some extent. The sedimentation process is omitted.


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.

distribution system

The equipment involved with the delivery of treated water from the treatment facility to the intended end-point user.


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


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


A waterborne intestinal parasite that causes a disease called giardiasis (GEE-are-DIE-uh-sis) in infected humans. Symptoms of the disease include diarrhea, cramps, and weight loss. Giardia contamination is found in most surface waters and some ground waters.

hardness, water

A characteristic of water caused mainly by the salts of calcium and magnesium, such as bicarbonate, carbonate, sulfate, chloride, and nitrate. Excessive hardness in water is undesirable because it causes the formation of soap curds, increased use of soap, deposition of scale in boilers, damage in some industrial processes, and sometimes objectionable tastes in drinking water.

pathogenic organisms

Organisms, including bacteria, viruses, or cysts, capable of causing diseases (giardiasis, cryptosporidiosis, typhoid, cholera, dysentery) in a host (such as a person). There are many types of organisms that do NOT cause disease. These organisms are called non-pathogenic.

plug flow

A type of flow that occurs in tanks, basins, or reactors when a slug of water moves through a tank without ever dispersing or mixing with the rest of the water flowing through the tank.


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


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

surface water

All water naturally open to the atmosphere (rivers, lakes, reservoirs, streams, impoundments, seas, estuaries, etc.); also refers to springs, wells, or other collectors that are directly influenced by surface water.


Total Organic Carbon. TOC measures the amount of organic carbon in water.


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.


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.


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


A wall or obstruction used to control flow (from settling tanks and clarifiers) to ensure a uniform flow rate and to avoid short-circuiting.

When designing a small water treatment system, ensure it, at a minimum, takes into account the following:

  • the quality of the water source
  • potential threats that the system may face from source to tap
  • the ultimate use of that water
  • the maximum treatment capacity of the system
  • operational requirements, including stand-by power
  • requirements for treatment equipment, including placement of sample taps
  • that it meets appropriate public health engineering standards for the type of system chosen
  • that it has sufficient ability to provide the quality and quantity of water appropriate to the intended user, including to meet the treatment objectives

We recommend that you begin with the BC Ministry of Health’s Small Water System Guidebook, which provides descriptions of all the components of a small water system, from source to tap, as well as the regulatory requirements, treatment objectives and more.

Design Guidelines for Small Water Treatment Systems

The design and construction of a small water treatment system must comply with all local and provincial bylaws and regulations and conform to good engineering practices. There are several widely-adopted design and construction guidelines for BC waterworks, which include the following:

Certification Requirements for the Systems Parts

When purchasing commercial materials, products and chemicals to build or use in a drinking water system, the owner or operator needs to ensure that the products are certified to be used in a drinking water system. You do this by looking for information, such as a trademark, that says the product meets the required NSF Standard, indicated by the NSF mark. NSF Standards provide products or emerging technologies with credibility and indicate industry acceptance. In drinking water systems, the standards assure the product materials and structural and performance claims are assured to be safe for use. BC health authorities require NSF certified products be used when new drinking water systems are built.

A more detailed discussion of NSF is available under another section of this website: NSF Standards. Lists of certified water treatment devices and systems are available on NSF International’s online product database, under the category “Drinking Water Treatment Units”.

Physical Removal of Pathogens

Viruses and protozoa can be partially removed physically, such as through clarification processes, followed by filtration. Guidance on filtration technologies, performance and pathogen removal credits is available in Health Canada’s technical document: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Turbidity.

Filtration systems consist of slow or rapid sand filtration and membrane technology. The relative effectiveness of different filtration systems, as found by the Government of Manitoba, is show in Table 1 below. "Log removal credits" are assigned based on accepted credits determined through the ongoing use, testing and validation of various technologies. 

Table 1: Log Removal Credits for Filtration Technologies

Filtration Technology

Log Removal Credit


Log Removal Credit


Log Removal Credit

Conventional treatment: coagulation, flocculation, clarification, filtration




Direct filtration: coagulation, filtration




Slow sand filtration




Cartridge filtration (1μm absolute stage, certified to NSF Standard 53) small systems only




Microfiltration and Ultrafiltration

3.0+ Demonstrated through challenge testing

3.0+ Demonstrated through challenge testing


1.0 if pre-coagulation

Nanofiltration and Reverse Osmosis
(only GUDI systems)

3.0+ Demonstrated through challenge testing

3.0+ Demonstrated through challenge testing


No filtration




Source: Chlorine and Alternative Disinfectants Guidance Manual (2005), Province of Manitoba Water Stewardship – Office of Drinking Water

More on filtration is available under the section Centralized Water Treatment Systems.

Common Water Disinfectants and How to Select Them

Disinfection removes, deactivates or kills pathogenic microorganisms by destroying the cell wall or interfering with its metabolic processes. Common water disinfectants include the following:

  • Sodium hypochlorite
  • Ultraviolet radiation
  • Calcium hypochlorite
  • Ozone
  • Chlorine dioxide

Each of above disinfectants has their own advantages and disadvantages. Their efficacy varies significantly depending upon the type of pathogens and the quality of the source water (e.g., pH, temperature, hardness, total organic carbon (TOC), etc.) and contact time. A summary of the effectiveness of various disinfectants on different pathogens is shown below, in Table 2:

Table 2: Effectiveness of Disinfectants on Different Pathogens


Microorganism Reduction Ability


E. Coli





Very effective

Moderately effective

Not effective

Very effective


Very effective

Very effective

Very effective

Very effective


Very effective

Moderately effective

Not effective

Moderately effective

Chlorine dioxide

Very effective

Moderately effective

Moderately effective

Very effective

Ultraviolet radiation

Very effective

Very effective

Very effective

Moderately effective

Source: Chlorine and Alternative Disinfectants Guidance Manual (2005), Province of Manitoba Water Stewardship – Office of Drinking Water.
Primary and Secondary Disinfections

Because no one disinfectant is effective on all pathogens equally, a multi-barrier approach combining primary and secondary disinfections is recommended:

Primary disinfection is intended to kill or inactivate pathogenic microorganisms that may be present in the source water. However, while some disinfectants are very effective as a primary disinfectant, they cannot be easily maintained throughout the distribution system. For example, UV applied during primary water treatment does not prevent pathogen re-growth.

Secondary disinfection maintains the residual of disinfectant throughout the distribution system to prevent the re-growth of microorganisms in the system, as well as to kill or inactivate microorganisms that may enter the distribution system. Because the distribution system can encompass a series of water mains, reservoirs, stations, valves, and other equipment, there are several points vulnerable to fecal contamination through leaks in pipes and improper connections, as well as re-growth of inactive pathogens that may remain in the system. Chlorine is the most commonly used water disinfectant for secondary (residual) disinfection.

Calculating the Disinfection Contact Time (CT)

The effectiveness of disinfection (e.g., chlorine, chlorine dioxide, ozone, or chloramines) is demonstrated through the concept of CT: concentration (C) and contact time (T). The CT disinfection value is the product of a disinfectant’s residual concentration (in mg/L) times the effective contact time (in minutes). For a water treatment system using surface water/GARP/GWUDI sources, the CT calculated value achieved must provide a minimum of 4-log virus reduction.

To complete the CT calculation, the following factors should be considered to determine the effective CT value:

  • The peak hourly flow rate
  • The minimum disinfectant residual found in the water during disinfection, measured at the furthest point in the disinfection chamber from where it was administered
  • The minimum temperature of the water during disinfection
  • The maximum pH of the water during disinfection
  • The minimum normal operating level of the storage reservoir
  • The baffling factor for the chlorine contact tank.

The baffling factor of a contact tank is used to adjust the theoretical detention time to a more realistic value of the CT and reduces the effective storage volume to account for potential short-circuiting. The baffling factors are determined based on the geometry and configurations of the contact tank, where the typical values are shown below, in Table 3.

Table 3: Typical Baffling Conditions

Baffle Condition

T10/T Ratio

Baffle Description

Unbaffled (mixed flow) separate inlet/outlet


No baffles, agitated basin, very low length to width ratio, high inlet and outlet flow velocities



Single or multiple unbaffled inlets and outlets, no intra-basin baffles



Baffled inlet or outlet with some intra-basin baffles



Perforated inlet baffle, serpentine or perforated intra-basin baffles, outlet weir or perforated launders

Perfect (plug flow)


Very high length to width ratio (pipeline fl

Source: Chlorine and Alternative Disinfectants Guidance Manual (2005), Province of Manitoba Water Stewardship – Office of Drinking Water (original source: AWWA (American Water Works Association). Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources, McGraw-Hill, New York, NY, 1991.)

A helpful tool and a user’s guide for calculating the CT are available on the Newfoundland Labrador Municipal Affairs and Environment website.

Selecting a Commercial UV Reactor for a Small Water System

Ultraviolet (UV) rays are part of the light that comes from the sun. UV rays, recreated in mercury vapour lamps (UV lamps), which resembles a fluorescent lamp, have proven to be an effective water disinfectant due to its ability to kill or inactivate microorganisms.

A UV reactor must be validated before it is installed for drinking water systems. The purpose of validation is to ensure that a UV reactor can meet the level of inactivation required for a specific application. However, the validation test for commercial UV equipment can be expensive. So, for small water systems consisting of a single connection, BC health authorities recommend they use UV equipment that is already certified to NSF Standards (NSF/ANSI Class55A), to deliver a minimum dose of 40mJ/cm2 for achieving a 3-log (99.9%) inactivation of protozoa.

To ensure the UV disinfection is effective, a small drinking water system should also consider measuring the UV transmittance and the flowrate. Typically, a higher transmittance value of 85% or higher is recommended. Under no circumstances should the UV transmittance be below than 75%. Since every UV system is designed to treat a maximum flow rate based on a specific transmittance value, it is important to install a system that is the adequate size to accompany maximum flow rates while achieving the required dose.

Further, small drinking water systems should choose UV equipment that requires minimum operator intervention in terms of installation and routine annual maintenance (e.g., replacement of lamps and quartz sleeves). UV equipment that is designed with instrumentation and other features, such as quartz sleeve wiper mechanisms, lamp out alarms, heat sensors and UV intensity monitors, are recommended.