By Mark Hutchinson

Summary: With the market for ultraviolet disinfection systems growing rapidly, many in the water treatment market are considering starting or expanding their sales of UV equipment. This article presents a primer on basic system sizing and installation.

Every year, thousands of ultraviolet (UV) disinfection systems are installed in North America. They provide protection from microbiological contaminants such as bacteria, viruses, and cysts (including Giardia and Cryptosporidium). While many systems are installed in commercial and institutional buildings (restaurants, schools, pre-schools, etc.), the vast majority are installed in the basements and crawl spaces of homes and cottages on private water supplies. Many are installed only as the result of receiving a bad water test, but more and more are wisely being installed as a preventive measure.

The sizing and installation of a typical, basic UV disinfection system is straightforward. Installation usually requires two hours or so. This article presents a step-by-step guide to sizing and installing a basic UV system.

The selection of a system of the proper capacity is accomplished by consideration of three variables: the maximum flow rate, dose required and UV transmittance (or UVT) of the water. Make sure the manufacturer of a system can provide you with sizing tables or otherwise help you size equipment under different combinations of these variables.

Step 1
Determine the maximum flow rate—The maximum flow rate is defined as the flow rate that would be achieved if all the water outlets in the house or facility were fully opened at once including faucets, toilets, dish washers, showers, sprinkler systems, etc. The maximum flow rate for the given site will depend on the pressure and size of the water lines, the number and nature of the water outlets, and other variables. Generally speaking, a typical home with ¾” lines will have a peak flow rate of 7 gallons per minute (gpm) or so, but a larger than average home with 1” lines might reach 15 gpm or more.

Step 2
Select the appropriate level of disinfection—The level of disinfection is a function of the UV dose applied. Dose is a measure of how much UV the water column is exposed to, and is a function of light intensity and time. The higher the output of the lamp and the cleaner the water, the higher the intensity. Similarly, the larger and more hydraulically optimized the chamber, the longer the exposure time (given a set flow rate). The units for dose are milliJoules per square centimeter (mJ/cm2). This also may be expressed as microWatt-seconds per square centimeter (µW-sec/cm2).
Generally speaking, installers choose from one of three target dose levels:

  • 16 mJ/cm2: An added level of confidence for safe water supplies such as municipal water. This dose provides 99.99 percent or 4-log reduction of common pathogens such as E. coli.
  • 30 mJ/cm2: A standard dose for industrial applications using safe water supplies.
  • 40 mJ/cm2: The dose required to assure the safety of water from potentially unsafe water supplies such as wells, lakes and rivers. It’s also the U.S. Environmental Protection Agency (USEPA) and NSF standard for such water supplies. A dose of 40 mJ/cm2 assures 99.99 percent or 4-log reduction of viruses, which are harder to kill than most bacteria and cysts.

Step 3
Establish the UV transmittance of the water—UV transmittance (UVT) is a measure of the water’s ability to transmit UV light. The majority of deep wells have a UVT of about 85 percent or greater. If
the UVT of the water is less than 75 percent, pre-treatment is generally required. It’s important to note that UVT may not necessarily correlate with how visibly discolored the water is, so it’s recommended the water be tested. Some manufacturers will do this at no cost.

The equipment you’ll need includes:

  • UV system and 5-micron (µm) sediment filter,
  • Plumbing: Pipe, 2 shut-off ball valves, hose bib, connector fittings, elbows, propane torch, solid-core solder, paste flux, emery cloth, pipe cutter, wrenches, pipe wrench, Teflon tape,
  • Electrical: GFCI (ground fault circuit interruption), electrical box, electrical wire, wire cutters and strippers, and
  • Other: Gloves, safety glasses, ruler, screw drivers, plywood.

In any installation of UV equipment, a 5-µm sediment filter is installed upstream of (before) the system to prevent larger particles from reaching the UV chamber, where they could potentially shield pathogens from the UV light through a process called shadowing.

The system chosen for this installation includes a sensor to monitor the amount of UV light getting through the water. Should the light intensity drop below a set point, a visual and audible alarm is triggered. The system may be connected to a solenoid valve, which is shut off during any alarm condition to prevent potential contamination passing downstream.

UV systems may include a number of other features including timers, diagnostics, digital displays, electronic ballasts, high-output lamps and a reduced footprint. These features are important to consider during the selection of the equipment to be used, but do not significantly affect installation procedures. Another factor to consider in selecting equipment is verification of product claims via third-party testing and certification (see EXTRA).

To facilitate maintenance, it’s suggested a ball valve be installed before the filter, and both a hose bib and ball valve should be installed after the UV system. This accomplishes several things: 1) It allows the complete system to be isolated; 2) it permits water sampling, and 3) it provides a pressure release mechanism.

A checklist
A step-by-step approach to the installation process follows:

  1. Attach a plywood backing board to the concrete to hold all the components required for the installation. Install a GFCI outlet to the board to supply power for the UV disinfection system.
  2. Attach the bracket clamp for the UV chamber to the plywood. Slide the UV chamber into the bracket and tighten the band clamp just enough to support the chamber. Attach the threaded connectors to the UV chamber. Attach the power supply to the plywood, positioning it so it won’t obstruct the plumbing.
  3. Attach the filter housing to the plywood. Attach the threaded connectors to the filter.
  4. Attach a ball valve to the incoming end of the filter, and a hose bib and ball valve to the outgoing end of the UV system. The valves will allow you to isolate the system and the hose bib will allow you to take samples.
  5. Take the necessary measurements for the copper plumbing, then dry-fit all of the piping before soldering the connections. Complete as much of the plumbing as possible before breaking into the main waterline.
  6. Insert the UV sensor into the chamber. Insert the UV lamp into the chamber and connect the lamp harness to the lamp. Plug-in the UV system.
  7. To disinfect the water lines, first remove the filter element and fill the housing with bleach. Replace the housing and allow water to flow to all outlets until you can smell bleach. Allow it to sit for at least two hours before flushing out the bleach and replacing the filter element.
  8. The system should look something like this (see Step 8) once installation is complete.

UV systems provide protection from microbiological contamination by bacteria, viruses and cysts such as Giardia and Cryptosporidium and, thus, an increasing number of water treatment professionals are including UV in their product and service offering. The sizing and installation of a UV system is relatively straightforward. This article is meant only to be an introduction to UV installation; however, it should be stressed that proper maintenance is also critical to system performance. See WC&P’s archives—www.wcponline. com—for more detailed information every dealer should know.


  1. Abboud, N., “Ultraviolet Disinfection for Small Systems,” WC&P, June 2002:
  2. Gadgil, A., “UV Waterworks 2.0: Answers to Ten Commonly Asked Questions about the Design, Operation, and Economics,” Indoor Environment Program, Energy & Environment Division, Lawrence Berkeley National Laboratory, Berkeley, Calif., November 1995:
  3. Hargy, T.M., “Status of UV Disinfection of Municipal Drinking Water Systems in North America,” WC&P, June 2002.
  4. Hutchinson, M., “UV: Selling Disinfection to the Public,” WC&P, January 2003: ArticleID=1908
  5. Meyer, E.A., “Water Purification Project by Ultraviolet Radiation,” Department of Molecular Microbiology and Immunology, Oregon Health & Science University, Portland, Oregon: html
  6. Reynolds, K.A., “On Tap: Ultraviolet Light—An Alternative Disinfectant,” WC&P, June 2002: ArticleID=1556

About the author
Mark Hutchinson works with the Residential Business Team at Trojan Technologies, a leading manufacturer of UV systems for residential, industrial and municipal applications. He can be reached at (519) 457-3400, (519) 457-3030 (fax) or website:



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