The major components of all UV disinfection systems are:
a dedicated stable source of electrical power (separate circuit) to deliver a sufficient UV dose
a cylindrical reactor vessel or chamber made of non-corrodible material such as stainless steel (the reactor contains the UV lamp)
a low-pressure mercury vapour UV lamp that is properly secured inside
a quartz sleeve
a clear quartz sleeve (with a high UV light transmission rate) to protect the UV lamp from being coated with minerals
a control unit (also called a ballast) to provide the energy needed to start and sustain gas discharges in the UV lamp
a safety control to shut off the UV lamp in case of low flow levels and elevated lamp temperature.
Class A and Class B UV systems
There are two classes of UV systems (as established by NSF/ANSI Standard 55).
Class B systems are designed for supplemental bacterial treatment of disinfected public water or other water supplies that have been tested and confirmed to meet drinking water standards.
Class A systems are intended to inactivate microorganisms, including bacteria, viruses, Cryptosporidium oocysts and Giardia cysts in water that does not meet drinking water standards (contaminated water). They are designed for use on virtually clear water and not for water that has obvious contamination such as raw sewage. For small water systems with surface water sources or groundwater at risk of containing pathogens (GARP), regional health authorities will require Class A systems which include UV dosage and fail-safe requirements that go above and beyond those of Class B systems.
With reference to the BC Guidelines for Ultraviolet Disinfection of Drinking Water in the Drinking Water Officers’ Guide (DWOG), a Class A system should have:
- a minimum UV dose of 40 mJ/cm2 at a wavelength of 254 nm
- a built-in flow restrictor or automatic fixed flow rate control to ensure appropriate radiation contact time with the flowing water
- a sensor to monitor the intensity of the UV light passing through the water
- an alarm system (visual and audio) to alert the operator in case of low UV intensity
- an emergency shut-off valve
- warning devices and/or automatic water shutoffs (a solenoid valve) that activate when UV light dosage reaches a fail-safe set point below 40 mJ/cm² (most regional health authorities will require the automatic shutoff in order to avoid the consumption of untreated water)
- a UV performance data sheet that includes the rated service flow of the reactor in litres/minute or litres/day.
UV reactors for small water systems are typically certified based on a recognized certification standard such as NSF/ANSI Standard 55 Class A. For applications involving surface water sources or groundwater at risk of containing pathogens (GARP), regional health authorities will require Class A systems which include UV dosage and fail-safe requirements that go above and beyond those of Class B systems. NSF Standard 55 Class B certified systems are not acceptable for the production of potable water.
NSF-certified equipment complies with the standards and procedures imposed by NSF including extensive product testing and material analyses. Reactor certification is not the same as reactor validation as the certification and validation processes are not equal (i.e. there are many variables that are not accounted for in reactor certification). Reactor certification is discussed in Section 7 of the BC Guidelines for Ultraviolet Disinfection of Drinking Water in the Drinking Water Officers’ Guide (DWOG).
Certification information for NSF/ANSI 55 Ultraviolet Microbiological Water Treatment Systems is available online via NSF’s website.
A UV reactor must be validated before it is installed for drinking water systems. 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/ANSI Standard 55 Class A.
Water systems that serve more than 500 people in any 24-hour period should use UV disinfection systems that have been validated using one of the validation protocols listed in Section 6 of the BC Guidelines for Ultraviolet Disinfection of Drinking Water.
These protocols validate a UV reactor to determine the operating conditions required to deliver a required UV dose for pathogen reduction under variable flow rates, UV transmittance and UV intensity settings.
Validation testing is based on reactor type/model and is typically conducted by a recognized third party at a facility specifically designed for reactor validation.
Recommended maximum pathogen log reduction credits for NSF Standard 55 Class A devices are listed in Table 4 of the BC Guidelines for Ultraviolet Disinfection of Drinking Water. The PHE and DWO of regional health authority will review each UV proposal and they can use discretion in assigning pathogen log reduction credits based on an assessment of risk for any specific application.
The NSF Standard 55 does not require Class A certified systems to have a UV monitor, which provides an online readout of UV intensity and/or dose delivered. However, provision of a UV monitor and a reference UV sensor may be requested by the regional health authority to allow for monthly calibration verification checks of the duty UV sensor.
In 2019, a revision was made to NSF/ANSI 55 Ultraviolet Microbiological Water Treatment Systems establishing new criteria for use of ultraviolet light-emitting diodes (UV-LED) technology for microbial reduction and provides a new test method to certify manufacturer claims.
Small 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-LEDs are emerging as a viable technology for drinking water disinfection. Compared to conventional mercury UV lamps, UV-LED lamps are mercury-free, compact, robust, suffer minimal damage from repeated cycling, have longer life and reach full power faster. These advantages, along with virtually instantaneous start-ups and tunable wavelengths, offer great flexibility in UV-LED reactor design. Many applications of UV-LED reactors have focused on small-scale, point-of-use
systems due to cost and power considerations; however, some larger-scale applications have been developed and approved for installation under the U.S. EPA UVDGM validation protocol.
To be considered for pathogen log reduction credit assignment, UV-LED equipment for drinking water disinfection should be validated under an approved validation protocol or have NSF Standard 55 Class A certification.
Other UV Design Considerations
UV Transmittance (UVT) is a very important parameter for the raw untreated water entering the UV reactor. As shown above in Table 1 (Recommended Water Quality for Water Entering a UV Reactor), the UVT must be at least 75%; however, a UVT of at least 85% is preferred. The effectiveness of a UV disinfection system is determined by the dose that the system is able to deliver to the target microorganisms in the water. The effective UV dose is dependent primarily on the combined effects of the UV light intensity at a wavelength of 254nm, the exposure time, and the water quality (including the UVT).
To ensure the UV disinfection is effective, a small water system should consider measuring the UV transmittance (UVT) and the flow rate. 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 sized adequately to accompany maximum flow rates while achieving the required dose.
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. To prevent untreated water reaching the consumers, UV reactor bypasses should not be installed unless specifically authorized by a DWO for the provision of emergency water supply.
Design and construction guidelines for BC waterworks
Guidelines for the Construction of Waterworks, Interior Health, BC, 2014
For more information on viruses, refer to Health Canada’s Guidelines for Canadian Drinking Water Quality: Guideline Technical Document -- Enteric Viruses