Ultraviolet disinfection technical bulletin
This document outlines the ministry’s position for employing ultraviolet (UV) equipment as primary disinfection in municipal residential drinking water systems.
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Issue #2 – July 2010
Ultra-violet light (UV) lamps are sensitive to fluctuation in electric power. Lamps may lose the arc, resulting in UV failure for periods ranging from a few seconds to minutes. During drinking water compliance inspections, Safe Drinking Water Branch (SDWB) inspectors face challenges when they observe UV equipment failures (due to a power blip or a sudden flow change etc.) and to determine if this constitutes a non-compliant event. This document outlines the ministry’s position on performance criteria for UV equipment. It is intended to provide clarity to inspection staff when assessing compliance where UV equipment is installed for primary disinfection in municipal residential drinking water systems.
Primary disinfection of drinking water using UV is gaining popularity amongst drinking water system (DWS) owners due to its ability to inactivate pathogens without forming disinfection by-products (DBPs). UV disinfection is a means of killing or inactivating pathogens by UV light, in a dedicated environment (i.e., the reactors) by damaging their nucleic acid (DNA and RNA), thereby, preventing them from replicating. UV disinfection may supplement, and sometimes replace, other primary disinfection processes, but it does not provide secondary disinfection .
UV light of certain wavelength damages DNA of harmful protozoa, bacteria and viruses. The UV spectrum ranges from 40 to 400 nanometers (nm) with the most effective spectral for germicidal purposes being between 250 to 265 nm. UV wavelength around 254 nm is commonly used by manufacturers of UV lights.
The amount of UV light delivered to pathogens in a reactor is called "UV dose" and is measured in mJ/cm2. UV dose depends on:
- UV intensity, or magnitude of UV light, measured by UV intensity sensors in Watts/cm2 or Watts/m2.
- UV transmittance (UVT).
- Water flow rate and hydraulics in the reactor.
UV dose = UV intensity (Watts/cm2) × Time of exposure (sec)
UV dose delivery depends on a number of factors including reactor design (hydrodynamics), flow rate, UV transmittance of water, UV intensity field within reactor impacted by lamp output, lamp placement, aging, fouling and microbe inactivation kinetics. A safety factor is added to establish a design dose and is established through UV validation.
UV validation testing is usually conducted by the UV manufacturer or a third party to pre-validate their reactors to determine the operating conditions under which a UV reactor would deliver the validated dose. The validation testing is conducted for the full-scale testing of the reactor that will actually be used in field and inactivation of a test micro-organism with dose-response characteristics quantified through bioassay tests. The operating conditions include flow rate, UV intensity, UV lamp status, an account for UV absorbance of the water, lamp fouling, aging, inlet and outlet piping configuration of the UV reactor and measurement of uncertainty of on-line sensors, etc.
The Safe Drinking Water Act, 2002 (SWDA) and the Drinking Water Systems Regulation (O. Reg. 170/03) came into effect in 2003. The Regulation requires that water treatment equipment is capable of achieving primary disinfection in accordance with the ministry’s Procedure for Disinfection of Drinking Water in Ontario . Neither the regulation nor the procedure provides the necessary testing frequency, minimum alarm standard or performance criteria for UV equipment when it is used for primary disinfection. This document outlines the ministry’s position for employing UV equipment in municipal residential drinking water systems. This document also provides clarification for drinking water system owners/operators in order that there is a common understanding of the requirements when assessing UV performance, and to ensure safe drinking water in Ontario. The following technical clarifications have been approved by the Director, Safe Drinking Water Branch, Drinking Water Management Division, Ministry of the Environment.
For Inquiries Contact:
Ministry of the Environment, Conservation and Parks
Compliance, Promotion and Support Branch
40 St. Clair Avenue West, Toronto, ON, M4V 1M2
Tel: (416) 212-7318 Fax: (416) 212-7576
Operational Challenges (3)
UV performance is highly dependent on water quality, flow rate, electrical power quality and other operational parameters. To receive inactivation credit, a UV reactor must operate within the validated limits. When the reactor is operating outside of these limits, the UV reactor is operating “ off specification ” (e.g., flow rate higher than the validated range or a UVT below the validated range).
The UV lamps are sensitive to fluctuation in electrical power. According to UV manufacturers, voltage that varies more than 10 to 15 percent above or below the normal volt - age for as little as two to five cycles (power blip of 0.03 to 0.08 seconds) may cause UV lamps to lose their arc, i.e., UV lamps would go out; this phenomena is called “ down time ” due to a power blip.
The occurrences of down time and off specification are common features. Additional redundancies and uninterrupted power supplies can be provided to increase the overall reliability of UV reactors.
The UV systems are required to have a shut off feature or an alarm when the equipment malfunctions, loses power or ceases to provide the appropriate level of disinfection. Although the alarms must be re-activated immediately when the system operates in off specification or down time, there is always a delay in responding to this change. This is inherent to the UV controls because of electronic noise, communication issues, signal de-bouncing, etc.
Existing Regulatory Requirements for Continuous Monitoring Equipments (4)
The continuous monitoring equipment, residual chlorine analysers and turbidity meters must test and record the result at a minimum frequency of five minutes and 15 minutes respectively (section 6-5 of Schedule 6) .
Equipment must have a shut off feature or sound an alarm when it malfunctions, loses power, or if a parameter test result is above the maximum alarm standard or below the minimum alarm standard (section 6-5 (1.1) Schedule 6).
Fluctuation in Power Quality
Within two minutes of an alarm sound, if a further test result indicates that the parameter is no longer above or below the alarm standard, an operator need not be present to take appropriate action. The two minute window allows any test result that was taken during a power quality fluctuation to undergo a check and to reset automatically (self diagnostic) (section 6-5 (1.1) 3 of Schedule 6) .
O. Reg. 170/03 does not address the following relating to continuous monitoring equipment used in a UV system:
- a minimum testing frequency
- minimum alarm standard
- time window for self diagnostic and reset (due to power fluctuation, etc.).
Therefore, the ministry has adopted criteria for continuous monitoring equipment, as outlined in the next section.
The Ministry’s Position (5)
The Ministry has adopted the following criteria for continuous monitoring equipments used in a UV system:
- testing frequency must be five minutes or less and recording of test data cannot exceed four hours;
- all off specification alarms to be recorded at a minimum of five minute intervals; and
- if the UV system does not correct itself (self- diagnostic) within two consecutive alarms (10 minute period), the event is considered an adverse condition and appropriate actions must be taken as per section 16-4 of O. Reg. 170/03
In addition, DWS owners are required to prepare a monthly summary record of off specification alarm records at the end of each calendar month. The summary record must incorporate the duration of all off specification alarms, volume of water treated during this period and the action taken.
Q. What would be considered as an observation that needs to be reported under section 16-4 of Schedule 16 of O. Reg. 170/03?
A. Duty to report other observations (section 16-4) In the absence of any performance criteria outlined in the Disinfection Procedure, the following is recommended.
If UV monitoring equipment operates off specification (outside the validated range) for a continuous period of 10 minutes, and a report under subsection 18(1) of the SWDA has not been made in respect of UV monitoring in the preceding 24 hours, the observation is considered as an adverse and must be reported under section 16-4 of Schedule 16.
- footnote[*] Back to paragraph UVT is the per cent of UV light passing through the water over a specified distance, usually 1 cm. UV light is absorbed by the lamp sleeves, reactor walls, and substances in the water, such as iron, manganese, natural organic matter etc. If UV light is absorbed, it is no longer available to inactivate pathogens. UVT of filtered water is commonly between 85 and 90 per cent.