A clear understanding of UV disinfection technology needed
While the benefits of UV disinfection are well established, a clearer understanding of the issues around system design and operation is still needed, argues Tom Hall of WRc.
Ultraviolet (UV) irradiation, first used for drinking water disinfection 100 years ago, seems straightforward enough – switch it on, the lamps glow and the blue light kills the bugs. What could be easier? If that is the case, though, why is it that the US Environmental Protection Agency (USEPA) UV Disinfection Guidance Manual runs to over 460 pages on how to design and operate UV systems.
Clearly there is more to this than meets the eye, and much of the complexity arises from the need to identify and maintain appropriate disinfection doses. The use of UV disinfection is increasing in the UK, mainly because it is now accepted as a treatment option for Cryptosporidium.
Before the 1990s, UV was believed to be ineffective for Cryptosporidium, because assessment of UV performance was based on in vitro assessment of oocyst viability rather than the more reliable direct measurement of infectivity more widely available later. In the UK drinking water regulations there was a standard for Cryptosporidium of one oocyst in 10l, which required removal of oocysts rather than inactivation.
This standard was removed in 2008, allowing water companies to install UV specifically for Cryptosporidium as well as for general disinfection. In February 2010 the Drinking Water Inspectorate (DWI) published guidance on the use of UV irradiation for the disinfection of public water supplies, and water companies have needed to develop a clear understanding of the technology to meet DWI requirements.
UV light is in the wavelength range 100-400nm of the electromagnetic spectrum. Wavelengths of 200-300nm, sometimes referred to as the practical germicidal range, are absorbed by nucleic acids in microbial cells. The damage caused to the nucleic acids prevents microorganisms from replicating. Maximum absorbance and damage occur at about 254nm.
The technology uses UV lamps in purpose-built reactors, which can be relatively quickly installed compared with other disinfection technologies where contact tanks are required. The mercury vapour gas discharge lamps currently used for water treatment are either low pressure (LP) or medium pressure (MP) dependent upon the mercury vapour pressure in the lamp.
The UV output of LP lamps is practically monochromatic at 254nm. Most LP lamps now available are either high output (LPHO), or LP-amalgam, which typically provide twice the UV output of conventional LP lamps.
In MP lamps, the UV light is generated over virtually the entire UV spectrum, but output in the germicidal range is up to 100 times greater than from LP lamps; far fewer MP lamps are therefore required for a given throughput. Modern MP lamps have efficiencies (germicidal UV output relative to electrical energy input) closer to those of LP lamps compared with older systems.
Good knowledge of inlet water quality is vital for design of UV plant. UK drinking water regulations requires turbidity during disinfection to be below 1 NTU. The minimum UV transmittance (UVT) of the water is a critical design parameter, and overestimating this value risks under-sizing the plant. Allowance needs to be made for materials which can cause fouling of the lamps, such as iron, manganese and hardness.
Because UV achieves disinfection in a few seconds, plant footprint is small. Electricity consumption, typically in the range 5-20 kWh/Ml provided UVT is 90% or more, represents the main operating costs, together with routine lamp replacement. From reported experience, UV at usual disinfection doses has negligible by-product formation potential. However, UV at wavelengths below 230nm reduces nitrate to nitrite: there is a drinking water standard for nitrite, and nitrite exerts a significant chlorine demand, so MP lamps treating water containing nitrate often have quartz sleeves which block wavelengths below 240nm.
It is not possible to directly monitor UV dose, which is a function of the distribution of UV intensity throughout the reactor and the retention time of the water. Performance has to be inferred from other measurements – UV intensity at control points, UVT, flow rate.
Determining the effective dose of a UV plant is not straightforward, because for a given UV reactor it depends on water quality (primarily UVT) and system hydraulics. Manufacturers of UV reactors have the performance envelope of their equipment independently validated, and the end user must operate the plant within this envelope to ensure adequate disinfection is achieved.
Identifying UV dose for older systems relied on a modelling approach based on estimated UV intensity at many points within the reactor and assumed flow patterns. However, the limitations to this approach have led to the development of dose validation methods based on biodosimetry. This involves the reactor being challenged with a microorganism of known UV sensitivity, and the UV dose is identified from the observed inactivation in the reactor under a range of conditions compared with inactivation data from laboratory tests.
Regulatory requirements and guidance
In America, the validation protocol described in the USEPA UV Disinfection Guidance Manual is based on demonstrating a target inactivation of Cryptosporidium or Giardia using surrogate test microorganisms. Any suitable challenge microorganism can be used, and a methodology is provided to determine from the inferred dose for the challenge microorganism whether the target inactivation will be achieved.
The principal European standards for UV plant intended for disinfection of drinking water – DVGW W294 (Germany) and ÖNORM 5873 (Austria) – stipulate a minimum inferred UV dose of 40 mJ/cm2 using Bacillus subtilis spores as the test microorganism. The USEPA guidance allows reactors with European validation to claim a 99.9% inactivation credit for Cryptosporidium or Giardia, although this is probably conservative.
In February 2010 the Drinking Water Inspectorate (DWI) published guidance on the use of UV irradiation for the disinfection of public water supplies. This requires dose validation, but leaves water companies to decide which protocol, USEPA or European, is more appropriate for a particular application.
Implementation and issues
In December 2011, a conference organised jointly by WRc and the International UV Association (IUVA) was held in London, aimed at sharing of experience on implementation of UV systems in the UK, mainland Europe and the USA. Speakers and topics at the conference were:
- Engineering consultancy Atkins’ Briony Grose reviewed the design of five new plants in the Bristol Water area, where issues over mercury and glass containment from lamp breakage, and measures to prevent accidental by-pass of reactors, had been addressed. Considerations had also been given to consistency between the alternative dose validation approaches in relation to DWI requirements
- A paper on the design and operation of new plant by DÅµr Cymru Welsh Water was presented by Andrew Elphinston, where fouling of lamps had been addressed in particular
- Richard Lake of Veolia Water described the company’s approach to UV as part of an overall disinfection strategy, including as an alternative to membranes for Cryptosporidium
- Andrew Campbell of United Utilities discussed the design of 12 new plants in United Utilities, mainly for general disinfection to reduce the need for chlorine contact time. A previously unrecognised issue of bromate formation had been identified when UV was used after chlorine dosing
- Mike Templeton of Imperial College London reviewed dose validation procedures, and potential approaches for carrying out on-site validation for existing plants
- Steve Lambert (on behalf of DWI), reviewed approaches to development and implementation of the UK regulatory guidance. The difference between the European and USEPA approaches to dose validation, and the relationship between these and the UV application (for Cryptosporidium or general disinfection), was highlighted
- As well as disinfection, the UK use of high dose UV for destruction of organic micropollutants such as pesticides was presented by Barrie Holden of Anglian Water and Chris Rockey of South West Water.
- Design and implementation of UV in the USA, Austria, Germany and Switzerland were reviewed by Bertrand Dussert (IUVA), Regina Sommer (Medical University of Vienna), Jutta Eggers of the Water Technology Centre (TZW) in Germany and Margarete Bucheli of Switzerland respectively. Similar concerns to those in the UK over the existence of older plant without formal dose validation occurred in other countries
In summary, UV offers a chemical free approach to disinfection, with relatively minor concerns over by-products compared with chemical disinfectants such as chlorine, ozone or chlorine dioxide. Its use in the UK and elsewhere is increasing, mainly to deal with Cryptosporidium, but also to reduce reliance on chlorine for disinfection.
For effective implementation, and to comply with regulatory requirements, a clear understanding of the complex issues around UV design and operation is needed, particularly in relation to establishment and maintenance of UV dose and the appropriateness of alternative dose validation procedures to the site-specific disinfection requirements.
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