Many of you working in the water and wastewater industry will have observed the emergence of Ultraviolet technology as a player in the world of disinfection over the past 10-15 years. This article will provide a brief introduction to UV technology and outline a recent advancement in the field: - the generation of UV light using microwave energy instead of electrical energy.
UV – A Natural Disinfectant
There is nothing inherently new about UV disinfection. Sunlight has long been recognised as a natural disinfectant. Ultraviolet light is absorbed by DNA disrupting the bonds between the DNA bases thereby inhibiting the ability of the organism to replicate. It is this same property of UV light that can lead to skin cancer.
This same phenomenon was occurring in the primordial soup 3.5 billion years ago, where it is speculated that high levels of UV radiation in earth’s early atmosphere caused mutations that led to development of more complex microorganisms.
Ultraviolet light is part of the electromagnetic spectrum which is a rather broad spectrum encompassing gamma rays at one end and radio waves at the other end. Visible light occupies an area towards the middle of this spectrum and to the left of this is the UV range. UV light is further divided into UVA UVB and UVC. For the purposes of disinfection UVC is the range of interest and in particular UVC at 254nm.
The first scientific discovery that UV light inactivated microorganisms was in 1878 (Downes and Blunt, 1878). The technology underwent a period of development with significant progress made by the 1940’s. However in North America chlorine became and remained the disinfectant of choice for water and wastewater applications as it proved to be both cost effective and reliable.
One of the drivers which stimulated interest in the use of UV disinfection technology in wastewater applications was concern regarding the impact of chlorination by-products such as trihalomethanes on the environment. In the late 1970’s the USEPA began to discourage the use of chlorine for the disinfection of wastewater and more restrictive limits began to be placed on chlorine residuals. UV, while still relatively unknown at the time in North America, emerged as a viable alternative as it does not generate any harmful disinfection byproducts (Whitby and Scheible 2004). It also eliminates the potential for overdosing as you can irradiate water with too much UV and there are none of the chemical handling and storage issues associated with chlorine. In the field of drinking water treatment UV has long been widely adopted in Europe and has received renewed attention in recent years due to the fact that it is effective against cyst forming bacteria such as Cryptosporidium and Giardia which are resistant to chlorine disinfection.