The rise and rise of UV disinfection
Ultraviolet disinfection is no longer an emerging technology. This is supported by the fact that it is used to treat more than three billion litres of water every day around the world. Craig Howarth reports.
The regulatory acceptance of UV for treating drinking water and regulatory standards for validating new UV reactor designs all signal a major shift in the acceptance of the technology into the mainstream. The UV industry has experienced double digit sales growth over the past 20 years, and combined annual sales of UV products worldwide will soon be in excess of £270M.
The use of computational fluid dynamics modelling has vastly improved manufacturers' ability to predict with confidence the level of treatment required using their proprietary equipment. System sizing is no longer a black art, as the selected manufacturer can work with the design engineer to accurately predict treatment levels under varying conditions of water quality and flow.
All UV equipment manufacturers will soon use this tool to optimise the dose delivery of their reactors and minimise energy costs. As manufacturers develop and improve optimised reactors, they will then validate the designs using European or USEPA validation protocols. These optimised reactors will be rolled out over the next several years.
Conventional UV lamp technology will also improve. Medium-pressure lamps will continue to see gains in energy efficiency, lamp life and power density, with quartz coating techniques extending lamp life to considerably more than 12,000 hours.
This approach will remain favoured for compact, small-footprint installations, particularly retrofit, or where automated wiping is required. Low-pressure, high-output lamps will also have increasing power, perhaps approaching 1kW, which will decrease the footprint and maintenance requirements for systems using this technology. Lamp disposal will emerge as a significant issue for low-pressure UV installations which use many thousands of low-pressure lamps.
New UV light sources such as light-emitting diodes (LED) claim to be a technology of the future. The advantages of LEDs are their ability to concentrate virtually all of the electrical power into a very narrow bandwidth of 260nm to 262nm, their vastly superior power efficiencies, a very long lamp life - reported to be greater than 100,000 hours - and, because of their point-source nature, they are not restricted to conventional cylindrical designs.
Likely drawbacks of this promising technology will be in the power supply drives for the lamps, which remain largely in the concept phase. Other lamp types such as Excimer lamps show some advantages, including being mercury free and having no warm-up time, but are currently limited by low power efficiency and high ballast costs. The Excimers are often also more toxic than the elements they propose to replace.
Another interesting technology involves the use of microwaves to energise a UV lamp without the use of electrodes. Developers claim to have produced power outputs of up to 1,000W with similar UV outputs to low-pressure lamps, which would dramatically improve the footprint and maintenance of low-pressure lamp-based systems.
The absence of electrodes also greatly increases the lamp life. This development could well see microwave power supply emerge as the consumable, with the lamp remaining in situ for four to five years. The long-term effects of using microwaves on sleeve wipers remains unknown.
UV sensor technology has also greatly improved over the past decade, with stable, reliable and germicidally accurate sensors now available and a well regulated calibration protocol now in place.
In addition, manufacturers have improved the proprietary control systems for taking information from the sensors, flow meters and other monitoring devices and using this information to optimise the performance of their equipment. They can also interface with the operator at a plant's control centre.
The D10 values (*see end note) of more and more micro-organisms is now known, with the list growing all the time. Most notably, research has confirmed the very low doses required to disinfect Cryptosporidium and Giardia, while also finding several viruses that have an unusually high D10. As new applications for UV are found, new microbes will be added to existing D10 tables.
A major concern to the UV industry is the issue of reactivation - the apparent ability of some micro-organisms to repair the damage done to their DNA by UV, reactivating their ability to infect. DNA repair can occur in a closed (dark) system, but is more likely in open systems under direct sunlight (photoreactivation).
The dose level and lamp type seem to affect the degree of reactivation, with low-pressure (single-wavelength) UV lamps appearing to be more susceptible to photoreactivation than medium-pressure (multi-wavelength) lamps. A much larger research effort into the area of photoreactivation is required and will most likely be forthcoming over the next five years.
A significant amount of research has also targeted the question of UV disinfection by-products, specifically the most common water constituents such as chlorine, bromide, nitrate, ozone, natural organic matter, and iron. At normal UV disinfection doses no significant disinfection by-products have been shown to form. Research continues with more exotic water constituents.
By far the greatest potential market for UV disinfection is drinking water. UV is now accepted as an available technology to deactivate Cryptosporidium and Giardia in surface water and other vulnerable sources.
From 1997 to the present, growth in this market has been generally slow due to several factors, including:
- The uncertainty of sensitivity of Cryptosporidium and Giardia to UV
- The lack of a regulatory framework for UV disinfection
- The lack of a guidance manual
- The lack of case histories and engineering knowledge in the application of UV in drinking water plants
- The general conservatism of the water industry
- The uncertainty of the outcome of several court cases considering a royalty on the use of UV for Cryptosporidium and Giardia destruction
Another UV application with much potential is wastewater re-use for irrigation and greywater applications. Re-use is already common in the south-western US and other areas of acute fresh water shortages such as southern Europe, the Middle East and North Africa.
UV systems for this market are validated to much higher doses than drinking water systems. Drinking water type product validation - with the accompanying rigor - will probably emerge as the dominant method of assessing suitability for these critical applications.
The ability to prevent photo repair will also emerge as key.
Another new market for UV is disinfecting water for aquifer storage and recovery. This involves pumping highly treated wastewater into aquifers to recharge drinking water supplies. California, Texas and Florida are three US states considering this approach, and there is growing interest in other parts of the world.
Finally, UV for advanced oxidation involves the use of UV, either by itself or in combination with a hydroxyl radical, to break down contaminants in water. This technology has already been successfully used for groundwater remediation, industrial wastewater treatment and drinking water treatment.
Most notably, several large advanced oxidation projects in the US have involved the use of advanced oxidation for N-nitrosodymethylamine, Methyl tertiary-butyl ether, pesticides, taste and odour compounds, and chlorinated solvents.
In conclusion, the UV industry has matured considerably over the past decade, and is now highly regulated and dominated by major water companies. Conventional UV technologies have been field-tested and now have considerable track records in a wide range of applications. Uncertainties surrounding regulations, royalties, technology and engineering have decreased and acceptance of UV is expected to grow rapidly over the next 20 years.
Conventional UV designs have been greatly aided by CFD, which will be used as a routine sizing tool for future designs. Incremental improvements in conventional lamps, sensors and controls will also continue over the next decade.
New technologies such as LED lamps and microwave lamps hold promise of further improvements in electrical efficiency, footprint and cost.
The stage is now set for dramatic growth in the drinking water market, especially if new technologies can bring increased efficiencies and lower costs. Other applications, such as wastewater reuse and aquifer storage and recovery are smaller, and will grow at slower rates, but are still attractive applications for UV.
The use of UV for advanced oxidation is still in its infancy and is highly dependent on energy costs. These markets will grow dramatically if newer, more energy efficient technologies are available.
Craig Howarth is managing director of Hanovia.
T: 01753 515300.
>* The D10 value for a micro-organism is the UV dose needed to cause a 99% reduction in colony forming units. The relationship between UV dose and kill rate is logarithmic. For a 99.99% kill rate of a micro-organism, the dose is determined by multiplying the D10 value by four.