Research Notes: LEDs and micropollutant removal
LEDs that emit ultraviolet light could play a valuable role in water treatment if used as an alternative to UV lamps. Researchers at Cranfield University are exploring the potential and limitations of large-scale deployment of LEDS for advanced oxidation
By Dr Irene Carra Ruiz, Research Fellow, Cranfield Water Science Institute
The Blackpool Illuminations have been getting bigger and brighter for over 130 years – but recently their electric bill has been going down. Light emitting diodes (LEDs), requiring a fraction of the power of their conventional or even ‘energy saving’ alternatives, are a green alternative for homes and businesses. What might their potential be for the water industry? New research is investigating how LEDs that emit ultraviolet (UV) light could improve treatment processes for removing micropollutants in water and wastewater, including harmful pesticides and organic matter.
UV-oxidation treatment in the water industry usually involves a combination of UV light and hydrogen peroxide (H2O2). This produces hydroxyl radicals in the water which have the power to oxidise many contaminants and break them down from harmful chemicals to more simple components. This is just one of a number of UV-based processes that can be used to remove persistent compounds from water, called Advanced Oxidation Processes (AOPs). Another AOP that has not been widely implemented at large scale but is equally interesting, involves the combination of UV and titanium dioxide (TiO2). This oxidation process is very attractive for water treatment since titanium dioxide can be easily separated at the end of the process and reused, making the treatment effectively chemical free. At present, research at Cranfield University, in collaboration with the water industry, is focussing on implementing this AOP at larger scale.
The existing legislation in the UK regarding micropollutants such as pesticides, drugs and endocrine disruptors means that these AOPs are only really used for specific water quality challenges. For now they are used mainly in drinking water treatment for targeting pesticides. However, the direction of travel is towards reducing micropollutants further, even in wastewater and where water is being reclaimed and reused. EU regulations such as the Water Framework Directive indicate this strongly. And our ongoing reliance on pesticides in food production and antibiotics in health mean that the challenges may get greater as micropollutants become more prevalent at all stages of the water cycle. The industry therefore needs to consider much wider and more sustained use of these AOP technologies.
Innovations in AOPs are therefore crucial to keep on top of the micropollutant issues. An example of this is metaldehyde. Metaldehyde is the most reported pesticide causing failures in the UK water companies (Drinking Water Inspectorate Annual Reports 2010-2014). This pesticide, used to control slugs and snails, is a persistent compound which is not easily removed by conventional processes such as adsorption. At Cranfield University, we’re currently working with a number of water companies and technology providers to evaluate different AOPs at a range of scales of investigation as the industry is keen to find solutions that work for the largest and smallest treatment systems.
If the time comes when UV-oxidation needs to be rolled out on a much larger scale across the water supply network, the potential limitations to the process as it stands will become clear; not least in the electricity bill. The UV lamps required are expensive to purchase at the outset, and expensive to run at scale. UV lamps also do not burn out as normal bulbs do. They ‘solarize’ which means their intensity can be reduced by up to 60% through just one year of continuous use. This is shortened significantly if the lamp is turned on and off frequently, meaning their effectiveness and feasibility is reduced. UV bulbs also contain mercury, which means their disposal must be properly managed. In terms of cost, energy use, degrading performance, and waste, they are not a green solution.
LED bulbs potentially offer significant advantages here. They are considerably more energy efficient as they convert less electricity to heat rather than light. They also have a long useful life with no degradation in performance, and they offer the opportunity to be much more flexible in reactor designs and geometry. In addition to these benefits there is also the fact that the LED market is growing rapidly, with many different manufacturers supplying them, so their price is becoming much more competitive. LEDs therefore present a very attractive option.
The use of LEDs in water treatment processes would be a real innovation for the sector. At Cranfield University, we are looking into addressing some of the limitations of applying LEDs. Their limitations come from their smaller size and output power, particularly when providing light in the UV light wavelengths. This is being addressed through investigations of new reactor designs. If we can engineer solutions which solve the issues, we can accelerate the development of UV LEDs in oxidation processes.
We do still need technology companies to help advance the capabilities of LEDs further, to become a fully scaled-up processes; this would better arm the industry to tackle the micropollutant challenge and help them to achieve some of their ambitious targets, such as zero water quality failures.
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