The rapidly growing role of UV-AOPs in the production of safe drinking water

A number of cities, facing increasing pressures on conventional water supplies, have begun to turn to alternative sources to meet demands for clean water. In some cases, the source waters available for purification may be of very low quality, having been reclaimed from wastewater effluents previously destined for disposal. Such low quality source waters would present unacceptable risks to public health if not treated by well understood, effective and reliable water treatment processes.

The most prominent examples of highly degraded source waters being considered for drinking water supply include the potable reuse of municipal wastewaters and the reuse of urban stormwater. In both cases, there is a high likelihood of contamination by toxic trace chemical contaminants. Furthermore, the specific chemical identities of the vast range of potential contaminants are predominantly unknown and, for the time being, largely unknowable. Consequently, the safe use of these highly degraded source waters requires broad-spectrum treatment processes, capable of effectively eliminating diverse chemical contaminants.

Conventional water treatment processes such as filtration and chemical disinfection have proved to be highly effective for the production of safe drinking water from most drinking water sources. Combined with source water protection strategies, these processes are relied upon throughout most of the world to produce drinking water free from unsafe chemical and microbial contamination. However, conventional water treatment is unlikely to be sufficient to effectively target the wide range of potentially hazardous substances which may be present in highly degraded source waters.

Advanced oxidation processes (AOPs) offer great promise for the challenge of treating contaminated water sources. Predominantly used as add-ons to conventional treatment trains, or to obtain water quality that is fit for a specific purpose within the drinking water supply chain (e.g. health care providers), AOPs are particularly effective at targeting the types of contaminants for which conventional treatment processes do not perform well. These include trace organic chemical substances such as pesticides, hormones, pharmaceutically active substances, and industrial organic chemicals.

In the context of drinking water treatment, the term ‘AOP’ has been applied primarily to systems specifically designed for the enhanced generation of radical oxidants. While a number of technologies are available, the last couple of decades have witnessed particular growth in interest relating to AOPs incorporating the use of ultraviolet (UV) radiation to produce the radical oxidants. By far the most well established UV-AOP system involves the use of UV radiation to produce hydroxyl radicals (˙OH) from hydrogen peroxide (H₂O₂).

The incorporation of UV/H₂O₂ AOP in treatment trains for potable reuse projects is a recent development. The Orange County Water Factory 21 project (California, USA) paved the way with the establishment of a UV irradiation process implemented specifically for the photolysis of N-nitrosodimethylamine (NDMA) in 2001. A similar treatment design was subsequently incorporated in a number of other projects including in Singapore. However, it was not until 2008 that true UV-AOP systems, incorporating the use of H₂O₂ for the production of secondary oxidants, were commissioned at the new Groundwater Replenishment System (GWRS) in California and the Western Corridor Water Recycling Project in Queensland, Australia.

The successful development of the Prairie Waters Project at Aurora, Colorado confirms the potential flexibility in potable reuse treatment train selection and that high-pressure membrane treatment preceding AOP treatment is not always necessary. In that case, natural treatment by riverbank filtration and soil aquifer treatment provide effective polishing steps prior to UV/H₂O₂ AOP.

More recently, interest in UV/Cl₂ AOPs for potable reuse projects has emerged, notably in current planning for the Terminal Island Water Reclamation Plant owned by the Los Angeles Department of Public Works Bureau of Sanitation.

Heterogeneous catalytic processes, such as those which use TiO₂ for the production of hydroxyl radicals, have not yet been applied to large-scale potable reuse projects. However, a tremendous volume of research has taken place in this field throughout the last few decades. Potential significant advantages of heterogeneous UV-AOPs include low-power operation and very high final water quality.

In addition to capital and operational costs, regulatory goals are a major driver for the selection and implementation of UV-AOPs for potable reuse projects. The trend toward more direct potable reuse may lead to increased requirements, or more rigorous performance assessment for chemical and pathogen control. UV-AOPs have been shown to be amenable to stringent performance validation and ongoing monitoring requirements. This has made UV-AOPs an attractive technology choice for potable reuse projects.

Areas of ongoing interest and research include identifying the oxidation products arising from UV-AOPs and their potential toxicity, and further optimising the design of AOP reactors and components in order to make them more affordable, scalable and adaptable to the installation of broad-spectrum water technologies.

In this themed issue of Environmental Science: Water Research & Technology, we have assembled a collection of papers presenting cutting-edge research on the development and application of UV-AOPs for advanced water treatment. We commend these papers to our readers as a spotlight on a topic which we expect will continue to rapidly grow in importance for the supply of safe drinking water in cities throughout the world.

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