A Fluorescent Future?

A new technology using fluorescence could reignite waning interest in wastewater recycling. Dr Gareth Evans lights up the new argument for recycled water systems that could change attitudes all around the world.

Endorsements of recycled water seldom come more blatant or compelling than when, at the end of 2008, Queensland's Deputy Premier Paul Lucas drank a glassful to mark the passing of a major milestone in the Western Corridor recycled water project.

It is then perhaps fitting that four months after he acknowledged the successful progress of this ambitious A$2.5bn scheme – designed to yield in excess of A$230m – he was also appointed new minister for health. In many ways, and albeit unintentionally, this new portfolio set alongside his pre-existing responsibility for infrastructure and planning, encapsulating both sides of the ongoing recycled water debate.

A question of perception

The innate appeal of the approach is obvious; reclaiming water allows the twin demands of an increasingly urbanised world – appropriate wastewater disposal and the ready availability of clean water – to be simultaneously and rather neatly met. However, if the uptake of recycled water is to grow to satisfy this need, it will have to be alongside concomitant assurances that the technology is safe enough to ensure water quality and safeguard public health while protecting both aquifers and the aquatic environment at large.

"The team's most immediate goal is to develop a practical,
hand-held fluorescence sensor."

Historically such cast-iron guarantees have been hard to find and public perception and the willingness to embrace the idea have suffered at times as a result.

Part of the problem lies in uncertainty over just what is meant by "recycled" or "reclaimed" water – the term being applied to both sanitised wastewater intended for irrigation and more comprehensively treated effluent destined for indirect potable use. The idea has been little explored in Europe, one of the main reasons cited being that "the concept is difficult for the regulators and wider public to understand and accept". So places such as Australia offer good points of reference for those seeking further solutions to meeting water demand.

If, as some in the industry believe, the key to changing some of these perceptional barriers lies in the ability to detect contamination accurately and swiftly, then the solution could just be on hand, in the form of fluorescence spectroscopy.

Fluorescent solution

Using fluorescence to characterise water is nothing new; dissolved organic material fluoresces in water when it is illuminated by a suitable type of lamp – a property that has led to it being successfully used as an indicator of various forms of water-borne pollution. As such, it has become an established technique for monitoring marine and river ecosystems, while its ability to detect farm wastes in agricultural run-off has been well documented by many institutions – the Université du Sud Toulon-Var, amongst others.

However, work at the University of New South Wales Water Research Centre – the subject of a recently published paper in the journal Water Research – suggests that the idea could potentially be extended to improve the performance of recycled water systems. If so, it could ultimately herald the development of the sensor technology that can provide the round-the-clock monitoring in real-time required to meet process-management needs of recycled water plants.

According to Dr Rita Henderson and her colleagues, monitoring based on fluorescence spectroscopy has two principal advantages over conventional chemical testing. Firstly, it offers high sensitivity. Under test conditions, Henderson and her team have consistently been able to detect the presence of recycled water in potable water at concentrations as low as 5%. Secondly, it requires no sample preparation prior to analysis, which should make it a faster and simpler solution to use in practice – making it even more attractive to the industry using it.

Much of the focus of the work to date has specifically been on dual reticulation systems, which supply both potable and recycled water to homes, the latter being used for flushing toilets, watering gardens and other expressly non-potable purposes. Although they are designed to be quite distinct, Henderson's evidence suggests that there can be accidental contamination between the two streams – hence the study's emphasis on detecting low-concentration mixes.

Developing the idea

The team's most immediate goal is to develop a practical, hand-held fluorescence sensor which can be held under running "potable" water in a dual reticulation household to safeguard against the possibility of cross over. But ambitions for the new application do not end there. If successful, the group hopes that one day detectors within the pipe-work of supplied houses will constantly monitor water quality, relaying information back to the reclamation plant and triggering alarms should pre-defined contamination parameters be detected.

"Dissolved organic material fluoresces in water when it is illuminated by a suitable type of lamp."

The idea is still very much in its infancy and inevitably there are hurdles to be overcome before this vision can even begin to make its way down the long road to becoming a future reality. As Henderson herself freely admits, the stability of the light source needs further work, while at present the impact of various water treatment processes on the fluorescence characteristics of dissolved organics is imperfectly understood. For their spectral analysis technology to function in the integrated way the team hope it eventually will, clearly these issues will need to be addressed. However, there is perhaps a more fundamental issue to consider.

With demand continuing to mount on the world's conventional resources and regulatory standards on wastewater becoming increasingly stringent, it is impossible to imagine that reclaimed water will not have a significant role to play in future provision. What is not yet clear is if this will predominantly see it involved in agricultural irrigation, the augmentation of potable-source reservoirs or direct householder use through more widespread dual reticulation supply systems. In the end, it seems likely that it is the answer to this question that will decide just how far, and in which direction, this particular application of fluorescence spectroscopy sensors will ultimately develop.