Dr Elfrida Carstea discusses the FLUORO-BOOST project's findings on the potential of fluorescence spectroscopy for improving the energy efficiency of wastewater treatment works.
As most chemical compounds contained in water are subjected to light interaction, fluorescence spectroscopy could rapidly and efficiently provide information about the quality of a sample of treated wastewater. The FLUORO-BOOST project is investigating its potential for improving the energy efficiency of wastewater treatment works.
Thanks to an investment of EUR 14.3 billion from 2007 to 2013 and tough legal action, the EU has seen considerable improvements in the collection and treatment of wastewater. But treating more wastewater and constantly improving post-treatment quality also puts a lot of pressure on current technologies, resulting in increased energy consumption. The aeration of settled sewage in the 'Activated sludge process' (ASP) alone contributes to over half of the energy costs associated with wastewater treatment.
What's more, quality control still has much room for improvement. According to Dr Elfrida Carstea from the University of Birmingham, who set out to find a solution to both problems thanks to EU funding under the FLUORO-BOOST (Fluorescence-Based Optimisation Of Sewage Treatment) project, current compliance testing methods have several shortcomings.
Instead of resorting to the widespread technique of monitoring the potential for removal of oxygen from water by aerobic heterotrophic bacteria, Dr Carstea suggests applying the recent progress made in the field of fluorescence spectroscopy to optimise wastewater treatment performance. As the project is getting close to an end, she agreed to discuss some of her findings.
What are, according to you, the main shortcomings of current wastewater treatment methods?
Wastewater treatment works are a great engineering achievement and the techniques used are highly effective in reducing the organic load from sewage. With the ongoing effort of engineers, the processes will become better and better. So, the problems do not lie in the treatment process itself, but rather in the methods used for quality control.
Treatment relies mostly on the off-line 'Five-day biological oxygen demand' (BOD5) test as proof of compliance with relevant legislation. BOD5 is defined as the potential for removal of oxygen from water by aerobic heterotrophic bacteria, which utilise organic matter for their metabolism and reproduction. Although this is a desirable measurement in treatment processes, it presents several disadvantages that make this technique unsuitable for on-line monitoring and process control: it is slow to yield information, it is labour-intensive, toxic substances affect bacteria, it may not reflect conditions in the treatment processes, it is insensitive and imprecise at low concentrations and it results in an uncertainty of 15-20 % in result accuracy. Because of these problems, the industry often has to over-treat to make sure they comply with regulation.
Your research, on the other hand, builds on the latest advancements in fluorescence spectroscopy. Why this choice?
This research is built on preliminary studies led by Professor John Bridgman and colleagues and published in Environmental Technology on the fluorescence of wastewater at a treatment works. They have shown that the technique offers significant potential for online characterisation and monitoring. Several studies, before this one, have argued that this technique may be suitable, but few focused on wastewater monitoring and none on real-time measurements. Fluorescence spectroscopy has been suggested for its multiple advantages: it is fast, inexpensive, reagentless, requires little sample preparation, is highly sensitive and is non-invasive. Therefore, this technique could provide rapid feedback, allowing dynamic and high-spatial and temporal resolution studies. Moreover, our research builds on studies that tested the portability of fluorescence on surface and drinking water, and that have generated a major step forward in the development of new technologies for implementing fluorescence in wastewater quality monitoring.
What were the main difficulties you faced in applying this new technique?
I wouldn't call them difficulties, but challenges. Establishing the relationship between fluorescence and BOD5 data turned out to be more difficult than we initially expected. We obtained surprising results, which led to more questions than answers. However, this will only help us to gain better understanding of fluorescence spectroscopy potential as a surrogate for BOD5 and as an effective tool for treatment process control.
How does your technology help bring down energy consumption in wastewater treatment?
Most energy usage in wastewater treatment arises from the vigorous aeration of settled sewage in the activated sludge process. The bacteria and microorganisms that form the activated sludge are fed with wastewater containing organic waste. In order to sustain the biological activity during the activated sludge process for BOD5 reduction, air is pumped into the tanks to provide sufficient quantities of dissolved oxygen. Aeration is one of the most energy intensive operations of wastewater treatment, with almost 65 % of energy being consumed for the activated sludge process. Energy consumption has increased significantly in the last two decades, and further increases by another 60 % are forecast in the forthcoming 10-15 years. These increases have been brought about primarily in response to tightened legislation and regulation surrounding the discharge of final effluent from treatment works to watercourses.
Replacing the outdated and inaccurate BOD5 with fluorescence spectroscopy provides the plant with the optimum tool for real-time control and remediation of plant performance failures. It is estimated that, by monitoring wastewater quality in treatment works, 40 % of the current energy costs could be saved. Thus, fluorescence may be used to optimise process control in treatment works and eliminate the unnecessary costs associated with overtreatment.
Is fluorescence spectroscopy also suitable for treatment of drinking water?
Yes, this technique is suitable for monitoring the treatment of drinking water. There are several studies that prove its potential. A very recent publication of Professor Bridgeman and colleagues, in Science of the Total Environment, describes a novel, LED-based instrument, developed by the authors at the University of Birmingham and University of Sheffield. The instrument is capable of detecting fluorescence peaks T and C in drinking water supply systems in situ. As shown by the research group, these peaks are of interest, as peak C provides a surrogate for the dissolved organic carbon present in the water, whilst peak T will identify any microbial growth, which occurs as a result of dissolved organic carbon presence and insufficient residual chlorine concentration in the water.
The project is close to its end. What are the next steps in your research?
The effort to implement fluorescence spectroscopy for real-time monitoring in treatment plants will continue through joint studies and collaborations with leading scientists in this field. Also, we have recently started to study the fate of nanoparticles in treated wastewater by various techniques, including fluorescence spectroscopy. I hope to continue on this path in the near future.
When do you expect florescence spectroscopy to be adopted in treatment plants?
This technology is already on the market; it is the application to real-time, in situ measurements, which is novel in our work. Although this technology is still in its infancy, it shows great promise for process control and energy reduction in treatment works.
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For further information, please visit FLUORO-BOOST: cordis.europa.eu/project/rcn/109162_en.html