Monitoring water quality

Bethany Fox from the University of the West of England explains what Natural Organic Matter – NOM – is, why it needs to be measured, and how in situ real-time fluorescence-based sensors from Chelsea Technologies are helping

According to the 2013 United Nations Joint Monitoring Programme for Water Supply and Sanitation, access to safe drinking water and adequate sanitation is the most efficient way to improve human health. Globally, however, 800 million people still do not have access to safe potable water, with more than one billion people still practicing open defecation.

Management of the world’s water resources is under increasing strain from a growing population, increased urban population density and climate change. Water contamination, particularly sewage and anthropogenic pollution, is a major threat to ever depleting potable water supplies, agricultural practices, resource sustainability and health across the world. As such, water quality monitoring has become an unavoidable and essential focus for societies and politics. To attempt to tackle these issues and improve water quality, many environmental policies have been put in place, such as the European Union Water Framework Directive.

Natural organic matter (NOM) is a complex, ubiquitous, heterogeneous mixture that originates from both the surrounding environment and the microorganisms within the system. Environmental NOM is influenced by the land-use, hydrology and geology of its source; whilst biologically sourced NOM is created in situ via microbial processes.

Aquatic dissolved organic matter (DOM) has been increasingly researched and characterised over the past two decades due to its importance in global biogeochemical cycling and as a major source of carbon transportation. Within the

DOM mixture there are some naturally-occurring fluorescent (aromatic) compounds that allow for analysis to be undertaken using fluorescence spectroscopy – a sensitive and non-destructive technique for water analysis.

The practice of benchtop fluorescence spectroscopic analysis has led to a greater understanding of aquatic DOM, particularly with the increased use of Excitation-emission matrices (EEMs). EEMs provide three dimensional data that can be ‘mapped’ to create a visual representation of the DOM present within a water sample. This methodology has now become standard within aquatic DOM analysis. EEM analysis of a sample provides high spectral resolution data with higher sensitivity compared with absorbance analysis, and is quicker to perform than separation techniques such as high-performance liquid chromatography. The major drawback with EEM acquisition is that the instruments are laboratory-based. Since fluorescence is highly susceptible to quenching during filtration, storage and over time, grab sampling compromises both the data quality and possible spatiotemporal data resolution.

The increased understanding of aquatic DOM provided by EEM analysis has led to a drive to monitor anthropogenic and environmental DOM across a wide range of applications, in situ. The ability to monitor water quality and microbial processes within freshwater systems in a cost-effective manner with in situ sensors is now understood to be essential for improving water quality assessment to achieve the spatiotemporal resolution required for accurate event monitoring.

An emerging technology

Novel instrumentation is urgently required which will enable the real-time determination of the microbial health and status of freshwater systems (surface and ground). Existing techniques require time consuming active sampling and laboratory analysis (18-36hr microbial enumerations), which lack spatiotemporal resolution and do not permit the real-time monitoring of environmental or ecosystem/ecological health. One such emerging technology is provided by the Chelsea Technologies Group (CTG) range of in situ real-time portable fluorescence-based sensors that are capable of high temporal

resolution monitoring. One major application of these fluorescence-based sensors is the identification of anthropogenic contaminants, such as polycyclic aromatic hydrocarbons (PAH) and optical brighteners.

CTG also offers in situ portable sensors with the ability to monitor sewage contamination, using microbially derived fluorescence signals that indicate the presence of proteinaceous material, as well as sensors that are targeted

to detect different groups of DOM. Recent developments within this area have seen the deployment of multi-parameter fluorescence sensors, which provide spectral information of multiple DOM regions, allowing for the improved identification of contamination events.

Changes in policy and public health concerns develop with increased knowledge and research into aquatic DOM, providing increasing opportunities for fluorescence sensing. One key application of growing interest is the ability of optical in situ sensors to monitor the formation potential of disinfection by-products (DBPs), including carcinogenic trihalomethanes (THMs). DBPs

are produced during chlorination of NOM. The well-established CTG CDOMfluorescence sensor is now being used to monitor the DOM precursors of these harmful by-products, helping to inform on the management of DBPs in treated water.

Research

Ongoing research is continuing to improve the understanding of fluorescent DOM and how it can be used to provide a quick and easy monitoring network with high temporal and spatial resolution. Growing acceptance of fluorescence monitoring enables the environmental observation of our water resources to be vastly improved. This has the potential to reduce the global impact of contaminated water on poor quality of health and sustainable agricultural practices, and have far reaching societal and economic benefits.

Chelsea Technologies Group