Multi-Parameter Compensation Method for Accurate In Situ Fluorescent Dissolved Organic Matter Monitoring and Properties Characterization

Guilherme De Oliveira,John Awad,Steve Bird,Kelvin O’Halloran,Aleicia Holland,Edoardo Bertone,Rodney Stewart

doi:10.3390/w10091146

Abstract

The recent deployment of fluorescent dissolved organic matter (fDOM) probes in dam catchments and drinking water treatment plants (DWTP) for water quality monitoring purposes has resulted in the production of a large amount of data that requires scientific evaluation. This study introduces a comprehensive, transferable methodological framework for scientists and water professionals to model fluorescence site-specific quenching on fDOM probe readings caused by temperature, suspended particles, and the inner filter effect (IFE) and applies it to an Australian subtropical reservoir. The findings revealed that quenching due to turbidity and IFE effects were best predicted by threshold autoregressive models. Raw fDOM probe measurements were validated as being more reliable if they were systematically compensated using the proposed procedure. The developed fDOM compensation procedure must consider the instrument features (i.e., wavelength broadband and responsiveness) and site-specific conditions (i.e., DOM characteristics and suspended particles). A finding of particular interest was that the compensated normalized fDOM readings had a high correlation with the low (<500 Da) molecular weight fraction of the DOM, which is more recalcitrant to removal by coagulation. As a consequence, there is potential to use compensated fDOM probe readings to provide real-time, in situ information on DOM properties in freshwater systems, which will enable water treatment plant operators to optimize the coagulation process.

Highlights

A sequential compensation method to account for in situ interferences on measurements of a fluorescent dissolved organic matter (fDOM) probe has been developed, based on raw water sampled from a South-East Queensland reservoir and data collected during laboratory experiments
An inner filtering effect played a role in self-quenching fDOM at ultraviolet absorbance at 254 nm (UV254) levels above 0.6 cm−1
The compensation framework relies on site-specific water quality rather than standards and it can be straightforwardly re-applied to other sites where the same probe is deployed

Summary

Introduction

Fluorescence probes can measure fluorescent dissolved organic matter (fDOM) amounts through a non-invasive, online approach [4] Such probes are typically installed in multi-parameter sondes such as vertical profiling systems (VPS), which are useful for real-time in situ water quality monitoring, as well as an input for prediction of other critical water quality parameters [5,6]. These fDOM probes emit a beam of UV light at a certain wavelength to excite determined organic molecules in a given sample matrix, and detect the UV signal at a longer wavelength emitted by the excited molecules. Field fDOM measurements, are subject to quenching caused by changes in temperature, turbidity, pH, salinity, metal ions and inner filter effect (IFE), and for this reason they should be compensated [7]

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