Real-time detection of faecally contaminated drinking water with tryptophan-like fluorescence: defining threshold values

Abstract

We assess the use of fluorescent dissolved organic matter at excitation-emission wavelengths of 280nm and 360nm, termed tryptophan-like fluorescence (TLF), as an indicator of faecally contaminated drinking water. A significant logistic regression model was developed using TLF as a predictor of thermotolerant coliforms (TTCs) using data from groundwater- and surface water-derived drinking water sources in India, Malawi, South Africa and Zambia. A TLF threshold of 1.3ppb dissolved tryptophan was selected to classify TTC contamination. Validation of the TLF threshold indicated a false-negative error rate of 15% and a false-positive error rate of 18%. The threshold was unsuccessful at classifying contaminated sources containing <10 TTC cfu per 100mL, which we consider the current limit of detection. If only sources above this limit were classified, the false-negative error rate was very low at 4%. TLF intensity was very strongly correlated with TTC concentration (ρ=0.80). A higher threshold of 6.9ppb dissolved tryptophan is proposed to indicate heavily contaminated sources (≥100 TTC cfu per 100mL). Current commercially available fluorimeters are easy-to-use, suitable for use online and in remote environments, require neither reagents nor consumables, and crucially provide an instantaneous reading. TLF measurements are not appreciably impaired by common intereferents, such as pH, turbidity and temperature, within typical natural ranges. The technology is a viable option for the real-time screening of faecally contaminated drinking water globally.

Microbial Processing and Production of Aquatic Fluorescent Organic Matter in a Model Freshwater System

“…This is in agreement with the daily samples from the 10-day experimental period previously discussed. From this, unlike groundwater systems [21,22], it can be recommended that Peak T fluorescence cannot be used to determine species specific enumeration within surface freshwater systems where there is high AFOM background [12,52]. From the data that is presented within this study, it can be suggested that the correlations identified in other work are limited by low temporal resolution (five days) used during sample analysis and not truly reflective of the complex microbial-OM interactions that occur within these dynamic systems.…”

“…This study has been limited to the application of Peak T as an independent water quality parameter within surface freshwaters. However, this research builds upon existing AFOM research undertaken in groundwaters [21,54,55], surface freshwaters [52,56,57], estuarine systems [50,58], and the marine environment [11,36,59]. The wide spread use of this approach throughout the hydrological continuum would need to account for the differing complexities of sample matrices.…”

“…The known relationship between Peak T and BOD has popularized the employment of Peak T intensities as an in situ indicator of BOD [13,19,20]. More recent freshwater research has focused on groundwater systems, with low organic loads, where Peak T and Escherichia coli counts have been well correlated [21,22]. Despite this, the underpinning microbial-AFOM interactions within environmental systems are still lacking a comprehensive understanding.…”

Tryptophan-like fluorescence as a measure of microbial contamination risk in groundwater

“…We followed manufacturer recommended sampling protocols, which have been used in previous groundwater studies (Sorensen et al, 2015(Sorensen et al, , 2016(Sorensen et al, , 2018a. Approximately three litres of unfiltered water were pumped or poured into a container kept a black box to prevent ambient light from interfering with the measurement.…”

The Future of Water: A Collection of Essays on “Disruptive” Technologies that may Transform the Water Sector in the Next 10 Years

“…P R Sorensen et al, 2015; James P.R. Sorensen et al, 2018;Verma & Gupta, 2015;Zamyadi, Choo, Newcombe, Stuetz, & Henderson, 2016;Zhou et al, 2018). Ketos is developing such an ecosystem.…”