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

• Published in: The Science of the total environment Vol. 622-623; pp. 1250 - 1257
• Main Authors: Sorensen, James P.R., Baker, Andy, Cumberland, Susan A., Lapworth, Dan J., MacDonald, Alan M., Pedley, Steve, Taylor, Richard G., Ward, Jade S.T.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.05.2018
• Subjects: Drinking water; Drinking Water - chemistry; Drinking Water - microbiology; Environmental Monitoring - methods; Faecal contamination; Feces - chemistry; Fluorescence; Fluorescence spectroscopy; India; Malawi; South Africa; Thermotolerant (faecal) coliforms; Tryptophan - analysis; Tryptophan - chemistry; Tryptophan-like fluorescence; Water Microbiology; Water Pollutants - analysis; Zambia; Fluorescence spectroscopy; Drinking water; Faecal contamination; Tryptophan-like fluorescence; Thermotolerant (faecal) coliforms
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2017.11.162

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 (ρs=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. [Display omitted] •Urgent need to screen drinking water for faecal contamination rapidly.•Multi-country assessment of TLF as an indicator of faecal contamination.•A 1.3ppb dissolved tryptophan threshold is effective to infer contamination.•TLF is strongly correlated with thermotolerant coliform concentration.•TLF is a commercially available, easy-to-use, reagentless, real-time option.