Rapidly monitoring microbial quality of water

Contaminated drinking water is estimated to cause nearly 0.5M diarrheal deaths each year, largely because 80 percent of wastewater is dumped untreated into the environment causing widespread water pollution. According to the World Health Organization (WHO), at least 2 billion people globally use a drinking water source contaminated with wastewater. Rapid measurement of drinking water quality is therefore essential in to support community monitoring, regulatory surveillance, emergency preparedness and behavior change, and thus preventing the spread of disease and increased mortality. However, the traditional method of culturing the sample and counting bacteria is expensive, centralized, and takes multiple days for results to be available.

The United Nations Children’s Emergency Fund (UNICEF) has recognized the gap in rapid, affordable testing for E. coli bacteria since E.coli is a indicator organism for fecal contamination) in drinking water, and they have created a target product profile (TPP) for new product development in this space. The minimum performance requirements include a time to result of less than 6 hours, a minimum limit of detection of 10 CFU/100 ml, with both a 10% false-positive and false-negative error rate. In addition, the product should be capable of being operated with a battery with a minimum 2-year lifespan for the hardware, operate in a range of turbidity, pH & salinity, use minimal amount of consumables & cost no more than $6000 and ideally around $1000. Products which meets these requirements could be used for >100K tests annually worldwide.

Fluorescence testing using UVC LEDs is emerging as a potential candidate for meeting the need for rapid, portable testing of the microbial quality of water. It has been known for a long time that under UVC excitation at 280 nm, the intensity of fluorescence emitted at 350 nm by dissolved organic matter in aquatic systems (tryptophan like fluorescence or TLF) correlates well with sewage and bacterial contamination. However, E.coli in the water also produces indole from the tryptophan which also has a similar fluorescent signature to tryptophan, with 33% higher intensity. This has been the concern with correlating the 350 nm fluorescence to bacterial concentration: namely that both the bacteria and the organic matter behave similarly. 

Recent studies with UV LED fluorometers have shown good correlation between TLF and E. coli concentration. Using a 280 nm UVC LED, it has been demonstrated that 10 CFU/100 ml for E.coli can be detected in a flow through sensor3 which corresponds to high risk fecal contamination, as per the WHO microbial risk classification. While the LOD needs to be lower for detection of lower levels of contamination, these results indicate that with design improvements, there is potential for TLF to detect contamination of drinking water in both waterbodies in developing countries and in distribution networks in developed countries.

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