Lume
Lume
A summary of tryptophan-like fluorescence research underpinning continuous, real-time microbial water quality monitoring.
Tryptophan-like fluorescence (TLF) is an optical water quality parameter centered on excitation/emission wavelengths of approximately 275/340 nm. It reflects concentrations of compounds with fluorescence characteristics similar to the amino acid tryptophan, which is associated with microbial activity and fecal contamination.
A positive relationship between TLF intensity and fecal indicator bacteria (FIB), particularly E. coli, has been demonstrated across groundwater, surface water, estuarine, and urban watershed contexts on multiple continents. This makes TLF a promising real-time proxy for microbial contamination risk—complementing or replacing traditional culture-based methods that require 24–48 hours for results.
TLF sensors provide readings in 60 seconds, compared to 24–48 hours for traditional culture-based methods.
The Lume sensor's minimum detection limit for tryptophan dissolved in deionized water.
Minimum detection limit of the correlation to E. coli present in wastewater effluent (per 100 mL).
Microbial water quality is most frequently assessed using E. coli as a risk indicator. However, relying exclusively on culture-based E. coli measurement is limiting: it is slow, expensive, requires trained personnel, and captures only a single point in time. Risk assessments can be significantly improved by integrating TLF as a complementary, continuously monitored parameter.
TLF sensing is purely optical. No consumables, reagents, or lab infrastructure needed for each measurement.
In Kenya groundwater studies, TLF sampling showed 14% average relative percent difference between duplicates, compared to ≥26% for culture-based methods.
Sensors can operate autonomously for months, capturing short-duration contamination events that weekly grab samples miss.
Research in Malawi showed TLF indicates broader contamination risk than microbial culturing, making it a useful high-level screening tool.
For the cost of a single lab-processed grab sample, a TLF sensor can deliver thousands of in-situ microbial estimates over weeks to months of continuous deployment.
TLF has been validated across groundwater, freshwater rivers, chlorinated piped systems, and saline coastal environments on multiple continents, with studies spanning Kenya, Malawi, the US, and France.
TLF cannot be used as a direct proxy for E. coli on an individual sample basis. The TLF signal can be influenced by dissolved organic carbon (DOC), humic-like fluorescence (HLF), turbidity, and temperature. Standardization of TLF thresholds associated with different risk levels remains an active area of research. Performance is best in groundwater, which typically has low DOC, consistent temperature, and negligible turbidity.
The Virridy Lume addresses many of the limitations identified in the TLF literature by coupling optical TLF sensing with machine learning models that account for environmental confounders. Rather than relying on static TLF thresholds, the Lume's algorithm adapts to site-specific conditions.
Site-specific calibrated categorical classifications of microbial contamination risk.
Out-of-the-box accuracy on a continuous scale across 0–1,000 CFU/100 mL.
Mean absolute percentage error in log-transformed concentration space vs. culture-based methods.
Machine learning quantification: Unlike earlier TLF sensors that report relative fluorescent units, the Lume quantifies actual E. coli concentrations. The ML model was validated across freshwater and coastal environments, including saline settings where enterococci are the regulatory indicator.
Multi-parameter correction: Built-in turbidity and temperature sensors allow the algorithm to correct for environmental interference that confounds raw TLF readings.
Drinking water performance: Binary classification at regulatory thresholds (1 and 10 CFU/100 mL) achieves 91–92% overall accuracy with Cohen's kappa of 0.82–0.84, indicating strong agreement with laboratory classifications.
Ocean transferability: In initial ocean deployments, the Lume achieved over 76% categorical accuracy using just six training samples, demonstrating transferability across microbial indicators and water matrices.
Baker et al. demonstrate in-situ tryptophan-like fluorescence as a real-time indicator of faecal contamination in drinking water supplies. (Water Research)
Sorensen et al. evaluate TLF as a measure of microbial contamination risk in Kenyan groundwater across 37 water points. (Sci. Total Environ.)
Khamis et al. establish real-time detection of faecally contaminated drinking water with TLF, defining threshold values. (Sci. Total Environ.)
Bedell, Sharpe, Purvis, Brown & Thomas demonstrate low-cost TLF sensor concepts for fecal exposure detection. (Sustainability) • Virridy
Nowicki et al. conduct a nine-month monitoring program in Malawi, finding TLF is a more precautionary risk indicator. (Sci. Total Environ.)
Bedell, Harmon, Fankhauser, Shivers & Thomas field-validate a continuous in-situ fluorescence sensor coupled with ML. 83% accuracy. (Water Research) • Virridy
Multiple groups evaluate TLF for combined sewer overflow watersheds and estuarine systems. (Sci. Total Environ.; ACS ES&T Water)
Knopp, Klaus, Wilson et al. advance continuous in-situ quantification with Lume V1.2 sensor design and multi-site validation. (EarthArXiv) • Virridy
Demaree, Fankhauser, Cole, Ross & Thomas develop sensor-informed predictive models for TOC and nutrients on the Upper Yampa River. (ES&T Water) • Virridy