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Project: Evaluation of Optical Brighteners as a Microbial Source Tracking Tool

Background and Objectives

Typical methods for measuring fecal contamination in receiving waters quantify the concentration of fecal indicator bacteria at the location tested, but are unable to identify the sources of the contamination. In order to achieve targeted and effective remediation, though, microbial source tracking methods are essential. These methods can be expensive, complex, and overly time-consuming. As an alternative, fluorometric measurements of optical brighteners (OBs) represent an inexpensive, simple, and fast method for distinguishing sources of human fecal contamination from non-human sources. OBs are fluorescent agents added to modern laundry detergents to provide a whitening effect. Because laundry effluents discharge into sewer or septic systems, mixing with sewage and other household wastewater, OBs in the environment can indicate presence of human waste.

Some natural organic matter (NOM) found in waterways also has natural fluorescent properties. However, since OBs decay much faster than NOM under ultraviolet (UV) exposure, a method was proposed to distinguish OB from NOM by comparing fluorescence measurements before and after five minutes of UV exposure. The method was originally developed in the southeastern US, and has not been widely tested. The objective of this study was to conduct an evaluation of the microbial source tracking method and test its applicability in the southern California region.


This study was conducted in 2007.


The evaluation was performed by conducting experiments to address the following series of questions:

 • Does the fluorometry alone work? This step involved measuring fluorescent signals from various detergents and examining the relationship between fluorometric measurements and concentrations of detergents.

• Does the fluorometry-UV approach work and what is its sensitivity? This component involved examining decay curves (under UV exposure) of various detergents, sewage and septage from southern California region to see whether the OBs decay much faster than NOM, and at what concentration they can be detected by the method.

• What are the potential factors that would influence or prevent application of this microbial source tracking method? These factors included (1) stability of the OB signal under natural sunlight exposure, (2) water turbidity, and (3) difference in sources of human contamination (e.g., differences between sewage and septage, and between raw and treated sewage)

Twenty-one detergents were obtained and diluted with natural stream water collected from Malibu Creek to a concentration of 5 ppm. A blank (deionized water) and standard preparation of 50 ppm Tide laundry detergent were used to quantify relative fluorescence. Sewage samples were obtained from local sanitation districts and tested at dilutions of 1:20 and 1:100 in deionized water.


Major findings included:

• All but one of the tested detergents (Biokleen, an environmentally friendly detergent which contains no OBs) showed fluorescent signals proportional to the concentration of the detergents. All OB sources tested produced photodecay curves characterized by sharp initial drops in fluorescence, while samples without OB had very little decay.

• The original method utilizing a fluorometry-UV approach was found to be insensitive due to conservative thresholds, failing to detect raw sewage and approximately one-half of the 5 ppm detergents in natural stream water. A method improvement was proposed utilizing the ratio of reduction after 10 minutes of UV exposure to the reduction after 5 minutes. This method modification led to detection of all detergents at 5 ppm, sewage at 1:10 dilution and septage at 1:100 dilution.

• OBs remained detectable in the morning and in shaded locations, but quickly dissipated around midday when UV intensity was high. Water turbidity decreased the method sensitivity but did not prohibit the detection of OBs. Concentration of OBs were higher in septage as compared to sewage; secondary treated sewage were not detectable even with the improved method.

In summary, the fluorometry-UV approach to microbial source tracking is valid and largely applicable in southern California region. However, even with the method improvement, it is constrained by its limited sensitivity. Thus, the improved approach is recommended for detecting septage in areas where dilution is restricted.

For more information on Evaluation of Optical Brighteners as a Microbial Source Tracking Tool, contact Yiping Cao at (714) 755-3241.
This page was last updated on: 7/2/2014