Optical Brightener Sensors: Water Quality Monitoring

Why would I want to measure Optical Brighteners?

Optical Brighteners (OBAs) or Fluorescent Whitening Agents (FWAs) are added to products, such as laundry soaps, detergents, or cleaning agents. Fabrics or materials absorb OBAs during the washing or cleaning process. When illuminated by ultraviolet light, they fluoresce and make products and fabrics appear brighter.

Laundry wastewater is the largest contributor of brighteners to wastewater systems. FWA or OBA contributions to the total volume of most laundry detergents is less than 0.5%, however a large portion (5-80%) can remain in discharged wastewater as dissolved compounds (molecular). The presence of brighteners in water systems to which wastewater is being discharged could mean failing septic systems, sewage leaks, or complete lack of water treatment. Therefore, detection of optical brighteners in aquatic systems can help water municipalities or researchers correct system failures and avoid increased anthropogenic input that may greatly impact ecosystems.

Studies have shown a correlation between fluorescence of brighteners and fecal coliform levels in aquatic environments. Water quality is currently being assessed by fecal coliform standards through federal, state, and municipal agencies. Public access to water bodies such as lakes, rivers, or beaches that do not meet these standards can be restricted, thereby impacting tourism and ultimately the local economy. The main problem in prevention of increased fecal coliform levels is the inability to determine the source of the contamination (human/non-human). Laundry wastewater is typically discharged through the household sewer or septic systems; thus brighteners are a component of sewage wastewater. Brightener correlation to fecal coliform levels can provide valuable information to help researchers determine if contamination sources are attributed to human waste.

Brighteners are highly susceptible to adsorption. They are removed from surface and ground water by adsorption onto soil and organic material. This allows researchers to also assess the effectiveness of natural cleansing of wastewater by determining brightener concentrations in ground or surface waters

How are Optical Brighteners Measured?

Optical brighteners, or fluorescent whitening agents (FWAs in the detergent industry), are compounds that are excited (activated) by wavelengths of light in the near-ultraviolet (UV) range (360 to 365 nm), and then emit light in the blue range (400 to 440 nm). Electrons in fluorescent molecules are excited into a higher energy state by absorption of light and then emit a small amount of heat plus fluorescence as the electrons return to their ground state. The fluorescence from the second excited state is measured by Eureka’s OBA fluorometer and converted to ppb of OBA up to 2500 ppb.

What should I know about Optical Brighteners measurement in the field?

Solinst Eureka’s Optical Brightener sensor has a large dynamic range (>5000 parts per million) which allows it to operate in most aquatic environments, and its high resolution increases its accuracy.  The MantaPlus with Optical Brightener sensor may be used in studies to locate specific sites where on-site wastewater systems may be entering rivers or other waterways.  Multiparameter probes with OBA sensors may be deployed for continuous monitoring or used for discrete sampling.  Fluorometric determinations of brightener concentrations in natural water samples have been useful in finding wastewater sources and fecal coliform contaminations for Health Department studies, as well as in general research.

Fouling is the biggest field problem for fluorescence sensors.  Any foreign material that accumulates on the active surface of the sensor will either reduce the amount of emitted light or received light, or both.  And sometimes the foreign material might provide a false signal by fluorescing at the same wavelength. Fouling typically does not pose a problem when profiling or spot checking for daily surveys; only when deployed for extended periods.  For continuous deployment, anti-fouling accessories may be added, such as Eureka’s universal wiper system, and copper mesh sensor guard.