April 2006Analytical Instrumentation & Environmental Monitoring
Dissolved oxygen (DO) is one of the most important of the dissolved gases found in water. In natural waters, DO exists in a dynamic equilibrium controlled by biochemical depletion and oxygenation through atmospheric diffusion, aeration, and photosynthesis. As a result, bacterial populations proliferate and provide a key input up the food chain. However, dissolved oxygen is subject to detrimental fluxes when a catastrophic event occurs such as the discharge of organic waste into natural waters. Depending on the severity of the insult, DO may be depleted to the point where higher trophic organisms such as macro invertebrates and fish are killed off.
In wastewater treatment, organic-based sewage is degraded under controlled aerobic conditions. Failure to maintain adequate supplies of DO result in anaerobic conditions that lead to offensive and corrosive sulphides. Excessive aeration, on the other hand, is wasteful and drives up unnecessary operational costs. Prudent monitoring of DO is essential for assessing environmental risk in natural waters and for optimal wastewater treatment performance and regulatory compliance. As a result, precision and accuracy of the DO measurement becomes a critical issue - not only for estimating the degree of water quality or purification, but in calculating industry discharge loading of treatment works. And while the two regulatory EPA methods of Winkler titration (360.2) and membrane probe (360.1) have been in use for 30 or more years, they often fall short in delivering interference-free readings that are accurate and precise.
In the Proposed EPA Method 360.3 (Luminescence) for the Measurement of Dissolved Oxygen in Water and Wastewater, 12 wastewater facilities, representative of the United States wastewater industry, analysed DO reference water samples and wastewater matrices by the two EPA approved methods and the luminescence technology. Results of the study clearly demonstrate that the luminescence procedure to be more accurate and precise than either the Winkler titration or the membrane probe techniques currently used for reporting DO. The primary benefit of the luminescence method is better performance with respect to accuracy and precision, regardless of wastewater matrix (influent-to-treatment and final effluent). Additional benefits of the luminescence method include elimination of toxic and or hazardous chemicals, no membrane to change, simplicity of use, and increased sample-throughput.
The Hach LDO probe (luminescence dissolved oxygen) has a luminescence sensor that consists of an indicator dye layer immobilised at the surface of an optically transductive support material. The indicator layer consists of an oxygen sensitive luminophore. A light emitting diode (LED) provides incident light required to excite the luminophore. The resulting dynamic lifetime of the excited luminophore is measured and equated to DO concentration.
There are no membranes to replace, calibration once a year, simple maintenance, full scale response within 30 seconds, no poisoning from H2S, heavy metals or other waste water chemicals and a three-year warranty on the probe.
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