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C.O.D., B.O.D., T.O.C. and UV absorbance in the measurement of the
contamination of water.
There are a number of ways of measuring the overall level of contamination in water. Often used are - Biological Oxygen Demand (B.O.D.), Chemical Oxygen Demand (C.O.D.), Total Organic Carbon (T.O.C.) and UV absorbance.
The underlying assumption is that the contamination is organic in origin, and carbon based. The methods were developed as laboratory methods over the last century or so and B.O.D., C.O.D. and T.O.C. have been described in the “Blue Book” series of Standard Methods. (Methods for the Examination of Waters and Associated Materials – created and maintained by the “Standing Committee of Analysts”). The existence and documentation of the methods opens up their use as legally supported consent limit parameters.
B.O.D. is a test which in the original laboratory format takes a sample into a volume of water containing aerobic biological organisms and saturated with oxygen – 9mg/lt at 20°C. The mixture is sealed to eliminate oxygen ingress and held at 20°C for five days (this is usual in the UK – other times are used) and the resulting oxygen level is measured. The result is expressed as oxygen demand in mg/lt which means milligrams of oxygen used up per litre of sample.
This test is designed to measure the amount of organic material in a body of water. The more contamination the more oxygen used to break it down biologically. The test was used as an indicator of the quality of treated waste water and is a very normal consent parameter for discharges to the environment.
The key advantage is that it measures the effect of water on the ecosystem into which it will be discharged. Taking the bugs from the local environment gives the test a direct relevance in that sense. Treatment of waste water using a biological process (which is overwhelmingly the case) again gives the test a direct relevance to the treatment process.
The key features of the test are
- 1. Reference method is described in the Standing Committee of Analysts series of Blue Books.
- 2. The biological nature of the test give it a relevance to environmental impact and treatment efficiency
- 3. the test result is dependent upon the biological organisms used
- 4. the temperature of the test is seldom the temperature of the environment
- 5. the results are not available in time to take any action
- 6. the test can involve high dilutions
- 7. the analyst needs to know approximately what the result will be in order to prepare the dilutions. The measured value after dilution should be 4 or 5 mg/lt and should not be too close to either 9 or 0 mg/lt O2.
- 8. It is relatively labour intensive and expensive
- 9. It is not accurate or precise.
- 10. Toxic samples give low readings.
- 11. Used in consent limits for direct discharge to the environment.
- 12. The standard laboratory method is not easy to set up in an on-line analyser, however using a different approach allows continuous B.O.D. to be measured.