Recent advances in oxygen demand research have expanded the number of options available for testing. The purpose of this technical bluebook is to 1) provide background information on oxygen demand testing, and 2) discuss the comparative advantages of methods cur- rently available, including the new Hach Manganese III COD Method designed to eliminate hazardous heavy metals waste.
Background on Oxygen Demand Testing
Oxygen demand is an important parameter for deter- mining the amount of organic pollution in water. The test has its widest application in measuring waste loadings of treatment plants and in evaluating the efficiency of treatment processes. Other applications include testing lake and stream water samples for organic pollution. Oxygen demand testing does not determine the concen- tration of a specific substance; rather, it measures the effect of a combination of substances and conditions. Because oxygen demand is not a pollutant, it poses no direct threat to fish or other life. It can, however, pose an indirect threat to living organisms by reducing the level of dissolved oxygen.
There are three widely-used methods of measuring oxygen demand. Two measure oxygen demand directly: Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). A third method— Total Organic Carbon (TOC)—measures oxygen demand indirectly.
BOD Test
Of the three test methods that determine oxygen demand (BOD, COD and TOC), the BOD test most closely models aerobic waste treatment and the aquatic ecosystem. In this test, microorganisms consume organic compounds for food while consuming oxygen at the same time. The standard BOD test measures the amount of oxygen consumed in a sample over a five-day period. Due to the length of time required to complete the test, results provide historical data only and do not facilitate rapid water quality assessment or optimal pro- cess control. The test is limited in some applications such as industrial wastewaters, which often contain heavy metal ions, cyanides, and other substances toxic to microorganisms. When microorganisms become poisoned by toxic substances, they are unable to oxidize waste, in which case the BOD test becomes an ineffective measure of organic pollution.