A Review on Detection Methods of Chemical Oxygen Demand in Water Bodies

COD is regarded as an important index for representing the degree of organic pollution in water bodies, which has attracted wide attention. The water quality of 10.2% of the 972 state-controlled sections exceeded Ⅴ class among ten water areas such as the Yangtze River and the Yellow River in China in 2012. COD is one of the major pollution indicators in those water bodies. Therefore, it is of great importance to accurately monitor COD in water bodies. However, standard determination methods of COD have some disadvantages including complexity, low efficiency, high cost and severe secondary pollution. Aiming at the difficulties of traditional methods, instruments and equipment (sample digestion and determination), digestion reagents, and determination methods have been optimized and improved. In addition, certain environmentally friendly detection techniques with higher detection efficiency and accuracy have also been developed and applied. The most recent research progress of the determination methods of COD, including two main parts: the improved and optimized methods according to the standard ones, and new technologies or methods are reviewed in this paper. The traditional detection method is suitable for determining COD with the content range of 30-700 mg/L in light or moderate polluted water bodies. The improved and optimized methods extend the detection limits from 8.6 mg/L to 1600 mg/L, with the detection time shortened and cost of detection decreased significantly. However, the environmental pollution caused by chemical reagents could not be avoided, and oxidation ability to the refractory organics was insufficient. Flow injection, chemiluminescence or a combination of several techniques, which had already been used as the mature detection methods in drugs and immune analysis, mineral and rock analysis, have also been employed for the determination of COD in water bodies. Lower detection limit (0.16 mg/L) and faster detection time could be achieved, and the environmental pollution caused by the reagents was reduced dramatically. With the development of science and technology, some new methods such as ozone oxidation, electrochemical ones, especially as photocatalytic and photoelectrocatalytic determination technologies, based upon the reaction between hydroxyl radicals and organic compounds, have been developed recently. A wider detection range (0-23200 mg/L) was achieved by these new methods, which are capable of determining COD in water bodies with different pollution levels. The new techniques could provide a reference for the accurate, rapid, low-cost and on-line monitoring determination of COD.