Natural organic detritus and organic waste from wastewater treatment plants, failing septic systems, and agricultural and urban runoff, acts as a food source for water-borne bacteria. Bacteria decompose these organic materials using dissolved oxygen. The determination of Chemical Oxygen Demand (COD) is widely used in municipal and industrial laboratories to measure the overall level of organic contamination in wastewater. The contamination level is determined by measuring the equivalent amount of oxygen required to oxidize organic matter in the sample.

In the COD method, the water sample is oxidized by digesting in a sealed reaction tube with sulphuric acid and potassium dichromate in the presence of a silver sulphate catalyst. The amount of dichromate reduced is proportional to the COD. A reagent blank is prepared for each batch of tubes in order to compensate for the oxygen demand of the reagent itself.

Over the range of the test a series of colors from yellow through green to blue are produced. The color is indicative of the chemical oxygen demand and is measured using a photometer. The results are expressed as milligrams of oxygen consumed per liter of sample [34].

Test Procedure

i. Mixed reagent. Potassium dichromate solution. To a 1000 mL volumetric flask, add 42.256 ± 0.001 g of potassium dichromate (previously dried for one hour at 140 — 150 oC). To the flask, add approximately 500 mL of water and mix the contents to dissolve. Then add 33.3 g de HgSO4 to the potassium solution. Add in an ice bath slowly 167 mL pure H2SO4. When the mixture has cooled, stir the mixture until the solid dissolves and dilute to one liter.

ii. Silver sulphate in sulphuric acid (10 g). To a glass bottle, add 10.0 ± 0.1 g of silver sulphate and 1000 ± 10 mL of sulphuric acid and stopper. To obtain a satisfactory solution, swirl the initial mixture and allow it to stand overnight. Swirl the contents again until all the silver sulphate dissolves. This solution may be stored in the dark at room temperature for up to an indefinite period.

iii. Pipette 2.0 mL sample into cuvette with 2.0 mL of the mixed reagent solution and 1.0 mL of the silver sulphate in sulphuric acid solution. Invert cuvettes carefully.

iv. Heating reactor for determination of COD for about 30 minutes.

v. Heat cuvettes for 2 h at 150 °C.

vi. read cuvettes in the spectrophotometer at 620 nm.