Monitoring Water Quality

Part 2. COD and BOD

Properties of water bodies vary based on natural factors such as landscape position and slope, watershed size, geology, and soil composition, as well as man-made pollution.

Before a water source can be used for human, animal, or plant consumption, it must be assessed and possibly treated to make sure it is safe for consumption.

When wastewater containing high levels of organic matter enters a natural stream, valuable dissolved oxygen is consumed during the breakdown of organic matter, adversely affecting the aquatic ecosystem.

Chemical oxygen demand (COD) and biochemical oxygen demand (BOD) are parameters used to determine the amount of oxygen consuming pollutants in water. 

To meet environmental protection regulations, wastewater treatment facilities remove COD and BOD from water prior to releasing it as effluent water.

Due to the speed of testing, COD is more commonly used than BOD and facilities usually establish a correlation between BOD and COD, then only run BOD occasionally. However, be sure to check with your local regulatory agency for detailed advice on BOD and COD testing regimens.

COD

Chemical oxygen demand (COD) is a critical waste treatment measurement in everything from municipal systems to food manufacturing waste streams. COD indirectly measures the amount of inorganic chemicals and organic pollutants by the amount of oxygen consumed during their chemical oxidation. COD is measured on both influent and effluent water. The efficiency of the treatment process is normally expressed as COD removal, measured as a percentage of the organic matter purified during the cycle.

Due to its faster turnaround time compared to BOD, COD is more commonly measured during wastewater treatment. 

The Hanna COD method is based on the well-established closed dichromate-reflux colorimetric method. The colorimetric measurement of COD is faster and easier to perform than the titrimetric analysis as additional reagents are not required. The sample is added to the reagent vial and digested under closed reflux conditions and allowed to cool before measurement is taken using a photometer. Reference standards can be made using potassium hydrogen phthalate (KHP), 1 mg of KHP is equal to 1.175 mg COD.

BOD

BOD measures the amount of oxygen consumed by microbes to breakdown organic matter. BOD is a more accurate quantification of the readily available organic matter than COD; however, BOD requires a five-day incubation to obtain a measurement, making it difficult to make real-time adjustments.

A very small amount of microorganism seed is added to each sample being tested. The samples are kept at 20°C in the dark for five days. The loss of dissolved oxygen during incubation is called the BOD5.The dissolved oxygen concentration is measured using a dissolved oxygen (DO) meter before and after the incubation and BOD is calculated in mg/L from the difference.

Hanna Instruments offers multiple solutions to measure COD and BOD. Pleas use the links below to explore all the available options.

COD

BOD