What are BOD and COD used for in wastewater characterization?

• A. Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) quantify organic pollutant loads; used to size treatment and evaluate efficiency.
• B. They measure metal concentration.
• C. They measure density of water.
• D. They measure salinity.

Answer: (A)

BOD and COD quantify the organic pollutant load in wastewater and are key for sizing treatment systems and judging how well a plant is doing. Biological Oxygen Demand (BOD) measures the amount of oxygen that microorganisms will consume to biologically oxidize the organic matter in the water under standard lab conditions (usually over five days at 20°C). It reflects the biodegradable portion of the waste, which is what aerobic treatment needs to process, so engineers use BOD to estimate the required aeration capacity and to design the biological stage.

Chemical Oxygen Demand (COD) measures the amount of oxygen equivalent needed to chemically oxidize both biodegradable and nonbiodegradable organic substances, as well as some reducing inorganic substances, using a strong oxidant. Because the chemical test can oxidize a broader range of substances, COD results are obtained much faster and are typically higher than BOD for the same sample. Using COD alongside BOD provides a quicker overall sense of organic loading and helps cross-check the more biologically meaningful BOD.

In practice, you compare influent and effluent BOD or COD to determine removal efficiency and to assess whether the treatment process is meeting design goals or regulatory requirements. These tests do not measure metals, density, or salinity, which are determined by other analyses.