Biochemical Oxygen Demand (BOD) is a fundamental analytical parameter used to assess the quality of water in streams, rivers, and wastewater. It quantifies the amount of dissolved oxygen consumed by microorganisms breaking down organic matter in a water sample. BOD serves as a direct indicator of organic pollution within an aquatic environment, such as contamination from sewage, industrial discharge, or agricultural runoff. Monitoring this value is essential in environmental management to determine the overall health of a water body and its ability to sustain aquatic life.
The Biochemical Process
BOD measurement is rooted in the natural process of aerobic respiration carried out by aquatic microorganisms, primarily bacteria. When organic substances enter a water body, microbes use them as a food source. To metabolize and break down this organic load into simpler, stable inorganic compounds, the microorganisms require molecular oxygen.
This biological oxidation process consumes the dissolved oxygen (DO) in the water. The rate and total amount of oxygen consumed are directly proportional to the quantity of biodegradable organic material available in the water sample. Therefore, BOD is an indirect measure, reflecting the concentration of organic matter by quantifying the oxygen demand that its microbial decomposition creates. A higher concentration of organic pollutants requires a greater amount of oxygen for microbial respiration.
The decomposition process begins immediately when the organic matter is introduced and continues until the biodegradable material is fully stabilized. This microbial metabolism places a respiratory strain on the aquatic system, drawing down the oxygen stores.
Standardized Measurement of BOD
BOD determination follows a standardized laboratory procedure known as the BOD5 test, which specifies the conditions for measuring oxygen consumption. The number “5” indicates that the measurement is taken after a five-day incubation period, which captures the majority of the carbonaceous oxygen demand. The test is performed by filling a sealed, airtight glass bottle with the water sample and incubating it in the dark at a constant temperature of 20°C. This temperature mimics a typical natural water environment and optimizes microbial activity.
The procedure requires two measurements of the dissolved oxygen (DO) concentration: one at the beginning of the test (initial DO) and one after the five-day incubation period (final DO). The BOD value, expressed in milligrams of oxygen consumed per liter (mg/L), is calculated by subtracting the final DO reading from the initial DO reading. For samples with a very high organic load, such as untreated wastewater, a dilution step is often necessary to ensure that some dissolved oxygen remains at the end of the five days for the final measurement.
If the sample contains a low population of indigenous microorganisms, a small amount of microbial seed is often added to the dilution water to ensure that the decomposition process occurs. The BOD5 test primarily measures the consumption of oxygen by bacteria breaking down carbon-based organic material, known as carbonaceous demand. Oxygen consumed by bacteria oxidizing nitrogen compounds, or nitrogenous demand, can also occur, but it is typically prevented in the BOD5 test by adding a chemical inhibitor to isolate the more immediate carbonaceous demand.
Significance of BOD Readings for Water Quality
Interpreting the BOD value provides a direct assessment of a water body’s health and its capacity to assimilate organic waste. A low BOD reading, typically less than 1 or 2 mg/L, signifies very clean water with minimal biodegradable organic material, such as that found in pristine rivers or lakes. Conversely, a high BOD value indicates substantial organic pollution; for instance, untreated municipal sewage can exhibit BOD levels ranging from 200 to 600 mg/L.
The most significant consequence of high BOD is the rapid depletion of dissolved oxygen (DO), which is the oxygen available for aquatic organisms. As the microbial population rapidly consumes oxygen to break down a large organic load, the DO concentration can fall below the level necessary to support aquatic life. When DO levels drop below approximately 4 mg/L, most fish species begin to struggle, and the water body enters a state of hypoxia.
Prolonged or severe oxygen depletion can lead to anoxia, a complete absence of oxygen, resulting in fish kills and the loss of sensitive invertebrate species. This disruption fundamentally alters the aquatic ecosystem, favoring organisms that can tolerate low oxygen conditions. Consequently, regulatory agencies and wastewater treatment plant operators use BOD readings as a key metric for monitoring and control.
Wastewater treatment plants are designed to reduce the BOD of effluent before discharge. The BOD test is routinely used to gauge the efficiency of the treatment process and to ensure that discharged water meets environmental quality standards and permits. Consistent monitoring of the BOD helps to protect the natural oxygen balance of receiving waters, ensuring that aquatic habitats remain viable and healthy.