Water quality management relies on standardized metrics that quantify the amount of pollution present in a sample. One of the most fundamental measurements used globally is Biochemical Oxygen Demand (BOD). This metric represents the amount of dissolved oxygen consumed by aerobic biological organisms, primarily bacteria, as they break down organic material in the water sample. The presence of organic matter, such as sewage or decaying plants, fuels the growth of these microorganisms. By measuring the oxygen they consume, scientists can indirectly determine the concentration of biodegradable organic pollutants.
Understanding Biochemical Oxygen Demand Over 5 Days
The specific measurement known as BOD5 is the standard method used to assess the oxygen demand over a standardized five-day period. This period is a practical compromise for capturing a substantial portion of the carbonaceous oxygen demand, which is the oxygen used to break down organic carbon compounds. Microorganisms require this oxygen to metabolize organic material into simpler, more stable compounds like carbon dioxide and water. The rate of this consumption is directly proportional to the amount of biodegradable waste present in the sample.
The five-day duration was chosen partly for practical laboratory scheduling. More scientifically, the 5-day mark generally precedes the onset of the second stage of oxygen consumption, known as nitrogenous demand. This second stage involves slower-growing nitrifying bacteria converting nitrogen compounds, like ammonia, into nitrates. By stopping the test at five days, the BOD5 value primarily reflects the oxygen demand from the breakdown of carbon-based organic matter.
The total amount of oxygen required for all organic matter to be completely degraded is called Ultimate BOD (BODu), which can take 20 to 30 days. This extended period is impractical for operational and regulatory monitoring purposes. BOD5 serves as a reliable, reproducible, and timely approximation of the immediate oxygen stress that a polluted water source would exert. The BOD5 result typically accounts for about 60% to 80% of the total degradable oxygen demand.
The Laboratory Measurement Process
The BOD5 test uses a dilution method, which requires specialized 300-milliliter glass bottles sealed with water locks. The procedure begins by measuring the water sample’s initial dissolved oxygen (DO) concentration. If the sample is expected to have a very high oxygen demand, such as raw sewage, it must be diluted with aerated, nutrient-rich dilution water. This dilution is necessary to ensure oxygen is not completely depleted before the five days are over, as the test requires a measurable decrease in DO but also a minimum residual amount remaining.
The sealed bottles are then placed in an incubator for five days at a constant temperature of 20 degrees Celsius. The incubation is performed in the dark to prevent photosynthetic organisms, like algae, from producing oxygen and artificially altering the results. After the five-day period, the final DO concentration in each bottle is measured. For a result to be considered valid, the DO depletion must be at least 2.0 milligrams per liter, with at least 1.0 milligram per liter remaining.
The final BOD5 value is calculated by subtracting the final DO reading from the initial DO reading. This difference represents the amount of oxygen consumed by the microorganisms. If the sample was diluted, this difference is then multiplied by the dilution factor to determine the oxygen demand of the original, undiluted water sample. The resulting value is expressed in milligrams of oxygen consumed per liter (mg/L).
Application in Water Quality Management
The BOD5 test is a fundamental tool used by environmental regulators and water treatment facilities worldwide. Its primary role is to evaluate the operational efficiency of wastewater treatment plants. A high BOD5 value in the water entering the plant (influent) is expected, while a significantly lower BOD5 value in the treated water (effluent) confirms that the plant’s biological processes are successfully removing organic pollutants.
Regulatory programs, such as the National Pollutant Discharge Elimination System (NPDES) in the United States, use BOD5 to set strict limits on the organic material that industries and municipalities can discharge into receiving waters. Monitoring BOD5 in the effluent ensures that dischargers comply with their permits and protect the health of the aquatic environment. If a facility exceeds its permitted BOD5 limit, it indicates inadequate treatment, which can lead to enforcement action.
The environmental implication of a high BOD5 discharge is the potential for oxygen depletion in the receiving water body. When a large load of organic material enters a river, the natural bacterial population rapidly consumes dissolved oxygen to break it down. This reduction in dissolved oxygen can quickly lead to hypoxic or anoxic conditions, which are harmful or deadly to fish, aquatic insects, and other oxygen-dependent organisms. Therefore, the BOD5 measurement is a direct indicator of the potential for a pollutant to suffocate aquatic life.