Biochemical Oxygen Demand (BOD) is a fundamental measurement in environmental science, representing the amount of dissolved oxygen consumed by microorganisms as they break down organic material in a water sample. This metric is expressed in milligrams of oxygen per liter (mg/L) and serves as an indirect measure of biodegradable organic pollution present in the water. The process mimics the natural biological consumption of oxygen that occurs in rivers and lakes when they receive effluent. Measuring BOD is a direct way to assess the potential environmental impact of wastewater before it is discharged.
Why Measuring BOD is Essential
BOD data provides a snapshot of the organic load in a water body, which directly correlates to its overall health and quality. High levels of organic matter, typically from sources like sewage, agricultural runoff, or industrial discharge, lead to a high BOD value. The rapid consumption of dissolved oxygen by microbes trying to decompose this load can quickly deplete the oxygen supply in a water body. This oxygen depletion stresses and can ultimately cause the death of fish and other aquatic organisms that rely on dissolved oxygen to survive. Environmental protection agencies use BOD measurements to monitor the health of natural waters and enforce discharge limits on wastewater treatment facilities.
Preparing the Water Sample for Testing
Accurate BOD measurement requires careful preparation of the sample to ensure the microbial consumption of oxygen proceeds correctly over the test period.
Dilution
One necessary step for highly polluted water, such as raw sewage, is dilution with high-quality, aerated water. This dilution prevents the microorganisms from consuming all the available oxygen too quickly, which would invalidate the five-day test. The goal is to ensure a measurable oxygen drop while still having at least 1 mg/L remaining at the end.
Seeding
Seeding involves adding a small, known population of healthy microorganisms to the sample. This step is performed if the original sample is sterile or lacks sufficient active microbes, which can occur with industrial wastes or chlorinated effluent. The added seed ensures that enough organisms are present to break down the organic matter within the test window.
Inhibition
A final preparatory step is inhibition, specifically the addition of a chemical compound like allylthiourea to suppress the activity of nitrifying bacteria. Inhibiting this process allows for the measurement of Carbonaceous BOD (cBOD), representing only the oxygen demand from the breakdown of carbon-based organic matter. This cBOD measurement is often the target for regulatory compliance.
The Standard 5-Day BOD Test Procedure
The standard test, known as \(\text{BOD}_5\), is a method-defined procedure that must be followed precisely. The first step is to prepare the sample and measure its initial dissolved oxygen (\(\text{DO}_i\)) concentration using an oxygen meter. The prepared sample, including any necessary dilution water and seed, is then transferred to a specialized 300 mL glass bottle, filling it completely to eliminate any air bubbles.
The bottle is sealed to prevent atmospheric oxygen from entering and placed in an incubator. Incubation must occur in the dark at a constant temperature of \(20^{\circ}\text{C}\) for exactly five days. The dark environment is important to prevent algae in the sample from producing oxygen through photosynthesis, which would artificially increase the oxygen level and skew the results.
After the five-day incubation period, the final dissolved oxygen (\(\text{DO}_f\)) concentration is measured. The difference between the initial and final DO measurements represents the amount of oxygen consumed by the microorganisms.
A quality control measure involves running a blank bottle with only dilution water and a seed control to account for any oxygen depletion not caused by the sample’s organic matter.
Calculating and Interpreting BOD Results
The BOD value in mg/L is calculated using a formula that accounts for the measured oxygen consumption and the extent of any dilution. The calculation is \(\text{BOD} = (\text{DO}_i – \text{DO}_f) \times \text{Dilution Factor}\). The dilution factor is the ratio of the total volume in the bottle to the volume of the original sample used.
A low BOD value, typically in the range of 1 to 2 mg/L, signifies relatively clean water with minimal organic pollution. This low demand means that the water body has a high capacity to support aquatic life.
Conversely, a high BOD value signifies a substantial organic load that will rapidly consume oxygen in the receiving water. Untreated domestic sewage, for example, can have BOD values well over 200 mg/L. This high oxygen demand indicates heavy pollution and a significant threat to the dissolved oxygen levels necessary for a healthy aquatic ecosystem.