Biochemical Oxygen Demand (BOD) quantifies the amount of dissolved oxygen consumed by microorganisms as they break down organic matter, such as sewage and natural waste, under aerobic conditions. A higher BOD value indicates a greater load of organic material present in the water, signifying a higher degree of organic pollution. This measurement is a primary indicator used to assess the overall quality of wastewater and the effectiveness of treatment processes.
The Biological Mechanism of Oxygen Demand
The consumption of oxygen measured by BOD is a result of natural biological processes carried out by heterotrophic aerobic bacteria. These microbes utilize dissolved oxygen (DO) in the water to metabolize and stabilize organic compounds found in the wastewater, such as proteins, carbohydrates, and fats. The aerobic decomposition process converts the complex organic molecules into simpler, stable inorganic substances like carbon dioxide and water.
The rate of oxygen consumption is directly proportional to the amount of biodegradable organic waste available to the microbial population. If the wastewater contains a large quantity of organic pollutants, the bacteria multiply rapidly and demand a substantial amount of oxygen for their respiration and growth. This biological demand for oxygen is distinct from Chemical Oxygen Demand (COD), which measures the total oxygen required for both biologically available and non-biodegradable organics to be chemically oxidized.
Measuring BOD: The Five-Day Standard
The standard method for quantifying this pollutant load is the five-day Biochemical Oxygen Demand test, abbreviated as \(\text{BOD}_5\). This procedure simulates the natural process of decomposition under controlled conditions to ensure a standardized result. The test begins by measuring the initial dissolved oxygen concentration of a sealed water sample.
The sample is then incubated in the dark for precisely five days at a constant temperature of \(\text{20}^\circ\text{C}\). This incubation period allows the microorganisms to consume a significant portion of the biodegradable organic matter. The final dissolved oxygen concentration is measured after the incubation period.
The difference between the initial and final DO readings represents the amount of oxygen consumed by the microbes over the five days. The result is expressed in milligrams of oxygen consumed per liter of water (mg/L), providing a quantifiable measure of the organic pollution load.
Environmental Impact of High BOD Levels
The release of untreated or inadequately treated wastewater with high BOD into natural water bodies like rivers and lakes can have severe environmental consequences. As the discharged organic material enters the receiving water, the indigenous microbial population rapidly breaks it down, consuming the naturally available dissolved oxygen. This consumption quickly depletes the limited supply of DO in the water.
When dissolved oxygen levels drop significantly, the water body can enter a state of hypoxia or anoxia, meaning very low or zero oxygen concentration. This condition starves aquatic life forms, such as fish, mussels, and sensitive invertebrates, which depend on sufficient DO for survival. The resulting stress leads to fish kills, disrupts the aquatic food web, and significantly reduces the biodiversity of the ecosystem. High BOD discharges are directly linked to the degradation of water quality and the creation of ecological “dead zones.”
Reducing BOD in Wastewater Treatment
Wastewater treatment facilities are primarily designed to reduce the BOD load before the water is discharged back into the environment. Initial treatment stages, known as primary treatment, physically remove large, settleable organic and inorganic solids from the wastewater stream.
The most significant reduction in BOD occurs during secondary treatment, which relies on biological processes to consume the remaining dissolved and suspended organic matter. This phase often involves large aeration basins where air is actively pumped into the wastewater to maintain a high concentration of dissolved oxygen. The oxygen supports a large, active population of aerobic bacteria, often in the form of activated sludge, which metabolizes the organic pollutants.
By providing an ideal environment for microbial growth, the treatment plant accelerates the natural decomposition process. The microorganisms convert the high-BOD organic compounds into carbon dioxide and new bacterial biomass, which is then separated from the clean water in a clarifier. This biological consumption can reduce the initial BOD of domestic sewage, typically around \(\text{300 mg/L}\), to low levels, often below \(\text{25 mg/L}\), ensuring the discharged water meets regulatory standards and minimizes environmental harm.