Biochemical Oxygen Demand (BOD) is a measure of the amount of dissolved oxygen required by aerobic microorganisms to decompose organic matter in water over a specified period of time, typically five days at a standardized temperature of 20°C, in which case it is referred to as BOD₅. In stormwater management, BOD is a key indicator of the organic pollution load carried by runoff and its potential impact on receiving water bodies.
BOD reflects the presence and concentration of biodegradable organic materials such as leaves, grass clippings, animal waste, sewage, food waste, and other carbon-based compounds that enter stormwater systems. When these materials are washed into streams, rivers, lakes, or wetlands, naturally occurring bacteria begin to break them down through aerobic respiration. This process consumes dissolved oxygen (DO) from the water column, which is essential for the survival of fish, invertebrates, and other aquatic organisms.
High BOD levels indicate that a large amount of oxygen will be consumed during decomposition, which can lead to oxygen depletion in receiving waters. If the rate of oxygen consumption exceeds the rate at which oxygen is replenished through atmospheric diffusion and photosynthesis, dissolved oxygen levels can drop to stressful or even lethal levels for aquatic life. This condition is commonly associated with degraded water quality, fish kills, and shifts in aquatic community structure toward more pollution-tolerant species.
In stormwater systems, sources of elevated BOD often include organic debris accumulated on impervious surfaces, illicit discharges, failing septic systems, sanitary sewer overflows, and runoff from agricultural or landscaped areas. Urban runoff can deliver pulses of high BOD during storm events, particularly following dry periods when organic materials have built up on surfaces.
BOD is typically measured through a laboratory test in which a water sample is incubated under controlled conditions, and the decrease in dissolved oxygen is measured over time. The result is expressed in milligrams per liter (mg/L), representing the amount of oxygen consumed. Higher values correspond to greater levels of organic pollution.
From a management standpoint, controlling BOD in stormwater involves both source control and treatment. Practices such as street sweeping, proper yard waste management, and prevention of illicit discharges help reduce the amount of organic material entering the system. Structural Best Management Practices (BMPs), including detention and retention basins, constructed wetlands, and bioretention systems, can further reduce BOD by promoting sedimentation, microbial degradation, and vegetative uptake before runoff is discharged to receiving waters.
In summary, biochemical oxygen demand is a fundamental water quality parameter that quantifies the oxygen-consuming potential of organic pollutants in stormwater. It serves as an important tool for assessing the health of aquatic systems and the effectiveness of stormwater management practices in protecting dissolved oxygen levels.