BOD5

BOD5 stands for five-day biochemical oxygen demand, a measure of the amount of dissolved oxygen consumed by aerobic microorganisms to decompose organic matter present in a water sample over a five-day incubation at 20 degrees Celsius. It is used as an index of organic pollution and wastewater strength, and is widely applied in water quality management, wastewater treatment design, permitting, and regulatory compliance. The standard test involves incubating a diluted sample with an inoculum of seed microorganisms in the dark at 20°C for five days, measuring the decline in dissolved oxygen from the initial to the final reading. The BOD5 value is expressed as milligrams of oxygen consumed per liter (mg/L). Dilution is often required for samples with high organic content, and the test uses dilution water and a standardized seed. Final results can be influenced by temperature, nitrification, sample preservation, and seed quality. Nitrification can cause overestimation of carbonaceous BOD unless inhibited, leading to CBOD5 where nitrification inhibitors are employed or nitrification is suppressed by adjusting the test conditions. BOD5 provides an estimate of readily biodegradable organics and is related to, but distinct from, chemical oxygen demand (COD) and total organic carbon (TOC). Typical untreated municipal wastewater yields BOD5 in the range of hundreds of mg/L, while well-treated effluent is often below 20 mg/L; high-quality natural waters are typically well below 5 mg/L. Limitations include the time required and its focus on biodegradable material only.