Practice question

Biochemical oxygen demand, or BOD, measures how much dissolved oxygen aerobic microorganisms consume while decomposing biodegradable organic matter under standardized conditions, commonly over five days at 20°C. A high BOD therefore means that the water contains a substantial load of decomposable material or other substances supporting intense microbial respiration. Sewage, manure, food-processing waste, and organic-rich runoff are common sources. As bacteria oxidize this material, dissolved oxygen can fall, stressing or killing fish and oxygen-sensitive invertebrates and favoring organisms tolerant of hypoxia. High BOD is consequently a widely used indicator of organic pollution rather than evidence of abundant oxygen. Clean water generally has little biodegradable substrate and therefore a low oxygen demand. Temperature affects both oxygen solubility and metabolic rate, so it must be controlled during testing; “cold water” is not what BOD itself measures. Chemical oxygen demand is a related but broader assay that chemically oxidizes susceptible compounds. The ecological link is direct: more microbial decomposition creates greater oxygen demand and signals poorer water quality.