生化需氧量Biochemical oxygen demand

稀释接种法：把水样用含营养盐的稀释水稀释，必要时接种微生物，测定初始溶解氧，在 20 ± 1 °C 暗处培养 5 天后再测溶解氧，两者之差乘以稀释倍数即为 BOD₅。溶解氧用碘量法（温克勒法）或膜电极测定。培养 5 天约可完成含碳有机物 60–70% 的降解；更长时间的“总 BOD”还包括硝化作用的耗氧。

In the dilution method the sample is diluted with nutrient-containing water, seeded with microbes if necessary, its dissolved oxygen measured, and then incubated in the dark at 20 ± 1 °C for five days before measuring again; the difference times the dilution factor is BOD₅. Dissolved oxygen is measured by iodometric (Winkler) titration or a membrane electrode. Five days covers roughly 60–70% of carbonaceous breakdown; longer 'ultimate BOD' also includes oxygen used by nitrification.

1908 年英国皇家污水处理委员会提出以 BOD 评价河流污染，选择 5 天是因为英国河流从源头到入海的最长时间约为 5 天，20 °C 接近夏季平均水温。1912 年委员会建议排放标准为 BOD₅ 20 mg/L、悬浮物 30 mg/L，这一“20/30 标准”影响深远。

In 1908 Britain's Royal Commission on Sewage Disposal proposed BOD to assess river pollution; five days was chosen because it was about the longest time water took to travel from source to sea in British rivers, and 20 °C was close to the average summer water temperature. Its 1912 recommendation of 20 mg/L BOD₅ and 30 mg/L suspended solids, the '20/30 standard', was hugely influential.

BOD 直接反映有机污染对水体溶解氧的影响：BOD 高的废水排入河流会造成缺氧、鱼类死亡和发黑发臭。它是设计和评价污水生物处理工艺的核心参数。由于测定需要 5 天，实践中常同时测 COD，并用 BOD₅/COD 判断可生化性。

BOD directly shows how organic pollution will deplete a river's oxygen: high-BOD wastewater causes anoxia, fish kills, blackening and odour. It is the central parameter for designing and assessing biological wastewater treatment. Because it takes five days, COD is usually measured alongside, and BOD₅/COD indicates biodegradability.