The rate of oxygen use is commonly referred to as Biochemical Oxygen Demand (BOD). Biochemical Oxygen Demand is a chemical procedure for determining how fast biological organisms use up oxygen in a body of water. It is used in water quality management and assessment, ecology and environmental science. BOD is not an accurate quantitative test, although it is considered as an indication of the quality of a water source. Biochemical oxygen demand is not a specific pollutant, but rather a measure of the amount of oxygen required by bacteria and other microorganisms engaged in stabilizing decomposable organic matter over a specified period of time. The BOD test is often used to estimate the impacts of effluents that contain large amounts of biodegradable organics such as that from food processing plants and feedlots, municipal wastewater treatment facilities and pulp mills. A high oxygen demand indicates the potential for developing a dissolved oxygen sag as the microbiota oxidize the organic matter in the effluent. A very low oxygen demand indicates either clean water or the presence of a toxic or non-degradable matter.
The BOD test was first used in the late 1800s by the Royal Commission on Sewage Disposal as a measure of the amount of organic pollution in British rivers. At that time, the test was standardized to run for 5 days at 18.30C. These numbers were chosen because none of the British rivers had headwater-to-sea travel times greater than 5 days and the average summer temperature for the rivers was 18.30C. Accordingly, this should reveal the "worst case" oxygen demand in any British river. The BOD incubation temperature was later rounded to 200C, but the 5-day test period remains the current as standard.
In its simplest version, the 5-day BOD test begins by placing water or effluent samples into two standard 60 - or 300 - mL BOD bottles (Fig.1). One sample is analysed immediately to measure the initial dissolved oxygen concentration in the effluent, often using a Winkler titration. The second BOD bottle is sealed and stored at 20°C in the dark. (The samples are stored in the dark to avoid photosynthetic oxygen generation). After 5 days the amount of dissolved oxygen remaining in the sample is measured. The difference between the initial and ending oxygen concentrations is the BOD.
BOD is used in water quality management and assessment, ecology and environmental science. BOD is not an accurate quantitative test, although it is considered as an indication of the quality of a water source. It is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20°C or 3 days of incubation at 27°C. The BOD test must be inhibited to prevent oxidation of ammonia. If the inhibitor is not added, the BOD will be between 10% and 40% higher than can be accounted for by carbonaceous oxidation.
Stages of Decomposition in the BOD test
There are two stages of decomposition in the BOD test: a carbonaceous stage and a nitrogenous stage. The carbonaceous stage represents oxygen demand involved in the conversion of organic carbon to carbon dioxide. The second stage or the nitrogenous stage represents a combined carbonaceous plus nitrogenous demand, when organic nitrogen, ammonia and nitrite are converted to nitrate. Nitrogenous oxygen demand generally begins after about 6 days. Under some conditions, if ammonia, nitrite and nitrifying bacteria are present, nitrification can occur in less than 5 days. In this case, a chemical compound that prevents nitrification is added to the sample if the intent is to measure only the carbonaceous demand. The results are reported as carbonaceous BOD (CBOD) or as CBOD5 when a nitrification inhibitor is used.
BOD – Dilution Method
BOD is the amount of oxygen (Dissolved Oxygen (DO)) required for the biological decomposition of organic matter. The oxygen consumed is related to the amount of biodegradable organics. When organic substances are broken down in water, oxygen is consumed
Organic Carbon + O2 → CO2
Where, organic carbon in human waste includes protein, carbohydrates, fats, etc. Measure of BOD = Initial oxygen- Final Oxygen after (5 days at 20 °C) or (3 days at 27 °C). Two standard 300 mL BOD bottles are filled completely with wastewater. The bottles are sealed. Oxygen content (DO) of one bottle is determined immediately. The other bottle is incubated at 20°C for 5 days or (or at 27 °C for 3 days) in total darkness to prevent algal growth. After which its oxygen content is again measured. The difference between the two DO values is the amount of oxygen consumed by micro-organisms during 5 days and is reported as BOD5.
Where, DOi and DOf are initial and final DO concentrations of the diluted sample, respectively. P is called as dilution factor and it is the ratio of sample volume (volume of wastewater) to total volume (wastewater plus dilution water).
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