What is Biochemical Oxygen Demand (BOD)?
Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen required by aerobic microorganisms in water to decompose organic matter into inorganic substances within a specific time, at a given temperature.
Although BOD is not a precisely quantitative test, because it indirectly reflects the relative content of organic matter in water, it has long been widely used as an environmental monitoring indicator. In water-environment modeling, where considering each compound individually is impractical, BOD is likewise used to simulate changes in organic matter in water.
The ratio of BOD to Chemical Oxygen Demand (COD) indicates the proportion of hardly biodegradable organic matter in water; organic pollutants that microorganisms struggle to decompose cause greater environmental harm. It is generally considered that a wastewater ratio above 0.3 is suitable for biological treatment.
In BOD measurement, test conditions of 20°C over 5 days are conventionally specified, with results expressed as mg/L of oxygen, denoted as the five-day BOD. This indicator was established by the British Royal Commission on Sewage Disposal.
Pollutants in surface water consume dissolved oxygen in water through microbial-mediated oxidation; the oxygen consumed is called Biochemical (or Biological) Oxygen Demand, BOD, in mg/L, indirectly reflecting the amount of biodegradable organic matter in water.
BOD, also called biochemical oxygen consumption, is abbreviated BOD in English (biochemical oxygen demand). It is a comprehensive indicator of the content of oxygen-demanding pollutants such as organic matter in water, showing the total dissolved oxygen consumed as organic matter is oxidized and decomposed by microbial biochemical action into inorganic or gaseous forms. The higher the value, the more organic pollution and the more severe the pollution. Hydrocarbons, proteins, oils, lignin, and other organic pollutants present in suspended or dissolved form in domestic sewage and industrial wastewater from sugar, food, paper, and fiber industries can be decomposed by aerobic bacteria. Because oxygen is consumed in the decomposition process, they are also called oxygen-demanding pollutants. If too much of such pollutants is discharged into water bodies, dissolved oxygen will be depleted; meanwhile, the organic matter causes putrefaction through anaerobic bacterial decomposition, producing foul-smelling gases such as methane, hydrogen sulfide, mercaptans, and ammonia, spoiling and stinking the water.
The time for various organic matter in sewage to be completely oxidized and decomposed totals about 100 days. To shorten testing time, BOD is generally represented by the oxygen consumed by a tested sample over five days at 20°C, called the five-day BOD, abbreviated BOD5. For domestic sewage, it is approximately 70% of the oxygen consumed for complete oxidation.
BOD is widely used to measure the pollution intensity of wastewater and the load and efficiency of wastewater treatment structures, and to study the oxygen balance of water bodies (see river self-purification). A sample or diluted sample is stored and incubated for a period; the difference in dissolved oxygen before and after incubation is its BOD. Both incubation time and temperature affect oxygen consumption. The incubation time adopted by all countries is now 5 days at 20°C, called the five-day BOD, denoted BOD5,20°C (the temperature subscript is often omitted, i.e., BOD5; sometimes BOD alone is used). Extending incubation can measure the total oxygen needed for microbial degradation of all organic matter in water, called total BOD, generally estimated from BOD5 per biochemical oxygen-consumption patterns. BOD measurement is not easy to do accurately. Sample storage, dilution, inoculation, and other procedures should follow standard methods. For toxic industrial wastewater, special equipment is often required, and sometimes measurement is impossible. High-concentration organic industrial wastewater can have BOD5 in the thousands or even millions of mg/L. Municipal sewage BOD5 is around 200 mg/L. Unpolluted water bodies often have BOD5 below 2 mg/L.
Generally, the five-day BOD of clean rivers does not exceed 2 mg/L; above 10 mg/L it emits a foul odor. Industrial, agricultural, and aquaculture water require BOD below 5 mg/L, while drinking water should be below 1 mg/L.
China's Integrated Wastewater Discharge Standard stipulates that at a factory outfall, the secondary-standard maximum allowable BOD concentration is 60 mg/L, and surface water BOD must not exceed 4 mg/L.
Municipal wastewater treatment plant standards: First-class A 10 mg/L; First-class B 20 mg/L; Second-class 30 mg/L; Third-class 60 mg/L.
The microbial sensor for measuring BOD in water consists of an oxygen electrode and a microbial membrane. The principle: when a sample saturated with dissolved oxygen enters the flow cell and contacts the microbial sensor, dissolved biodegradable organic matter in the sample is acted upon by the strains in the membrane, consuming a certain amount of oxygen and reducing the mass of oxygen diffusing to the oxygen electrode surface. When the diffusion rate (mass) of biodegradable organic matter from the sample to the membrane becomes constant, the oxygen mass diffusing to the electrode also becomes constant, producing a constant current. Because a quantitative relationship exists between the constant-current difference and the oxygen reduction, the sample's BOD can be calculated. This is a rapid microbial-sensor method for determining BOD in water and wastewater. The standard defines BOD as the dissolved oxygen consumed by dissolved, biodegradable organic matter in water and wastewater under microbial action.
This method applies to the determination of BOD in surface water, domestic sewage, and industrial wastewater that does not contain substances markedly toxic to microorganisms.
The maximum allowable amounts of the following substances in water that do not significantly interfere with this method are: CO2+ 5 mg/L; Mn2+ 5 mg/L; Zn2+ 4 mg/L; Fe2+ 5 mg/L; Cu2+ 2 mg/L; Hg2+ 5 mg/L; Pb2+ 5 mg/L; Cd2+ 5 mg/L; Cr6+ 0.5 mg/L; CN- 0.05 mg/L; suspended solids 250 mg/L. For samples containing free or combined chlorine, add 1.575 g/L sodium sulfite solution to deactivate the chlorine, avoiding excess. This method is unsuitable for high-concentration bactericides or pesticide wastewater that are toxic to the membrane strains.
Under the peristaltic pump's action, the sample or cleaning solution is continuously fed into the measurement cell at a fixed ratio per unit time.
Add buffer solution to the measurement cell to keep the microbial sensor (microbial membrane) in contact with the buffer, then add a fixed amount of the tested sample to measure its BOD value.
The microbial electrode's response depends on certain temperature conditions, so a stable temperature field is required during the test. This device is called the constant-temperature control device in the instrument.
The cleaning solution (buffer) is prepared from potassium dihydrogen phosphate and disodium hydrogen phosphate. Its main roles are to regulate sample pH as a buffer, clean and maintain the microbial sensor for normal operation, and precipitate heavy-metal ions.
Analytical-grade reagents and distilled water; the distilled water should be boiled for about 2–5 minutes before use and cooled to room temperature. Phosphate buffer solution, 0.5 mol/L: dissolve 68 g potassium dihydrogen phosphate (KH2PO4) and 134 g disodium hydrogen phosphate (Na2HPO4·7H2O) in distilled water and dilute to 1000 mL; reserve. This solution has pH about 7. Phosphate buffer working solution (cleaning solution): 0.005 mol/L. Hydrochloric acid (HCl) solution: 0.5 mol/L. Sodium hydroxide (NaOH) solution: 20 g/L. Sodium sulfite (Na2SO3) solution: 1.575 g/L (unstable; prepare before use). Glucose–glutamic acid standard solution: weigh 1.705 g each of anhydrous glucose (C6H12O6) and glutamic acid (HOOC–CH2–CH2–CHNH2–COOH) dried at 103°C for 1 hour and cooled, dissolve in 4.2 of phosphate buffer working solution, dilute to 1000 mL, mix to obtain a 250 mg/L BOD standard solution. Glucose–glutamic acid standard working solution (prepare before use): take 10.00 mL of the standard solution from 4.6, place in a 250 mL volumetric flask, dilute to the mark with 0.005 mol/L phosphate buffer working solution, mix; this solution is 100 mg/L.
D1: Initial dissolved oxygen of the diluted sample (mg/L).
If a sample cannot be analyzed within 2 hours of collection, it should be preserved at 0–4°C and analyzed within 6 days; if it cannot be analyzed within 6 hours, the storage time and temperature should be reported with the results. Under no circumstances should storage exceed 24 hours.
The calculation of BOD is as follows:
BOD（mg / L）=（D1-D2） / P
Glassware and plastic containers used should be cleaned carefully; no toxic or biodegradable compounds may remain on container walls, and contamination should be prevented during operation. Microbial-sensor rapid BOD meter. Microbial membrane: strains in the membrane should be uniform and as consistent as possible between membranes. It can be preserved wet or dry at room temperature. The microbial membrane's continuous service life should exceed 30 days. Membrane activation: soak the microbial membrane in 0.005 mol/L phosphate buffer working solution for over 48 hours, then install it on the microbial sensor. 10 L polyethylene plastic bucket.
D2: Dissolved oxygen of the diluted sample after 5-day incubation in a 20°C constant-temperature incubator (mg/L).
P = [Sample volume (mL)] / [Final volume of diluted sample (mL)].
Difference between BOD and COD (Chemical Oxygen Demand): COD is the amount of oxygen needed to oxidize reducible matter in a water sample, measured chemically. Under certain conditions, the amount of oxidant consumed to oxidize reducible substances in 1 L of sample is taken as the indicator, converted to the milligrams of oxygen needed to fully oxidize the sample per liter, expressed as mg/L. It reflects the degree of pollution by reducible substances. This indicator is also one comprehensive indicator of relative organic content. The BOD/COD ratio indicates how much of the organic pollutants in water are hard for microorganisms to decompose. Organic pollutants that microorganisms struggle to decompose cause greater environmental harm.
