Wastewater Terminology: 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 certain temperature.
Although BOD is not a precise quantitative test, because it indirectly reflects the relative content of organic matter in water, BOD has long been widely used as an environmental monitoring indicator. In water-environment modeling, since 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 are difficult for microorganisms to decompose cause greater environmental harm. It is generally considered that when this ratio in wastewater exceeds 0.3, biological treatment is suitable.
In BOD measurement, test conditions of 20°C and 5 days are normally specified, and the result is expressed as mg/L of oxygen, recorded as the 5-day BOD. This indicator was established by the British Royal Commission on Sewage Disposal.
Pollutants in surface water consume dissolved oxygen in water during microorganism-mediated oxidation; the dissolved oxygen consumed is called Biochemical Oxygen Demand (or biological oxygen demand, i.e., BOD, in mg/L), indirectly reflecting the amount of biodegradable organic matter in water.
BOD, also called biochemical oxygen consumption, abbreviated BOD from "biochemical oxygen demand," is a comprehensive indicator of the content of oxygen-demanding pollutants such as organic matter in water. It shows the total dissolved oxygen consumed when organic matter in water is oxidized and decomposed into inorganic or gaseous form by the biochemical action of microorganisms. The higher the value, the more organic pollution in the water and the more serious the pollution. Hydrocarbons, proteins, oils, lignin, etc., present in suspended or dissolved state in domestic sewage and industrial wastewater from sugar, food, paper and fiber industries are organic pollutants that can be decomposed by the biochemical action of aerobic bacteria; because they consume oxygen during decomposition, they are also called oxygen-demanding pollutants. If such pollutants are discharged into water bodies in excess, dissolved oxygen will be depleted; meanwhile, organic matter causes putrefaction through anaerobic bacterial decomposition, producing foul gases such as methane, hydrogen sulfide, mercaptans and ammonia, making the water putrid and malodorous.
The time for all organic matter in sewage to be completely oxidized and decomposed is about 100 days in total. To shorten the test time, BOD is generally represented by the oxygen consumption of the tested sample at 20°C over 5 days, called the 5-day BOD, abbreviated BOD5; for domestic sewage it is about 70% of the oxygen consumed for complete oxidation.
It is widely used to measure the pollution intensity of wastewater and the load and efficiency of wastewater-treatment structures, and also to study the oxygen balance of water bodies (see river self-purification). The BOD is the difference in dissolved oxygen of a sample before and after incubation for a period. Both incubation time and temperature affect oxygen consumption. The incubation time adopted by all countries is now 5 days at 20°C, expressed as BOD5 at 20°C; the temperature subscript is often omitted, i.e., BOD5, and sometimes only BOD is used. Extending the incubation time allows measurement of the total oxygen required for microorganisms to degrade organic matter in water, called total BOD, generally estimated from BOD5 according to biochemical oxygen-consumption patterns. BOD testing is not easy to do accurately; sample storage, dilution and inoculation should follow standard methods. For toxic industrial wastewater, special equipment is often required and sometimes measurement is impossible. The BOD5 of high-concentration organic industrial wastewater can reach thousands or millions of mg/L. Municipal sewage BOD5 is around 200 mg/L. For unpolluted water bodies, BOD5 is often below 2 mg/L.
For a clean river, the 5-day BOD generally does not exceed 2 mg/L; if above 10 mg/L, a foul odor is emitted. For industrial, agricultural and aquacultural water, BOD should be below 5 mg/L, while for drinking water it should be below 1 mg/L.
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.
Municipal wastewater treatment plant discharge standards: Grade 1A: 10 mg/L; Grade 1B: 20 mg/L; Grade 2: 30 mg/L; Grade 3: 60 mg/L.
The microbial sensor for BOD measurement consists of an oxygen electrode and a microbial membrane. When a sample containing saturated dissolved oxygen enters the flow cell and contacts the microbial sensor, the 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 oxygen diffusing to the oxygen electrode surface. When the diffusion rate (mass) of biodegradable organic matter to the membrane becomes constant, the oxygen diffusing to the electrode surface also becomes constant, producing a constant current. Because there is a quantitative relationship between the constant-current difference and the oxygen reduction, the BOD of the sample can be calculated. This is the rapid microbial-sensor method for BOD in water and wastewater. The biochemical oxygen demand defined by this standard is the dissolved oxygen consumed by dissolved biodegradable organic matter in water and wastewater under microbial action.
This method is applicable 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 cause obvious interference with this method: 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 sodium sulfite solution at 1.575 g/L to deactivate the free or combined chlorine, avoiding excess. This method is not suitable for sewage containing high concentrations of bactericides or pesticides toxic to the membrane strains.
Under the action of a peristaltic pump, the sample or cleaning solution is continuously fed into the measurement cell at a fixed ratio per unit time.
China’s Integrated Wastewater Discharge Standard stipulates that at the factory outfall the maximum allowable concentration of BOD in wastewater for Grade 2 is 60 mg/L, and the BOD of surface water must not exceed 4 mg/L.
The response performance of the microbial electrode depends on a certain temperature condition, 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 role is to adjust the sample pH as a buffer, clean and maintain the microbial sensor for normal operation, and to settle heavy-metal ions.
Analytical-grade reagents and distilled water; distilled water should be boiled for about 2–5 minutes before use and cooled to room temperature. Phosphate buffer (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; 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 h and cooled, dissolve and dilute 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 into a 250 mL volumetric flask, dilute to the mark with 0.005 mol/L phosphate buffer working solution, mix well; this solution is 100 mg/L.
Glassware and plastic containers must be cleaned carefully; no toxic or biodegradable compounds may remain on the walls, and contamination must be avoided during operation. Microbial-sensor rapid BOD analyzer. Microbial membrane: strains in the membrane should be uniform and membranes as consistent as possible; it can be kept wet or dried at room temperature. Continuous service life of the membrane should exceed 30 days. Membrane activation: soak the microbial membrane in 0.005 mol/L phosphate buffer working solution for over 48 h, then install it on the microbial sensor. 10 L polyethylene plastic bucket.
If samples cannot be analyzed within 2 hours after collection, store at 0°C–4°C and analyze within 6 days; if not analyzable within 6 hours, report storage time and temperature with the results. Under no condition may storage exceed 24 hours.
The BOD calculation is as follows:
BOD (mg/L) = (D1 − D2) / P
D1: initial dissolved oxygen of the diluted sample (mg/L)
D2: dissolved oxygen of the diluted sample after 5-day incubation at 20°C (mg/L)
P = [volume of water sample (mL)] / [final volume of diluted sample (mL)]
Difference between BOD and COD (Chemical Oxygen Demand): COD is the amount of reducing substances in a water sample that need to be oxidized, measured chemically. Under certain conditions, the amount of oxidant consumed to oxidize reducing substances in 1 L of sample is taken as the indicator, converted to the milligrams of oxygen required to fully oxidize per liter of sample, expressed as mg/L. It reflects the degree of pollution by reducing substances, and is also one of the comprehensive indicators of relative organic content. The ratio of BOD to COD indicates how much of the organic pollutants in water are difficult for microorganisms to decompose; such pollutants cause greater environmental harm.
