BOD test (Biochemical Oxygen Demand) is a standard laboratory test. It is used for measuring the amount of dissolved oxygen which is consumed by aerobic microorganisms (bacteria) when the organic matter present in water sample is broken down and decomposed. It is the process where oxygen is utilized during biodegradation and this is referred to as biochemical oxygen demand.
In this test the initial dissolved oxygen (DO) of the sample is measured first. After this the water sample is taken in an airtight bottle and it is sealed properly so that air cannot enter and photosynthesis is prevented. The bottle is incubated at constant temperature of 20°C for exactly five days. After five days the final dissolved oxygen is measured and the difference between initial DO and final DO gives the BOD value.
BOD value is used as an indicator of water quality and organic pollution. It is measured for natural water bodies industrial effluents and wastewater. If BOD value is high it indicates more organic waste in water and microorganisms will consume more oxygen for decomposition and this can cause rapid depletion of oxygen which can stress suffocate or kill fish and other aquatic life. If BOD value is low it indicates the water is cleaner and less polluted.
Objectives of BOD Test
- To determine the biodegradable organic matter present in wastewater industrial effluents and natural waters.
- To evaluate efficiency of wastewater treatment plant by measuring waste load entering and BOD removal before and after treatment process.
- To assess ecological health and water quality and to know impact of effluent discharge on receiving water bodies (rivers and lakes).
- To measure oxygen depletion potential by calculating dissolved oxygen required by aerobic microorganisms to break down organic matter and oxygen utilized to oxidize inorganic materials like sulphides and ferrous iron.
- To ensure regulatory compliance by monitoring effluent discharge so that it meets environmental regulations standards and discharge permits (NPDES in United States).
- To support water quality modeling by providing data like ultimate BOD values and decay kinetics for stream assimilative capacity and managing oxygen demand of aquatic habitats.
Principle of BOD Test
Principle of BOD test (Biochemical Oxygen Demand) is based on the measurement of dissolved oxygen which is consumed by aerobic microorganisms during biochemical degradation of organic matter present in water sample. In this test the water sample is taken in a BOD bottle and it is filled completely so that no air space is present (it may be diluted and seeded with microbial population when required). The initial dissolved oxygen (DO) is measured first and then bottle is sealed airtight. The sealed sample is incubated in dark at constant temperature of 20°C for a fixed period mostly five days (this is referred to as BOD5). After incubation the final DO is measured and BOD is calculated by difference between initial DO and final DO. The drop in oxygen gives estimation of biodegradable organic waste load and it is used as direct index of degree of organic pollution in water.
Materials required
Materials required for BOD test are–
Equipment and apparatus
- Incubation bottles– 300 mL glass BOD bottles with flared mouth and ground glass stopper for airtight sealing.
- Incubator or water bath– thermostatically controlled at 20 ± 1°C and light should be excluded to prevent algae photosynthesis during 5 day test.
- Dissolved oxygen (DO) instrument– DO meter with electrochemical or optical sensor probe (stirrer attachment may be used). If Winkler titration method is used then titration glassware and chemicals are required.
- pH meter– it is used to check and adjust pH of sample.
- Aeration equipment– aquarium air pump plastic air tubing and air diffusion stone for saturating dilution water with oxygen.
- Large container– 10 to 20 L carboy or large glass bottle for preparation and storage of dilution water.
- Volumetric glassware– graduated cylinders (50–250 mL) and wide tipped pipets for measuring and transferring samples and seed.
- Thermometer– calibrated for checking correct temperature of incubator and sample.
- Glass beads– solid borosilicate beads (5 mm) used for displacing water in BOD bottle and to remove trapped air bubbles before sealing.
- Overcaps– paper plastic cups or aluminium foil placed over stopper to prevent evaporation of water seal during 5 day incubation.
Chemicals reagents and solutions
- Source water– distilled or deionized water free from heavy metals chlorine and organic materials (used as base for dilution water).
- Nutrient and mineral solutions– phosphate buffer magnesium sulphate calcium chloride and ferric chloride solutions added to dilution water for supporting bacterial growth.
- pH adjustment solutions– 1N sulphuric acid (H2SO4) and 1N sodium hydroxide (NaOH) for neutralizing sample to pH 6.5 to 7.5.
- Dechlorination solution– sodium sulphite or sodium thiosulphate solution used for destroying residual chlorine in sample.
- Microbial seed– bacteria population such as settled raw domestic wastewater primary effluent or commercial polyseed capsules for providing enough microorganisms to consume organic matter.
- Glucose glutamic acid (GGA) solution– standard check solution used to verify accuracy of test and health of microbial seed.
- Nitrification inhibitor– TCMP or Allylthiourea (ATU) added only when carbonaceous BOD (CBOD) is tested to stop oxidation of nitrogen compounds.
Procedure of BOD Test
Procedure of BOD test are–
- The water sample is taken and it is brought to temperature of 20 ± 3°C. The pH is checked and it is adjusted between 6.5 to 7.5 by using sulphuric acid or sodium hydroxide. If residual chlorine is present then it must be neutralized by using dechlorinating chemical like sodium sulphite.
- Dilution water is prepared by taking distilled or deionized water and it is aerated properly so that dissolved oxygen saturation is obtained. This water is fortified with nutrient and mineral solutions like phosphate buffer magnesium sulphate calcium chloride and ferric chloride.
- If sample is lacking the microbial population (chlorinated water high temperature wastes or some industrial effluents) then microbial seed suspension is added. If carbonaceous BOD (CBOD) is measured then nitrification inhibitor is also added in this step.
- Different dilutions of sample is prepared by using dilution water so that oxygen will not be completely depleted during test. An airtight BOD bottle (generally 300 mL) is filled to overflowing with the diluted mixture.
- The initial dissolved oxygen (DO) concentration of the prepared mixture is measured within 30 minutes. It can be measured by DO meter/probe or by Winkler titration method.
- The glass stopper is inserted tightly and it is ensured that no air bubble is trapped inside bottle. Water is added to flared mouth for creating water seal. The bottle is incubated in complete darkness at constant 20 ± 1°C for exactly 5 days.
- After 5 days (± 6 hours) incubation the bottle is removed and final dissolved oxygen (DO) is measured.
- BOD value is calculated by subtracting final DO from initial DO. The difference is multiplied with dilution factor and oxygen depletion due to seed correction factor is subtracted if seed was added. The test is valid when sample depletes at least 2.0 mg/L DO and residual DO should be at least 1.0 mg/L remaining.
Calculation
Calculation of BOD test are–
Calculation for unseeded sample
- Formula– BOD (mg/L) = (Initial DO – Final DO) / P
- In this the dissolved oxygen depletion is calculated by subtracting final DO from initial DO. The value is divided by volumetric fraction (P) of sample used in bottle.
Calculation for seeded sample
- Formula– BOD (mg/L) = [(Initial DO – Final DO) – S] / P
- When microbial seed is added then oxygen consumed by seed is also included in total depletion. So seed correction factor (S) is subtracted first and then the value is divided by P.
Terms used in calculation
- Initial DO– dissolved oxygen (mg/L) of diluted sample immediately after preparation.
- Final DO– dissolved oxygen (mg/L) of diluted sample after exactly 5 days incubation at 20°C.
- P (volumetric fraction)– fraction of sample used in bottle. Example– if 100 mL sample is taken in 300 mL BOD bottle then P = 100/300 = 0.33.
- S (seed correction factor)– oxygen depletion due to added seed. It is calculated by seed control (DO uptake per mL seed) and it is multiplied with volume of seed added in sample bottle.
Criteria for valid calculation
- Minimum depletion– sample should deplete at least 2.0 mg/L dissolved oxygen in 5 days.
- Minimum residual– at least 1.0 mg/L dissolved oxygen should remain after 5 day incubation.
- Blank check– dilution water blank should not show DO depletion more than 0.20 mg/L.
Uses of BOD Test
Uses of BOD test are–
- It is used for measuring organic pollution and waste loading by determining biodegradable organic matter present in sewage industrial effluents and polluted water.
- It is used for evaluating wastewater treatment efficiency by measuring waste load entering treatment plant and by testing water before and after treatment.
- It is used for ensuring regulatory compliance by monitoring effluent discharge and to check that it meets environmental regulations standards and discharge permits (EPA).
- It is used for assessing ecological impact and water quality and it acts as indicator of health of aquatic ecosystem. It is also used to evaluate impact of effluent discharge on receiving waters like rivers and lakes.
- It is used for determining biodegradability of wastewater. When it is compared with COD then BOD to COD ratio is used to know how easily wastewater can be treated biologically and it helps in selecting treatment technology.
- It is used for supporting water quality modeling by providing kinetic data and ultimate BOD (UBOD) values for stream assimilative capacity and deoxygenation rate in aquatic habitat.
- It is used for evaluating toxicity and sludge stability by continuous respirometric BOD methods. It is used for assessing biodegradation of specific chemicals treatability of industrial wastes inhibitory effect of toxic compounds and stability of sludges.
Limitations of BOD Test
Limitations of BOD test are–
- Lengthy testing time– it requires 5 days incubation period so results are obtained late and it gives only historical data. It cannot be used for rapid real time operational adjustment and process control.
- Interference from toxic substances– the test depends on living microorganisms. Toxic materials like heavy metals chlorine and cyanides can inhibit or kill bacteria so BOD value becomes low and inaccurate.
- Measures only biodegradable organics– it measures only biologically degradable fraction of organic matter. Total organic pollution is not measured (COD measures total oxidizable matter).
- Low precision and high variability– it is an empirical biological bioassay so variability is present. Error may occur due to seed quality microbial health and sample handling (about 15% to 28%).
- Does not reflect actual stream conditions– the incubation is done in laboratory at 20°C in dark so sunlight temperature fluctuation and water movement are not present. So it gives only approximate estimate of actual stream oxygen demand.
- Nitrogenous demand interference– oxidation of reduced nitrogen compounds like ammonia by nitrifying bacteria also consumes oxygen. It may cause overestimation of carbonaceous organic load unless nitrification inhibitor is added.
- Only fraction of ultimate demand is measured– BOD5 does not represent ultimate biochemical oxygen demand (BODu). Complete oxidation of biodegradable material may take 20 to 30 days or more.
- Requires complex preparation for some sample– high organic strength samples must be diluted because oxygen solubility in water is limited (about 7–9 mg/L) and oxygen can be depleted completely. Sterilized sample or extreme pH samples require microbial seed addition and it increases complexity and variability.
References
- Analytical framework and comprehensive assessment of biological oxygen demand (BOD) in water quality management and wastewater engineering. (n.d.).
- Aquatech. (n.d.). Essential guide: Biochemical oxygen demand – BOD testing and measurement in water. https://www.aquatechtrade.com/water-stories/water-treatment/essential-guide-biochemical-oxygen-demand-measure-test
- Hanna Instruments. (n.d.). A beginner’s guide to dissolved oxygen measurement. https://blog.hannainst.com/beginners-guide-to-dissolved-oxygen-measurement
- OPSSYS. (2008, March 22). BOD manager example calculations [Q10841]. https://www.opssys.com/InstantKB/Article.aspx?id=10841
- Standard Methods Committee. (2001). 5210 Biochemical oxygen demand (BOD). In Standard Methods for the Examination of Water and Wastewater. https://polyseed.com/wp-content/uploads/2020/07/SM-22nd-Edition.pdf
- The Wastewater Blog. (2020, May 10). BOD, COD, and TOC. https://www.thewastewaterblog.com/single-post/2019/01/13/bod-cod-and-toc
- U.S. Geological Survey. (2018, June 5). Biochemical oxygen demand (BOD) and water. Water Science School. https://www.usgs.gov/water-science-school/science/biochemical-oxygen-demand-bod-and-water
- Wisconsin Department of Natural Resources. (n.d.). BOD quality control. https://dnr.wisconsin.gov/topic/labCert/BODQC.html
- Wisconsin Department of Natural Resources. (n.d.). BOD seed. https://dnr.wisconsin.gov/topic/labCert/BODSeeding.html