BOD measurement is a water-quality analysis used to determine how much oxygen is consumed when organic matter contained in water is decomposed by microorganisms.
BOD is called Biochemical Oxygen Demand and is an important indicator for evaluating organic pollution in domestic wastewater, food-processing wastewater, river water, lake water, and other types of water.
The more organic matter water contains, the more readily dissolved oxygen is consumed through microbial decomposition.
In a discussion of BOD measurement, it is not sufficient simply to write that “the BOD was high” or that “dissolved oxygen decreased.”
It is necessary to explain why dissolved oxygen decreases, how microbial decomposition and the amount of organic matter are related, what effects water with high BOD has on aquatic organisms, and where measurement errors arise.
This article clearly explains, as examples of discussions that can be used in laboratory reports on BOD measurement, microbial decomposition and changes in dissolved oxygen, how to interpret BOD values, dilution factors, incubation conditions, differences from COD, sources of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of BOD measurement results obtained in environmental chemistry experiments, analytical chemistry experiments, and basic chemistry experiments at universities and similar institutions.
For the actual BOD measurement method, incubation temperature, incubation time, dilution method, dissolved oxygen measurement method, calculation formula, units, handling of environmental standards and effluent standards, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is BOD Measurement?
- Main Items to Include in the Results
- Reference Experimental Values and Calculation Examples for BOD Measurement
- Reference Experimental Conditions
- Concept of BOD Measurement
- Example Measurements Without Dilution
- Example Calculation of BOD
- Example Measurements of Diluted Samples
- Example Calculation Including the Dilution Factor
- Effect of Using an Incorrect Dilution Factor
- Comparison of Appropriate Dilution Factors
- Example of Blank Correction
- Comparison of DO Decrease With and Without Microorganisms
- Effect of Oxygen Consumption by Nitrification
- Decrease in DO Over Time
- Comparison of Errors Caused by Sampling and Storage
- Example of How to Write the Results
- Points for Connecting the Results to the Discussion
- Example Discussion
- Summary
- Relationship Between Microbial Decomposition and Dissolved Oxygen
- What Is Dissolved Oxygen?
- Discussion When the BOD Value Is High
- Discussion When the BOD Value Is Low
- Difference Between BOD and COD
- Discussion of Dilution Factor
- Effect of Incubation Temperature
- Effect of Incubation Time
- Effect of Microbial Activity
- Effect of Nitrification
- Discussion When Toxic Substances Are Present
- Effects of Sampling and Storage
- Relationship Between Water Temperature and Dissolved Oxygen
- Discussion When Oxygen Is Excessively Depleted
- Discussion When the Decrease in Dissolved Oxygen Is Small
- Sources of Error in BOD Measurement
- Sources of Error in Dissolved Oxygen Measurement
- When the BOD Measurement Can Be Considered Good
- Example Discussion When the Experiment Did Not Go Well
- How to Write Points for Improvement
- Difference Between a Superficial Discussion and a Good Discussion
- Examples of Expressions That Can Be Used in Reports
- Points to Check When Discussing BOD Measurement
- Summary
What Is BOD Measurement?
BOD is an abbreviation for Biochemical Oxygen Demand.
It represents the amount of oxygen consumed when organic matter contained in water is decomposed by microorganisms.
In general, the higher the BOD, the more organic matter that microorganisms can decompose is considered to be present.
In BOD measurement, the sample water is incubated under fixed conditions, and the difference in dissolved oxygen before and after incubation is measured.
Because microorganisms consume oxygen when decomposing organic matter, the amount of dissolved oxygen after incubation decreases.
BOD is determined from this decrease in oxygen.
Example Discussion:
BOD measurement evaluates the amount of oxygen consumed when organic matter in water is decomposed by microorganisms.
Because the dissolved oxygen decreased after incubation in this experiment, the sample water was considered to contain organic matter that microorganisms could use.
The BOD value is important as an indicator showing the degree of water pollution caused by readily biodegradable organic matter.
Main Items to Include in the Results
In the results of BOD measurement, organize the type of sample water, sampling location, sampling date and time, dissolved oxygen before measurement, dissolved oxygen after incubation, incubation time, incubation temperature, dilution factor, calculated BOD value, and other information.
Because BOD is readily affected by measurement conditions, it is important to clearly state the incubation and dilution conditions.
Main Items to Include in the Results
- Type of sample water
- Sampling location
- Sampling date and time
- Weather at the time of sampling
- Water temperature
- Appearance of the sample
- Dissolved oxygen before measurement
- Dissolved oxygen after incubation
- Decrease in dissolved oxygen
- Incubation temperature
- Incubation time
- Dilution factor
- Presence or absence of seeding
- Blank measurement value
- Calculated BOD value
- Comparison with other indicators such as COD
- Sources of error and points for improvement
Example of How to Write the Results:
The sample water was incubated under fixed conditions, and the amounts of dissolved oxygen before and after incubation were measured.
After incubation, the amount of dissolved oxygen decreased, and the BOD value was determined from this decrease.
From this result, the sample water was considered to contain organic matter decomposed by microorganisms.
Reference Experimental Values and Calculation Examples for BOD Measurement
Here, reference experimental values are organized for measuring biochemical oxygen demand (BOD) and discussing the relationship between organic-matter decomposition by microorganisms and the decrease in dissolved oxygen (DO).
BOD is an indicator representing the amount of oxygen consumed when organic matter in water is decomposed by microorganisms.
In general, water containing large amounts of organic matter shows a greater decrease in DO because of microbial respiration and has a higher BOD.
Water with high BOD readily consumes oxygen in the water and may cause oxygen deficiency for aquatic organisms.
Reference Experimental Conditions
| Item | Details |
|---|---|
| Measurement target | Tap water, river water, pond water, domestic wastewater, food-factory wastewater |
| Measurement method | BOD measurement by dilution method |
| Measurement period | 5 days at 20°C |
| Measurement items | Initial DO, DO after 5 days, decrease in DO, BOD |
| Container used | BOD bottle |
| Dilution water | Oxygen-saturated dilution water |
| Storage conditions | 20°C, dark |
| Evaluation items | Oxygen consumption, dilution factor, BOD, microbial decomposition, sources of error |
Concept of BOD Measurement
In BOD measurement, sample water is stored under fixed conditions, and the difference in DO before and after storage is measured.
Because microorganisms consume oxygen when decomposing organic matter, the DO after 5 days becomes lower than the initial DO.
BOD = Initial DO − DO after 5 days
If the sample is diluted before measurement, the decrease in DO is multiplied by the dilution factor to convert it to the BOD of the original sample.
BOD = (Initial DO − DO after 5 days) × Dilution factor
Example Measurements Without Dilution
For water with relatively little pollution, BOD may be measured without dilution.
Here, reference examples are shown for measuring tap water, river water, and pond water without dilution.
| Sample | Water Sample | Initial DO | DO After 5 Days | Decrease in DO | Dilution Factor | BOD | How to Interpret Water Quality |
|---|---|---|---|---|---|---|---|
| A | Tap water | 8.8 mg/L | 8.4 mg/L | 0.4 mg/L | 1× | 0.4 mg/L | Contains little organic matter |
| B | River water | 8.5 mg/L | 6.9 mg/L | 1.6 mg/L | 1× | 1.6 mg/L | Relatively good |
| C | Pond water | 8.2 mg/L | 4.8 mg/L | 3.4 mg/L | 1× | 3.4 mg/L | Contains somewhat more organic matter |
Example Calculation of BOD
For river water, the initial DO is 8.5 mg/L and the DO after 5 days is 6.9 mg/L.
Decrease in DO = 8.5 − 6.9 = 1.6 mg/L
Because the sample was measured without dilution, the dilution factor is 1.
BOD = 1.6 × 1 = 1.6 mg/L
Therefore, in this reference example, the BOD of the river water is determined to be 1.6 mg/L.
Example Measurements of Diluted Samples
For samples containing large amounts of organic matter, such as domestic wastewater and factory wastewater, measuring them directly may cause all of the DO to be consumed, making it difficult to determine BOD accurately.
Therefore, the sample is diluted before measurement, and the result is multiplied by the dilution factor at the end to convert it to the BOD of the original sample.
| Sample | Water Sample | Dilution Factor | Initial DO | DO After 5 Days | Decrease in DO | BOD | How to Interpret the Result |
|---|---|---|---|---|---|---|---|
| D | Domestic wastewater | 10× | 8.7 mg/L | 5.6 mg/L | 3.1 mg/L | 31 mg/L | Contains much organic matter |
| E | Food-factory wastewater | 50× | 8.6 mg/L | 3.9 mg/L | 4.7 mg/L | 235 mg/L | Very high |
| F | Sample containing sugar solution | 100× | 8.8 mg/L | 4.2 mg/L | 4.6 mg/L | 460 mg/L | Large oxygen consumption |
Example Calculation Including the Dilution Factor
Consider a case in which domestic wastewater is diluted tenfold and its BOD is measured.
The initial DO is 8.7 mg/L and the DO after 5 days is 5.6 mg/L.
Decrease in DO = 8.7 − 5.6 = 3.1 mg/L
This is the amount of oxygen consumed in the tenfold-diluted measurement solution.
To convert it to the BOD of the original sample, it is multiplied by the dilution factor of 10.
BOD = 3.1 × 10 = 31 mg/L
Therefore, in this reference example, the BOD of the domestic wastewater is determined to be 31 mg/L.
Effect of Using an Incorrect Dilution Factor
For samples measured after dilution, forgetting to include the dilution factor in the calculation causes BOD to be greatly underestimated.
| Calculation Condition | Calculated BOD | Problem |
|---|---|---|
| Tenfold dilution correctly considered | 31 mg/L | Correct calculation |
| Dilution factor omitted | 3.1 mg/L | Underestimated to one-tenth |
| Incorrectly calculated as a 50-fold dilution | 155 mg/L | Overestimated |
In BOD calculations, it is necessary to always confirm how many times the original sample was diluted.
Comparison of Appropriate Dilution Factors
In BOD measurement, it is desirable for DO to remain after 5 days while still showing a sufficient decrease.
Here, an example is shown in which the same domestic wastewater was measured at different dilution factors.
| Dilution Factor | Initial DO | DO After 5 Days | Decrease in DO | Calculated BOD | Evaluation of Measurement |
|---|---|---|---|---|---|
| 2× | 8.7 mg/L | 0.2 mg/L | 8.5 mg/L | 17 mg/L | Unsuitable because DO is almost exhausted |
| 5× | 8.7 mg/L | 2.4 mg/L | 6.3 mg/L | 31.5 mg/L | Decrease in DO is somewhat large |
| 10× | 8.7 mg/L | 5.6 mg/L | 3.1 mg/L | 31 mg/L | Appropriate |
| 20× | 8.7 mg/L | 7.1 mg/L | 1.6 mg/L | 32 mg/L | Decrease is somewhat small |
| 50× | 8.7 mg/L | 8.0 mg/L | 0.7 mg/L | 35 mg/L | Decrease is small and error is large |
If the dilution factor is too low, DO is almost entirely consumed and accurate BOD becomes difficult to determine.
Conversely, if the sample is diluted too much, the decrease in DO becomes small and the effect of reading errors becomes large.
Example of Blank Correction
The dilution water itself may show slight oxygen consumption.
Therefore, the dilution water alone is stored under the same conditions and used as a blank for correction.
| Sample | Initial DO | DO After 5 Days | Decrease in DO | How Correction Is Handled |
|---|---|---|---|---|
| Dilution-water blank | 8.8 mg/L | 8.6 mg/L | 0.2 mg/L | Blank decrease |
| Tenfold-diluted domestic wastewater | 8.7 mg/L | 5.6 mg/L | 3.1 mg/L | Decrease in measurement solution |
When blank correction is performed, the decrease in DO of the dilution-water blank is subtracted from the decrease in DO of the measurement solution.
Corrected decrease in DO = Decrease in DO of measurement solution − Decrease in blank DO
Corrected decrease in DO = 3.1 − 0.2 = 2.9 mg/L
Corrected BOD = 2.9 × 10 = 29 mg/L
If oxygen consumption in the blank cannot be ignored, failing to perform the correction may cause the BOD to be estimated somewhat too high.
Comparison of DO Decrease With and Without Microorganisms
BOD measurement makes use of oxygen consumption caused by microbial decomposition of organic matter.
Therefore, under conditions where microbial activity is weak, the decrease in DO may be small even when the same amount of organic matter is present.
| Condition | Initial DO | DO After 5 Days | Decrease in DO | How to Interpret the Result |
|---|---|---|---|---|
| Normal conditions | 8.7 mg/L | 5.6 mg/L | 3.1 mg/L | Microbial decomposition progresses |
| Sterilized sample | 8.7 mg/L | 8.2 mg/L | 0.5 mg/L | Microbial effect is small |
| Stored at low temperature | 8.7 mg/L | 7.4 mg/L | 1.3 mg/L | Microbial activity decreases |
| With seeding | 8.7 mg/L | 5.3 mg/L | 3.4 mg/L | Decomposition proceeds more readily |
The decrease in DO is small under sterilized and low-temperature conditions.
From this, oxygen consumption in BOD measurement is considered to depend strongly on microbial activity.
Effect of Oxygen Consumption by Nitrification
In BOD measurement, oxygen may be consumed not only by decomposition of organic matter but also by nitrification, in which ammoniacal nitrogen is oxidized to nitrate.
If nitrification progresses, BOD may be estimated as higher.
| Condition | Initial DO | DO After 5 Days | Decrease in DO | BOD | Direction of Discussion |
|---|---|---|---|---|---|
| Without nitrification inhibition | 8.6 mg/L | 3.9 mg/L | 4.7 mg/L | 47 mg/L | Effect of organic-matter decomposition + nitrification |
| With nitrification inhibition | 8.6 mg/L | 4.8 mg/L | 3.8 mg/L | 38 mg/L | Closer to carbonaceous BOD |
| Difference | – | – | 0.9 mg/L | 9 mg/L | Approximate oxygen consumption caused by nitrification |
Because BOD was higher without nitrification inhibition, oxygen consumption caused by oxidation of ammoniacal nitrogen may also have been included.
Decrease in DO Over Time
BOD measurement usually uses the DO after 5 days, but examining how DO decreases over time makes it easier to discuss the progress of microbial decomposition.
| Elapsed Days | DO | Cumulative Decrease in DO | How to Interpret the Result |
|---|---|---|---|
| Day 0 | 8.7 mg/L | – | Initial condition |
| Day 1 | 7.6 mg/L | 1.1 mg/L | Decomposition begins |
| Day 2 | 6.8 mg/L | 1.9 mg/L | Oxygen consumption progresses |
| Day 3 | 6.2 mg/L | 2.5 mg/L | Decomposition continues |
| Day 5 | 5.6 mg/L | 3.1 mg/L | Treated as BOD5 |
| Day 7 | 5.2 mg/L | 3.5 mg/L | Further oxygen consumption |
DO decreases greatly by Day 5 and continues to decrease slightly afterward.
BOD5 is an indicator representing the amount of oxygen consumed over 5 days and does not represent the amount required for complete decomposition of all organic matter.
Comparison of Errors Caused by Sampling and Storage
In BOD measurement, the results may change depending on the storage condition after sampling and the time before measurement.
| Condition | Initial DO | DO After 5 Days | Calculated BOD | Effect |
|---|---|---|---|---|
| Measurement started immediately after sampling | 8.7 mg/L | 5.6 mg/L | 31 mg/L | Reference |
| Left at room temperature for 6 h after sampling | 7.9 mg/L | 5.3 mg/L | 26 mg/L | Oxygen consumption progresses before measurement |
| Strongly shaken during sampling | 9.2 mg/L | 5.8 mg/L | 34 mg/L | Initial DO becomes high because of air mixing |
| Stored with bubbles present | 8.8 mg/L | 6.3 mg/L | 25 mg/L | May appear low because oxygen is supplied |
If time passes after sampling, oxygen consumption may already have progressed before measurement begins.
In addition, if bubbles are present, oxygen from the air dissolves into the water and changes in DO may not be reflected correctly.
Example of How to Write the Results
The initial DO and DO after 5 days were measured for each water sample and BOD was determined.
For river water, the initial DO was 8.5 mg/L and the DO after 5 days was 6.9 mg/L, giving a BOD of 1.6 mg/L.
On the other hand, for pond water, the initial DO was 8.2 mg/L and the DO after 5 days was 4.8 mg/L, giving a BOD of 3.4 mg/L.
The pond water showed a greater decrease in DO and may have contained more organic matter.
Domestic wastewater was measured after tenfold dilution.
The initial DO was 8.7 mg/L and the DO after 5 days was 5.6 mg/L, giving a decrease in DO of 3.1 mg/L in the diluted solution.
Multiplying this by the dilution factor of 10 gave a BOD of 31 mg/L for the domestic wastewater.
The BOD of the domestic wastewater was higher than that of the river water, indicating that it contained more organic matter decomposed by microorganisms.
In food-factory wastewater, the decrease in DO was 4.7 mg/L even after 50-fold dilution, and the BOD was 235 mg/L.
This was considered to be because the wastewater contained large amounts of food-derived organic matter such as sugars and proteins and a large amount of oxygen was consumed through microbial decomposition.
Points for Connecting the Results to the Discussion
In a discussion of BOD measurement, it is important not only to calculate the decrease in DO but also to explain microbial decomposition, dilution factor, pollution, amount of organic matter, and errors caused by measurement conditions in relation to one another.
- Has the amount of oxygen consumed been determined from the difference between initial DO and DO after 5 days?
- If the sample was diluted, has the dilution factor been applied to convert the value to the BOD of the original sample?
- Can it be explained that water with high BOD contains large amounts of organic matter decomposed by microorganisms?
- Can the possibility that the dilution factor was inappropriate be discussed when DO was completely consumed?
- Can it be explained that excessive dilution makes the decrease in DO small and increases the effect of measurement error?
- Can the meaning of correcting for oxygen consumption in the blank be explained?
- Can changes in BOD caused by microbial activity, temperature, seeding, and sterilization be discussed?
- Can it be explained that oxygen consumption caused by nitrification may cause BOD to be overestimated?
- Can bubbles during sampling, shaking, storage time, and temperature changes be explained as sources of error?
Example Discussion
In this experiment, water samples were stored at 20°C for 5 days and BOD was determined from the difference between initial DO and DO after 5 days.
The BOD was 0.4 mg/L for tap water, 1.6 mg/L for river water, and 3.4 mg/L for pond water.
Because BOD represents the amount of oxygen consumed when microorganisms decompose organic matter, the pond water, which showed a higher value, was considered to contain more readily biodegradable organic matter than the tap water and river water.
Domestic wastewater was measured after tenfold dilution, and the decrease in DO in the diluted solution was 3.1 mg/L, giving a BOD of 31 mg/L in the original sample.
If the dilution factor were not considered, the BOD would be 3.1 mg/L and would be greatly underestimated.
Therefore, for high-concentration samples, it is important to correctly include the dilution factor in the calculation.
The BOD of food-factory wastewater was high at 235 mg/L.
Food-factory wastewater may contain large amounts of organic matter such as sugars, proteins, and fats, and large amounts of oxygen were considered to have been consumed during microbial decomposition of these substances.
If such water enters a river, the DO in the water may decrease and adversely affect aquatic organisms.
In the comparison of dilution factors, under conditions of insufficient dilution, the DO after 5 days became nearly zero and accurate BOD was difficult to determine.
On the other hand, under conditions of excessive dilution, the decrease in DO was small and the effect of reading errors from the burette or DO meter became larger.
Therefore, in BOD measurement, the dilution factor must be selected so that sufficient DO remains after 5 days while the decrease in DO is still within a measurable range.
Possible sources of error include introduction of bubbles during sampling, leaving the sample standing after sampling, deviations in temperature control, differences in microbial activity, and oxygen consumption in the blank.
If bubbles enter, oxygen from the air dissolves into the water and the decrease in DO may appear smaller.
In addition, if time passes before measurement begins after sampling, microbial decomposition proceeds during that period and the initial DO may already have decreased.
Therefore, in BOD measurement, it is important to process the sample promptly after sampling and store it under standardized conditions at 20°C in the dark.
Furthermore, in samples containing ammoniacal nitrogen, oxygen is also consumed through nitrification.
Because BOD was higher under conditions without nitrification inhibition, oxidation of nitrogen components may have contributed to oxygen consumption in addition to organic-matter decomposition.
When interpreting BOD values, oxygen consumption caused by carbonaceous organic matter and oxygen consumption caused by nitrification must be distinguished.
Summary
BOD is a water-quality indicator representing the amount of oxygen consumed when organic matter in water is decomposed by microorganisms.
The amount of oxygen consumed is determined from the difference between initial DO and DO after 5 days, and for diluted samples the result is multiplied by the dilution factor to convert it to the BOD of the original sample.
In this reference example, BOD was low in tap water and river water and high in domestic wastewater and food-factory wastewater.
In a report, it is useful to discuss the decrease in DO, dilution factor, microbial decomposition, organic pollution, blank correction, nitrification, and errors caused by sampling and storage operations in relation to one another.
Relationship Between Microbial Decomposition and Dissolved Oxygen
Microorganisms in water use organic matter as a nutrient source and consume oxygen during the decomposition process.
The oxygen consumed at this time is dissolved oxygen in the water.
Therefore, the more readily biodegradable organic matter the sample water contains, the greater the decrease in dissolved oxygen during incubation and the higher the BOD value becomes.
BOD is an indicator reflecting how readily organic matter in water can be decomposed by organisms.
Even when the total amount of organic matter is the same, BOD may be high if a large proportion of the substances are readily decomposed by microorganisms, while BOD may be low if a large proportion is difficult to decompose.
Example Discussion:
Because the amount of dissolved oxygen decreased greatly after incubation, organic matter in the sample water was considered to have been decomposed by microorganisms, consuming oxygen in the process.
The greater the decrease in dissolved oxygen, the greater the amount of organic matter that microorganisms can use.
Therefore, the BOD value reflects the degree of pollution caused by readily biodegradable organic matter rather than the total amount of organic matter.
What Is Dissolved Oxygen?
Dissolved oxygen is oxygen dissolved in water.
Dissolved oxygen is important for fish, aquatic organisms, and aerobic microorganisms.
When organic matter in water is decomposed by microorganisms, dissolved oxygen is consumed.
When the amount of dissolved oxygen decreases, aquatic organisms become less able to survive.
In water bodies with severe organic pollution, oxygen may be consumed through microbial decomposition and hypoxic conditions may occur.
BOD measurement is used to evaluate the possibility of such oxygen consumption.
Example Discussion:
Dissolved oxygen is oxygen dissolved in water and is necessary for the activity of aquatic organisms and aerobic microorganisms.
When the sample water contains a large amount of organic matter, dissolved oxygen is consumed through microbial decomposition.
Therefore, water with high BOD may cause oxygen deficiency in natural water bodies.
Discussion When the BOD Value Is High
When the BOD value is high, the sample water is considered to contain large amounts of organic matter that microorganisms can readily decompose.
BOD tends to increase when domestic wastewater, food-processing wastewater, human waste, agricultural wastewater, decomposition products of plants and animals, and similar substances are mixed into the water.
In water with high BOD, dissolved oxygen is greatly consumed through microbial decomposition.
If dissolved oxygen becomes insufficient, fish, aquatic insects, and other organisms may be adversely affected.
In environments with even less oxygen, anaerobic decomposition may progress and substances causing unpleasant odors may be generated.
Therefore, water with high BOD is discussed as water with advanced organic pollution.
Example Discussion:
Because the BOD value was high, the sample water was considered to contain large amounts of organic matter readily decomposed by microorganisms.
In such water, dissolved oxygen may be consumed during organic-matter decomposition, creating an unfavorable environment for aquatic organisms.
The inflow of organic matter originating from domestic wastewater or food sources can be considered a possible cause of the increased BOD.
Discussion When the BOD Value Is Low
When the BOD value is low, the amount of organic matter readily decomposed by microorganisms is considered small.
Clean river water, tap water, and treated wastewater tend to have low BOD.
In water with low BOD, dissolved oxygen consumption through microbial decomposition is considered small.
However, low BOD does not necessarily mean that there are no water-quality problems.
Refractory organic matter, inorganic ions, heavy metals, and substances that inhibit microorganisms may not be sufficiently reflected in BOD.
It is necessary to evaluate the water together with COD, electrical conductivity, pH, and other indicators.
Example Discussion:
Because the BOD value was low, the sample water was considered to contain little organic matter readily decomposed by microorganisms.
Therefore, the amount of dissolved oxygen consumed during incubation was also considered small.
However, BOD is an indicator related to biodegradable organic matter, so the effects of refractory substances and inorganic ions must be judged together with the results of other analyses.
Difference Between BOD and COD
BOD and COD are both indicators used to evaluate water pollution, but their measurement principles differ.
BOD is the amount of oxygen consumed when microorganisms decompose organic matter, while COD is a value expressing the amount of substances chemically oxidized by an oxidizing agent in terms of oxygen.
In other words, BOD reflects biodegradability, while COD reflects chemical oxidizability.
If both BOD and COD are high, there may be large amounts of organic matter that microorganisms can readily decompose.
If COD is high but BOD is low, there may be large amounts of refractory organic matter or reducing inorganic substances.
Comparing BOD and COD makes it easier to consider the properties of organic matter in the water.
| Result | Possible Meaning | Point for Discussion |
|---|---|---|
| High BOD / High COD | Large amount of readily biodegradable organic matter | Consider the effects of domestic wastewater and food-processing wastewater |
| Low BOD / High COD | Possibility of refractory substances or reducing substances | Consider substances that are difficult for microorganisms to decompose |
| Low BOD / Low COD | Possibility of little organic pollution | Possibility of clean or treated water |
| High BOD / Low COD | Measurement conditions must be checked | Review BOD dilution and DO measurement or insufficient oxidation in COD measurement |
Example Discussion:
If both BOD and COD were high, the sample water may have contained large amounts of organic matter readily decomposed by microorganisms.
On the other hand, if COD was high despite low BOD, the presence of refractory organic matter or substances inhibiting microbial activity can be considered.
Therefore, comparing BOD with COD makes it possible to evaluate the properties of organic matter in water in greater detail.
Discussion of Dilution Factor
In BOD measurement, sample water may be diluted so that dissolved oxygen is not completely consumed during incubation.
If a sample containing large amounts of organic matter is incubated without dilution, oxygen may become excessively depleted and accurate BOD may become difficult to determine.
Therefore, selecting an appropriate dilution factor is important.
If the sample is diluted too much, the decrease in dissolved oxygen becomes small and the effect of measurement error becomes large.
If dilution is insufficient, dissolved oxygen is almost completely consumed and the measurement may exceed the usable range.
In BOD measurement, dilution conditions are adjusted so that the decrease in oxygen falls within an appropriate range.
Example Discussion:
In BOD measurement, an appropriate dilution factor must be selected according to the amount of organic matter in the sample.
If dilution is insufficient, dissolved oxygen may be almost completely consumed during incubation, making it difficult to determine an accurate BOD value.
On the other hand, if the sample is diluted too much, the decrease in dissolved oxygen becomes small and the effect of measurement error increases, so appropriate dilution conditions are important.
Effect of Incubation Temperature
Because BOD measurement is based on microbial activity, it is strongly affected by incubation temperature.
At higher temperatures, microbial activity may become more vigorous and decomposition of organic matter may proceed more rapidly.
At lower temperatures, microbial activity decreases and oxygen consumption may become smaller.
Therefore, it is important to incubate samples for a fixed period at the specified temperature in BOD measurement.
If the incubation temperature deviates, the BOD value may change even for the same sample.
When comparing multiple samples, the incubation conditions must be standardized.
Example Discussion:
Because the BOD value depends on microbial decomposition activity, it is affected by incubation temperature.
If the incubation temperature is high, microbial activity may increase and oxygen consumption may become larger.
In contrast, if the temperature is low, microbial decomposition may proceed less readily and the BOD value may be measured as lower.
Effect of Incubation Time
In BOD measurement, the decrease in dissolved oxygen is measured after incubation for a fixed period.
The longer the incubation time, the more microbial decomposition may progress and the greater the oxygen consumption may become.
On the other hand, if the incubation time is short, organic-matter decomposition may not proceed sufficiently and the BOD value may be underestimated.
In standard measurements, values are compared using a specified incubation time.
Data obtained with different incubation times cannot simply be compared.
When comparing BOD, it is necessary to confirm that the incubation time, temperature, and dilution conditions are the same.
Example Discussion:
BOD values change depending on incubation time.
If the incubation time is too short, organic-matter decomposition by microorganisms may not proceed sufficiently and the decrease in dissolved oxygen may become small.
Therefore, in BOD measurement, it is important to follow the specified incubation time and not simply compare values obtained under different conditions.
Effect of Microbial Activity
In BOD measurement, the ability of microorganisms to decompose organic matter greatly affects the result.
If the number of microorganisms is small or if they are not sufficiently active, oxygen consumption becomes small even when organic matter is present.
As a result, the BOD value may be lower than the actual value.
If the sample contains few microorganisms, microorganisms may be added through an operation called seeding.
On the other hand, if toxic substances or strong acids or alkalis are present in the sample, microorganisms may be inhibited and BOD may be measured as low.
Example Discussion:
One possible reason the BOD value was lower than expected is that microbial activity was insufficient.
If there are few microorganisms or if the sample contains substances that inhibit microbial activity, decomposition may not proceed readily even when organic matter is present.
Therefore, when interpreting BOD results, microbial activity and the presence or absence of seeding must be considered.
Effect of Nitrification
In BOD measurement, oxygen may be consumed not only through organic-matter decomposition but also through oxidation of ammoniacal nitrogen, that is, nitrification.
If nitrification proceeds, oxygen consumption not originating from organic-matter decomposition may be included in the BOD value.
Therefore, depending on the measurement conditions and sample type, the BOD value may be estimated as higher.
The effect of nitrification must be considered particularly for domestic wastewater, sewage, and water containing large amounts of nitrogen compounds.
To distinguish oxygen consumption caused by organic matter from that caused by oxidation of nitrogen compounds, the method and treatment of inhibitors specified in the laboratory manual must be checked.
Example Discussion:
One possible reason the BOD value became high is oxygen consumption caused by nitrification in addition to organic-matter decomposition.
In samples containing ammoniacal nitrogen, nitrification proceeds through microbial activity and dissolved oxygen is also consumed in this process.
Therefore, in samples such as domestic wastewater and sewage, oxygen consumption caused by organic matter and oxygen consumption caused by nitrogen compounds must be distinguished in the discussion.
Discussion When Toxic Substances Are Present
Because BOD is a measurement based on microbial activity, correct values may not be obtained when substances harmful to microorganisms are present.
Heavy metals, organic solvents, strong acids, strong alkalis, disinfectants, detergent components, and similar substances may reduce microbial activity.
If toxic substances are present, microorganisms may be unable to decompose organic matter even when large amounts are present, and BOD may be measured as low.
In such cases, COD may be high while BOD is low.
The difference between COD and BOD provides a clue for considering the possibility of refractory substances or toxicity.
Example Discussion:
If COD was high despite low BOD, the sample may have contained substances that inhibited microbial activity.
Heavy metals, disinfectants, strong acids, and strong alkalis may interfere with microbial decomposition and reduce oxygen consumption even when organic matter is present.
Therefore, a low BOD result should not simply be interpreted as indicating little organic matter, and the possibility of microbial inhibition must also be considered.
Effects of Sampling and Storage
Water samples continue to change after sampling because of microbial activity and chemical reactions.
If organic-matter decomposition progresses during storage, dissolved oxygen may already have been consumed before measurement or the amount of organic matter may change.
As a result, the measured BOD value may no longer accurately reflect the condition at the time of sampling.
In BOD measurement, it is desirable to perform the measurement as soon as possible after sampling.
If storage is necessary, temperature and storage time are controlled according to the conditions specified in the laboratory manual.
Contamination of the sampling container also affects the BOD value, so a clean container must be used.
Example Discussion:
One possible source of error in the BOD value is a change in water quality during storage after sampling.
If microbial decomposition progresses during storage, the amount of dissolved oxygen and organic matter may differ from those at the time of sampling, causing the BOD value to change.
Therefore, in BOD measurement, it is important to perform the measurement as soon as possible after sampling and keep storage conditions constant.
Relationship Between Water Temperature and Dissolved Oxygen
Water temperature affects both the amount of dissolved oxygen and microbial activity.
In general, the higher the water temperature, the smaller the amount of oxygen that can dissolve in the water.
At the same time, microbial activity tends to become more active at higher temperatures, making organic-matter decomposition more likely to progress.
Therefore, in high-temperature water, dissolved oxygen tends to be lower, and when large amounts of organic matter are present, oxygen deficiency is even more likely to occur.
In BOD measurement, it is important to keep the incubation temperature constant so that the measurement conditions are standardized.
Example Discussion:
Water temperature is an important factor affecting BOD measurement results.
The higher the water temperature, the less readily oxygen dissolves in water, and microbial activity may also increase, causing greater oxygen consumption.
Therefore, when comparing BOD values, the water temperature at the time of sampling and the incubation temperature must be checked.
Discussion When Oxygen Is Excessively Depleted
In BOD measurement, it is important that some dissolved oxygen remains after incubation.
If dissolved oxygen is almost completely depleted during incubation, decomposition may stop because of oxygen deficiency even though further organic matter could still be decomposed.
In this case, the actual BOD cannot be measured correctly.
If oxygen becomes excessively depleted, the sample must be diluted more and remeasured.
For water with high concentrations of organic pollution, it is important to prepare multiple dilution factors and select conditions that produce an appropriate decrease in oxygen.
Example Discussion:
If almost no dissolved oxygen remained after incubation, the sample may not have been diluted sufficiently for the amount of organic matter it contained.
If oxygen is completely consumed, any further oxygen consumption cannot be measured, so an accurate BOD value cannot be obtained.
In such a case, the sample must be further diluted and remeasured under conditions in which dissolved oxygen remains after incubation.
Discussion When the Decrease in Dissolved Oxygen Is Small
If the difference in dissolved oxygen before and after incubation is small, the sample water may contain little organic matter readily decomposed by microorganisms.
However, the decrease in dissolved oxygen may also be small if the sample is diluted too much, microbial activity is low, toxic substances are present, or the incubation time is too short.
If the decrease in dissolved oxygen is too small, the effect of measurement error becomes large.
Therefore, for samples with low BOD, attention must be paid to the dilution factor and measurement accuracy.
Even when the value is small, it is important to separately consider the meaning of the result and the possibility of measurement error.
Example Discussion:
Because the difference in dissolved oxygen before and after incubation was small, the sample water was considered to contain little organic matter readily decomposed by microorganisms.
However, oxygen consumption may also become small if the sample is diluted too much or if microbial activity is low.
Therefore, for a low BOD result, both the possibility of clean water quality and the effects of measurement conditions must be considered.
Sources of Error in BOD Measurement
Sources of error in BOD measurement include inappropriate dilution factors, fluctuations in incubation temperature, deviations in incubation time, errors in dissolved oxygen measurement, introduction of bubbles, changes during sample storage, insufficient microbial activity, and the presence of toxic substances.
Because BOD uses microbial reactions, it may be more dependent on measurement conditions than chemical measurements.
In addition, if bubbles enter the BOD bottle, oxygen from the air is added and may affect the measured amount of dissolved oxygen.
Insufficient calibration of the dissolved-oxygen meter and endpoint-determination errors in titration methods also affect the BOD value.
It is important to keep the measurement procedures consistent.
Example Discussion:
Possible sources of error in the BOD value include inappropriate dilution factors, fluctuations in incubation temperature, and errors in dissolved oxygen measurement.
In addition, if bubbles remain in the BOD bottle, oxygen may be supplied to the sample water, making it impossible to accurately evaluate oxygen consumption.
Because BOD depends on microbial activity, the presence of toxic substances and differences in storage conditions may also affect the measured value.
Sources of Error in Dissolved Oxygen Measurement
In BOD measurement, the amounts of dissolved oxygen before and after incubation must be measured accurately.
When a dissolved-oxygen meter is used, insufficient calibration, dirty electrodes, insufficient temperature correction, and insufficient stirring are possible sources of error.
When a titration method is used, reagent concentration, titration endpoint, introduction of bubbles, and the redox condition of the sample may have an effect.
Because BOD is calculated from the difference in dissolved oxygen, an error in either the initial value or the post-incubation value affects the entire BOD result.
Particularly in samples with low BOD, even a slight measurement error in dissolved oxygen becomes relatively large.
Example Discussion:
Because the BOD value is determined from the difference in dissolved oxygen before and after incubation, errors in dissolved-oxygen measurement directly affect the result.
If the dissolved-oxygen meter is insufficiently calibrated, the electrode is dirty, or temperature correction is inadequate, the decrease in oxygen cannot be evaluated correctly.
Particularly in low-BOD samples, even a slight DO measurement error may greatly affect the BOD value.
When the BOD Measurement Can Be Considered Good
BOD measurement can be considered to have produced good results when there is an appropriate decrease in dissolved oxygen before and after incubation, dissolved oxygen still remains after incubation, and variation among repeated measurements is small.
In addition, if the trend does not contradict the sampling location, appearance of the sample, COD value, and other information, the results are easier to consider valid.
For example, if BOD is high in water considered to be affected by domestic wastewater and low in clean river water, the measurement results may reflect the characteristics of the water quality.
However, because BOD is readily affected by measurement conditions, it is important to compare values under standardized conditions.
Example Discussion:
In this experiment, an appropriate decrease in dissolved oxygen was observed after incubation, and dissolved oxygen still remained after incubation.
In addition, the BOD values showed a trend that did not contradict the surrounding environment of the sampling locations or the COD values.
From these results, the BOD measurement results in this experiment were considered to approximately reflect the amount of biodegradable organic matter in the sample water.
Example Discussion When the Experiment Did Not Go Well
When BOD measurement does not go well, possible causes are considered from results such as almost no dissolved oxygen remaining after incubation, a decrease in dissolved oxygen that is too small, variation in measured values, difficulty explaining the relationship with COD, or unnatural trends among samples.
Organizing the causes according to dilution, incubation, microbial activity, DO measurement, and storage conditions makes the discussion easier.
Example Discussion:
In this experiment, almost no dissolved oxygen remained after incubation.
This was considered to be because the dilution was insufficient for the amount of organic matter in the sample and almost all of the oxygen was consumed during incubation.
In this case, additional oxygen may actually have been consumed, but this further oxygen consumption cannot be evaluated by the measurement.
To obtain a more accurate BOD value, the sample must be remeasured at a higher dilution factor.
How to Write Points for Improvement
In a discussion of BOD measurement, including not only sources of error but also points for improvement makes the report easier to organize.
Points for improvement can be divided into sampling and storage, dilution, incubation, dissolved oxygen measurement, and analysis.
Improvements to Sampling and Storage
- Use a clean sampling container
- Measure as soon as possible after sampling
- Keep storage time and storage temperature constant
- Mix the sample thoroughly before aliquoting
- Record the sampling location and sampling time
- Record the weather and water temperature at the time of sampling
Improvements to Dilution and Incubation
- Prepare multiple dilution factors
- Select conditions in which dissolved oxygen remains after incubation
- Keep the incubation temperature constant
- Accurately follow the incubation time
- Perform seeding when necessary
- Fill the BOD bottle so that no bubbles enter
Improvements to Measurement and Analysis
- Calibrate the dissolved-oxygen meter
- Check temperature correction during DO measurement
- Carefully determine the endpoint when using a titration method
- Perform blank correction
- Correctly reflect the dilution factor in the calculation
- Perform multiple measurements and calculate the average value
- Compare with COD, pH, and electrical conductivity
Example of How to Write Points for Improvement:
To improve the accuracy of BOD measurement, an appropriate dilution factor must be selected according to the amount of organic matter in the sample, and the measurement must be performed under conditions in which dissolved oxygen remains after incubation.
In addition, it is important to keep the incubation temperature and incubation time constant and operate the BOD bottle so that no bubbles enter.
In dissolved oxygen measurement, the measuring instrument must be calibrated and temperature correction performed, and the results should be discussed together with other water-quality indicators such as COD.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of BOD measurement, simply writing that “oxygen decreased” or “BOD was high” results in a superficial discussion.
A good discussion relates microbial decomposition, amount of organic matter, dissolved oxygen, effects on water quality, and measurement conditions.
| Superficial Discussion | Good Discussion |
|---|---|
| BOD was high. | Because the BOD was high, the sample water was considered to contain large amounts of organic matter readily decomposed by microorganisms, and dissolved oxygen was greatly consumed during decomposition. |
| Oxygen decreased. | During incubation, microorganisms decomposed organic matter and consumed dissolved oxygen through respiration, so the amount of dissolved oxygen decreased after incubation. |
| BOD was low. | Because the BOD was low, the amount of readily biodegradable organic matter was considered small. However, insufficient microbial activity or inhibition of decomposition by toxic substances must also be considered. |
| The measured value was strange. | Possible causes of the unnatural measured value include an inappropriate dilution factor, fluctuations in incubation temperature, bubbles in the BOD bottle, errors in dissolved oxygen measurement, and changes during sample storage. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of BOD measurement.
Adjust the necessary parts according to your own experimental results.
- BOD indicates the amount of oxygen consumed when organic matter in water is decomposed by microorganisms.
- Because the amount of dissolved oxygen decreased after incubation, oxygen was considered to have been consumed through microbial decomposition.
- The higher the BOD value, the greater the amount of readily biodegradable organic matter that may be present.
- Water with high BOD may cause dissolved-oxygen deficiency in natural water bodies.
- BOD values are affected by microbial activity, incubation temperature, incubation time, and dilution factor.
- If COD is high and BOD is low, the presence of refractory substances or substances that inhibit microorganisms can be considered.
- If almost no dissolved oxygen remains after incubation, the dilution factor may have been insufficient.
- If the decrease in dissolved oxygen is too small, the effect of measurement error may become large.
- Bubbles in the BOD bottle must be avoided because they affect the measured amount of dissolved oxygen.
- BOD should not be evaluated alone but together with COD, pH, electrical conductivity, and other indicators.
Points to Check When Discussing BOD Measurement
Checking the following points before writing the report makes the discussion easier to write.
- Is it explained what BOD represents?
- Are the amounts of dissolved oxygen before and after incubation compared?
- Is the relationship between microbial decomposition and oxygen consumption described?
- Are the reasons for high or low BOD values explained?
- Is the appropriateness of the dilution factor considered?
- Are the effects of incubation temperature and incubation time considered?
- Are the effects of microbial activity and toxic substances considered?
- Is the possibility of oxygen consumption caused by nitrification considered?
- Are bubbles in the BOD bottle and DO measurement errors considered?
- Are changes in water quality caused by sampling and storage considered?
- Are the results compared with other indicators such as COD?
- Do the points for improvement correspond to the sources of error?
Summary
BOD measurement is a method used to measure the amount of oxygen consumed when organic matter in water is decomposed by microorganisms and to evaluate the degree of pollution caused by biodegradable organic matter.
The greater the difference in dissolved oxygen before and after incubation, the greater the amount of organic matter readily decomposed by microorganisms is considered to be.
Therefore, BOD is an important indicator for examining the effects of domestic wastewater and organic pollution.
In water with high BOD, microbial decomposition in natural water bodies may consume dissolved oxygen and adversely affect aquatic organisms.
On the other hand, even when BOD is low, the effects of refractory substances, inorganic ions, and toxic substances cannot be determined from BOD alone.
It is important to consider the results together with COD, pH, electrical conductivity, and other parameters.
In a report, rather than simply writing that “BOD was high or low,” organize and discuss microbial decomposition, the decrease in dissolved oxygen, dilution factor, incubation temperature, incubation time, microbial activity, and measurement errors.
BOD measurement is effective for water-quality evaluation, but because it is based on microbial reactions, the effects of measurement conditions must be fully considered.
