Discussion Examples for Dissolved Oxygen Measurement | Relationship Between DO, Water Temperature, and Pollution Level

What Is Dissolved Oxygen?

Dissolved oxygen is oxygen dissolved in water.
In English, it is called Dissolved Oxygen and is abbreviated as DO.
Oxygen in water is used for respiration by fish, aquatic insects, aerobic microorganisms, and other organisms.
Therefore, the DO value is an important indicator for evaluating the biological environment of a water body.

In water with a sufficiently high DO value, aerobic organisms can grow readily and the water quality may also be relatively good.
On the other hand, in water with a low DO value, oxygen deficiency may make it difficult for aquatic organisms to survive, and anaerobic decomposition may proceed.
When anaerobic decomposition progresses, substances that cause unpleasant odors may also be generated.

Main Items to Include in the Results

In the results of dissolved oxygen measurement, organize the sampling location, sampling date and time, weather, water temperature, DO value, measurement method, pH, electrical conductivity, COD, BOD, and other related items.
Because the DO value readily changes depending on the water temperature at the time of sampling, contact with the atmosphere, and the state of water flow, recording the sampling conditions is important.

Main Items to Include in the Results

• Sampling location
• Sampling date and time
• Weather at the time of sampling
• Water temperature
• Air temperature
• Presence or absence of water flow
• Color and turbidity of the water
• Presence or absence of odor
• DO value
• Measurement method
• DO-meter calibration conditions
• Presence or absence of temperature correction
• Presence or absence of salinity correction
• pH
• Electrical conductivity
• Comparison with COD and BOD
• Sources of error and points for improvement

Reference Experimental Values and Calculation Examples for Dissolved Oxygen (DO) Measurement

Here, reference experimental values are organized for measuring dissolved oxygen (DO), the amount of oxygen dissolved in water, and discussing water temperature, pollution, oxygen saturation, and errors caused by water-sampling operations.

DO is an important indicator used by organisms in water for respiration.
In general, DO is high in clean or well-aerated water, while DO is low in water containing large amounts of organic matter in which decomposition is progressing.
In addition, because oxygen becomes less soluble in water as water temperature increases, the DO value changes with water temperature even when the water quality is the same.

Reference Experimental Conditions

Relationship Between Water Temperature and Saturated DO

The amount of oxygen that can dissolve in water changes with water temperature.
The lower the water temperature, the more readily oxygen dissolves, while the higher the water temperature, the less readily oxygen dissolves.
Here, reference values for saturated DO in water near 1 atm are shown.

If only DO values are compared, it becomes difficult to determine whether differences are caused by water quality or water temperature.
Therefore, in DO measurement, water temperature should always be recorded and discussed together with saturated DO and oxygen saturation.

DO Measurement Results by Water Sample

Reference examples are shown for measurements of each water sample using a DO meter.
Approximate saturated-DO values corresponding to the water temperature are used.

Example Calculation of Oxygen Saturation

Because DO is affected by water temperature, comparing measured DO with saturated DO and calculating oxygen saturation makes it easier to evaluate the condition of the water.

Oxygen saturation (%) = Measured DO ÷ Saturated DO × 100

For river water, the measured DO is 8.2 mg/L and the saturated DO at a water temperature of 18°C is 9.5 mg/L.

Oxygen saturation = 8.2 ÷ 9.5 × 100 = 86.3%

Therefore, this river water is considered to contain a relatively large amount of oxygen, although it is slightly below saturation.

Changes in DO With Increasing Water Temperature

When the same water is heated, saturated DO decreases as water temperature increases, and measured DO may also decrease.
Here, a reference example is shown for heating tap water.

Measured DO decreases as water temperature increases, but oxygen saturation does not change greatly.
This is considered to be because the solubility of oxygen decreased as water temperature increased rather than because the water quality deteriorated.

Example of DO Decrease Due to Pollution

In water containing large amounts of organic matter, microorganisms consume oxygen when decomposing the organic matter, causing DO to decrease.
Here, a reference example is shown in which organic matter was added to river water and the decrease in DO over time was measured.

Because DO decreases as time passes, oxygen is considered to have been consumed during decomposition of organic matter.
Such a decrease in DO is important when discussing water pollution and effects on organisms.

Example Calculation of Oxygen Consumption

The amount of oxygen consumed in the water can be determined from the difference between the initial DO and the DO after a fixed period.

Oxygen consumption = Initial DO − DO after a fixed period

In river water to which organic matter was added, the initial DO was 8.2 mg/L and the DO after 24 hours was 3.6 mg/L.

Oxygen consumption = 8.2 − 3.6 = 4.6 mg/L

Because 4.6 mg/L of oxygen was consumed in 24 hours, organic-matter decomposition was considered to have progressed and the DO decreased greatly.

Example of DO Recovery by Aeration

Even in water with low DO, aeration by supplying air causes oxygen to dissolve in the water and DO to recover.
A reference example is shown for aerating polluted water.

As the aeration time increases, DO rises and approaches saturated DO.
From this result, aeration is considered effective as a method for improving low-DO conditions.

Changes in DO Between Day and Night

In ponds and water bodies with abundant algae, DO changes with time of day because of photosynthesis and respiration.
During the daytime, oxygen is generated by photosynthesis, while at night oxygen is consumed by respiration, so DO tends to decrease.

DO tends to become high during the daytime and low in the early morning and at night.
In water bodies with abundant algae, there may be large differences between times of high DO and low DO, so the measurement time is also important.

Comparison Between a DO Meter and the Winkler Method

In addition to direct measurement using a DO meter, DO can also be measured by chemical methods such as the Winkler method.
Here, a reference example is shown in which the same samples were measured by the two methods.

The two methods give similar values for clean water, but differences are more likely to occur in water containing turbidity and reducing substances.
The characteristics of each measurement method must be understood when comparing the results.

Comparison of Errors Caused by Water-Sampling Operations

In DO measurement, the value may change if air enters during sampling or if time passes before measurement.

Because DO readily changes even after sampling, it is important either to measure immediately in the field or, in the Winkler method, to perform the fixation procedure immediately after sampling.

Example of How to Write the Results

The DO and water temperature of each water sample were measured.
In tap water, the water temperature was 20°C and the DO was 8.6 mg/L, giving an oxygen saturation of 94.5% relative to the saturated DO of 9.1 mg/L.
In river water, the water temperature was 18°C, the DO was 8.2 mg/L, and the oxygen saturation was 86.3%, indicating water containing a relatively large amount of oxygen.

In pond water, the DO was 5.4 mg/L and the oxygen saturation was 63.5%, which were lower than those of tap water and river water.
In polluted water, the DO was 2.1 mg/L and the oxygen saturation was 25.3%, indicating a state of oxygen deficiency.
In water containing large amounts of organic matter, microorganisms consume oxygen when decomposing the organic matter, so the DO may have decreased.

In polluted water subjected to aeration, the DO was 2.1 mg/L before aeration and increased to 8.1 mg/L after 30 minutes of aeration.
From this, oxygen from the air was considered to have dissolved into the water through aeration, improving the low-DO condition.

Points for Connecting the Results to the Discussion

In a discussion of DO measurement, it is important to explain not only whether the measured DO is high or low but also to relate water temperature, saturated DO, oxygen saturation, pollution, and water-sampling operations.

• Can it be explained that the unit of DO is mg/L and that it represents the amount of oxygen dissolved in water?
• Can it be explained that oxygen becomes less soluble as water temperature increases and that saturated DO decreases?
• Has oxygen saturation been calculated from measured DO and saturated DO?
• Can the reason DO decreases in water containing large amounts of organic matter be explained in relation to microbial decomposition and oxygen consumption?
• Can the reason DO increases through aeration be explained in relation to dissolution of oxygen from the air?
• Can the reason DO changes between day and night be explained in relation to photosynthesis and respiration?
• Can bubbles during sampling, shaking, standing, and temperature changes be discussed as sources of error?
• Can insufficient DO-meter calibration, membrane contamination, and insufficient response time be explained as measurement errors?

Example Discussion

In this experiment, the amount of dissolved oxygen in water samples was measured and compared with water temperature and water quality.
In tap water, the DO was 8.6 mg/L and the oxygen saturation was 94.5%, indicating that oxygen was sufficiently dissolved.
On the other hand, in polluted water, the DO was 2.1 mg/L and the oxygen saturation was 25.3%, indicating a state of oxygen deficiency.

One possible reason for the low DO in the polluted water is oxygen consumption associated with decomposition of organic matter.
When a large amount of organic matter is contained in water, microorganisms consume oxygen while decomposing it, causing DO to decrease.
In fact, in river water to which organic matter was added, DO decreased from 8.2 mg/L to 1.8 mg/L over time.
This result indicates that as organic pollution progresses, oxygen in the water is consumed and the environment becomes unfavorable for organisms.

Looking at the effect of water temperature, measured DO decreased as water temperature increased.
This is because the solubility of oxygen in water decreases as water temperature rises.
Therefore, when comparing DO, not only the measured value but also water temperature and saturated DO must be considered.
Using oxygen saturation makes it possible to evaluate the oxygen condition of water while correcting to some extent for differences in water temperature.

In the aeration experiment, the DO of the polluted water increased from 2.1 mg/L to 8.1 mg/L.
This was considered to be because oxygen in the air dissolved in the water through aeration and the DO recovered.
However, if a large amount of organic matter remains, microorganisms continue to consume oxygen even after aeration, so DO may decrease again after temporarily increasing.

Possible measurement errors include the introduction of bubbles during sampling, leaving the sample standing, temperature changes, and insufficient DO-meter calibration.
If bubbles enter or the sample is shaken strongly, oxygen from the air dissolves and DO may be overestimated.
On the other hand, if time passes after sampling, oxygen may be consumed by microbial respiration and DO may be underestimated.
Therefore, in DO measurement, it is important to measure immediately after sampling or, in the Winkler method, to perform fixation immediately.

Summary

Dissolved oxygen (DO) is an important water-quality indicator representing the amount of oxygen dissolved in water.
Oxygen becomes less soluble as water temperature increases, and in water containing large amounts of organic matter, oxygen is consumed by microbial decomposition and DO decreases.

In this reference example, DO was relatively high in tap water and river water and low in pond water and polluted water.
In a report, it is useful to relate measured DO, water temperature, saturated DO, oxygen saturation, organic-matter decomposition, aeration, day-night changes, and errors caused by water-sampling operations.

What Can Be Learned From the DO Value?

The DO value indicates how much oxygen is dissolved in water.
When the DO value is high, sufficient oxygen may be present in the water, creating an environment in which aerobic organisms can readily function.
In flowing rivers and water that is in frequent contact with air, the DO value tends to be relatively high.

When the DO value is low, oxygen may be insufficient.
Possible causes include high water temperature, large amounts of organic matter causing oxygen consumption through microbial decomposition, stagnant water with little contact with air, high respiration by algae and microorganisms, and decomposition progressing in bottom sediment.

Relationship Between Water Temperature and DO

Water temperature greatly affects the DO value.
In general, the lower the water temperature, the more readily oxygen dissolves in water, while the higher the water temperature, the less readily oxygen dissolves.
Therefore, even with the same water quality, the DO value tends to be low during high summer temperatures and high during low winter temperatures.

In addition, microbial activity may become more active as water temperature increases, increasing oxygen consumption associated with organic-matter decomposition.
In other words, under high-temperature conditions, both “oxygen becomes less soluble” and “oxygen may be consumed more readily,” making DO more likely to decrease.

Relationship Between Organic Pollution and DO

In water containing large amounts of organic matter, microorganisms consume oxygen when decomposing the organic matter.
Therefore, DO values tend to decrease in water with severe organic pollution.
When domestic wastewater, food wastewater, fallen leaves, decomposed algae, and similar substances enter the water, microbial decomposition progresses and dissolved oxygen is consumed.

Particularly in ponds, lakes, and drainage channels where water flow is weak, oxygen is not readily replenished from the air and DO may become low because of organic-matter decomposition.
When DO is low while BOD and COD are high, the effect of organic pollution can be considered strong.

Relationship Between BOD and DO

BOD indicates the amount of oxygen consumed when organic matter in water is decomposed by microorganisms.
Therefore, water with a high BOD is considered likely to consume oxygen even in a natural environment and to experience a decrease in DO.
BOD can also be considered an indicator of the potential for future oxygen consumption.

DO indicates the amount of oxygen currently present in the water, while BOD indicates the amount of oxygen that will be consumed by decomposition of organic matter.
In other words, DO represents the current oxygen condition, while BOD represents how much oxygen may be consumed.
Comparing both makes it possible to discuss the oxygen environment of a water body more specifically.

Relationship Between COD and DO

COD indicates the amount of oxygen required to chemically oxidize organic matter and reducing substances in water using an oxidizing agent.
Water with high COD may contain large amounts of easily oxidized substances.
If these substances are decomposed by microorganisms, DO may be consumed.

However, high COD does not necessarily mean that DO is low.
If the water flows well or is aerated, oxygen may be supplied and DO may remain high.
In addition, some substances reflected in COD are difficult for microorganisms to decompose.

Discussion When the DO Value Is High

When the DO value is high, sufficient oxygen is considered to be dissolved in the water.
DO may become high in flowing rivers, shallow water that easily contacts air, water well mixed by wind or waves, and water in which photosynthesis by algae or aquatic plants is active.

However, a high DO value alone does not prove that the water quality is completely good.
DO may be temporarily high because of daytime photosynthesis, or DO may remain high because of water flow even when COD or BOD is high.
It is important to evaluate DO together with other indicators.

Discussion When the DO Value Is Low

When the DO value is low, oxygen in the water may be insufficient.
Possible causes include oxygen consumption through organic-matter decomposition, increased water temperature, water stagnation, decomposition in bottom sediment, respiration by algae and microorganisms, and cessation of photosynthesis at night.
DO tends to be low in heavily polluted water bodies.

If low DO continues, aerobic organisms such as fish and aquatic insects have difficulty surviving.
In environments where oxygen is extremely scarce, decomposition by anaerobic microorganisms may progress and foul-smelling substances such as hydrogen sulfide may be generated.

Relationship Between Water Flow and DO

Water flow greatly affects the DO value.
In flowing water, there are more opportunities for contact with air, making it easier for oxygen to enter the water.
At waterfalls, riffles, and rapidly flowing rivers, air readily mixes into the water and the DO value may become high.

On the other hand, in stagnant ponds, deep lake water, drainage channels, and bottom water, oxygen supply tends to be low.
If organic-matter decomposition also progresses, the DO value tends to decrease.
Recording whether water flow is present at the sampling location is useful for discussing DO.

Relationship Between Photosynthesis and DO

Algae and aquatic plants perform photosynthesis during the daytime and produce oxygen.
Therefore, DO may become high near the water surface during the daytime.
Particularly in water bodies rich in nutrients where algae are proliferating, DO may become high during the daytime.

However, photosynthesis stops at night, while algae, microorganisms, and aquatic organisms continue to consume oxygen through respiration.
Therefore, DO may vary greatly between day and night even in the same water body.
The sampling time is important information in DO measurement.

Relationship Between Bottom Sediment and DO

Mud containing organic matter may accumulate at the bottom of ponds, lakes, and rivers.
Organic matter is decomposed in bottom sediment, and oxygen is consumed during this process.
Particularly where water flow is weak, the DO value in bottom water tends to become low.

When oxygen is consumed in bottom sediment, bottom water may become oxygen-poor or anaerobic.
Under anaerobic conditions, hydrogen sulfide, methane, and other substances may be generated.
In water bodies with large amounts of bottom sediment, DO may differ between surface water and bottom water.

Water Pollution and Oxygen Deficiency

As water pollution progresses, organic matter and nutrients increase, and the activity of microorganisms and algae becomes more active.
Oxygen is consumed when organic matter is decomposed, so DO tends to decrease in polluted water.
This becomes a major stress for aquatic organisms.

As oxygen deficiency progresses, fish may open their mouths near the water surface, benthic organisms may decrease, and foul odors may occur.
DO is an important indicator for quantitatively evaluating such conditions in water bodies.

Concept of DO Saturation

DO varies depending on water temperature, atmospheric pressure, and salinity.
Therefore, in addition to simple DO concentration, the condition may be evaluated using DO saturation, which is the proportion of oxygen actually contained in the water relative to the amount of oxygen the water can hold under those conditions.
The higher the DO saturation, the more fully oxygen is dissolved under those conditions.

Low-temperature water can dissolve more oxygen, so even the same DO value may have a different meaning in terms of saturation.
In addition, water with active photosynthesis may become supersaturated with oxygen.
DO saturation is useful when comparing oxygen conditions while considering the effect of water temperature.

Discussion of Measurement Using a DO Meter

A DO meter is an instrument that measures dissolved oxygen in water using electrodes or optical sensors.
It is convenient because measurements can be performed in a short time, but the value may change depending on calibration, temperature correction, electrode condition, adhesion of bubbles, and the state of stirring during measurement.

When measuring with a DO meter, calibration is performed before measurement and care is taken to prevent bubbles from adhering to the sensor.
It is also important to read the value only after it has stabilized.
Measuring water temperature at the same time makes discussion easier.

Discussion of the Winkler Method

The Winkler method may be used to measure dissolved oxygen.
In the Winkler method, dissolved oxygen in water is fixed through a chemical reaction and the amount of oxygen is then determined by titration.
Unlike a DO meter, titration operations and reagent handling affect the results.

In the Winkler method, if air enters during sampling, the DO value may become high.
In addition, addition of reagents, determination of the titration endpoint, concentration of sodium thiosulfate solution, and timing of starch-indicator addition are also sources of error.
Accurate operation is important.

Errors Caused by Water-Sampling Operations

Water-sampling operations are extremely important in dissolved oxygen measurement.
If water comes into strong contact with air during sampling or bubbles enter the container, oxygen from the air may dissolve and the DO value may be measured as higher.
Conversely, if time passes after sampling, oxygen may be consumed by microorganisms and chemical reactions, causing the DO value to decrease.

The sampling container should be filled as completely as possible without introducing bubbles, and measurement should be performed promptly after sampling.
Because DO may also differ depending on water depth, recording the sampling depth is important.

Sources of Error in DO Measurement

Sources of error in DO measurement include introduction of air during sampling, adhesion of bubbles, time elapsed before measurement, insufficient DO-meter calibration, insufficient temperature correction, sensor contamination, insufficient stirring, nonuniformity of the sample, and reading of the titration endpoint.
Because DO reflects the condition at that moment, operations from sampling through measurement greatly affect the result.

In addition, DO values vary even within the same water body depending on water depth, time of day, weather, and presence or absence of flow.
Rather than concluding that a single measurement represents the entire water body, it is important to clearly state the measurement conditions in the discussion.

When the DO Measurement Can Be Considered Good

DO measurement can be considered to have produced good results when the water temperature and sampling conditions at the time of measurement are clear and the values do not contradict the condition of the water body.
For example, if DO is high in flowing river water and low in stagnant water containing large amounts of organic matter, the results are easier to consider reasonable.

In addition, if similar values are obtained from multiple measurements at the same location, the reproducibility can be considered high.
If the results are also consistent with BOD, COD, pH, electrical conductivity, and other values, the water-quality discussion becomes more persuasive.

Example Discussion When the Experiment Did Not Go Well

When DO measurement does not go well, possible causes can be considered from results such as unnaturally high values, variation in the same sample, difficulty explaining the relationship with BOD and COD, and unstable measured values.
Organizing the causes according to sampling, measuring instruments, temperature correction, bubbles, storage time, and differences in measurement location makes the discussion easier.

How to Write Points for Improvement

In a discussion of dissolved oxygen 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, measurement operations, instrument management, and analysis.

Improvements to Sampling

• Do not introduce bubbles during sampling
• Fill the sampling container completely
• Measure as soon as possible after sampling
• Standardize the sampling depth
• Record the sampling location and time
• Record the weather, water temperature, and presence or absence of flow

Improvements to Measurement Operations

• Calibrate the DO meter
• Check temperature correction
• Perform salinity correction when necessary
• Clean the sensor
• Prevent bubbles from adhering to the sensor
• Read the value only after it stabilizes
• Perform multiple measurements and calculate the average value

Improvements to Analysis

• Consider water temperature and DO together
• Compare with BOD and COD
• Consider water flow and stagnation at the sampling location
• Consider the sampling time, such as daytime or nighttime
• Consider the effects of algae and aquatic plants
• Consider the effects of bottom sediment and organic-matter decomposition
• Do not determine the overall water quality from the DO value alone

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of dissolved oxygen measurement, simply writing that “DO was high” or that “there was little oxygen” results in a superficial discussion.
A good discussion relates water temperature, organic-matter decomposition, BOD, COD, water flow, and effects on organisms.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of dissolved oxygen measurement.
Adjust the necessary parts according to your own experimental results.

• Dissolved oxygen is oxygen dissolved in water and is an important indicator for the growth of aquatic organisms.
• When the DO value is high, sufficient oxygen is considered to be present in the water.
• When the DO value is low, oxygen deficiency caused by organic-matter decomposition, increased water temperature, or water stagnation can be considered.
• The higher the water temperature, the less readily oxygen dissolves in water, so the DO value tends to decrease.
• In water with high BOD, DO tends to decrease through microbial decomposition.
• If COD is high and DO is low, the effects of pollution by organic matter and reducing substances can be considered.
• In flowing water, oxygen is readily supplied through contact with air.
• In stagnant water, oxygen supply is small and DO tends to decrease.
• Photosynthesis by algae and aquatic plants may affect the increase in DO during the daytime.
• In DO measurement, introduction of bubbles during sampling and time elapsed before measurement are sources of error.

Points to Check When Discussing Dissolved Oxygen Measurement

Checking the following points before writing the report makes the discussion easier to write.

• Is it explained what the DO value indicates?
• Is the relationship between water temperature and DO considered?
• Is the relationship between organic pollution and DO decrease described?
• Is the relationship with BOD and COD discussed?
• Are water flow and stagnation at the sampling location considered?
• Are the effects of photosynthesis by algae and aquatic plants considered?
• Are the effects of bottom sediment and organic-matter decomposition considered?
• Are the sampling time and weather recorded?
• Is the introduction of bubbles during sampling considered?
• Are DO-meter calibration and temperature correction checked?
• If multiple measurements were performed, is the variation checked?
• Do the points for improvement correspond to the sources of error?

Summary

Dissolved oxygen measurement is an analysis used to determine the amount of oxygen dissolved in water and evaluate the oxygen condition and biological environment of a water body.
When the DO value is high, sufficient oxygen may be present, while when the DO value is low, oxygen may be deficient because of organic-matter decomposition, increased water temperature, or water stagnation.

The higher the water temperature, the less readily oxygen dissolves in water, and microbial activity may also become more active, increasing oxygen consumption.
If BOD and COD are high and DO is low, microbial decomposition may have progressed because of organic pollution, consuming dissolved oxygen.
However, DO may also remain high because of photosynthesis or water flow.

In a report, rather than simply writing that “the DO value was high or low,” organize and discuss water temperature, organic pollution, BOD, COD, water flow, photosynthesis, bottom sediment, water-sampling operations, and measurement errors.
Dissolved oxygen is an extremely important indicator for water-quality evaluation, but it is important to evaluate it in combination with other water-quality-analysis results rather than by itself.