Water quality analysis examines organic matter, inorganic ions, the degree of acidity or alkalinity, the ease of microbial decomposition, and other properties contained in water.
COD, BOD, pH, and electrical conductivity are representative indicators used to evaluate water quality.
They are commonly used when comparing river water, lake water, wastewater, tap water, groundwater, pond water, and other types of water.
In a discussion of water quality analysis, it is not sufficient simply to write that “COD was high,” “the pH was neutral,” or “the electrical conductivity was high.”
It is necessary to explain what COD and BOD indicate, how pH and electrical conductivity are related to components in water, and what type of water quality can be considered when the values are high or low.
This article clearly explains, as examples of discussions that can be used in water quality analysis laboratory reports, what can be learned from COD, BOD, pH, and electrical conductivity, evaluation of pollution, deviations in measured values, sources of error, and points for improvement.
Note:
This article is a reference intended to assist with discussions of water quality analysis results obtained in environmental chemistry experiments, analytical chemistry experiments, and basic chemistry experiments at universities and similar institutions.
For the actual measurement methods, reagent concentrations, types of water to be measured, environmental standards, effluent standards, units, calculation methods, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is Water Quality Analysis?
- Main Items to Include in the Results
- Reference Experimental Values and Example of an Overall Discussion of Water Quality Analysis
- Reference Experimental Conditions
- Overall Measurement Results by Water Sample
- Comparison of COD and BOD
- Example Calculation of the BOD/COD Ratio
- Relationship Between pH and Water Quality
- Electrical Conductivity and the Amount of Dissolved Ions
- Relationship Between DO and Organic Pollution
- Differences in Meaning Among Measurement Items
- Example of Overall Evaluation
- Comparison of Measurement Errors
- Example of How to Write the Results
- Points for Connecting the Results to the Discussion
- Example Discussion
- Summary
- What Is COD?
- What Is BOD?
- Difference Between COD and BOD
- What Can Be Learned From pH?
- What Can Be Learned From Electrical Conductivity?
- Discussion When Values Are High or Low
- Discussion Combining COD, BOD, pH, and Electrical Conductivity
- Discussion of Differences Among Sampling Locations
- Effects of Weather, Season, and Water Temperature
- Sources of Error in Water Quality Analysis
- Sources of Error in COD Measurement
- Sources of Error in BOD Measurement
- Sources of Error in pH and Electrical Conductivity Measurements
- When Water Quality Analysis Results 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 Water Quality Analysis
- Summary
What Is Water Quality Analysis?
Water quality analysis is an analysis used to measure chemical components and physical properties contained in water and evaluate the condition of the water.
Appearance alone is not sufficient to determine whether water is clean or polluted.
Even water that appears clear may contain organic matter, inorganic ions, acidic substances, alkaline substances, nutrients, and other components.
COD and BOD are used as indicators for evaluating pollution caused by organic matter in water.
pH indicates whether water is acidic, neutral, or alkaline, while electrical conductivity serves as an approximate indicator of the amount of ions dissolved in water.
By combining these indicators, water quality can be evaluated from multiple perspectives.
Example Discussion:
In water quality analysis, multiple indicators such as COD, BOD, pH, and electrical conductivity can be used to evaluate the amount of organic matter in water, biodegradability, acidity or alkalinity, and the amount of dissolved ions.
In this experiment, the pollution state and water-quality characteristics of the sample water were discussed by comparing the measured values.
Because water quality cannot be adequately judged from a single indicator alone, it is important to consider multiple measurement results in relation to one another.
Main Items to Include in the Results
In the results of water quality analysis, organize the sampling location, sampling date and time, weather, water temperature, appearance, odor, COD, BOD, pH, electrical conductivity, measurement method, dilution factor, calculated values, and other information.
Because values in environmental water readily change depending on sampling conditions, it is important to record the conditions at the time of sampling.
- Sampling location
- Sampling date and time
- Weather
- Water temperature
- Color and odor of the water
- Presence or absence of turbidity
- COD value
- BOD value
- pH value
- Electrical conductivity
- Measurement method
- Titration volume or absorbance
- Dilution factor
- Blank value
- Calculation process
- Comparison with other samples
- Sources of error and points for improvement
Example of How to Write the Results:
COD, BOD, pH, and electrical conductivity were measured for the collected water samples.
COD and BOD were evaluated as indicators of pollution caused by organic matter, pH as an indicator of acidic or alkaline conditions, and electrical conductivity as an approximate indicator of the amount of dissolved ions.
By comparing each measured value, the characteristics of the water quality of the sample water were discussed.
Reference Experimental Values and Example of an Overall Discussion of Water Quality Analysis
Here, COD, BOD, pH, and electrical conductivity, which are commonly examined in water quality analysis, are measured in the same water samples, and the way each indicator can be related in a discussion is organized using reference experimental values.
Rather than judging water quality from a single measurement item, it is important to comprehensively examine organic-matter pollution, oxygen consumption by microorganisms, acidity or alkalinity, the amount of dissolved ions, and other factors.
Water with high COD or BOD may be affected by organic pollution, while water with high electrical conductivity may contain large amounts of inorganic ions or salts.
Reference Experimental Conditions
| Measurement Item | Measurement Method | Main Meaning |
|---|---|---|
| COD | Potassium permanganate method | Amount of chemically oxidizable organic matter and reducing substances |
| BOD | Decrease in DO over 5 days at 20°C | Amount of organic matter that microorganisms can decompose |
| pH | pH meter | Acidity, neutrality, or alkalinity |
| Electrical conductivity | Electrical conductivity meter | Approximate amount of ions dissolved in water |
| DO | DO meter | Amount of oxygen dissolved in water |
Overall Measurement Results by Water Sample
Reference examples are shown for tap water, river water, pond water, domestic wastewater, fertilizer leachate, and food-factory wastewater.
| Sample | Water Sample | pH | Electrical Conductivity | COD | BOD | DO | How to Interpret Water Quality |
|---|---|---|---|---|---|---|---|
| A | Tap water | 7.2 | 145 μS/cm | 0.5 mg/L | 0.4 mg/L | 8.6 mg/L | Clean and low in organic matter |
| B | River water | 7.4 | 210 μS/cm | 2.0 mg/L | 1.6 mg/L | 8.2 mg/L | Relatively good |
| C | Pond water | 8.1 | 320 μS/cm | 4.4 mg/L | 3.4 mg/L | 5.4 mg/L | Effects of algae and organic matter |
| D | Domestic wastewater | 7.8 | 850 μS/cm | 96.9 mg/L | 31 mg/L | 2.1 mg/L | Severe organic pollution |
| E | Fertilizer leachate | 6.4 | 1850 μS/cm | 28.0 mg/L | 8.5 mg/L | 6.8 mg/L | Very high amount of inorganic ions |
| F | Food-factory wastewater | 6.8 | 980 μS/cm | 656 mg/L | 235 mg/L | 1.4 mg/L | Extremely high amount of organic matter |
Comparison of COD and BOD
COD reflects substances that are chemically oxidized, while BOD reflects organic matter that microorganisms can decompose.
By comparing both, not only the amount of organic matter but also its biodegradability can be discussed.
| Water Sample | COD | BOD | BOD/COD Ratio | Direction of Discussion |
|---|---|---|---|---|
| River water | 2.0 mg/L | 1.6 mg/L | 0.80 | Relatively readily biodegradable organic matter |
| Pond water | 4.4 mg/L | 3.4 mg/L | 0.77 | Possibility of algae-derived organic matter |
| Domestic wastewater | 96.9 mg/L | 31 mg/L | 0.32 | Contains many chemically oxidizable components as well |
| Fertilizer leachate | 28.0 mg/L | 8.5 mg/L | 0.30 | Effects of inorganic or less readily biodegradable components |
| Food-factory wastewater | 656 mg/L | 235 mg/L | 0.36 | Extremely high amount of organic matter |
Example Calculation of the BOD/COD Ratio
The BOD/COD ratio is an approximate indicator of how much BOD there is relative to COD.
BOD/COD ratio = BOD ÷ COD
In domestic wastewater, BOD is 31 mg/L and COD is 96.9 mg/L.
BOD/COD ratio = 31 ÷ 96.9 = 0.32
When this value is low, the sample may also contain components that are chemically oxidized but are difficult for microorganisms to decompose in a short period.
Relationship Between pH and Water Quality
pH indicates the acidity or alkalinity of water.
Many natural waters are near neutral, but pH may change because of algal photosynthesis, acidic wastewater, fertilizer components, alkaline detergents, and other influences.
| Water Sample | pH | Example of Main Factors | Direction of Discussion |
|---|---|---|---|
| Tap water | 7.2 | Adjusted near neutral | Stable water quality |
| River water | 7.4 | Effects of rocks, soil, and domestic wastewater | Near neutral |
| Pond water | 8.1 | CO2 consumed by algal photosynthesis | Slightly alkaline |
| Fertilizer leachate | 6.4 | Fertilizer components and acidic components | Slightly acidic |
| Food-factory wastewater | 6.8 | Organic acids and food-derived components | Slightly acidic to neutral |
The degree of organic pollution cannot be judged from pH alone.
For example, domestic wastewater has a pH of 7.8 and is near neutral, but its COD and BOD are high, so it can be judged to have severe organic pollution.
Electrical Conductivity and the Amount of Dissolved Ions
Electrical conductivity is an approximate indicator of the amount of ions dissolved in water.
When inorganic salts, fertilizer components, domestic wastewater, seawater, and similar substances are mixed into water, electrical conductivity tends to become high.
| Water Sample | Electrical Conductivity | Possible Components | Relationship With COD and BOD |
|---|---|---|---|
| Tap water | 145 μS/cm | Small amounts of inorganic ions | COD and BOD are also low |
| River water | 210 μS/cm | Naturally derived ions | Relatively little organic matter |
| Pond water | 320 μS/cm | Nutrients and algae-derived components | COD and BOD are also somewhat high |
| Fertilizer leachate | 1850 μS/cm | Nitrate ions, phosphate ions, potassium ions, and others | Electrical conductivity is particularly high |
| Food-factory wastewater | 980 μS/cm | Food-derived components and salts | COD and BOD are extremely high |
Fertilizer leachate has very high electrical conductivity, whereas its COD and BOD are not as high as those of food-factory wastewater.
This may indicate that fertilizer leachate contains more inorganic ions than organic matter.
Relationship Between DO and Organic Pollution
In water containing large amounts of organic matter, microorganisms consume oxygen when decomposing the organic matter, so DO tends to become low.
| Water Sample | BOD | DO | How to Interpret the Relationship |
|---|---|---|---|
| Tap water | 0.4 mg/L | 8.6 mg/L | Low BOD and high DO |
| River water | 1.6 mg/L | 8.2 mg/L | Relatively good |
| Pond water | 3.4 mg/L | 5.4 mg/L | DO decreases because of organic-matter decomposition |
| Domestic wastewater | 31 mg/L | 2.1 mg/L | Oxygen deficiency |
| Food-factory wastewater | 235 mg/L | 1.4 mg/L | Strong oxygen consumption |
Water with high BOD tends to have low DO.
This is considered to be because oxygen was consumed when organic matter in the water was decomposed by microorganisms.
Differences in Meaning Among Measurement Items
Each measurement item represents a different aspect of water quality.
In a report, it is important to distinguish what each indicator represents.
| Measurement Item | What May Be Considered When High | What May Be Considered When Low |
|---|---|---|
| COD | Large amount of readily oxidizable organic matter or reducing substances | Few chemically oxidizable substances |
| BOD | Large amount of organic matter decomposed by microorganisms | Small amount of biodegradable organic matter |
| pH | Effects of alkaline components, photosynthesis, detergents, and similar factors | Effects of acidic components, organic acids, acidic wastewater, and similar factors |
| Electrical conductivity | Large amounts of dissolved ions and salts | Small amount of ions |
| DO | Sufficient oxygen; effects of aeration, low temperature, and photosynthesis | Effects of organic-matter decomposition, pollution, high temperature, and stagnation |
Example of Overall Evaluation
By combining each indicator, the characteristics of water quality can be judged more specifically.
| Water Sample | Main Characteristics | Example of Overall Evaluation |
|---|---|---|
| Tap water | Low COD, BOD, and electrical conductivity; high DO | Clean and stable water |
| River water | Low to moderate COD and BOD; high DO | Relatively good natural water |
| Pond water | Slightly high pH, increased COD and BOD, decreased DO | Water affected by algae and organic matter |
| Domestic wastewater | High COD, BOD, and electrical conductivity; low DO | Water with severe organic pollution |
| Fertilizer leachate | Very high electrical conductivity and slightly acidic pH | Water containing large amounts of inorganic nutrients |
| Food-factory wastewater | Extremely high COD and BOD; low DO | Water with an extremely high organic load |
Comparison of Measurement Errors
In water quality analysis, sources of error differ depending on the measurement item.
Organizing which operations affect which measured values makes it easier to write the discussion.
| Measurement Item | Main Sources of Error | Effect on the Result |
|---|---|---|
| COD | Insufficient heating time, deviation in titration endpoint, chloride ions, reagent concentration | COD may be overestimated or underestimated |
| BOD | Inappropriate dilution factor, bubbles, temperature control, microbial activity | BOD may be overestimated or underestimated |
| pH | Uncalibrated pH meter, dirty electrode, temperature differences, suspended matter | pH value deviates |
| Electrical conductivity | Insufficient temperature correction, dirty cell, bubbles, sample evaporation | Amount of ions may be evaluated incorrectly |
| DO | Bubbles during sampling, leaving the sample standing, temperature changes, insufficient sensor calibration | DO may be overestimated or underestimated |
Example of How to Write the Results
COD, BOD, pH, electrical conductivity, and DO were measured for each water sample.
In tap water, COD was 0.5 mg/L, BOD was 0.4 mg/L, electrical conductivity was 145 μS/cm, and DO was 8.6 mg/L, indicating water containing little organic matter or dissolved ions and sufficient oxygen.
In river water, COD was 2.0 mg/L and BOD was 1.6 mg/L, which were slightly higher than those of tap water, but DO was relatively high at 8.2 mg/L.
From this, the river water was considered to contain a certain amount of organic matter but not enough to cause severe oxygen deficiency.
In pond water, pH was 8.1, COD was 4.4 mg/L, BOD was 3.4 mg/L, and DO was 5.4 mg/L.
Because the pH was somewhat high, CO2 may have been consumed by algal photosynthesis.
In addition, because COD and BOD were higher than in river water and DO was lower, oxygen consumption caused by decomposition of algae and organic matter can also be considered.
In domestic wastewater, COD was 96.9 mg/L, BOD was 31 mg/L, electrical conductivity was 850 μS/cm, and DO was 2.1 mg/L.
Because COD and BOD were high and DO was low, the water was considered to contain large amounts of organic matter and to readily consume oxygen through microbial decomposition.
In fertilizer leachate, electrical conductivity was very high at 1850 μS/cm, but COD and BOD were not as high as those in food-factory wastewater.
From this, fertilizer leachate may contain large amounts of inorganic ions such as nitrate ions, phosphate ions, and potassium ions rather than organic matter.
Points for Connecting the Results to the Discussion
In comprehensive water quality analysis, it is important not only to explain each measured value individually but also to read the characteristics of the water quality from the relationships among the indicators.
- Can the difference between COD and BOD be explained as the difference between chemical oxidation and microbial decomposition?
- Can the reason DO decreases in samples with high COD and BOD be related to oxygen consumption through organic-matter decomposition?
- Can the reason electrical conductivity is high be explained in relation to dissolved ions, fertilizer components, and salts?
- Can changes in pH be related to acidic components, alkaline components, photosynthesis, and wastewater?
- Can biodegradability be discussed from the BOD/COD ratio?
- Can it be explained that the meaning of water quality changes when BOD or electrical conductivity differs even if COD is the same?
- Can the sources of error for each measurement item be explained separately?
- Can water quality be judged by combining multiple indicators rather than drawing a conclusion from a single indicator?
Example Discussion
In this experiment, COD, BOD, pH, electrical conductivity, and DO were measured for tap water, river water, pond water, domestic wastewater, fertilizer leachate, and food-factory wastewater, and their water quality was comprehensively compared.
In tap water, both COD and BOD were low and DO was high, so the water was considered to have little organic pollution and to contain sufficient oxygen.
In river water, COD was 2.0 mg/L and BOD was 1.6 mg/L, which were higher than those of tap water.
However, DO was high at 8.2 mg/L, and sufficient oxygen remained in the water.
From this, the river water was considered to contain some naturally or domestically derived organic matter but not to be in a state of severe pollution.
In pond water, pH was somewhat high at 8.1, and COD and BOD were also higher than those of river water.
In ponds, water tends to stagnate, and when algae proliferate, CO2 may be consumed through photosynthesis, causing the pH to rise.
In addition, because oxygen is consumed when algae and organic matter decompose, the DO was considered to have decreased to 5.4 mg/L.
COD and BOD increased greatly in domestic wastewater and food-factory wastewater.
In particular, COD was 656 mg/L and BOD was 235 mg/L in food-factory wastewater, indicating an extremely high organic load.
In such water, microorganisms consume large amounts of oxygen when decomposing organic matter, so DO decreases and aquatic organisms may be adversely affected.
In fertilizer leachate, electrical conductivity was extremely high at 1850 μS/cm, but BOD was not as high as that in food-factory wastewater.
This was considered to be because fertilizer leachate contained large amounts of inorganic ions such as nitrate ions, phosphate ions, and potassium ions, while the amount of organic matter decomposed by microorganisms was relatively small.
Thus, electrical conductivity must be interpreted not as an indicator of organic pollution but mainly as an indicator reflecting the amount of dissolved ions.
Possible measurement errors include heating time and titration endpoint for COD, dilution factor and bubble contamination for BOD, electrode calibration for pH, temperature correction for electrical conductivity, and leaving the sample standing after sampling for DO.
In particular, BOD and DO readily change even after sampling, so it is necessary to avoid introducing bubbles during sampling and to measure or fix the sample as soon as possible.
To correctly evaluate water quality, it is important not to judge from a single measured value but to consider multiple indicators together.
Summary
In water quality analysis, COD, BOD, pH, electrical conductivity, and DO each represent different aspects of water quality.
COD and BOD indicate organic pollution, pH indicates acidity or alkalinity, electrical conductivity indicates the amount of dissolved ions, and DO indicates the oxygen condition in the water.
In this reference example, COD and BOD were high and DO was low in domestic wastewater and food-factory wastewater, while electrical conductivity was particularly high in fertilizer leachate.
In a report, it is useful to comprehensively discuss the meaning of each measurement item, relationships among indicators, type of pollution, and measurement errors.
What Is COD?
COD is chemical oxygen demand and represents the amount of oxygen required when organic matter and similar substances in water are chemically oxidized using an oxidizing agent.
The higher the COD, the more readily oxidizable organic matter and reducing substances may be present.
It is used as an indicator for evaluating organic pollution in river water, lake water, wastewater, and other types of water.
COD is a value evaluated by chemical oxidation rather than microbial decomposition.
Therefore, organic matter that is difficult for organisms to decompose and reducing inorganic substances may also affect COD.
A high COD does not necessarily mean that there is a large amount of organic matter that microorganisms can readily decompose.
Example Discussion:
Because the COD value was high, the sample water was considered to contain large amounts of organic matter or reducing substances oxidized by the oxidizing agent.
Because COD reflects the amount of substances in water that are chemically readily oxidized, it serves as an indicator for evaluating the degree of organic pollution.
However, because some reducing inorganic substances may also affect COD, the type of organic matter cannot be identified from the COD value alone.
What Is BOD?
BOD is biochemical oxygen demand and represents the amount of oxygen consumed when organic matter in water is decomposed by microorganisms.
Water with high BOD contains large amounts of organic matter that microorganisms can decompose, and dissolved oxygen is readily consumed during the decomposition process.
Therefore, BOD is important for evaluating domestic wastewater and organic pollution.
If water with high BOD flows into rivers or lakes, oxygen in the water may become insufficient and adversely affect fish and aquatic organisms.
Because BOD depends on microbial activity, it is affected by temperature, measurement time, the presence or absence of toxic substances, dilution of the sample, and the amount of microorganisms.
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 is consumed as organic matter is decomposed, so the habitat conditions for aquatic organisms may deteriorate.
Because BOD is an indicator dependent on microbial activity, attention must also be paid to measurement temperature, dilution conditions, and the presence or absence of inhibitory substances.
Difference Between COD and BOD
COD and BOD are both indicators used to evaluate organic pollution in water, but their meanings differ.
COD is the amount of oxygen required for chemical oxidation, while BOD is the amount of oxygen required for microbial decomposition.
COD can be measured in a relatively short period, whereas BOD is generally measured by incubating the sample for a fixed period and determining oxygen consumption.
If both COD and BOD are high, the sample may contain large amounts of organic matter that is readily decomposed both chemically and biologically.
If COD is high but BOD is low, the sample may contain refractory organic matter or reducing inorganic substances.
The ease of decomposition of organic matter can be discussed by examining the relationship between BOD and COD.
| Item | Main Meaning | Point for Discussion |
|---|---|---|
| COD | Amount of chemically oxidizable substances | Examine the amount of organic matter and reducing substances |
| BOD | Amount of oxygen consumed during microbial decomposition | Examine the amount of readily biodegradable organic matter |
| High COD / High BOD | Possibility of severe organic pollution | Consider the effects of domestic wastewater and similar sources |
| High COD / Low BOD | Possibility of refractory substances | Consider substances that are chemically oxidized but difficult for microorganisms to decompose |
Example Discussion:
COD and BOD are both indicators of water pollution, but COD is based on chemical oxidation and BOD on microbial decomposition.
If both COD and BOD were high in this experiment, the sample water was considered to contain large amounts of organic matter, some of which was readily decomposed by microorganisms.
On the other hand, if BOD was low relative to COD, the effects of refractory organic matter or reducing inorganic substances can be considered.
What Can Be Learned From pH?
pH is a value indicating whether water is acidic, neutral, or alkaline.
A pH near 7 is neutral, a pH below 7 is acidic, and a pH above 7 is alkaline.
Acids, bases, carbon dioxide, hydrogen carbonate ions, industrial wastewater, domestic wastewater, soil, and rock components in water can affect pH.
pH is also strongly related to the habitat conditions of aquatic organisms.
In extremely acidic or alkaline water, organisms may have difficulty growing and the dissolution behavior of metal ions may change.
However, because the amount of organic matter or ions cannot be directly determined from pH alone, it must be considered together with COD, BOD, and electrical conductivity.
Example Discussion:
pH measurement can be used to evaluate whether the sample water is acidic, neutral, or alkaline.
If the pH is near neutral, strong acidic or alkaline pollution is considered to be small.
On the other hand, if the pH is strongly shifted toward acidic or alkaline conditions, the effects of acidic wastewater, alkaline wastewater, soil components, or dissolved carbon dioxide must be considered.
What Can Be Learned From Electrical Conductivity?
Electrical conductivity is a value indicating how readily water conducts electricity.
Pure water hardly conducts electricity, whereas water containing dissolved electrolytes such as salts, acids, and bases conducts electricity more readily.
Therefore, electrical conductivity serves as an approximate indicator of the amount of ions dissolved in water.
When electrical conductivity is high, the sample may contain large amounts of dissolved ions such as sodium ions, calcium ions, magnesium ions, chloride ions, sulfate ions, and nitrate ions.
However, electrical conductivity alone does not reveal which ions are present.
To determine the types of ions, separate qualitative analysis, ion chromatography, or other methods are necessary.
Example Discussion:
Because the electrical conductivity was high, the sample water was considered to contain a relatively large amount of dissolved ions present as electrolytes.
Electrical conductivity serves as an approximate indicator of the amount of ions in water, but it cannot identify the types of ions.
Therefore, to investigate the cause of high electrical conductivity in detail, separate analyses of chloride ions, nitrate ions, hardness components, and other substances are necessary.
Discussion When Values Are High or Low
When COD is high, large amounts of readily oxidizable organic matter or reducing substances may be present.
When BOD is high, large amounts of organic matter readily decomposed by microorganisms are present and dissolved oxygen is considered likely to be consumed.
When pH is low, the effects of acidic substances are considered, while when pH is high, the effects of alkaline substances or algal photosynthesis are considered.
When electrical conductivity is high, the amount of dissolved ions may be large.
Conversely, when COD and BOD are low, organic pollution is considered small.
However, even when COD and BOD are low, high electrical conductivity may indicate that large amounts of inorganic salts are present.
Even when electrical conductivity is low, nonelectrolyte organic matter or microbial contamination may not be adequately reflected.
Example Discussion:
When COD and BOD are high, pollution caused by organic matter is considered to be severe.
On the other hand, when COD and BOD are low and only electrical conductivity is high, the effect of inorganic ions may be greater than that of organic pollution.
Thus, in water quality analysis, it is necessary to judge the characteristics of water from combinations of multiple indicators rather than from individual values.
Discussion Combining COD, BOD, pH, and Electrical Conductivity
In water quality analysis, it is important not to judge from a single indicator but to combine multiple indicators.
If COD and BOD are high, organic pollution may be severe, while high electrical conductivity suggests the effects of dissolved ions.
If pH is strongly shifted, acidic or alkaline components may be affecting the water quality.
| Combination of Measurement Results | Possible Water Quality | Point for Discussion |
|---|---|---|
| High COD / High BOD | Large amount of readily biodegradable organic pollution | Consider organic matter originating from domestic wastewater or food |
| High COD / Low BOD | Large amount of refractory or reducing substances | Consider pollution that is difficult for microorganisms to decompose |
| High conductivity / Low COD | Large amount of inorganic ions | Consider salts, fertilizer components, and geological effects |
| Strongly shifted pH | Effects of acidic or alkaline components | Consider wastewater, soil, algae, and carbon dioxide |
Example Discussion:
In this experiment, both COD and BOD were high, and electrical conductivity was also relatively high.
From this, the sample water may have contained large amounts of both readily biodegradable organic matter and dissolved ions.
When domestic wastewater or agricultural drainage is mixed into water, organic matter and inorganic ions may increase at the same time, producing such a result.
Discussion of Differences Among Sampling Locations
Water quality varies greatly depending on the sampling location.
In upstream sections of rivers, anthropogenic pollution is small, and COD, BOD, and electrical conductivity tend to be low.
On the other hand, downstream sections after flowing through residential, agricultural, or industrial areas are more likely to be affected by domestic wastewater, agricultural drainage, industrial wastewater, and road runoff.
In ponds and lakes, water tends to remain stagnant, so organic matter and nutrients may accumulate.
If algae proliferate, pH, BOD, and COD may be affected.
Relating the surrounding environment of the sampling location to the results leads to a more persuasive discussion.
Example Discussion:
If COD, BOD, and electrical conductivity were high in a downstream sample, the water may have been affected by domestic wastewater, agricultural drainage, and road runoff while flowing from upstream to downstream.
In areas with greater human activity, organic matter and inorganic ions are more likely to enter the water.
Therefore, considering the surrounding environment of the sampling location makes it possible to interpret water quality analysis results more specifically.
Effects of Weather, Season, and Water Temperature
Water quality changes with weather and season.
After rainfall, components may flow into the water from soil, roads, and agricultural land, causing changes in electrical conductivity and COD.
On the other hand, large amounts of rainwater may dilute the water and lower concentrations.
Which effect is stronger depends on the sampling location and amount of rainfall.
Water temperature is also important.
When water temperature is high, microbial activity becomes more active and may affect organic-matter decomposition and BOD.
In addition, electrical conductivity tends to be measured higher at higher temperatures, so temperature correction and recording the measurement temperature are important.
Example Discussion:
If rainfall occurred before sampling, organic matter and inorganic ions may have flowed into the water from the surrounding soil and roads and affected the water-quality measurements.
In addition, under conditions of high water temperature, microbial activity may increase and affect BOD and pH.
Because electrical conductivity is also affected by temperature, the measurement temperature must be checked when comparing multiple samples.
Sources of Error in Water Quality Analysis
Sources of error in water quality analysis include contamination during sampling, contamination of containers, changes during storage, concentration errors in measurement reagents, titration errors, insufficient calibration of pH meters and electrical conductivity meters, insufficient temperature correction, sample nonuniformity, and dilution errors.
Because water samples continue to change after sampling through microbial activity and chemical reactions, storage conditions are also important.
In COD measurement, heating conditions, progress of the oxidation reaction, titration endpoint, and blank correction are important.
In BOD measurement, dilution factor, incubation temperature, incubation time, and microbial activity have an effect.
In pH measurement, electrode calibration and exchange of carbon dioxide have an effect, while in electrical conductivity measurement, temperature and electrode contamination are sources of error.
Example Discussion:
Possible causes of variation in measured values include contamination of the sampling container, changes during sample storage, insufficient calibration of measuring instruments, and titration errors.
Because water-quality samples continue to change after sampling through microbial activity and exchange of carbon dioxide, it is important to perform measurements as soon as possible after sampling.
In addition, pH meters and electrical conductivity meters must be calibrated with standard solutions before measurement.
Sources of Error in COD Measurement
In COD measurement, reagent concentration, heating time, reaction temperature, titration procedures, endpoint determination, blank correction, sample amount, and effects of coexisting substances are sources of error.
If the oxidation reaction is insufficient, COD becomes low, while if large amounts of reducing substances are present, COD may become high because of effects other than organic matter.
Example Discussion:
Possible sources of error in the COD value include differences in heating conditions, incomplete oxidation reactions, and errors in determining the titration endpoint.
If the sample contains reducing substances other than organic matter, these substances also consume the oxidizing agent and may cause COD to be estimated as higher than the actual amount of organic matter.
In addition, inappropriate blank correction causes systematic error in the entire COD calculation.
Sources of Error in BOD Measurement
In BOD measurement, dilution factor, incubation temperature, incubation time, dissolved oxygen measurement, microbial activity, the presence or absence of toxic substances, bubbles in the container, and sample storage time are sources of error.
If dilution is inappropriate, oxygen may become excessively depleted or the change may become too small, making accurate determination of BOD difficult.
Example Discussion:
Possible sources of error in the BOD value include inappropriate dilution factor, fluctuations in incubation temperature, differences in microbial activity, and errors in dissolved oxygen measurement.
If the sample contains substances that inhibit microbial activity, BOD may be measured as low even when organic matter is present.
Therefore, BOD results must be discussed while considering their strong dependence on measurement conditions.
Sources of Error in pH and Electrical Conductivity Measurements
In pH measurement, insufficient calibration of the pH meter, dirty electrodes, temperature changes, absorption of carbon dioxide by the sample, insufficient stirring before measurement, and insufficient cleaning of the electrode are sources of error.
In electrical conductivity measurement, temperature changes, dirty electrodes, deviation in the cell constant, insufficient calibration with standard solutions, adhesion of bubbles, sample nonuniformity, and contamination of containers are sources of error.
Example Discussion:
Possible sources of error in pH measurement include insufficient calibration of the pH meter, contamination of the electrode, and changes in measurement temperature.
In addition, because electrical conductivity tends to increase as temperature rises, temperature correction is important when comparing multiple samples.
If electrodes are not sufficiently cleaned, components from the previous sample may be introduced, causing pH and electrical conductivity to differ from their actual values.
When Water Quality Analysis Results Can Be Considered Good
Water quality analysis results can be considered good when the sampling and measurement conditions are clear and the COD, BOD, pH, and electrical-conductivity values can be explained consistently as characteristics of the water quality.
For example, if COD and BOD are high in water expected to contain large amounts of organic matter and electrical conductivity is high in water expected to contain many dissolved ions, the results are easier to consider valid.
Example Discussion:
In this experiment, the measured values of COD, BOD, pH, and electrical conductivity showed trends that did not contradict the surrounding environment of the sampling locations.
COD and BOD were high in samples where inflow of organic matter could be considered, while electrical conductivity was high in samples where the effects of dissolved ions were expected.
From these results, the water quality analysis in this experiment was considered to approximately reflect the characteristics of the sample water.
Example Discussion When the Experiment Did Not Go Well
When water quality analysis does not go well, possible causes can be considered from results such as large variation in measured values, difficulty explaining the relationship between COD and BOD, unstable pH, abnormally high or low electrical conductivity, a large blank value, or difficulty determining the titration endpoint.
Organizing the causes according to sampling, storage, measuring instruments, reagents, operations, and calculations makes the discussion easier.
Example Discussion:
In this experiment, variation was observed in measured values for the same sample.
Possible causes include insufficient homogenization of the sample, residue of the previous sample in the sampling container or on the electrode, and insufficient calibration of the measuring instruments.
In addition, because water samples continue to change after sampling through microbial activity and exchange of carbon dioxide, differences in storage time may also have affected the measured values.
How to Write Points for Improvement
In a discussion of water quality analysis, including not only sources of error but also points for improvement makes the report easier to organize.
Improvements can be divided into sampling, storage, measurement, and analysis.
Improvements to Sampling and Storage
- Use a clean container
- Record the sampling location and time
- Record the weather at the time of sampling
- Mix the sample thoroughly before measurement
- Measure as soon as possible after sampling
- Store in a cool, dark place when necessary
- Avoid air and contamination in the container
Improvements to Measurement Procedures
- Calibrate the pH meter with standard solutions
- Calibrate the electrical conductivity meter with standard solutions
- Clean the electrode for each sample
- Check temperature correction
- Perform blank correction in COD measurement
- Carefully determine the titration endpoint
- Select an appropriate dilution factor in BOD measurement
Improvements to Analysis
- Compare COD and BOD
- Consider the relationship between pH and components in the water
- Treat electrical conductivity as an approximate indicator of the amount of dissolved ions
- Relate the results to the environment of the sampling location
- Consider the effects of weather and season
- Perform multiple measurements and calculate the average value
- Evaluate the results in combination with other water-quality parameters
Example of How to Write Points for Improvement:
To improve the reliability of water quality analysis, the sampling container must be kept clean and measurements performed as soon as possible after sampling.
In addition, pH meters and electrical conductivity meters must be calibrated with standard solutions, and the electrodes must be cleaned for each sample.
In the analysis, COD, BOD, pH, and electrical conductivity should not be judged independently but should be comprehensively discussed in relation to the sampling location and surrounding environment.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of water quality analysis, writing only that “COD was high” or “pH was low” results in a superficial discussion.
A good discussion explains what each indicator means and how it relates to the sampling location, surrounding environment, and other measured values.
| Superficial Discussion | Good Discussion |
|---|---|
| COD was high. | Because COD was high, the sample water was considered to contain large amounts of readily oxidizable organic matter or reducing substances. Possible causes include mixing of domestic wastewater or plant-derived organic matter. |
| BOD was high. | Because BOD was high, the sample water was considered to contain large amounts of organic matter readily decomposed by microorganisms, making dissolved oxygen in the water more likely to be consumed. |
| The pH was neutral. | Because the pH was near neutral, strong acidic or alkaline pollution was considered small. However, organic pollution and the amount of dissolved ions cannot be judged from pH alone. |
| Electrical conductivity was high. | Because electrical conductivity was high, the sample water was considered to contain large amounts of dissolved ions. If COD and BOD are low, the effects of inorganic salts may be greater than those of organic pollution. |
Examples of Expressions That Can Be Used in Reports
- COD is an indicator reflecting the amount of readily oxidizable organic matter and reducing substances in water.
- BOD is an indicator used to evaluate the amount of organic matter decomposed by microorganisms.
- If both COD and BOD are high, organic pollution may be severe.
- If COD is high and BOD is low, the effects of refractory substances or reducing inorganic substances can be considered.
- pH indicates whether water is acidic, neutral, or alkaline.
- If pH is strongly shifted, the effects of acidic or alkaline wastewater or geology can be considered.
- Electrical conductivity serves as an approximate indicator of the amount of dissolved ions in water.
- If electrical conductivity is high, the effects of salts, fertilizer components, domestic wastewater, or industrial wastewater can be considered.
- Water quality changes depending on the sampling location, weather, season, and water temperature.
- In water-quality evaluation, multiple indicators must be combined rather than relying on a single indicator.
Points to Check When Discussing Water Quality Analysis
- Have the sampling location and date and time been recorded?
- Have the weather and water temperature at the time of sampling been recorded?
- Is it explained what COD indicates?
- Is it explained what BOD indicates?
- Is the difference between COD and BOD understood?
- Is the meaning of pH explained?
- Is electrical conductivity treated as an approximate indicator of the amount of dissolved ions?
- Are the measured values related to the environment of the sampling location?
- Are the effects of weather and season considered?
- Are calibration of measuring instruments and temperature correction considered?
- Are sources of error considered separately for each measurement item?
- Do the points for improvement correspond to the sources of error?
Summary
In water quality analysis, measuring COD, BOD, pH, and electrical conductivity makes it possible to evaluate organic pollution, biodegradability, acidity or alkalinity, and the amount of dissolved ions in water.
COD indicates the amount of chemically oxidizable substances, BOD indicates the amount of organic matter decomposed by microorganisms, pH indicates acidity, neutrality, or alkalinity, and electrical conductivity serves as an approximate indicator of the amount of dissolved ions.
When both COD and BOD are high, organic pollution may be severe, while when COD is high and BOD is low, refractory substances may be present.
When electrical conductivity is high, large amounts of inorganic ions may be present, while when pH is strongly shifted, the effects of acidic or alkaline components can be considered.
By combining these indicators, the characteristics of water quality can be explained more specifically.
In a report, rather than simply writing that “the value was high or low,” organize and discuss the meaning of each indicator, the environment of the sampling location, weather and season, water temperature, measurement errors, and relationships with other indicators.
In water quality analysis, it is important not to draw conclusions from a single numerical value but to comprehensively evaluate the condition of the water using multiple measurement results.
