Understanding Chemical Oxygen Demand (COD) in Wastewater: Importance, Analysis, and Environmental Impact

 The physical, chemical, and biological properties of water are referred to as its quality. It matters because it has an impact on how effectively people, animals, and plants can endure life. Pollutants, climate change, land use, and other factors can all have an impact on water quality.

Water that has been used for bathing, cooking, or toilet flushing is referred to as wastewater. It contains a wide range of pollutants, such as human waste, leftover food, and detergents that contain organic, inorganic, and pathogenic material. If wastewater is not properly treated, it can contribute significantly to water pollution. Wastewater's COD acts as a measure of the level of organic matter present.

The COD of wastewater is significant because it can show whether pollution is a possibility. High COD levels can harm aquatic life by reducing the amount of dissolved oxygen in the water. COD may contain pathogens that can lead to disease, which makes it a potential health risk. 

What is Chemical Oxygen Demand(COD):

Chemical oxygen demand is a measurement of the quantity of oxygen needed to chemically oxidize the organic and inorganic nutrients, like ammonia or nitrate, that are present in water. The earliest techniques for measuring COD involved observing colour changes in permanganate solutions when combined with water samples.

COD is a crucial indicator of water quality and is used in many different contexts, such as:

1. To ensure that the process for treating waste and discharging wastewater complies with regulatory standards

2. To calculate the BOD/COD ratio of the biodegradable fraction of wastewater effluent.

3. To assess how well water treatment methods are working.

4. To track how industrial waste discharges affect water quality

Milligrammes of oxygen per litre (mg/L) is the unit used to express COD.

Discovery of Chemical Oxygen Demand

Several scientists, including the following, are credited with the discovery of chemical oxygen demand (COD):

French chemist Jean-Baptiste Boussingault (1802–1887), is widely regarded as the founder of agricultural chemistry. Using a technique that involved oxidising organic matter with potassium permanganate, Boussingault was the first to measure how much oxygen was consumed by organic matter in water.

A Belgian-American chemist named Emile Verbeck (1844–1921) is credited with creating the first standardised technique for determining COD. Verbeck's technique, which still serves as the foundation for the most widely used technique for determining COD today, used a solution of potassium dichromate as the oxidising agent.

Ralph H. Aiken, an American chemist who lived from 1900 to 1982, is credited with enhancing the COD test's precision. Aiken created a technique for measuring COD that employed a closed reflux system to stop oxygen loss during oxidation.

The first standardised method for measuring COD was created in the early 1900s, and the first published reports of COD measurements appeared in the 1860s.

To evaluate water quality and track the efficacy of water treatment procedures, COD has become a widely used water quality parameter.

Additional information regarding the discovery of COD is provided below:

In 1949, the phrase "chemical oxygen demand" was first used.

The COD test is a straightforward and quick procedure.

The presence of specific inorganic substances, such as ammonia and nitrate, can have an impact on the COD test.

Different kinds of organic matter cannot be differentiated using the COD test.

Where did chemical oxygen demand analysis enter the industrial setting?

Chemical oxygen demand (COD) analysis was first introduced in the pulp and paper industry in the early 1900s. The paper industry was one of the first industries to recognize the need to measure the amount of organic matter in wastewater, as this could impact the quality of the receiving water. The COD test was also used to monitor the effectiveness of wastewater treatment processes in the paper industry.

The pulp and paper industry first used chemical oxygen demand (COD) analysis in the early 1900s. One of the first industries to realise the importance of measuring the amount of organic matter in wastewater because it could affect the quality of the receiving water was the paper industry. The effectiveness of wastewater treatment procedures in the paper industry was also monitored using the COD test.

Other industries, including the textile, food, and petroleum industries, started using COD analysis in the 1950s. The amount of organic matter in wastewater needs to be measured because it may affect both the effectiveness of wastewater treatment procedures and the quality of the receiving water, as recognised by these industries.

Today, a variety of industries, including the following, use COD analysis:

Paper and pulp, Food, Textiles, Petroleum, Metal Finishing, Pharmaceuticals, and Municipal Wastewater Treatment

Analytical methods:

The dichromate method is the most popular and is based on the oxidation of organic matter by potassium dichromate in an acidic solution. Besides, the closed reflux method is more accurate and uses a sealed vessel to prevent the loss of oxygen during the oxidation process.

The following materials are used.

sample of water

Solution of potassium dichromate

Solution of sulfuric acid

Solution for starch indicators

Conical burette flask

Thermometer

Glass beads on a hot plate

Procedure:

Use distilled water to rinse the burette and conical flask.

Conical flask with 10 mL of the water sample add.

Fill the flask with 1 mL of mercuric sulphate solution.

The flask should now contain 5 mL of potassium dichromate solution.

The flask should now contain 15 mL of sulfuric acid solution.

Place the flask on a hot plate after attaching it to a condenser.

The flask is heated at 150°C for two hours.

Remove the flask from the hot plate and let it cool for another two hours.

Fill the flask with 1 mL of the starch indicator solution.

Once the blue colour has vanished, titrate the solution with a standardised sodium thiosulfate solution.

Use the following equation to determine the COD of the water sample:

Where: COD (mg/L) = (V * N * 1000) / M

V is the sodium thiosulfate solution used in volume (in millilitres).

The sodium thiosulfate solution is normal, or N.

M is the potassium dichromate molecular weight (294.19 g/mol).

Notes:

To get rid of suspended solids, the water sample needs to be filtered before analysis.

Before analysis, the water sample's temperature should be between 20°C and 25°C.

Before the analysis, the potassium dichromate solution needs to be standardised.

Before the analysis, the starch indicator solution needs to be freshly made.

Results Interpretation:

A water sample's COD can be used to evaluate the water's quality and track the efficiency of wastewater treatment procedures. For instance, a high COD value in a wastewater sample would suggest ineffective wastewater treatment.

The amount of oxygen needed to treat a sample of water can also be calculated using the COD. For instance, a water sample with a COD of 100 mg/L would need to be treated with 100 mg of oxygen.

It is significant to note that a variety of elements, including water temperature, pH, and the presence of specific inorganic compounds, can have an impact on the COD of a water sample. Therefore, to compare the outcomes of various samples, it is crucial to standardise the COD analysis's conditions.

Effective variables on COD

These extra variables can influence the COD acceptance standards as well:

Water temperature: As the temperature rises, the COD of the water rises as well.

Water's pH: As the pH rises, COD in the water decreases.

The presence of specific inorganic substances: The COD of water can be impacted by the presence of specific inorganic substances like ammonia and nitrate.

The technique used to calculate COD: Results from various COD measurement techniques may vary.

When interpreting the outcomes of a COD test, it is critical to be aware of these factors.

Role of Reagent in COD

Sulfuric Acid in COD Analysis: 

Sulfuric acid is utilised in the Chemical Oxygen Demand (COD) analysis of water for a few reasons.

It is a strong acid.  This indicates that protons, which are required for the oxidation of organic matter, can be donated by it with ease. Because they are weaker, other acids like hydrochloric acid or nitric acid might not be able to completely oxidise all of the organic material in a sample.

One stable acid is sulfuric acid. This indicates that it doesn't interact with any other substances present in the sample, which might cause problems for the analysis. Because they react more easily with other substances in the sample, some acids, like citric or acetic acid, may produce unreliable results.

A cost-effective acid is a sulfuric acid. It is therefore an affordable option for COD analysis. There are more expensive acids, like hydrochloric acid and nitric acid.

It doesn't evaporate easily because it is a non-volatile acid. This is significant because it guarantees that the sample will contain the same amount of sulfuric acid throughout the analysis.

Since the acid is transparent, it is simple to observe how the colour of the indicator used in the COD analysis changes.

As long as it is used correctly, handling this acid is not too dangerous.

Mercury sulphate in COD analysis: 

The dichromate oxidant can oxidise chloride ions, interfering with the COD analysis and producing an unnaturally high COD reading. To remove this interference, mercury sulphate forms a complex with chloride ions that the dichromate does not oxidise.

The following is the reaction that occurs between mercury sulphate and chloride ions:

The mercury chloride complex, 

HgSO4 + 2Cl- → Hg2Cl2 + SO42-

is a very stable compound that is not oxidised by the dichromate. This indicates that the chloride ions do not add to the COD reading because they are essentially "masked" by the dichromate.

The concentration of chloride in the sample determines how much mercury sulphate needs to be added to remove chloride interference. Mercury sulphate-to-chloride ratios of 10:1 are typically enough to eliminate interference.

It is crucial to remember that mercury sulphate is a hazardous material and needs to be handled carefully. It's important to dispose of the waste solution containing mercury sulphate correctly.

There are some more specifics regarding the application of mercury sulphate in COD analysis to remove chloride interference:

Before adding the dichromate oxidant to the sample, the mercury sulphate must be added.

A high temperature of 148–150 °C is required to form the mercury chloride complex in the sample.

Before taking the COD reading, the sample needs to cool.

The standard procedure in the COD analysis involves the removal of chloride interference using mercury sulphate. It is a dependable and practical way to guarantee precise COD readings.

The standard procedure in the COD analysis involves the removal of chloride interference using mercury sulphate. It is a dependable and practical way to guarantee precise COD readings.

There are a few reasons why COD analysis does not make use of chemicals other than mercury sulphate.

Mercury sulphate is a very good mask for interference caused by chlorides. Some substances, like silver sulphate, might not be able to cover up the interference because they are less potent.

The molecule mercury sulphate is comparatively stable. This indicates that it is difficult to decompose, which is crucial because it guarantees that the sample contains the same amount of mercury sulphate throughout the analysis.

Sulphate of mercury is not too expensive. It is therefore an affordable option for COD analysis.

Mercury sulphate, however, is a hazardous material that needs to be handled carefully. It's important to dispose of the waste solution containing mercury sulphate correctly.

The risks that mercury poses to human health and the environment have come to light more recently. Consequently, there is a movement in favour of using mercury-free techniques for COD analysis.

The following are a few substitute substances that have been looked into for use in mercury-free COD analysis:

oxide of silver

Sulphur Dioxide

Tin chloride (IV)

Iodine

While all of these substances are good at hiding chloride interference, they are not without limitations. For instance, tellurium dioxide is more costly and silver oxide is not as stable as mercury sulphate.

Role of Indicator

Ferroin is a complex combination containing iron(II) and 1,10-phenanthroline, which is employed as an indicator in COD analysis. In its reduced state, it has a deep red colour, and in its oxidized state, it has a pale blue tint. The COD analysis of water samples uses this colour shift as an indicator.

An intense oxidizing agent, like potassium dichromate, is applied to a water sample to perform the COD assay. Any organic matter in the sample will be oxidized by the oxidizing agent, and the amount of oxidizing agent employed indicates how much organic matter there is in the sample.

After adding the oxidizing agent, ferroin is added to the sample. Ferroin will change from its red to its blue form when any organic substance is present in the sample and is oxidized by the oxidizing agent.

The ferroin's colour shift can be utilized to calculate the titration's end point. The endpoint is reached when the ferroin has fully oxidized to its blue form and all of the organic stuff in the sample has undergone oxidation.

One accurate and dependable way to measure the amount of organic matter in water samples is to utilize ferroin as an indicator in COD measurement.

Here are some more specifics on ferroin's function as an indicator in COD analysis:

The sample is mixed with a highly diluted solution of ferroin. This is because ferroin when it is not in a diluted solution, decomposes quickly and is unstable in its oxidized form.

The ferroin exhibits a fairly quick colour change. This indicates that the titration's end point is seen.

Chloride ions or any other component in the sample did not affect ferroin. It is therefore a trustworthy signal for COD analysis.

Additional indications for COD analysis include the following:

1,10-Phenanthroline: This is ferroin's parent chemical, although it's employed here in its simpler form. In its reduced form, it has a red colour; in its oxidized form, it has a blue colour.

1,5-diphenylcarbazide: The indicator 1,5-diphenylcarbazide is coloured red in its oxidized state and yellow in its reduced form. Since it is more oxidation-sensitive than ferroin, samples with low quantities of organic matter are frequently treated with it.

Methylthymol blue: This indicator has two colours: blue when it's oxidized and yellow when it's reduced. Since it is less oxidation-sensitive than ferroin, samples with a high concentration of organic materials are frequently treated with it.

The particular application will determine which indicator is best for COD analysis. For instance, ferroin might not be a wise choice if the sample contains a lot of chloride ions since it can be impacted by them. 1,10-phenanthroline or 1,5-diphenylcarbazide would be a better option in this situation.

It is significant to remember that other substances in the sample could interfere with any of these indications. Thus, before doing the COD analysis, it's crucial to utilize a blank sample to look for interferences.

Interference in COD analysis:

 A variety of substances have the potential to cause issues when analyzing COD in water samples. Among the most typical interferences are the following:

Chlorides: The dichromate oxidant used in the COD measurement can oxidize chloride ions, which can result in an unnaturally high COD signal. By adding mercury sulphate to the sample, which combines with the chloride ions to produce a complex that stops them from oxidizing, this interference can be removed.

Sulfites: The dichromate oxidant can also oxidize sulfite ions, which can result in an unnaturally elevated COD measurement. By reducing the sulfite ions to sulphate ions in the sample with sodium thiosulfate, this interference can be removed.

Nitrites: The dichromate oxidant can oxidize nitrites, which might cause an unnaturally high COD value. By converting the nitrites in the sample to nitric acid, sulfuric acid can be added to remove this interference.

Ammonia: Ammonia and the dichromate oxidant can react to produce nitrogen gas, which is undetectable by the COD test. By introducing hydrochloric acid to the sample, which turns ammonia into ammonium chloride, this interference can be removed.

Ferrous ions: The dichromate oxidant can oxidize ferrous ions, which might provide an unnaturally elevated COD measurement. By oxidizing the ferrous ions in the sample to ferric ions with the addition of potassium permanganate, this interference can be removed.

Numerous other substances can also interfere with the COD analysis in addition to these particular interferences. To get reliable COD readings, it's critical to be aware of these interferences and take action to remove them.

Additional advice for avoiding interferences in COD analysis is provided below:

Use distilled water and premium reagents.

Make sure the samples are properly prepared by following the right processes.

To look for interferences, use a blank sample

.

Consistently calibrate the device.

You may contribute to ensuring that your COD measurements are precise and trustworthy by paying attention to these pointers.

A variety of parameters, such as the concentration of ammonia in the sample, the reaction temperature, and the kind of dichromate oxidant used, might affect how much ammonia interferes with COD detection.

Ammonia generally does not cause significant interference in COD measurement because of its poor potential to decrease dichromate. Nevertheless, ammonia and dichromate can react at high amounts to produce nitrogen gas, which the COD analysis won't pick up on. An artificially low COD reading may result from this.

By utilizing a low quantity of dichromate oxidant and heating the reaction mixture to a high temperature, ammonia interference can be minimized. A chemical reagent, such as hydrochloric acid, may also need to be added in some circumstances to change ammonia into ammonium chloride, which is more resistant to dichromate oxidation.

Nitrite and Nitrate as Interference in COD Analysis:

The interference of nitrite and nitrate in COD measurement occurs when nitrite is oxidized to nitrate during the COD test. Because oxygen is used during this oxidation process, the COD measurement is falsely inflated. The concentration of nitrite in the sample determines the extent of interference. In samples where the content of nitrite is more than 2 mg/L, there may be a notable interference.

It is necessary to remove nitrite from the sample before the test to eliminate its interference with COD analysis. Sulfamic acid can be added to the sample to achieve this. Nitrous oxide and water are produced when sulfamic acid and nitrite react; oxygen is not used in the process. Usually, 10 mg of sulfamic acid must be given for every milligram of nitrite in the sample.

The following procedures can help you get rid of nitrite's influence with COD analysis:

Ten milligrams of sulfamic acid should be added for every milligram of nitrite in the sample.

Blend the sample well.

To give the reaction time to finish, let the sample stand for 15 minutes.

To filter the sample if needed.

Run the COD analysis.

Remember that sulfamic acid can also cause issues with COD analysis, so make sure you only use the very minimal amount required. In addition, nitrous oxide, a combustible gas, can be produced by the sulfamic acid reaction. It is crucial to carry out the reaction in a space with enough ventilation as a result.

Here are some more pointers for removing nitrite interference from the COD analysis:

Make use of a COD test kit made especially for samples with high levels of nitrate.

If the concentration of nitrite in the sample is too high, dilute it.

Observe the guidelines provided by the manufacturer for the COD test kit you are using.

By taking these precautions, you can make sure that nitrite interference is removed and that the COD measurement you are receiving is accurate.

Chemical Oxygen Demand's acceptance standards 

Depending on the particular application, different chemical oxygen demand (COD) acceptability criteria apply. But usually, the following standards are applied:

Wastewater: The COD of wastewater is commonly used to evaluate the wastewater's quality and track how well its treatment procedures are working. The exact standards in existence will determine the permissible level of COD in wastewater. Nonetheless, it is generally accepted that a COD level of less than 100 mg/L is acceptable.

Surface water: The COD of surface water is commonly used to evaluate the water's quality and pinpoint possible pollution sources. Depending on how the water is specifically used, different COD levels are allowed for surface water. Nonetheless, it is generally accepted that a COD level of less than 10 mg/L is appropriate.

Drinking water: To make sure the water is safe to drink, the COD is usually measured. For drinking water, a COD level of less than 2 mg/L is usually appropriate.

It is significant to remember that the water's temperature and pH, the presence of specific inorganic compounds, and the technique used to detect COD can all have an impact on the acceptance requirements for the nutrient. It's also critical to remember that various businesses or applications may have varied COD acceptance standards.

Limits on the amount of COD that wastewater can release into natural resources 

The maximum levels of COD in wastewater that can be released into saltwater vary according to the nation or area. That being said, the standard limit is usually established at 250 mg/L. This is due to the potential harm that elevated COD levels may do to the marine ecosystem.

The amount of oxygen needed to chemically oxidize the organic stuff in wastewater is measured by the COD. When large concentrations of COD wastewater are dumped into saltwater, the organic matter in the wastewater can break down and take up oxygen in the surrounding water. This may cause the water's oxygen content to drop, which could be harmful to marine life.

Fish deaths, for instance, may result from low oxygen levels. Low oxygen levels can also make it difficult for marine life to breathe, which can have an effect on their development and ability to reproduce.

Reducing the amount of COD discharged into saltwater is crucial for the preservation of the marine ecosystem. We can contribute to maintaining healthy water oxygen levels and safeguarding the marine environment by lowering the quantity of COD in wastewater.

The following are some negative effects of high COD levels in saltwater:

Reduced oxygen levels: As was previously indicated, excessive COD concentrations can eat up oxygen in the water, resulting in a reduction in oxygen levels. Given that many organisms depend on oxygen to exist, this could have a detrimental effect on marine life.

Increased eutrophication: An excessive amount of nutrients, such as phosphorus and nitrogen, can enter a body of water and cause eutrophication. Algae and other water plants may thrive as a result, but they may ultimately wither and disintegrate. The oxygen in the water may be consumed by this decomposition process, which could have the same detrimental effects as previously stated.

Beach and shoreline pollution: Elevated COD levels have the potential to cause contamination on beaches and shorelines. This is due to the fact that the organic stuff in wastewater might wash up on shorelines and beaches, where it will break down and generate odorous fumes. This may negatively affect the marine species that reside in these areas and make beaches and shorelines uncomfortable for people to utilize.

The limitations of COD in wastewater that can be discharged into saltwater are subject to change, it is crucial to remember this. Depending on the particular site and the kind of wastewater being released, the limitations could change. Nonetheless, the 250 mg/L guideline is often a reasonable place to start when it comes to safeguarding the marine environment.

Why the limits of COD in wastewater to discharge into natural Resources is typically set at 250 mg/L?

The standard threshold of 250 mg/L for COD in wastewater intended for ocean discharge is established for several reasons.

For most aquatic creatures, it is a safe level. The majority of marine species can withstand COD levels up to 250 mg/L, according to studies. It is crucial to maintain COD levels below this threshold because greater levels can begin to negatively affect marine life.

It's a level that can be attained using practical therapeutic methods. Wastewater levels of COD can be lowered using a variety of treatment techniques. The majority of wastewater treatment facilities can use these reasonably priced technologies.

It is at a level that is compliant with global norms. The maximum amount of COD in wastewater that can be released into saltwater is 250 mg/L, as defined by numerous nations and international organizations. This makes it easier to protect the marine environment by ensuring that wastewater treatment plants across national borders are operating under the same standards.

Naturally, depending on the particular location and kind of wastewater being discharged, there may be differences in the precise limit of COD in wastewater to be discharged into saltwater. And yet, the 250 mg/L limit is a solid place to start when it comes to safeguarding the marine ecosystem.

When determining the maximum level of COD in wastewater that can be discharged into saltwater, the following other considerations may be taken into account:

The type of marine life that exists there: Different marine species are more or less vulnerable to COD. In regions with delicate marine life, the COD limit might be less than 250 mg/L.

The volume of wastewater released:  The limit of COD that may be required to safeguard the marine environment increases with the volume of wastewater that is released.

The seawater's natural COD concentrations: The limit will also be impacted by the seawater's inherent COD levels. It could be necessary to set a limit greater than 250 mg/L in regions with high natural COD levels.

Finally, the amount of COD in wastewater that can be discharged into saltwater is determined by striking a compromise between treatment costs and safeguarding the marine environment. Although the 250 mg/L limit is a reasonable place to start, it could need to be changed based on the particular site and the kind of wastewater being released.