COD Test - Test for Chemical Oxygen Demand In Wastewater

COD Test (Chemical Oxygen Demand test) is a water quality test. It is used to determine the amount of oxygen required to chemically oxidize the organic and inorganic pollutants present in a water sample.

It is based on oxidation of pollutants by a strong chemical oxidant. Generally potassium dichromate (K2Cr2O7) is used in heated and highly acidic condition. In this process the pollutants is broken down into carbon dioxide (CO2) and water (H2O). The amount of oxidant consumed in this reaction gives indirect but accurate idea about concentration of organic contamination in the water sample.

This test is widely used in environmental and industrial facilities. It is used to assess pollution level of wastewater surface water and industrial effluents. High COD value indicates severe pollution and it can deplete oxygen in aquatic ecosystem. It is therefore used for monitoring environmental health and also for evaluating efficiency of wastewater treatment plant.

One of main advantage of COD test is its speed. The test is completed within few hours and it helps in making rapid adjustment in water treatment process without waiting for many days.

Principle of COD Test

Principle of COD Test is based on measuring the oxygen required to chemically oxidize the organic and inorganic pollutants present in water sample. It is the process where a known excess amount of strong oxidizing agent is added to the sample and the consumption of oxidant is measured.

In this test the sample is boiled (refluxed) in strongly acidic solution. Generally sulphuric acid (H2SO4) is used and potassium dichromate (K2Cr2O7) is taken as oxidant. During digestion heat is applied and silver sulphate (Ag2SO4) is added as catalyst so that resistant organic compound are also oxidized.

Mercuric sulphate (HgSO4) is also added in this step. It is used to mask chloride ion which can interfere in oxidation process and it gives wrong COD value. In digestion phase the organic carbon is oxidized to carbon dioxide (CO2) and water (H2O) and it causes reduction of dichromate in proportional manner.

After completion of digestion the remaining unreacted dichromate is estimated. It is measured by titration using ferrous ammonium sulphate (FAS) or by colorimetric and spectrophotometric methods. The amount of dichromate consumed is used to calculate oxidizable matter of sample in terms of oxygen equivalent.

Objectives

Purpose or objective of COD Test are as follows–

It is used for assessing water pollution level. It is used to measure concentration of chemically oxidizable organic and inorganic pollutant in surface water groundwater and industrial wastewater.
It is used for evaluating environmental impact. It is used to determine oxygen depleting potential of wastewater effluent and it indicate how much dissolved oxygen will be consumed after discharge in aquatic environment.

It is used for monitoring efficiency of treatment plant. It is applied to measure waste loading and to evaluate daily performance and overall purification efficiency of wastewater treatment process.
It is used for real time process control. COD test gives rapid result (within about 2–3 hour) so immediate daily adjustment in treatment operation can be done.
It is used for regulatory compliance. COD is used as discharge standard parameter so that effluent meets environmental regulation and discharge limit.

It is used in environmental modeling and design. COD value is used in engineering design of wastewater treatment plant and also for environmental modeling.
It is used for estimating BOD. COD gives rapid rough estimate of BOD and it helps in selecting proper sample volume for actual 5 day BOD test.

Solutions and other material required

Solutions and other material required for COD Test are as follows–

Chemical reagents and solutions

Potassium dichromate (K2Cr2O7) – It is the strong oxidizing agent used in known excess for oxidation of organic matter.
Concentrated sulphuric acid (H2SO4) – It is used to provide highly acidic medium and it also increases boiling point.
Silver sulphate (Ag2SO4) – It is used as catalyst for oxidation of resistant organic compounds.

Mercuric sulphate (HgSO4) – It is used as masking agent to bind chloride ion and avoid interference.
Silver nitrate (AgNO3) – It is sometimes used for precipitation of chloride in highly saline sample.
Ferrous ammonium sulphate (FAS or Mohr’s salt) – It is used as standard titrant for remaining unreacted dichromate.

Ferroin indicator – It is added during titration and it gives colour change at end point.
Potassium hydrogen phthalate (KHP) – It is used as primary standard to check accuracy and calibration.
Sulfamic acid (optional) – It is used when nitrite interference is present in sample.

Apparatus and equipment

Digestion or reflux apparatus – It include reflux flask with condenser or closed reflux tubes or vials (with Teflon lined cap).
Heating device – Hot plate or reactor heating block is used to maintain about 150°C for 2 hour digestion.
Spectrophotometer or colorimeter – It is required when COD is measured by colorimetric or photometric method.

Standard laboratory glassware – Burette pipette and other glassware are required and boiling aid (rough glass bead) is also used to prevent bumping.

Procedure of COD Test

Procedure of COD Test are as follows–

The water sample is taken and it is homogenized (by blender) so that suspended solid size is reduced and sample becomes uniform.
COD reactor block or digestion apparatus is preheated at 150°C.

A measured volume of sample is transferred into digestion flask or sealed digestion vial.
Potassium dichromate (K2Cr2O7) is added as oxidant. Sulphuric acid (H2SO4) containing silver sulphate (Ag2SO4) is added as catalyst and mercuric sulphate (HgSO4) is added for masking of chloride ion. (In closed reflux vial these reagent are already present).
The content is mixed properly by swirling the flask or by inverting the vial several times.

The mixture is digested by heating at 150°C for 2 hour. In this step the organic matter is oxidized.
After digestion the vessel is removed and it is cooled to room temperature.
The COD value is measured by any one method.
- Colorimetric method – The vial is placed in colorimeter or spectrophotometer and absorbance is measured (at 420 nm or 600 nm).
- Titrimetric method – Ferroin indicator is added and unreacted dichromate is titrated with standard ferrous ammonium sulphate (FAS) till colour changes from blue green to reddish brown.
Reagent blank is also run simultaneously. Distilled or deionized water is taken as blank and it is treated in same manner. COD concentration is calculated by comparing sample reading with blank.

Calculation and result of COD Test

Determining the result

Titration method – The unreacted excess dichromate is measured by titration. The digested sample is titrated with ferrous ammonium sulphate (FAS) using ferroin indicator. The end point is obtained when colour changes from blue green to reddish brown.
Colorimetric method – The absorbance change is measured by colorimeter or spectrophotometer. It is measured either by increase of green trivalent chromium (Cr3+) at 600 nm or by decrease of orange hexavalent chromium (Cr6+) at about 420 nm or 350 nm.

Calculation (Titrimetric method)

The COD value is calculated by following formula–

COD (mg O2/L) = ((A – B) × M × 8000) / mL of sample

Where,
A = Volume of FAS used for blank (mL).
B = Volume of FAS used for sample (mL).
M = Molarity (or normality) of FAS solution.
8000 = Conversion factor (8 × 1000) for oxygen equivalent.
mL of sample = Volume of water sample taken for test.

Result

The final COD result indicate amount of oxygen required to oxidize the pollutants present in sample. It is expressed in mg/L or mg O2/L.

Uses of COD Test

Uses of COD Test are as follows–

It is used for assessing water pollution level. It is used to measure concentration of organic and inorganic contaminant in lakes river stream rainwater and groundwater.

It is used for monitoring efficiency of wastewater treatment plant. The waste loading and daily performance of municipal and industrial plant is evaluated by COD test.
It is used for real time process control. COD test is completed within few hours and it gives rapid feedback as compared to BOD test (5 day).
It is used for evaluating environmental impact. It indicate oxygen depleting potential of wastewater discharge and it shows how much dissolved oxygen will be consumed in aquatic ecosystem.

It is used for estimating BOD. After establishing relationship COD result is used for rough estimation of BOD or for deciding suitable sample volume for BOD test.
It is used for determining biodegradability of wastewater. COD/BOD ratio is used as diagnostic tool and it indicate proportion of non biodegradable or toxic matter in wastewater and it helps in selecting treatment technology.
It is used for regulatory compliance. It is taken as standard parameter for environmental modeling wastewater treatment plant design and for checking effluent discharge within legal limit.

Limitations of COD Test

Limitations of COD Test are as follows–

It does not distinguish types of organic matter. COD measures total chemically oxidizable matter so biodegradable and non biodegradable (refractory) organic material is not separated. It also does not give information about biological stabilization rate of wastewater.
Some organic compound are not completely oxidized. Pyridine and its derivatives and some aromatic hydrocarbon are oxidized partially or not oxidized so COD value can be underestimated. Some volatile organic substance may evaporate before reaction so lower COD is obtained.
Chloride ion interference is major problem. Dichromate can oxidize chloride ion and it gives falsely high COD. Chloride also reacts with silver sulphate catalyst and precipitate is formed which reduce catalytic activity.

Highly saline sample are difficult to test. Mercuric sulphate is used for masking but it becomes insufficient when chloride concentration is very high (about more than 1000–2000 mg/L) so seawater brackish water and high chloride wastewater gives inaccurate COD without dilution or modification.
Other inorganic species also interfere. Nitrite sulphide ferrous iron and manganous manganese are also oxidized in COD test and it gives false positive oxygen demand unless correction is done or specific reagent (sulfamic acid for nitrite) is added.
Hazardous chemical are required in COD test. Potassium dichromate (hexavalent chromium) concentrated sulphuric acid silver sulphate and mercuric sulphate are toxic and corrosive reagent.

Hazardous waste is generated. Heavy metal containing waste is produced and disposal is difficult and costly and it can also cause secondary environmental pollution.

What is the difference between BOD and COD in wastewater?

Difference between BOD and COD in wastewater are as follows–

Full form – BOD is Biochemical Oxygen Demand. COD is Chemical Oxygen Demand.

Basic principle – In BOD the oxygen is consumed by living microorganism during biological degradation of organic matter. In COD the organic matter is oxidized chemically by strong oxidizing agent (generally potassium dichromate) in acidic medium with heating.
Type of matter measured – BOD measures only biodegradable organic matter present in sample. COD measures total chemically oxidizable matter including biodegradable and non biodegradable (refractory) substance.
Value comparison – COD value is generally higher than BOD value because more compound are oxidized in COD test.

Time required – BOD test requires 5 day for completion. COD test is completed within few hour (about 1.5–3 hour).
Use in plant control – BOD gives slow result so it is not suitable for immediate process adjustment. COD gives rapid result so it is used for daily monitoring and real time operational control.
Effect of toxic substance – BOD test is affected by toxic chemical (heavy metal cyanide etc). These toxin can poison bacteria and it gives falsely low BOD. COD test is not much affected by toxic chemical so it gives reliable result for industrial effluent.

Significance of COD/BOD ratio – COD/BOD ratio is used to determine biodegradability of wastewater. If COD is much higher than BOD (more than twice) then non biodegradable organic matter is present in high amount and it is not removed by normal biological treatment process.

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COD Test – Test for Chemical Oxygen Demand In Wastewater