Chemical oxygen demand, or COD, indicates the amount of oxygen that can be used by processes in a measured solution. It is often expressed as the mass of oxygen utilised per unit of solution volume, or milligrammes per litre (mg/L) in SI units. The most common application of COD is determining the quantity of oxidizable pollutants present in surface water (such as lakes and rivers) or wastewater. Chemical Oxygen Demand is useful for analysing the quality of water since it provides a metric for determining the impact of an effluent on the receiving body.

What is Chemical Oxygen Demand (COD)?

• Chemical oxygen demand refers to the amount of oxygen required for the chemical oxidation of organic and inorganic components present in wastewater with oxidising agents such as potassium permanganate, potassium dichromate, etc.
• When COD is present, the chemical oxidation of organic matter is accelerated without the need for extra equipment.
• This is the only approach for determining the organic load in very toxic sewage.
• To measure COD, the water sample is placed in a closed container and subjected to a potent oxidant, such as potassium dichromate and sulfuric acid, for the specified duration and temperature.
• The COD is the quantity of oxygen required by both the inorganic and organic components of the wastewater sample.
• When treated wastewater is put back into the environment, it can put organic materials into the water nearby.
• High COD concentrations in wastewater indicate the presence of organic substances that may lower the amount of dissolved oxygen.
• Typically, this has negative impacts on the environment and regulatory obstacles.
• Oxygen demand is a critical metric for analysing the impact of organic pollution in water and, eventually, reducing its amount.

Measurement of COD

• There are a number of approaches that can be used to calculate COD.
• Online testing and off-line laboratory approaches employing environmental analyzers are examples.
• In the presence of a powerful oxidising agent in an acidic environment, virtually any organic component will oxidise to carbon dioxide, according to the COD testing method.
• The COD analysis will determine the equivalent oxygen concentration required to chemically oxidise organic molecules in water.
• Environmental analyzers are sophisticated scientific instruments utilised in contemporary COD testing processes.
• The EasyPREP COD-200 is a technological pioneer in this field. COD sample mixes are heated, cooled, and then evaluated throughout this procedure.

Methods to Reduce COD in Wastewater

There are many ways that have been shown to reduce COD in the management of wastewater. Wastewater Separation (coagulation and flocculation) and COD removal by microbial action are two of the most common ways to get rid of COD in wastewater.

Wastewater Separation

• Using coagulation and flocculation during the treatment of wastewater, wastewater separation techniques remove colloidal particles.
• During coagulation, a non-toxic agglomerating agent is added to the water to clump together all the suspended particles so that they may be easily filtered out of the wastewater. Such agents include ferric chloride (FeCl) and alum, among others.
• By creating bigger particles, or flocs, flocculation uses a chemical polymer (flocculating agent) to remove clumped particles from water.
• After being placed in a sedimentation tank for extra treatment prior to disposal, flocs undergo sedimentation in the wastewater treatment process.
• By eliminating organic materials from wastewater with coagulants and flocculants, the competition for dissolved oxygen between marine life and bacteria is lessened, as microorganisms no longer have “food” to thrive.

By Microbial Action

• Another successful method for COD removal is the addition of bacteria or other microbes that degrade organic components in wastewater.
• There are both aerobic and anaerobic bacteria in sewage treatment.
• Introducing bacteria or other microorganisms that decompose organic waste-derived compounds into carbon dioxide and water when air is present is aerobic COD removal.
• Ideal COD treatment for wastewater with COD levels below 3000 mg/L is aerobic COD treatment.
• For anaerobic COD removal, microorganisms are used to turn organic waste components into biomass in the absence of oxygen.
• This technique produces ethanol that may be utilised as an alternative energy source for power, heating, and drying, making it a highly useful method.
• COD levels more than 2,000 mg/L are suitable for anaerobic COD treatment.

COD – Uses

COD is a key indicator of water quality and is applied in a variety of applications, including the following:

• Chemical Oxygen Demand measures are used to quantify the biodegradable component of wastewater effluent; COD measurements are also used as an estimate of the size of a wastewater treatment facility required for a specific area.

Challenges Associated with COD Monitoring

• Due to the two-hour delay between delivery to the laboratory and testing, environmental harm can occur before the results are available.
• The exam is expensive and time-consuming.
• The test entails the use of hazardous chemicals that must be disposed of properly and may constitute a risk to the operators.
• It does not simulate natural processes (for example, the test involves an artificial incubation with a strong oxidising agent).
• It is inaccurate and has a low limit of detection. Therefore, it is inapplicable to river samples that are clean and uncontaminated.

Determination of Chemical Oxygen Demand of Wastewater

Principle

In the presence of sulfuric acid, silver sulphate, and mercury sulphate, potassium dichromate oxidises the organic matter contained in the water sample to produce carbon dioxide (CO2) and water (H2O). The amount of potassium dichromate utilised is determined by comparing the volumes of ferrous ammonium sulphate consumed during blank and sample titrations. The amount of potassium dichromate used in the reaction is equivalent to the amount of oxygen (O2) required to oxidise the wastewater’s organic content.

Requirement

Preparation of Potassium dichromate (K2Cr2O7) Solution

1. Add 6,13 g of dried at 105 °C for at least two hours potassium dichromate to 800 ml of distilled water.
2. Shake the flask vigorously to dissolve the contents, then add 1000 ml of water and combine thoroughly.

Preparation of Silver sulfate-Sulfuric acid Solution

1. Dissolve 10 grammes of silver sulphate (Ag2SO4) in 500 millilitres of concentrated sulfuric acid and bring the total volume to one thousand millilitres.
2. Allow the solution to stand for 24 hours before to use.

Preparation of Mercury sulfate Solution

• Dissolve with care 0.1 grammes of HgSO4 in 5 millilitres of sulfuric acid.

Preparation of Ferrous ammonium sulfate Solution (0.025 M)

1. Dissolve 9.8 grammes of ferrous ammonium sulphate in 100 millilitres of distilled water and 20 millilitres of sulfuric acid.
2. Cool the solution and dilute it with 1,000 millilitres of distilled water.
3. Determine the actual concentration of the solution in order to compute the chemical oxygen demand by standardising it.

Preparation of Ferroin Indicator

1. 400 ml of purified water should be mixed with 3.5 grammes of Iron Sulfate heptahydrate and 7.5 grammes of Phenanthroline monohydrate.
2. Mix thoroughly to dissolve, and add enough pure water to reach 500 ml.

Test Procedure for Chemical Oxygen Demand:

1. Place 10 ml of sample in a flask with a circular bottom.
2. Add some glass beads to keep the solution from contacting the flask while it is being heated.
3. Mix 1 ml of Mercury sulphate (HgSO4) solution by rotating the flask.
4. Add 5 ml of a solution of potassium dichromate (K2CrO7).
5. Now add 15 ml of Silver sulfate-sulfuric acid solution slowly and cautiously.
6. Connect the reflex condenser and digest the contents for two hours on a hot plate.
7. After digestion, rinse the condenser with 25 ml of distilled water collected in the same flask.
8. Add 2-4 drops of ferroin indicator to the flask and titrate to the endpoint with 0.025 M ferrous ammonium sulphate solution.
9. Prepare the blank in the same manner as the sample, substituting distilled water for the sample.

Calculation

Determine the chemical oxygen requirement using the following formula:

COD = 8x1000xDFxMx(VB – VS)/Volume of sample (in ml)

Where,

• DF – Factor of Dilution (if applicable)
• M  – Molarity of a standard solution of Ferrous Ammonium Sulfate
• VB – Volume consumed during titration with blank solution
• VS –  Volume utilised during titration sample preparation

Example Calculation

Sample (VS) ferrous ammonium sulphate volume = 23,8 ml

Ferrous ammonium sulphate volume for Blank VB) = 25.6 ml

1 is the Dilution Factor (DF) (sample used as it is)

COD = 8x1000x1x0.025x(25.6-23.8)/10

 = 8000×0.025×1.8/10

 = 360/10 = 36 mg/lit or ppm

References

• Hu, Z., & Grasso, D. (2005). WATER ANALYSIS | Chemical Oxygen Demand. Encyclopedia of Analytical Science, 325–330. doi:10.1016/b0-12-369397-7/00663-4 
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