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COD procedure for wastewater

In this COD procedure for wastewater post we have briefly explained about COD analysis principle, COD procedure, requirements, and results of COD procedure.

COD procedure for wastewater

Under precise conditions of oxidizing agent, temperature, and time, the Chemical Oxygen Demand (COD) procedure calculates the amount of oxygen required to oxidise the organic materials in a waste sample.

The result has no definite relationship to the waste’s Biochemical Oxygen Demand (BOD) or Total Organic Carbon (TOC) level because the test uses a specialized chemical oxidation. The test result should be regarded as a separate assessment of organic matter in the sample rather than a replacement for the BOD or TOC tests.

Principle

The organic matter in the water sample is first oxidised with a known volume of potassium dichromate, and then excess oxygen is allowed to react with potassium iodide, liberating iodine in amounts equal to the excess oxygen, which is measured titrimetrically using sodium thiosulphate as an indicator.

Requirements

Reagents Required

1. Water sample

2. Potassium dichromate solution (0.1N)

3. Sodium sulphate (0.1N)

4. Potassium iodide solution                  

5. Starch solution (1%)

Materials Required

1. Water bath

2. Titration apparatus

3. 100ml conical flask

Sample Collection

If possible, collect the samples in glass vials. Plastic containers may be used if no organic pollutants are known to be present in the containers. Samples that are biologically active should be analysed as soon as possible.

To allow for the removal of representative aliquots, samples containing settle able material should be well mixed, ideally homogenised. Samples should be stored in sulfuric acid with a pH of 2 and kept at 4°C until they are analysed.

COD procedure

1. A waste water sample is collected. Ten millilitres of waste water sample were poured into three 100 millilitre conical flasks labelled Test1, Test2, and Test3.

2. Simultaneously, 100mL conical flasks designated Blank1, Blank2, and Blank3 are filled with distilled water.

3. In each of the six conical flasks, 5mL potassium dichromate solution was added. For 1 hour, place the flask in a water bath at 100°C (boiling temperature).

4. Allow 10 minutes for the samples to cool. In each flask, 5 mL potassium iodide was added in each flask, 10 mL sulphuric acid was added.

5. 0.1N Sodium Thiosulphate was added to the contents of each flask until the blue shade disappeared completely.

Result

COD procedure is expressed in mg/L, which indicates the mass of oxygen consumed per liter of solution. Unlike for the BOD test, toxic compounds (such as heavy metals and cyanides) in the samples to be analysed do not have an effect on the oxidants used in the COD procedure test.

Therefore, the COD procedure test can be used to measure the strength of wastes that are too toxic for the BOD test. Some organic molecules (e.g., benzene, pyridine) are relatively resistant to dichromate oxidation and may give a falsely low COD.

Interference

Organic substance from the glassware or the environment can generate a gross, positive mistake. Organic compounds should be avoided in the distilled water used for reagent preparation or sample dilution with extreme caution.

When the sample temperature rises during the sulfuric acid addition process, volatile components may be lost. The flask should be chilled before adding the sulfuric acid solution to reduce this loss.

Dichromate oxidises chlorides quantitatively, resulting in a positive interference. In all but brine and estuarine samples, mercuric sulphate is added to the digestion flask to complex the chlorides, thereby eliminating interference.

Further Readings

Reference

  1. https://realtechwater.com/parameters/chemical-oxygen-demand/
  2. https://www.hach.com/parameters/chemical-oxygen-demand
  3. https://www.h2olabcheck.com/blog/view/chemical-oxygen-demand
  4. https://pubmed.ncbi.nlm.nih.gov/11558305/
  5. https://www.lar.com/products/lar/cod-analysis/cod-chemical-oxygen-demand/