How to measure COD in raw collection

 Measuring COD (Chemical Oxygen Demand) in raw sewage involves quantifying the amount of oxygen required to oxidize both organic and inorganic matter chemically. COD testing is faster than BOD testing and provides a snapshot of the total pollution load in wastewater. Here's how to measure COD in a raw collection system:

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1. Collecting the Sample

• Sample Collection:
  • Use clean, glass or plastic containers to collect raw sewage.
  • Avoid excessive aeration or agitation during collection to prevent oxidation of organic matter.
• Preservation:
  • If testing cannot be done immediately, store the sample at 4°C to prevent microbial activity.
  • Analyze within 24 hours to ensure accuracy.

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2. Preparing the Reagents

• Reagents Required:
  1. Potassium Dichromate (K₂Cr₂O₇):
     • Strong oxidizing agent to oxidize organic matter.
  2. Sulfuric Acid (H₂SO₄):
     • Provides an acidic environment and prevents volatile organic compounds from escaping.
  3. Silver Sulfate (Ag₂SO₄):
     • Catalyst for oxidation of certain organic compounds.
  4. Mercuric Sulfate (HgSO₄):
     • Suppresses chloride interference (important in sewage with high chloride content).
  5. Ferroin Indicator or Titration Standard:
     • Used for back titration to measure excess oxidizing agent.

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3. Digestion Process

1. Dilute the Sample:
   • If COD levels are expected to be very high, dilute the sample to bring it within the testable range.
2. Add Reagents:
   • Pipette a measured volume of the raw sewage sample (e.g., 2 mL) into a digestion tube or flask.
   • Add a measured amount of potassium dichromate and sulfuric acid mixture with silver sulfate.
   • Add mercuric sulfate to suppress chloride interference if necessary.
3. Sealing:
   • Seal the digestion tube or reflux flask tightly to prevent evaporation.

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4. Digestion (Oxidation of Organics)

• Heat the sample at 150°C in a COD digestion unit or reflux apparatus for 2 hours.
• The digestion process oxidizes organic and inorganic matter, reducing potassium dichromate to chromium (III).

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5. Titration or Spectrophotometry

Titration Method:

1. After digestion, cool the sample to room temperature.
2. Add a ferroin indicator.
3. Titrate the sample with ferrous ammonium sulfate (FAS) to measure the unreacted potassium dichromate.

Spectrophotometric Method:

1. Measure the absorbance of the digested sample at a specific wavelength (e.g., 600 nm) using a spectrophotometer.
2. Compare the results against a calibration curve to determine COD concentration.

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6. Calculating COD

For the Titration Method, use the following formula:

$$\text{COD (mg/L)} = \frac{(a - b) \times N \times 8000}{\text{Sample Volume (mL)}}$$

Where:

• $a$ = Volume of FAS used for blank (mL)
• $b$ = Volume of FAS used for the sample (mL)
• $N$ = Normality of FAS

For Spectrophotometric Method, calculate COD directly from the calibration curve.

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Example Calculation

• Sample Volume: 2 mL
• Volume of FAS for Blank ($a$): 10.0 mL
• Volume of FAS for Sample ($b$): 5.0 mL
• Normality of FAS: 0.1 N

$$\text{COD} = \frac{(10 - 5) \times 0.1 \times 8000}{2} = 200 \, \text{mg/L}$$

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Precautions

1. Chloride Interference:
   • Raw sewage often has high chloride concentrations. Use mercuric sulfate to prevent overestimation of COD.
2. Safety:
   • Handle chemicals (e.g., sulfuric acid and potassium dichromate) with care, as they are corrosive and toxic.
3. Calibration:
   • Ensure instruments and reagents are calibrated and standardized before testing.
4. Replicates:
   • Test multiple samples for accuracy and reproducibility.

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Advanced Methods

• Use COD Test Kits:
  • Pre-packaged kits simplify the process and include reagents in pre-measured quantities.
• Use Automated COD Analyzers:
  • Provide faster and more accurate results with minimal manual intervention.

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Conclusion

COD measurement is a critical tool for assessing the organic and inorganic load in raw sewage. The process requires careful handling of reagents and equipment, but it provides quick and reliable data for wastewater treatment system design and performance monitoring.