Biological analysis of water involves measuring parameters that reflect the presence and activity of biological organisms and the effects of organic and inorganic substances on water quality. Three critical parameters often analyzed are dissolved oxygen (DO), biochemical oxygen demand (BOD), and chemical oxygen demand (COD).

1. Dissolved Oxygen (DO)

Dissolved Oxygen is the amount of oxygen present in water, essential for the survival of aquatic organisms.

• Measurement Methods:

  • Winkler Titration Method:
    • Procedure:
      1. Add manganese sulfate and alkaline iodide to a water sample, which reacts with oxygen to form a brown precipitate.
      2. Add sulfuric acid to dissolve the precipitate, releasing iodine equivalent to the amount of dissolved oxygen.
      3. Titrate the released iodine with sodium thiosulfate using a starch indicator to determine the DO concentration.
    • Significance: Provides accurate results and is a standard method for DO measurement.
  • Electrochemical Methods:
    • Membrane Electrode (Clark Electrode): Measures oxygen diffusion through a membrane.
    • Optical DO Sensors: Use fluorescence quenching by oxygen to measure concentration.
    • Significance: Provides real-time measurements and is useful for continuous monitoring.
  • Units: Milligrams per liter (mg/L) or parts per million (ppm).

• Significance:

  • Ecological: Essential for the respiration of aquatic organisms; low DO levels (hypoxia) can lead to fish kills and affect biodiversity.
  • Water Quality: Indicates the balance between oxygen-producing and oxygen-consuming processes, such as photosynthesis and decomposition.

2. Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand measures the amount of oxygen required by microorganisms to decompose organic matter in water.

• Measurement Methods:

  • Standard 5-Day BOD Test:
    • Procedure:
      1. Collect a water sample and incubate it in the dark at 20°C for five days.
      2. Measure the DO level before and after incubation.
      3. The difference in DO levels indicates the amount of oxygen consumed by microorganisms.
    • Dilution: High BOD samples may need dilution to bring oxygen consumption within measurable limits.
  • BOD Bottles: Special bottles designed to minimize air exchange and prevent DO contamination.
  • Units: Milligrams of oxygen consumed per liter of water (mg/L).

• Significance:

  • Water Pollution: High BOD indicates high levels of organic pollution, which can deplete oxygen and harm aquatic life.
  • Wastewater Treatment: Used to assess the effectiveness of treatment processes in reducing organic matter.

3. Chemical Oxygen Demand (COD)

Chemical Oxygen Demand measures the total amount of oxygen required to oxidize both organic and inorganic substances in water.

• Measurement Methods:

  • Open Reflux Method:
    • Procedure:
      1. Digest the water sample with a strong oxidizing agent (potassium dichromate) and sulfuric acid under heat.
      2. Measure the remaining oxidizing agent by titration with ferrous ammonium sulfate using a ferroin indicator.
    • Significance: Can handle samples with high levels of organic matter.
  • Closed Reflux Method:
    • Procedure:
      1. Similar to the open reflux method but performed in a sealed container to prevent volatilization of organic compounds.
      2. Measure the remaining dichromate spectrophotometrically.
  • Colorimetric Method:
    • Procedure: The digestion product's color intensity is measured using a spectrophotometer.
  • Units: Milligrams of oxygen per liter (mg/L).

• Significance:

  • Pollution Monitoring: COD provides a rapid and comprehensive measure of water pollution by organic and inorganic compounds.
  • Treatment Efficiency: Used to assess the performance of wastewater treatment plants in removing organic pollutants.

Detailed Measurement Techniques and Significance

1. Dissolved Oxygen (DO) Measurement

   • Winkler Titration Method:
     • Steps: Manganese sulfate and alkaline iodide-azide are added to the sample, forming a precipitate. Sulfuric acid is then added to dissolve the precipitate, releasing iodine equivalent to the oxygen concentration. The released iodine is titrated with sodium thiosulfate.
     • Significance: Accurate but requires careful handling of reagents.
   • Electrochemical Methods:
     • Clark Electrode: Measures the current produced by the reduction of oxygen at the cathode.
     • Optical Sensors: Measures the quenching of fluorescence by oxygen.
     • Significance: Provides real-time, continuous monitoring suitable for various applications.

2. Biochemical Oxygen Demand (BOD) Measurement

   • Standard 5-Day BOD Test:
     • Procedure: Measure initial DO, incubate the sample for five days at 20°C, and measure the final DO.
     • Significance: Reflects the amount of biodegradable organic matter, indicating pollution levels.
   • BOD Bottles: Ensures accurate measurements by preventing oxygen exchange with the atmosphere.

3. Chemical Oxygen Demand (COD) Measurement

   • Open Reflux Method:
     • Procedure: Sample digestion with potassium dichromate and sulfuric acid under heat, followed by titration.
     • Significance: Measures both biodegradable and non-biodegradable substances, providing a comprehensive pollution assessment.
   • Closed Reflux Method:
     • Procedure: Similar to the open reflux but in a sealed container, followed by spectrophotometric measurement.
     • Significance: Prevents loss of volatile organic compounds, ensuring more accurate results.
   • Colorimetric Method: Measures the color intensity of the digestion product using a spectrophotometer.