Sewage Treatment Processes

Why it matters

Sewage treatment processes are essential for managing wastewater generated from domestic, industrial, and commercial sources. Proper treatment ensures that harmful pollutants are removed before the water is released back into the environment, protecting public health and ecosystems.

Key ideas

• Primary Treatment: Primary sedimentation removes settleable solids; preliminary screening and grit removal normally precede it.
• Secondary Treatment: Biological processes are used to degrade organic matter. Common methods include activated sludge processes and trickling filters.
• Tertiary Treatment: Advanced treatment methods to remove remaining inorganic compounds and pathogens. Techniques include filtration, lagooning, and nutrient removal.
• Sludge Treatment: The byproduct of sewage treatment, sludge, is treated through digestion, dewatering, and disposal.
• Disinfection: Final step to kill pathogenic microorganisms, often using chlorine or UV radiation.

Formulas

• BOD_5 = (D_0 - D_5) / P
  • BOD_5: Biochemical Oxygen Demand over 5 days (mg/L)
  • D_0: Initial dissolved oxygen (mg/L)
  • D_5: Dissolved oxygen after 5 days (mg/L)
  • P: Decimal sample fraction V_sample/V_total (dimensionless), the inverse of the dilution factor

Worked example

Problem: Calculate the BOD_5 of a wastewater sample with an initial dissolved oxygen of 9 mg/L and a dissolved oxygen of 3 mg/L after 5 days. Use an undiluted valid unseeded test at 20°C in the dark, so the sample fraction P = 1 and no seed correction is needed.

1. Identify given data:

   • Initial dissolved oxygen, D_0 = 9 mg/L
   • Dissolved oxygen after 5 days, D_5 = 3 mg/L
   • Sample fraction, P = 1

2. Apply the formula:

   • BOD_5 = (D_0 - D_5) / P

3. Substitute the values:

   • BOD_5 = (9 - 3) / 1 = 6 mg/L

4. Final answer: 6 mg/L

Common mistakes

• Confusing BOD with COD (Chemical Oxygen Demand).
• Incorrectly calculating the dilution factor.
• Neglecting the units in calculations, leading to errors.

For GATE CE

• Focus on understanding the different stages of sewage treatment and their purposes.
• Practice calculations involving BOD, COD, and other key parameters.
• Be familiar with the design and operation of treatment units like sedimentation tanks and aeration basins.

Quick check

1. What is the primary purpose of secondary treatment in sewage treatment?
2. Name one method used in tertiary treatment.
3. What does BOD stand for?

Answers: 1. To degrade organic matter biologically. 2. Filtration. 3. Biochemical Oxygen Demand.