Sewage Treatment in Environmental Engineering – Ultimate Guide for JKSSB JE Civil 2025

🗕️ Updated: July 2025
📌 Topic Category: Environmental Engineering
📘 Target Audience: JKSSB JE Civil, SSC JE, RRB JE, AE/JE State Exam Aspirants

📖 Introduction

Sewage treatment, also referred to as wastewater treatment, is a systematic process designed to remove physical, chemical, and biological contaminants from wastewater, making it suitable for safe disposal or reuse. The process is vital in maintaining a healthy environment by preventing the pollution of natural water bodies and protecting public health. In the context of Environmental Engineering, especially for JKSSB JE, SSC JE, RRB JE, and other competitive exams, understanding this process is critical.

With the rapid pace of urbanization and industrialization, the volume of wastewater generated from domestic and industrial sources has increased significantly. If left untreated, this sewage can cause serious environmental hazards such as eutrophication, groundwater contamination, and the spread of waterborne diseases. Therefore, modern sewage treatment plants (STPs) are engineered using advanced technologies and designed to meet strict effluent quality standards.

This topic not only tests your conceptual clarity for exams but also builds your practical understanding of real-world civil engineering applications.

💧 Classification of Sewage

🔪 Detailed Explanation of Sewage Treatment Stages

Sewage treatment is done in multiple steps. Each stage plays a role in removing a specific type of pollutant or contaminant.

🔹 1. Preliminary Treatment

Purpose: To remove large floating materials and coarse solids, such as rags, plastics, sticks, and other debris that could clog or damage downstream equipment like pumps and valves. This stage also helps in preventing wear and tear on subsequent mechanical units and improves overall treatment efficiency by reducing the initial load.

Components:

• Bar Screens: Capture large solids like plastics, rags, and bottles.
• Grit Chambers: Remove heavy inorganic particles like sand and gravel.
• Skimming Tanks: Remove grease and oils floating on the surface.

Note: No significant reduction in BOD at this stage.

🔹 2. Primary Treatment (Physical Treatment)

Purpose: To remove suspended solids through sedimentation and reduce the organic load in subsequent biological treatment processes. This stage also helps in improving the efficiency of secondary treatment by settling out settleable organic and inorganic matter.

Process:

• Sedimentation Tanks or Primary Clarifiers are used.
• Wastewater is retained long enough for solids to settle at the bottom as primary sludge.
• Floating materials are skimmed from the top.

✅ Removes ~60% of suspended solids and ~30% of BOD.

🧠 Remember: Primary treatment is physical only, no biological processes involved.

🔹 3. Secondary Treatment (Biological Treatment)

This is the most critical stage in sewage treatment for the removal of organic pollutants. In this biological process, microorganisms—primarily bacteria—utilize the dissolved oxygen in the wastewater to break down biodegradable organic matter. This metabolic activity transforms complex organic compounds into simpler, stable end products like carbon dioxide, water, and biomass (sludge), significantly reducing the Biochemical Oxygen Demand (BOD) and improving the overall water quality.

Methods of Secondary Treatment:

1. Activated Sludge Process (ASP):
   • Air is pumped into an aeration tank.
   • Microorganisms consume organic matter.
   • Mixed liquor flows into a secondary clarifier, where sludge settles and is recycled.
2. Trickling Filter:
   • Wastewater is sprayed over a bed of stones or plastic media.
   • Biofilm (bacteria) on media degrades organic pollutants.
3. Oxidation Ponds (Lagoons):
   • Large open ponds where natural bacteria and algae treat the sewage.
   • Algae provide oxygen during photosynthesis.
4. Rotating Biological Contactors (RBC):
   • Rotating discs support biofilm growth.
   • Discs rotate partially submerged, alternately exposing biofilm to sewage and air.

✅ Reduces BOD by 85–90%
✅ Converts organic matter into stable, non-harmful products

🔹 4. Tertiary (Advanced) Treatment

Purpose: To remove residual contaminants including:

• Dissolved nutrients (N, P)
• Pathogens
• Refractory organic compounds

Common Processes:

• Filtration: Sand, carbon, or membrane filters
• Chemical Treatment: Coagulation and flocculation
• Disinfection:
  • Chlorination – Most common and economical
  • UV Irradiation – Fast and chemical-free
  • Ozonation – Strong oxidizing agent, but costly
• Nutrient Removal: Biological or chemical methods

✅ Used in sensitive areas or for reuse of treated water

📊 Design Parameters & Unicode-Compatible Formulas

✔️ 1. BOD First-Order Reaction Formula:

𝓁ₜ = 𝓁₀ × (1 − e⁻ᵏᵗ)

✔️ 2. Organic Loading Rate (OLR):

OLR = (Q × S₀) ÷ V

✔️ 3. Sludge Volume Index (SVI):

SVI = (Settled Sludge Volume in mL/L) ÷ (MLSS in mg/L) × 1000

✔️ 4. Mean Cell Residence Time (MCRT):

MCRT = (V × X) ÷ (Qw × Xw + Qe × Xe)

✔️ 5. Food to Microorganism Ratio (F/M):

F/M = (Q × S₀) ÷ (V × X)

✔️ 6. Effluent BOD:

BODₜ = BOD₀ × e⁻ᵏᵗ

✔️ 7. Chlorine Dosage:

Chlorine dosage = (Volume × Residual chlorine) ÷ 1000

🧬 Sludge Treatment Stages

🔸 Thickening

• Gravity thickeners reduce the water content.

🔸 Digestion

• Anaerobic digestion decomposes organic solids into methane and stabilized sludge.

🔸 Dewatering

• Sludge drying beds or mechanical processes like centrifuges.

🔸 Disposal

• Landfilling, incineration, or use as fertilizer.

🌐 Real-World Examples (India)

• Okhla STP (Delhi): Among Asia’s largest.
• Nagpur (MH): Reuses sewage for power plant cooling.
• Bangalore (KA): Uses treated water for gardens and flushing.

📚 Exam Practice Questions (JKSSB Style)

Q1. What is the function of a grit chamber?
✅ B. Remove sand and silt

Q2. BOD is measured in:
✅ C. mg/L

Q3. Activated sludge process is:
✅ C. Biological aerobic process

🔹 Summary Table of Processes

📂 Conclusion

Understanding sewage treatment is essential for JKSSB JE Civil and other technical exams. Focus on:

• Stages of treatment
• Unicode-compatible formulas
• Design parameters
• Practical applications

Stay consistent with your revision, and practice MCQs daily to strengthen retention.

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Sahil Digra

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