📅 Updated: July 2025
📚 Subject: Environmental Engineering
🎯 Exam Focus: JKSSB JE Civil, SSC JE, RRB JE, State AE/PSC Exams
📘 What is Sewage?
Sewage is wastewater that originates primarily from residential areas but also includes discharges from institutions, commercial establishments, and industries. It is a complex mixture comprising water (about 99.9%) and various solids (about 0.1%), which may be dissolved, suspended, or colloidal. These solids include:
- Organic matter (from human and animal waste, food residues, plant material)
- Inorganic matter (such as sand, grit, salts, and metallic compounds)
- Pathogens (disease-causing microorganisms like bacteria, viruses, protozoa)
- Nutrients (such as nitrogen and phosphorus, which can lead to eutrophication in water bodies)
- Toxic substances (originating from industrial waste, including heavy metals and hazardous chemicals)
The Sewage Characterstics vary significantly depending on the source, time of collection, and temperature. Accurate knowledge of these components is essential for the planning and design of sewage treatment plants (STPs) to ensure environmental safety and compliance with pollution control norms.
🚨 Key Point: Sewage ≠ Pure domestic wastewater. It includes a mix of blackwater, greywater, and possibly stormwater runoff.
📊 Categories of Sewage Characteristics
Sewage characteristics are grouped into:
- 🔹 Physical Characteristics
- 🔹 Chemical Characteristics
- 🔹 Biological Characteristics
1️⃣ Physical Characteristics of Sewage
These are directly observable or measurable without complex chemical analysis.
a. Color
- Fresh sewage: Light grey
- Stale sewage: Dark grey → Black (due to anaerobic decomposition)
- Indicates: Age and presence of septic conditions.
b. Odor
- Fresh: Musty or earthy
- Stale/septic: Offensive, due to H₂S (hydrogen sulfide) gas → smells like rotten eggs.
c. Temperature
- Typically 2–3°C higher than ambient air.
- Affects microbial activity and BOD rate.
d. Turbidity
- Due to suspended organic/inorganic particles.
- Measured using a Nephelometer or turbidity rods.
e. Solids
- Total Solids (TS) = Suspended Solids + Dissolved Solids
- Suspended Solids (SS): Can be removed by sedimentation
- Dissolved Solids (DS): Require advanced treatment
💡 Important Formula:
- ⌒ TS = SS + DS
- Volatile Solids (VS): Portion of TS that burns off at 550°C, mostly organic.
2️⃣ Chemical Characteristics of Sewage
These describe the chemical makeup of sewage.
a. pH
- Ideal range: 6.5 to 8.5
- Acidic (<6.5): Could indicate industrial discharge
- Alkaline (>8.5): May reduce biological treatment efficiency
b. Alkalinity
- Helps in buffering and neutralizing acids.
- Mainly due to bicarbonates (HCO₃⁻).
c. Chlorides
- From human excreta, kitchen waste, and industrial discharges.
- Excess chlorides (>250 mg/L) may harm aquatic life.
d. Nitrogen Forms
- Organic Nitrogen (Norg)
- Ammonia Nitrogen (NH₃-N)
- Nitrite (NO₂⁻)
- Nitrate (NO₃⁻)
e. Phosphates
- From soaps, detergents, fertilizers
- Contribute to eutrophication in natural water bodies.
f. Dissolved Oxygen (DO)
- DO in raw sewage is nearly 0 mg/L
- Essential in secondary treatment stages
- DO > 4 mg/L is good for aquatic life
g. Biochemical Oxygen Demand (BOD)
- BOD₅ = Oxygen required by microbes to decompose biodegradable organics over 5 days at 20°C.
🧮 BOD Rate Equation (Unicode Format):
BODₜ = BOD₅ ⋅ (1 − e^( −k ⋅ t ))
Where:
- BODₜ = BOD exerted at time t
- BOD₅ = 5-day BOD
- k = Reaction rate constant (per day)
- t = Time in days
h. Chemical Oxygen Demand (COD)
- Oxygen needed to chemically oxidize both biodegradable and non-biodegradable organics.
- Measured using potassium dichromate digestion.
◾ Relation:
BOD < COD
- If BOD/COD > 0.5 → Easily biodegradable
- If BOD/COD < 0.3 → Poorly biodegradable
3️⃣ Biological Characteristics of Sewage
a. Pathogens
- Disease-causing organisms (bacteria, viruses, protozoa)
- Common: E. coli, Salmonella, Giardia
b. Microorganisms
- Help in biological treatment
- Two types:
- Aerobic Bacteria: Need oxygen
- Anaerobic Bacteria: Thrive without oxygen
c. Biodegradable Organics
- Provide food for microorganisms
- Measured as BOD
d. Non-Biodegradable Organics
- Plastics, heavy metals, pesticides
🔍 Typical Values for Domestic Sewage
| Parameter | Range |
|---|---|
| BOD | 200–300 mg/L |
| COD | 400–600 mg/L |
| Total Solids (TS) | 500–1000 mg/L |
| Suspended Solids (SS) | 150–400 mg/L |
| pH | 6.5 – 8.5 |
| Temperature | 20°C – 30°C |
| DO (untreated sewage) | 0 – 1 mg/L |
| Chlorides | 100–250 mg/L |
🔎 Exam-Oriented Points for JKSSB/SSC/JE
📋 Expected MCQs:
- What is the typical BOD range in domestic sewage?
- Which gas gives rotten egg smell in sewage?
- What does BOD/COD ratio > 0.5 signify?
🧠 Memory Tricks:
- BOD = Biological, COD = Chemical
- Grey → Black = Anaerobic
- DO low = Pollution high
📋 Bonus Practice
Q: A sewage sample has a BOD of 300 mg/L and COD of 500 mg/L. What is the BOD/COD ratio? Is it biodegradable?
Solution:
BOD/COD = 300 / 500 = 0.6
✅ Yes, it’s easily biodegradable (since > 0.5)
✅ Conclusion
Understanding the characteristics of sewage is fundamental to the successful design, operation, and management of wastewater treatment systems. Each category—physical, chemical, and biological—offers insights into the composition and behavior of sewage, allowing engineers to select appropriate treatment processes.
- Physical characteristics help in preliminary treatment processes such as screening and sedimentation.
- Chemical parameters like BOD, COD, pH, and nutrient content are crucial for designing biological and chemical treatment stages.
- Biological characteristics, especially the presence of pathogens and organic load, influence public health and environmental impact.
For civil engineering aspirants preparing for JKSSB, SSC JE, and RRB JE, mastering this topic is essential as it frequently appears in both theoretical and numerical formats. From direct MCQs to BOD/COD calculations and treatment decisions based on sewage content, questions are drawn from real-life applications.
In the context of sustainable development and increasing urbanization, sewage treatment is no longer just a sanitation requirement—it’s a critical component of urban environmental planning. Therefore, a solid grasp of sewage characteristics lays the foundation for becoming an effective and responsible civil/environmental engineer.
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