Sewage treatment is the process of removing contaminants from domestic and industrial wastewater before it is discharged into natural water bodies or reused. With India generating over 60,000 MLD of sewage but treating only ~25%, there is a massive infrastructure gap — and consequently, vast career and business opportunity in this sector. This guide covers the design of a conventional activated sludge process (ASP) sewage treatment plant, the most widely used technology in India and globally.
Sewage Characteristics and Design Parameters
| Parameter | Typical Raw Sewage (India) | Treated Effluent (CPCB standard) |
|---|---|---|
| BOD₅ (20°C) | 150–300 mg/L | < 30 mg/L (for river discharge) |
| COD | 300–600 mg/L | < 250 mg/L |
| Suspended Solids (SS) | 150–300 mg/L | < 100 mg/L |
| Total Kjeldahl Nitrogen (TKN) | 35–65 mg/L | < 10 mg/L (sensitive zones) |
| Total Phosphorus | 5–10 mg/L | < 2 mg/L (sensitive zones) |
| pH | 6.5–8.0 | 6.0–8.5 |
| Total Coliform | 10⁷–10⁹ /100 mL | < 1000 /100 mL (unrestricted irrigation) |
Per Capita Sewage Generation
Sewage flow = 80% of water supply (CPHEEO Manual)
For Indian cities: 80–120 lpcd sewage flow
Dry weather flow (DWF) = average daily flow
Peak flow = 3× DWF (for design of sewers and primary settling)Conventional STP — Unit Operations
1. Preliminary Treatment
- Coarse bar screen: 25–75 mm spacing, removes large solids
- Fine screen: 5–15 mm spacing, mechanically raked
- Grit chamber: Removes inorganic grit (sand, gravel) that would otherwise damage pumps; horizontal-flow design, velocity = 0.3 m/s; detention 60–90 sec; removes particles > 0.2 mm
- Flow measurement: Parshall flume or magnetic flow meter
2. Primary Sedimentation (Clarifier)
Removes 60–70% of suspended solids and 25–35% of BOD by gravity settling.
| Design Parameter | Criteria |
|---|---|
| Surface overflow rate (SOR) | 24–36 m³/m²/day (average); 48–60 m³/m²/day (peak) |
| Detention time | 1.5–2.5 hours at average flow |
| Tank depth | 2.5–4.5 m (excluding sludge zone) |
| L:W ratio (rectangular) | 3:1 to 5:1 |
| Sludge scraper speed | 2–3 m/min (circular), continuous |
3. Secondary Treatment: Activated Sludge Process (ASP)
ASP is the most widely used biological treatment. Microorganisms oxidise dissolved and colloidal organic matter in an aeration tank, forming biological floc (activated sludge) which is then settled in a secondary clarifier.
Aeration Tank Design:
Key design parameters:
F/M ratio (Food to Microorganism) = S₀ / (θc × X) = 0.2–0.4 g BOD/g VSS·d (conventional ASP)
= 0.05–0.15 (extended aeration)
Sludge Retention Time (SRT, θc): 5–10 days (conventional); 20–30 days (extended aeration)
MLSS (Mixed Liquor Suspended Solids): 2000–4000 mg/L (conventional); 3000–6000 mg/L (extended)
MLVSS = 0.75–0.80 × MLSS
Hydraulic Retention Time (HRT):
θ = θc × Y × (S₀ - S) / [X × (1 + kd × θc)]
Typically: HRT = 4–8 hours (conventional); 16–24 hours (extended aeration)Oxygen Requirements:
O₂ needed = Q × [1.5 × BOD removed - 1.42 × Px]
Where Px = net sludge production (kg VSS/day)
Aeration efficiency of diffused air: 1.5–2.5 kg O₂/kWh
Air supply: 1.5–2.5 m³ air per m³ sewage treated4. Secondary Clarifier (Final Settling Tank)
| Design Parameter | Criteria |
|---|---|
| Surface overflow rate (SOR) | 16–28 m³/m²/day (average); ≤ 40 m³/m²/day (peak) |
| Solids loading rate | ≤ 120 kg SS/m²/day |
| Detention time | 1.5–2.5 hours |
| Depth | 3.5–5.0 m |
| Return Activated Sludge (RAS) | 50–100% of inlet flow |
| Waste Activated Sludge (WAS) | 1–2% of inlet flow (for sludge age control) |
5. Disinfection
Chlorination (sodium hypochlorite, NaOCl) at 5–15 mg/L with contact time 30 minutes. UV disinfection (preferred for reuse quality effluent): 40 mJ/cm² dose eliminates coliform to <1000/100 mL.
6. Sludge Management
Sludge from primary and secondary clarifiers must be stabilised, thickened, dewatered, and disposed:
| Stage | Technology | Purpose |
|---|---|---|
| Thickening | Gravity thickener / dissolved air flotation | Reduce sludge volume (to 3–6% DS) |
| Stabilisation | Aerobic digestion, anaerobic digestion | Reduce volatile solids, pathogens, odour |
| Dewatering | Sludge drying beds, belt filter press, centrifuge | Reduce water content (15–30% DS cake) |
| Disposal | Land application (compost), landfill, incineration | Final disposal |
Biogas recovery: Anaerobic digestion of sludge produces biogas (~0.5–0.7 m³/kg VS destroyed, 60% methane). Large STPs can generate significant electricity — CPHEEO mandates energy audit for STPs >10 MLD.
SBR vs Activated Sludge Process
| Feature | Conventional ASP | Sequential Batch Reactor (SBR) |
|---|---|---|
| Footprint | Larger | 30–40% smaller |
| Secondary clarifier | Required (separate) | Not needed (settling in aeration tank) |
| Operation | Continuous flow | Batch (Fill-Aerate-Settle-Decant) |
| Flexibility | Less | Higher (nutrient removal easier) |
| Preferred for India | Very large STPs (>50 MLD) | Small-medium STPs (1–50 MLD) |
Frequently Asked Questions
What is the CPCB standard for STP effluent discharge in India?
CPCB (Central Pollution Control Board) General Standards (Schedule VI, Environment Protection Act): BOD ≤ 30 mg/L, SS ≤ 100 mg/L, pH 6.5–9.0 for discharge to inland surface water. For treated wastewater reuse in agriculture (CGWB): BOD ≤ 100 mg/L, coliform ≤ 1000 MPN/100 mL. Zero liquid discharge (ZLD) is mandated for heavily polluted rivers.
What is the typical energy consumption of an activated sludge STP?
Conventional ASP: 0.3–0.5 kWh per m³ of sewage treated. Extended aeration: 0.5–0.7 kWh/m³. Aeration accounts for 60–70% of total STP energy. Moving Bed Biofilm Reactor (MBBR) and SBR processes are typically 10–20% more energy-efficient than conventional ASP.
What is the difference between BOD and COD?
BOD (Biochemical Oxygen Demand) measures the oxygen consumed by microorganisms in degrading biodegradable organic matter over 5 days at 20°C. COD (Chemical Oxygen Demand) measures total oxidisable organic matter using a chemical oxidant (dichromate). COD/BOD ratio: 1.5–2.5 for domestic sewage (biodegradable), >3.0 for industrial effluent with refractory compounds. All designs use BOD₅ (5-day BOD at 20°C) per IS 3025 Part 44.