Sewage Treatment Plants (STPs) are essential in managing wastewater generated by residential, commercial, and industrial facilities. Properly sizing an STP is crucial to ensure effective wastewater treatment without overloading the system or underutilizing infrastructure. If you’re planning to install an STP, understanding how to calculate its capacity is the first and most vital step.
This guide will walk you through everything you need to know—from daily sewage flow estimates to peak factor considerations and key design parameters.
Why Calculating STP Capacity Matters
Before diving into calculations, it’s important to know why STP capacity calculations are important:
- Ensures Compliance: Correct sizing ensures treated water meets environmental norms.
- Prevents Overflows: Underestimating capacity can cause overflows and environmental damage.
- Cost-Effective: Oversized plants waste resources and increase maintenance costs.
- Design Accuracy: Influences tank sizes, pipeline design, and equipment selection.
- Operational Efficiency: A well-sized plant operates more efficiently and with lower energy use.
Key Factors in STP Capacity Calculation
Several parameters influence the capacity of an STP. These include:
- Population Equivalent (PE)
- Per Capita Wastewater Generation
- Influent Flow Variations (Peak and Average Flows)
- Type of Facility or Premises
- Future Load Projections
- Industrial Wastewater Load (if applicable)
- Reuse Options (e.g., irrigation, flushing)
Let’s break these down for better clarity.
Step-by-Step Guide to Calculating STP Capacity
Step 1: Determine Population Equivalent (PE)
Population Equivalent refers to the number of people that contribute to the wastewater generation. This could include:
- Residents in a housing complex
- Employees in an office building
- Students in an educational institution
- Guests in a hotel
Formula:
PE = Number of users per day
For example, if a residential complex has 300 flats, each with 4 residents,
PE = 300 x 4 = 1,200 people
Step 2: Estimate Per Capita Wastewater Generation
Wastewater generation depends on water usage habits and geographic location. In India, the Central Public Health and Environmental Engineering Organization (CPHEEO) recommends:
- 135 liters per capita per day (lpcd) in urban areas
This figure can be adjusted for:
- Commercial use: 50–75 lpcd (offices)
- Hotels: 180–250 lpcd
- Hospitals: 400–500 lpcd
Formula:
Wastewater Generation = PE × Per Capita Usage
Using our previous example (1,200 people):
Wastewater = 1,200 × 135 = 1,62,000 liters/day or 162 KLD (Kilo Liters per Day)
Step 3: Include a Safety/Peak Factor
Water usage varies throughout the day. Hence, a peak factor accounts for sudden increases in wastewater flow. A factor between 1.5 to 2.5 is often used depending on the facility type.
Formula:
Peak Flow = Average Daily Flow × Peak Factor
If average daily flow is 162 KLD and peak factor is 2.0:
Peak Flow = 162 × 2.0 = 324 KLD
While designing tank capacities and pumps, this peak flow is important.
Step 4: Consider Sludge and Inflow/Infiltration
- Sludge Volume: Consider an additional volume for sludge accumulation in sedimentation tanks.
- Infiltration: For open systems, consider rainwater inflow or groundwater infiltration (usually 10% of total flow).
Additional Load = Wastewater Flow × 10%
Total STP Flow = Main Wastewater + Infiltration Load
Calculating Design Capacities for Major STP Units
Now that we know the daily sewage load, we must size the main components of an STP, such as:
1. Equalization Tank
Purpose: To store influent and regulate flow into treatment units.
- Size: 25% to 30% of daily flow.
- Retention Time: 8–12 hours
For 162 KLD:
Equalization Tank = 25% of 162 = 40.5 KLD
2. Aeration Tank (in ASP)
Purpose: To promote bacterial breakdown of organic matter using air.
- Retention Time: 6–8 hours
- MLSS (Mixed Liquor Suspended Solids): 2,000–3,500 mg/l
Volume = Flow × Retention Time
For 162 KLD and 8 hours:
Volume = (162,000 / 24) × 8 = 54,000 liters or 54 m³
3. Secondary Clarifier
Purpose: To settle biological solids after aeration.
- Surface loading rate: 20–30 m³/m²/day
- Typical depth: 3.5 – 4 meters
Area = Daily Flow / Surface Loading Rate
162 KLD = 162 m³/day
If loading rate is 25 m³/m²/day:
Area = 162 / 25 = 6.48 m²
4. Sludge Holding Tank
Purpose: To collect and stabilize sludge before disposal.
- Typical Size: 1–2% of total daily wastewater flow
- For 162 KLD:
Sludge Tank = 1.5% × 162 = 2.43 KLD
Factors to Adjust for Long-Term Planning
1. Population Growth
- Assume 2–3% annual growth
- Design for a horizon of 15–20 years
Future PE = Current PE × (1 + Growth Rate)^Years
For 1,200 people and 2% growth over 15 years:
Future PE = 1,200 × (1.02)^15 ≈ 1,616 people
2. Occupancy Rate
- In housing projects or commercial spaces, consider actual occupancy
- Use 70–80% occupancy for early phases of development
3. Reuse Potential
- If treated water will be reused (e.g., flushing, gardening), recalculate net discharge load
Common Mistakes to Avoid in STP Capacity Calculation
- Ignoring Peak Loads: Can lead to system overload and poor treatment.
- Underestimating Population Growth: Causes early system failure or expansion needs.
- Incorrect Flow Estimates: Using inaccurate per capita values can affect sizing.
- No Sludge Volume Allowance: Leads to overfilled tanks and increased maintenance.
- Overlooking Industrial Loads: Adds to COD/BOD and flow volume unexpectedly.
Tools and Techniques for STP Design
Engineers use a mix of manual calculations and software tools to determine STP sizing:
- Manual Calculations using design norms
- AutoCAD for layouts
- STP Simulation Software: BioWin, GPS-X
- Excel Templates for flow, load, and tank sizing
Partnering With the Right Manufacturer
Choosing the right sewage treatment plant manufacturer helps bring these calculations to life. Manufacturers like 3D AQUA not only provide accurate design assistance but also help with:
- Customized STP design and capacity planning
- Installation and commissioning
- Automation and energy-saving features
- Long-term operation and maintenance support
They offer a variety of STP models—MBR, SBR, MBBR, ASP—that can be tailored to your calculated capacity.
Read here about MBR Vs MBBR
Conclusion
Calculating the right capacity for an STP plant is both a science and an art. It requires a clear understanding of wastewater generation, population estimates, and design principles. A misstep in calculation can lead to operational inefficiencies or environmental harm. By following a methodical approach, factoring in population growth, peak flows, and system expansion, you can build an STP that serves your facility reliably for years.
If you’re planning a new project or upgrading an existing plant, work with trusted experts like 3D AQUA to ensure your plant is correctly sized and fully compliant with modern wastewater standards.
Need Help with STP Capacity Design?
Contact 3D AQUA for custom sewage treatment plant solutions across residential, commercial, and industrial sectors.
📞 Phone: +91-6262629090
📧 Email: info@3daqua.in
🌐 Website: www.3daqua.in