STP Load Calculator
Residential High-Rise Building • Step-by-Step Calculations • Export to PDF
Based on CPHEEO Manual, NBC 2016, CPCB/SPCB & IS 18997:2024
1. Project Inputs
STP Load Calculation Report
Residential High-Rise Building • Generated by STP Load Calculator
2. Project Summary
| Parameter | Value |
|---|
3. Step-by-Step Calculations
4. Pollution Load
| Parameter | Concentration | Load Calculation | Load (kg/day) |
|---|
5. Flow Summary
| Description | KLD | m³/hr | Remarks |
|---|
6. Key Design Notes
- Average Daily Flow (ADF) is used for biological process design and overall STP capacity.
- Peak Flow is used only for hydraulic design of inlet works, screens, channels, pumps and pipelines.
- Always provide 10–20% safety margin but avoid excessive over-sizing.
- Final design must be verified by a qualified STP technology provider as per site conditions and local SPCB norms.
References: CPHEEO Manual on Sewerage & Sewage Treatment (2013) • NBC 2016 Part 9 • CPCB/SPCB Guidelines • IS 18997:2024
This is a preliminary estimation tool. Actual design shall be carried out by a competent professional.
STP Load Calculation for Residential High-Rise Building
Solved Example with Formulas, Design Basis & Practical Guidance
Based on CPHEEO Manual, NBC 2016, CPCB/SPCB Norms & IS 18997:2024
1. Introduction
A Sewage Treatment Plant (STP) is essential for any residential high-rise building to treat wastewater generated from toilets, kitchens, bathrooms and utility areas before reuse or safe disposal. Proper load calculation ensures the plant is neither under-designed (leading to poor performance) nor over-sized (wasting capital and operating cost).
This article presents a complete, step-by-step worked example for a typical 500-flat residential high-rise project, covering hydraulic design, pollution load, capacity selection, unit sizing and treated water reuse.
Golden Rule of STP Design:
• Average Daily Flow (ADF) is used for biological process design and overall capacity selection.
• Peak Flow is used only for hydraulic components (inlet works, screens, channels, pumps, pipes).
• Average Daily Flow (ADF) is used for biological process design and overall capacity selection.
• Peak Flow is used only for hydraulic components (inlet works, screens, channels, pumps, pipes).
2. Project Details (Assumed)
| Parameter | Value |
|---|---|
| Type of Building | Residential High-Rise |
| Number of Flats | 500 Nos. |
| Average Occupancy | 4 Persons / Flat |
| Total Population | 500 × 4 = 2,000 Persons |
| Water Supply Basis | 135 LPCD (Litres Per Capita per Day) – Residential |
| Sewage Generation | 80% of Water Supply (as per CPHEEO) |
| Design Standards | CPHEEO Manual on Sewerage & Sewage Treatment (2013), NBC 2016 Part 9, CPCB/SPCB Norms, IS 18997:2024 |
3. Water Demand Calculation
1
Daily Water Demand
Daily Water Demand = Population × LPCD
= 2,000 × 135
= 270,000 L/day = 270 KLD
= 2,000 × 135
= 270,000 L/day = 270 KLD
LPCD = Litres Per Capita per Day
4. Sewage Generation (Domestic)
2
Average Sewage Flow (ADF)
Sewage Generation = 80% of Water Supply
= 270 KLD × 0.80
= 216 KLD
= 270 KLD × 0.80
= 216 KLD
Average Daily Flow (ADF) = 216 KLD
This is the most important number used for process design of the biological reactor, secondary clarifier and overall plant capacity.
Reference: As per CPHEEO Manual, domestic sewage is typically taken as 80% of water supply. IS 18997:2024 also recommends 108 LPCD wastewater generation for residential buildings (which is 80% of 135 LPCD).
5. Average Sewage Flow – Hourly Conversion
3
Average Hourly Flow = 216,000 L/day ÷ 24
= 9,000 L/hour = 9.0 m³/hr
= 9,000 L/hour = 9.0 m³/hr
6. Peak Sewage Flow (Hydraulic Design)
Peak Factor depends on contributing population as per CPHEEO Manual:
| Population (Contributing) | Peak Factor (PF) |
|---|---|
| Up to 20,000 | 3.0 |
| 20,001 – 50,000 | 2.5 |
| 50,001 – 5,00,000 | 2.0 |
| Above 5,00,000 | 1.5 |
4
Calculation for our project (Population = 2,000)
Peak Factor (PF) = 3.0
Peak Sewage Flow = ADF × Peak Factor
= 216 KLD × 3.0
= 648 KLD
Peak Sewage Flow = ADF × Peak Factor
= 216 KLD × 3.0
= 648 KLD
Peak Flow = 648 KLD = 27 m³/hr = 7.5 L/sec
Important Note:
• Peak Flow (648 KLD) is used only for hydraulic design of inlet works, bar screens, grit chambers, channels, pumps and pipelines.
• STP biological process capacity is always designed on Average Daily Flow (216 KLD).
• Peak Flow (648 KLD) is used only for hydraulic design of inlet works, bar screens, grit chambers, channels, pumps and pipelines.
• STP biological process capacity is always designed on Average Daily Flow (216 KLD).
7. STP Capacity Selection
While the average sewage generation is 216 KLD, practical capacity selection considers:
- Future load (staff, visitors, club house, commercial areas)
- Safety margin (10–20%)
- Possible increase in occupancy or water use
Proposed STP Capacity = 250 KLD (Preliminary Selection)
This provides approximately 15–16% extra margin over the calculated ADF of 216 KLD.
8. Pollution Load Calculation
Assumed characteristics for domestic sewage (typical values):
| Parameter | Unit | Typical Value | Load Calculation (216 KLD) | Load (kg/day) |
|---|---|---|---|---|
| BOD | mg/L | 250 | 216,000 L/day × 250 / 1000 | 54.0 |
| COD | mg/L | 500 | 216,000 × 500 / 1000 | 108.0 |
| TSS | mg/L | 200 | 216,000 × 200 / 1000 | 43.2 |
| Oil & Grease | mg/L | 20 | 216,000 × 20 / 1000 | 4.32 |
General Formula:
Load (kg/day) = Flow (L/day) × Concentration (mg/L) / 1000
Load (kg/day) = Flow (L/day) × Concentration (mg/L) / 1000
Design of the biological reactor is primarily based on BOD Load = 54.0 kg/day.
9. Flow Summary
| Description | Qty (KLD) | In m³/hr | Remarks |
|---|---|---|---|
| Average Daily Flow (ADF) | 216 | 9.0 | For Process Design |
| Peak Hydraulic Flow (PF = 3.0) | 648 | 27.0 | For Inlet works, Pumps, Channels |
| Peak Discharge | — | 7.5 L/sec | For Hydraulic Components |
| Proposed STP Capacity | 250 | 10.4 | Preliminary Selection |
10. Factors to Consider Before Final STP Capacity
- Actual water consumption pattern (metered data preferred)
- Flat occupancy variation (weekdays vs weekends)
- Common area usage (landscaping, clubhouse, swimming pool backwash)
- Staff / Kitchen / Restaurant / Commercial areas (if any)
- Future expansion (10% to 20% additional capacity)
- Reuse requirement of treated water
- Local SPCB norms and discharge standards
- Quality of influent water (low / high strength sewage)
- Availability of space and power
- O&M strategy and skilled manpower availability
Always select STP capacity based on engineering judgement with adequate safety margin, but avoid over-sizing.
11. Design Basis Summary
| Item | Basis | Value / Assumption |
|---|---|---|
| Population | Flats × Occupancy | 500 × 4 = 2,000 Persons |
| Water Supply | As per Project | 135 LPCD (Residential) |
| Sewage Generation | As per CPHEEO | 80% of Water Supply |
| Average Sewage Flow | Calculation | 216 KLD (9.0 m³/hr) |
| Peak Factor | CPHEEO Manual | 3.0 (for Pop. up to 20,000) |
| Peak Sewage Flow | ADF × PF | 648 KLD (27 m³/hr) |
| Peak Discharge | Conversion | 7.5 L/sec |
| Proposed STP Capacity | Selection | 250 KLD |
12. Typical STP Process Flow Diagram
A modern residential high-rise STP typically follows this treatment train:
- Bar Screen (Channel) – Removes large solids (rags, plastics, sticks)
- Grit Chamber – Removes grit, sand and small stones
- Equalization Tank – Balances flow & load; homogenizes sewage
- Biological Treatment (SBR / MBBR / MBR) – Microorganisms break down organic matter (BOD, COD, TSS reduction)
- Secondary Clarifier – Settles biomass (sludge); clarified water overflows
- Tertiary Treatment (Filter) – Removes fine suspended solids and residual organics
- Disinfection (UV / Chlorine) – Kills pathogenic microorganisms
- Treated Water Tank – Stores treated water for reuse or final disposal
Sludge Line: Secondary Clarifier → Sludge Holding Tank → (Optional Thickener) → Sludge Dewatering (Belt Filter Press) → Dry Cake / Biosolids Disposal
Typical Removal Efficiency
| Parameter | % Removal (Typical) |
|---|---|
| BOD | 90 – 95 % |
| COD | 85 – 95 % |
| TSS | 90 – 95 % |
| Oil & Grease | 80 – 90 % |
| Pathogens | > 99 % |
13. Indicative Unit Sizing (for 250 KLD STP)
| Unit | Design Criteria | Qty / Size (Approx.) | Remarks |
|---|---|---|---|
| Bar Screen | Clear bar opening 20–40 mm | Manual / Mech. | Channel width \~450 mm |
| Grit Chamber | Detention Time 30–45 sec | Size: 1.0 m (L) × 1.0 m (W) × 2.0 m (SWD) | — |
| Equalization Tank | Detention Time 6–8 hr | Volume ≈ 250 × 8/24 ≈ 83 m³ (Say 85 m³) | With submersible mixers |
| Biological Reactor | SBR: 4–6 cycles/day MBBR: 30–40% Media |
Volume: 250–300 m³ | Based on technology provider |
| Secondary Clarifier (if MBBR system) |
Surface Loading 20–25 m³/m²/day | Dia.: 8.0 m SWD: 3.0 m |
Central Drive / Rake |
| Tertiary Filter | Filtration Rate 8–10 m³/m²/hr | Dual Media Filter | Sand + Gravel Filter |
| Disinfection | Chlorine Contact Time 20–30 min | Contact Tank Volume ≈ 250 × 30/(24×60) ≈ 5.2 m³ | Provide baffle arrangement |
| Sludge Drying Bed (if used) |
Loading 30–50 kg/m² | Area: 15–20 m² | As per sludge quantity |
Note: Actual sizing shall be as per detailed design and technology provider data.
14. Treated Water Reuse Calculation
Treated Water Available ≈ 216 KLD (Average)
Assumed Reuse Requirement:
- Gardening / Landscaping → 120 KLD
- Flushing (Common area) → 40 KLD
- DG Set / Utility / Others → 20 KLD
Total Reuse = 180 KLD
Balance 36 KLD can be kept as buffer / surplus for future demand or discharge (as per norms).
Balance 36 KLD can be kept as buffer / surplus for future demand or discharge (as per norms).
15. Discharge Norms (Typical – As per CPCB / SPCB)
| Parameter | Unit | Permissible Limit* |
|---|---|---|
| BOD | mg/L | ≤ 10 |
| COD | mg/L | ≤ 50 |
| TSS | mg/L | ≤ 20 |
| Oil & Grease | mg/L | ≤ 10 |
| pH | — | 6.5 – 9.0 |
| Total Coliform | MPN/100 mL | < 230 |
* As per CPCB / SPCB norms for inland surface water discharge (may vary as per local authority).
16. Pumps & Blowers (Indicative for 250 KLD)
A) Sewage Pump (Peak Flow 27 m³/hr)
- Duty: 27 m³/hr at 12 m head
- Quantity: 2 Working + 1 Standby
- Type: Submersible / Dry well pump
B) Treated Water Pump
- Duty: 12 m³/hr at 20 m head
- Quantity: 2 Working + 1 Standby
C) Blowers (For Aeration in SBR / MBBR)
- Air Demand: 0.4 – 0.6 m³/min per 100 KLD
For 250 KLD → 1.0 – 1.5 m³/min - Quantity: 2 Working + 1 Standby
- Type: Roots Blower
Selection shall be as per process requirements, head losses and technology provider data.
17. O&M Considerations
- Ensure regular cleaning of screens and grit chamber
- Monitor DO, MLSS (if applicable), pH and sludge volume
- Check chemical dosing system and chlorine residual regularly
- Ensure blower, pump and mixer preventive maintenance
- Maintain log book for operating parameters and maintenance
- Provide skilled operator and proper training
18. Key Points to Remember
- Average Sewage Flow (ADF) is used for STP process design.
- Peak Flow is used for hydraulic design of inlet works, screens, channels and pumps.
- Proper screening, equalization and aeration are essential for efficient STP performance.
- Always provide 10–20% safety margin but avoid excessive over-sizing.
- Final design must comply with NBC, CPHEEO, SPCB/CPCB norms and technology provider recommendations.
19. References
- CPHEEO Manual on Sewerage and Sewage Treatment Systems (2013)
- National Building Code (NBC) 2016 – Part 9, Section 2 – Sanitation, Drainage & STP
- CPCB / SPCB Guidelines for STP and Treated Water Reuse
- IS 18997:2024 – Sewage Treatment Systems – Code of Practice
Disclaimer: This calculation is for preliminary estimation and planning purposes only. Final design shall be carried out by a qualified STP technology provider / consultant considering actual site conditions, local regulations and selected treatment technology.
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