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).

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

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

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

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 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).

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

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:

1. Bar Screen (Channel) – Removes large solids (rags, plastics, sticks)
2. Grit Chamber – Removes grit, sand and small stones
3. Equalization Tank – Balances flow & load; homogenizes sewage
4. Biological Treatment (SBR / MBBR / MBR) – Microorganisms break down organic matter (BOD, COD, TSS reduction)
5. Secondary Clarifier – Settles biomass (sludge); clarified water overflows
6. Tertiary Treatment (Filter) – Removes fine suspended solids and residual organics
7. Disinfection (UV / Chlorine) – Kills pathogenic microorganisms
8. 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).

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.

Prepared for educational & reference purposes based on standard Indian design practices