How to Calculate STP Capacity for Housing Society, Hotel & Factory
Getting your STP capacity wrong is one of the most expensive mistakes you can make in a water treatment project. An undersized plant produces non-compliant treated water within months, triggering show-cause notices from HSPCB, enforcement visits, and the cost of retrofitting at an occupied building — which runs 2–3 times what correct sizing would have cost at the start. An oversized plant wastes capital, consumes unnecessary electricity, and struggles to maintain biological treatment efficiency at low loads.
This guide gives you the exact calculation method used by Meridian Micro Technology's engineers across 300+ installations in Faridabad, Delhi NCR and India — using CPCB and CPHEEO standard norms that your State Pollution Control Board will expect to see in your consent application.
Whether you are a housing society committee member, hotel facility manager, factory owner or architect — by the end of this guide you will have a defensible STP capacity figure in KLD, ready to get a quotation.
What Is KLD and How Is STP Capacity Measured?
STP capacity is measured in KLD — Kilolitres Per Day. One KLD equals 1,000 litres of sewage treated per day.
You will also encounter LPD (Litres Per Day): 1 KLD = 1,000 LPD. For smaller plants (residential quarters, clinics, small offices), capacity may be expressed in LPD. For large plants (townships, hospitals, factories), KLD or MLD (Million Litres per Day) is used.
Unit | Full form | Equivalent | Typical use |
|---|---|---|---|
LPD | Litres per day | 1 LPD = 0.001 KLD | Small installations up to 10,000 LPD |
KLD | Kilolitres per day | 1 KLD = 1,000 LPD | Most residential and commercial STPs |
MLD | Million litres per day | 1 MLD = 1,000 KLD | Large townships and municipal plants |
The Master Formula for STP Capacity Calculation
The CPCB and CPHEEO (Central Public Health and Environmental Engineering Organisation) use a per-capita sewage generation norm as the basis for all STP sizing in India. The standard formula is:
STP Capacity (KLD) = Population × LPCD × Sewage Factor ÷ 1,000
Where:
Population = Number of people generating sewage at peak occupancy
LPCD = Litres Per Capita Per Day — the standard water consumption per person per day for your facility type (see table below)
Sewage Factor = Fraction of water consumed that becomes sewage, typically 0.80 (80%). The remaining 20% is lost to evaporation, garden watering, and other non-drainage uses.
÷ 1,000 converts litres to kilolitres (KLD)
Add 15–20% safety buffer to the calculated figure before selecting plant size
LPCD Norms by Facility Type — CPCB and CPHEEO Standards
The LPCD rate varies significantly by facility type. Using the wrong LPCD is the most common sizing error. Always use the correct figure for your specific application:
Facility Type | LPCD (L/person/day) | Sewage Factor | Notes |
|---|---|---|---|
Urban residential (flats/apartments) | 135 | 0.80 | CPHEEO / National Building Code standard. Use 135 for design; 150 as conservative upper limit. |
Individual houses / villas | 150 | 0.80 | Higher due to garden watering and car washing |
Hotels (guests, overnight stay) | 180 | 0.80 | CPHEEO norm. Add 45 LPCD for staff separately. |
Resorts / luxury properties | 200–250 | 0.85 | Higher consumption from pools, spa, gym |
Hospitals (general ward beds) | 400 | 0.85 | High water use for sterilisation, laundry, OT |
Hospitals (ICU / special ward) | 450 | 0.85 | Even higher for life support and critical care |
Offices / IT parks / commercial | 45–50 | 0.80 | Day-use only. No overnight stay. |
Schools / colleges (day scholars) | 15–25 | 0.80 | Low consumption; no overnight stay |
Hostels / residential schools | 135 | 0.80 | Same as residential since students live on campus |
Restaurants (full service) | 70 | 0.80 | Per seat capacity. Add kitchen effluent separately. |
Factories / industries | 45 | 0.80 | For workers only. Industrial process water is separate (ETP). |
Shopping malls | 15 | 0.80 | Per visitor. Base on peak daily footfall. |
Cinemas / multiplexes | 10 | 0.80 | Per seat capacity |
Important: For facilities with multiple user types (e.g. a hospital with in-patients, out-patients, staff and canteen), calculate each population segment separately and add the results together.
Step-by-Step Worked Examples — 4 Common Scenarios
Example 1 — Housing Society (100 Flats)
Housing Society — STP Capacity Calculation |
Total flats: 100 |
Average residents per flat: 4 persons |
Total population: 100 × 4 = 400 persons |
LPCD (residential): 135 litres/person/day |
Total water consumption: 400 × 135 = 54,000 LPD |
Sewage generation (80%): 54,000 × 0.80 = 43,200 LPD = 43.2 KLD |
Add 20% safety buffer: 43.2 × 1.20 = 51.8 KLD |
→ Select plant size: 55 KLD (next standard size above 51.8) |
Note: If your society has a swimming pool, club house, or large garden irrigation system, add an additional 5–10 KLD to account for increased water use.
Example 2 — 3-Star Hotel (80 Rooms)
Hotel — STP Capacity Calculation |
Total rooms: 80 rooms |
Occupancy (80%): 80 × 0.80 = 64 occupied rooms |
Guests per room (average): 1.8 persons |
Total guest population: 64 × 1.8 = 115 guests |
Guest LPCD: 180 litres/person/day |
Guest sewage: 115 × 180 × 0.80 = 16,560 LPD |
Hotel staff (estimated): 60 persons |
Staff LPCD: 45 litres/person/day |
Staff sewage: 60 × 45 × 0.80 = 2,160 LPD |
Kitchen / banquet addition: 5,000 LPD (fixed estimate for restaurant) |
Total daily sewage: 16,560 + 2,160 + 5,000 = 23,720 LPD = 23.7 KLD |
Add 25% buffer (peak season): 23.7 × 1.25 = 29.6 KLD |
→ Select plant size: 30 KLD |
Example 3 — 100-Bed Hospital
Hospital — STP Capacity Calculation |
In-patients (general ward): 70 beds × 400 LPCD × 0.85 = 23,800 LPD |
In-patients (ICU): 30 beds × 450 LPCD × 0.85 = 11,475 LPD |
Out-patients (daily footfall): 200 persons × 25 LPCD × 0.80 = 4,000 LPD |
Staff (doctors, nurses, etc.): 150 persons × 45 LPCD × 0.80 = 5,400 LPD |
Canteen / kitchen: 3,000 LPD (fixed estimate) |
Total daily sewage: 23,800 + 11,475 + 4,000 + 5,400 + 3,000 = 47,675 LPD = 47.7 KLD |
Add 20% safety buffer: 47.7 × 1.20 = 57.2 KLD |
→ Select plant size: 60 KLD |
Example 4 — Factory with 500 Workers
Factory — STP Capacity Calculation (Workers Only) |
Total workers (two shifts): 500 persons |
Worker LPCD: 45 litres/person/day |
Worker sewage: 500 × 45 × 0.80 = 18,000 LPD |
Office staff: 50 persons × 50 LPCD × 0.80 = 2,000 LPD |
Canteen: 2,000 LPD (fixed estimate) |
Total domestic sewage: 18,000 + 2,000 + 2,000 = 22,000 LPD = 22 KLD |
Add 15% buffer: 22 × 1.15 = 25.3 KLD |
→ Select plant size: 25 KLD |
|
IMPORTANT: Industrial process wastewater requires a separate ETP — not this STP. |
The STP at a factory treats only domestic sewage from workers and offices. Any wastewater from the manufacturing process — cooling water, wash water, chemical effluent — must go through a separate Effluent Treatment Plant (ETP). Mixing process water into the STP overloads the biology and causes HSPCB compliance failures.
Peak Flow Factor — Why Average Daily Flow Is Not Enough
Sewage generation is not uniform throughout the day. It peaks sharply in the morning (6–9 AM) and evening (6–9 PM) in residential complexes, and at lunchtime in offices and factories. An STP designed only for average daily flow will be hydraulically overloaded during peak hours, causing short-circuiting and non-compliant effluent quality.
The peak flow factor accounts for this variation. For Indian facilities, CPCB recommends:
Facility Type | Peak Flow Factor | How to apply |
|---|---|---|
Residential societies | 1.5–2.0× | Multiply average hourly flow by 1.5–2.0 to size inlet screens, equalisation tank, and pumps |
Hotels | 2.0–2.5× | High peak during check-in/check-out and meal service hours |
Hospitals | 1.5× | More uniform but critical — uptime cannot be compromised |
Offices / factories | 2.0–3.0× | Extreme morning peak when all workers arrive simultaneously |
Practical implication: Size your equalisation tank (the first tank in the STP) to buffer 6–8 hours of average daily flow. This smooths the peak before it hits the biological treatment stage, protecting treatment quality.
When Is an STP Mandatory? CPCB and HSPCB Thresholds
In Haryana, the HSPCB enforces CPCB guidelines strictly. The following thresholds determine when an STP is legally mandatory for your project:
Facility | CPCB / HSPCB Mandatory STP Threshold |
|---|---|
Residential apartments / housing societies | 50+ units OR generating 10+ KLD of sewage |
Hotels | Generating 10+ KLD of sewage (typically 50+ rooms) |
Hospitals | Generating 10+ KLD of sewage (typically 25+ beds) |
Industries / factories | All units generating any domestic sewage (separate from ETP requirement) |
Commercial complexes / malls | Generating 10+ KLD of sewage |
Educational institutions | Generating 10+ KLD of sewage |
In practice, HSPCB requires STP installation as a condition of the Consent to Establish (CTE) — which you need before construction begins. Without a functioning STP, the Consent to Operate (CTO) will not be issued, meaning your building, hotel or factory legally cannot operate.
Which STP Technology Is Right for Your Capacity?
Once you have your capacity in KLD, the next decision is technology. The right choice depends on available space, budget, output quality requirement, and whether you need continuous 24/7 operation.
Capacity | Recommended Technology | Why | Space needed | |
Up to 10 KLD | Packaged MBBR or SBR | Compact, factory-built, installs in 3–5 days, no civil work | 15–25 m² | |
10–50 KLD | MBBR or SBR (civil or packaged) | Good balance of cost, footprint, and output quality | 30–70 m² | |
50–200 KLD | MBBR or ASP (civil-based) | Civil construction for permanent installation, reliable operation | 80–250 m² | |
200–500 KLD | ASP or MBR (civil-based) | MBR if space is very constrained or treated water reuse quality is critical | 200–600 m² | |
500 KLD+ | MBR or ASP with tertiary treatment | Large townships, hospital complexes, or industrial campuses | Site-specific design |
For housing societies in Delhi-NCR where basement space is limited, we recommend underground STP systems — the entire plant is installed below the parking basement slab, leaving the surface completely free. Underground STPs also eliminate odour complaints from residents.
Quick Reference — STP Capacity by Building Type and Size
Use this table for rapid preliminary estimates. Always verify with a formal calculation before purchasing.
Building / Facility | Size / Occupancy | Estimated STP Capacity |
|---|---|---|
Housing society | 50 flats (200 residents) | 25–30 KLD |
Housing society | 100 flats (400 residents) | 50–55 KLD |
Housing society | 200 flats (800 residents) | 100–110 KLD |
Housing society | 500 flats (2,000 residents) | 250–275 KLD |
Hotel | 30 rooms | 10–15 KLD |
Hotel | 80 rooms | 28–35 KLD |
Hotel | 150 rooms | 55–65 KLD |
Hotel | 300 rooms | 110–130 KLD |
Hospital | 30 beds | 15–20 KLD |
Hospital | 100 beds | 55–65 KLD |
Hospital | 250 beds | 130–150 KLD |
Office building | 200 employees | 10–12 KLD |
Office building | 500 employees | 25–30 KLD |
Factory | 300 workers | 12–15 KLD |
Factory | 1,000 workers | 40–50 KLD |
School (day scholars) | 1,000 students + 100 staff | 20–25 KLD |
Frequently Asked Questions — STP Capacity Calculation
Q1. What is LPCD and who sets the standard in India?
LPCD stands for Litres Per Capita Per Day — the average quantity of water used by one person per day. In India, the CPHEEO (Central Public Health and Environmental Engineering Organisation) under the Ministry of Housing and Urban Affairs publishes the standard LPCD norms used for water supply and sewage planning. CPCB adopts these norms for STP sizing guidelines. The standard residential LPCD in urban India is 135 litres per person per day, which means the standard sewage generation per person is 135 × 0.80 = 108 LPD.
Q2. Why is 80% used as the sewage factor?
Not all water supplied to a building becomes sewage. Approximately 20% of the total water is lost to evaporation, garden watering, car washing and other non-drainage uses. The remaining 80% flows into the drainage network and becomes sewage that needs treatment. This 80% sewage factor is the CPCB and CPHEEO standard for residential and commercial buildings. For hospitals and hotels where more water is consumed in washing and sterilisation, a higher factor of 85% is sometimes used.
Q3. What is the minimum STP capacity required for a 100-flat housing society in Delhi-NCR?
For a 100-flat housing society with an average of 4 residents per flat (400 residents total): Sewage generation = 400 × 135 × 0.80 = 43,200 LPD = 43.2 KLD. Adding a 20% safety buffer gives 51.8 KLD. The minimum recommended plant size is therefore 55 KLD. This assumes no swimming pool, large club house or commercial kitchen on the premises — if these exist, add 5–15 KLD accordingly.
Q4. Can I install a smaller STP now and expand it later?
Yes, modular expansion is possible but comes with significant caveats. The STP civil structure (equalisation tank, aeration tank, clarifier) needs to be sized for the ultimate capacity from day one — only the mechanical and electrical equipment can be staged. Expanding the civil tanks at an occupied building is extremely disruptive and expensive. Meridian Micro Technology recommends designing for the 10-year projected population from day one, even if you commission a reduced mechanical capacity initially. The incremental cost of building larger tanks upfront is small compared to the disruption cost of future expansion.
Q5. How much space does an STP need in a Delhi-NCR housing society?
Space requirements depend on technology and capacity. As a rough guide: a 50 KLD MBBR-based STP needs approximately 50–70 m² of above-ground space or can be installed underground beneath a parking area with no surface footprint. A 100 KLD civil-based STP needs 100–150 m². Underground STPs add 20–30% to cost but are increasingly preferred in urban areas societies where every square metre is valuable and odour complaints from residents are a concern.
Q6. How long does it take to get HSPCB consent for an STP in Delhi-NCR?
HSPCB consent timelines for STP systems in Faridabad typically follow this path: Consent to Establish (CTE) — required before construction — takes 30–90 days after application with full documentation including STP design drawings, process flow diagram, and layout plan. Consent to Operate (CTO) — required before occupancy — takes 15–45 days after the STP is commissioned and a treated water quality test report from an accredited laboratory is submitted.
Q7. What happens if my STP is undersized?
An undersized STP causes a cascade of problems: the biological treatment stage becomes overloaded within months, BOD and COD in treated water exceed CPCB discharge limits, HSPCB issues a show-cause notice during the next inspection, you are required to upgrade the plant immediately, and retrofitting at an occupied building costs 2–3 times what correct sizing would have cost originally. In severe cases, HSPCB can issue a closure notice to the housing society, hotel or factory.
Not sure which capacity or technology is right for your housing society, hotel, hospital, or factory? Meridian Micro Technology's engineers have been sizing and commissioning STP systems in Faridabad and Delhi NCR since 2000. Share your project details — number of flats, rooms, or workers and available space — and we will provide a no-obligation capacity calculation and preliminary cost estimate within 24 hours.
Call: +91-9811561063 | Email: meridianmicro05@gmail.com
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