STP Size Calculator
How the formula works
The size of a sewage treatment plant (STP) is its daily capacity, usually given in KLD (kilolitres per day, which is the same as cubic metres per day). It is found from the number of people using the building, the water they use, and the part of that water that becomes sewage.
1. Water demand
Water demand (KLD) = persons x litres per person per day / 1000
If the building has more than one use, add the demand of each use.
2. Sewage flow
Sewage flow = water demand x sewage generation factor
A typical factor is 80 % of the water supplied.
3. Design capacity
Design capacity = sewage flow x (1 + design margin)
The next standard STP size equal to or above the design capacity is recommended.
4. Flows
Average flow (m3/h) = capacity / 24
Peak flow = average flow x peak factor
5. Tank volumes (optional)
Biological reactor volume = capacity x retention time (hours) / 24
Equalization tank volume = capacity x detention time (hours) / 24
BOD load (kg/day) = capacity x inlet BOD (mg/L) / 1000
6. Persons served from a known STP
Persons = (capacity / (1 + margin)) x 1000 / (litres per person per day x sewage factor)
Where
- 1 KLD = 1 m3/day = 264.17 US gallons per day
- Persons can also mean beds, seats or workers, depending on the occupancy type
Typical water demand (NBC India style)
- Residential apartment: 135 L per person per day
- Office and day school: 45 L per person per day
- Hostel: 135 L per person per day
- Hotel: 180 L per bed per day
- Hospital: 340 L per bed per day up to 100 beds, 450 L per bed per day above 100 beds
- Restaurant: 70 L per seat per day, cinema or theatre: 15 L per seat per day
- Factory: 45 L per worker per day with bath, 30 L without bath
Typical process retention times (aeration or reactor)
Extended aeration (ASP) about 20 hours, SBR about 16 hours, MBBR about 6 hours, MBR about 5 hours. The process supplier design is final.
How to use the calculator
- Select the calculation and the occupancy type, then enter the number of persons, beds or seats.
- Add a second occupancy if the building has mixed use.
- Select the sewage generation. Add a design margin for future expansion if needed.
- Optionally select the peak factor, process, equalization time and inlet BOD to get flows and tank volumes.
- Click Calculate, then Download PDF Result if you need a record.
Important notes
- Typical values only. The local code, the pollution control board norms and the process vendor design are final.
- Tank volumes use the recommended standard STP size, so they match the plant that will actually be built.
- Flushing water recycling, rainwater and grey water reuse can change the real sewage flow.
Worked example
Example 1 – Residential society with process details
500 residents, 135 L per person per day, sewage 80 %, design margin 10 %, peak factor 3, extended aeration (20 h), equalization 6 h, inlet BOD 250 mg/L.
Water demand = 500 x 135 / 1000 = 67.5 KLD
Sewage flow = 67.5 x 0.80 = 54.0 KLD
Design capacity = 54.0 x 1.10 = 59.4 KLD, so the standard size is 60 KLD
Average flow = 60 / 24 = 2.5 m3/h (0.69 L/s), peak flow = 2.5 x 3 = 7.5 m3/h
Reactor volume = 60 x 20 / 24 = 50 m3, equalization tank = 60 x 6 / 24 = 15 m3
BOD load = 60 x 250 / 1000 = 15 kg/day
Example 2 – Mixed use: office with restaurant
1200 office staff at 45 L and 150 restaurant seats at 70 L.
Water demand = 54.0 + 10.5 = 64.5 KLD
Sewage flow = 64.5 x 0.80 = 51.6 KLD
No margin, so the design capacity is 51.6 KLD and the standard size is 60 KLD.
Example 3 – Hospital with MBBR
150 beds (above 100 beds), 450 L per bed per day, sewage 80 %, peak factor 2.5, MBBR (6 h).
Water demand = 150 x 450 / 1000 = 67.5 KLD, sewage flow = 54.0 KLD
Standard size = 60 KLD, peak flow = 2.5 x 2.5 = 6.25 m3/h
Reactor volume = 60 x 6 / 24 = 15 m3
Example 4 – How many people can a 100 KLD plant serve?
Residential, 135 L per person per day, sewage 80 %, design margin 20 %.
Usable sewage flow = 100 / 1.20 = 83.33 KLD
Sewage per person = 135 x 0.80 = 108 L per day
Persons = 83,333 / 108 = 771.6, rounded down to 771 persons.
Common mistakes
- Using the water supply figure as the sewage flow. Not all water becomes sewage, so a sewage factor (typically 80 %) is applied.
- Using one water demand for the whole building. Offices, restaurants, hospitals and hostels use very different amounts of water per person.
- Counting only residents and forgetting visitors, staff, shops, clubhouse, swimming pool backwash or canteen load.
- Choosing the exact calculated capacity with no margin. An STP running at 100 % from day one has no room for expansion or peak loads.
- Sizing the plant on a smaller size than needed because the calculated value is just above a standard size. Always round up to the next standard size.
- Forgetting the peak factor. The plant treats average flow, but pumps, pipes and the equalization tank must handle the peak.
- Mixing KLD with litres per hour. 1 KLD is 1000 litres per day, not per hour.
- Using the same retention time for every process. Extended aeration, MBBR and MBR need very different reactor volumes.
- Ignoring local norms. Discharge standards, treated water reuse and the pollution control board approval can change the required plant size and process.
Frequently asked questions
What is the formula for STP capacity?
STP capacity (KLD) = number of persons x water demand per person per day x sewage generation factor / 1000, plus a design margin. For example, 500 residents at 135 L per day and 80 % sewage give 54 KLD before margin.
What is KLD in an STP?
KLD means kilolitres per day. One KLD equals 1000 litres per day, or one cubic metre per day.
How much sewage does one person produce per day?
As a typical value, a person in an apartment uses about 135 litres of water per day, and about 80 % of it, which is about 108 litres, becomes sewage.
How is the sewage generation factor decided?
Some water is lost in gardening, cleaning and evaporation, so only part of the supplied water reaches the sewer. A typical factor is 80 %. Some designs use 85 % to 100 % when little water is lost.
What is the water demand for offices and hospitals?
As typical NBC India style values, offices use about 45 L per person per day, hospitals about 340 L per bed per day up to 100 beds, and about 450 L per bed per day above 100 beds.
What are the standard STP sizes?
Common sizes are 10, 15, 20, 25, 30, 40, 50, 60, 75, 100, 125, 150, 200, 250, 300 and 500 KLD, and higher. This calculator recommends the next standard size at or above the design capacity.
What is the peak factor in an STP?
The peak factor is the ratio of the highest hourly flow to the average hourly flow. For small plants a value of about 3 is common. It is used to size pumps, pipes and the equalization tank.
What is the retention time of an STP?
It is the time the sewage stays in a tank. Typical reactor retention times are about 20 hours for extended aeration, 16 hours for SBR, 6 hours for MBBR and 5 hours for MBR. The process supplier gives the final value.
Why is an equalization tank needed?
Sewage flow is high in the morning and evening and low at night. An equalization tank stores the flow and sends it to the plant at a steady rate, so the treatment process works better.
Should I add a margin to the STP capacity?
Yes, many designers add 10 % to 20 % for future expansion and uncertain occupancy. Use the optional design margin field.
Can I use this for a mixed-use building?
Yes. Choose the main occupancy, then add a second occupancy, such as a restaurant in an office building. For more than two uses, add the demands by hand and use the Custom option.
Is this a replacement for a process design?
No. It gives a quick preliminary STP size and tank volumes. Final design depends on the process supplier, the inlet quality, the discharge norms and the approval of the authority.