WTP Size Calculator
How the formula works
A water treatment plant (WTP) cleans raw water, such as borewell, municipal or tanker water, so that it is fit for domestic use. Its size is the treated water flow it must deliver, usually given in cubic metres per hour (m3/h). It is found from the daily water demand and the hours the plant runs.
1. Treated water demand
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. Design capacity and treated flow
Design capacity = demand x (1 + design margin)
Treated flow (m3/h) = design capacity / operating hours
The next standard WTP size equal to or above this flow is recommended.
3. Raw water flow
Raw water flow = treated flow / (1 – plant water loss)
The plant loses some water in filter backwash, softener regeneration or RO reject, so more raw water is needed than treated water.
4. Filter size (optional)
Filter area = raw water flow / filtration rate
Diameter per filter = square root of (4 x area per filter / pi)
The next standard vessel diameter is recommended, and the actual filtration rate is shown.
5. Raw water tank and chlorine (optional)
Raw water tank volume = raw water flow x storage hours
Chlorine (kg/day) = raw water per day x dose (mg/L) / 1000
6. Persons served from a known WTP
Persons = (capacity x hours / (1 + margin)) x 1000 / litres per person per day
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 values used
- Water demand (NBC India style): residential 135 L per person per day, office and day school 45, hostel 135, hotel 180 per bed, hospital 340 per bed up to 100 beds and 450 above, restaurant 70 per seat, cinema 15 per seat, factory 45 with bath and 30 without
- Plant water loss: about 5 % for filtration only, about 8 % with a softener, about 30 % with RO (reject water)
- Filtration rate of pressure sand and activated carbon filters: about 10 m/h, in the range 8 to 12 m/h
- Chlorine dose: about 1 to 3 mg/L, depending on the water quality
- Operating hours: about 16 to 20 hours per day
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 plant operating hours and the treatment train.
- Optionally add a design margin, a filtration rate, number of filters, raw water storage and chlorine dose.
- Click Calculate, then Download PDF Result if you need a record.
Important notes
- Typical values only. The raw water quality, the local code and the process vendor design are final.
- Raw water flow, filter size, tank volume and chlorine use the recommended standard size, so they match the plant that will actually be built.
- This calculator does not size the softener, RO membranes or the treated water storage tank. Use the manufacturer data for those.
Worked example
Example 1 – Residential society with filters, tank and chlorine
500 residents at 135 L per person per day, 16 operating hours, filtration + softener (8 % loss), design margin 10 %, filtration rate 10 m/h, 2 working filters, raw water storage 6 hours, chlorine 2 mg/L.
Demand = 500 x 135 / 1000 = 67.5 KLD
Design capacity = 67.5 x 1.10 = 74.25 KLD
Treated flow = 74.25 / 16 = 4.64 m3/h, so the standard size is 5 m3/h
Raw water flow = 5 / (1 – 0.08) = 5.43 m3/h, which is 86.96 KLD per day
Filter area = 5.43 / 10 = 0.543 m2, or 0.272 m2 per filter. Diameter = 0.59 m, so the standard vessel is 0.6 m and the actual filtration rate is 9.61 m/h
Raw water tank = 5.43 x 6 = 32.6 m3
Chlorine = 86.96 x 2 / 1000 = 0.174 kg/day
Example 2 – Office with restaurant
1200 office staff at 45 L and 150 restaurant seats at 70 L, 12 operating hours, filtration only (5 % loss).
Demand = 54.0 + 10.5 = 64.5 KLD
Treated flow = 64.5 / 12 = 5.38 m3/h, so the standard size is 7.5 m3/h
Raw water flow = 7.5 / 0.95 = 7.89 m3/h (94.74 KLD per day).
Example 3 – Hospital with RO
200 beds (above 100 beds) at 450 L per bed per day, 20 operating hours, filtration + RO (30 % loss), filtration rate 12 m/h, 1 filter.
Demand = 200 x 450 / 1000 = 90 KLD
Treated flow = 90 / 20 = 4.5 m3/h, so the standard size is 5 m3/h
Raw water flow = 5 / 0.70 = 7.14 m3/h
Filter area = 7.14 / 12 = 0.60 m2, diameter 0.87 m, so the standard vessel is 0.9 m, with an actual rate of 11.23 m/h.
Example 4 – How many people can a 10 m3/h plant serve?
Residential, 135 L per person per day, 16 operating hours, design margin 20 %.
Daily capacity = 10 x 16 = 160 KLD
Usable demand = 160 / 1.20 = 133.33 KLD
Persons = 133,333 / 135 = 987.6, rounded down to 987 persons.
Common mistakes
- Sizing the plant on the daily demand without dividing by the operating hours. A WTP is rated in m3/h, not in litres per day.
- Forgetting the plant water loss. Filter backwash, softener regeneration and especially RO reject mean you need more raw water than treated water.
- Ignoring the raw water source. Borewell, municipal and tanker water have very different quality, and the treatment train depends on it.
- Using the same plant for drinking and flushing without checking the demand split. Flushing water often does not need RO treatment.
- Choosing the exact calculated flow with no margin. A plant at 100 % load from day one has no room for growth or maintenance downtime.
- Running the plant for 24 hours in the calculation but not in practice. Plants need time for backwash, regeneration and maintenance.
- Using a very high filtration rate to save cost. A high rate gives poor filtrate quality and shorter filter runs.
- Using one very large filter instead of several smaller ones. A single filter cannot be backwashed without stopping the plant, so use at least two filters where supply cannot stop.
- Forgetting the raw water tank and the treated water storage tank, which are needed to run the plant at a steady rate.
Frequently asked questions
What is a WTP?
A WTP (water treatment plant) treats raw water so that it is safe and suitable for use. Typical steps are filtration, softening, RO and disinfection.
How is WTP capacity calculated?
Find the daily treated water demand from the number of people and their water use per day, then divide by the operating hours of the plant. For example, 74.25 KLD over 16 hours gives about 4.64 m3/h.
What is the water demand per person?
As typical NBC India style values, a residence needs about 135 litres per person per day, an office about 45, and a hospital about 340 to 450 litres per bed per day.
Why is the raw water flow higher than the treated flow?
Some water is lost in filter backwash, softener regeneration and RO reject. A plant with an RO unit may lose about 30 % of its feed water, so the raw water flow must be higher.
What is a typical filtration rate for a pressure sand filter?
About 10 m/h is typical, usually in the range 8 to 12 m/h. A lower rate gives better water quality and longer filter runs.
How is the filter diameter calculated?
Filter area = raw water flow / filtration rate. For one filter, diameter = square root of (4 x area / pi). For several filters, divide the area by the number of working filters first.
How many filters do I need?
Use at least two working filters if the supply must continue during backwash. Many plants also add a standby unit. Use the optional filter field to see the effect of more filters.
What chlorine dose is typical?
A dose of about 1 to 3 mg/L is common, but the real dose depends on the water quality, the contact time and the residual chlorine required at the end user.
How many hours should a WTP run?
Many plants are designed for about 16 to 20 hours per day, which leaves time for backwash, regeneration and maintenance.
Does this calculator size the softener or RO?
No. It gives the flow the softener or RO has to handle and the water loss for planning. The vendor must size the resin volume, membranes and pumps from the raw water analysis.
What is the difference between a WTP and an STP?
A WTP treats incoming fresh water before use, while an STP treats the used water (sewage) after use. Both are often needed in the same building. See the STP Size Calculator for sewage.
Is this a replacement for a process design?
No. It gives a quick preliminary plant size. Final design depends on the raw water test report, the treated water standard and the process vendor.