How to use this calculator
- Enter the flow and the number of vessels sharing it.
- Choose a filtration rate suited to the raw water.
- Set backwash rate and time.
- Read the vessel size, backwash flow and media quantities.
How do you calculate the size of a pressure sand filter?
Area (m²) = Q (m³/h) ÷ filtration rate (m/h)
Diameter = √(4 × Area ÷ π), rounded up to 50 mm
Backwash flow = Area × backwash rate
Media mass uses bulk density 1,600 kg/m³ for sand and 1,700 kg/m³ for gravel.
Worked example
With flow to filter 10 m³/h, number of working vessels 1, filtration rate 12 m/h, backwash rate 35 m/h, backwash time 10 minutes, filter sand depth 0.6 m, support gravel depth 0.3 m:
Vessel diameter: 1,050 mm
The calculator also gives filter area per vessel 0.866 m²; actual filtration rate 11.5 m/h; backwash flow 30.3 m³/h.
Assumptions and limits
- Feed turbidity under about 20 to 50 NTU; higher needs a clarifier first.
- Vessel sizes in practice follow standard shell diameters; pick the next standard size up.
Frequently asked questions
What is the filtration rate of a pressure sand filter?
Most pressure sand filters run at 8 to 15 m³ per m² per hour. Multigrade filters can run at 15 to 20 m/h. Rapid gravity filters in municipal plants run slower, at about 5 to 7.5 m/h.
How often should a sand filter be backwashed?
Backwash when pressure drop rises by about 0.5 to 0.7 bar above the clean value, or daily for most industrial filters.
What media is used in a pressure sand filter?
Typically 0.45 to 0.55 mm effective size filter sand over graded gravel layers. Multigrade filters add anthracite on top.
References
- CPHEEO, Manual on Water Supply and Treatment (1999)
- BIS IS 8419, Requirements for filtration equipment
- AWWA, Water Treatment Plant Design, 5th ed.