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Sand and multimedia filter sizing

Enter flow and media type to see how many filters to use, what size, the rate they run at with one in backwash, and how much water and pump the backwash needs.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

Pre-sizing
Flow and filter type

The design rate applies to the filters left in service while one is backwashed. Industrial process water filters often run at 4 to 8 gpm/ft²; for potable water, follow the state rules.

Backwash
Backwash pump
—Recommended filters
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Filtration rate versus number of filters

Rate in the filters in service with one in backwash, for each filter count and the size chosen for it, compared with the reference limit.

Tap the chart to see the values.

Backwash water versus run length

Share of daily production spent on backwash and rinse.

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Typical layers and grading

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Method, assumptions and references

Equations used

Area per filter: A = Q ÷ ((N − 1) · design rate); pressure filter: Ø = √(4A/π), rounded up to the next standard diameter
Actual rates: all in service Q ÷ (N · A); one in backwash Q ÷ ((N − 1) · A)
Backwash: Q_bw = bw rate · A; V = Q_bw · t_bw + rate · A · t_rinse; washes/day = N · 24 ÷ run length; % = V · washes/day ÷ (24 Q)
Fluidized bed loss: Δh = Σ L (1 − ε)(ρs − ρ) ÷ ρ; pump: H = lift + underdrain loss + Δh; P = ρ g Q H ÷ η

Assumptions and limits

  • Identical filters washed one at a time; the design rate applies to N − 1 filters. Porosity 0.42 for sand, 0.50 for anthracite and 0.45 for garnet; densities 2,650, 1,500 and 4,100 kg/m³ (165, 94 and 256 lb/ft³).
  • Rate limits: U.S. common practice of 3 gpm/ft² for sand and 4 gpm/ft² for dual or mixed media (state rules govern); NBR 12216 without pilot tests (7.5 and 15 m/h); industrial, 15 and 20 m/h (6 and 8 gpm/ft²).
  • The pump estimate excludes washwater piping losses and air scour; compute them in the pump calculator.

Standards and technical documents

  • ABNT NBR 12216:1992: Projeto de estação de tratamento de água para abastecimento público
  • Recommended Standards for Water Works (Ten States Standards), Part 4: Treatment
  • AWWA B100: Granular Filter Material
  • EN 12904: Products used for treatment of water intended for human consumption — Silica sand and silica gravel; EN 12909: Anthracite
  • MWH. Water Treatment: Principles and Design, 3rd ed., Wiley, 2012
  • Kawamura, S. Integrated Design and Operation of Water Treatment Facilities, 2nd ed., Wiley, 2000

U.S. regulations

  • EPA 40 CFR 141 Subpart P and T (IESWTR, LT1ESWTR): combined filter effluent turbidity ≤ 0.3 NTU in 95% of samples and never above 1 NTU
  • ASME Boiler and Pressure Vessel Code, Section VIII, and state rules for pressure filter vessels
  • OSHA 29 CFR 1910.147: control of hazardous energy during filter maintenance

Frequently asked questions

What filtration rate should a sand filter use?

Common U.S. design is 2 to 3 gpm/ft² for single sand and up to 4 gpm/ft² for dual or mixed media, with higher rates only when the state approves pilot data. Industrial process water filters often run 4 to 8 gpm/ft².

How do I size a pressure filter diameter?

Divide flow by rate to get area and use D = √(4A/π). For 500 gpm at 4 gpm/ft² with 3 filters and one in backwash, each needs 62.5 ft², or 107 in; the next standard size is 108 in.

What backwash rate does Ten States Standards require?

At least 15 gpm/ft², or the rate that expands the bed 50%, with washwater for at least 15 minutes of one filter at the design rate. With air scour, about 8 gpm/ft² of water is typical.

How much water does backwashing use?

Usually 2% to 5% of production. A 64 ft² filter washed at 16 gpm/ft² for 10 min uses about 10,200 gal; with 3 filters on 24 h runs, that is about 31,000 gal per day.