What is the recommended ratio of orifice area to filter bed area?

The total area of perforations or orifices should be between 0.2 to 0.3 percent of the filter bed area (Clause 5.2.1).

What is the maximum allowable head loss through the underdrain system during backwash?

The head loss should not exceed 1.0 meter of water at the maximum backwash rate (Clause 5.2.2).

What are the maximum permitted water velocities in the manifold and laterals?

Velocity should not exceed 1.8 m/s in the manifold and 2.4 m/s in the laterals during maximum discharge (Clause 5.2.3).

What is the typical spacing for laterals in a pipe grid underdrain system?

The laterals are typically spaced between 15 cm and 30 cm from centre to centre (Clause 5.3.1).

IS 8419

: 2000

Requirements for filtration equipment, Part 1: Filtration media - sand and gravel

CurrentSpecializedSpecificationBIMWater Resources · Public Health Engineering

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This standard specifies the requirements for the underdrainage system of rapid sand gravity filters used in water treatment. It covers pipe-and-nozzle grid systems and false bottom with nozzle systems, outlining design criteria, material specifications, and construction requirements to ensure uniform collection of filtered water and efficient distribution of backwash water.

Requirements for filtration equipment, Part 1: Filtration media - sand and gravel

Quick Reference Values

Ratio of total orifice area to filter area0.2 to 0.3 percent

Maximum head loss during backwash1.0 m of water

Maximum velocity in manifold1.8 m/s

Maximum velocity in laterals2.4 m/s

Centre to centre spacing of laterals15 to 30 cm

Centre to centre spacing of nozzles on false bottom15 to 30 cm

Practical Notes

Proper leveling of the underdrain system is critical to ensure uniform filtration and backwashing; even minor deviations can lead to channeling and poor filter performance.

The choice between a pipe grid and a false bottom system depends on the plant scale, cost, and maintenance. False bottom systems generally provide better backwash distribution but can be more expensive.

Clogging of orifices or nozzles is a common operational issue. Ensure the design facilitates easy inspection and cleaning where possible.