What is the standard rate of filtration for a slow sand filter?

The rate of filtration should be between 0.1 and 0.2 m³/m²/h (Clause 5.2.1).

What is the required effective size (D10) for the filter sand?

The effective size should be between 0.15 mm and 0.35 mm (Clause 6.2.1).

When should a slow sand filter be cleaned?

When the head loss reaches the maximum design limit, typically 0.8 to 1.2 m (Clause 8.3).

What is the minimum depth of the sand bed before it needs replenishment?

The sand bed should be replenished with new sand when its depth is reduced to about 0.5 to 0.6 m after repeated scrapings (Clause 9.2).

IS 11401

: 2000

General requirements for slow sand filters, Part 2: Design, construction, operation and maintenance

CurrentFrequently UsedCode of PracticeBIMWater Resources · Public Health Engineering

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This standard provides guidelines for the design, construction, operation, and maintenance of slow sand filters used in water treatment. It specifies critical parameters like filtration rates, characteristics of filter media (sand and gravel), and procedures for filter operation and cleaning. The code is essential for designing effective and reliable water purification systems, particularly for small to medium-sized communities.

General requirements for slow sand filters, Part 2: Design, construction, operation and maintenance

Quick Reference Values

Rate of filtration0.1 to 0.2 m/h

Effective size of sand (D10)0.15 to 0.35 mm

Uniformity coefficient of sand (Uc)1.8 to 3.0

Initial depth of sand bed900 to 1200 mm

Minimum depth of water over sand bed1.0 m

Maximum allowable head loss0.8 to 1.2 m

Minimum sand bed depth after scraping500 to 600 mm

Key Formulas

Filter bed area (A) = Q / v — Where Q is flow rate (m³/h) and v is rate of filtration (m/h)

Practical Notes

Slow sand filters are highly effective for microbiological purification but are best suited for raw water with low turbidity (ideally < 20 NTU, max 50 NTU).

The biological layer, or 'Schmutzdecke', is crucial for filtration efficiency and takes several days to weeks to develop (filter ripening). The filtered water should be run to waste during this period.

Cleaning is done by scraping off the top 1.5-2.5 cm of sand, not by backwashing. The sand bed must be replenished when it reaches the minimum permissible depth after several scrapings.