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IS 11401:2000 (Part 1) is the Indian Standard (BIS) for requirements for slow sand filters, part 1: general guidelines. 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.
Requirements for slow sand filters, Part 1: General guidelines
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
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.
Consolidated list per BIS. For the text of each amendment, refer to the BIS portal link above.
sandgravelconcreteunderdrainage system
International Equivalents
Similar International Standards
Slow Sand Filtration for Community Water Supply (1978)WHO / IRC (International Reference Centre for Community Water Supply and Sanitation), Netherlands
HighCurrent
Slow Sand Filtration for Community Water Supply, Technical Paper No. 11
Provides comprehensive design, construction, and operation guidelines for slow sand filters, mirroring the IS code's intent.
Slow Sand Filtration (1991)American Water Works Association (AWWA), USA
HighCurrent
Slow Sand Filtration
A detailed manual of practice covering the principles, design, and operation of slow sand filters, highly aligned with the IS code.
BS EN 12904:2005British Standards Institution (BSI), UK / European Committee for Standardization (CEN)
LowCurrent
Products used for treatment of water intended for human consumption - Sand and gravel
Specifies requirements for sand and gravel filter media, which is only one component of the overall IS 11401 guidelines.
Guidelines for Drinking-water Quality, 4th EditionWorld Health Organization (WHO), Switzerland
MediumCurrent
Guidelines for Drinking-water Quality, Fourth Edition
Provides the foundational principles and performance targets for water treatment processes, including slow sand filtration, but not detailed design specifications.
Key Differences
≠IS 11401 provides more prescriptive limits for raw water turbidity (preferably <10 NTU, max 50 NTU), whereas international guidelines (e.g., WHO) focus on performance and strongly recommend pre-treatment for water consistently above 10-20 NTU.
≠The Indian standard specifies a narrower range for sand effective size (0.20-0.40 mm) compared to some international guidelines like WHO/IRC (0.15-0.35 mm).
≠IS 11401 can be more prescriptive in its construction details, such as underdrainage system layout, while AWWA and WHO manuals often present a variety of acceptable design options to achieve the same hydraulic performance.
≠International guidelines often place a stronger emphasis on the biological maturation period ('ripening') being performance-based (i.e., until effluent quality is met), whereas IS 11401 provides more of a time-based guideline.
Key Similarities
≈Both IS 11401 and international guidelines recognize the 'schmutzdecke' (biological layer) as the primary mechanism for filtration and purification.
≈The fundamental operational principle of using a low filtration rate (typically 0.1-0.2 m/h) is a core similarity across all standards.
≈All standards specify a similar method for cleaning the filter, which involves draining the unit and manually scraping off the top 1-2 cm of sand and the schmutzdecke.
≈The general structure of the filter is consistent, comprising a supernatant water layer, a main sand bed, a graded gravel support layer, and an underdrainage system.
Parameter Comparison
Parameter
IS Value
International
Source
Filtration Rate
0.1 to 0.2 m/h
0.1 to 0.4 m/h (rates >0.2 m/h only for very clean raw water)
WHO/IRC Technical Paper No. 11
Sand Bed Effective Size (ES)
0.20 to 0.40 mm
0.15 to 0.35 mm
WHO/IRC Technical Paper No. 11
Sand Uniformity Coefficient (UC)
Less than 3, preferably less than 2
Generally < 3, preferably 1.5 to 2.5
AWWA Slow Sand Filtration (1991)
Initial Sand Bed Depth
900 to 1200 mm
800 to 1200 mm (1000 mm is typical)
WHO/IRC Technical Paper No. 11
Minimum Sand Bed Depth (before resanding)
600 mm
500 to 600 mm
AWWA Slow Sand Filtration (1991)
Depth of Supernatant Water
1.0 to 1.5 m
0.8 to 1.5 m
WHO/IRC Technical Paper No. 11
Maximum Raw Water Turbidity
Should not exceed 50 NTU; preferably below 10 NTU
Should ideally be below 10-20 NTU for consistent operation
WHO Guidelines for Drinking-water Quality
Gravel Support Layer Depth
300 to 500 mm (in 3 to 5 graded layers)
Approx. 300 mm (in 3 to 4 graded layers)
WHO/IRC Technical Paper No. 11
⚠ Verify details from original standards before use
Key Values7
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)
Tables & Referenced Sections
Key Tables
Table 1 - Recommended Standards for Raw Water Quality for Slow Sand Filtration