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

IS 11401 Part 2: Scope & Key Design Criteria for Slow Sand Filters

Scope Summary

• Applies to design and construction of slow sand filters for rural water supply.
• Ensures compliance with IS 2:1960 for rounding off numerical values.

Key Design Criteria (Table 1)

Additional Notes

• Layout should allow easy future expansion (Clause 4.4).
• Design ensures continuous 24-hour operation.
• Filter sizing based on population water demand and filtration rate.

flowchart TD
    A[Water Supply] --> B[Slow Sand Filter]
    B --> C[Filter Bed: 0.8-0.9 m Sand Depth]
    B --> D[Underdrain + Gravel: 0.3-0.5 m]
    B --> E[Supernatant Water: 1 m]
    C --> F[Filtration Rate: 0.1-0.2 m/h]
    F --> G[Water Demand: 40 lpcd]

This concise scope and design criteria ensure effective rural water filtration per IS 11401 Part 2.

IS 11401 Part 2: Referenced Indian Standards - Key Points

• Rounding Off (IS 2:1960)

  • Final test/analysis values must be rounded as per IS 2 rules.
  • Retain the same number of significant digits as the specified value.

• Relevant IS Codes for Water Retaining Structures (Clause 6.1)
  Construction must comply with applicable IS standards such as:

  • IS 456:2000 – Plain and Reinforced Concrete Design
  • IS 3370 (Part 1-4) – Concrete Structures for Storage of Liquids
  • IS 800:2007 – General Construction in Steel
  • IS 875 (Part 3) – Wind Loads
  • IS 1893 – Earthquake Resistant Design

Summary:
Follow IS 2 for rounding off results. Use relevant IS codes (especially IS 3370 and IS 456) for design and construction of water retaining structures.

flowchart TD
    A[IS 11401 Part 2] --> B[Construction of Water Retaining Structures]
    B --> C[Follow Relevant IS Codes]
    C --> D[IS 3370 - Concrete Storage Structures]
    C --> E[IS 456 - Concrete Design]
    C --> F[IS 2 - Rounding Off Rules]

IS 11401 Part 2: Suitability & Design Criteria for Slow Sand Filtration

Key Design Parameters (Table 1, Clause 5.1)

Suitability Notes (Clause 3)

• Slow sand filtration is ideal for rural water supplies with low turbidity.
• Effective for biological purification due to biofilm development.
• Requires low maintenance and continuous operation.

Summary Formula for Flow Rate (Q):

[ Q = A \times v ]

Where:

• ( Q ) = flow rate (m³/h)
• ( A ) = filter bed area (m²)
• ( v ) = filtration rate (m/h, 0.1 to 0.2)

flowchart TD
    RawWater -->|Pre-treatment| SettlingTank --> SlowSandFilter --> TreatedWater
    SlowSandFilter -->|Filtration rate 0.1-0.2 m/h| TreatedWater
    SlowSandFilter -->|Filter bed area ≥ 200 m²| Underdrain
    Underdrain --> GravelLayer

References: IS 8419 (Part 1) for filtration media specifications.

IS 11401 Part 2: Plant Location and Lay-Out – Key Points

Location Criteria (Clause 4.1)

• Raw water should be gravitated to filters to avoid pumping costs.
• Select sites with stable ground and good drainage.
• Avoid flood-prone or contaminated areas.

Plant Lay-Out (Clauses 4.3 & 4.4)

• Compact layout to facilitate operator tasks and minimize travel.
• Design for easy future expansion without disrupting existing operations.
• Logical sequence: raw water intake → pre-treatment → slow sand filters → clear water tank.

Design Considerations (Clause 3.1)

• Slow sand filters suitable if raw water turbidity ≤ 30 NTU.
• For turbidity > 30 NTU, pre-treatment (e.g., sedimentation, coagulation) required.

Typical Layout Flow (Mermaid Diagram)

flowchart LR
    A[Raw Water Source] --> B[Pre-treatment (if turbidity >30 NTU)]
    B --> C[Slow Sand Filters]
    C --> D[Clear Water Tank]
    D --> E[Distribution]

Summary Table: Turbidity & Treatment

Tip: Ensure site allows gravity flow to reduce operational energy costs and design layout for easy access and maintenance.

IS 11401 Part 2: Annotated Design Criteria for Slow Sand Filters

Key Design Criteria (Clause 5.1, Table 1)

Notes:

• Layout should allow easy future expansion (Clause 4.4).
• Design ensures continuous operation and adequate filtration capacity for rural water supply.
• Sand quality critical for filtration efficiency; uniformity coefficient indicates particle size distribution.

Simple Filtration Rate Formula:

[ Q = A \times v_f ]

Where:

• ( Q ) = Flow rate (m³/h)
• ( A ) = Filter bed area (m²)
• ( v_f ) = Filtration rate (m/h) (0.1 to 0.2 m/h)

flowchart TD
    A[Water Inlet] --> B[Supernatant Water (1 m)]
    B --> C[Sand Filter Bed (0.8-0.9 m)]
    C --> D[Underdrain + Gravel (0.3-0.5 m)]
    D --> E[Filtered Water Outlet]

This summarizes essential design parameters for slow sand filters per IS 11401 Part 2.

IS 11401 Part 2: Construction Key Points for Slow Sand Filters

General Design Criteria (Table 1, Clause 5.1)

Construction Notes:

• Follow relevant IS specifications for materials and workmanship.
• Ensure uniform sand grading for effective filtration.
• Maintain minimum filter sand depth for longevity and performance.
• Provide adequate underdrain height for drainage and support.

flowchart TD
    A[Water Supply] --> B[Slow Sand Filter]
    B --> C[Supernatant Water (1 m)]
    C --> D[Filter Sand Layer (0.8 - 0.9 m)]
    D --> E[Underdrain + Gravel (0.3 - 0.5 m)]
    E --> F[Filtered Water Collection]

This summarizes construction essentials per IS 11401 Part 2 for slow sand filters in rural water supply.

IS 11401 Part 2: Operation and Maintenance Key Points

Valve Installation (Clause 6.5)

• Outlet valve, inter-connecting valve, and filter drain-out valve must be in dry sluice valve chambers for easy maintenance.
• Other valves preferably in dry chambers.
• Valves in wet chambers require extension spindles and capstan heads for operation.

General Design Criteria for Slow Sand Filters (Clause 5.1, Table 1)

Maintenance Tips:

• Frequent valve operation in dry pits to prevent sticking.
• Monitor sand depth; replenish when below minimum.
• Maintain filtration rate within specified limits for optimal performance.

flowchart LR
    A[Water Inlet] --> B[Filter Bed (Sand Layer)]
    B --> C[Underdrain + Gravel Layer]
    C --> D[Outlet Valve in Dry Chamber]
    D --> E[Distribution]
    style D fill:#f9f,stroke:#333,stroke-width:2px

This ensures ease of operation and maintenance as per IS 11401 Part 2.

Throwing Over Method of Resanding (IS 11401 Part 2: Clause 7.5 & Annex A)

Purpose:
Restore sand depth from minimum permissible 0.5–0.6 m to design level 0.8–1.0 m without cumulative fouling.

Key Specifications & Process:

• Minimum sand depth before resanding: 0.5–0.6 m above gravel.
• Design sand depth after resanding: 0.8–1.0 m.
• Avoid placing new sand directly over old sand to prevent fouling and increased initial head loss.
• Throwing Over Process:
  • Excavate bottom sand layer in strips without disturbing gravel.
  • Move excavated sand from one strip aside as a ridge.
  • Fill excavated trench with new sand.
  • Excavate adjacent strip and throw its removed sand over the new sand in the previous strip.
  • Continue strip-wise until whole bed is resanded.
  • Use ridge sand from first strip to cover new sand in last strip.

Benefits:

• Prevents cumulative fouling.
• Ensures bottom sand layer is scraped eventually.
• Leaves biologically active sand on top for quick re-ripening.

Gravel Layer Specifications (Clause 5.5):

Diagram: Throwing Over Method

flowchart LR
    A[Excavate strip 1 sand] --> B[Stack sand in ridge]
    B --> C[Fill strip 1 with new sand]
    C --> D[Excavate strip 2 sand]
    D --> E[Throw strip 2 sand over new sand in strip 1]
    E --> F[Repeat for all strips]
    F --> G[Use ridge sand from strip 1 to cover last strip]

Note: Always ensure the gravel layer remains undisturbed during excavation. This method is crucial for maintaining filter efficiency and longevity.