Requirements for slow sand filters, Part 1: General guidelines

IS 11401 Part 1 — Scope & General Guidelines

• Scope: Defines general principles and guidelines for the standard’s application, without specific formulas or tables.
• Rounding Off (Clause 0.4):
  Final test or calculation results must be rounded per IS 2:1960 rules:
  • Round to the same number of significant digits as the specified value.
  • Ensures consistency and clarity in compliance checks.

Key Points:

IS 2:1960 Rounding Rules Summary:

• If the digit after the last significant digit is >5, round up.
• If <5, round down.
• If =5, round to the nearest even digit (to avoid bias).

flowchart TD
    A[Test/Calculation Result] --> B{Round off as per IS 2:1960?}
    B -->|Yes| C[Round to specified significant digits]
    B -->|No| D[Use raw value]
    C --> E[Compliance decision]

For detailed formulas or tables, refer to subsequent parts of IS 11401.

IS 11401 Part 1: Description of Basic Elements of Slow Sand Filters

This part covers general principles and basic elements of slow sand filters used in water treatment.

Key Elements & Specifications

• Filter Bed:

  • Depth: Typically 0.6 to 1.2 m
  • Media: Clean, uniformly graded sand with effective size (D10) ~ 0.15 to 0.35 mm
  • Uniformity coefficient (Cu) ≤ 3

• Filter Rate:

  • Typical filtration rate: 0.1 to 0.3 m³/m²/hr

• Underdrain System:

  • Designed to evenly collect filtered water without disturbing the sand bed.

• Head Loss:

  • Initial head loss: ~ 0.3 to 0.5 m
  • Maximum allowable head loss before cleaning: ~ 1.0 m

Basic Formula for Filtration Rate (Q):

[ Q = A \times v ]

Where:

• ( Q ) = flow rate (m³/hr)
• ( A ) = filter surface area (m²)
• ( v ) = filtration velocity (m/hr)

Typical Sand Specifications Table

flowchart TD
    RawWater -->|Inlet| FilterBed[Slow Sand Filter Bed]
    FilterBed -->|Filtered Water| Underdrain[Underdrain System]
    Underdrain --> Outlet[Outlet for Clean Water]

This summarizes the basic elements and their typical parameters as per IS 11401 Part 1.

Principle of Purification of Water (IS 11401 Part 1)

Key Concepts:

• Purification begins in the supernatant raw water layer where large particles settle and smaller particles agglomerate into flocs.
• Biological activity in the Schmutzdecke (top 0.4-0.5 m of filter bed) breaks down organic matter and pathogens.
• Water changes direction through the bed, enhancing particle contact and attachment to filter grains covered with a sticky biofilm.
• Biological oxidation converts organic matter into water, CO₂, and harmless salts.
• Deeper layers remove impurities by adsorption and chemical oxidation.

Important Parameters & Specifications

Performance Summary (Table 1 from Clause 4.1)

Filtration Rate Formula

[ Q = A \times v ]

Where:

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

Diagram: Water Purification Process in Slow Sand Filter

flowchart TD
    A[Raw Water] --> B[Supernatant Layer]
    B --> C[Schmutzdecke (Bioactive Layer)]
    C --> D[Filter Bed]
    D --> E[Filtered Water Outlet]
    
    subgraph Biological Processes
        C -->|Bio-oxidation| D
    end
    
    subgraph

IS 11401 Part 1: Performance of Slow Sand Filter – Key Points

Filtration Rate

• Recommended filtration rate: 0.1 to 0.2 m³/m²/hr (equivalent to 0.1 to 0.2 m/hr)
• This low filtration rate ensures effective transport, attachment, and purification mechanisms within the filter bed.

Oxygen Content

• Minimum dissolved oxygen: 0.5 mg/l in filter influent water
• Below this, biomass activity reduces, causing anaerobic conditions and release of impurities.
• Prevent anaerobic conditions by:
  • Aeration of raw water
  • Pre-sedimentation
  • Recycling aerated effluent to the filter inlet
  • Ventilation of outlet weir chamber

Detention Time

• Sufficient detention time in the filter bed is critical for purification.
• Detention time (t) can be approximated by:

[ t = \frac{V}{Q} = \frac{A \times d}{Q} ]

Where:

• (V) = volume of filter bed (m³)
• (A) = filter bed area (m²)
• (d) = effective sand depth (m)
• (Q) = flow rate (m³/hr)

Summary Table

flowchart LR
  RawWater --> Aeration[Pre-Aeration]
  Aeration --> FilterBed[Slow Sand Filter Bed]
  FilterBed --> Outlet[Filtered Water Outlet]
  Outlet --> Ventilation[Outlet Weir Ventilation]
  FilterBed --> Recycling[Recycle Aerated Effluent]
  Recycling --> RawWater

Note: Ensure proper maintenance and monitoring of oxygen levels and filtration rates for optimal slow sand filter performance.

Applicability and Limitations of Slow Sand Filters (IS 11401 Part 1)

• Filtration Rate:
  Maintain a slow filtration rate between 0.1 to 0.2 m³/m²/hr (equivalent to 0.1 to 0.2 m/h) to ensure effective purification.

• Detention Time:
  Sufficient detention time in the filter bed is critical for biological and physical purification mechanisms to function properly.

• Oxygen Content:
  Oxygen concentration must be ≥ 0.5 mg/l entering the filter bed. Below this, biomass activity reduces, potentially causing anaerobic conditions and release of undesirable impurities.

• Aeration:
  To prevent anaerobic conditions:

  • Pre-aerate raw water
  • Use pre-sedimentation
  • Recycle aerated effluent to the reservoir
  • Provide ventilation in outlet weir chamber

Summary Table: Key Parameters

flowchart LR
    RawWater --> PreAeration
    PreAeration --> Sedimentation
    Sedimentation --> SlowSandFilter
    SlowSandFilter --> FilteredWater
    FilteredWater --> OutletWeirChamber
    OutletWeirChamber --> Ventilation
    OutletWeirChamber --> RecycleEffluent
    RecycleEffluent --> RawWater

This ensures effective slow sand filtration per IS 11401 Part 1 guidelines.