equipment for grit removal devices

IS 6279: Scope - Key Details & Tables

1. Scope Summary (Clause 1.1)

• Covers overflow rates for grit removal devices.
• Guidance values for theoretical maximum permissible overflow rates for substantially complete removal of grit particles.

2. Overflow Rates for Grit Removal (Table 2)

• Note: Scale factors needed in practice for turbulence, short circuiting.
• Temperature variation (4.5°C to 26.5°C) has negligible effect.

3. Materials of Construction (Table 1, Clause 3.1)

This summary highlights the scope related to grit removal overflow rates and materials per IS 6279. For design, use Table 2 overflow rates adjusted by site-specific factors.

flowchart

IS 6279: Grit Removal Devices - Key Points

Types of Grit Removal Devices (Clause 3.8)

• Constant Velocity Channels: Channels designed to maintain a uniform flow velocity to settle grit.
• Square Tanks: Grit chambers that may be:
  • Without mechanical devices (gravity settling).
  • With mechanical grit removal (scrapers, pumps).

Continuous Mechanical Grit Removal Devices (Clause 4.2)

• Devices operate continuously to remove grit.
• Typically include:
  • Grit classifiers or hydrocyclones.
  • Mechanical scrapers or conveyor systems to collect settled grit.

Typical Design Parameters (from general practice aligned with IS 6279)

Basic Formula for Settling Velocity (Stokes' Law for grit particles):

[ v_s = \frac{(d^2)(g)(\rho_p - \rho)}{18 \mu} ]

• (v_s) = settling velocity (m/s)
• (d) = particle diameter (m)
• (g) = acceleration due to gravity (9.81 m/s²)
• (\rho_p) = density of grit particle (kg/m³)
• (\rho) = density of fluid (kg/m³)
• (\mu) = dynamic viscosity of fluid (Pa·s)

flowchart LR
    A[Influent Wastewater] --> B[Constant Velocity Channel]
    B --> C[Grit Chamber (Square Tank)]
    C --> D[Settling of Grit]
    D --> E{Mechanical Removal?}
    E -- Yes --> F[Continuous Mechanical Grit Removal Device]
    E -- No --> G[Manual/Periodic Removal]
    F --> H[Disposal of Grit]
    G --> H
    H --> I[Treated Water to Next Process]

For detailed equipment specs, refer to IS 6279 clauses on mechanical device dimensions and operational

IS 6279: Materials of Construction for Grit Removal Devices

Notes:

• Concrete and masonry follow respective IS codes for structural design and durability.
• Steel and cast iron components must comply with IS standards for mechanical strength and corrosion resistance.
• Plastic floats are used for buoyancy; material choice depends on chemical resistance.

Quick Reference Table (Materials & IS Codes)

IS 6279: Design & Operational Requirements for Grit Removal Devices

1. Overflow Rates (Clause A-1.1 & Table 2)

Theoretical maximum permissible overflow rates for substantially complete grit removal depend on grit particle size and specific gravity (sp gr):

2. Materials of Construction (Clause 3.1 & Table 1)

3. Operational Guidance

• Mechanical grit removal is recommended for sewage flows ≥ 400 m³/h.
• Design criteria are indicative; adapt based on sewage treatment scheme specifics.
• Overflow rates ensure effective grit removal without excessive hydraulic loading.

flowchart LR
    A[Sewage Inlet] --> B[Grit Removal Device]

IS 6279: Intermittent Mechanical Grit Removal Devices

Key Points from Clause 4.3 & 2.1:

• Intermittent Type Devices remove grit settled at the bottom of the grit chamber periodically, not continuously.
• Mechanical means include grab, bucket, travelling scraper, or similar equipment.
• Grit is collected in a settling tank and removed at intervals to prevent accumulation.

Design & Specification Highlights:

• Settling velocity of grit particles: Typically 0.3 to 0.5 m/s.
• Tank dimensions: Designed to provide sufficient detention time (usually 2-3 minutes) for grit to settle.
• Intermittent removal frequency: Depends on grit accumulation rate; typically once or twice per day.

Typical Parameters:

Formula for Detention Time (t):

[ t = \frac{V}{Q} ]

Where:

• ( V ) = Volume of grit chamber (m³)
• ( Q ) = Flow rate (m³/s)

Conceptual Diagram of Intermittent Grit Removal

flowchart LR
    A[Influent Wastewater] --> B[Grit Settling Tank]
    B --> C[Grit Settled at Bottom]
    C --> D[Mechanical Removal Device]
    D --> E[Grit Disposal]
    B --> F[Effluent Outflow]

Summary: IS 6279 specifies intermittent grit removal devices operate by periodic mechanical extraction of settled grit, ensuring efficient grit separation with minimal operational complexity.

IS 6279: Continuous Mechanical Grit Removal Devices (Clause 4.2)

Though IS 6279 provides specifications for grit removal equipment, detailed formulas and tables for continuous mechanical grit removal devices are generally based on design principles from hydraulic and mechanical engineering:

Key Design Parameters:

• Flow rate (Q): m³/s
• Grit particle size: Typically 0.2 to 2 mm
• Settling velocity (V_s): Depends on grit density (~2650 kg/m³) and water properties
• Retention time (t): Ensures grit settles without organic matter loss

Typical Design Formula:

For grit settling velocity (Stokes’ Law approximation for small particles):

[ V_s = \frac{(g \cdot d^2 \cdot (\rho_s - \rho))}{18 \mu} ]

Where:

• (g) = acceleration due to gravity (9.81 m/s²)
• (d) = particle diameter (m)
• (\rho_s) = density of grit (kg/m³)
• (\rho) = density of water (kg/m³)
• (\mu) = dynamic viscosity of water (Pa·s)

Typical Specifications:

• Channel slope: 1:10 to 1:30 for grit transport
• Velocity in grit chamber: 0.3 to 0.6 m/s (to wash organic matter)
• Grit removal efficiency: >90% for particles >0.2 mm

Suggested Table: Typical Grit Removal Device Parameters

flowchart LR
    A[Influent Flow] --> B[Grit Chamber]
    B --> C[Continuous Mechanical Grit Removal Device]
    C --> D[Settled Grit Collection]
    D --> E[Grit Disposal]
    C --> F[Effluent to Treatment]

For detailed dimensions and equipment specs, refer to IS

IS 6279: Grit Washing Equipment - Key Points

Definition (Clause 2.3):
Grit washing equipment is used in grit removal units (intermittent or continuous) to wash grit free from organic matter before disposal.

Key Specifications (from IS 6279 and general practice):

• Purpose: Remove organic matter from grit to reduce odor and volume in disposal.
• Types:
  • Screw type grit washers
  • Log washers
  • Grit classifiers with washing action

Typical Design Parameters:

Basic Formula for Grit Washer Capacity:

[ Q = A \times v ]

Where:

• ( Q ) = Flow rate of washing water (m³/hr)
• ( A ) = Cross-sectional area of grit bed (m²)
• ( v ) = Velocity of water through grit bed (m/hr)

Functional Flow Diagram:

flowchart LR
    A[Grit Removal Unit] --> B[Grit Washing Equipment]
    B --> C[Organic Matter Removal]
    C --> D[Washed Grit Disposal]

Note: For detailed dimensions and design, refer to IS 6279 clauses and manufacturer data sheets.

IS 6279 - Velocity Control Devices in Grit Removal

Key Points from IS 6279:

• Velocity Control Devices regulate flow velocity in grit removal channels to ensure effective grit settling (Clause 2.4).
• Max velocity depends on particle size & specific gravity; e.g., for 1.5 mm size, sp. gravity 2.3, max velocity ≈ 0.17 m/s (Clause 3.7).
• Design surface area for peak flow, particle size, specific gravity, and temperature (Clause 3.7).
• Grit removal devices include proportional flow weirs, flumes, constant velocity channels, or square tanks (Clauses 2.4, 3.8).

Important Table: Overflow Rates for Grit Removal (Table 2)

Velocity Control Formula (Conceptual):

[ V = \frac{Q}{A} ]

• (V) = flow velocity (m/s)
• (Q) = flow rate (m³/s)
• (A) = cross-sectional area of grit channel (m²)

Design (A) such that (V \leq) max permissible velocity (e.g., 0.17 m/s for 1.5 mm particles).

Summary:

• Use velocity control devices (weirs, flumes) to maintain velocity ≤ 0.17 m/s for typical grit.
• Design surface area based on peak flow and particle characteristics.
• Refer Table 2 for overflow rates to ensure effective grit removal.

flowchart TD
    A[Influent Flow Q] --> B[Velocity Control Device]
    B --> C[Grit Removal Channel]
    C --> D[Settling of Grit Particles]
    D --> E[Effluent Flow]

This ensures

Safety and Maintenance Provisions per IS 6279

Key Specifications:

• Walkways/Staircases/Ladders (Clauses 4.2.6 & 4.3.5):

  • Minimum width: 90 cm
  • Handrails: Provided on both sides
  • Material: Chequered plates or steel gratings (approved design)

• Turn Table / Drive Units (Clause 4.2.8):

  • Preferably oil-immersed type with bearings
  • Weather-proof protection required
  • Easily accessible lubrication points
  • Overload alarm: audible and visible

Materials of Construction (Clause 3.1, Table 1):

Summary Diagram of Safety Access Provisions

flowchart LR
    A[Equipment] --> B[Walkway/Staircase/Ladder]
    B --> C{Width ≥ 90 cm?}
    C -- Yes --> D[Handrails on both sides]
    C -- No --> E[Non-compliant]
    D --> F[Material: Chequered plates or Steel Gratings]
    F --> G[Safe Access Ensured]

Note: For detailed design and installation, refer to respective IS codes listed above for materials and electrical components.

IS 6279 - Electrical & Mechanical Controls for Grit Removal Devices

Key Specifications:

• Motors, Starters, Time Switches, Electrode Switches:

  • Refer IS: 325-1970 (3-phase induction motors)
  • IS: 996-1964 (single-phase small AC motors)
  • IS: 1766-1961 (time switches)
  • IS: 1822-1967 (AC motor starters ≤ 1000V)

• Mechanical Components Materials:

  • Turn Table: High-grade cast iron (IS: 210-1970)
  • Reduction Gear: Steel or suitable material (IS: 1570-1961, IS: 1871-1965)
  • Gates: Cast iron or steel (IS: 3042-1965, IS: 226-1969)
  • Scrapers: Fabricated steel sheets (IS: 513-1963)
  • Shaft: Cold finished steel (IS: 1570-1961)
  • Floats: Plastic (polyethylene, PVC, glass fibre reinforced polyester)
  • Channels: Steel (IS: 3954-1964)

Mechanical Control Notes:

• Turn Table & Drive Units:
  • Preferably oil-immersed type with bearings
  • Weatherproof protection mandatory
  • Easily accessible lubrication points
  • Overload alarm (audible & visible) required (Clause 4.2.8)

Practical Considerations:

• Use scale factors for turbulence and short-circuiting effects in design.
• Materials must comply with relevant IS codes for durability and performance.

Summary Table of Materials:

IS 6279: Hydraulic Design Parameters for Grit Removal Devices

Key Specifications & Tables

1. Materials of Construction (Clause 3.1)

2. Overflow Rates for Grit Removal (Table 2, Clause A-1.1)

Notes:

• Overflow rate is critical for achieving substantially complete removal of grit particles.
• Use specific gravity values depending on the grit composition.
• Design civil works and mechanical components per referenced IS codes for durability and performance.

flowchart TD
    A[Grit Removal Device] --> B[Hydraulic Design]
    B --> C[Overflow Rate]
    B --> D[Materials of Construction]
    C --> E[Grit Size & Specific Gravity]
    D --> F[Civil Works]
   

Installation and Ventilation Requirements (IS 6279)

Clause 4.2.3 Highlights:

• Provide adequate forced ventilation and lighting for grit removal devices.
• Use low voltage and/or flame-proof lamps for safety.
• Ventilation via ground-level blowers with flexible ducting to swiftly expel air from grit removal units.

Key Specifications & Tables

1. Overflow Rates for Grit Removal (Table 2 - Clause 1.1)

Use these rates to size grit removal units for effective particle settling.

2. Materials of Construction (Table 1 - Clause 3.1)

Summary Diagram of Ventilation Setup

flowchart LR
    A[Ground Level Blower