Code of practice for installation of septic tanks, Part II: Secondary treatment and disposal of septic tank effluent

IS 2470 Part 2: Scope - Key Specifications & Tables

Seepage Pit Design (Clause 5.2.1):

• Minimum cross-sectional dimension: 0.90 m
• Minimum depth below invert of inlet pipe: 10 m
• Lining: stone, brick, or concrete blocks with dry open joints backed by ≥ 75 mm clean coarse aggregate
• Inlet pipe depth: taken down to 0.90 m from top (anti-mosquito measure)
• Masonry ring at top to prevent surface run-off flooding

Allowable Rate of Effluent Application (Table 2, Clause 5.1):

• Note: Soils with percolation > 30 min unsuitable for soakaways; > 60 min unsuitable for any soil absorption system.
• Absorption area for seepage pits = effective side wall area (from 150 mm below inlet pipe invert to pit bottom).

Percolation Rate (Clause 1.6):
Time in minutes for water to fall 25 mm in soil test.

flowchart TD
    A[Seepage Pit] --> B[Min dimension 0.90 m]
    A --> C[Depth ≥ 10 m below inlet invert]
    A --> D[Lining with open joints + 75 mm aggregate]
    A --> E[Inlet pipe depth 0.90 m from top]
    A --> F[Masonry ring at top]
    
    G[Effluent Application Rate] --> H[Based on percolation rate]
    H --> I[Refer Table 2 for max rates]
    I --> J[Soil suitability based on percolation]

Summary: IS 2470 Part 2 governs septic tank effluent disposal, emphasizing seepage pit dimensions

IS 2470 Part 2 - Key Definitions & Specifications

Definitions (Clause 2.0 & 2.8)

• Seepage Pit: A pit for effluent disposal, minimum cross-section 0.90 m, and depth ≥ 10 m below inlet pipe invert. Lined with stone/brick/concrete blocks with dry joints backed by 75 mm coarse aggregate. Top portion may be narrowed for RCC cover slabs.
• Surface Water: Run-off from precipitation and other water flowing over the ground surface.

Seepage Pit Specifications (Clause 5.2.1)

• Depth below inlet invert: ≥ 10 m
• Minimum cross-sectional dimension: 0.90 m
• Lining: Stone/brick/concrete blocks with dry joints + 75 mm coarse aggregate backing
• Anti-mosquito inlet pipe depth: 0.90 m from top
• Masonry ring at top to prevent flooding

Allowable Effluent Application Rates (Table 2, Clause 5.1)

• Note:
  • Absorption area for trenches = trench bottom area
  • Absorption area for seepage pits = effective side wall area (from 150 mm below inlet pipe invert to bottom)
  • Soils with percolation > 30 min unsuitable for soakaways; > 60 min unsuitable for any absorption system

flowchart TD
    A[Effluent Inlet Pipe] --> B[Seepage Pit]
    B --> C[Side Walls with Dry Joints + 75 mm Aggregate]
    B --> D[Bottom of Pit]
    E[Surface Water] --> F[Masonry Ring at Pit Top]

This summary captures essential definitions, pit design, and effluent application rates per IS 2470 Part 2.

Preliminary Data for Design (IS 2470 Part 2)

Key Specifications:

• Seepage Pit Dimensions (Clause 5.2.1):
  • Minimum cross-sectional dimension: 0.90 m
  • Minimum depth below invert of inlet pipe: 10 m
  • Lining: stone, brick, or concrete blocks with dry open joints backed by 75 mm clean coarse aggregate
  • Inlet pipe depth: 0.90 m from top (anti-mosquito measure)
  • Masonry ring at top to prevent flooding

Soil & Site Data (Clause 3.1):

• Soil exploration to determine horizons, permeability, water table, and drainage
• Percolation test to find percolation rate (time in minutes for water to fall 25 mm)
• Soil classification as per IS:1498-1970
• Site plan with buildings, ground levels, septic tank discharge, and nearby water bodies

Percolation Rate & Effluent Application (Table 2, Clause 5.1):

• Soils with percolation >30 min unsuitable for soakaways; >60 min unsuitable for any soil absorption system.

Formula for Percolation Rate:

[ \text{Percolation Rate} = \frac{\text{Time for water to fall 25 mm (min)}}{25 \text{ mm}} ]

Notes:

• Absorption area for trenches = trench bottom area
• Absorption area for seepage pits = effective sidewall area (from 150 mm below inlet pipe invert to bottom)

flowchart TD
    A[Soil Exploration] --> B[Percolation Test]
    B --> C{Percolation Rate}
    C -->|<30 min| D[Suitable for Soakaways

IS 2470 Part 2: Methods of Treatment and Disposal of Septic Tank Effluent

Key Specification:

The disposal method depends on subsoil water level, soil type, and percolation rate (time for water to percolate 25 mm soil):

Notes:

• Percolation Rate: Time in minutes for 25 mm water to infiltrate soil.
• Disposal methods vary to prevent groundwater contamination.
• Administrative approval required before work (Clause 3.3).

Summary Table (from Clause 6.0):

| Subsoil Water Level | Soil Type & Percolation Rate | Disposal Method                                  |
|---------------------|------------------------------|-------------------------------------------------|
| Within 1.8 m        | Porous, ≤ 30 min             | Dispersion trench in mound                       |
|                     | Porous, 30-60 min            | Dispersion trench in mound                       |
|                     | Dense/clay, > 60 min         | Biological filter or upflow anaerobic filter    |
| Below 1.8 m         | Porous, ≤ 30 min             | Seepage pit or dispersion trench                 |
|                     | Porous, 30-60 min            | Dispersion trench                                |
|                     | Dense/clay, > 60 min         | Subsurface biological filter or upflow anaerobic filter |

Key Specifications & Formulas for Design of Soil Absorption Systems (IS 2470 Part 2)

1. Types of Soil Absorption Systems

• Seepage Pit: Minimum cross-section 0.90 m; depth ≥ 10 m below invert of inlet pipe.
• Dispersion Trench

2. Seepage Pit Construction (Clause 5.2.1)

• Pit lined with stone, brick, or concrete blocks with dry open joints.
• Backfill with ≥ 75 mm clean coarse aggregate.
• Masonry ring at top to prevent flooding.
• Inlet pipe depth: 0.90 m from top (anti-mosquito measure).

3. Allowable Rate of Effluent Application (Table 2, Clause 5.1)

• Note:
  • For seepage pits, absorption area = effective side wall area (depth from 150 mm below inlet pipe invert).
  • For dispersion trenches, absorption area = trench bottom area.
  • Soil unsuitable if percolation > 30 min for soakaways, > 60 min for any soil absorption system.

4. Design Formula for Effluent Application Rate

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

Where:

• (Q) = allowable effluent application rate (l/m²/day) from Table 2
• (V) = volume of effluent (l/day)
• (A) = absorption area (m²)

Additional Recommendations (Clause 3.1)

• Conduct percolation test as per Appendix A to determine soil permeability.
• Obtain detailed soil profile and water table info.
• Use IS 1498-1970 for soil classification.

flowchart TD

IS 2470 Part 2 - Biological Filters: Key Points & Formulas

1. Purpose & Application

• Treat septic tank effluent in impervious soils, waterlogged areas, or limited land availability.
• Biological filters use a stone/sand medium supporting microbial films that oxidize organic matter.
• Achieves ~70% BOD removal; effluent is clear and odor-free.

2. Capacity

• Filter volume = 0.04 to 0.05 m³ per capita
• Or 1/3 to 1/2 of septic tank liquid capacity.

3. Filter Types & Distribution

• Shapes: Rectangular (Fig. 4A) or Circular (Fig. 4B).
• Distribution:
  • Fixed serrated channels or
  • Rotary arm distributor for even effluent spread.
• Air vent must be 150 mm above ground level.
• Flexible joints recommended on inlet/outlet rigid pipes.

4. Design Considerations

• Ample ventilation and efficient underdrain system are essential.
• Upflow (reverse) submerged filters suit dense soils and high water table.
• Effluent introduced from bottom; microbial growth retained on media.

5. Typical Dimensions & Notes

6. Flow Concept Diagram

flowchart TD
    A[Septic Tank Effluent] --> B[Distribution System]
    B --> C[Biological Filter Media]
    C --> D[Microbial Film Oxidation]
    D --> E[Underdrain System]
    E --> F[Treated Effluent Discharge]

Summary: Design biological filters with proper volume, ventilation, and distribution to ensure efficient secondary treatment of septic tank effluents, especially in challenging soil conditions.

Upflow Anaerobic Filters (IS 2470 Part 2 - 1985) Key Points

Types (Clause 7.1)

• Single chambered rectangular
• Double chambered rectangular
• Circular type

Design & Operation (Clause 7.0, 7.2.2)

• Effluent from septic tank enters bottom of filter.
• Microbial growth retained on stone media (20 mm coarse aggregate).
• Capacity:
  [ 0.04 \text{ to } 0.05 , m^3/\text{capita} \quad \text{or} \quad \frac{1}{3} \text{ to } \frac{1}{2} \text{ of septic tank liquid capacity} ]
• BOD removal: ~70%
• Media depth (Double chambered rectangular):
  • 1st chamber: 0.55 m depth (20 mm coarse medium)
  • 2nd chamber: 0.45 m depth (20 mm coarse aggregate)
• Effluent flows up through media, exits over a V-notch weir 75 mm above media top.
• Gas vents must be provided (air vent 150 mm above ground).

Specifications

• Filter media: 20 mm coarse aggregate
• Flow control: Perforated trays and valves for desludging
• Effluent disposal: Surface drain or grass plot; disinfection if contamination risk (Clause 6.6)

Typical Dimensions (mm)

Flow Diagram (Simplified Upflow Anaerobic Filter)

flowchart TD
    A[Septic Tank Effluent] --> B[Bottom Inlet of Filter]
    B --> C[Filter Media (Stone Aggregate)]
    C --> D[Microbial Digestion & BOD Removal]
    D --> E[V-notch Weir Outlet]
    E --> F[Effluent Discharge or Disinfection

IS 2470 Part 2 (1985) — Installation Practices for Septic Tanks

Key Specifications:

• Seepage Pit Dimensions (Clause 5.2.1):
  • Minimum cross-sectional dimension: 0.90 m
  • Minimum depth below inlet invert: 10 m
  • Lining: stone, brick, or concrete blocks with dry joints, backed by 75 mm clean coarse aggregate.
  • Inlet pipe depth: 0.90 m from top (anti-mosquito measure).
  • Masonry ring at top to prevent flooding.

Percolation Rate & Effluent Application (Table 2, Clause 5.1):

• Note:
  • Absorption area for seepage pits = effective side wall area (from 150 mm below inlet invert to pit bottom).
  • Soil with percolation >30 min unsuitable for soakaways; >60 min unsuitable for any soil absorption.

Percolation Rate Calculation (Clause 1.6):

• Time (minutes) for water to fall 25 mm in soil.

Summary Diagram of Seepage Pit Installation:

graph TD
A[Inlet Pipe] -->|Depth 0.90 m| B[Seepage Pit]
B --> C[Lining: stone/brick/concrete blocks]
C --> D[Backfill: 75 mm coarse aggregate]
B --> E[Masonry ring at top]
E --> F[Prevents surface flooding]

Use these guidelines for proper septic tank effluent disposal ensuring soil suitability and effective seepage pit design per IS 2470 Part 2.

IS 2470 (Part 2) - Maintenance & Operation Key Points

1. Percolation Rate Calculation (Clause 1.6)

• Time in minutes for water to fall 25 mm in soil.
• Used to assess soil suitability for effluent absorption.

2. Allowable Effluent Application Rates (Table 2, Clause 5.1)

• Note: Soil unsuitable if percolation > 30 min for soakaways; > 60 min unsuitable for any soil absorption.

3. Seepage Pit Specifications (Clause 5.2.1)

• Minimum cross-section: 0.90 m
• Minimum depth: 10 m below inlet invert.
• Lining: stone/brick/concrete blocks with dry joints + 75 mm coarse aggregate backing.
• Top masonry ring to prevent flooding.
• Inlet pipe depth: 0.90 m (anti-mosquito).

4. Recommended Disposal Methods (Table 1, Clause 6.0)

Percolation Test Procedure (IS 2470 Part 2)

Test Setup:

• Pour water to 300 mm depth over gravel.
• Wait 24 hours for soil swelling.
• If water remains, reduce depth to 150 mm.
• Measure water level drop over 30 minutes from a fixed reference point.
• If no water remains, refill to 150 mm and measure drop every 30 minutes for 4 hours.
• Use drop in the final 30 minutes to calculate percolation rate.

For Sandy/Porous Soils:

• If 150 mm water seeps in less than 30 minutes, measure drop every 10 minutes for 1 hour.
• Use drop in final 10 minutes for rate calculation.

Key Formula for Percolation Rate (P)

[ P = \frac{\text{Time interval (minutes)}}{\text{Drop in water level (mm)}} ]

• Expressed in minutes per mm or converted to minutes per inch as needed.

Table: Allowable Rate of Effluent Application (Clause 5.1)

• Note: Soils with percolation > 30 min unsuitable for soakaways; > 60 min unsuitable for any soil absorption.

Seepage Pit Specifications (Clause 5.2.1)

• Minimum cross-section: 0.90 m
• Minimum depth: 10 m below inlet pipe invert
• Lining: stone/brick/concrete blocks with 75 mm coarse aggregate backing
• Top portion may be narrowed for RCC cover slabs
• Inlet pipe depth: 0.90 m from top (anti-mosquito)
• Masonry ring to prevent surface flooding

flowchart TD
    A[