Code of practice for installation of septic tanks, Part I: design, criteria and construction

IS 2470 Part 1: Scope - Key Specifications & Tables

This standard covers design criteria for septic tanks based on number of users and fixture units.

1. Fixture Units & Peak Discharge (Clause 3.4.1.2)

• Note: 70% of fixture units assumed to discharge simultaneously.

2. Residential Colonies Peak Discharge

• Note: 60% fixtures discharge simultaneously; 1 WC, 1 bath, 1 basin per household.

3. Hostels & Boarding Schools

• Note: 70% fixture units discharge simultaneously.

IS 2470 Part 1 - Key Definitions & Specifications

Definitions (Clause 2.0)

• Fixture Unit: A unit representing the probable discharge from a plumbing fixture.
• Peak Discharge: Maximum expected flow rate during peak usage.

Peak Discharge Tables (Clause 3.4.1.2)

• Note: 70% of fixture units assumed discharging simultaneously.

• Note: 60% fixture units discharging simultaneously.

IS 2470 Part 1 — General Requirements Key Points

1. Septic Tank Sizes (Appendix A, Clause 3.4.5.2)

Recommended sizes for 20 users:

• Freeboard: 300 mm recommended.
• Capacity assumes discharge from WC only.

2. Peak Discharge Estimation (Clause 3.4.1.2)

Small tanks (up to 20 users):

Residential Colonies (up to 300 users):

• Peak discharge based on 60% simultaneous discharge.

Host

Septic Tank Design as per IS 2470 Part 1

Key Design Criteria (Clause 2.1 & 3.4)

• Designed for discharge primarily from water closets (WC).
• Freeboard provision: 300 mm above liquid depth.
• Cleaning interval affects liquid depth (1 or 2 years).

Recommended Sizes for 20 Users (Clause 3.4.5.2, Appendix A)

Additional Specifications

• Freeboard: 0.3 m above liquid depth.
• Capacity: Based on WC discharge only.
• Shape: Rectangular tanks are typical.
• Volume Calculation:

[ V = L \times B \times D ]

Where:

• (L) = Length (m)
• (B) = Breadth (m)
• (D) = Liquid depth (m)

Summary Diagram

flowchart TD
    A[Inlet Wastewater] --> B[Septic Tank]
    B --> C[Settling of Solids]
    B --> D[Scum Layer Formation]
    B --> E[Effluent Outlet]
    E --> F[Soak Pit / Drain Field]

This provides a concise reference for septic tank sizing and design per IS 2470 Part 1.

IS 2470 Part 1: Septic Tank Construction Key Points

1. Design Criteria (Clause 2.1 & 3.4)

• Septic tank design is based on assumptions like discharge from WC only.
• Freeboard of 300 mm must be provided above liquid depth.

2. Recommended Sizes (Clause 3.4.5.2, Appendix A, Table 5)

• Liquid depth varies with cleaning interval; longer intervals require deeper tanks.

3. Specifications (Clause 3.4.5)

• Length to breadth ratio typically around 2:1 to 3:1.
• Minimum liquid depth usually 1.0 to 1.5 m.
• Provision for sludge storage and scum layer considered.

Formula for Volume (approximate):

[ V = L \times B \times D ]

Where:

• (L) = Length (m)
• (B) = Breadth (m)
• (D) = Liquid depth (m)

Add freeboard (0.3 m) above liquid depth for total tank height.

flowchart TB
    A[Wastewater Inlet] --> B[Septic Tank]
    B --> C[Settling of Solids]
    C --> D[Sludge Layer at Bottom]
    C --> E[Scum Layer at Top]
    B --> F[Effluent Outlet]
    F --> G[Soak Pit or Drain Field]

Summary: Use Table 5 for sizing based on users

IS 2470 Part 1 (1985) — Sludge Withdrawal & Maintenance

Key Specifications:

• Sludge Withdrawal Pipe:
  • Minimum diameter: 150 mm
  • Hydrostatic pressure head: ≥ 450 mm
  • Equipped with a delivery valve for controlled sludge withdrawal.
• Alternative Method: Portable pumps can be used instead of fixed sludge pipes/pumps, avoiding manual handling.

Structural Requirements for Sludge Tank Walls:

• Brick walls: Minimum thickness 200 mm
• Stone masonry walls: Minimum thickness 370 mm
• Plastering: Cement mortar 1:3, minimum thickness 12 mm (inside and outside)

Sludge Volume:

• Digested sludge volume generated per capita per day:
  [ V_s = 0.00021 , m^3/\text{capita/day} ]

Summary Table:

flowchart TD
    A[Digested Sludge] -->|Withdraw via dip pipe (≥150 mm dia)| B[Sludge Pipe]
    B -->|Hydrostatic head ≥450 mm| C[Sump with Delivery Valve]
    C -->|Sludge Pump or Portable Pump| D[Sludge Disposal]

Note: Avoid manual sludge handling for hygiene and safety. Use valves or portable pumps for controlled desludging.

IS 2470 Part 1: Commissioning of Septic Tank - Key Points

1. Design Criteria (Clause 2.1 & 3.4)

• Septic tank sizing is based on user count and assumed discharge (usually from WC).
• Freeboard of 300 mm must be provided above liquid depth.
• Liquid depth depends on cleaning interval (1 or 2 years).

2. Recommended Sizes for 20 Users (Appendix A, Clause 3.4.5.2, Table 5)

3. Commissioning Checklist

• Ensure tank dimensions match Table 5 for user load.
• Provide 300 mm freeboard above liquid level.
• Confirm inlet and outlet pipes are correctly positioned to avoid scum and sludge escape.
• Verify tank is watertight and properly ventilated.
• Fill with water before use to check for leaks.

Formula for Volume (approximate):

[ V = L \times B \times D ]

Where:

• (L) = Length (m)
• (B) = Breadth (m)
• (D) = Liquid Depth (m)

This ensures proper retention time and sludge accumulation for effective treatment.

IS 2470 Part 1 (1985) - Disposal of Septic Tank Effluent: Key Points

1. Effluent Disposal Methods (Clause 7.1):
   Effluent from septic tanks must be disposed of as per methods in IS 2470 Part 2 (1985), which includes:

   • Soak pits
   • Subsurface irrigation trenches
   • Open drains (where permitted)

2. Septic Tank Design (Clause 3.4 & 2.1):

   • Design based on assumptions like retention time, sludge volume, and flow rate.
   • Typical retention time: 24 to 48 hours.
   • Volume of tank calculated by:
     [ V = Q \times t ]
     where ( V ) = volume of tank (m³),
     ( Q ) = daily sewage flow (m³/day),
     ( t ) = retention time (hours) / 24.

3. Preparation Before Use (Clause 6.2):

   • Fill tank with water up to outlet level before use.
   • Seed with digested sludge or organic matter to initiate bacterial action.

Typical Soak Pit Design Parameters (from IS 2470 Part 2):

flowchart TD
    A[Septic Tank Effluent] --> B[Disposal Methods as per IS 2470 Part 2]
    B --> C[Soak Pit]
    B --> D[Subsurface Irrigation]
    B --> E[Open Drains (if permitted)]

Summary:
Dispose septic tank effluent using IS 2470 Part 2 methods; design septic tanks with 24-48 hrs retention; seed tanks before use; soak pits are common disposal structures with specified dimensions and filtration.

IS 2470 Part 1: Recommended Sizes of Septic Tanks

Key Specifications (Clause 3.4.5.2 & Appendix A)

• Length to Breadth ratio: 2 to 4 times (Length = 2 to 4 × Breadth)
• Freeboard: 300 mm above liquid depth

Table 5: Recommended Sizes for 20 Users

Notes:

• Capacities assume discharge from WC only.
• Liquid depth varies based on cleaning frequency (1 or 2 years).

Design Formula Summary:

• Volume (V) ≈ Length × Breadth × Liquid Depth
• Ensure freeboard = 0.3 m above liquid depth.

flowchart LR
    A[Users] --> B[Select Length & Breadth]
    B --> C[Length = 2 to 4 × Breadth]
    C --> D[Determine Liquid Depth based on cleaning interval]
    D --> E[Calculate Volume = L × B × Depth]
    E --> F[Add Freeboard 0.3 m]
    F --> G[Finalize Septic Tank Dimensions]

This ensures efficient settling and retention time per IS 2470 Part 1.

IS 2470 (Part 1) - Typical Layouts & Specifications Summary

1. Typical Septic Tank Layouts (Clause 2.4 & Figs. 3-5)

• Two-compartment septic tank dimensions:
  • For population up to 50: max width 525 mm, min width 300 mm
  • For population over 50: same width limits apply
• Twin inlet recommended for tanks wider than 1200 mm

2. Sewage Flow & Fixture Units (Clause 3.4.1.2)

• Peak discharge estimation based on fixture units and number of users.
• Fixture units represent typical sanitary fittings (WC, bath, wash basin).

3. Key Tables for Peak Discharge:

4. **Layout Recommendations (Clause