Code of practice for installation of septic tanks, Part I: design, criteria and construction
1985 Edition

Overview of Scope and Key Parameters

This section defines the design parameters for septic tanks based on user count and fixture units.

1.1 Fixture Units and Peak Flow Rates (Clause 3.4.1.2)

• Note: 70% of fixtures assumed active simultaneously.

1.2 Peak Discharge for Residential Settlements

• Note: 60% of fixtures assumed discharging simultaneously; each household includes 1 toilet, 1 bath, and 1 basin.

1.3 Hostels and Boarding Facilities

• Note: 70% simultaneous fixture use assumed.

Key Definitions and Peak Flow Specifications

2.1 Definitions (Clause 2.0)

• Fixture Unit: A measure representing anticipated discharge from a plumbing fixture.
• Peak Discharge: The maximum flow rate expected during peak usage periods.

2.2 Peak Discharge Tables (Clause 3.4.1.2)

• Note: 70% fixtures active simultaneously.

• Note: 60% fixtures assumed discharging simultaneously.

General Guidelines and Specifications

3.1 Recommended Tank Dimensions (Appendix A, Clause 3.4.5.2)

Suggested sizes for septic tanks serving up to 20 users:

• Freeboard height recommended: 300 mm.
• Design capacity assumes discharge mainly from water closets.

3.2 Peak Flow Calculations (Clause 3.4.1.2)

For small tanks (≤ 20 users):

Residential colonies (≤ 300 users):

• Calculations based on 60% simultaneous fixture use.

Design Elements According to IS 2470 Part 1

4.1 Design Foundations (Clauses 2.1 & 3.4)

• Tank designed primarily for wastewater from water closets.
• Provide a freeboard of 300 mm above liquid level to prevent overflow.
• Liquid depth varies depending on cleaning frequency, typically 1 or 2 years.

4.2 Standard Dimensions for 20 Users (Clause 3.4.5.2 & Appendix A)

4.3 Additional Details

• Freeboard of 0.3 m above the liquid surface is mandatory.
• Capacity calculations assume discharge solely from toilets.
• Tanks are generally rectangular.
• Volume determined by:

[ Volume = Length \times Width \times Liquid\ Depth ]

4.4 Process Flow Diagram

flowchart TD
    Input[Wastewater Inlet] --> Tank[Septic Tank]
    Tank --> Settling[Solid Sedimentation]
    Tank --> Scum[Scum Layer Formation]
    Tank --> Output[Effluent Outlet]
    Output --> Disposal[Soak Pit or Drain Field]

This section summarizes the sizing and design recommendations provided by the standard.

Core Construction Guidelines of IS 2470 Part 1

5.1 Design Assumptions (Clauses 2.1 & 3.4)

• Tank dimensions are based on discharge from water closets only.
• A freeboard of 300 mm above liquid level must be maintained.

5.2 Recommended Dimensions (Clause 3.4.5.2, Appendix A, Table 5)

• Depth varies with cleaning frequency; longer intervals require deeper tanks.

5.3 Construction Requirements (Clause 3.4.5)

• Length-to-width ratio generally between 2:1 and 3:1.
• Minimum liquid depth ranges between 1.0 and 1.5 meters.
• Provision for sludge and scum accumulation is essential.

5.4 Volume Calculation

[ Volume = Length \times Width \times Liquid\ Depth ]

Add a freeboard of 0.3 m above the liquid depth to obtain total tank height.

5.5 Process Flow Illustration

flowchart TB
    Inlet[Wastewater Inlet] --> Tank[Septic Tank]
    Tank --> Settling[Solids Settling]
    Settling --> Sludge[Sludge Accumulation]
    Settling --> Scum[Scum Layer]
    Tank --> Outlet[Effluent Outlet]
    Outlet --> Drain[Soak Pit or Drain Field]

This ensures structural integrity and effective treatment as per IS 2470 Part 1.

Guidelines for Sludge Evacuation and Upkeep per IS 2470 Part 1 (1985)

6.1 Sludge Withdrawal System

• Minimum sludge withdrawal pipe diameter: 150 mm.
• Hydrostatic head of at least 450 mm must be maintained.
• Delivery valve should be installed to regulate sludge removal.
• Portable pumps may be used as an alternative to fixed pipes to avoid manual handling.

6.2 Structural Requirements for Sludge Tank Walls

• Brick walls should be at least 200 mm thick.
• Stone masonry walls require a minimum thickness of 370 mm.
• Both inside and outside walls must have a cement mortar plaster finish of at least 12 mm thickness (1:3 mix).

6.3 Sludge Volume Estimation

• Digested sludge production per person per day is approximately 0.00021 cubic meters.

6.4 Summary Table

6.5 Sludge Withdrawal Process Flow

flowchart TD
    Sludge[Digested Sludge] -->|Withdraw via ≥150 mm dip pipe| Pipe[Sludge Pipe]
    Pipe -->|Maintain hydrostatic head ≥450 mm| Sump[Sump with Delivery Valve]
    Sump -->|Use fixed or portable pump| Disposal[Sludge Disposal]

Note: Manual sludge handling is discouraged for health and safety. Use valves or portable pumps for controlled desludging.

Procedures for Septic Tank Commissioning as per IS 2470 Part 1

7.1 Design Basis (Clauses 2.1 & 3.4)

• Tank size is selected based on expected number of users and typical discharge (usually from toilets).
• Maintain a freeboard of 300 mm above liquid level.
• Liquid depth depends on planned cleaning interval (1 or 2 years).

7.2 Standard Dimensions for 20 Users (Appendix A, Clause 3.4.5.2)

7.3 Checklist for Commissioning

• Confirm tank dimensions comply with Table 5 for the expected load.
• Ensure 300 mm freeboard above liquid level.
• Verify inlet and outlet pipe positions to prevent scum and sludge escape.
• Check for watertightness and proper ventilation.
• Pre-fill tank with water to detect leaks before use.

7.4 Volume Calculation

[ Volume = Length \times Width \times Liquid\ Depth ]

Proper commissioning guarantees effective retention time and sludge accumulation for optimal treatment.

Effluent Management from Septic Tanks (IS 2470 Part 1, 1985)

1. Disposal Methods (Clause 7.1): Effluent should be managed following IS 2470 Part 2 guidelines, including:

   • Soak pits
   • Subsurface irrigation trenches
   • Open drains where allowed

2. Septic Tank Design Considerations (Clauses 3.4 & 2.1):

   • Designed based on retention time, sludge volume, and flow rate.
   • Typical retention duration: 24 to 48 hours.
   • Tank volume computed by: [ V = Q \times \frac{t}{24} ] where (V) = tank volume (m³), (Q) = daily sewage flow (m³/day), (t) = retention time (hours).

3. Pre-Use Preparation (Clause 6.2):

   • Fill tank with water to outlet level before operation.
   • Seed with digested sludge or organic material to promote bacterial activity.

Typical Soak Pit Specifications (from IS 2470 Part 2):

flowchart TD
    Effluent[Septic Tank Effluent] --> Disposal[Disposal Methods per IS 2470 Part 2]
    Disposal --> SoakPit[Soak Pit]
    Disposal --> Irrigation[Subsurface Irrigation Trenches]
    Disposal --> OpenDrains[Open Drains (if permitted)]

Summary: Effluent disposal must adhere to IS 2470 Part 2 standards, ensuring proper retention and treatment before release.

Standard Dimensions for Septic Tanks (IS 2470 Part 1)

Design Parameters (Clause 3.4.5.2 & Appendix A)

• Length to width ratio from 2:1 to 4:1.
• Freeboard of 300 mm above liquid level.

Table 5: Suggested Sizes for 5 to 20 Users

Notes:

• Capacity based on toilet discharge only.
• Depth varies with cleaning frequency.

Design Process Flow

flowchart LR
    Users[Number of Users] --> SelectDimensions[Determine Length & Width]
    SelectDimensions --> Ratio[Length = 2 to 4 × Width]
    Ratio --> Depth[Set Liquid Depth per Cleaning Interval]
    Depth --> Volume[Calculate Volume = L × W × Depth]
    Volume --> AddFreeboard[Add 0.3 m Freeboard]
    AddFreeboard --> Finalize[Finalize Tank Dimensions]

This ensures effective retention and settling capacity.

Illustrative Layouts and Design Details (IS 2470 Part 1)

B.1 Typical Tank Configurations (Clause 2.4 & Figures 3-5)

• Two-chamber septic tanks have width limits:
  • For populations up to 50, max width 525 mm, minimum 300 mm.
  • Same width limits apply above 50 users.
• For tanks wider than 1200 mm, twin inlet pipes are advised.

B.2 Sewage Flow and Fixture Units (Clause 3.4.1.2)

• Peak flow rates are calculated using fixture units per number of users.
• Fixture units correspond to typical sanitary fittings such as toilets, baths, and basins.

B.3 Peak Flow Tables

B.4 Layout Notes (Clause 2.4)

• Tank compartments and inlet/outlet arrangements should follow specified dimensions to ensure proper sedimentation and flow.