How to build septic tank

 🏗️ Septic Tank Calculation & Construction Process (Complete Guide)

📌 Introduction

A septic tank is an essential part of a household wastewater management system, especially in areas where a sewer system is not available. Proper design and calculation of a septic tank ensure efficient waste treatment, long life, and minimal maintenance issues.

In this blog, we’ll cover:

Septic tank size calculation

Design principles

Step-by-step construction process

📐 1. Basics of Septic Tank Design

A septic tank works on the principle of sedimentation and anaerobic digestion:

Solid waste settles at the bottom (sludge)

Oil/grease floats on top (scum)

Liquid wastewater flows out to soak pit or drainage.

📊 2. Septic Tank Size Calculation

✅ Step 1: Calculate Water Consumption

Average water usage per person:

135 liters/day/person (as per Indian standards)

👉 Example:

For a family of 5 members:

Total water = 5 × 135 = 675 liters/day

✅ Step 2: Retention Time

Standard retention time:

24 hours (1 day)

👉 So, tank capacity = daily wastewater = 675 liters

✅ Step 3: Add Sludge Storage

Sludge storage = 30–50% extra volume

👉 Total tank capacity:

675 + 40% = ~950 liters (approx.)

✅ Step 4: Final Tank Size (Dimensions)

Typical proportions:

Length : Width = 2:1 to 3:1

Depth = 1.2 m to 1.8 m

👉 Example Size:

Length = 2.0 m

Width = 1.0 m

Depth = 1.2 m

🧱 3. Standard Septic Tank Dimensions (Quick Table)

No. of Users

Tank Size (L × B × H in meters)

3–5 persons

2.0 × 1.0 × 1.2

6–10 persons

2.5 × 1.2 × 1.5

10–20 persons

3.0 × 1.5 × 1.8

🏗️ 4. Septic Tank Construction Process

🔹 Step 1: Excavation

Dig pit as per required size

Provide extra space for wall thickness

🔹 Step 2: Base Preparation

Lay 100–150 mm PCC (Plain Cement Concrete)

Ensure proper leveling

🔹 Step 3: Wall Construction

Use brick masonry or RCC walls

Thickness: 200–230 mm (brick wall)

🔹 Step 4: Partition Wall

Divide tank into 2 chambers

First chamber: 2/3 length

Second chamber: 1/3 length

👉 Important: Provide opening at mid-height for flow

🔹 Step 5: Plastering & Waterproofing

Internal plaster: 1:3 cement mortar

Add waterproofing compound

🔹 Step 6: Inlet & Outlet Pipe

Inlet pipe slightly higher than outlet

Use PVC pipes (100 mm dia)

Provide T-joint for proper flow

🔹 Step 7: Slab Casting

RCC top slab with manholes

Provide 2 inspection covers

🔹 Step 8: Vent Pipe

Install vent pipe (50 mm dia)

Height: 2–3 meters

Cover with mosquito mesh

⚠️5. Important Design Tips

✔ Never make septic tank too small

✔ Always provide partition (very important)

✔ Maintain slope in inlet pipe

✔ Do not connect rainwater to septic tank

✔ Clean tank every 2–3 years

❌ Common Mistakes to Avoid

No partition inside tank

Wrong slope in pipes

Using poor waterproofing

No vent pipe

Direct discharge without soak pit

🏁 Conclusion

A properly designed septic tank ensures:

Efficient wastewater treatment

Less maintenance

Long-lasting performance

By following the correct calculation and construction process, you can avoid common problems like choking, overflow, and bad odor.