Rainwater Harvesting System Design for a Residential House

Given Data:

• Roof Area = 150 m²
• Average Annual Rainfall = 1000 mm
• Runoff Coefficient = 0.8
• Daily Water Requirement = 500 liters/day
• Duration for Storage Tank Supply = 3 months (~90 days)

1. Calculate Total Rainwater Harvested per Year

2. Design the Size of the Storage Tank

Water needed for 3 months =

Storage Tank Capacity Required = 45,000 liters

You can choose to install:

• One large tank of 45,000 liters
  or
• Three tanks of 15,000 liters each (modular design for easy cleaning and maintenance)

3. Overflow System Design

• When rainfall exceeds storage capacity (after the tank fills), excess water should be safely diverted to avoid flooding.
• Overflow Pipe:
  • Should be slightly lower than the tank rim.
  • Divert water to a soak pit, recharge well, or storm drain.
• Use PVC/HDPE pipe of minimum 110 mm diameter.

4. Filtration System Design

Before Storage (Pre-filtration):

• Mesh Filter at roof outlets to block leaves and debris.
• First Flush Diverter to discard the first few liters of dirty rainwater.

During Storage:

• Store in covered tank to avoid contamination, mosquito breeding.

After Storage (Post-filtration):

• Sand-Charcoal Filter or UV Purifier if water is used for domestic purposes like drinking or cooking.

5. Overall Layout Design

Rain falls on roof
↓
Gutters collect water
↓
Mesh filters remove debris
↓
First flush diverter removes initial runoff
↓
Filtered water enters storage tank (45,000 liters)
↓
Overflow pipe diverts excess to soak pit or drain
↓
Pump system (if needed) for internal household distribution

Summary Table