Rainwater Harvesting for Indian Buildings — Design Guide
Rainwater harvesting (RWH) is mandatory for new constructions above specified plot sizes in most Indian states, and is a prerequisite for IGBC, LEED, and GRIHA green building ratings. Beyond compliance, a well-designed RWH system significantly reduces municipal water dependence and groundwater withdrawal. This guide covers collection, filtration, storage, and recharge system design for Indian commercial and residential projects.
1. RWH Regulations Across Indian States
State / City | RWH Mandatory For | Authority |
Tamil Nadu | All buildings with plot >300 m² | TWAD / ULB |
Karnataka (Bangalore) | All buildings with plot >2400 m²; 30 m+ height | BWSSB / BDA |
Delhi | All buildings with plot >100 m² | DJB / MCD |
Maharashtra (Mumbai, Pune) | Plots >300 m² or floor area >500 m² | MCGM / PMC |
Gujarat | All buildings with plot >300 m² | GWSSB / ULB |
Rajasthan | All buildings with plot >500 m² | PHED / ULB |
Telangana | All buildings with plot >300 m² | HMWSSB |
West Bengal | All buildings >15m height | KMC / state |
2. RWH System Components
Component | Function | Key Specification |
Catchment area (roof) | Collects rainfall | Impervious roof = 0.85–0.95 runoff coefficient |
Gutters and downpipes | Convey water from roof | GI / UPVC — sized for peak intensity |
First flush diverter | Discards first 1–2mm of rainfall (removes contamination) | Capacity: 15–25 L per 100 m² of catchment |
Pre-filter (mesh + gravel) | Removes coarse debris | 100 micron mesh + gravel bed |
Storage tank | Stores harvested water | HDPE / RCC / GRP — sized for dry season demand |
Pump and distribution | Supplies stored water to use points | As per demand — toilet flushing, garden irrigation |
Groundwater recharge pit | Recharges aquifer — mandatory in many states | Perforated casing + gravel backfill — minimum 3m deep |
3. Sizing — Rooftop Catchment and Storage
Step 1 — Annual Harvestable Volume
V_annual = A × R × C
Where: A = roof catchment area (m²), R = annual rainfall (m), C = runoff coefficient (0.8–0.9 for impervious roof)
Step 2 — Storage Tank Sizing
Storage should cover the demand during the dry period between rainfall events. For Indian cities:
City | Annual Rainfall (mm) | Distinct Dry Season | Recommended Storage (days supply) |
Mumbai | 2400 mm | Nov–May (6 months) | 15–30 days |
Delhi | 780 mm | Oct–Jun (9 months) | 30–60 days |
Bangalore | 900 mm | Dec–Feb + Jun–Jul | 20–30 days |
Chennai | 1400 mm | Mar–Sep | 30–45 days |
Kolkata | 1600 mm | Nov–Apr | 20–30 days |
Hyderabad | 810 mm | Oct–May | 30–45 days |
Ahmedabad | 780 mm | Oct–Jun | 45–60 days |
Pune | 700 mm | Oct–May | 30–45 days |
Worked Example — IT Park, Bangalore, 5000 m² Roof
Parameter | Value |
Roof catchment area | 5000 m² |
Annual rainfall (Bangalore) | 900 mm = 0.9 m |
Runoff coefficient | 0.85 |
Annual harvestable volume | 5000 × 0.9 × 0.85 = 3825 m³ |
Toilet flushing demand | 500 persons × 20 L/day = 10,000 L/day = 10 m³/day |
Irrigation demand | 2000 m² landscaping × 5 L/m²/day = 10 m³/day |
Total daily demand | 20 m³/day |
Required storage (20 days) | 400 m³ |
Selected storage tank | 2 × 200 m³ RCC underground tanks |
RWH self-sufficiency | 3825 / (20 × 365) = 52% of annual demand from RWH |
4. Water Quality and Treatment
Use | Treatment Required | Notes |
Toilet flushing | First flush diverter + settling | No chlorination needed — not potable |
Cooling tower makeup | First flush + 100 micron filter + chlorination | Legionella risk — must treat |
Garden irrigation | First flush + settling | Minimal treatment needed |
Car washing | First flush + settling + 50 micron filter | |
Potable use (emergency) | Full treatment: settle + filter + UV + chlorine | Not recommended as primary potable supply |
5. Groundwater Recharge Systems
In states and cities where direct aquifer recharge is mandated, the RWH system includes recharge structures:
- Recharge pit: 1m diameter × 3–6m deep — perforated casing surrounded by gravel filter
- Recharge trench: for large plots — 0.6m wide × 2m deep trench filled with gravel, covered with filter media
- Recharge well (borewell): deep aquifer recharge — requires hydrogeological assessment
- Capacity: 1 recharge pit per 100 m² of catchment area — multiple pits for large buildings
- Overflow: recharge pit must have overflow to stormwater drain — prevents flooding
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