In a rainwater harvesting system, the storage tank plays a vital role in collecting and storing the harvested rainwater. The size and capacity of the tank must be determined carefully to match the rainfall, catchment area, and water usage needs. This ensures optimal use of rainwater without overflow or underutilization.
1. Calculate the Potential Rainwater Harvest
To determine the required storage tank size, the following formula is used:
- Roof Area: Total surface area available to collect rainwater.
- Annual Rainfall: Average rainfall received in mm.
- Runoff Coefficient: Efficiency of the collection surface (typically between 0.8–0.95 for hard roofs).
2. Decide the Storage Capacity Needed
- Determine the purpose of usage (e.g., drinking, toilet flushing, irrigation).
- Calculate monthly or daily water demand.
- Choose a storage capacity that meets the demand during dry periods while minimizing wastage.
3. Choose the Tank Material
- Concrete: Durable and suitable for underground storage.
- Plastic (HDPE): Lightweight, corrosion-free, and ideal for above-ground use.
- Metal (Galvanized iron): Strong but prone to rusting without coatings.
The choice depends on:
- Budget
- Site conditions
- Lifespan required
- Ease of installation
4. Decide the Tank Location
- Accessibility: Tank should be easily accessible for cleaning and maintenance.
- Near Collection Point: Preferably located close to the downspouts to reduce pipe length.
- Should not block building access or affect the structure.
5. Design an Overflow System
- An overflow pipe must be installed to direct excess water safely during heavy rains.
- The outlet should lead to a soak pit, stormwater drain, or percolation pit to prevent waterlogging and tank damage.
6. Include Other Considerations
- Provide ventilation and a cover to prevent mosquito breeding.
- Install water level indicators to monitor storage.
- Ensure the tank sits on a stable foundation, especially for larger volumes.