Understanding Rainwater Storage Lifespan
The longevity of stored rainwater depends on its intended use: potable (drinking) or non-potable (gardening, flushing). Different applications have varying standards for safety and storage methods.
Non-Potable Rainwater Storage
For non-potable uses, rainwater can be stored indefinitely if the collection system is high-quality. The main goal is to prevent stagnation and the growth of algae and mosquitoes. This involves using sealed, opaque tanks that block sunlight and screens that keep out debris.
Potable Rainwater Storage
Making stored rainwater safe for drinking requires a multi-stage treatment process. Rainwater can contain pathogens from roof surfaces and air pollutants. It needs to be filtered to remove sediment and disinfected to kill harmful microorganisms using methods like UV light, boiling, or chlorination. Even with treatment, it's wise to test the water and rotate the supply.
Factors Affecting Rainwater Storage
Several factors affect rainwater quality and lifespan. Ignoring these factors can make the water unsafe.
Quality of Collection System
A proper rainwater harvesting system includes a first-flush diverter, which prevents the initial, dirtiest runoff from entering the storage tank. The roof material also matters; metal and tile are cleaner than asphalt shingles.
Sunlight Exposure
Sunlight encourages algae growth, which degrades water quality. Opaque or dark-colored tanks are better than translucent ones. Underground cisterns naturally avoid light exposure.
Storage Container Material
The tank's material affects water quality. Food-grade polyethylene tanks are durable and won't leach chemicals. Concrete can neutralize rainwater's acidity. If you plan to drink the water, use materials certified for potable water.
Regular Maintenance
Maintain gutters and screens to keep them clear of debris. Over time, sediment can accumulate at the bottom of the tank, which needs to be flushed out periodically.
Comparison of Storage Methods
| Feature | Non-Potable Use (e.g., Garden) | Potable Use (e.g., Drinking) |
|---|---|---|
| Storage Container | Opaque tanks; screens | Opaque, food-grade tanks |
| Pre-filtration | Gutter guards, diverter | Multi-stage filtration system |
| Treatment | Not usually needed | Disinfection (UV, chlorine) |
| Maintenance | Clean gutters, tank | Regular filter and system checks |
| Storage Lifespan | Indefinite | Rotate supply; treat fresh water |
| Water Quality | Suitable for plants | Must meet drinking water standards |
Rainwater Maintenance Best Practices
- Install Gutter Guards and Screens: Prevent debris from entering the system.
- Use a First-Flush Diverter: Divert the initial runoff to remove contaminants.
- Keep Tanks Covered and Opaque: Prevent sunlight from promoting algae growth.
- Inspect and Clean Gutters: Regularly clear gutters to prevent stagnant water.
- Flush Out Sediment: Clean the tank every few years.
- Maintain Treatment Systems (for potable use): Replace UV lamps and filters as needed.
Conclusion
How long you can keep rain water depends on proper management. Non-potable uses can have indefinite storage with a sealed collection system. Potable use needs a system with filtration and disinfection. By following best practices for collection and maintenance, you can maximize the lifespan of your stored rainwater.
For more information on system design, see Brad Lancaster's books on Harvesting Rainwater.
