When researching water purification, you will inevitably encounter Reverse Osmosis (RO). It is often touted as the gold standard of filtration, capable of stripping water down to its bare H2O molecules. However, when it comes to harvested rainwater, imposing an RO system isn't always the best—or most necessary—approach. Let's break down exactly what RO is and whether your system actually needs it.
What is Reverse Osmosis?
In simple terms, RO works by using high pressure to force water through a semi-permeable membrane. The pores in this membrane are so unimaginably small (often 0.0001 microns) that practically nothing but water molecules can pass through. It removes heavy metals, dissolved salts (minerals), fluoride, lead, and pharmaceuticals.
The Unique Nature of Rainwater
To understand if RO is necessary, we must look at the source water. Groundwater from a well has spent decades slowly dissolving rocks and minerals, becoming "hard." Municipal water is treated with chlorine, chloramines, and fluoride. In both of those cases, an RO system is excellent for removing those dissolved solids.
Rainwater, however, is naturally distilled by the sun. When it falls from the sky, it is incredibly soft and contains virtually zero dissolved minerals or heavy metals (aside from whatever minute amounts it might pick up from your specific roofing material). Because rainwater is already naturally soft and free of the specific contaminants RO is designed to remove, putting rainwater through an RO system is often redundant.
When is RO Actually Recommended for Rainwater?
While a standard multi-stage filter array (Sediment -> Carbon -> UV) is sufficient for 99% of rainwater setups, there are a few specific scenarios where RO is recommended:
- Asphalt Shingle Roofs: If you are harvesting drinking water from a standard asphalt composite shingle roof, there is a minor risk of petroleum-based chemical leaching over time. An under-sink RO system provides peace of mind by stripping out any potential complex hydrocarbons that a standard carbon filter might miss.
- Heavy Industrial Areas: If you live downwind of heavy manufacturing, coal plants, or major highways, the smog and particulate matter in the air can bring heavy metals onto your roof. RO is highly effective at removing heavy metals from the water supply.
- Personal Preference: Some people simply prefer the "flat," ultra-pure taste of RO water, or require it for specific medical machinery (like CPAP humidifiers).
The Drawbacks of RO in Rainwater Systems
Before installing an RO system, consider the downsides:
- Water Waste: RO systems are incredibly inefficient. To produce 1 gallon of pure water, they usually flush 3 to 4 gallons of "brine" (waste water) down the drain. When you are relying on a finite rainwater tank, throwing away 75% of your drinking water is often unacceptable.
- Over-Purification: By removing everything, RO also removes trace minerals that give water its "crisp" taste. RO water can also be slightly acidic, requiring a remineralization cartridge to buffer the pH before it enters your copper plumbing to prevent corrosion.
Conclusion
For a metal roof in a clean environment, an RO system is vastly over-engineered and wastes precious harvested water. A high-quality sediment, carbon, and UV array will produce exceptional drinking water without the waste. Reserve RO only for situations where chemical contamination from the roof or air is a genuine concern.