Rainwater Harvesting
Why Is Untreated Rainwater Corrosive?
Harvested rainwater is naturally soft, acidic, and under-mineralized, making it inherently corrosive, especially to metals and infrastructure. This corrosiveness stems from its low pH, low alkalinity, and low hardness, resulting in an aggressive LSI profile. To make rainwater suitable for household or potable use, it must be treated to raise pH, add buffering capacity, and restore mineral balance to reduce both aesthetic and structural corrosion risks.
1. Naturally Low pH (Acidic Nature)
-
Freshly fallen rainwater typically has a pH of 4.0 – 6.0, due to dissolved atmospheric carbon dioxide forming carbonic acid (H₂CO₃).
-
In industrial or urban areas, this can be further lowered by acid rain contaminants like sulfur dioxide (SO₂) and nitrogen oxides (NOx), leading to even more acidic rainwater.
-
Low pH accelerates corrosion in metals like copper, galvanized steel, and iron, especially in plumbing and storage systems.
2. Low Alkalinity
-
Alkalinity (typically bicarbonates) acts as a buffer to neutralize acids.
-
Rainwater has very low or no alkalinity (<30 mg/L CaCO₃), offering no buffering capacity against pH fluctuations.
-
Without this buffering, any introduced acid (even mild CO₂ exposure) can drastically lower the pH, increasing corrosion potential.
3. Low Hardness
-
Rainwater is classified as “soft” water, containing very little calcium (Ca²⁺) or magnesium (Mg²⁺).
-
These minerals help form a protective scale layer inside pipes that reduces corrosion by limiting water-to-metal contact.
-
Without this scale, metal surfaces are more exposed to the corrosive action of acidic water.
4. Aggressiveness (Negative LSI Index)
-
The Langelier Saturation Index (LSI) is often highly negative for rainwater (commonly between -2.0 and -4.0).
-
A negative LSI indicates water is under-saturated with calcium carbonate and thus tends to dissolve metals and minerals in an attempt to reach balance.
-
This makes the water aggressive and scaling-inhibitive, increasing the risk of pinhole leaks, metal leaching, and infrastructure degradation.
Common Signs of Corrosive Rainwater
- Greenish stains on taps or basins (copper corrosion).
- Pitting or rusting of galvanized or steel pipes.
- Metallic or bitter taste in water (due to dissolved metals).
- Leaks or pinholes in pipework or tanks over time.