Why does swimming pool water turn green?
What causes swimming pool water to turn green?
2026-04-30
Green algae blooms are the ultimate result of chemical imbalance in pool water, primarily caused by the simultaneous fulfillment of two conditions:
(1). Disinfection ineffectiveness: This is the root cause. When free residual chlorine is insufficient (below 0.5 mg/L) or the pH value is imbalanced (above 7.2-7.8), chlorine's bactericidal and algae-killing abilities will significantly decrease or even become ineffective. When the pH value is frequently below 7.0, disinfectants evaporate too quickly, resulting in a prolonged period where bacteria and algae proliferate due to the lack of residual chlorine.
(2). Eutrophication: Swimmers' sweat, skin flakes, rainwater-introduced spores, and fallen leaves are rich in nitrogen and phosphorus, providing "nutrients" for algae.
Hypochlorous acid (HOCl) and hypochlorite (OCl-) will exist in different ratios under different pH conditions.The percentage of free available chlorine varies at different pH values, as shown below:
|
pH value
|
Effective activity of free available chlorine |
|
4.0 |
100.0% |
|
7.0 |
75.0% |
|
7.5 |
48.0% |
|
8.0 |
22.0% |
|
11.0 |
0.03% |
|
12.0 |
0.01% |
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