Algae are simple, plant-like organisms, often microscopic, found in various aquatic environments. Chlorine is a widely used chemical disinfectant. While it may seem algae “consume” chlorine when water turns green, algae do not biologically “eat” it. Their presence and growth impact chlorine levels through chemical reactions.
How Chlorine Acts on Algae
Chlorine acts as a powerful oxidizing agent against algae. When introduced into water, it forms hypochlorous acid, the active form for disinfection. This acid attacks and disrupts algal cell walls and membranes, damaging internal structures. This process effectively inactivates or destroys algal cells. Maintaining adequate chlorine levels and proper water chemistry ensures sufficient hypochlorous acid is available.
Why Algae Can Still Appear in Chlorinated Water
Despite chlorine’s ability to destroy algae, these organisms can still appear and thrive in water for several reasons.
Organic Matter and Insufficient Chlorine
A primary factor is the consumption of chlorine by organic matter. Algae, along with other organic debris like leaves and dirt, react with chlorine, using it up. A significant algae bloom or a high load of organic material can rapidly deplete chlorine levels, making the disinfectant ineffective.
Insufficient chlorine levels are a direct consequence of this consumption, or from inadequate initial dosing. If the concentration of chlorine falls below effective levels, typically between 1-3 parts per million (ppm) for pools, algae can begin to grow and multiply unchecked. This creates an environment where residual chlorine is too low to prevent algal proliferation.
Environmental Conditions
Environmental conditions also play a substantial role in reducing chlorine’s effectiveness and promoting algae growth. Sunlight, particularly ultraviolet (UV) radiation, rapidly degrades chlorine through a process called photolysis. Up to 90% of chlorine can be lost in just two hours, reducing its sanitizing power.
Water pH is another critical environmental factor. Chlorine’s effectiveness is highly dependent on pH, with hypochlorous acid being far more potent than its less effective counterpart, the hypochlorite ion. When pH levels are too high, the less effective form of chlorine becomes predominant, rendering active chlorine useless against algae. Many types of algae also prefer slightly alkaline environments, with some thriving in pH ranges of 8.2 to 8.7.
Nutrient Levels and Circulation
High nutrient levels in water provide a ready food source for algae. Phosphates and nitrates, often originating from agricultural runoff, fertilizers, pet waste, or decomposing organic matter, fuel rapid algal growth. Even with chlorine present, abundant nutrients can support a thriving algae population.
Poor water circulation and inadequate filtration contribute to algae problems by allowing “dead spots” where algae can settle and grow undisturbed. Filtration systems are designed to remove algae cells and other organic matter, but if they are not functioning correctly or are clogged, these particles can accumulate and foster algal blooms.