The myth appears to be more prevalent here in the sun-belt region and with mustard algae issues.  In part, it is probably a misunderstanding between the term "chlorine consumption" and the idea of chlorine actually being "eaten" as a food source.   Chlorine is certainly "consumed" by organic material, but what this refers to is the amount of chlorine that is used up in the process of dissinfecting and/or oxidizing.  The more organic material in a pool, the more chlorine will have to be added to bring the chemistry into balance.  This consumption has nothing to do with being used as food.  

Due to the ideal growth conditions for Algae that are found here in Southern Arizona during spring and summer months, it also may appear that the Algae is simply having a feast.  This just is not the case.  In particular, Black Algae and Mustard Algae can be quite resistant to Chlorine (if it is not eradicated completely.)  The fact is, Chlorine will kill pretty much everything bad that can get into your pool, if it is used properly.  However, pool chemistry and Algae infections can be significantly more complex than what can be cured by simply pouring chemicals into the water.

Even the most resistant parasitic beasts such as cryptosporidium, frequently found in public pools, will succumb (die) at 15-30 ppm or so of Chlorine just in water around them, given enough time.  Virtually all other types of bacterial or viral invaders die at levels between 1 and 4 ppm, which is the norm for a typical Chlorinated swimming pool.  Algae are technically simple plants, and will suffer the same fate if the treatment addresses other important factors as well.

Types of Chlorine Used in Pools: Most Chlorinated pools are treated with calcium hypochlorite tablets or granuals; but also used are chlorine gas, sodium hypochlorite, lithium hypochlorite or chlorinated isocyanurates.  When any of these compounds mixes with water, hypochlorous acid (HOCl) is released.  This is what kills the unwanted organisms.  No component in any of these products can be metabolized, photosynthesized (such as the case with Algae), or in any way can be used as a "food-item."

But, as suggested above, controlled experimentation has been done which clearly suggested that "resistance" is a much more plausible explanation as to why certain Algae infestations can be hard to treat.  Algae and slime bacteria can form "protective barriers" which may keep the HOCl from reaching the parasite.  After a lengthy study of the effects of various algaecides in 1964, The American Society for Microbiology concluded, "It is apparent that algae can develop, which are resistant to the application of certain toxic chemicals."  In order to combat this, more than chemical treatment is usually required.

Mustard algae and black algae are both known to have the ability to show resistance to certain levels of Chlorine.   A pool operator may shock, and kill nearly all of the organisms, but if only a few remain, hiding in cracks, behind light fixtures, and other hard-to-reach places, they can quickly take hold again.  After many generations of this, resistance can increase.   The fact is, sometimes dealing with Algae issues requires a multi-step systematic and methodical approach which includes:  Shocking with Chlorine, manual scrubbing of hard to reach places, brushing sides and bottoms of the pool to disrupt the protective barriers, being sure that all pool equipment and toys are also sanitized, double-checking the balance in water chemistry, use of an algaecide, taking measures to remove REAL food sources such as phosphate, removing Carbon Dioxide (which as any other plant, Algae actually breath.), and improving circulation to problem areas.

When all these things are taken into account, the idea that Algae actually eat chlorine will soon fade away, along with the invasion itself.   Reoccurring Resistant Algae infestations usually require professional methods.  Don't let a chemical salesman try to convince you that pouring a bottle of their magical cure will work (by itself.)  It rarely ever will. If you have a question or problem with Algae in your pool, please give Angelfish Pool and Spa Services and Pool Supply a call or email us today!