In order to kill algae cells, chlorine

must pass through the cell walls and disrupt the inner workings of

the organisms. Thus, it’s most effective against green algae,

whose cell walls are relatively vulnerable.
In order to kill algae cells, chlorine must pass through the cell walls and disrupt the inner workings of the organisms. Thus, it’s most effective against green algae, whose cell walls are relatively vulnerable.

When a pool’s sanitation goes out-of-whack, some of the most obvious signs are those all-too-familiar green, yellow or black blooms that begin to creep across walls, steps and other surfaces.

Every pool service technician will battle algae at some point in his or her career, and most every pool will face an infestation at some point in its lifetime. The key to dealing with this common problem is a solid grasp of the chemistry of algae treatments.

Here, we talk with professional chemists who’ve spent years studying the life and death of algae’s many forms.

With the help of their expertise, we’ll examine the industry’s most widely used algaecides, discuss the science behind their workings, and lay out some situations where each one is most useful.


The most common algaecide often isn’t described as an algaecide at all. Though chlorine is useful for oxidizing all sorts of contaminants, a residual of this chemical in the water (experts recommend 1 to 3 ppm free available chlorine) is an ideal first line of defense against an algae invasion.

However, a heightened bather load — or even a missed service call — can allow that residual to become depleted, which gives algae the opportunity to take hold.

If this is the case, chlorine may still be effective for treating the problem if it’s caught at an early stage. The first step is to use a test kit to verify that the water’s residual of free available chlorine (FAC) is low. If the test indicates a low residual, it may be possible to banish the algae with a shock treatment. Once the shock has cleaned out the bloom, return the residual to its normal level.

For such a widely used chemical, the actual mechanism by which chlorine kills algae remains surprisingly poorly understood. Chemists know that it diffuses through the walls of algae cells and disrupts their inner workings, but they’re still investigating how exactly this works.

“The predominant theories are that it either oxidizes material inside of the cell, or disrupts the metabolic reactions that happen within the cell,” says Karen Rigsby, leader of technical services at BioLab Inc. in Lawrenceville, Ga.

In either case, because chlorine must pass through the cell wall in order to complete its work, it’s most effective against algal blooms that don’t produce layers of protective covering — in other words, it’ll often kill green algae, but yellow and black algae may prove tougher to wipe out.

Finally, avoid adding chlorine to a pool whose water contains a hydrogen peroxide oxidizer. Hydrogen peroxide is used in pools sanitized with PHMB, commonly known as biguanide. Instead of acting as an oxidizer in these pools, the chlorine will simply be neutralized by the hydrogen peroxide, resulting in a useless sticky substance.

Sodium bromide

In and of itself, sodium bromide isn’t actually an algaecide — instead, it’s a chemical delivery system for introducing inactive bromide into the water. As the sodium dissolves away, the bromide is oxidized by chlorine into hypobromous acid (HBrO) — a potent algaecide that’s a close cousin of the hypochlorous acid (HClO) that forms when chlorine dissolves in water.

“So, for a period of time, both free bromine and free chlorine are present and active in the pool,” says John Puetz, director of technology at Advantis Technologies in Alpharetta, Ga. “That synergy gives you improved oxidation performance.”

Though this added boost is sometimes all it takes to rid a pool of algae, the addition of sodium bromide to the water leads to some new chemical properties of which it’s important to be aware. At continual high doses, the process of converting sodium bromide to hypobromous acid can consume much of a pool’s free chlorine, leading to a lower residual. And like chlorine, bromine is neutralized by PHMB sanitizers, creating a gooey mess with no oxidative properties.

Quats and polyquats

If oxidizers aren’t doing the trick, the next wave of attack should come from quaternary ammonium compounds, commonly known as “quats.” Like chlorine, quats aren’t particularly effective at penetrating the protective sheaths of black and yellow algae — but they do possess some special chemical properties that can make them more effective at hitting the invaders where it hurts.

Chemically, quats consist of two main sections: A positively charged end that’s attracted to the negatively charged cell membranes of algae, and a “tail” composed of a chain of carbon atoms, which diffuses through the cell membranes, tearing them open and causing the cells to die. In fact, the length of this carbon chain makes quite a difference in a quat algaecide’s effectiveness.

“A chain of 14 carbon atoms has been found to be the most effective,” Rigsby says. “So check the label of a container of ammonium chloride quats — the more 14-carbon it contains, the more effective it’ll be as an algaecide.”

Quats can bring several advantages over chlorine — for one thing, their targeted effect on cell membranes means they’re effective at much lower doses than chlorine — typically 8 to 20 ounces per 10,000 gallons of water. Also, because their chemical structure is somewhat similar to that of surfactants like dishwashing detergent, quats help coagulate algae as they kill it, making the algae easier for the pool’s filtration system to clean up.

However, this surfactant property also has a downside: Quats tend to foam, especially in bubbly areas like spas and waterfeatures. This has led many manufacturers to develop polymeric quats, or “polyquats” — chains of quaternary ammonium compounds that function in much the same way as ordinary quats, but don’t produce foam. The downside of polyquats is that their dosage requirements tend to be slightly higher than those of non-polymeric quats.

Because quats and polyquats don’t react unpleasantly with PHMB, they’re often ideal algae solutions for a pool using this type of chemical regimen.

Copper and silver

When algae just won’t quit, it’s time to pull out the big guns: metal algaecides. These are deadly enough to destroy even the most stubborn black slime — but like any powerful weapon, they also carry some risks. Overestimating the dose, or leaving the metal in the water too long, can result in surface stains that are tricky to remove. Thus, it’s crucial to follow manufacturers’ directions precisely, and get the water back to its normal chemistry as soon as possible.

Metal algaecides attack algae by interfering with many chemical processes that are necessary to living cells. “Copper disrupts enzymatic activity within the cell,” Rigsby explains. “Enzymes are what make the cell’s metabolism work, so when those aren’t working properly, the cell can’t stay alive.”

Silver algaecides work on a very similar principle.

Copper and silver algaecides are effective at much lower doses than quats — often as little as 1 to 3 ounces per 10,000 gallons of water. This power comes at a price, though. “A lot of consumers assume that if some is good, more is better,” Puetz says. “So if they don’t read the label directions, and they assume the potency is comparable to that of a quat algaecide, they end up adding many times too much, and that can stain the pool.”

Metal staining from algaecides is easy to avoid, though, with a little forethought. After carefully measuring the dose, it’s important to monitor the algaecide’s progress daily, if possible — or explain to the homeowner that it’s necessary to check in with the pool often. Once the treatment is completed according to the manufacturer’s instructions, add a sequestrant to the water so the metal can be filtered out. Through diligent monitoring and quick cleanup, it’s possible to keep the pool free of both algae and stains.

One exception, however, is a pool that uses a PHMB sanitizer. Like chlorine and bromine, metals react with PHMB to create a sticky mess, so it’s best to avoid using these algaecides in such a pool.

An understanding of chemical interactions like these will go a long way toward keeping algae treatments effective and trouble-free. Obviously no two algae infestations are exactly alike, but with a working knowledge of each type of algaecide, it’s not too hard to choose the ideal treatment.