This discussion covers different types of algae and methods of treating them.
Algae requires sunlight, water and nutrients to grow. The rate of algae growth is ultimately limited by sunlight and temperature regardless of nutrient level and in ideal conditions takes 3-8 hours to double in population. Algae can be killed by chlorine alone, but it is the active form of chlorine, hypochlorous acid, that kills algae and this form is much lower in quantity in the presence of Cyanuric Acid (CYA). In fact, the minimum chlorine level for residential pools is that required to kill algae and not that of killing pathogens that are mostly much easier to kill.
There are many different species of algae, but they can roughly be grouped into the following types with associated methods of treatment (some are not really algae and are noted as such).
The first step in clearing a pool with algae is to physically remove as much debris as possible. It is a waste of chlorine or other products to try and get rid of leaves and other material that can be more readily removed through physical means. Some people use ProTeam® System Support to help raise organics to the surface for easier physical removal. The next step is to shock the pool with a high chlorine level (if algaecides noted below are to be used, these are often added prior to shocking, except be careful of copper as raising the pH can cause staining). Shocking to kill algae and to clear a pool is completed when the overnight chlorine loss test loses <= 1 ppm FC, when the Combined Chlorine (CC) level is <= 0.5 ppm and when the water is clear. The key to shocking is maintaining a sustained high chlorine level along with regular brushing and 24/7 circulation with backwashing/cleaning of the filter as appropriate. Note that some pools have poor circulation so need more regular brushing. For speedier clearing, some people use clarifiers or flocculants, but these vary in how well they work depending on the type of contamination in the pool (they do tend to work well for algae).
The above discussion talked about how to prevent and kill algae using chlorine alone. At extra cost, one can also use algaecides and/or phosphate removers which are described below.
The only one of the aforementioned algaecides that is guaranteed to prevent algae even with zero chlorine levels is copper, but it would need to be at levels that would stain unless the pH is low. All the others significantly slow down algae growth (the bromide/bromine and ammonium/monochloramine kill algae, but are not permanent) and may appear to prevent it completely depending on growth conditions. For pool services, weekly Polyquat, one-time borates, and maintenance-level phosphate removers are the most practical choices with the least side effects. Nevertheless, even these can fail if the active chlorine level gets low enough. For Polyquat that failure may occur around an FC that is 1.0-1.5% of the CYA level. Borates may be similar. Phosphate remover failure depends on the level or organic phosphates where at zero chlorine levels bacteria can rapidly convert organic phosphate to orthophosphate for algae to use.
For pools with an existing algae outbreak and high CYA levels where one cannot do a partial drain/refill to lower that level, the use of ammonium sulfate is a reasonable "get in, get out" approach since the monochoramine is temporary and can be readily removed by additional chlorine. Though monochloramine kills bacteria slowly, it is reasonably effective against algae.
Nice! I will be following this one very closely, as we get an awful lot of calls on algae.
Thank you, Richard!
Good write-up on "algae" Richard.
I would to like to offer the following formula for those who want to add borax for algae control. 20 Mule Team Borax is usually sold as sodium tetraborate decahydrate (10 Mol water). If I have my numbers right, the equation is as follows: Volume of pool water, divided by 13,600 (formula number for borax 10 mol), times 50 (ppm of boron suggested in Richard's write-up) equals pounds of borax to add. For example, 20,000 gallons of water, divided by 13,600, times 50, equals 73.5 pounds of borax to add.
Some chemical companies sell a stronger strength of borax, which is sodium tetraborate pentahydrate (5 mol water). If adding that strength, then substitute 13,600 (the formula number for 10 mol) with 17,800 (for formula number for 5 mol).
I personally think that it would be better to add that amount of borax over a period of several weeks.
The following are amounts needed to get to 50 ppm Borates (technically ppm Boron) per 10,000 gallons:
36.8 pounds (7-3/4 76-ounce boxes) of 20 Mule Team Borax (sodium tetraborate decahydrate) plus 282 fluid ounces (2.2 gallons) full-strength Muriatic Acid (31.45% Hydrochloric Acid)
28.1 pounds of Proteam Supreme (sodium tetraborate pentahydrate) plus 282 fluid ounces (2.2 gallons) full-strength Muriatic Acid (31.45% Hydrochloric Acid)