Dealing with black algae (Part 1 of 2)

[smallpooldad]

My apologies for the long post but otherwise it is too hard to understand.

After having been a member of this forum for some time I now hope that I can give back to the community in the future. My two main personal areas of experience are iron stains and Black Algae, both of which can be major problems in Hawaii. Recently I installed a Salt Water Generator and while I am very happy with it (Pool Pilot Digital) I am not very experienced with it. I have learnt by trial, error, and experience. I am not a pool specialist but in these two areas my pool which was once a poster child for these problems is now free and clear. Those that visit our home think we have a new pool and often comment that it is one of the best looking pools they have seen – if only they knew.

The following I hope will help those of you with black algae with more than a few spots. Having lived both in Florida and now in Honolulu, Hawaii I have experienced a number of older pools, including my own, that are in various stages of delaminating, from minor plaster loses to major, and thanks to this forum have learnt how to deal with this issue. There may be other methods that I am not aware of but these methods have proven successful for me and those I have helped. Much/most of this knowledge was garnered from ”Chem Geek’s” postings, who might wish to correct or comment on this posting some of which may not be as accurate as I believe, I will take no offense and would welcome such constructive criticism. I have no experience with Fiberglass pools and cannot comment on them.

Method for controlling black algae in older delaminating PLASTER/CONCRETE POOLS ONLY:

Killing Black Algae:

Black Algae is thought to be a form of marine algae and loves to live in cracks. It has a thick outer membrane that cannot be broken by chlorine at normal levels (read near impossible). Main sources are swimming trunks that have been in seawater and not washed prior to entering pool, wind born debris blown up from beaches and carried over land and mountains, and dogs swimming in the ocean and not properly washed down – there are other sources. And because they come from the ocean they love salt water pools. There are at present only two methods for controlling with which I have had success. Note: Pouring Chlorine directly on it in a pool full of water does not work as it dissipates too fast and the membrane is not broken for long enough. These methods and calculation do not apply to those of you that do not have this problem.

Method One:

The membrane needs to be broken using a wire brush 3 times a day for at least 4 days, possibly 7 days. The pool would need a maintained shock level of 25 ppm chlorine to kill it for 4 to 7 days. This ideally should be done every 8 hours, and best of all every 6 hours. Hiring some local children during the holidays to do the scrubbing ONLY can reduce your manual labor at a reasonable cost. Kill rate varies with scrubbing effort.

Method 2:

An alternative method, far less labor intensive, and simpler, is to drain the pool so it is empty by nightfall. Then scrub the affected areas with a wire brush, or wet/dry 1000 grit sandpaper, and cover with towels soaked in pure chlorine in darkness (chlorine loses most of its strength in sunlight). One would also need to remove the lights from their sockets and soak the enclaves as it tends to hide in there. This works very well with a near 100% kill rate. Refill in the morning, preferably before sunrise and leave the towels in the pool, retreiving them after they start floating.

Note: To enhance this method a short acid wash of the affected areas with 1 part muriatic acid to 5 parts water (1:5) may help break down the cell wall. Wash off immediately with lots of water and then cover immediately with chlorine soaked towels.

Controlling Black Algae:

It can be controlled from growing (re-growing) if one uses a combination of all of the following method:

1. An initial dose of Algaecide 60 at 14 ozs per 10,000 gals. And then a maintenance dose of 3 oz per week per 10,000 gals.
2. Boron maintained at level of 50 ppm (Common Household Borax sold in 76 oz units) in the pool (a small replenishment is usually only necessary every 3 months). See the BBB method on this sight and the China Shop on how to do this. LaMotte has the best test strips.
3. A chlorine level to achieve an HOCL level of 0.075 or higher (HOCL levels are pH and CyA dependant so they need to be calculated to find the optimum chlorine value need to be maintained in the pool. HOCL levels below 0.075 cannot kill marine algae so it will slowly re-generate. My experience is that when the HOCL number falls below 0.075 black algae will very slowly creep back. You can download the excellent “Pool Solutions” Excel Spreadsheet here and play with your numbers:

http://richardfalk.home.comcast.net/pool/PoolEquations.zip

Or use Ben’s rough guide depending on your CyA level as follows for chlorine (use the higher max number):
Stabilizer CyA . . . . . . Min. FC . . . . Max FC . . . 'Shock' FC
=> 0 ppm . . . . . . . . 1 ppm . . . . . 3 ppm . . . . 10 ppm
=> 10 - 20 ppm . . . . 2 ppm . . . . . 5 ppm . . . . 12 ppm
=> 30 - 50 ppm . . . . 3 ppm . . . . . 6 ppm . . . . 15 ppm
=> 60 - 90 ppm . . . . 5 ppm . . . . . 10 ppm . . .. 20 ppm
=> 100 - 200 ppm . . . 8 ppm . . . . . 15 ppm . . .. 25 ppm

Why do some Salt Water Generators not kill off black algae?

Some salt water generators sold and purchased can be under powered. It is easier to sell to the public when the units are cheaper. This is especially so when they are sold by some less experienced or less than honest pool maintenance person as their clients would balk at having to pay for the proper sized unit as they are more expensive, also your pool person’s profit could be lower or you might think that person too expensive, become suspicious, and get yourself a new pool person, so it is partly us the general public to blame. It is akin to the old painters’ saying “What’s the most expensive paint job. Answer – The cheapest”. So it is with saltwater generators. One can always upgrade one’s unit to a larger size to generate more chlorine.

Note: Paint labelled "Professional Grade" is normally the lowest quality and the cheapest so study before you buy. But Professional sure sounds good!

Most units sold can generate 1-3 ppm chlorine and they do an excellent job of keeping the water clean and can control some other algaes; but to kill black algae the water would need to have a higher chlorine residue level as it need to sit on the algae at these levels to stop it growing more. So if your unit cannot generate 5 ppm of chlorine at a a CyA of 30 you will have a hard time contolling black algae.

The above chart from Ben illustrates the residue levels at which the chlorine needs to be maintained based on the CyA level (Note: Use the higher Max FC):

As most saltwater generators require a CyA level of around 60 ppm, one would need to maintain the chlorine at a level of at least 9 ppm and possibly 10 ppm. This is way beyond the capability of most under-sized Salt Water Generators, even some properly sized units.

Is Chlorine too high at 5ppm

The answer is no as its real strength is mitigated, controlled and measured by the affects of the CyA level. The higher the CyA the lower the killing power of the chlorine.