I noticed that waterbear hadn't responded yet to your questions. I can give you some more information and links to understand better the chlorine/CYA relationship.
As for your questions, algae is like a single-celled plant (though technically it is in the kingdom Protista for most algae or Monera for blue-green algae which is more like a kind of photosynthetic bacteria) and by itself is more of a nuisance (unsightly) and is not harmful in the sense of being a pathogen. Nevertheless, most people don't want algae in their pools and in quantity it will clog filters requiring more constant cleaning and may also be an indication of unsanitary water, though technically the amount of sanitizer it takes to kill algae is far higher than that needed to kill most pathogens (most bacteria and viruses). Algae will also use up chlorine so the bottom line is that for practical purposes it's something you don't want in a pool.
There is no such thing as "chlorine lock". The term came about because at higher Cyanuric Acid (CYA) levels, chlorine appeared to be less effective and no longer prevent algae. In reality what happens is that the amount of actual disinfecting and oxidizing chlorine -- the "active" chlorine known as hypochlorous acid -- has a concentration that is roughly proportional to the ratio of FC to CYA -- that is, FC/CYA. So if you don't raise the FC level when the CYA level climbs, you get lower and lower amounts of "active" chlorine. The result is that algae can start to grow and at first this happens in clear water where there seems to be a mysterious chlorine demand, but in fact it's just algae growth and chlorine getting used up keeping it from turning into a full bloom. If the CYA continues to climb, then at the same FC level the "active" chlorine amount is less so the algae growth gets ahead of the rate of chlorine killing it and the water starts to turn dull, then cloudy, then into a full green algae bloom. Some types of algae don't clump or form strands as readily in which case the water may skip directly to turning a dull green (a clear green usually indicates copper, not algae).
The technical chemical details for the chlorine/CYA relationship are described in the this post
at Trouble Free Pool
. The basic chemical equilibrium can be described as follows:
Chlorine + CYA <--->>> Clorine-CYA
where I try and indicate that the equilibrium is far to the right. In fact, in typical pool water that has enough chlorine to kill algae with some safety margin, namely at an FC that is 10% of the CYA level, and at a pH of 7.5, 97% of the chlorine is bound to CYA in a series of related compounds that are not effective sanitizers nor oxidizers. 1.5% is in the form of hypochlorite ion (a much weaker sanitizer) and only 1.5% is the "active" form of sanitizing and oxidizing chlorine, hypochlorous acid. The traditional industry graph showing chlorine concentration vs. pH is wrong in the presence of Cyanuric Acid (CYA) and I show the wrong and correct graphs in this post
The science behind chlorine and CYA was known definitively as far back as 1974 when a paper done in 1973 was published and presented at a conference and is in a used book you can buy here
if it's still available. The equilibrium constants from that paper can be seen in this EPA document
(document page 12; PDF page 18). The only reason that this information is not well known is that it has not been taught, period. That is, important information that pool stores, consumers and commercial/public pool health officials would need to know has been intentionally withheld. This is not unusual -- remember that the tobacco industry denied the health effects of tobacco since the first large-scale studies showed a link in the 1950's and it wasn't until the 1990's when Jeffrey Wigand exposed the truth of what the industry knew. That's a 40-year deceit. This one in the pool industry isn't even 34 years old yet, but then again people aren't dying from cancer. Who cares that people get algae in their pools, go to pool stores that sell them lots of chemicals to combat it, all the while that this could be prevented by proper use of chlorine alone and an understanding of the true role of CYA? Some manufacturers DO recommend that for "guaranteed no-algae" one use a weekly algaecide along with their Trichlor, but they don't tell you why (that use of Trichlor itself builds up CYA making chlorine less effective nor how quickly CYA builds up).
There are basic chemical facts that aren't even taught, nor put on labels, such as the following that are independent of the concentration of product:
For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.
For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.
For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.
If you do some basic math, you can see that continued use of Trichlor can readily lead to very high CYA levels. This is especially true if one has an oversized cartridge filter that doesn't need cleaning more than once or twice a year. Without the weekly backwashing of a sand filter in a smaller pool with a short swim season, there isn't enough dilution of the water to prevent CYA from building up. In my own 16,000 gallon pool with an oversized cartridge filter and an electric opaque safety cover, I had a low chlorine usage of somewhat less than 1 ppm FC per day and used Trichlor pucks in a floating feeder. I started out with 30 ppm FC, but 1 ppm FC per day of Trichlor adds 18 ppm CYA per month so after one 7-month swim season I added 126 ppm CYA. After one and a half swim seasons, I had unusual chlorine demand and couldn't keep up with the Trichlor tabs, so I used more which just made the problem worse over time. The water got dull and this was in spite of using PolyQuat 60 algaecide, though I was only using it every other week. I found that my CYA was at 150 ppm (there was some amount of splash-out and one cartridge filter cleaning) and that's when I decided I needed to learn pool water chemistry since the local pool store was of no help in understanding exactly what was going on other than wanting to sell lots of additional chemicals.
One CAN use Trichlor pucks/tabs even with CYA rising, but need to use a supplemental algaecide such as PolyQuat 60 and to do so weekly (not every other week as I had done) or they need to use a phosphate remover also with regular maintenance. This costs around $2-3 per week extra. Also, Trichlor is very acidic so pH Up and/or Alkalinity Up (or grocery store equivalents) need to be added regularly and these increase the true cost of Trichlor. In fact, my Trichlor floating feeder "parked" itself near some stainless steel bars in my pool and rusted the mounts!
There are several grocery store equivalents for commonly needed chemicals such as the following:
pH Up -- sodium carbonate, same as Arm & Hammer Super Washing Soda (careful: NOT the laundry detergent)
Alkalinity Up -- sodium bicarbonate (or sodium hydrogen carbonate), same as Arm & Hammer Baking Soda
Proteam Supreme -- sodium tetraborate pentahydrate, nearly identical to 20 Mule Team Borax except for hydrated water content (Borax is ...decahydrate). Can be used to raise pH with less of a rise in Total Alkalinity (TA).
Calcium Hardness Increaser -- calcium chloride, Dow Peladow or DowFlake
Chlorinating Liquid -- sodium hypochlorite, same as unscented bleach (e.g. Clorox regular or off-brand Ultra) except for strength
In spite of this, I purchase 12.5% chlorinating liquid from my local pool store because they charge a reasonable price and they reuse (not just recycle) the containers (with a small refundable deposit for the jugs crate that holds 4 jugs). I try and use my local pool store when I can, but I pretty much just buy the chlorinating liquid which for my pool costs me less than $15 per month and my pool is crystal clear (even with 2000-3000 ppb of phosphates!). That's all I add except for some Muriatic Acid about once a month since my pH is very stable. Another fact that isn't taught in the industry is that the usage of chlorine is an acidic process and this exactly compensates for the initial rise in pH from hypochlorite sources of chlorine (e.g. bleach, chlorinating liquid, Cal-Hypo, lithium hypochlorite) except for the small amount of "extra lye" in the liquid chlorine products. This is described more technically in this post
Yet another myth in the industry is that of using the "slug" or "acid column" method to lower Total Alkalinity (TA). Even though part of the industry debunked this here
, these minimally effective methods are still used. The efficient way of lowering TA is through a combination of acid addition and aeration (or mixing of the water with the air, including surface turbulence) at a lower pH (usually around 7.0 or 7.2).
As for saltwater chlorine generator (SWG) pools and their salt levels, one can simply require the use of cartridge filters and eliminate the backwashing issue. Where I live, there are sometimes water restrictions so cartridge filters are used here, though that isn't because of salt but rather to reduce water consumption. There are potential issues in higher salt-level pools, such as SWG pools, where the higher conductivity of the water can lead to shocking if there are stray voltages already present around the pool, can lead to increased metal corrosion if inferior materials are used and especially if CYA is not used (as with most indoor pools), can lead to greater degradation of softer or more absorbent stones, etc. These issues can be mitigated (sealing stone, using stainless steel and avoiding copper and especially aluminum, using a sacrificial anode, etc.), but the SWG industry isn't very forthcoming about discussing them.
The above just scratches the surface of what pool owners have had to figure out on their own because the industry has not been forthcoming. Ben Powell started The PoolForum
to disseminate what he had figured out from his experience and talking to some chemists and others. The sad thing is that all it would take for this industry to change is for people to wake up in the morning, snap their fingers, and say to themselves "what can I do to help people today?" instead of "what can I do to maximize my personal gain or the profit of my company through not disclosing certain information?". The irony is that more people have flocked to installing SWG systems because of the problems with their pools that would have been avoided if the industry had been more forthcoming in the first place. Of course, that might have put some dent into 25-30 years of profits, but I would hope that people would have an easier time sleeping at night with a clear conscience.