There are several approaches to removing phosphorous - biologically and chemically.
Biological removal can be achieved by growing "stuff" such as water hyacinths or other fast growing plants which you then harvest and remove from your pond. You can enhance the rate of phosphorous removal by adding "stump remover" aka: potassium nitrate.
But your question is regarding the chemical approaches. There are a limited number of approaches: precipitation, adsorption/absorption, ion-exchange. ThereÂs probably something IÂve missed but those are the big three. Precipitation is accomplished by adding a compound that forms an insoluble cation-phosphate compound. For example, alum (aluminum sulfate), ferric chloride, and calcium sulfate can be used to knock down the phosphorous levels. However, this approach is not really viable for use in your pond for several reasons: a.) adding alum or ferric salts will cause the pH to drop dramatically and would probably overwhelm any alkalinity you have in the pond. Secondly, b.) any precipitate would result in significant amounts of sludge being formed which you would have to be able to remove from you pond. c.) This approach typically will not drop the phosphorous levels much below 0.5 mg/L - well above the point at which you get algae blooms (assuming phosphorous is the limiting growth agent).

ÂSorption processes, such as activated carbon will ad/absorb some phosphorous. In some cases the carbon may be pre-treated with an agent to help "grab" the phosphorous. This approach has been used, to the best of my knowledge, in aquariums and similar relatively small volumes of water. Application on a pond of moderate size would, I suspect, be cost prohibitive as well as not necessarily dropping the levels sufficiently to inhibit algae growth (which I think is the reason for your question).

Ion-exchange is effective at removing phosphorous levels to very low concentrations. Things such as "green sands" and other natural zeolites have been used. They tend to be pricey per volume treated but can be regenerated to some extent (usually using saline/brine solutions). The way they work is they remove phosphorous and exchange it for another anion, usually chloride or sulfate. The efficiency, however, decreases with time as the material becomes coated with bacteria or if the pores become plugged with sediment. Incidentally, this is (surface contamination) also a problem for long-term use of carbons.

My choice in the process is to use the biological approach: I think itÂs cheaper in the long run (thatÂs my New England DNA speaking!). Hope I've given you a general overview of what might be going on.
ÂDavid

"You can enhance the rate of phosphorous removal by adding "stump remover" aka: potassium nitrate."

Jumping in on your post, Albert. Hope you don't mind.

Hi, Professor. So adding the stump remover would help lower my phosphate levels?
I am having a massive algae bloom. Yikes. My WH are doing terrible and I'd been wanting to add some, but was doubtful because my phosphate levels are so high. Everything else is normal such as ammonia, KH, PH, and nitrate. Thank you both.

Prof, that's exactly what I would have said if you hadn't beaten me to it. :)
Thanks for responding.
"Horton"

Hardin, as you know, the three major growth nutrients or requirements for plants and algae are N-P-K (and carbon and oxygen are assumed to always be available). By adding potassium (also known as potash) and nitrate then phosphorous becomes the definite, limiting growth nutrient; plants such as water hyacinths, water lettuce, etc. will then actively compete for all available phosphorous. When you buy "Stump remover" make sure it is the potassium nitrate variety (there are a few out there that are not). You might be able to find some packages of potassium nitrate fertilizer but I prefer the "stump remover" package since it comes in small quantities and it will cake up a bit on storage (I'm too lazy to keep busting up a 20 or 50 pound bag!). One can usually lasts me a couple of years. I add about 1 tablespoon to my pond about every 5-7 days until things are where I want them. This creates - in my pond - a nitrate concentration of approximately 1.5 ppm. This is a short-burst concentration which is rapidly assimilated by the plants and not harmful to the fish. You would probably see a similar burst in nitrate if you monitored nitrates on a continuous basis for a few hours after you feed them (ammonia excreted by fish, converted to nitrites and then nitrates). I would adjust the amount I add if I had any significant background concentration of nitrates prior to adding stuff. Below is a little spread sheet in Excel you can use to calculate what you need. If that format doesn't work please let me know. This should also help your water hyacinths look much, much better.
As I stated earlier, I prefer this approach...it may be a bit slower but I like the idea of stimulating biology to deal with a biological problem whenever possible. Plus it's cheaper!!!! :-))
Hope this helps.
---David
P.S. Hey there Horton! I knew you'd say the same thing but I didn't want you to get a cramp in your typing finger!

Here is a link that might be useful: Pond chemical addition calculator in Excel

Horton and David talking chemistry. Takes me back to 2001. Glad to see you experienced gents are still sharing knowledge.

I hope to read more from both of you.

Regards, Kevin

Hi there Kevin, it has been a long time since we saw you posting here. You will have to start posting again, as you always had good information to share. Plus you liked to join in on the friendly banter as I remember.

Yep the "Prof" and I are still happily haranguing each other just for fun.
David has a great deal of experience and information to share and we have to draw it out of him fast.
He is getting on in life you know!!
Thanks for saying hello,
"Horton"

Kevin! Wow, glad to see you're here! Too bad "He-who-must-not-be-mentioned" isn't around for us to have fun with!!! As to getting on with life...hmmm, something about certain people in Canada may also be getting on with whatever it is they do up there.

But back to chemistry 101 and half. Ion exchange resins. How do they work? What are they? Ion exchange is literally what it says: the resin or material removes one ion and releases another. The material can be natural as in the case of some zeolites, green sands, etc. or they can be synthetic or manufactured resins. The surfaces of the resins have sites on them where an ion will be loosely bonded. There are cation and anion resins - obviously cation resins release cations for cations removed from the water and similarly for anions. In the context of the original question assume that you are working with an anion resin. Whether or not it will remove phosphate depends on what is bound to the resin since there is a hierarchy of the order in which ions are removed. Essentially the higher the charge on the ion (this is VERY approximate) then the higher the affinity for that ion to be bound to the surface of the resin. It will therefore "release" or replace an ion with lower affinity. A typical affinity relationship might look like this: OH-1 ---David

Ok ...I think I got it. I have a large bloom of string algae. The water hyacinths are doing so-so .....if I add the potassium nitrate I would cause the hyacinths to grow faster thereby starving the string algae....right?

I certainly don't want to step on the excellent knowledge already shared, but I do want to share my experience.

Hair or string algae used to be a minor problem for me. It was easily dealt with by adding a barley bale as soon as the ice thawed. I hated the barley bale floating around but it soon sank and remained out of site for most of the season.

An early season, and sometimes repeat mid-season algae bloom was another annual struggle. Every spring I would buy 6 or 8 hyacinths well before the weather suited them and would rotate 2 or 3 at a time between the pond and an aquarium.

Several years ago I put 3 marsh marigolds in a pot and tucked them into the shady back corner where they have thrived since. These plants are the first to awake each spring and have nice yellow blooms for a number of weeks and seem to use up the excess nutrients before the algae bloom can kick in.

Since the addition of these 3 plants I haven't had a single algae bloom or a single strand of hair/string algae. Could be coincidence but I don't think so. Whichever method you choose, I hope you have success.

Yes there is some very good information here and some information left out. The city I live in adds phosphate to the drinking water to keep any lead from leaching out of any piping systems. I change 3% of the pond water every single day all year long even during the winter. The pond hasn't turned green anytime in the last 15 years. String algae hasn't been a problem in the main pond for the 13 years or in the streams or waterfalls. 5 years ago I redid the connecting pond and every spring before the system was started up it would get a lot of string algae but once system was started up it would disappear. For the last 3 years this pond hasn't had any string algae anytime. These ponds have never been emptied or cleaned since they were put in. The main pond is 19 years old. On the main pond there plants only around the pond as the pond is 6 ft deep and the home of koi. So why don't I get string algae or green water any more? It is not because there isn't enough sunlight as these ponds are in full sun with no shade cloth covering them. The answer is very simple. The ponds are old enough that they have grown the right bacteria. In my area it takes years for this bacteria to grow and as long as there is enough circulation to get the pond water to come in contact with this bacteria and enough space for this bacteria to grow.
Hi David, nice to see your informative posts here as always.
Mike

Scaley-tailey, you interpretation is correct. As to how you deal with string algae,etc.-- the original question was regarding the chemistry of phosphorous removers. I was not attempting to get into the usual discussion on string/suspended algae. However, check the internet on using Daphnia as a biological source for algae-eaters. Yes, many towns/cities add zinc polyphosphate to the water supply to help reduce corrosion as well as reduce release of lead from plumbing fixtures. The levels of phosphate created from that source are modest - approximately 1/2 ppm or so (if memory serves me correctly). On the other hand, if you take a mass balance approach to sources of phosphorous you may well find that the amount and type of food being added will have an equal or more significant impact. Fish food suppliers will claim that it is low in phosphorous - but it is not zero. In the Spring, after the fish have been in the depths pooping, etc. you have a rather high reservoir of phosphorous in the pond even with periodic removal with your bottom drain (none of the bio-systems which would normally take up phosphorous are terribly active at that point). You've got increasing sunlight (change in the zenith angle of the sun), increasing temperatures, etc. It's a race to see which will get going the fastest - algae, algae predators, etc. But the original question was on phosphorous removal chemistry. Rsingley, whether you use water hyacinths or marsh marigolds....anything that consumes the phosphorous is GREAT!
---David