Dripping kalk into the skimmer to remove Phosphate

[jstraka1]

I was told this is a method of phosphate removal. I guess (with my primative understanding of chemistry) the Hydroxide ions form bonds with the Phosphate ions, and are carried away by the bubbles.
1) Is this an effective way to remove phosphate?
2) I use Randy's two part to dose Alk & Cal, could I continue with this and add a very slow drip of kalk to the skimmer input without adverse affects? ( I am thinking this would stabilize my pH also?)

Thanks in advance for any input.

[Amphiprion]

My only thoughts are that the constant change of salinity in the entering water may interfere with proper foam production, given how sensitive some models can be. I don't really know for sure, though.

[Billybeau1]

I would not consider it a method of PO4 removal although limewater does have a small effect on phosphates as shown in Randy's limewater article.

"What Else Does Limewater Do In An Aquarium? Reduce Phosphate

Many reefkeepers accept the concept that adding limewater reduces phosphate levels. This may be true, but the mechanism remains to be demonstrated. Craig Bingman has done a variety of experiments related to this hypothesis, and has published them in the old Aquarium Frontiers. While many aquarists may not care what the mechanism is, knowing it would help to understand the limits of this method, and how it might best be employed.

Habib Sekha (Salifert) has pointed out that limewater additions may lead to substantial precipitation of calcium carbonate in reef aquaria. This idea makes perfect sense. After all, it is certainly not the case that large numbers of reef aquaria will exactly balance calcification needs by replacing all evaporated water with saturated limewater. And yet, many find that calcium and alkalinity levels are stable over long time periods with just that scenario. One way that can be true is if the excess calcium and alkalinity that such additions typically dump into the aquarium are subsequently removed by precipitation of calcium carbonate (such as on heaters, pumps, sand, live rock, etc.).

It is this ongoing precipitation of calcium carbonate, then, that may reduce the phosphate levels: phosphate binds to these growing surfaces, and becomes part of the solid precipitate. The absorption of phosphate from seawater onto aragonite is pH dependent, with the binding maximized at around pH 8.4 and with less binding at lower and higher pH values. If the calcium carbonate crystal is static (not growing), then this process is reversible, and the aragonite can act as a reservoir for phosphate. This reservoir can inhibit the complete removal of excess phosphate from a reef aquarium that has experienced very high phosphate levels, and may permit algae to continue to thrive despite having cut off all external phosphate sources. In such extreme cases, removal of the substrate may even be required.

If the calcium carbonate deposits are growing, then phosphate may get buried in the growing crystal, which can act as a sink for phosphate, at least until that CaCO3 somehow dissolves. Additionally, if these crystals are in the water column (e.g., if they form at the local area where limewater hits the tank water), then they may become coated with organics and be skimmed out of the aquarium.

An alternative mechanism for phosphate reduction via limewater may simply be the precipitation of calcium phosphate, Ca3(PO4)2. The water in many reef aquaria will be supersaturated with this material, as the equilibrium saturation concentration in normal seawater is only 0.002 ppm phosphate. The supersaturation of calcium phosphate will be even higher in the high pH/high calcium fluid present where limewater enters reef aquaria. The locally high pH converts much of the HPO4-- to PO4---, and it is the concentration of PO4--- that ultimately determines supersaturation. That high supersaturation may tip the balance to precipitation of calcium phosphate, just as too much limewater all at once can tip the balance to precipitation of calcium carbonate. As with CaCO3, the precipitation of Ca3(PO4)2 in seawater may be limited more by kinetic factors than by equilibrium factors, so it is impossible to say how much might precipitate under reef tank conditions (without, of course, somehow determining it experimentally).

As with the precipitation of CaCO3 containing some phosphate, if these calcium phosphate crystals are in the water column (e.g., if they form at the local area where limewater hits the tank water), then they may become coated with organics and be skimmed out of the aquarium."

It will however help you with your pH and probably allow you to use less of the 2-part to maintain cal and alk.

PO4 removal is better done with the methods in this article.

Phosphorus: Algaes best friend
http://www.advancedaquarist.com/issues/sept2002/chem.htm

[David Grigor]

If your using a recirculating type skimmer, it may cause buildup on the pump and possibly on the venturi area where it comes in contact with water.

My personal opinion it is probably negligable whether you add it directly to the skimmer vs. to an area in the sump.

[Boomer]

Skimmer is not a good idea at all and about the last place to add it. There is to much going on in a skimmer with CO2 input and output. And then there is factor of changing surface tension.

[jstraka1]

I figured this was too good to br true, thanks guys

[Gary Majchrzak]

I've never heard of dripping kalkwasser into a skimmer.
Dripping kalk into a sump before the skimmer (and water return) would help knock down it's high initial pH and may enhance protein skimming as per the aforementioned article by Randy.

[outy]

if its not true then we need to change this in the reefkeeping facts section of the front page.

Aside from growing macroalgae and limiting the phosphate entering the tank, two excellent methods for phosphate removal are vigorous protein skimming, and the use of kalkwasser (lime water). Kalkwasser can cause phosphate to precipitate out of solution as calcium phosphate. If kalkwasser is dripped into a protein skimmer, the precipitate can be skimmed right out of the water column, which will prevent it from potentially re-dissolving at a later date.

[i agree there are better methods then dripping kalk]

[Boomer]

It is not dripped in to the skimmer or is not suppose to be dripped in. Some add kalk near the skimmer input line where there is a moderate flow rate, more or less as Gary suggested. It aids in a chemical process called Saponification.

Saponification is a process where the OH- from the kalk, before it dissolves, causes the hydrolysis of an ester. In short, meaning the OH- reacts with a fat or oil which forms a fatty acid, aka==> soap. So, it increases the skimmer efficiency of removing organics. How effective this process is I do not really know.

Calcium phosphate, is a salt and and has little if any hydrophobic action, like organics. Sure some may/will end up in the skimmate just like anything else in seawater may/will end up in the skimmate. However, the skimmer is not skimming it out really, any more than it is skimming out sodium chloride, which is also in your skimmate. I do not know who put that part in Reefkeeping FAQ.