Kalkwasser going bad in reactor?
 

The main question is why would the pound of pickling lime in my reactor go bad? Skip to the summary at the end if you don't want to read this. Sorry for the long post, I'm trying not to leave anything out. Last I checked Mg was north of 1500, Ca was mid 400, alk was 7, but the symptoms are consistent across a range of Ca and alk levels.



I have a small kalkwasser reactor, a Reef Octopus KS100. I'm turning the drip and stir functions on and off with an Apex using two pH probes. Both probes are calibrated monthly. I do not normally notice a change in pH pre/post cal. The reactor at the start of this had ~4 pounds of Mrs. Wages Pickling Lime

Initially this was on a 170 gallon mixed reef with a 75 gallon sump. I turned the drip on, pH jumped up to my target value (currently 8.24, but that isn't the point).
In the process of setting up the 600 gallon reef I tied the two together. This ~800 gallons of combined water volume took more effluent per day, but still worked, and at this point I'm down to 1-2 lbf of powder in the reactor.

Recently I started having issues keeping the pH up and fixed this by putting a fan on the tank. More evaporation allows more kalk, which had reached 100% of my top off water. A month later it was no longer keeping up. To check that the reactor was still working I disconnected the 600, and put the kalk drip right at the pH probe location, disconnected most of the circulation in the sump, and proceeded to drain 5 gallons of kalk (~1 gallon per hour, stirrer on the whole time). There was ZERO change to my pH. A couple weeks of frustration later I emptied the reactor, put in fresh powder, and it's back to working as expected.

Summary
-Old mix in reactor, well stirred: pH 11.3, zero noticeable change in tank parameters. Same tank pH if I dump it in to the tank until the water goes cloudy, although this only happens if I dump a bunch at once manually.
-New mix in reactor: pH 12.4, pulled the tank pH up by 0.3 in less than 5 minutes (note, circulation in sump, but drip still near the probe).

What happened? It's only been a couple months, and the reactor is not open to the air.

Additional note, I really don't care about pH, I'm using a kalk drip as a balanced method of maintaining Ca and alk. I've tried 2 part (still have the pumps) and find that I prefer kalk. When the pH was holding I found that I was had stable Ca and alk levels. Once I stopped being able to control pH with my kalk drip I was also unable to keep my Ca and Alk where I wanted. My hope is that by understanding the kalk issue I will once more be able to keep my parameters in line.

Note 2: The stirrer has been modified to mix a bit better. Taking the goop out of the bottom and shaking it up in a jar showed the same pH as the effluent.

Note 3: One possibility I've considered is that I could have accidentally dumped salt water into my ATO instead of fresh. It seems unlikely and if so would have been near the start of my kalk dosing, but perhaps that precipitated something inside the reactor that remained as the standard lime went out in effluent?

Over time, the carbon dioxide in the input water should convert the lime to calcium carbonate (sand). The sand will build up in the reactor. That is unavoidable, but I don't know the amount of water that went through the reactor or its carbon dioxide content, so I can't be sure exactly what happened. I would guess that some or most of the solid in the reactor is calcium carbonate, though, from your description. A conductivity meter that read in the 10 mS range or so would be more accurate than a pH meter, but I don't think it's worth the investment.

Is the input to the reactor RO-only, or RO/DI?

The input water is RO only. I make it 300 gallons at a time, then manually carry it to a 5 gallon screw top bottle with a tap at the bottom. This was gravity fed through the reactor via airline, although I recently switched this to a doser so that I could leave the container on the floor. Every couple weeks when the water gets to 150 gallons remaining I make more. The screw top lid is on, but cracked a quarter turn loose to let air in as the water drains.

But his effluent was over 11. If it was Calcium sand, it wouldn't be so high.

I agree that it's not all calcium sand, although I'm not sure how much to trust the pH reading. I have never understood exactly what's happening, but Kalk reactors seem to have problems delivering saturated Kalk. The fact that fresh lime resulted in a higher pH shows that the meter is reacting to something.

I understand the hesitance to trust probes, but I think these are reading correctly:
1) There are two probes, one standard Apex probe and one double junction sci. probe. Both agree within 0.1 at worst, normally better than that (0.02) in the tank
2) Both probes do just fine in monthly calibration at 7 and 10.
3) Both read the same 11.3/4 in the old mix (now down the drain).
4) Both read the same (12.4) in the new kalk mix.
5) In the absence of dosing, both rise and fall together during the light cycle/dark cycle of my tank.
6) It's been a while since I did a quick dip in vinegar, but they were in the ballpark there too.
7) The effluent pH from the reactor drip is the same as the pH when I pull goop (wet kalk powder) out of the reactor and shake a bunch of it up in a jar of RO water).

Chemistry has been a while, but is there a weak base that can commonly form out of kalk?

Yes, some of the calcium carbonate will remain dissolved as carbonate (and calcium ions) in the water column.

But would that drive a theoretically saturated blob down a full point in pH? It seems it must have converted to something else. That 11.3/4 number was also stable through several dozen gallons of kalk over a couple weeks.

Thinking through this : A pH of 11 is 1/10 the ions of OH- than a pH of 12


That means that to effect the same pH change in tank volume, you would need 10x the volume of effluent. I think that what you're seeing.

Basically, the kalk was 90% bad if it was only generating a pH of 11, not 12.... 90% Calcium sand?

By the way, I think fine calcium carbonate is still good for the tank, just not for increasing alkalinity.

I don't think I've ever read a convincing description of why Kalk ends up at less than full saturation. The effect seems to be real. I could guess, but it'd just be guessing. For example, the presence of a significant amount of carbonate in the water might affect the solubility of the hydroxide, but I don't know why that would happen. I'm not much of a chemist.

He used the same water source with the old and new batches.

I suspect the old got humidity in it...

The kalk came from new sealed containers both times, although not the same source (Mrs. Wages the first time, BRS the second). It was all a light powder when added. Neither were kept in unsealed containers. Along those lines though, why would humidity getting in be an issue? CO2 I could see, but why moisture?

I think H2O would react with the Ca(OH)2 making it (aqueous) more likely to react with the CO2 and make chalk.

Calcium hyrdoxide dissasociates in water. The hydroxide uses CO2 to form carbonate alkainity .Alkalinity is a measure of the solution's ability to nuetralize acid .

Tom - any idea what could have happened to Danny's kalk?

CO2 from the air or stirred water depending on whether the reactor is sealed to the air, loosley covered or open with just a crust on top.. Maybe input water sourcing CO2 . Increased proportion of precipitated impurities and/or calcium carbonate in the powder in the reactor limiting saturation.

The water and reactor were not changed. Only the source kalk was different.

The reactor is sealed at the top.
I do think it somehow went bad in the reactor as the old mix did initially work.

Your incoming water is from RODI, right?

Also, when you replenish, do you purge the old white mud completely?

RO, not DI. This is the first time I've had to replenish and I did clean it out.

RO can contain a far amount of carbon dioxide. That's one of the problems people have when dealing with some water sources: the high carbon dioxide content kills the DI rapidly.

Is there that much CO2 even after a couple weeks of sitting there? My RO goes into large storage containers where it sits. I do not have the RO line plumbed in to ATO. That may still be the cause though as I remember the DI going quickly the time I hooked it up. Is pH out if the RO filter, then again after degassing and acceptable way to estimate dissolved CO2?

If the water is aerated for a few hours, especially outside, it will outgas carbon dioxide if it contains more carbon dioxide than the equilibrium level with the air. The water always will have some carbon dioxide, in your situation.

Measuring the pH of RO water accurately is difficult for reasonable TDS levels. Technically, I don't remember how to do it. If the DI media was being exhausted rapidly, and the TDS out of the RO was reasonably low, the issue might well have been carbon dioxide. I have forgotten how strongly carbon dioxide ionizes in water. Is this well water?