Powerhead question

[Anonymous]

Randy,
Your article mentioned that calcium carbonate will tend to precipitate at heaters and powerheads due to the inverse solubility with temperature and the shift towards carbonate and I've got a few related questions specifically regarding powerheads:

I get very little precipitation on my heaters (150W), but a lot on my powerhead internals (7W, they tend to seize after about 4 weeks). From your explaination, it seems as if I should see more on the heater and less in the powerheads. Would the pressure drop at the impellor come into play here w/ regards to solubility? I guess it also is possible that the duty cycle of my heaters might be pretty low (haven't paid attention to them in a while).

Am I correct in inferring that a totally clean impellor will have a lower precipitation rate than one that has had only a cursory cleaning?

Thanks,
Ty

 

[Anonymous]

One other quicky that I'd like to explore--if I look at an ocean, as far as concentrations and solubility goes, I'd call it a very uncoupled situation as you can easily have wide differences in concentration and solubility from one spot to the next due to "big ocean theory" for a lack of a better word.

It seems that an aquarium would be the opposite--due to it's small size and typically high turnover, I'd guess it's a very closely coupled system--there isn't much I can do in one local spot that won't affect the entire volume.

Could I possibly create an area of low solubility such that I would have controlled precipitation at a point of my choosing instead of someplace undesirable like my powerhead impellor? I guess it would be the equivalent of a sacrificial anode if we were discussing corrosion instead of calcium.

Ty

 

[Anonymous]

hi.
A perfectly clean impeller without any site of nucleation (sp?) will not get precipitation. But that will be impossible for something spinning all the time.

Regarding second post, I don't think it will work unless none of the livestocks in the tank needs calcium. Most corals are used to live in a calcium-supersaturated water that if you pull down the calcium artifically low, they will not grow, and your aragonite sand and coral skeleton may dissolve slowly.

 

[randy holmes-farley]

Sorry, I was on vacation when these posted.

"I get very little precipitation on my heaters (150W), but a lot on my powerhead internals (7W, they tend to seize after about 4 weeks). From your explaination, it seems as if I should see more on the heater and less in the powerheads. "

I get more on the heaters. I don't think pressure is the explanation for the powerhead precipitation. Maybe it has to do with the pH in these settings? For example, if you use a CaCO3/CO2 reactor, and the pH is lower int he sump than in the main tank, that might be an explanation. The heater duty cycle could certainly be an issue. Mine are on a lot.

""Am I correct in inferring that a totally clean impellor will have a lower precipitation rate than one that has had only a cursory cleaning?""

I don't know. Once the CaCO3 gets coated with organics/magnesium/phosphate, it is a rather poor nucleation site for precipitation of CaCO3. Whether that site is better or poorer than the native impeller surface is unknown to me.

"if I look at an ocean, as far as concentrations and solubility goes, I'd call it a very uncoupled situation as you can easily have wide differences in concentration and solubility from one spot to the next due to "big ocean theory" for a lack of a better word."

You could, but generally don't because even in a big ocean, circulation is faster than local removal or addition of most ions, except in places like coastal areas, river mouths, mostly closed lagoons, or even mostly closed seas, like the Red Sea.

"It seems that an aquarium would be the opposite--due to it's small size and typically high turnover, I'd guess it's a very closely coupled system--there isn't much I can do in one local spot that won't affect the entire volume."

True except in places of poor circulation, like in sand beds and inside of live rock.

"Could I possibly create an area of low solubility such that I would have controlled precipitation at a point of my choosing instead of someplace undesirable like my powerhead impellor? I guess it would be the equivalent of a sacrificial anode if we were discussing corrosion instead of calcium."

That will only be useful if the goal is to have less than natural levels of calcium and alkalinity. Otherweise, you are stuck with supersaturation and the precipitation that it entails.

 

[CraigBingman]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by tyoberg:
<strong>One other quicky that I'd like to explore--if I look at an ocean, as far as concentrations and solubility goes, I'd call it a very uncoupled situation as you can easily have wide differences in concentration and solubility from one spot to the next due to "big ocean theory" for a lack of a better word.

It seems that an aquarium would be the opposite--due to it's small size and typically high turnover, I'd guess it's a very closely coupled system--there isn't much I can do in one local spot that won't affect the entire volume.

Could I possibly create an area of low solubility such that I would have controlled precipitation at a point of my choosing instead of someplace undesirable like my powerhead impellor? I guess it would be the equivalent of a sacrificial anode if we were discussing corrosion instead of calcium.

Ty</strong><hr></blockquote>

Calcium carbonate saturation in the ocean is largely, or perhaps I should say proximally controlled by photosynthesis. The ocean is supersaturated in the photic zone largely because phytoplankton is pulling carbon dioxide out of the water and raising the pH.

It is possible to create local zones of undersaturation in an aquarium system. I did exactly this when I was prototyping the UV/H2O2 reactor that I described in the most recent Reef Annual (or whatever Fancy decided to call that annual publication.) Because the flow through the reactor was slow, and because the UV light dissapates a fair amount of heat into the water, I knew that calcium carbonate scaling was going to be a potential problem. So I chose to inject carbon dioxide into the reactant stream to lower the pH of the water in the body of the reactor.

Please note that this works only in situations where you have limited flow, and where you can handle the excess carbon dioxide. With the reactor prototype, the excess carbon dioxide was handled by a single-pass CaCO3 reactor. In the full system, the excess carbon dioxide will be used for phytoplankton growth.

There would be no way of dealing with the excess carbon dioxide if you CO2 injected a circulation pump, because it handles several system volumes per hour, and the overall carbonate saturation state of the entire aquarium system would decrease.

Craig