slow sand filter pore water ca/alk question

[dendronephthya]

Some further information about my system discussed a few weeks earlier:

This is a 120 gallon tank with 10 inches of fine aragonite over a plenum.
Approximately 75 gallons/day is drained from the plenum and returned to the bulk water via TOM drip pump. The tank is illuminated 24 hours/day with 52 5 watt LEDs, and there is very little water flow. Topoff is with tap water and Kent’s Iron and Manganese supplement, and fed 5-10 ml of a yeast based invertebrate food on occasion (q 3-7 days). Approx. 10 ml/week of silica “water glass” has been added. There is a heavy growth of microalgae and microalgae in the tank, being grazed down by Trochus snails. The purpose of this unusual setup is to attempt to generate a continuous plankton generation, as I have not been satisfied with the quantity of plankton generated by other refugium designs. There is excellent growth of plankton in this tank, and nitrates and phosphates are undetectable. The growth of plankton is especially heavy in floating mats present on the surface, and water drained from the surface (such as would occur in an overflow) is highly populated with plankton.

The problem I am having has to do with calcium and alkalinity. I would have thought that slow diffusion through a deep sand bed would DISSOLVE aragonite due to generation of carbon dioxide. At this time the exiting pH from the plenum (that is, the pore pH) is 8.4, and calcium is 300 and alkalinity is 5 kH even though I have been “pouring in” the baking soda and some calcium chloride supplementation (and sometimes vinegar). In fact, these values seem to have been better since I stopped adding them, although not sure of this. Puzzling, yes?

This paper discusses pH of anoxic pore water in marine sediments http://www4.ee.bgu.ac.il/~pel/pdf-files/jour24.pdf The conclusion section states that pore pH in marine anoxic segments is controlled by 1) weak acids and bases which are the products of decomposition, 2) sulfate to sulfide reaction, 3) precipitation of metal sulfides, 4) precipitation of calcium carbonate. The first two drive the pH toward 7.0, the second two toward 8.4.

So, here’s my question: will this system continue to precipitate calcium carbonate in the sand bed, or will it reach some equilibrium? (It hasn’t after three months...). And- if no equilibrium will be reached, is there a way to fix this kind of system so it will perhaps liberate some ca/alk, such as switching to a quartz sand base, or adding elemental sulfur like a sulfur reactor and spare the sulfate utilization?

I appreciate any help here. I have great hopes for this type of simple natural system both for plankton generation and for filtration.

[dendronephthya]

Some further info in addition to above: Borneman discusses sand beds and some experiments he did here:

http://reefkeeping.com/issues/2007-02/eb/index.php

From this article, "Because of interstitial sediment dynamics, calcite saturation is 20-30x that of seawater and the result is that calcium carbonate actually precipitates in the sediments.”

The mystery remains: what “interstitial dynamics”? Would they occur in a quartz sand bed? Why do they not occur in a sulfur reactor? Does this pore water precipitation reach some steady state if some form of calcium and alkalinity is added to the bulk water? Overall, is there a slow sand filter I can construct that is not a sink to calcium and alkalinity, or perhaps a slight source such as adding sulfur?

[Randy Holmes-Farley]

Hopefully you are not getting too much metal sulfide. Is the sand turning black?

Not sure what you mean by a process driving toward a specific pH. Precipitation of calcium carbonate lowers pH no matter where it starts. Other processes, raise it, I agree. I would not interpret their model as saying the process drives the pH to a specific value and then stops in real world settings.

What is the tank calcium and pH level? No amount of single pass calcium carbonate precipitation in the sand could drop calcium from 420 to 300 ppm, since there is not enough carbonate available for that. That would consume 17 dKH. But over time it may have accomplished that if you keep adding alkalinity supplements and not enough calcium.

Certainly, the conversion of nitrate to N2 in the sand bed will result in a boost to pH. That may be why you observe the pH so high there. Adding vinegar may increase that reaction. Normally, pH in the sand is low due to degradation of organics in the upper, more oxygenated regions. My sand pH was lower than the water column pH in an aragonite sand bed.

[Randy Holmes-Farley]

I see Eric's assertion:

"Because of interstitial sediment dynamics, calcite saturation is 20-30x that of seawater and the result is that calcium carbonate actually precipitates in the sediments."

But I do not see any rationale or data supporting it. It may be true, but I cannot see how based on the numbers he gives. That would require, for example, either the calcium level to increase by 20x (hardly likely), alkalinity to increase by 20x (hardly likely), or the pH to rise by more than a full pH unit (above pH 9). Neither he nor you seem to suggest any of these are true, so I cannot see how the supersaturation increases at all, and certainly not 30x.

Perhaps what he meant so say is that it "could" increase that much if there was no precipitation of calcium carbonate. That is somewhat believable if there was a constant boost to pH. And perhaps that is what you are observing.

There are many people who add an awful lot of calcium and alkalinity supplements to reef tanks and really do not know where it is ending up. Some precipitation in the sand bed seems a likely possibility. Hardening sand is one indication that such processes may happen.

[dendronephthya]

No blackness in the sand. Early on there were some black spots under a dead snail and some rice grains; previously there was a precipitation line about two inches down- it’s higher if I turn off the plenum siphon; in the last few weeks this has essentially gone and the sand bed looks pretty uniform.
Tank calcium and pH are now the same as the plenum; which I would expect given the 75 gallons daily that fluxes through.
The bulk water continually passes through the sand at 75 gallons/day, so it’s not a problem of what’s extracted in a single pass, but rather the steady state of the closed system. I have added considerable baking soda to this system (maybe close to a cup-!)
and maybe get up to alk 6 but it falls back to 4-5 again.
There’s this paper http://www.nature.com/ngeo/journal/v.../ngeo2070.html which discusses significant “authigenic” deposition of calcium carbonate in areas where organic deposition is likely high, I guess suggesting that this effect is biogenic.
So if biogenic, the problem with maintaining calcium and alkalinity in this closed system without visible calcifiers is just an effect of the microbial decomposition of the organic inputs, and not an effect of the sand bed?

So can I conclude that I can forget about this sand bed continuing to be a problem for maintaining Ca and Alk (except for a lag phase as the sand bed microbially equilibrates), and then the tank will correct to desirable values with the usual things we do to maintain Ca and Alk?

[Randy Holmes-Farley]

I couldn't read past the abstract, so I'm not sure how it relates to your system.

How much alkalinity are you adding each day? Just trying to understand the ballpark values.

What do you feed this tank?

Do you think the sand ever becomes low in O2 with that flow rate?

[triggreef]

Just a normal non mad scientist guy here, but I was under the impression that baking soda caused a good deal of precipitation of Ca. I was actually directed to use a small amount of baking soda when my system was out of balance and Ca was high, alk was low. It worked perfectly and then equal parts of two part kept everything perfect since then. In two days it brought the Ca from like 500 to 440. And the alk went up maybe .04 dkh (ish)

[dendronephthya]

I added about a cup of baking soda over a two weeks or so, alk rose from 3 or 4 up to 6, I quit adding for a few weeks and it’s back to 4 now.
I just checked, both plenum and bulk water alk 4 and calcium 320 and pH 8.4 or 8.5.
I overstated the description of the sand bed, there’s some darker areas beginning about two inches down, not much different from usual sand beds.
Regarding oxygen, there is a precipitation link about 2-3 inches down- it’s more like one inch if I turn the plenum pump off for several hours.

One observation is that there is no resistance of the sand bed to water flow- there is no drop in the level of the pipe plumbed to the plenum compared to the bulk water at this rate of flow- it seems unlikely there is much binding going on.

I’m no longer feeding this tank (one week without food). I have fed vinegar up to 100 ml/day, and also as much as 10 ml of yeast-based food twice daily, and for awhile a fertilizer containing sodium nitrate. I think overall things have stabilized since stopping food in terms of bulk and plenum water equilibrating.

Could high pH from low CO2 in the bulk water be supplying the push for precipitation? In that case, why not on the heater?

[bertoni]

Baking soda won't cause precipitation unless it's overdosed or the calcium, alkalinity, or magnesium levels are fairly far out of the recommended zones. I dosed it for years. That measured drop (60 ppm) is fairly large, but given the limited accuracy of our test kits and the time frame, it's possible that the drop was due to consumption by corals or coralline. My tanks would consume close to 20 ppm per day.

[dendronephthya]

From http://en.wikipedia.org/wiki/Microbi...precipitation:

Several mechanisms have been identified by which bacteria can induce the calcium carbonate precipitation, including urea hydrolysis, denitrification, sulphate production, and iron reduction. Two different pathways, or autotrophic and heterotrophic pathways, through which calcium carbonate is produced have been identified. There are three autotrophic pathways exist. However, all three pathways result in depletion of carbon dioxide and favouring calcium carbonate precipitation.[5] In heterotrophic pathway, two metabolic cycles can be involved: the nitrogen cycle and the sulfur cycle.

Maybe CO2 is further depleted at the surface by algae predominating over bacteria, and thus the bulk water is more basic...?

[dendronephthya]

I”m sorry, I meant to say that the plenum water would be more depleted of CO2 and therefore more basic...

[dendronephthya]

Wow, I have to stop having these conversations with myself, but in fact the pore water isn’t more basic- I just measured it! :-)

[Randy Holmes-Farley]

IMO, that demand you mention is not that much, and I'd probably just live with it since there isn't anything obvious you can do about it. A DIY two part is pretty cheap.

[triggreef]

Originally Posted by bertoni

Baking soda won't cause precipitation unless it's overdosed or the calcium, alkalinity, or magnesium levels are fairly far out of the recommended zones. I dosed it for years. That measured drop (60 ppm) is fairly large, but given the limited accuracy of our test kits and the time frame, it's possible that the drop was due to consumption by corals or coralline. My tanks would consume close to 20 ppm per day.

But its a 3 month old system. I believe mine was out of whack but I think just that small amount in my tank dropped the Ca from about 500 to 440 or so.

[bertoni]

I don't see how that's possible. Baking soda doesn't change the pH very much, and it can't precipitate 60 ppm of calcium unless 8-9 dKH is dosed in one shot, which is not a small amount.

[dendronephthya]

One trouble with the 2-part is that the pH is already 8.4 or a little higher due to the continuous lighting and slow flow. I was hoping eventually the grazing would catch up to primary production and the pH would stabilize at lower levels- and maybe it will...

I have returned to returning to adding vinegar and was thinking of titrating that to keep the pH lower and dissolve some of this 10 inch sand bed. In the past there seemed to be too much bacterial growth before the pH starts falling (and the pearling stops at the bottom, so an oxygen problem could develop in this slow flow system). Perhaps if I went slowly enough with the vinegar to allow more time for grazers to catch up with the bacterial films it would work. (Note that there is no skimming or exchange in this system.)

[Randy Holmes-Farley]

One trouble with the 2-part is that the pH is already 8.4 or a little higher due to the continuous lighting and slow flow.

You can buy or make a two part that actually lowers the pH a bit. it uses baking soda only without any washing soda/sodium carbonate/baked baking soda. B-ionic Bicarboante or my DIY Recipe #2.

[dendronephthya]

Thanks, Randy!

[Randy Holmes-Farley]

Good luck.