maintaining alkalinity in recirculatinplankton tank

[dendronephthya]

This is an experimental tank for maintaining a continuous poly culture of microalgae, rotifers, and protozoa. (And yes, it's working except for the alkalinity).

Tank parameters:

120 gallon still water plankton continuous culture tank
8 inch sand bed over plenum
plenum drained by TOM 75 gallons/day to top of tank
other than 75 gallon circulation through the sand bed daily there is no other circulation
Lighting: 52 5 W LED on 24 hours/daily
no skimmer or other filtration, no water changes
Effluent from the plenum has undetectable nitrate, phosphate, no odor; pH is rock solid at 8.0. I have not checked bulk water since it's essentially replaced daily by the effluent. I assume the low pH is due to CO2 but haven't tested it.
Additives Florida Macroalgae Grow (Guillard F/2 containing sodium nitrate, phosphate, trace minerals)
Organisms: Moderate growth of microalgae (Tetraselmis primarily), rotifers 10/lpf, Euplotes, Euglena, other ciliates (sufficient to produce haze but can barely see the long way in the tank)
Salt is ESV's three part salt
There is a tremendous amount of constant oxygen pearling.

This tank has potential for raising marine larvae and (by overflowing to a more rapid flow regime tank) the non photosynthetic filter feeders (the last is my major interest).

Here's the alkalinity problem: Soon after starting up the tank and before biology got going, there was a drop in alkalinity to 4 dKh and drop in calcium to about 360. Magnesium 1500. The calcium has been raised by calcium chloride, and stays at seawater levels now. But, the alkalinity keeps dropping; in spite of putting in maximum "œbuffer" (Two Little Fishies part B and other products) at twice maximum daily (I could get you figures on this exactly but I don't think it's germane to the problem) the alkalinity just doesn't get above 5 and really with daily buffer additions it's staying at 4dKh.

I added 600 ml vinegar/day with if anything even more rapid decline in alkalinity.

Not sure what went on early (before all the photosynthesis and pearling) and we can skip what happened early on. But I'm perplexed by the tremendous demand for alkalinity; what's taking it up, and will this continue.

At the end of the day, one way to think about this is similar to the depletion of alkalinity in the ocean with CO2 acidification; there is a net influx of CO2 in the tank, which draws down alkalinity. But shouldn't it reach some recycling equilibrium? (The tank has been up for three months).

Practically speaking, I would be OK with a normal seawater value of dKh 6-7 especially since the pH is so steady. So, my question is mostly about where the alkalinity is going, is it really a problem say for non photosynthetics (which have skeletons or spicules), and what changes to occur over the long haul.
Thanks for your attention to this very interesting and complex problem.
Charles Matthews M.D.

 

[Randy Holmes-Farley]

Forget what the max dose on the label says. If you have confidence in the testing, keep dosing as needed. Baking soda in RO/DI is a fine choice.

This shows what is needed:

http://reef.diesyst.com/chemcalc/chem_calc3.html

The generation of nitrate will deplete alkalinity. That might be it.

At the end of the day, one way to think about this is similar to the depletion of alkalinity in the ocean with CO2 acidification; there is a net influx of CO2 in the tank, which draws down alkalinity. But shouldn’t it reach some recycling equilibrium? (The tank has been up for three months).

No, CO2 never, ever has an effect on alkalinity. It is an axiom of alkalinity. It is called The Principle of Conservation of Alkalinity.

I discuss it here:
What is Alkalinity
http://www.advancedaquarist.com/2002/2/chemistry

from it:


Alkalinity Facts
There are several facts about total alkalinity that follow directly from the definition. Unfortunately, some of these have been misunderstood by some hobby authors.

One of these facts is termed The Principle of Conservation of Alkalinity by Pankow ("Aquatic Chemistry Concepts", 1991). He shows mathematically that the total alkalinity of a sample CANNOT be changed by adding or subtracting CO2. Unfortunately, there is an article available on line, which claims otherwise, and encourages people to "lower alkalinity" by adding CO2 in the form of seltzer water. This is simply incorrect.

Forgetting the math for the moment, it is easy to see how this must be the case. If carbonic acid is added to any aqueous sample with a measurable alkalinity, what can happen?

Well, the carbonic acid can release protons by reversing equations 1 and 2:

(5) H2CO3 ==> H+ + HCO3-

(6) HCO3- ==> H+ + CO3--

These protons can go on to reduce alkalinity by combining with something that is in the sample that provides alkalinity (carbonate, bicarbonate, borate, phosphate, etc). However, for every proton that leaves the carbonic acid and reduces alkalinity, a new bicarbonate or carbonate ion is formed that adds to alkalinity, and the net change in total alkalinity is exactly zero. The pH will change, and the speciation of the things contributing to alkalinity will change, but not the total alkalinity.

This is not true for strong acids, however. If you add hydrochloric, sulfuric or phosphoric acids (or any acid with a pKa lower than the carbonic acid endpoint), there will be a reduction in the alkalinity.

Another interesting result of the Principle of Conservation of Alkalinity is the equation for determining the total alkalinity when two different aqueous solutions are mixed together. If you mix (a) parts of a solution with total alkalinity A with (b) parts of a solution of total alkalinity B, the resulting alkalinity is just the weighted average of the two samples:

TAmix = [a(A) + b(B)]/[a + b]

Equation 7 can be used to calculate changes in TA for water changes in a tank, for additions of limewater, for dilution of tank water with pure water, and a host of other situations where you might want to know what the final alkalinity will be. It can also be used for calculating reductions in alkalinity caused by strong acids, where the alkalinity of the acid is just the normal strength of the acid as a negative number.

 

[dendronephthya]

Randy, thank you for the thoughtful reply and clearing up the CO2 part of it.

There are no protein inputs in the tank (except very recently); it's dosed with sodium nitrate in the Guillard's F/2. So the nitrate should make the alkalinity go up. Two types of test kits have been used.

So, where's the alkalinity going?

And, should there be some maximum or equilibrium? If I keep going I'll end up with a bicarbonate archipelago or something...

 

[Randy Holmes-Farley]

Yes, I agree, adding nitrate should make it go up if the consumption is higher than the production.

Adding vinegar can reduce alkalinity in the short term (that is, before it is consumed). When are you measuring in relation to the dosing?

 

[dendronephthya]

Vinegar dosing is via float valve/topoff pump, so close to continuous.
In microalgae culture, one can add either CO2 or bicarbonate to provide a source of carbon to the culture and prevent pH rise. Could the alkalinity left after dissociation of carbonic acid be temporarily stored in the microalgae culture as carbon? (I may be saying something stupid here.)

 

[Randy Holmes-Farley]

I do not think that most microorganisms can consume significant alkalinity (even temporarily) unless they deposit something like calcium carbonate or they are nitrifiers (converting ammonia into organics and nitrate) or some other weird chemical consumer (like sulfur oxidizing bacteria).

I'm not sure what's going on.

 

[dendronephthya]

Randy, thanks for hanging in here and figuring this out.

Tonight, pH rose from 8.0 to 8.5 (over two days or less since I got the 8.0 value) and alkalinity was 3dKH (!), calcium 360.

I increased the vinegar in topoff and started spooning the baking soda intp the topoff water, and briefly turning out the lights (I'm afraid to run this still water tank long without lights).

Other than sodium nitrate in "œMicroAlgae Grow", vinegar, alkalinity supplements, previously some silica "œwater glass", and now some yeast-based rotifer food, there's nothing in or out.

Photosynthesis is really intense under 52 5 watt LEDs. There is a floc on the surface, on microscopy containing oxygen bubbles, and microalgae and a lot of paramecia and some copepods and rotifers (and, I think, maybe some trochophore larvae after the spawning that occurred two nights ago).

In addition to about 1200 ml of Two Little Fishies, I now have put in 12 heaping tablespoons of Arm and Hammer into about ten gallons of topoff, and it's using about one gallon daily roughly.

Recall that this tank has 8 inches of fine substrate and 75 gallons passes through this substrate every 24 hours.

The fact that the calcium has settled down to NSW value while the alkalinity continues to fall- perhaps this means the calcium is in equilibrium with aragonite dissolution and formation and coming out the plenum effluent, but for some reason the alkalinity (including that generated by dissolution of aragonite and conversion of nitrate to nitrogen) is remaining in the sand bed? The only other place to put it is in the floc on the top.

The parameters I am testing is from the effluent from the plenum.

 

[Randy Holmes-Farley]

What is the salinity?

 

[dendronephthya]

1.024 measured two ways.

Isn't this a situation that's seen in algae culture? When CO2 is exhausted the pH climbs as plants begin to draw carbon from alkalinity-? The pH climb is also seen with huge growth of plants in under pruned refugia...

 

[Randy Holmes-Farley]

Organisms use HCO3- as a source of CO2 for photosynthesis in seawater, but they must spit out the OH- that results since they have no use for it

HCO3- (in seawater) ---> taken up ---> CO2 + OH-

 

[dendronephthya]

Randy, some new information:

pH of plenum water is now 8.4
dKh of plenum water is 3 and calcium 360
dKh of bulk water is 6 (!)

As you recall, there is a continuous flow of 75 gallons per day through the 8 inch sand bed into the bulk water (via TOM pump); and the bulk water is only circulated by thermal currents and rising gas bubbles.

I would have thought that with this amount of flow the bulk water would have the same dKh.

What the hell?

At least it's clear that alkalinity is lost in transit across the sand bed, but why does the difference persist? Perhaps alkalinity is being trapped at the surface of the sand bed and recycled into the bulk water, leaving water depleted of alkalinity to pass through the sand bed?

 

[dendronephthya]

Here's a reference that describes calcium and carbonate precipitation in interstitial water:

see http://www4.ee.bgu.ac.il/~pel/pdf-files/jour24.pdf
"œpH buffering of pore water of recent anoxic marine sediments"

It's an old reference and won't cut and paste. The conclusion is that sulfide precipitation drives the pH towards 8.3, which drives the precipitation of calcium carbonate.

shouldn't this happen in a sulfur denitrator? That is, isn't that what I am effectively doing by slow flow through the sand bed?

Could the precipitation be reversed by increasing the input of vinegar into the bulk water? Isn't this slow flow setup both a calcium reactor and a sulfur denitrator with competing effects on calcium and alkalinity- and I may get net dissolution rather than precipitation by increasing the carbon to nitrate input ratio...

Thanks for hanging in here with these questions. I am very interested in this slow sand filter approach to keeping zooplankton in our systems.