Calcium Carbonate Reactors
Sanjay Joshi: http://wimpy1.psu.edu/aquarium/180G.html

Although calcium carbonate reactors have been around for a while, I first saw calcium carbonate reactors about two years ago, when Jeff Turner of Exotic Aquaria showed me videotape of some German reef tanks. Several of them were using these devices to maintain the carbonate hardness levels in their predominantly small polyp stony (SPS) corals.

I was very intrigued by the idea of using a reactor, since I was fighting a loosing battle with trying to maintain the Ca and alkalinity levels in my 180G SPS coral tank through the use of Kalkwasser only. Not only did this device have the potential to address my problem; it seemed like a dream for a consummate lazy aquarist like me. No adding and mixing Kalkwasser daily, no hassles with trying to increase evaporation and adding humidity to basement, and other hassles that I am sure you are all aware of.

Before delving into the reactors let us first take a look at how it works. The basic idea of the reactor is the reverse process of calcification. A calcium carbonate media is dissolved using carbonic acid (generated by addition of CO2 to water) to provide the Calcium and bicarbonate ions, in the same proportion that is used during calcification.

CaCO3 + H2O + CO2 <-> Ca2+ + 2HCO3-

So the calcium carbonate reactor is basically a device that brings the three ingredients together in a manner so as to allow efficient production of Ca and bicarbonate.

Basic Design

There are two basic types of designs that have been used to construct the reactor. The open circulation kind and the recirculating kind. The current trend is to use the recirculating type of design, so I will not spend much time with the open circulation kind.

In a recirculating type of reactor the mixture of CO2 and H2O is continuously circulated in the chamber through the calcareous medium in a closed circulation loop. Water from the tank/sump enters the reactor through an inlet and exists through a outlet, both of which are connected to the main recirculating loop.

Typical recirculation rates are in several hundred gph, while typical input/output rates are in several liters/hr. Water entering the reactor comes in at tank alkalinity levels and effluent leaving the reactor has much higher alkalinity (depending on the settings). The amount of CO2 added is typically measured in bubbles/min (unfortunately there is no standard bubble size, so this is a dubious measure when comparing different reactors).

All the commercially available reactors basically use a variation of techniques to create the circulation loop, the manner in which CO2 is injected, how the water is input to the reactor, and how the effluent is drawn. The Knop reactor for example, has a top to bottom circulation loop, whereas the MKR reactor has a reverse flow circulation loop through the media. Some require an external pump to input the water into the reactor, whereas others use gravity or siphon to introduce water into the reactor.

You can easily see the variations in design by looking at the commercially available designs, but they all essentially do the same thing - provide a mixing chamber for the three ingredients - calcareous medium, CO2 and water. So let us stick to the basic principles and not get into a product review, at this point.

There are basically two adjustments that can be made to the reactors - the amount of CO2 being added and the effluent flow rate. Keeping the same amount of CO2 and increasing the effluent flow rate will result in a reduction in the alkalinity of the effluent, Increasing the amount of CO2 while maintaining the same effluent flow rate will result in a increase in the effluent alkalinity. So, using these parameters we can adjust the resulting alkalinity of the tank.

The most important ingredient in the Ca reactor is the Calcareous Medium used. It is the only ingredient whose "quality" is attributed to the problems one could encounter when using a reactor. The factors of concern are

(a) the chemical composition of the medium

Since the calcareous medium is dissolved in the reactor, it is reasonable to expect that all the compounds contained in the medium are susceptible to dissolution and subsequent introduction into the tank. Magnesium, Strontium, Phosphates, and other heavy metal salts present in the medium will also dissolve and enter the tank via the effluent. This has both positive and negative effects. On the positive side, one may be able to stop adding additional Strontium and other trace elements. On the negative side we may be adding large amounts of undesirables such as phosphate, copper, heavy metals, etc that will contribute to the problems that I will discuss later. I wish the companies selling these would provide a complete chemical assay of their medium (To be fair to the companies - I have not tried calling them to get it either).

(b) the particle size of the media.

The particle size of the media impacts the total surface area of the calcareous media exposed to the carbonic acid formed in the reactor. The larger the particle size the less total surface area available for the carbonic acid to react with the medium. So a medium with a larger particle size will dissolves more slowly, and has larger inter particle space which results in less trapping of powdered particles in the medium, and easier flow through the medium.

Smaller particles provide a much large surface area, and will dissolve at a faster rate. This has its own disadvantages. The medium turns to powder (mushy) much faster than the larger grained particles. This powder mush being light in weight is easily suspended in the water in the reactor and over a short period of use (2-3 months) results in chalky effluent being introduced into the tank. Further, the powder particles tend to stay trapped in the medium and sometimes when the reactor "burps" it spews a lot of chalky effluent into the tank. Using finer grained particles also puts more back pressure on the pump used for circulation.

Given that the circulation rate in the reactor is very high compared to the rate at which the effluent is drawn I don't think that the rate of dissolution due to the particle size (with in certain range) is a sensitive parameter in the operation of the reactor.

I found with the Caribsea Sea Flor as the medium in the reactor, it would turn powdery in a about 2-3 months of use, and start creating a haze in my tank. I am right now experimenting with Carbisea Geomarine and have been using it for a month so far.

Problems often Identified with the use of a Calcium Carbonate Reactor

(1) Depressed tank pH

For an aquarium at equilibrium conditions there is a certain quantity of CO2 that can be maintained without reducing the pH. As this amount of CO2 is increased, additional CO2 will decrease the pH. Thus when adding the effluent back to the tank, there will always be some additional CO2 that will be added to the tank, that can result in a drop in the tank pH. This drop in pH will be higher if the tank initially has low alkalinity.

In my tank I have not seen any depression in tank pH. In fact, the tank runs at a higher pH since I added the reactor. This is possibly due to the fact that the tank is now running at higher alkalinity values and hence less susceptible to pH drop. I also think that since I am running a downdraft skimmer it may help to "blow off" the excess CO2.

(2) Increase in Phosphates

A common criticism leveled against these reactors is that they can increase the phosphate levels in the tank. The reasons cited are:

(a) the medium used contains phosphates which redissolve and are introduced into the tank

I have not done any chemical assays of calcareous mediums available to determine which one is high in phosphates. In an Aquarium Frontiers (May/June 1997) article Greg Schiemer stated that the effluent using Caribsea Sea Flor tested at 0.25-.30 ppm of phosphate, but there was no measurable increase in the phosphate levels in his tank. I have seen similar results in my tank.

At the rate at which this extra phosphate is being added to the water, it is either being removed by the skimmer or being bio assimilated. If the phosphate issue is a concern to you and is in fact increasing the phosphate levels in your tank, then you may want to try some of the "purer" brands of calcareous medium that are being advertised. I have not used them so I cant really say much about them or their claims.

(b) Since most people using these reactors stop using Kalkwasser, they give up the capability of using the phosphate precipitating properties of Kalkwasser.

This may be of some concern in tank where the phosphate transport/removal mechanisms are poor to begin with. Using a "better quality" substrate can minimize the impact of this.

(c) The drop in the pH of the tank, causes a release of phosphates already bound in the tank substrates (live sand, etc). I think this is essentially a non-issue. If your tank pH is dropping so much so as to cause the substrate in your tank to redissolve then you should be in big trouble already.

(3) Increase in Algae and Diatoms

The increase in available CO2 in the tank often acts like a fertilizer for the and coupled with the phosphate additions from the medium can fuel an increase in algae growth. Further, the calcareous medium may also contain silicates that are released into the tank, thus increasing the diatom growth.

Whether or not these problems will manifest themselves in an aquarium depends to a large extent on the prevailing conditions of the tank - its ability to degass excess CO2, transport/removal of phosphate, availability of herbivores to keep the algae in check, etc.

My advice to people thinking of using Calcium Carbonate reactors is to first get your tank running in peak condition without the reactor. Once you are over the typical tank problems such as red slime, hair algae, diatom blooms, and you have a trouble free tank then evaluate whether you really need or want a Ca reactor. Most tanks can be run very well without them, and the only reason to use one on these tanks is the lower maintenance factor.

Adjusting the Reactor (and some reactor Math)

Once you have decided to add a reactor, how do you go about adjusting it, i.e. determining the effluent flow rate and the amount of CO2 to be added.

The following discussion is based on several personal conversations with Dr. Craig Bingman, and the experience of several others in the Fishroom.

Let us assume that the reef system contains T liters of water. Set the CO2 flow rate to approx. 20-30 bubbles/min and the effluent flow rate to a slow enough drip and let the reactor run for several hours until it reaches a steady state. Measure the effluent flow rate in liters/hr - say its L liters/hr.

Now measure the alkalinity in the tank, and the alkalinity of the effluent. The difference between the two values will give you the increase in alkalinity due to the reactor. Let us say this is d meq/L.

Assuming no calcification and use of alkalinity, this will result in a an increase in tank alkalinity that is given by the following formula:

Increase in tank alk/day due to the reactor = (d x L x 24)/T - (1)

Now measure the tank alkalinity after a day. The difference between the increase in alkalinity due to the reactor and the actual increase in alkalinity will give you the daily consumption of alkalinity for your tank. Let us say this value is c meq/L.

So now we need to adjust the reactor so that the daily increase due to the reactor is approximately c meq/L. This will give us the setting at which the reactor will replenish the alkalinity that is consumed daily.

Looking at the equation (1), we can see that there are 2 ways in this can be achieved.

• (1) adjusting d - the increase in effluent alkalinity
• (2) increasing L - effluent flow rate

The effluent alkalinity can be increased by increasing the amount of CO2, and keeping the effluent flow rate constant. Increasing the flow rate will result in a decrease in effluent alkalinity if the CO2 flow rate is not simultaneously increased.

Which one of these is a better adjustment?. My opinion is that increasing the amount of CO2, and keeping the flow rate is a better choice, since it adds less CO2 to the tank.

Similarly to reduce the alkalinity of the effluent, it is better to reduce the amount of CO2 added.

I don't want to bore you all with the math and chemistry details but I feel that a basic understanding is necessary to avoid the trial and error, and test and adjust solutions. You can use the above equation to calculate, for example, what the value of d should be, given a certain effluent rate and the desired increase in alkalinity - rather than making wild guesses and adjustments.

Conclusion

Calcium carbonate reactors are a very useful device, and when used with the proper care and understanding of its advantages, and associated problems addresses a constant struggle faced by most aquarists - Ca and alkalinity.

For a consummate lazy aquarist like me it has been worth every penny. I built myself a DIY one, and I know I will be deluged with requests for plans. I have not taken the time to write and sketch the plans, but I will refer to the plans on the web by John Payter http://www.concentric.net/~Jfpnn3/calcium.htm. Mine is very similar, but I used acrylic tube so I can see inside the reactor. A 4.5" OD acrylic tube works with 4" PVC fittings.

I am sure I have not covered all aspects of Ca reactors in this talk, but I am sure we can cover those when we get to the Q&A.

In case you need to get in touch with me after this talk, you can send me email at sjoshi@psu.edu.