[untamed]

I'm really interested in this.

Sorry for tons of questions...
1) The sulfer based reaction has no need for low pH or CO2 addition, right?
2) Do you add CO2 to the Ca reaction chamber?
3) Is the Ca reaction chamber recirculated? Is the Nitrate reaction chamber recirculated? Both?
4) Do you have an additional Ca reactor, or is this your only Ca reactor?
5) Why the ORP sensor in the Nitrate chamber? Does the sulpher reaction also need to be in the absence of oxygen? If so, how is oxygen removed from this reactor?
6) Do you attempt to vent the N2 produced?...

Here's what Holmes-Farley had to say about it. I believe this was written in 2003.

Use a sulfur denitrator. In these systems, bacteria use elemental sulfur and produce N2 from it and nitrate according the following equation (or something similar):

2 H2O + 5 S + 6 NO3- à 3 N2 + 5 SO4-- + 4 H+

It has also been suggested to pass the effluent of such a reactor through a bed of aragonite to use the acid (H+) produced to dissolve the calcium carbonate, and thereby provide calcium and alkalinity to the aquarium.

While that is a fine idea, it doesn’t add much calcium and alkalinity to most aquaria.

To estimate the magnitude of the effect, we start with a liberal estimate of how much nitrate might be removed. Say 10 ppm of nitrate per week.

10 ppm nitrate = 0.16 mmole/L of nitrate

Since 4 moles of H+ are produced for every 6 moles of nitrate consumed, this will produce

0.107 mmoles/L of H+ per week

How much calcium this could produce?
Assume that it takes one proton to dissolve one calcium carbonate:

CaCO3 + H+ ßà Ca++ + HCO3-

Clearly, this is a substantial overestimate because much of the acid will be used up driving the pH down to the point where CaCO3 can even begin to dissolve. Consequently, we have an upside limit of

0.107 mmoles of Ca++ per week

since calcium weighs 40 mg/mmol, that's

4.3 ppm Ca++ per week.

For comparison, an aquarist adding 2% of the tank volume in saturated limewater daily is adding on the order of 16 ppm of calcium per day. Consequently, this method may not be especially useful for maintaining calcium and alkalinity levels. On the other hand, the acid produced will have a long term lowering effect on the alkalinity, so if you use it, watch the alkalinity.

As to its actual ability to reduce nitrate, I cannot say for sure. I expect that it can be made to work, but the only aquarist that I have spoken to that uses one has had considerable difficulty with it.