Sulfur reactor

kevensquint · Mar 10, 2016

Hi, I clean a guy's 500 gallon reef and his only issue is high nitrates. 100 ppm. There is nutrient export via weekly 50 gallon WC's, a 40 gallon chaeto sump, a bio pellet reactor, large skimmers etc. Basically a variety of good methods. But he has predators and needs to feed a lot. So my last idea is installing a Sulfur nitrate reactor. Are there any cons chemically speaking? Are they reef safe? Thanks.

Randy Holmes-Farley · Mar 10, 2016

The con is depletion of alkalinty, but it is reef safe.

I compare methods here:

Nitrate in the Reef Aquarium - REEFEDITION
https://www.reef2reef.com/blog/nitrate-in-the-reef-aquarium

from it:

Sulfur Denitrators

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

2 H2O + 5 S + 6 NO3– → 3 N2 + 5 SO42- + 4 H+

The production of acid (H+) in this reactor can tend to reduce the aquarium alkalinity. 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+ → Ca2+ + 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 Ca2+per week. Since calcium weighs 40 mg/mmol, that’s 4.3 ppm Ca2+ 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. Additionally, the acid produced will have a long term lowering effect on the alkalinity. In fact, it is double dipping on the alkalinity depletion since alkalinity is consumed when the nitrate is produced, and again when it is removed in the denitrator. So if you use a sulfur denitrator, be sure to monitor the alkalinity in the aquarium.

kevensquint · Mar 10, 2016

OK, so fast Alk depletion is an issue. In your opinion is it an effective method?since nitrates are the only problem in this particular tank. I am left wondering how huge public reef displays control nitrate with often many large fish.

Randy Holmes-Farley · Mar 11, 2016

Some people use them and are big proponents of them, so yes, they can work very well.

As to large aquariums, sometimes they use nothing special:

http://www.advancedaquarist.com/2007/2/aquarium

"The reef tank is 30’long x 14’ wide x 6.5 deep (water depth) and contains over 20,000 gallons."

"PO4’s are a concern and require effort to control them, but luckily NO3’s have been below 1ppm with no work on my end. There must be enough denitrification within the rock and substrate to handle the large bio load and for that I’m grateful."