Sulphur Denitrator

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Diagram of a simple sulphur denitrator combined with a calcium carbonate reactor


Uses sulphur to remove nitrates and phosphates, and add calcium.

The sulphur denitrator works by creating an anaerobic (zero oxygen) condition inside the filter, where anaerobic bacteria will grow and process the nitrate to nitrogen. It is a very slow process, takes some time to establish a functional sulphur denitrator, so do not expect instant results. The water flowrate through the reactor is similar to that of a calcium reactor. A by-product is the production of carbon dioxide, which will lower the alkalinity and pH. The production of carbon dioxide is minor so will have minimal effect on the system as a whole. Calcium carbonate can also be added to the sulphur denitrator, taking advantage of the depressed pH to dissolve the calcium carbonate and introduce calcium and alkalinity to the water at the same time.

The overall chemical reaction for conversion of nitrate, sulphur and carbon dioxide to nitrogen, sulphate, acid and tissue is[1]:

1.06NO3- + 1.11S + 0.3CO2 + 0.785H2O -> 0.5N2 + 1.11SO42- + 1.16H+ + 0.06C5H7O2N

See denitrator for the other type of denitrator, which is based on just using anoxic conditions to convert nitrates to nitrogen.


Some commercially available unit have a design flaw, the recirculating pump is located at the top of the column. In this position any excess air / gass in the column will accumulate here. It is difficult to prime the pump and bleed the excess gas from the column.

The denitrator can be set up with media for the bacteria, such as the ceramic noodles common in canister filters, to populate as well as the sulphur media. Once it has been loaded with the media it will not be opened for 1 to 2 years, depending on the amount of nitrate being processed. If it is opened up, then it is exposed to oxygen and the bacteria is killed and the initial cycle has too be repeated.

The recirculating units are recommended to have gravity feed or a small feed pump. However, they can be operated by using the recirculating pump only, they can supply sufficient suction to flow the water through the unit on their own. This is only after the denitrator has been primed, by filling it up using another pump and expelling all the air. Turn on the recirculating pump and water will start to be drawn through.

Operation and Tuning

To start with, flush the reactor with full flowrate for a day. This is to ensure any trapped air is removed, shaking the unit occassionaly to remove any air pockets is recommended.

A redox meter is very useful for tuning the denitrator. However, the meter must be able to provide negative values, since the denitrator operates under anoxic conditions. The target ORP reading is between -100mV and -250mV. Above -50mV indicates that tehre is too much oxygen in the water and the bacteria will not be able to grow correctly. Below -300mV is too low and the water has reached anaerobic conditions, at this point hydrogen sulphide will be produced and the reactor will not work. To tune the redox value, simply adjust the water flowrate through the denitrator. Too high, decrease the flowrate. Too low, increase the flowrate.

It can take some time for anoxic conditions to be established within the denitrator. It is possible to speed up this process by the addition of a carbon source to the water. This carbon source will be processed by bacteria within the unit, using oxygen up and generating the low oxygen levels required. This can be done by:

  1. add 25 millilitres of vodka or sugar solution to the denitrator column.
  2. seal up column and insert redox probe.
  3. start recirculating pump but don't allow water to flow through, so no new water is entering the unit.
  4. the ORP will drop quickly and within a day it should be down to negative values.

Once the value stops below -100mV start water flowing through the denitrator slowly. Since an outside carbon source may have been added to start things, a significant flowrate may be possible while the reading still falls. But, keep an eye on the value, since the carbon source will be flushed from the denitrator and oxygen levels may start to increase again. If the ORP starts to climb, decrease the flowrate.

Now all that is required is to make adjustments to the flowrate until the ORP value is between -50mV and -300mV. In this range nitrate will be utilised by the bacteria. Target the -100mV to -250mV range, with -170mV being optimal.

Once the sulphur denitrator is tuned and running, keep an eye on alkalinity and pH. Alkalinity will be used up more than without the denitrator in operation, so adjustment of alkalinity additions may be required.




  1. ( ): Koenig, A., and Liu, L. H., Kinetic model of autotrophic denitrification in sulphur packed-bed reactors , Water Research, 35(8), (2001), 1969-1978.