Denitrification--sand bed too deep or flow too slow

wooden_reefer

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http://reefkeeping.com/issues/2005-12/rhf/index.php

Here is an article cited by Bertoni that warns of flow in a denitrifier, coil, DSB and the like, being too slow (implied is the bed being too deep IMO).

H2S comes from too sources: decomposition of protein with insufficent oxygen and the reduction of sulfate.

If no protein gets into the denitrification filter, it is not necessary to have much discoloration, it seems to imply. The dark coloration frequently seen in a denitrification filter is solid iron sulfide.

A minute amount of protein will get into the nitrification filter, causing some discoloration.

To point is to avoid ORP so high that sulfate reduction takes place. This is the author's point of avoiding too slow a flow rate (sand bed too deep implied IMO) thru a denitrifier.

Another consideration is whether the iron sulfide would disappear even if H2S is no longer present. I think it won't. This is hard to gauge the current condition. Smelling the water coming out of a denitrifier for H2S may be important. For closed denitrifier like the coil, it may be good to make sure that the denitrification loop is open so that one can smell the water coming out of it.

Just a thought. Since the flow rate is slow, the amount of H2S introduced into the tank may be small, regardless.
 
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From the article:

As mentioned above, this order of electron acceptors used to oxidize organic material is oxygen (O2), then nitrate (NO3-), then manganese (Mn++++), then iron (Fe++), then sulfate (SO4--). Researchers can plot the concentrations of these chemicals as a function of depth, and can also associate an ORP with each transition, although some overlap of the chemistries takes place in each zone. The oxygen zone has an ORP of 0 to 600 mV, the nitrate zone is -150 to 550 mV, the manganese zone is -50 to 400 mV, the iron zone is -700 to -150 mV and the sulfate reduction zone is -850 to 0 mV. Consequently, if hydrogen sulfide is being formed from sulfate, the ORP is likely below 0 mv in that region.

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It is good to know the order, but reduction of sulfate seems to occur in a wide range of ORP.
 
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