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6. From time to time, measurement of nitrite and nitrate concentrations in the outlet of the
Nitratereductor
has to take place.
9. Options
With a redox potential control, the function of the
Nitratereductor
can be optimized and the
reliability can be increased.
The optimal working point of the
Nitratereductor
can be determined by a measurement of the
redox potential.
Denitrification and redox potential
The redox potential is a parameter which can be measured electronically. The value is a
measurement for the equilibrium between reducing and oxydizing reactions in the water.
The positive redox potential in the aquarium itself is kept at a few hundred Millivolt. In the
seawater tank, it should be between 300 and 440 mV. This high redox potential indicates that
oxydation reactions dominate over reduction reactions. Oxydation reactions are biochemical
reactions where a substance is oxydized, e. g. by oxygen.
A negative redox potential indicates the absence of oxygen and is lethal for most aquarium
inhabitants.
The biochemical conditions in the
Nitratereductor
differ completely from those in the aquarium:
Nitrate has to be reduced to nitrogen gas. This is only possible if there is no oxygen dissolved in
the water.
The redox potential is low or even negative. The ideal range is between -50 and -250 mV.
If it exceeds -50 mV, the denitrification reaction may stop at the nitrite stage!
If it falls below -300 mV, all the nitrate is reduced. The bacteria then start to use sulphate. This is a
very undesired process because the end product of this reaction is Hydrogensulfide.
Hydrogensulfide (H
2
S) is toxic and smells very strange like fouling eggs.
If a little bit of Hydrogensulfide is entering the aquarium, this is not critical. It is immediately
oxydized to sulphate. The closed version of the Nitratereductor causes no problems with bad smell.
Controlling the Nitratereductor
The
Nitratereductor
can be controlled by the rate of feeding or the flow rate of water:
If the redox potential exceeds -50 mV or even gets positive, the dosage of food can be increased or
flow rate decreased.
If the redox potential sinks below -300 mV, the feeding can be reduced or flow rate increased.
Feeding with
Denimar-Powder
: You should keep the flow rate constant and vary the food supply.
10. Failures
Problems with denitrification are mostly caused by wrong adjustion of the flow and feeding rate.
They can only be determined by measurements of nitrite and nitrate concentrations in the filter or
by measurements of the redox potential.
- The
pump produces noise:
If the pump housing contains air or gas, this causes a strong noise.
In this case, the pump is pumping little or no water and its cooling is insufficient. The pump may
overheat and be destroyed. The plastic elbow at the pump outlet has a small hole where air and
gas can escape. If this hole is blocked, it has to be cleaned by using a needle.
-
Nitrite in the outlet of the filter:
If the outlet of the filter contains high amounts of nitrite, the
feeding rate is too low. Increase feeding or lower flow rate. In most cases, the redox potential is
too high (above -50 mV).
-
Nitrate in the outlet of the filter.
High residual concentrations of nitrate often occur together
with high nitrite values.
Caution!
Most nitrate tests are disturbed by high nitrite concentrations! In
this case, the redox potential is also too high. Increase feeding rates, decrease flow rate.
