EAZA News 57-12 - European Association of Zoos and Aquaria

The disadvantage of heterotrophic bacteria is the production of large
amounts of extra-cellular biomass. The filter easily clogs if it is not
cleaned on a regular basis. Secondly, part of the produced biomass may
exit the filter and foul the aquarium water. Extra filtration with a protein
skimmer and a mechanical filter such as a sand filter may be necessary.
The positive side of this type of filter is the easy regulation of its activity
via the amount of incoming ethanol. Though care must be taken when no
nitrate is left and ethanol is still dosed, toxic substances like hydrogen
sulphur or sulphite can be produced. The flow through the filter is very
slow (around 150 l/h), making it a system which only has a long-term effect.
The fish aquaria at Burgers’ Ocean are not heavily stocked, to give a
realistic impression of a coral reef. Thus food for the fish added to
the aquarium is limited, which in turn limits production of nitrate.
The tunnel display for example, is fed with 25 kg wet weight per week.
Nitrate levels have been kept constant with the denitrification filter for
the last three years at 35 mg NO3--N/l, with a 3% water change per year.
Denitrification with elemental sulphur
During the European Union of Aquarium Curators (EUAC) Conference in
1997, Michel Hignette of Aquarium de la Porte Dorée in Paris, presented
a new de-nitrification technique (Hignette et al., 1997). This technique is
based on drinking water filtration, in which the bacteria convert nitrate
and elemental sulfur into nitrogen gas and sulfate. A disadvantage of
this technique is that it also produces sulfuric acid, which will deplete
the buffer capacity and decrease the pH. This technique is also used at
Burgers’ Ocean to deplete nitrate in fish aquaria.
Sulfur-based denitrification units photo burgers’ zoo
sustainability
Methanol-based denitrification unit photo burgers’ zoo
The effluent of the systems has a pH between 6.2 and 6.5. To prevent
depletion of the pH in the aquaria, this water is first run through a
calcium carbonate or a coral sand bed. This increases the pH to between
7.3 and 7.6. Burgers’ Ocean uses three parallel columns of 300 l each
(including 250 l sulfur) for an aquarium with a volume of 600 m3 . The
volume of the serial coral sand buffers is the same as the sulfur columns.
The flow through the sulfur columns is approximately 150 l/h. During
the three years this filter is used, the nitrate level is kept constant at
40 mg NO3--N/l, with a 4% water change per year and a feeding regime
of approximately 15 kg wet weight per week.
Phosphate removal
When nitrate levels are controlled by the techniques described earlier, a
second important nutrient like phosphate may become a problem. It is
commonly known that, in low concentrations, phosphates are a problem
to corals (> 0.05 mg PO4 3--P/l). While the effect on fish is unknown it
may also be negative. Therefore, it was decided to look for techniques
to remove phosphates as so little water change takes place. Granulates
work perfectly for small systems, but are extraordinarily expensive to use
in large volumes of water.
Aluminium, iron or lanthanum chloride may be better alternatives. These
elements will draw on the phosphates to precipitate into aluminium-,
iron- or lanthanum phosphates. The first pilot experiments with the
non-toxic iron chloride addition look promising. An iron phosphate and
iron hydroxide complex is formed, which settles down in a settlement tank.
Residuals are filtered out with a high-rate sand filter. A disadvantage of
this is the usage of hydroxide and thus a decreasing pH. It may therefore
be necessary to increase the pH with a sodium hydroxide or sodium
carbonate addition.
It seems that the techniques described above make it possible to re-use
water in fish aquaria in nearly closed systems. Cascading techniques and
backwashing sand filters with freshwater help keep the seawater usage
to a minimum. It may be necessary to apply other techniques at Burgers’
Ocean in the future, if other problems relating to the small amount of
water change appear. •
eaza news 57
2007
15