Background: The activated sludge treatments combined to the addition of
ferric chloride is commonly used to eliminate nitrate and phosphate from waste water
in urban area. These processes that need costly infrastructures are not suitable for
rural areas and passive treatments (lagoons, reed bed filters…) are more frequently
performed. Reed bed filters are efficient for removing organic matter but are not
suitable for treating phosphate and nitrate as well. Passive water treatments using
various materials (hydroxyapatite, slag…) were already performed, but those allowing
the elimination of both nitrate and phosphate are not actually available.
Methods: The goal of this work is to identify the most suitable iron based materials
for such treatments and to determine their optimal use conditions, in particular in
hydrodynamic mode. The reactivity of the iron based minerals was measured either by using free particles
in suspension or by depositing these particles on a solid substrate.
Results: Pozzolana, a volcanic rock, that is characterized by a porous sponge-like structure suits for settling
a high amount of iron oxides. In addition, the experimental conditions enabling to avoid any ammonium
formation when green rust encounters nitrate were also determined within the framework of a full factorial
design. The dephosphatation and denitrification process is divided into two steps that will be performed inside
two separated reactors. Indeed, the presence of phosphate inhibits the reduction of nitrate by green rust
and the dephosphatation process must precede the denitrification process.
Conclusion: In order to remove phosphate, ferrihydrite coated pozzolana is the best materials. The kinetics
of reaction of green rust with nitrate is relatively slow and often leads to the formation of ammonium.
In this process, it is interesting to favor the accumulation of nitrite in a first step, these species react
much more quickly with green rust and do not transform into ammonium.