Removal of phosphorus takes place through substrate adsorption,
chemical precipitation, microbial activity, plant uptake and
incorporation into organic matter [42]. As mentioned in Section
1, zeolite and bauxite are expected to enhance removal of ammonia
nitrogen and phosphorus, respectively, since they are rich in
Ca, Al and Fe oxides. Adsorption is the main removal mechanisms
and is close connected with material properties. The pH values of
the units (7–7.5; Table 3) are favorable for the physical adsorption
to Fe and Al oxides. Since the phosphorus source used was
KH2PO4, the major portion of phosphorus was soluble and inorganic,
which favors the precipitation of OP ions by Ca. Despite the
adsorption capacity of the materials used, ion exchange on zeolite
(selective to the ammonium ion) as also on bauxite (selective to
OP) was not enhanced compared to the carbonate material, probably
due to the relative short contact time between the material
and the fluid, as the wastewater drains vertically by gravity, as
also noticed by Lüderitz and Gerlach [7] Analyses of gravel samples
from all units confirmed this, since practically no differences
occurred in nitrogen and phosphorus content among the units with
the two special materials and the units with the carbonate material
(Table 2). It has also been reported that fast phosphate adsorption
is completed within a few months or a year [36]. Thus, it is
assumed that nitrogen and phosphorus removal is not only an issue
of the porous media used, but also of the microbial activity, biological
hydrolysis and plant uptake as well. As mentioned above, the
removal by plant uptake is small (Table 2); however, it is probably
the only sustainable removal mechanism [43]. Therefore, microbial
consumption and precipitation with Ca seem to dominate on phosphorus
removal in all beds, independently of the substrate used,
due to limited contact time.