The dependence of the biomass P concentration on the N
supply in the medium had implications for the removal of P
from the medium. The removal of P was limited when the N
supply was low. P was removed below the EU emission level
standard of 2 mg L1 only when the P supply was 4 mg L1 or
less. When the N supply was high, however, P could also be
removed below 2 mg L1 in treatments with a P supply of 6 or
even 8 mg L1
. On the contrary, N removal was independent
of the P supply. N was removed to below 10 mg L1 (the EU
emission level for N) in all treatments where the N supply
was 40 mg L1 and even in some of the treatments with a
104 water research 77 (2015) 98 e106
supply of 50 mg N L1
. Because the model species used here
are often dominant in HRAP systems used for microalgaebased
wastewater treatment, Chlorella and Scenedesmus, it is
likely that the same phenomenon may apply to large-scale
systems. Further research is needed to investigate to what
extent this link between N and P accumulation in the
biomass is influenced by C assimilation, which is in turn
influenced by irradiance and CO2 supply. An increase in C
assimilation will most likely increase the C:N and C:P ratio in
the biomass (Sterner and Elser, 2002), but it is not known
whether this will influence the balance between N and P in
the cell.