This study confirmed that the N and P concentration in
microalgal biomass can deviate strongly from the concentrations
predicted by the Redfield ratio. For Chlorella, the N:P ratio
in the biomass varied between 15e42 and for Scenedesmus
between 7e32. Models for nutrient uptake by microalgae
based on a fixed Redfield stoichiometry are therefore not
reliable for estimating the capacity of microalgae to remove N
and P from wastewater. More flexible models that take into
account a variable N and P concentration in the biomass
should be applied, such as the Droop model (Droop, 1973). In
these models, the N concentration in the biomass is only
dependent on the N supply in the medium. According to our
results, these models may be refined in a way that the N
concentration in the biomass not only depends on the N
supply in the medium, but also on the P supply.