The prospect of treating wastewater and at the same time producing microalgae biomass is receiving increasing attention. Mechanistic models for microalgae growth in wastewater are currently being developed for new systems design as well as to improve the understanding of the involved biokinetic processes. However, mathematical models able to describe the complexity of microalgal cultures are still not a common practice. The aim of the present study is to present and calibrate a new mechanistic model built in COMSOL Multiphysics™ platform for the description of microalgae growth. Carbon-limited algal growth, transfer of gases to the atmosphere; and photorespiration, photosynthesis kinetics and photoinhibition are included. The model considers the growth of microalgae as a function of light intensity and temperature, as well as availability of nitrogen and other nutrients. The model was calibrated using experimental data from a case study based on the cultivation of microalgae species in synthetic culture medium. The model was able to reproduce experimental data. Simulation results show the potential of the model to predict microalgae growth and production, nutrient uptake, and the influence of temperature, light intensity and pH on biokinetic processes of microalgae.Peer ReviewedPostprint (author's final draft

Horizontal subsurface Flow Constructed Wetlands (HF CWs) are biofilters planted with aquatic macrophytes within which wastewater is treated mostly through contact with bacterial biofilms. The high concentrations of organic carbon and nutrients being transported leads to high bacterial biomass production, which decreases the flow capacity of the porous material (bioclogging). In severe bioclogging scenarios, overland flow may take place, reducing overall treatment performance. In this work we developed a mathematical model using COMSOL Multiphysics™ and MATLAB(®) to simulate bioclogging effects in HF CWs. Variably saturated subsurface flow and overland flow were described using the Richards equation. To simplify the inherent complexity of the processes involved in bioclogging development, only one bacterial group was considered, and its growth was described using a Monod equation. Bioclogging effects on the hydrodynamics were taken into account by using a conceptual model that affects the value of Mualem's unsaturated relative permeability. Simulation results with and without bioclogging were compared to showcase the impact of this process on the overall functioning of CWs. The two scenarios rendered visually different bacteria distributions, flow and transport patterns, showing the necessity of including bioclogging effects on CWs models. This work represents one of the few studies available on bioclogging in variably saturated conditions, and the presented model allows simulating the interaction between overland and subsurface flow occurring in most HF CWs. Hence, this work gets us a step closer to being able to describe CWs functioning in an integrated way using mathematical models.