Modelling anaerobic digestion of microalgae grown in wastewater treatment systems using ADM1 Fabiana Passos*, Doris Brockmann**, Jean-Phillipe Steyer**, Ivet Ferrer* *GEMMA - Group of Environmental Engineering and Microbiology, Department of Hydraulic, Maritime and Environmental Engineering, Universitat Politècnica de Catalunya·BarcelonaTech, c/ Jordi Girona 1-3, Building D1, E-08034, Barcelona, Spain. (E-mail: fabiana.lopes.del.rei@upc.edu, ivet.ferrer@upc.edu) **INRA, UR0050, Laboratoire de Biotecnologie de l’Environnement, Avenue des Etangs, Narbonne F - 11100, France (E-mail: doris.brockmann@supagro.inra.fr, jean-philippe.steyer@supagro.inra.fr) Abstract: This study aimed at evaluating the anaerobic digestion performance and biogas production from microalgal biomass grown in high rate algal ponds (HRAP) for wastewater treatment. Experimental results obtained in lab-scale reactors indicated that the methane yield increased with HRT, reaching 0.21 L CH 4 /g COD when anaerobic reactors were operated at 20 days HRT. Variations in anaerobic digestion performance were detected in accordance with harvested biomass characteristics, which showed a strong seasonality. The Anaerobic Digestion Model No. 1 (ADM1) was used to model anaerobic digestion of microalgae grown in HRAPs. After adjusting the hydrolysis rates and the fraction of particulate inerts in the influent, the model described well the performance of the lab-scale reactors. This was the first time that experimental data from wastewater treatment HRAP was used for model calibration and, therefore, this study may contribute to the understanding and optimization of anaerobic digestion performance. Keywords: ADM1; Algae; Biogas; High rate algal pond; Methane; Wastewater INTRODUCTION In the last decades, microalgae have been in the focus of research regarding new environmentally-friendly energy sources. Although these photoautotrophic organisms have many advantages compared to oil crops for biofuel production, microalgae biomass production and optimization at full-scale is not yet feasible due to high costs of harvesting and downstream processing. On the contrary, wastewater treatment by symbiosis between microalgae and bacteria grown in high rate algal ponds (HRAP) has already been demonstrated [1]. In fact, this treatment technology could be preferred to activated sludge systems for small communities, since no aeration is needed in the biological step. As for activated sludge, microalgal biomass produced in HRAPs during the wastewater treatment process could also undergo anaerobic digestion to produce methane and electricity through cogeneration. So far, only a few studies on microalgae anaerobic digestion in continuous reactors have been conducted, most of them with pure cultures and/or with residual biomass after lipid extraction for biodiesel production. Nevertheless, experimental results indicate that the methane yield from microalgae is similar values to that of secondary sludge (around 0.25 L CH 4 /g VS at 28-30 days HRT) [1, 2]. Therefore, microalgae anaerobic digestion could be coupled to HRAP treating wastewater. Mathematical models are commonly used for predicting anaerobic digestion performance and methane production, reducing time and cost associated with experimental procedures. Modelling anaerobic digestion of organic substrates has been intensively developed, in particular for sewage sludge. The Anaerobic Digestion Model no.1 (ADM1) is a well-accepted biokinetic model used to describe the main processes taking place during anaerobic digestion [3]. A study on modelling microalgae anaerobic digestion indicated that ADM1 showed good agreement with