ENERGY RECOVERY FROM IMMOBILISED CELLS OF SCENEDESMUS OBLIQUUS AFTER WASTEWATER TREATMENT M. Gomes San Juan*, F. Ometto**, R. Whitton*, M. Pidou, B. Jefferson* and R. Villa *† . *Cranfield University, CWSI, Cranfield, UK **Scandinavian Biogas Fuels AB, Linköping, SE † Corresponding author Anaerobic digestion, biogas, wastewater treatment Summary Biomethane batch test of alginate beads and beads with algae at different stages of utilisation in the wastewater treatment plants showed that immobilised S. obliquus yield similar biogas and biomethane than freely suspended algae (between 60.51 ± 4.19 and 82.32 ± 2.17 mL g- 1 VSadd) and that a pre-treatment stage was not necessary for the digestion process. Introduction Microalgae have shown to be able to remediate nutrients effectively from secondary wastewater, their use in WWT processes and biogas production by anaerobic digestion (AD) was first reported by Golueke at al. (1957) and Oswald and Golueke (1960). At the time, the authors’ main conclusion was that, although the algae removed nutrients to satisfactory levels, the overall process was not economically and energetically viable, and regrettably this is still the case today (Ometto, 2014b). Process intensification can be achieved using cells entrapped into a resin or gelatinous media, such as alginate or synthetic polymers (3.3 g·L-1 DW). Ruiz- Marín et al. (2010 and 2011) and Whitton et al (2014) all demonstrated good nutrient removals with immobilised algae. However, even when immobilised, the inclusion of microalgae in the WWT process for nutrients absorption could only be justified if biomass is processed to recover energy. Of the currently available biomass-to-energy technologies, gasification, thermochemical liquefaction, direct combustion and anaerobic digestion (AD), AD provides the most feasible process for large scale application which, depending on the chemical composition, has the potential to yield up to 800 mlCH4 gVS -1 (Heaven et al., 2011). However, microalgae species have the ability to resist microbial degradation, their structure and chemical composition identified the cell wall as the main limiting factor to microbial degradation (Atkinson et al., 1972; Burczyk et al., 1999). High energy (thermal and ultrasound) and low energy (mechanical and biological) pre-treatments can be used to: (1) degrade the cell wall, (2) release AOM and hence (3) enhance methane production (Alzate et al., 2012; González-Fernández et al., 2012b; Cho et al., 2013a). Batch anaerobic digestion experiments were used to assess the effect of thermal and biological pre-treatment on the methane production of immobilised Scenedesmus obliquus, after nutrients removal process. Material and methods Algae culture and immobilisation The S. obliquus (276/42) culture was obtained from the Culture Collection for Algae and Protozoa (CCAP), (Oban, UK). Microalgae was cultured in batch in 100 L tanks containing 50 L Jaworski media as reported in Ometto et al., 2014c and b . Immobilisation conditions were reported in Whitton et al., 2016. Batch anaerobic digesters