Current development of new anaerobic digester designs and configurations can play an important role in the future of hybrid microalgae production and anaerobic digestion facilities. New digester reactor designs that decouple the hydraulic and solid retention times can be extremely beneficial for anaerobic digestion of micro­algae (Ward et al. 2014) and can be achieved by upflow anaerobic sludge blanket reactors, anaerobic membrane reactors, anaerobic filters and bed reactors, in-pond fermentation pits and also by two-stage anaerobic digestion (Gao et al. 2007; Goodwin et al. 2001; Green et al. 1995a, b; Haridas et al. 2005; Inglesby and Fisher 2012; Shin et al. 2010; Varel et al. 1988; Vergara-Fernandez et al. 2008; Zamalloa 2012; Zhou et al. 2009). New reactor designs allow better control, breakdown and conversion of organic matter within the digester (Vergara-Fernandez et al. 2008). In particular, two-stage anaerobic digestion physically separates stages of digestion into separate reactors, allowing much better control over the anaerobic digestion process (Dinsdale et al. 1996; Dugba and Zhang 1999; Shin et al. 2010; Varel et al. 1988; Vergara-Fernandez et al. 2008; Yu et al. 2002). Furthermore, molecular research underway is providing new insights into the microbial communities associated with the anaerobic digestion process and aim to understand the impact of internal envi­ronmental conditions and change within the digester community (Keyser et al. 2006; Shin et al. 2010; Supaphol et al. 2011). For example, new molecular methods in the anaerobic digestion process (such as “poly chain reaction denaturing gel gradient electrophoresis” and “real time poly chain reaction”) are under investigation to improve metabolic pathways within the digester and the abundance and species composition of bacterial populations (Patil et al. 2010a, b; Shin et al. 2010; Skilman et al. 2009; Supaphol et al. 2011; Ward et al. 2015; Zhang et al. 2012; Ziganshin et al.

2011) . A greater understanding of the anaerobic digestion processes at the molecular level can optimise the anaerobic digestion process to specific microalgal biomass and co-digestate substrates (Ward et al. 2014).

16.2 Conclusions

The integration of anaerobic digestion with microalgae-based biofuels production is able to attain higher efficiency and improve sustainability of the production of microalgae-based biofuels. Several of the technical issues including the low con­centration of biodegradable (digestible) microalgae substrates, cell wall disruption and high lipid concentrations can be overcome by the pre-treatment methods used to process microalgae for liquid and gaseous biofuels. Gas produced by the anaerobic digestion of residual microalgae biomass can be used for electrical or thermal energy within the microalgae biofuels bio-refinery, while the high density microalgae cultures can provide efficient biogas purification. The resulting digestate has been shown to be an ideal nutrient source for the continued growth of additional microalgae biomass and help to close the nutrient loop associated with large-scale microalgae biomass production. With a greater understanding of the different algae species and their characteristics, the anaerobic digestion of microalgae and their residues can be optimised to play an essential role in the sustainable future of clean energy derived from microalgae biomass.