16.3.1 Low Concentration of Biomass

One of the major factors hampering anaerobic digestion of microalgae biomass is the low concentration of algal biomass under culture conditions. Most outdoor microalgae cultures are very dilute and may only contain 1 g L-1 of solid biomass (Golueke et al. 1957; Stephans et al. 2010); too dilute for anaerobic digesters, potentially causing bacterial washout due to biomass and excessive water addition to achieve the required VS solid loading rates (De Schamphelaire and Verstraete 2009; Golueke et al. 1957; Parkin and Owen 1986). This problem is overcome by harvesting, concentrating and dewatering microalgae cultures to concentrate the biomass. This is a relatively expensive and time consuming production requirement for most microalgae biofuel production methods (Harun et al. 2010; Pragya et al. 2013; Stephans et al. 2013; Ward et al. 2014). However, when anaerobic digestion is integrated with a microalgae biofuel production system, the energy requirement for harvesting and dewatering processes is offset by resultant methane production from the anaerobic digestion process (Sialve et al. 2009). Many different methods are used for the harvesting, concentration and dewatering microalgae, including settling of biomass (Collet et al. 2010), use of chemical flocculants (Golueke et al. 1964; Kalyuzhnyi et al. 1998), centrifugation (Benemann et al. 1977), alum (Golueke and Oswald 1963) and electro-flocculation (Pragya et al. 2013). The above listed harvesting methods have been shown to be nontoxic to anaerobic digestion and may improve digester performance due to better retention of solids (Callander and Barford 1983; Campos et al. 2008; Golueke and Oswald 1963; Kalyuzhnyi et al. 1998; Krishnan et al. 2006). However, with the high cost asso­ciated with some of these harvest methods, many new laboratory and pilot scale methods are under development, the resulting impact to anaerobic digestion is yet to be assessed for toxicity effects causing inhibition of anaerobic digestion processes (Ward et al. 2014).