Microalgae have a relatively low carbon to nitrogen (C/N) ratio when compared to other biomass substrates and wastewaters utilised in the anaerobic digestion process (Sialve et al. 2009; Vergara-Fernandez et al. 2008). The ideal C/N ratio for anaerobic digestion is approximately in the range of 20-30 (Parkin and Owen 1986). In contrast, microalgae have a C/N ratio, between 4 and 8 (Ward et al. 2014). When a substrate has a C/N ratio below 20, it can cause an imbalance in an accumulation of NH3-N within the digester and associated inhibition (Sialve et al.

2009) . To overcome low C/N ratios associated with microalgae, co-digestion with other waste streams high in carbon (i. e. paper, glycerol, pig manure, cow manure, lipid rich fats oils and greases, municipal waste, soybean oil) emolliates the high NH3-N build-up (Ehimen et al. 2009; Gonzalez-Fernandez et al. 2011; Saxena et al. 1984; Shouquan et al. 2009; Yen and Brune 2007). For example, when paper was added to microalgae in a digester, the C/N ratio increased from 6.7 to 36.4 with the best co-digestion biogas production with a mix of 50 % paper and 50 % microalgae at a C/N ratio of 18.0 (Yen and Brune 2007). When the biogas production from the C/N ratio treatment of 18 was compared to the lower C/N ratio of 6.7, a 50 % increase in biogas productivity was recorded (Yen and Brune 2007). Such co­digestion ratio productivity is highly dependent on the species of microalgae being digested as well as the secondary substrate being co-digested (Ward et al. 2014). When a high C/N ratio is used, there is a risk that the bacterial population may become NH3-N limiting, causing inhibition with the methanogen bacterial com­munity (Chen et al. 2008; Parkin and Owen 1986). When considering the co­digestion of microalgae with other substrates, the availability, seasonality and location of the secondary co-digestion substrate must also be considered to ensure continuous availability and low cost associated with procuring and transportation (Ward et al. 2014). Furthermore, balancing both the C/N ratio and all co-digested substrate, degradation rates are essential to balance the carbon and nitrogen release within the digester. To balance the C/N ratio, the co-digestion substrate should have a similar degradation rate as the primary substrate being digested (Ehimen et al. 2009; Kayhanian 1994).