The use of alga biomass for
biogas generation has been studied for over fifty years but until today, several distinct features, like inefficient degradation and low C/N ratios, limit the applicability of algal biomass for
biogas production in larger scale. In this work we investigated a novel, one-stage combined cultivation/fermentation strategy including inherently progressing
nitrogen starvation conditions to generate improved microalgal biomass substrates. For this strategy, comparable low amounts of
nitrogen fertilizers were applied during cultivation and no additional enzymatic, chemical or physical pretreatments had to be performed. The results of this study demonstrate that progressing
nitrogen limitation leads to continuously increasing C/N ratios of the biomass up to levels of 24-26 for all three tested alga strains (Chlamydomonas reinhardtii, Parachlorella kessleri and Scenedesmus obliquus). Importantly, the degradation efficiency of the algal cells increased with progressing
starvation, leading to strain-specific cell disintegration efficiencies of 35%-100% during the fermentation process.
Nitrogen limitation treatment resulted in a 65% increase of
biogas yields for C. reinhardtii biomass (max. 698±23mL
biogas g(-1) VS) when compared to replete conditions. For P. kessleri and S. obliquus, yields increased by 94% and 106% (max. 706±39mL and 586±36mL
biogas g(-1) VS, respectively). From these results we conclude that this novel one-stage cultivation strategy with inherent
nitrogen limitation can be used as a pretreatment for microalgal biomass generation, in order to produce accessible substrates with optimized C/N ratios for the subsequent anaerobic fermentation process, thus increasing methane production and avoiding the risk of
ammonia inhibition effects within the fermenter.