applied sciences  
Acticle  
Co-Fermentation of Microalgae Biomass and  
Miscanthus X giganteus Silage--Assessment of the Substrate, Biogas Production and Digestate Characteristics  
[PERSON](c), [PERSON],*(c), [PERSON](c) and [PERSON](c)  
1 Department of Environmental Engineering, Faculty of Geoengineering, University of Warmia and Mazury in Olsztyn, 10-720 Olsztyn, Poland; [EMAIL_ADDRESS] (M.D. ); [EMAIL_ADDRESS] (M.Z.) 2 Department of Water Supply and Sewage Systems, Faculty of Civil Engineering and Environmental Sciences, Bialystok University of Technology, 15-351 Bialystok, Poland; [EMAIL_ADDRESS]  
* Correspondence: [EMAIL_ADDRESS]  
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microalgal biomass can be converted into many types of biofuels--biodiesel, biohydrogen or biogas--or can be combusted as is [15; 16; 17]. Some species of microalgae accumulate large amounts of lipids, starches and proteins, while containing little to no hard-to-degrade lignin, making them prime candidates for biomethane production feedstocks [18; 19]. Algae can generate 140 to 360 \(\mathrm{dm^{3}/kgVS}\) biomethane, depending on the species [20]. The biogas produced from microalgal biomass contains 55 to 75% methane, matching or even exceeding the calorific value of biogas produced from other plant substrates [21]. Anaerobic digestion is currently widely used to produce renewable energy from organic biomass [22; 23]. However, the performance of this process can be hampered by several limiting factors, one of which is an inadequate ratio of carbon (C) to nitrogen (N) in the reactor feedstock [24]. The optimal C/N ratio in the feedstock is between 20/1 and 30/1 [25]. By contrast, the C/N ratio of algal biomass is generally below 10, which may induce high levels of ammonia nitrogen and volatile fatty acids, thus impeding methane generation [26]. Most agricultural biogas plants in the EU use maize silage as the primary organic substrate for biogas generation [27]. However, the use of maize--a food and feed crop--for biogas production has raised controversy and concerns due to the crop's rising price and the adverse impact of maize monoculture on water resources, soil properties and flora/fauna biodiversity [28]. It would therefore be reasonable to pursue biogas production from energy-efficient crops as an alternative to maize. Due to the high yield of biomass, low environmental requirements and the possibility of cultivation on low-quality and marginal soils, _Miscanthus_\(\times\)_giganteus_ is a fully justified choice [29]. Its suitability for producing many types of biofuels has been verified. In the case of methane fermentation, limitations related to the high content of lignocellulosic complexes, low hydration or inappropriate C/N ratios were indicated [30]. One of the possibilities of reducing or eliminating the weaknesses of _Miscanthus_\(\times\)_giganteus_ biomass may be co-fermentation with microalgae biomass. A detailed analysis of the anaerobic digestion of such a substrate composition has not been presented so far, which has become the inspiration and the basis for conducting research to verify this hypothesis. Co-digestion of the two substrates may be a way to solve both problems: limiting the land required for growing energy crops while optimizing the digestion of microalgal biomass [31; 32]. The main benefits of co-digestion include more favorable moisture levels, optimization of the pH and C/N ratios, greater buffer capacity and the dilution of toxic compounds, thus improving biogas/methane yields [33].