Biogas production has been suggested as a valid valorization solution for microalgal/bacteria biomass (MAB) grown on wastewater. This research is aimed at assessing the feasibility to use MAB grown in an outdoor raceway fed on piggery wastewater for biogas production. Batch and continuous anaerobic tests were conducted on the sole MAB and on a blend of MAB and carbonaceous substrates (deproteinated cheese whey and cellulose) to improve the carbon/nitrogen ratio. Results of batch biochemical methane potential tests confirmed that the sole microalgal/bacteria biomass was poorly degradable (119 NmLCH4·gCOD−1), while blending it with deproteinated cheese whey or cellulose (80% of carbonaceous material and 20% of MAB, as chemical oxygen demand (COD)) had no synergistic effects on the methane yield, although slight improvements in the degradation kinetics were observed. Continuous anaerobic degradation tests (at an organic loading rate of 1.5 gCOD·L−1·d−1, 35 °C and 30 days of hydraulic retention time) increased the overall methane yield from 81 NmLCH4·gCOD−1 (sole MAB) to 216 NmLCH4·gCOD−1 (MAB and deproteinated cheese whey) and 122 NmLCH4·gCOD−1 (MAB and cheese whey). However, data confirm that no evident synergistic effects were obtained.