It is an economical way to co-digest microalgae with food waste and sludge in their existing digestion facilities. However, the synergies between these wastes are not clear at high solid concentrations. In this study, Spirulina platensis was co-digested with food waste and sludge in batch experiments, and the effects were compared in terms of methanogenic rates and yields. The results showed that co-digestion with food waste improved methane production by 37.5% at most and accelerated the methanogenic rate from 0.16 to 0.35 d(-1), but the ratio of microalgae in the co-substrate should be lower than 25% to maximize synergy. Co-digestion with sludge improved the methane production by 10.3%, but the methanogenic rate remained unchanged at 0.16 d(-1). The experimental methane productions from co digestion were compared with the calculated values based on mono digestion of microalgae, food waste, and sludge. In terms of methane production, the synergy occurred at any blending ratio of microalgae and sludge, but the best performance resulted from 66% microalgae in the co-substrate. Overall, the synergistic effect of co-digestion varied in a range of-5-15% when microalgae accounted for 6-38% in the co-substrate with food waste or 33-67% in the co-substrate with sludge. (C) 2019 Elsevier Ltd. All rights reserved.