Inclusion of Saccharina latissima in conventional anaerobic digestion systems

F. Ometto, A. Berg, A. Björn, L. Safaric, B. H. Svensson, A. Karlsson, J. Ejlertsson

Loading macroalgae into existing anaerobic digestion (AD) plants allows us to overcome challenges such as low digestion efficiencies, trace elements limitation, excessive salinity levels and accumulation of volatile fatty acids (VFAs), observed while digesting algae as a single substrate. In this work, the co-digestion of the brown macroalgae Saccharina latissima with mixed municipal wastewater sludge (WWS) was investigated in mesophilic and thermophilic conditions. The hydraulic retention time (HRT) and the organic loading rate (OLR) were fixed at 19 days and 2.1 g l(-1) d(-1) of volatile solids (VS), respectively. Initially, WWS was digested alone. Subsequently, a percentage of the total OLR (20%, 50% and finally 80%) was replaced by S. latissima biomass. Optimal digestion conditions were observed at medium-low algae loading (≤50% of total OLR) with an average methane yield close to 220 Nml g VSalgae(-1) and 250 Nml g VSalgae(-1) in mesophilic and thermophilic conditions respectively. The conductivity values increased with the algae loading without inhibiting the digestion process. The viscosities of the reactor sludges revealed decreasing values with reduced WWS loading at both temperatures, enhancing mixing properties.