TECHNOLOGY FOCUS - BIOLOGY - Preferred Components: The brown algae includes at least one of kelp, gulfweed, hizikia fusiforme, Ecklonia cava, brown seaweed, Analipus japonicus, Coelocladia arctica Rosenvinge, Ishige okamurai, Scytosiphon, Petalonia binghamiae, Ecklonia stolonifera, rhizoma rhei, Costaria costata, Sargassum horneri, and Sargassum thunbergii. The mesophilic aerobic mixed strain includes at least one of Clostridium tyrobutyricum, Prevotella maculosa, Dialister succinatiphilus, Rahnella aquatilis, Bifidobacterium thermacidophilum, Bacteroides uniformis, Yersinia intermedia, Shewanella baltica, Pseudomonas veronii, Clostridium celerecrescens, Spirochaeta coccoides, Enterobacter ludwigii, Clostridium saccharoperbutylacetonicum, Clostridium xylanolyticum, Clostridium saccharolyticum, Raoultella terrigena and Xanthomonas oryzae. The mesophilic aerobic mixed strain is isolated from sewage sludge. Preferred Method: The solid-liquid separation process is carried out by crushing the biomass and extracting the crushed biomass in monosaccharide solvent, preferably mannitol solvent. The biological preprocessing includes sedimentation or centrifugation process. The biological preprocessing is carried out by introducing air at 30-35 degrees C with a supply rate of 1-5 l/minute under mesophilic aerobic condition using an air pump. The biological preprocessing includes batch culture, fed-batch culture or batch continuous culture (preferred). The retention time of solid phase in the continuous culture is greater than or equal to 1 day. The method further comprises physico-chemical treatment, generating useful product or bioenergy by using biologically preprocessed saccharified solution, and decomposing polysaccharide containing alginate using the mesophilic aerobic strain within a reactor equipped with an air aeration tank at 30-35 degrees C. The mesophilic aerobic mixed strain is enrichment cultured in the culture tank containing brown algae, The mesophilic aerobic mixed strain is isolated by primarily culturing sewage sludge under anaerobic condition and performing secondary enrichment culture under aerobic condition. EXAMPLE - No suitable example given.