This study investigated the effect of biochar in the psychrophilic anaerobic co-digestion regarding biomethane production potential (BMP), metabolic efficiencies, and microbial population. BMP tests of cheese whey and cattle manure as substrates were conducted at different gasified pine wood biochar concentrations (Bc: 10g/L, 30g/L, 50g/L); and particle sizes (Ps: ~0.15mm, ~0.45mm, ~1mm). The most favourable conditions of Ps < 0.575mm and Bc of 30g/L, allowed BMP values to go from 0.23m3CH4/kgVSadd to 0.34m3CH4/kgVSadd. The study of metabolic stages shows the carrier modulates hydrolysis and methanogenesis and favours the acetoclastic metabolism to improve methane yield even at 15°C. The positive effect of biochar is reinforced in the fact that it boots the growth of methanogen psychrotrophs microbial populations up to 520% compared with a BMP with no biochar added. The study showed that psychrophilic AD + biochar might overcome mesophilia's energy needs by improving yields with no extra energetic requirements.