To impose the zero-waste strategy through the diverse valorization route, this study used bread waste (BW) as the model compound. To realize these grand technical challenges, BW was enzymatically hydrolyzed to obtain a monomer sugar compound (glucose), and glucose was used as the carbon substrate in heterotrophic cultivation of Euglena gracilis ( E. gracilis ). Cultivation of E. gracilis using cultivation medium derived from BW was evaluated from a perspective of an economic viability . In addition, the more production of the target compound (paramylon, β-1,3-glucan) stemmed from E. gracilis was achieved (1.93 g L −1 d −1 ; 24% higher productivity than that of control). To approach zero waste disposal, bread waste residue (BWR) derived from enzymatic hydrolysis of BW was valorized into syngas . To offer a greener pyrolysis platform for BWR, CO 2 was used as a raw material. Here in this study, the mechanistic functionality of CO 2 was disclosed. In detail, CO 2 reacted with volatile matters (VMs) evolved from BWR, thereby resulting in CO 2 reduction. Simultaneously, reduced CO 2 also led to oxidation of VMs. Such consecutive gas-phase-reactions (GPRs) played a critical role to enhance CO formation. Lastly, the identified GPRs induced by CO 2 were tried to expedite the reaction kinetics in the presence of 5 wt% of Ni/SiO 2 catalyst. As a result, the molar concentrations of H 2 and CO in gaseous pyrolytic products derived from catalytic pyrolysis in CO 2 environment were 2- and 6-times higher, respectively, than that from pyrolysis without catalyst in N 2 environment. • Zero waste disposal platform for food waste (FW) was experimentally practiced. • Glucose derived from FW was used as carbon substrate for the growth of E. gracilis . • High yield of paramylon was achieved from E. gracilis grown with FW medium. • FW residue after glucose extraction was further valorized into energy. • CO 2 was converted into gaseous fuel in catalytic pyrolysis of FW residue.