Valorisation of restaurant food waste under the concept of a biorefinery

This paper explores the challenge to turn restaurant food waste from environmental issue to energy asset. The transformation of the dehydrated restaurant food waste to used oil, ethanol and methane was investigated. Each process unit was studied and optimised. Thermal dehydration was adopted producing a dried feedstock suitable for the downstream processes as well as a condensate rich in volatile fatty acids with a biomethane potential yield of 1.24 L CH₄/L. In the oil extraction process, the application of hexane, 20 Soxhlet cycles and solvent to feedstock ratio equal to 2 provided 81.76% oil yield with quality suitable for transesterification. Different loadings of non-commercial amylolytic enzyme were examined so as to enhance the starch bioconversion efficiency. Five microlitres of enzyme per gramme of starch and 90 min of saccharification time were selected as the optimum operating conditions for producing a hydrolysate that could be efficiently fermented to ethanol. The anaerobic biodegradability of the resulting stillage (415 L CH₄/kg VS) could further promote the viability of the whole treatment train. The experimental results were integrated in a material flow diagram in order to enlighten the assessment of the biorefinery scheme.