New insights in food waste, sewage sludge and green waste anaerobic fermentation for short-chain volatile fatty acids production: A review

50.3 M tons of wastes are annually produced at urban level in the EU-27. Sewage sludge, Organic Fraction of Municipal Solid Wastes (OFMSW) and food industrial wastewaters, are the major typologies of wastes produced at urban level. OFMSW and sewage sludge account for the 28 % and 23 % of the EU-27 wastes streams, respectively. Their abundance and the high content of nutrients (nitrogen and phosphorous) make them very interesting as substrates in a biorefinery loop to produce biofuels and bio-based products. This review provides an overview on the conversion of urban wastes into Volatile Fatty Acids (VFAs) at different operational condi- tions, from small laboratory scales to full industrial plants. Mono-fermentation of no pretreated substrates OFMSW and Thickened Primary Sludge (TPS) led to low VFAs yields of 0.25–0.30 and 0.50 gVFA-COD/gCOD, respectively. The co-fermentation of OFMSW and sewage sludge achieved higher VFAs yields (0.38 gVFA-COD/ gCOD). Co-fermentation yields was further improved (0.85 gVFA-COD/gCOD) by the adoption of thermophilic temperature (55 ◦C). Regarding VFAs profile, it was observed that substrates with lower VFAs yields presented a higher concentration of acetic acid, while the improvement of the acidogenic fermentation’s yield had as consequence the increasing of propionic and butyric acids’ concentrations. Finally, innovative electro-driven approaches, electro-fermentation and electrodialysis, employing polarized electrodes have been investigated to favor the production of desired VFAs or to enhance acids separation from the fermentation broth.