Pilot-scale multi-purposes approach for volatile fatty acid production, hydrogen and methane from an automatic controlled anaerobic process

A combined two-levels control method has been developed and tested on a long term operation of a two-phases pilot-scale anaerobic process for the concurrent production of volatile fatty acids, hydrogen and methane. The latter was designed for the treatment of food waste of urban origin (namely, the organic fraction of municipal solid waste). The optimized control method was set on the base of the inputs of three online probes: a pH-meter in the fermentation reactor, a pH-meter and a conductivity probe in the digestion reactor. The first control level managed the pH in the fermentation reactor while the second control level managed the ammonia concentration in the digestion reactor. This combination established the volume of the digestate to be recycled from the digestion to the fermentation reactor, optimizing the yield of volatile fatty acid (0.31–0.32 kg COD_VFA/kg COD_fed) and the specific hydrogen production (SHP; 0.070–0.074 m³ H₂/kg TVS_fed) in the fermentation reactor and the specific methane production (SMP; 0.48–0.55 m³ CH₄/kg TVS_fed) in the digestion reactor. A new process configuration was also proposed and applied over the course of the long operation period. This configuration allowed to remove part of the volatile fatty acid-rich liquid stream from the fermenter effluent, maintaining the corresponding solid-rich effluent in the whole system (as feed for the digestion reactor) by using a solid/liquid separation unit. In this way, the concentration of volatile fatty acids in the digester was kept at a low level, even with high loading rates, so maintaining a satisfying efficiency of methane production and utilizing the excess volatile fatty acids (out of the system) as building blocks for other purposes. The optimized two-levels control method for the anaerobic treatment of food waste provides new perspectives for the valorisation of such waste stream; the production of building blocks namely volatile fatty acids supports new innovative bio-refinery platforms for the production of bio-products.