Initiating methanogenesis of vegetable waste at low inoculum-to-substrate ratio: Importance of spatial separation

With the goal of starting-up the methanogenesis of easily biodegradable waste with minimum inoculum, the present work evaluated different inoculum-to-substrate ratios (rI/S) in completely mixed systems and in the systems with spatial separation of inoculum and waste. It was found difficult to initiate methanogenesis in the completely mixed systems, even at high rI/S 1.105 on a volatile solid basis. Fermentation efficiencies were independent of rI/S. In the spatial-separation systems with a low total rI/S 0.053, the ultimate methane yield (35 °C, 1 atm) reached 445 mL/g-VS added for the inoculum-waste initially completely separated system. The yields decreased to 285, 181, and 34 mL/g-VS added, respectively, for partially separated systems with the ratios controlled at 1.105, 0.254, and 0.113 in the inoculum-containing reactors. This demonstrates the importance of setting spatial separation between inoculum and waste when inoculation is employed. An appropriate inoculation method would initiate methanogenesis rapidly even at low inoculum-to-substrate ratios.

► Spatial separation is more important than inoculum-to-substrate ratio (rI/S).
► High rI/S was not enough to initiate methanogenesis in completely mixing system.
► Methanogenesis can be initiated at low rI/S in spatial separation system.
► The efficiencies of hydrolysis and acidogenesis were independent of the rI/S.