Comparative evaluation of digestate and reject water as nutrient media for syngas biomethanation in thermophilic trickle-bed reactors

Syngas biomethanation facilitates the utilization of thermal gasification products. This study evaluated the performance of two liquid organic waste streams (manure-based digestate and reject water from digested sewage sludge) as nutrient media in thermophilic trickle-bed reactors (TBRs) over more than one year. Digestate achieved a Methane Evolution Rate (MER) of 4.5 L/(L-pbvd) with the highest so far published H-2 and CO conversion rates (>95 %). Reject water only led to a maximum MER of 1.0 L/(L-pbvd), while the addition of sulfur and phosphorus to the reject water resulted in improved MER of up to 3.1 L/(L-pbvd). The microbial analysis illustrated a similar microbial community structure and methanogenic abundance for both TBRs, with Methanothermobacter as the dominating methanogen both in the liquid phase and biofilm of the carriers. Carbon monoxide was likely converted to both methane and acetate.