Thermophilic hydrogen and methane production from sugarcane stillage in two-stage anaerobic fluidized bed reactors

This study evaluated the feasibility of H-2 and CH4 production in two-stage thermophilic (55 degrees C) anaerobic digestion of sugarcane stillage (5,000 to 10,000 mg COD.L-1) using an acidogenic anaerobic fluidized bed reactor (AFBR-A) with a hydraulic retention time (HRT) of 4 h and a methanogenic AFBR (AFBR-S) with HRTs of 24 h-10 h. To compare two-stage digestion with single-stage digestion, a third methanogenic reactor (AFBR-M) with a HRT of 24 h was fed with increasing stillage concentrations (5,000 to 10,000 mg COD.L-1). The AFBR-M produced a methane content of 68.4 7.2%, a maximum yield of 0.30 +/- 0.04 L CH4.g COD-1, a production rate of 3.78 +/- 0.40 L CH4.day(-1).L-1 and a COD removal of 73.2 +/- 5.0% at an organic loading rate (OLR) of 7.5 kg COD.m(-3).day(-1). In contrast, the two-stage AFBR-A system produced a hydrogen content of 23.9 +/- 5.6%, a production rate of 1.30 +/- 0.16 L H-2.day(-1)L(-1) and a yield of 0.34 +/- 0.08 mmol H-2.g COD.m(-1). Additionally, the decrease in the HRT from 18 h to 10 h in the AFBR-S favored a higher methane production, improving the maximum methane content (74.5 +/- 6.0%), production rate (5.57 +/- 0.38 L CH4.day(-1).L-1) and yield (0.26 +/- 0.06 L CH4.g COD-1) at an OLR of 21.6 kg COD.m(-3).day(-1) (HRT of 10 h) with a total COD removal of 70.1 +/- 7.1%. Under the applied COD of 10,000 mg L-1, the two-stage system showed a 52.8% higher energy yield than the single-stage anaerobic digestion system. These results show that, relative to a single-stage system, two-stage anaerobic digestion systems produce more hydrogen and methane while achieving similar treatment efficiencies. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.