Addition of biomass ash as a promising strategy for high-value biohythane production from the organic fraction of municipal solid waste

In this work, the effect of adding ash from combustion of six locally available biomass types, namely common reed (Phragmites australis), corn stover, eucalyptus sawdust, hazelnut shells, pine chips, and rice husks at different concentrations (2.5, 5, 10, and 20 g/L) on H-2 production from the organic fraction of municipal solid waste (OFMSW) was first investigated through batch experiments. The maximum cumulative H-2 production of 1124 +/- 3.21 mL was achieved using 5 g/L of rice husk ash (RHA), increasing by 644% compared to the control. Then, in a semi-continuous two-stage biohythane production process, the first reactor dedicated to H2 production was supplemented with 5 g/L of RHA and its performance was evaluated at three HRTs (3, 2, and 1 day). The results showed that the highest steady-state volumetric H-2 production rate (554 +/- 5.48 mL/L/d) and H-2 yield (184.7 +/- 1.83 mL/g-VSadded) were obtained at 2-day HRT. In the subsequent CH4 production stage, of the three HRTs tested (12, 8, and 4 days), the 12-day HRT resulted in a maximum volumetric CH4 production rate and CH4 yield of 321 +/- 2.39 mL/L/d and 727.2 +/- 5.46 mL/g-VSadded, respectively, under steady-state conditions. Throughout both stages, the pH remained within a suitable range without the addition of any commercial reagents. The overall energy yield of the two-stage process was found to be 31.2 kJ/g-VSadded, which corresponded to a total COD removal efficiency of 96%. Moreover, the composition of biohythane (H-2 + CH4) obtained met the standards of a clean hythane fuel.