Hydrogen and Methane Production from Anaerobic Co-Digestion of Sorghum and Cow Manure: Effect of pH and Hydraulic Retention Time

Abstract

The need for alternative energy sources is constantly growing worldwide, while the focus has shifted to the valorization of biomass. The aim of the present study was to determine the optimal pH and hydraulic retention time (HRT) values for treating a mixture of sorghum biomass solution with liquid cow manure (in a ratio 95:5 v/v) through anaerobic digestion, in a two-stage system. Batch tests were initially carried out for the investigation of the pH effect on bio-hydrogen and volatile fatty acids (VFA) production. The highest hydrogen yield of 0.92 mol H2/mol carbohydratesconsumed was obtained at pH 5.0, whereas the maximum degradation of carbohydrates and VFA productivity was observed at pH 6.0. Further investigation of the effect of HRT on hydrogen and methane production was carried out. The maximum yield of 1.68 mol H2/mol carbohydratesconsumed was observed at an HRT of 5 d, with H2 productivity of 0.13 L/LR·d. On the other hand, the highest CH4 production rate of 0.44 L/LR·d was achieved at an HRT of 25 d, with a methane yield of 295.3 mL/g VSadded, whereas at a reduced HRT of 20 d the process exhibited inhibition and/or overload, as indicated by an accumulation of VFAs and decline in CH4 productivity.

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“…This VFA/alkalinity ratio could serve as an indicator of the buffering capacity of the system, as the increase in VFAs and a decline in alkalinity signifies a diminished pH buffer capacity. Generally, a maximum threshold VFA/alkalinity ratio of 0.6 is considered suitable to maintain a healthy AD process without the risk of acidification [ 19 ]. Therefore, the high VFA/alkalinity ratio could potentially emerge as a critical factor contributing to the instability observed in the 20-days HRT.…”

“…Therefore, the high VFA/alkalinity ratio could potentially emerge as a critical factor contributing to the instability observed in the 20-days HRT. In contrast, it was observed that the VFA/alkalinity ratio, between 0.63 and 0.64, detected within the HRTs of 40, 30 and 25 days, stayed within a range that closely aligns with favorable conditions (0.6) [ 19 ]. This observation is consistent with previous research that indicated a VFA/alkalinity ratio of 0.6, which was detected at the optimal HRT and resulted in effective biogas production from biogas effluent and filter cake [ 54 ].…”

“…In the context of methane production through AD, the continuous stirred tank reactor (CSTR) is widely used [ 18 ]. Key factors influencing methane production include pH, temperature, organic loading rate (OLR), and hydraulic retention time (HRT) [ 19 ]. Maintaining an appropriate HRT is vital for optimizing methane production rate (MPR) and ensuring the stability of microbial communities in CSTR systems [ [20] , [21] , [22] ].…”

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“…These correlations were similar to those observed in the study by Dareioti et al (2022), who found that at a lower HRT of 20 days, the process showed evidence of inhibition and/or overload, such as an accumulation of VFAs and a decline in MPR and MY. Moreover, the maximum MPR and MY of 0.44 L-CH 4 /L•d and 295.3 mL-CH 4 /g-VS added, respectively, were obtained at the HRT of 25 days [30]. Likewise, Ramos and Silva (2020) reported an increased MPR with a higher OLR achieved by reducing HRT to the optimal level in a two-stage hydrogen and methane production process from sugarcane silage in a sequential fluidized bed reactor [31].…”

Biogas Production Optimization in the Anaerobic Codigestion Process: A Critical Review on Process Parameters Modeling and Simulation Tools

“…In the process of producing methane, most bacteria prefer a neutral pH. Kornaros et al [70] experimented with the AcoD of CM and sorghum aimed at studying the infuence of initial pH value on CH 4 and H 2 production in a two-stage digester. Tey achieved the highest H 2 yield of 0.92 mol H 2 /mol carbohydrates consumed at pH 5.…”