Bio-hythane production from cassava residue by two-stage fermentative process with recirculation
Graphical abstract
Introduction
Hythane (a mixture of methane and hydrogen) is considered as a bridge energy carrier in the “decarbonization” between natural gas (consisting mainly of methane) and the final goal of hydrogen. Because it can markedly reduce exhaust emissions of hydrocarbon compounds and drastically improve the efficiency of conventional spark- or compression-ignition engines (Muradov, 2014, Porpatham et al., 2007), the development of hythane fermentation method appears inevitable as the direction for future energy emerges.
Theoretically, the highest hydrogen yield is restricted to 4 mol-H2/mol-Hexose and the COD removal efficiency gets bogged down at 33.3%, accompanied by extremely unpleasant odor from the accumulation of VFAs in the one-stage hydrogen dark fermentation (Khanal, 2011). On the other hand, the hydrolysis of lignocellulosic biomass has been considered as the rate-limiting step for methane production (Noike et al., 1985, Tong et al., 1990, Fey and Conrad, 2003). The phase separation of hydrolysis and hydrogen fermentation from methanogenesis in different reaction environments has been proposed as a strategy to improve overall process performances, in terms of stability, degradation efficiencies and overall energy recovery from biomass (Luo et al., 2011). Wang et al. estimated that 5.78% of the influent COD of food waste was converted to hydrogen in the first stage, and 82.2% of COD was converted to methane in the second stage (Wang and Zhao, 2009). The two-stage hythane fermentation had the ability to enhance 8%–43% of the total energy recovery compared with the one-stage methane fermentation, when the lignocellulosic materials were used as substrates (Nathao et al., 2013, Nielsen et al., 2004, Schievano et al., 2014). Recently, the several successful operations of two-stage hythane fermentation with the recirculation (pumping a part of the methanogenic sludge back into the first stage hydrogen reactor) were reported, which could maintain appropriate condition in the first stage with little or no addition of alkaline and NH4+ (Lee et al., 2010), and dilute the concentration of substrate without adding any extra water (Ohba et al., 2005). Consequently, recirculation contributed to the improvement of carbohydrate degradation and the increasing of biogas production (Kobayashi et al., 2012). However, the improper recirculation ratio could upset the balance in the first stage, since certain amount of hydrogenotrophic methanogens and homoacetogens was sent back to the first stage, as hydrogen consumers, with the hydrolysis bacteria and hydrogen producing bacteria (HPB) at the same time. Thus, it is necessary to investigate the effect of operational modes (hydraulic retention times (HRTs) and recirculation) on the practical efficiency of two-stage hythane fermentation.
Cassava (Manihot esculenta) as one of the world’s fastest expanding crops is widely planted in tropical areas and used as food, animal feed and industrial materials. The total production of cassava has continued to rise in the last two decades, mainly due to the development of industrial manufacture on starch and ethanol (Food and Agriculture Organization of the United Nations, 2016). However, 40%–90% of fresh cassava roots were converted to residue in the industrial processing (Edama et al., 2014), and this kind of residue is extremely high in carbohydrate which requires urgent treatment. In recent years, agro-industrial cassava residue, including processing wastewater (Intanoo et al., 2014), cassava stillage and cassava excess sludge (Luo et al., 2010) has gained prominence along with the rapid development of cassava starch processing manufacture. However, because of its low protein concentration, there are certain limits to the practical utilization of cassava residue. To compensate for this, some researchers have considered co-digestion with pig manure (Ren et al., 2014) or adding ammonium-nitrogen to optimize the C/N ratio. A sufficient N-source has been required to guarantee microflora proliferation and the metabolism of fermentation. It has also been shown that the addition of trace metals has a significant effect on hydrogen and methane fermentation (Qiang et al., 2012, Qiang et al., 2013, Lin and Lay, 2005).
In this study, both the continuous operation of two-stage hythane fermentation with recirculation and the repeated batch experiment were carried out in order to (i) demonstrate the stability of long-term continuous hythane production from cassava residue using recirculation, (ii) clarify the effect of HRTs on hydrogen and methane production, and (iii) investigate the effect of nitrogen, nickel, cobalt and sulfur supplements.
Section snippets
Inoculum and substrate
The anaerobic mixed microflora as seed sludge for both hydrogen and methane fermentation was obtained from a mesophilic sewage sludge digester at Sendai municipal sewage treatment plant in Japan. In the beginning, inoculum for hydrogen fermenter was adjusted pH from 7.3 into 5.5 by feeding 2 mol/L HCl solution without heat-pretreatment, due to that the hydrogen consuming bacteria can be effectively inhibited by low pH and thermophilic operational temperature.
Cassava residue was collected from a
The time course of the operational performance under different HRTs
The two-stage hythane fermentation with 1:1 ratio of recirculation was operated over 440 days, which can be divided into three major parts. The first part was further separated into five phases (Phase 1–5) depending on HRT variation. Fast start-up strategy was performed in the second part (Phase 6), while Phase 7–9 were defined as the third part according to the addition of different TEs. Fig. 2, Fig. 3 illustrate that the time course of the operational performance of pH, biogas production rate,
Conclusion
The two-stage process treating cassava residue failed after 104 days caused by the lack of nitrogen and micronutrients. The fast restart-up strategy of operational modes and the effect of nutrients addition were verified by the continuous and the repeated batch experiment. The combination of 0.7 g-N/L, 1.0 mg-Co/L, 1.0 mg-Ni/L and 30 mg-S/L guaranteed the microorganism proliferation and the fermentative metabolism, due to covering the gap between theoretical demands and existing values of nutrients
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