Biohythane production of post-hydrothermal liquefaction wastewater: A comparison of two-stage fermentation and catalytic hydrothermal gasification
Graphical abstract
Introduction
Hydrothermal liquefaction (HTL) can convert wet organic wastes into biocrude oil at temperatures between 280 and 370 °C and pressures between 10 and 25 MPa (Toor et al., 2011). HTL has been reported as a promising conversion pathway for human feces. One study reported that HTL of human feces led to a biocrude oil yield of 34.44% and a heating value of 40.29 MJ·kg−1 (Lu et al., 2017). Furthermore, HTL is a relatively quick thermochemical process, as it can be finished within one hour, and it is also much more efficient than compost (over 15 days) (Anand and Apul, 2014) and anaerobic digestion (60 days) (Owamah et al., 2014). In addition, HTL avoids the safety risk caused by pathogens in human feces because of the high temperature adopted during the HTL process. However, high-strength post-hydrothermal liquefaction wastewater (PHW) (total organic carbon content reaching up to 3–80 g·L−1) is produced as a by-product during HTL (Leng et al., 2018), which usually contains 35–40% of the carbon and 65–70% of the nitrogen in the feedstock (Yu et al., 2011). In addition, PHW contains a high concentration of hazardous and highly cytotoxic organics which would cause serious pollution if it is directly discharged into the environment (Pham et al., 2013, Si et al., 2018). Hence, developing efficient methods to recover energy from PHW is critical for scaling up HTL technology.
Anaerobic fermentation is one of the most widely used conversion technologies that has been implemented worldwide (Zheng et al., 2014). In particular, utilizing anaerobic digestion as a means of treating PHW while concomitantly producing methane has been extensively studied in recent years (Fernandez et al., 2018, Posmanik et al., 2017, Tommaso et al., 2015, Zheng et al., 2017, Zhou et al., 2015). However, anaerobic fermentation for methane production suffers from a long retention time (over 30 days) and low methane yield (123–169 mL·g−1 chemical oxygen demand (COD)), which is caused by the inhibition of toxic compounds in the PHW (Tommaso et al., 2015, Zheng et al., 2017, Zhou et al., 2015). In comparison to conventional one-stage fermentation for methane production, two-stage fermentation (TF) consists of separate hydrogen and methane production steps. In the first step, the complex substrates are hydrolyzed, leading to the production of organic acids and hydrogen. The produced organic acids are then converted into methane in the second step. The produced mixture of hydrogen and methane, which is referred to as hythane, has been acknowledged as one of the most important fuels for transitioning from a fossil fuel-based society to a terminal hydrogen-based society (Liu et al., 2013). TF not only can produce a cleaner biofuel, but also enhance the conversion efficiency (Schievano et al., 2014). Hydrogen production has proven to be able to degrade furfural and 5-hydroxymethyl furfural (5-HMF), which can be used as a detoxification step for methane production (Liu et al., 2015). Si et al. (2016) compared one-stage fermentation and TF using PHW from cornstalk. The detoxification of hydrogen production and an enhancement of energy recovery in TF were observed.
Catalytic hydrothermal gasification (CHG) has also drawn increasing attention as it offers advantages for treating high-moisture content feedstock (Sikarwar et al., 2017). CHG has been combined with HTL to improve the energy recovery and produce hythane from PHW, and the hydrogen and methane in the produced gas can reach up to 54.9% (vol.%) and 15.8% (vol.%), respectively (Cherad et al., 2016, Zhang et al., 2011). Recently, Watson et al. (2017) studied the CHG of PHW, and a hydrogen rich gas (56.3%, vol.%) was achieved. However, CHG operates at high temperatures (300–700 °C) and incorporates transition metal catalysts, which leads to a high energy demand and large economic input (Cherad et al., 2016, Watson et al., 2017, Zhang et al., 2011).
Previous studies have investigated the performance of biohythane production from PHW via TF and CHG individually, including PHW from human feces (Watson et al., 2017), cornstalk (Si et al., 2016), microalgae (Cherad et al., 2016) and municipality sludge (Zhang et al., 2011). However, there is a lack of information to compare these two methods and benchmark commercial applications. In this study, the effect of PHW concentration on TF, and the influence of temperature and retention time on CHG were studied. By doing that, a comparison of biohythane production from human feces PHW using TF and CHG, including gas production and techno-economic analyses, was conducted.
Section snippets
HTL process and characteristics of PHW
HTL of human feces was conducted at the University of Illinois at Urbana-Champaign. Human feces was collected from volunteers. The characteristics of the collected human feces is shown in Table 1. Stainless steel reactors (100 mL, Model 4593, Parr Instrument Co.) were used to perform HTL. HTL experiments were conducted at 280 °C with a retention time of 60 min. The feedstock was placed directly into the reactor, and then the reactor was sealed and purged with nitrogen three times to replace the
Biohythane production from PHW via TF
TF of PHW was conducted with a fermentation concentration of 10, 7, 4 and 1 g COD·L−1, respectively (Fig. 2). The hydrogen production finished within 12 days. The hydrogen content at 10 g COD·L−1 and 7 g COD·L−1 reached up to 25%. A hydrogen yield of 29.3 mL H2·g−1 COD was achieved at a 7 g COD·L−1 fermentation concentration, which was the highest value in the anaerobic fermentation test. Hydrogen production of the human feces PHW mainly came from the metabolic pathway of glycerol bioconversion
Conclusion
TF and CHG were compared to produce biohythane from the PHW generated from human feces. A hydrogen yield of 29.3 mL·g−1 COD for TF was achieved, and a methane yield of 254.3 mL·g−1 COD was reached. Compared with TF, a higher hydrogen yield (116.2 mL·g−1 COD) and hydrogen content (38.0%) was achieved for CHG. The techno-economic analysis based on the experimental data determined that TF with conventional reactors had a higher net energy return but higher cost than CHG. Further improvement of TF
Acknowledgements
This work was financially supported by National Key Research and Development Program of China (2016YFD0501402), the Bill & Melinda Gates Foundation (RTTC-C-R2-01-001), Beijing Youth Top-notch Talents Program (2015000026833ZK10) and International Postdoctoral Exchange Fellowship (20170086).
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