Anaerobic treatment of hydrothermally solubilised sugarcane bagasse and its kinetic modelling
Introduction
To maximise bioethanol production at factories, an integrated approach to utilise whole plant biomass has been paid keen attention, where bagasse biomass after extraction of sugarcane juice is converted to the additional source for saccharification followed by ethanol fermentation under a biorefinery concept (Dias et al., 2009, Rabelo et al., 2011a, Rabelo et al., 2011b). This is based on the potential of ethanol production from the cellulosic compounds in the bagasse, which is supposed to be comparable to that from the juice (Cardona et al., 2010). Since bagasse is a lignocellulosic compound, pre-treatments are needed to recover the cellulosic materials whilst wastewater containing organic acids, solubilised carbohydrate and solubilised lignin is also generated from the process. At present, various kinds of pre-treatment methods are extensively studied and developed (Balat et al., 2008). In this regard, steam-explosion processes using a high-pressure vessel followed by an instantaneous depressurisation module are supposed to be a new and promising option because of high cellulose recovery efficiency (Kuo and Lee, 2009, Zheng et al., 2014, Romaní et al., 2016).
On the other hand, apart from the developments of bioethanol production system, studies for the treatment of the wastewater generated from the pre-treatments are still very limited (Cardona and Sánchez, 2007). In fact, most of available technical information for the anaerobic digestion of plant biomass are limited to list the biogas yields on the basis of material (Rabelo et al., 2011a, Rabelo et al., 2011b, Zhang et al., 2016). Since kinetic factors are not included in the information, process designing and system operation cannot be performed in precise manner (Damayanti et al., 2010). Consequently, conservative low-loaded systems, e.g. chemostat anaerobic reactor and/or conventional activated sludge process are mentioned for the anaerobic wastewater treatments (Steinwinder et al., 2011), which might result in overdesign of the capital and operational costs. Considering the organic-rich property of the wastewater, kinetic evaluation of anaerobic degradability of the wastewater to methane is of interest. Furthermore the biogas produced from the anaerobic process may be utilised as alternative fuel to generate the steam for the steam-explosion process (Zheng et al., 2014).
As the anaerobic decomposition of organics is a consequence of a sequential biochemical reactions composed of (1) hydrolysis of the polymers to monomers, (2) acidogenesis and acetogenesis from the monomers to acetate and formate/hydrogen, and (3) methanogenesis to produce methane from acetate and formate/hydrogen, it is desired to formulate a biochemical reaction map with listing individual kinetics and stoichiometries. This would provide a mechanistic platform to design high-rate anaerobic reactors, e.g. anaerobic membrane bioreactor, anaerobic fixed-bed bioreactor and upward anaerobic sludge blanket reactor.
Recently considerable efforts are being paid to model anaerobic digestion of organic solids, especially in the fields of energy crops and co-digestion (Lübken et al., 2007, Weinrich and Nelles, 2015), whilst the fate of readily fermentable soluble organics are also studied to control acidic failure (Hinken et al., 2014). However, it seems that kinetic studies for multiple soluble substrates present in the wastewater are still limited. This makes it difficult to estimate the anaerobic digestion of organics present in the steam-explosion wastewater. Based on this background, the study was conducted to model kinetic biochemical reactions for the steam-explosion wastewater organics obtained from sugarcane bagasse. The organics in the wastewater and its intermediates from the biological reactions were analysed under a batch condition. The developed kinetic model was then applied to simulate the process performances of a high-rate anaerobic system. In the study activated carbon adsorption treatment was also preliminarily examined to remove the remaining unbiodegradable fraction of the wastewater after the biological treatment.
Section snippets
Biological batch test
The steam-explosion of sugarcane bagasse collected in Vietnam was carried out under 3.0 MPa for 5 min at about 230 °C followed by instantaneous depressurisation to ambient pressure. The solid product was washed with tap water as much as 5 times of the bulk volume of the bagasse material before filtration, and the steam-explosion wastewater (liquid fraction) was separated from the composite accordingly. Since the wastewater was highly acidic (pH = 2.87), the pH was neutralised to 7.3 by adding NaOH
Biological decomposition of the steam-explosion wastewater organics
The biological decomposition of the soluble organics in the steam-explosion wastewater and methane production rate in the batch test were summarised in Fig. 2, together with each simulation curve. The decomposition of soluble carbohydrate continued until day 5, and about 2/3 of the fed organics was decomposed in the period (Fig. 2a). At the end of the experiment (day 18), About 560 mg-COD/L of soluble carbohydrate remained in the batch reactor, which was slightly lower than the initial
Conclusions
About 50% of soluble organics in the steam-explosion wastewater from sugarcane bagasse was identified to be anaerobically degradable whilst furfural and hydroxyl-methyl-furfural (known as the inhibitor on ethanol fermentation) were also biologically degraded in the methane fermentation process. To express the biological reactions, a kinetic model was developed. The model suggested 5–7 kg-COD/m3/d of volumetric loading rate could be applicable to an anaerobic reactor. To remove unbiodegradable
Acknowledgements
This research was conducted under New Energy Industry Development Organization (NEDO), Japan.
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