A mass diffusion-based interpretation of the effect of total solids content on solid-state anaerobic digestion of cellulosic biomass
Introduction
Anaerobic digestion (AD) is a biological process that converts organic matter into methane-rich biogas as a renewable energy source (Motte et al., 2013). Based on the total solids (TS) content of feedstocks used in digester operations, AD can be categorized into liquid AD (L-AD) and solid-state AD (SS-AD) (Xu et al., 2013). L-AD operates at TS ranging from 0.5% to 15%, and is a traditional technology that has been widely used to treat sewage sludge, food waste, and animal manure; while SS-AD operates at TS greater than 15%. SS-AD has gained more attention in recent years due to the increasing market demand for treating solid organic wastes, such as the organic fraction of municipal solid waste and crop residues (Bollon et al., 2011). SS-AD is advantageous over L-AD in solid waste handling as it allows much higher loading in a smaller volume with less energy input and water addition (Karthikeyan and Visvanathan, 2013). Moreover, the compost-like digestate remaining after SS-AD is easier and less costly to transport (Li and Wang, 2011). The fibrous biomass that may cause floating and stratification problems in L-AD can also be easily handled by SS-AD (Xu et al., 2013). Due to these advantages and its simple design, SS-AD has been widely used in Europe, representing about 60% of the total AD treatment capacity in 2010 (De Baere and Mattheeuws, 2010).
While the application of SS-AD holds promise for solid waste treatment and economical bioenergy production, it has been widely observed that the volatile solids (VS)-based methane production rate (mL CH4 g VSfeed−1 d−1) tends to decrease as the TS increase in SS-AD, especially when using cellulosic biomass as a substrate. Abbassi-Guendouz et al. (2012) investigated the effects of TS on AD by using cardboard as a substrate, and found that the VS-based methane production rate continued to decrease when the reactor TS increased from 10% to 35%. Similarly, Motte et al. (2013) used wheat straw as a substrate to study the TS effect on SS-AD, and their results also showed a declining VS-based methane production rate as TS increased from 15% to 25%. Many other studies have reported the same observation when using various types of substrates such as cardboard, paper, municipal solid waste, cotton stalks, rice straw, and others (Fernandez et al., 2010, Fujishima et al., 2000, Le Hyaric et al., 2012, Li and Wang, 2011, Pommier et al., 2007).
If not carefully controlled, the adverse effect of TS on SS-AD may offset the expected advantages of SS-AD. Therefore, an improved understanding of the fundamental mechanism that has compromised the methane production rate due to the effect of TS content is essential for optimizing the SS-AD process. Traditional perceptions derived from L-AD studies have considered either hydrolysis or methanogenesis as the possible rate-limiting step that constrains the methane production rate (Cirne et al., 2007, Vavilin et al., 2008). Recent SS-AD studies showed that, when feeding recalcitrant cellulosic biomass as a substrate, if enough inoculum was provided, there was very little volatile fatty acid (VFA) accumulation or pH drop, suggesting that the cellulosic biomass hydrolysis has constrained SS-AD reactions (Abbassi-Guendouz et al., 2012, Bollon et al., 2011, Li and Wang, 2011). In an attempt to fit SS-AD experimental data into the L-AD-based Anaerobic Digestion Model No. 1 (ADM1), Abbassi-Guendouz et al. (2012) found it is necessary to lower the first-order hydrolysis rate coefficient used in the model to mathematically explain the reduced methane production rate of cardboard at elevated TS content. The mechanism behind this reduced rate, however, was not elucidated.
Currently, limited information can be found about the compromised hydrolysis rate under high TS in the AD process (Vavilin et al., 2008), although it has been commonly observed in studies of enzymatic hydrolysis of cellulosic biomass for ethanol production (Ballesteros et al., 2002, Cara et al., 2007, Kristensen et al., 2009, Lu et al., 2002, Ramachandriya et al., 2013). Kristensen et al. (2009) proposed that this phenomenon is an “intrinsic or generic” effect of enzymatic hydrolysis at increased TS content. The inhibition to enzyme adsorption as a result of excessive sugar accumulation could be responsible for the decreased hydrolysis rate at high TS content during enzymatic hydrolysis (Kristensen et al., 2009).
Although this sugar inhibited hydrolysis in high TS content sounds like a plausible explanation to the existing SS-AD perplexity, some key components are still missing. A major difference between enzymatic hydrolysis and SS-AD is that the latter contains a large number of sugar consumers that quickly remove hydrolytic products upon their production, especially when hydrolysis is a rate-limiting step. Only when a barrier exists that prevents microbial access to sugars, could such a sugar accumulation phenomenon be manifested in an SS-AD system. Bollon et al. (2011) employed ADM1 in the examination of a hydrolysis-limited SS-AD system, and reached the conclusion that the Monod half-saturation coefficient was always larger when the TS content was higher, indicating increased mass diffusion resistance in the dry media of SS-AD. In line with their prediction, a recent measurement did show that the mass diffusion coefficients in SS-AD were two orders of magnitude below the normal range in L-AD (Bollon et al., 2013).
The extent of internal mass diffusion resistance is a primary difference between SS-AD and L-AD, however, this factor has not yet been considered in traditional modeling systems such as ADM1. The objective of this study was to develop an SS-AD tailored model to interpret the effect of TS content on the methane production rate, based on the hypothesis that limited mass diffusion in SS-AD causes hydrolysis inhibition due to accumulation of hydrolytic products such as sugar.
Section snippets
Physical process and assumptions
The model for this study was developed for the AD of cellulosic biomass in a batch bioreactor, with feedstocks and inoculum completely premixed before loading. It was assumed that the inoculum, upon complete mixing with the substrate, dispersed into many pinpoint microflora that were evenly distributed in the sludge bed (Fig. 1). For mathematical simulation, each microflora was assumed to be a small sphere enclosed in a substrate layer, with each thin substrate layer in contact with several
Model verification
This study attempted to mathematically interpret the effect of TS content on the methane production rate, based on the hypothesis that limited mass diffusion in SS-AD will cause hydrolysis inhibition due to accumulated hydrolytic products, namely sugar. This hypothesis was verified by data from different origins: experimental data from this study, literature data from experiments, and literature data from statistical methods.
The model was first verified with the experimental data from this
Conclusion
The influence of TS content on SS-AD performance is two-faceted. There is a TS content threshold between 15% and 20% for AD of lignocellulosic biomass, below which the methane production rate tends to increase with TS, and decrease when exceeding it. Incorporating a mass diffusion-caused hydrolysis inhibition mechanism into the AD model was able to provide a reasonable interpretation of this two-faceted effect of TS content on SS-AD. The proposed model is capable of giving a theoretical basis
Acknowledgements
This project was funded by USDA NIFA Biomass Research and Development Initiative Program (Award No. 2012-10008-20302). The authors wish to thank Mrs. Mary Wicks (Department of Food, Agricultural and Biological Engineering, OSU) for proofread and suggestions.
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