Recent trends in biochar integration with anaerobic fermentation: Win-win strategies in a closed-loop
Introduction
As fossil resources are finite and have a detrimental effect on the environment, finding alternative resources and developing effective utilization is essential for creating sustainable bioprocesses [1]. Three distinct types of ongoing biomass conversion processes exist in the field: biological, chemical, and thermochemical. Biological processes utilize microorganisms to decompose biomass and are sensitive to biomass composition and carbon loading [[1], [2], [3]]. Conversely, chemical processes utilize acids or alkalis for biomass pretreatment to enable sugar solubilization. However, low sugar solubilization, formation of sugar-degraded compounds, and acid separation/neutralization act as limiting factors [4]. Meanwhile, thermochemical processes are less sensitive to feedstock composition but require high temperatures, typically above 350 °C to decompose biomass into solid (biochar), liquid, and gaseous products [5,6]. As each process has advantages and limitations, this review discusses the thermochemical conversion of biomass into biochar and its involvement in increasing AF process efficiency to establish sustainable integration.
AF is a well-recognized and promising technology that has been established in commercial plants worldwide. AF produces biogas that is primarily used as a fuel for combined heat and power generation (CHP) [7,8]. In particular, the number of biomethane plants in Europe recently increased by 51%, from 483 (2018) to 729 (2020) [9]. AF organic bioconversion involves several steps such as hydrolysis, acidogenesis, acetogenesis, and methanogenesis [10,11]. Each step of the AF process has different factors that affect its performance. For instance, hydrolysis solely depends on the substrate type and composition, whereas acidogenic and acetogenic processes rely on microbial activity, redox conditions, nutrient availability, and organic loading [12]. Finally, methanogenesis is dependent on the preceding steps, and complete microbial decomposition is a major rate-limiting step for the overall process efficiency [13,14].
Options to improve AF performance (i.e., improve biogas yield) have been studied extensively, including assessments regarding the impacts of substrate pre-hydrolysis, two-stage processes, bioaugmentation, buffering control, and activated carbon addition [15,16]. Biochar, which is produced through the thermochemical conversion of organic waste in oxygen-depleted conditions and it is a porous solid carbonaceous material that consists of minerals in the form of ash [17,18]. Recently, biochar inclusion to improve AF efficiency has gained increasing attention [12,19] as it can boost AF performance by facilitating microbial colonization, electron transfer, acid shock minimization, buffering maintenance, and increased nutrients retention [15,[20], [21], [22]]. Moreover, the addition of biochar reduces the duration of the lag phase and can enhance the process efficiency by improving the overall methane yield [23].
Biochar enables the formation of biofilms by improving direct interspecies electron transfer (DIET), wherein cell-to-cell direct electron transfer occurs due to its conductive nature [24,25]. Conductive pili or microbial nanowires and cytochrome-c play key roles in inducing DIET-based electron mobility [26,27]. The mediated interspecies electron transfer (MIET) understanding is limited with the presence of formate and hydrogen. However, MIET represents thermodynamic barriers that only occur at low metabolite concentrations [28]. On the other side, DIET transformation occurs via biological pili or mineral transformation molecules and more efficient than MIET [25,29]. Several studies have been conducted to understand the electron transfer mechanism and how electrotrophic methanogens allow DIET-associated transfer [[30], [31], [32], [33]]. However, the aggregation of bacterial species is important for facilitating effective electron transfer, which mainly depends on the inoculum source, operation type, and reactor configuration [26,31]. The rapidly increasing number of publications on biochar and AF are shown in Fig. 1, for which the data were collected from the Clarivate Analytics Web of Science search report (accessed on May 15, 2021). The significance and momentum of research concerning the integration of biochar and AF processes rationalize the need for a comprehensive review and a discussion regarding future research trajectories. Despite the rapidly increasing research on the application of biochar in AF, few studies focus on discerning the specific roles of biochar in AF. In this context, this review aims to consolidate emerging research on waste biomass-based biochar production using different pyrolysis methods and the benefits of integrating biochar into the AF process. This review first addresses the aspects of biochar production from various types of biomass and organic waste. Thereafter, the AF performance benefits that result from biochar applications are discussed, in which special emphasis is placed on topics such as microbial colonization, DIET electron transfer, control of inhibitors, buffering maintenance, nutrient recycling, and soil conditioning. Finally, a summary of approaches for the sustainable integration of pyrolysis and AF to achieve effective resource utilization in a closed-loop system is outlined, as well as the associated limitations and future scope.
Section snippets
Biochar production and characterization
Biochar can be produced via various thermochemical processes including pyrolysis, gasification, and hydrothermal carbonization [5]. Pyrolysis is the most common method for biochar production and can be subdivided into three main categories: slow, intermediate, and fast pyrolysis, depending on the heating rate and biomass residence time [19,34]. Pyrolysis conditions significantly influence the yield and physicochemical properties of the resulting biochar [4,35]. Specifically, a slow pyrolysis
Biomethane
Biomethanization is a well-established and promising technology that converts organic waste into biogas [11]. Extensive studies have been conducted to optimize various aspects of biomethanization, including inoculum selection, utilization of different types of waste/wastewaters, different reactor configurations, optimizing various process parameters, and analyzing metabolic pathways [52,53]. Despite these developments, biomethane production has several limitations, including organic load
Control of inhibitors
The formation of inhibitors in AF occurs mainly via an excess substrate concentration, the presence of sugar-degraded products, or an accumulation of intermediate metabolites [78]. Furfurals, ammonia, phenols, heavy metals, and fatty acids (short-chain/long-chain) have been identified as common inhibitors [79]. The application of external adsorbents is likely to reduce the concentration of inhibitors and thereby enhance the bioprocess performance [74]. Factors that influence the performance of
Nutrients recycling
The sustainable management of fermentation residues (digestate) is vital because of their nutrient- and carbon-rich characteristics. The direct discharge of fermentation residues can cause an ecosystem nutrient imbalance and contribute to eutrophication [94]. The addition of biochar to AF supplies embedded nutrients for microbial growth. Moreover, biochar can absorb excess nutrients from the AF process, including nitrates, phosphates, sulfates, and organic carbon. Therefore, the fermentation
Closed-loop integration of pyrolysis and anaerobic fermentation
Bio-based circular economy requires the complete decoupling of materials (organic waste/biomass) and the simultaneous synthesis of commercially viable products [113,114]. The progressive integration of pyrolysis with AF improves the recovery of net energetic yields from biomass by facilitating effective utilization in a closed-loop approach (Fig. 3). The use of biomass or waste-derived biochar to augment biogas production could be considered a non-engineered sustainable amendment [115,116].
Challenges and future scope
Biochar inclusion in AF has several advantages, multiple factors must be thoroughly investigated before pilot-scale studies. The quantity, composition, and nutrient content of biochar have direct impacts on AF performance, specifically on electron transfer, buffering maintenance, and microbial community enrichment [126]. In addition, its substrate composition and organic loading significantly impact the AF performance. The application of fermented residues as fertilizer also requires in-depth
Conclusions
The integration of pyrolysis and AF maximizes resource recovery and creates a novel platform to mitigate the biomass sensitivity limitations of AF along with improving process performance. Further, biochar addition in AF promotes microbial colonization, facilitate DIET, minimizes organic acid inhibition, and contribute to buffering maintenance. As the physicochemical properties of biochar greatly impact AF performance, selecting the appropriate biochar feedstock and production conditions is
Credit author statement
A. Naresh Kumar: Conceptualization, Data curation, Writing - Original draft. Pavani Dulanja Dissanayake: Writing- Part of the original draft. Ondrej Masek: Conceptualization, Reviewing, and Editing. Anshu Priya: Writing- Part of the original draft. Carol Sze Ki Lin: Writing- Reviewing and Editing. Yong Sik Ok: Conceptualization, Visualization, Resources, Writing- Reviewing and editing. Sang-Hyoun Kim: Supervision, Resources, Project administration, Funding acquisition, Writing- Reviewing and
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Korea Environment Industry & Technology Institute (KEITI) through a project for developing innovative drinking water and wastewater technologies, funded by the Korea Ministry of Environment (MOE) (ARQ202001174001) and the Cooperative Research Program for Agriculture Science and Technology Development (Effects of plastic mulch wastes on crop productivity and agroenvironment, Project No. PJ01475801), Rural Development Administration, Republic of Korea and the
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2022, Industrial Crops and ProductsCitation Excerpt :Solid digestate can be converted into fertilizer through composting, while the liquid fraction of digestate could be used for hydroponics offering nutrients to the plants (Ronga et al., 2019). Digestate valorization also includes routes for biofuels (Monfet et al., 2018), biochar (Kumar et al., 2021), and biomaterials (Kaur et al., 2020). The development of further studies regarding thin stillage digestate would help nutrient recycling while avoiding environmental side effects and promoting a circular economy.
Combined effects of liquid digestate recirculation and biochar on methane yield, enzyme activity, and microbial community during semi-continuous anaerobic digestion
2022, Bioresource TechnologyCitation Excerpt :As the OLR increased, the archaeal communities of R2 and R3 gradually moved away (Fig. 5B). Furthermore, the distance between R3_D100 and R3_D120 was significantly shorter than that between R2_D100 and R2_D120, indicating that biochar application in LDR enhanced the stability of archaeal communities in anaerobic systems (Kumar et al., 2021). Principal coordinate analyses (PCoA) for the bacterial and archaeal communities are presented in Fig. 5C and D.