A review on facilitating bio-wastes degradation and energy recovery efficiencies in anaerobic digestion systems with biochar amendment
Introduction
Since the industrial age coming, overuse of fossil fuel worldwide has resulted in high levels of greenhouse gases (GHG) in the atmosphere, which leading to the global climate change. Meanwhile, the increasing volume of bio-wastes generated by human activities severely threatened environmental safety. To cope with that, technologies which extracting renewable energy from massive bio-wastes as an alternative to fossil fuel have received increased attention in recent years. In particular, anaerobic digestion (AD) was regarded as a promising one for energy recovery from bio-wastes (Li et al., 2019b), and extensively developed for the treatments of domestic wastewater, high strength industrial wastewater, municipal solids waste, and agricultural residues (Kong et al., 2019, Kumar and Samadder, 2020, Ye et al., 2018). However, various disadvantageous factors existing in AD system always disturbed microbial activity and lead to low energy recovery efficiency. For instance, accumulation of free ammonia or volatile fatty acids (VFA) as intermediate metabolites frequently inhibited methanogenic archaea activity, and in turn, restricted the maximum organic loading rate (OLR) of systems (Jiang et al., 2019). Phenol and polymeric aromatic hydrocarbons contained in industrial wastewaters were refractory to be biodegraded and even performed toxicity for microbes (Chen et al., 2014a). The overuse of antibiotics in livestock industry also increased the risk of microbial inactivation in AD systems for treating this type of manure wastes (Lee et al., 2020a). Thus, developing practical strategies to overcome these obstacles is essential for improving energy production efficiency and extending real-world applications of AD technology.
Biochar is a carbon-rich solid derived from biomass pyrolysis. The unique carbon sequestration function and versatile characteristics made biochar become research hotspot in the fields of soil amendment, functional material preparation, environmental remediation, and bio-wastes management (Liu et al., 2019b, Palansooriya et al., 2019, Wan et al., 2020). As a low-cost product, multi-functional biochar could be used as an additive to enhance the performance of AD systems (Qiu et al., 2019). Firstly, the eco-compatibility and developed porous structures on biochar surface not only provide active sites for the adsorptions of ammonium, gaseous byproducts and toxic pollutants, but also give the shelters for microbial attachment (Ahmad et al., 2014). In addition, both alkali metal salts and organic functional groups containing in biochar showed strong buffering capacity, which was helpful to balance pH decreasing caused by VFA accumulation in AD systems. Meanwhile, the electrochemical properties enable biochar as the mediator to accelerate interspecies electron transfer in syntrophic metabolism. Using biochar as an additive in AD system was firstly conducted by (Inthapanya et al., 2012). Since then, extensive researches studied the performances of biochar addition on AD systems, and discussed the associated mechanisms. Particularly, compared to amounts of initial studies focusing on batch experiments, more and more recent studies were conducted by a long-term (semi-)continuous operating model to evaluate the performance of biochar-amended AD systems, which was meaningful to uncover the real or dominant function of biochar in AD systems (Giwa et al., 2019, Wang et al., 2020b, Zhang et al., 2020c). Aside from applying biochar to conventional AD systems for degradable organic waste treatment, expanding the use of biochar for refractory organics treatment with advanced AD systems, such as anaerobic membrane bioreactors (AnMBR) and microbial electrochemical system (MES), were also drawn more and more attention recently (Cai et al., 2020, Chen et al., 2020). By this way, both novel insights and problematic issues about the mechanisms of multi-functional biochar regulating AD system performances were proposed.
This review aimed to summarize recent progress, discuss current research gaps, and propose the outlooks for future development in the field of biochar-amend AD technology. Potential mechanisms of biochar as an additive to facilitate the performance of AD system were reviewed firstly. After that, conflicted results of biochar’s multi-functions in AD systems were discussed based on the different experimental operation models and substrate types, and knowledge gaps were elucidated to trigger more deeply understanding of biochar’s real function in AD systems. Furthermore, future perspectives were proposed to boost the development of biochar-amended AD towards real-world applications. Finally, a novel scheme with biochar-amended AD as a core process was developed towards energy-resource recovery from various bio-wastes.
Section snippets
Ammonium-N adsorption to alleviate biotoxicity
As a product of protein-containing organics, accumulated ammonium in AD systems consequently lead to a high concentration of free ammonia, which was toxic to methanogenic archaea (Jiang et al., 2019). Various studies confirmed that adding biochar into AD system as an adsorbent was a feasible strategy to improve methanogenesis under high ammonium concentration condition. Studies of (Shen et al., 2016, Shen et al., 2015) evaluated the effect of biochar addition on total ammonium concentration
Buffering capacity of biochar for pH maintenance in AD systems
As a bio-waste treatment process, stable operation of AD systems under high OLR condition were significant to improve the energy recovery efficiency. However, one crucial issue limiting OLR rising was VFA accumulation, which caused by an imbalance in the metabolic rates between acidogenesis and methanogenesis processes. Severely VFA accumulation could lead to system pH decrease and inhibit methanogenesis process. To alleviate pH decrease, some buffering agents, such as CaCO3, NaHCO3, and lime
Boosting microbial activity to enhance organics degradation
As an eco-compatible material, biochar has been widely used as a microbial activity booster in soil amendments (Palansooriya et al., 2019). The positive role of biochar for microbial activity boosting was significant to accelerate refractory substances degradation process in AD. (Zhao et al., 2020a) studied fermentation process of cornstalk hydrolysate with biochar assistance, and reported that biochar triggered high H2 yield by increasing the microbial utilization degree of hydrolysate. With
Biochar as an antifouling additive in AnMBR
As an emerging advanced system for bio-waste treatment, AnMBR has drawn more and more attention recently (Zhen et al., 2019). Compared with conventional AD systems, AnMBR integrated the advantages of microbial biomass long time retention and high-quality effluent achievement (Chen et al., 2018, Lei et al., 2019b). Nonetheless, membrane fouling was a major obstacle limiting the treatment efficiency of AnMBR. Adding carbon-based materials into AnMBR was an effective strategy to in-situ
Research gaps of understanding the dominant functions of biochar in AD systems
As the solid product of pyrolysis, biochar added into AD systems for energy recovery enhancement has been widely confirmed. However, there are still many unclear issues waiting to be elucidated for uncovering the real mechanisms of biochar responsible for performance promotion in AD systems. In this section, three major disputable issues concerning the biochar’s functions in AD systems were discussed systematically. Meanwhile, opportunities were elucidated to fill the knowledge gaps of
Future perspectives for biochar-amended AD technology towards real-world applications
Although using biochar in AD has been developed from batch experiments to long-term continuous systems, real-world scale applications of biochar-amended AD is still lack (Cooney et al., 2016). Future perspectives and outlooks of advanced developing biochar-amendment AD technology for real-world applications were discussed in this section, which including optimizing energy recovery strategy between biochar preparation and AD, evaluating the potential of biochar-rich digestate as soil amendment
Conclusions
Multi-functional roles of biochar in AD systems have been widely confirmed for performances enhancement. Nonetheless, the bottlenecks of understanding sophisticated mechanisms of biochar influencing AD process are still waiting to be broken though. Further efforts should be made to evaluate the trade-off between biochar adsorption behaviors and AD performances, elucidate the electron transfer pathway between syntrophic partners, and uncover the principles of biochar alleviating membrane fouling
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.
Acknowledgements
Thanks to supports from National Natural Science Foundation of China (Grant No. 51978560), National Key Research and Development Program of China (No. 2017YFE0127300), and Shaanxi Provincial Program for Innovative Research Team (No. 2019TD-025).
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