Deeper insights into effect of activated carbon and nano-zero-valent iron addition on acidogenesis and whole anaerobic digestion
Graphical abstract
Introduction
Anaerobic digestion (AD) presented promising advantage in industrial and agricultural waste treatment via converting organic waste into biogas in presence of microorganism (Li et al., 2019). However, research showed that the process was still limited by low acidification efficiency and the accumulation of volatile fatty acids (VFAs) caused by slow syntrophic metabolism (Appels et al., 2011). Many studies have shown that adding conductive materials to anaerobic digesters can accelerate and stabilize the conversion of organic matter to methane (Dang et al., 2016, Zhang et al., 2018). Previous research indicated that carbon-based materials including biochar, activated carbon (AC) and carbon cloth or iron-based material including nano-zero-valent iron (nZVI), magnetite and iron powder could promote the performance of AD (Lim et al., 2020, Zhou et al., 2019, Li et al., 2019, Meng et al., 2013). Nevertheless, the potential mechanism involving different conductive materials to improve the performance of different stages of AD remains unclear.
Carbon materials are always used directly as an electron carrier to improve AD efficiency by promoting electron transfer between syntrophic and methanogenic partners (Liu et al., 2012a). Zhao et al. (2017) reported that after supplementing granular activated carbon (GAC) in methanogenic phase, the methane production rate improved by about 34%. Moreover, with a honeycomb pore structure, large specific surface area and good adsorption performance, AC provides attachment sites for microorganisms and helps reduce the impact of organic shock loads on the methane production process (Aziz et al., 2011, Liu et al., 2012a). Also, it was reported that AC could be used directly as an electron carrier to improve AD efficiency by promoting electron transfer between syntrophic and methanogenic partners (Zhao et al., 2017, Dang et al., 2016). nZVI, as a reducer, has high specific surface area, which is also often used to enhance AD. Su et al. (2013) observed that adding 0.1% nZVI in anaerobic system increased CH4 production by 40.4%. As a reducing substance, it could lower the ORP to provide a better anaerobic environment for methanogens (Liu et al., 2011). On the other hand, nZVI can produce hydrogen through chemical corrosion (Eq. (1)), which improves the efficiency of hydrogenotrophic methanogenesis (Hao et al., 2017). The produced Fe2+ from nZVI is an important element to many oxidoreductases, which can improve the metabolic activity of microorganisms.Fe0 + 2H2O → Fe2+ + H2 + 2OH– ΔG0 = -5.02 kJ/moL Fe0CO2 + 4H2 → CH4 + 2H2O ΔG0 = -131 kJ/moL8H+ + Fe0 + 4CO2 → CH4 + 4Fe2+ + 2H2O ΔG0 = −150.5 kJ/moL CH4
Previous studies have compared the mechanisms between different types of conductive materials that enhance AD. However, the potential mechanisms of the conductive materials in each stage of AD are also different. In general, AD mainly involves four steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Firstly, the complex organic matter is hydrolyzed into simple organic matter by fermentation bacteria, and then these simple organics are converted into VFAs (lactic acid, acetic acid, formic acid, propionic acid, butyric acid and H2/CO2). VFAs can be converted into acetic and formic acids by acetogens, and finally they will be used by methanogens to produce methane in methanogenesis. However, there are few specifically produced substrates that can be used by methanogens, including H2 (Eq. (4)), formic acid (Eq. (5)) and acetic acid (Eq. (6)). It can be seen that VFAs are important intermediates produced in the process of acidogenesis, acetogenesis and methanogenesis. Therefore, the concentration and composition of VFAs directly affects the efficiency of AD.4H2 + CO2 → CH4 + 2H2O ΔG0 = −135 kJ/moL4HCOOH → CH4 + 3CO2 + 2H2O ΔG0 = −130 kJ/moLCH3COOH → CH4 + CO2 ΔG0 = −33 kJ/moL
According to our knowledge, previous researches mainly focused on the influence of conductive materials on the performance of AD, especially for the methanogenesis. However, there has been less investigations into the potential connection between the acidogenesis and the methanogenesis. Xie et al. (2019) found that AC increased short-chain fatty acids production from algae during alkaline anaerobic fermentation. Furthermore, it was reported that nZVI promoted VFAs production, and acetic acid dominated in the batch system (Jin et al., 2019). AC improves the activity of key hydrolase enzymes and the number of coding genes. nZVI changes the community and metabolism of microorganisms by adjusting pH and reducing ORP. However, there was no consensus on how conductive materials affect the conversion of various products in the acidogenic phase. Additionally, the relationship between microorganisms and materials are still unclear.
According to the above, AC and nZVI were selected as conductive materials to deeply explore their effects on acidogenesis and whole AD in this study. One aim was to explore the effects of different conductive materials on the conversion of metabolic products during AD. Another objective was to dissect the microbial community structure and metabolic mechanism in response to the addition of conductive materials.
Section snippets
Substrates and inoculum
The granular sludge used as seed sludge was collected from the starch wastewater treatment plant in Xuzhou, China. The mixed liquor volatile suspended solid (MLVSS) of inoculum was 23.33 g/L, and mixed liquor suspended solid (MLSS) was 40.22 g/L.
In this experiment, artificial wastewater was used as the substrate. Glucose was the carbon source and the chemical oxygen demand (COD) concentration was 2000 mg/L. In addition, NH4Cl and KH2PO4 were used as nitrogen source and phosphorus source,
Effect of conductive materials on VFAs, pH and gas content in acidogenesis
The composition of VFAs and biogas content in each group were measured (Fig. 1). As shown in Fig. 1a-b, the main products were VFAs (including lactic acid, formic acid, acetic acid and propionic acid), and H2/CO2 after blocking the methanogenesis. In the initial 9 h, the pH of blank, AC and nZVI groups dropped sharply from 8.2, 8.1 and 8.1 to 6.2, 6.0 and 6.7 (Fig. 1c), respectively. It indicated that glucose was quickly converted into acidic products after experiments. The lactic and formic
Conclusions
The present study systematically investigated the effect of AC and nZVI addition on acidogenesis and whole anaerobic digestion process. AC enriched the Trichococcus and norank_f__Bacteroidetes_vadinHA17 by its pore structure, indicating that AC promoted the production of lactic and propionic acids in acidogenic phase. nZVI significantly improved the activity of functional enzymes by buffering pH and releasing Fe2+, thereby increased the production of formic acid in acidogenic phase. In
CRediT authorship contribution statement
Ruming Wang: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing - original draft. Chunxing Li: Resources, Formal analysis, Validation, Writing - review & editing. Nan Lv: Supervision, Validation. Xiaofang Pan: Resources, Funding acquisition, Supervision. Guanjing Cai: Writing - review & editing. Jing Ning: Supervision, Formal analysis. Gefu Zhu: Project administration, Funding acquisition, Supervision.
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
This work was supported by the National Natural Science Foundation of China (grant numbers: 51678553, 21876167 and 52070176) and the National Key Research and Development Program of China (grant number: 2017YFD0800804-03).
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