Carbon-based conductive materials facilitated anaerobic co-digestion of agro waste under thermophilic conditions
Graphical abstract
Introduction
Agro-waste management, including biomass burning, is a challenging issue in agrarian countries. Of 488 million tons crop residues generated in 2017 in India, about 24% were burnt at farmlands. It led to the emission of 824 Gg PM2.5, 58 Gg of elemental carbon (EC), and 239 Gg of organic carbon (OC). Overall, it led to 211 Tg of CO2-equivalent greenhouse gases (GHGs) emissions. Wheat contributed 29% to the total crop residue and 26% to the crop residue burned in India in 2017. With this trend, there could be a 45% increase in crop-residue emission by 2050. However, the crop-residue can be used to satisfy 10% (120 TWh of electricity production) of the immediate energy production in India (Ravindra et al., 2019).
Agro-waste (rice, wheat, mustard, sorghum, corn, maize: straw, stalk, husk), having lignocellulosic biomass (heating value 16 MJ/kg) can be a rich source of bioenergy production through anaerobic digestion (AD), i.e., bio-methanation. Anaerobic digestion of crop-residues is a promising approach to concurrently fulfil a percentage of the community’s energy requirements, curb greenhouse gas (GHG) emissions, and cope with agricultural residues. However, it lacks specific characteristics (high carbon but low nitrogen content, >50 C/N ratio), which may restrict its potential as such a source for methane (CH4) production. Anaerobic co-digestion (ACoD) of agro-waste with the nitrogen-rich substrate (sewage sludge, cattle manure) could enhance the process potential to achieve higher biogas yield while offering combined treatment of two problematic wastes (Tyagi et al., 2018). Nevertheless, the opportunities are missed to maximize the methane recovery from the facilities due to presence of a high fraction of lignocellulosic matter in agro-wastes, i.e., cellulose (30–60% of dry matter), hemicellulose (14–40% of dry matter) and lignin (7–25% of dry matter). It obstruct the biotransformation of the substrate, i.e., rate-limiting step of the process (Hendriks and Zeeman, 2009), and leading to low methane recovery. For this reason, conventional bio-methanation systems experienced low digestion efficiency, thus hampering broader applications. The thermophilic digestion of the mixed substrate (crop-waste and sewage sludge) can be an attractive way to attain high substrate solubilization; thus improve substrate biodegradability and resulting enhancement in biogas yield.
Direct interspecies electron transfer (DIET) has recently emerged as a potential solution for many challenges associated with conventional anaerobic digestion (Yan et al., 2017, Gahlot et al., 2020). DIET refers to the process of electron transfer between syntrophic microorganisms without the need for the electron carriers or redox mediators (Indirect Interspecies Electron Transfer, IIET). The intermediate chemical reactions associated with redox mediators are thermodynamically less favourable and induce instability in the overall AD process (Baek et al., 2018). Three fundamental mechanisms through which DIET occurs are membrane-bound electron transport proteins, conductive pili (nanowires), and abiotic conductive materials like iron oxides and carbon-based materials (Baek et al., 2018). Kato et al. (2012) first demonstrated that DIET is possible through abiotic conductive materials (hematite or magnetite) between the Geobacter and Methanosaeta. Subsequently, Liu et al. (2012) conclusively showed that DIET was possible via the conductive material (granular activated carbon (GAC), in this case) and ruled out the role of IIET, conductive pili, or electron transport protein in the methane formation process. Later, many studies have used various carbon-based conductive materials (like GAC, biochar, carbon cloth, graphene, and carbon nanotubes), in both defined cocultures and mixed populations to study DIET and its effects (Gahlot et al., 2020).
Thermophilic AD amended with conductive material has shown greater biogas production than the amended mesophilic AD or non-amended AD (Zhang et al., 2020). However, some studies reported the inhibition of methanogenesis due to the additions of conductive materials (CMs), mainly due to their toxic effects on microbes and substrate competition (Wu et al., 2020). Many studies that showed a positive effect due to conductive materials were carried out on simple substrates like acetate and ethanol and not on a complex substrate like agro-waste. Thus, even though numerous papers have been published on DIET-enhanced AD, the research gap is quite evident. To the best of authors’ knowledge, this is the first comparative study on conductive material mediated DIET in anaerobic co-digestion of lignocellulosic waste with municipal sludge under thermophilic condition. The key objectives of this study are: (i) to understand whether the addition of carbon-based CMs enhances the process efficiency of anaerobic co-digestion of lignocellulosic biomass (ii) the optimum concentration of CMs in increasing the efficiency of AD. We broadly studied and compared the effects of two different carbon based conductive materials, i.e., GAC and GBC with five different dosage of each including: (a) Effects of conductive material addition on soluble COD fraction, and pH, alkalinity and VFAs profile and biogas yield, (b) Interrelationships among VFAs, Alkalinity, pH, VFA/alkalinity ratio and effect on biogas yield, (c) Principle Component Analysis (PCA) to determine the correlation between different variables studied, and (d) preliminary energy and economic assessment.
Section snippets
Raw wastes and conductive materials
Cow dung (CD) was obtained from Tyagi dairy (29.87°N, 77.89°E), outside the IIT Roorkee campus. It had a high total solids (TS) concentration, which made mixing difficult and increased the overall TS of the mixture (substrate + inoculum). Thus, it was diluted using the Milli-Q water in the ratio 1:1 (w/w). Anaerobic digester sludge (ADS) was procured from 38 MLD (million litres per day) Up-flow Anaerobic Sludge Blanket (UASB)-based sewage treatment plant (STP) in Saharanpur (29.94°N, 77.55°E).
Physical and chemical characterization of inoculum, feedstock and condutive materials
The co-digestion blend used in this experiment has helped to attain optimum operational parameters (Table 1). The initial pH of the raw wastes is complementary, i.e., while MS has low pH, others have neutral or slightly higher pH. Similarly, a relatively higher pH of inoculum can be balanced by MS. The optimum pH range for stable anaerobic digestion is 6.6–7.8, and this was achieved in all the CM amended samples as well as in control assay at the beginning of the BMP (Fig. 4). The bulk density
Conclusions
Use of CMs in AD is an adaptable and simple method to increase process productivity. GBC can effectively serve this purpose for even recalcitrant lignocellulosic materials. GAC has also shown promising result, but it is not as consistent as GBC in terms of daily biogas production and physico-chemical characterization. The highest per cent increase in biogas yield for GAC and GBC amended samples compared to the control was 22 and 27%, respectively, for 51 days of digestion. This study was marked
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
Authors are thankful to Department of Biotechnology-GoI (GrantNo. BT/RLF/Re-entry/12/2016) for financial support to this research.
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