A review on the impact of conductive nanoparticles (CNPs) in anaerobic digestion: Applications and limitations
Graphical abstract
Introduction
Organic wastes are the most profitable source of renewable energy over the depleting fossil fuels due to hazardous environmental impacts such as incineration of organic waste and carbon dioxide (CO2)/ methane (CH4) emission that results in global warming effect and climate changes (Baniamerian et al., 2019). The various renewable sources include solar, wind, hydropower, geothermal energy, and waste-to-energy. Anaerobic digestion (AD) is promising and globally developing waste management method. AD is a bioprocess through redox reaction for the treatment of organic waste/ biomass to produce biogas (Methane and CO2) which can be used as fuel and electricity generation (Khalid et al., 2019). In terms of global warming potential, anaerobic digestion (AD) poses the greater performance. The incineration process comes with some massive downsides for raising recycling rates, releases carbon into the atmosphere, and can be a major health risk due to flue emissions which need to be cleaned-up to a very high degree. Flue gas clean-up by technologies are essential otherwise dangerous substances such as dioxin will be emitted, as serious health risk factors. However, on the other hand anaerobic digestion facilities to extract full value, makes materials inside of the circular economy, and does not release carbon into the atmosphere (Parthasarathy and Narayanan, 2014). The biogas generated by the anaerobic digestion, possess a high caloric value of 21–25 MJ/m3, which is an excellent replacement of natural gas and fossil fuel which can reduce more than 80% green gas emission to the environment. This renewable energy technique, AD opts to be a favorable and sustainable organic waste management system that moderately reduces sludge formation, bad odors, harmful pathogens, nutrient demands, and energy input (Hussein, 2015). AD is a graceful balance of complex microbial communities in a multistage series. The physiological parameters of the biochemical process could be failed by the fluctuation of microbial growth in every stage, modes of operation, concentration, pH, and temperature (Diehl and Lapara, 2010). In AD, organic waste is converted into methane (CH4) through four stages of biochemical reactions, that are initiated with hydrolysis in which complex organic materials converted into simple organic materials. Acidogenesis is the second process where the simpler organics converted into volatile fatty acids (VFA). The third stage is the acetogenesis in which the volatile fatty acids VFA are converted into acetic acid, hydrogen (), and CO2. The final stage is methanogenesis in which acetic acid and convert into methane (CH4) and CO2. AD process is often limited by its restrictive steps from hydrolysis to methanogenesis due to high ammonia inhibition that reduces biomass hydrolysis rate at the first stage and methane formation at the last stage with poor viability of the anaerobic digestion (Abdelsalam et al., 2016). Hence the methane improvement processes have been intensively scrutinized to raise the biomass utilization to promote co-digestion for boosting inter-microbial communications by the addition of conductive nanoparticles (CNPs) as an additive to develop adequate microbial growth and its metabolic processes (Luo et al., 2015).
Nano-sized CNPs (size of 1–100 nm) having special physicochemical properties such as high activity, high reactive surface area, high specificity for improving the performance, and the ability to increase the microbial growth in the AD process. The addition of CNPs boost the AD process and speeds up the hydrolysis or acidification stage to enhance the biogas output by strong degradation of biomass through direct or indirect interspecies electron transfer. The indirect interspecies electron transfer (IET) through and formate or acetate in acetogenesis and methanogenesis is a bottleneck stage to digest complex organic substances. In contrast, direct interspecies electron transfer (DIET) is the alternative and sustainable route to enhance interspecies /formate transfer in methanogenesis. DIET mainly arises from the co-culture between Geobacter metal-reduces species and Geobacter sulfur-reduces species for the convenient technique of genetic operation in both species. The mechanism of DIET for these Geobacter species with conductive pili possess high conductivity in the presence of CNPs to facilitate the interspecies electron transfer with the help of cytochromes (Cheng and Call, 2016) Most of the studies have demonstrated the enhanced performance of DIET by using various types of conductive nanoparticles like zero-valent metals (ZVM), metal oxide, and carbon-based nanoparticles in biogas production (Vyrides et al., 2018).
In the subsequent years, researchers have investigated the accumulation of CNPs in organic waste which in turn also increases the application of these nanoparticles in various industries. This investigation into potential effects of various CNPs on AD of organic sludge showed the excellent results for methane production. At certain concentrations, some CNPs exert no harmful effect on methane production and involve the microbe’s growth rate with their diversity in AD. Therefore, the present review intended to provide a depth insight into the new state of the art on the understanding of the direct interspecies electron transfer (DIET) and interspecies hydrogen transfer (IHT) in the AD process for enhanced biomass conversion for improved biogas production. Interaction between microbes in anaerobic conditions for electron transfer with the help of CNPs is briefly discussed. Application of a variety of CNPs as an additive discussed with their potential biogas production enhancement in the anaerobic digestion process. The scope of metal oxides, zero-valent metals (ZVM), and carbon-based nanoparticles as CNPs with their positive and negative impacts on the AD process are critically evaluated and reviewed. Furthermore, the role of kinetic factors, (temperature, pH, C:N ratio), the inhibition effect of CNPs on biogas productivity in the AD process are also briefly described. Finally, this review highlighted the sustainability of economic analysis and the environmental effects of CNPs for growing energy requirements and efficient biomass conversion in the anaerobic digestion (AD) process.
Section snippets
Application of conductive nanoparticles (CNPs)
Conductive nanoparticles with their nano-sized structure (1–100 nm) have tremendous applications including energy, agriculture, coating, catalysts, automotive sensors, food products, eco-friendly environmental process, and still research is in progress for other applications (Cheng and Call, 2016) CNPs have excellent conductivity and specific physicochemical properties (i.e. nano-size, high reactivity, specific surface area, catalytic properties, and solubility) that having huge applications in
Limitation of physical parameter in AD process
The Anaerobic digestion process depends on several kinetic parameters for the optimal performance of biogas (Öktem, 2019). Different groups of microorganisms are involved in methane formation and favorable conditions must be established to make a balanced chain of microbes transfer from one stage to another. However, temperature, pH, biomass substrate, C/N ratio are these pivotal kinetic parameters for hydraulic retention time (HRT) through direct and indirect interspecies electron transfer (
Kinetic limitation study of CNPs on AD process
The kinetics study of AD is an important for the understanding of bacterial growth and biogas production in a different medium. This study helps to investigate the rate of bacterial growth, substrate degradation, and product formation in the AD system (Yang et al., 2017, Zhang et al., 2018). A number of kinetic models were studied, for example, the first-order kinetic model for biogas production rate in batch and continuous process for organic material and municipal waste (Öktem, 2019). The
Current trends and future recommendations
The growing trends of the sustainable and feasible application of methane production through biomass and wastewater utilization by using different kinds of nanoparticles in the AD process enhances the biogas production rate. Moreover, some challenging activities need to perform in lab scale to industrial scale-up level. The aspects of AD can be maintained by the catalytic role of CNPs, methanogenesis activity, and microbe’s stability in the system. The biomass degradation time, hydrolysis step
Conclusion
The various application of CNPs on different biomass and wastewater has shown a positive effect on the AD process. It is observed that the CNP’s role in AD to increase the capacity of biogas production from biomass and industrial wastewater which depends on operating parameters. It is noticed that carbon to nitrogen ratio, temperature, pH, and CNPs size play important roles in anaerobic digestion, and several studies have illustrated to find the optimum set of these operating parameters. Very
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.
Acknowledgment
We would like to acknowledge Ministry of Higher Education Malaysia for finical support via Fundamental Research Grant Scheme (FRGS/1/2019/TK07/UMP/02/5) through University Malaysia Pahang (RDU1901202).
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