Research article
Energy efficient sludge solubilization by microwave irradiation under carbon nanotube (CNT)-coated condition

https://doi.org/10.1016/j.jenvman.2020.110089Get rights and content

Highlights

  • Carbon nanotube (CNT)-coated microwave (MW) vessel was developed to enhance MW energy efficiency and sludge solubilization.

  • The consumed energy of the CNT-coated vessel for the target temperature was reduced by 10.1% compared to the uncoated one.

  • The temperature on the catalytic sites in the CNT-coated vessel was 30.6% higher than that of sludge in the same vessel.

  • In the CNT-coated vessel, the specific sCOD increase was 94.83 mg sCOD/kJ, which was 1.80 times that of the uncoated one.

Abstract

Microwaves (MW) have great potential for sludge solubilization, and carbon materials can act as good microwave absorbers and heat transfer media because of their high dielectric loss tangent and thermal conductivity. In this study, carbon nanotube-coated MW vessels were developed by preparing a silane-CNT mixture and spray coating. In addition, sludge solubilization by microwave irradiation was performed to evaluate the effects of the CNT-coating at different initial total suspended solid (TSS) concentrations, target temperatures, and MW irradiation times in the uncoated and CNT-coated MW vessels. The sludge solubilization efficiency increased with increasing MW irradiation time and temperature and followed a first-order reaction in both vessels. However, the energy requirement to maintain the temperature was reduced in the CNT-coated MW vessel compared to the uncoated vessel. In addition, the Arrhenius equation revealed the catalytic site in the CNT-coated MW vessel to have a temperature of around 130 °C at an average sludge temperature of 100 °C. The maximum chemical oxygen demand (COD) solubilization and soluble COD (sCOD) increase per MW energy used were 1.64 and 1.67 times higher in the CNT-coated MW vessel than in the uncoated vessel, respectively. The increase in soluble total nitrogen and phosphorus in the CNT-coated MW vessel was attributed to cell wall destruction and intracellular protoplast dissolution, because of the acceleration of the MW thermal effect and high conductivity of CNTs, as well as the MW-induced cell wall and membrane disruption by hot spots on the CNT surface. This suggests that CNTs can be applied to increase the energy efficiency in MW-based pretreatment methods.

Introduction

Anaerobic digestion enables a decrease in wastewater sludge generation and pathogens growth and the recovery of energy from organic matter (Ge et al., 2013; Batstone et al., 2014; Appels et al., 2011). On the other hand, the anaerobic digestion of wastewater sludge has not been operated efficiently with a low digestion efficiency of less than 50%, long hydraulic retention time, and undeveloped operation technology (Jang et al., 2014; Cho et al., 2014).

Studies have stimulated and improved sludge solubilization for stable anaerobic digestion by chemical pretreatment, heating, ozonation, and ultrasound (Lin et al., 2007; Abelleira-Pereira et al., 2015; Cheng et al., 2012; Pham et al., 2009). The microbial cells in wastewater sludge are destroyed by these pretreatment methods so that the intra- and extra-cellular cell substances, such as soluble organic matter, are more available in the anaerobic digestion process (Chang et al., 2011). These pretreatment methods, however, have some problems in that the chemical pretreatment process, such as alkali or acid pretreatment, requires high operation and maintenance costs (Ren et al., 2014), the conventional thermal treatment requires long reaction time (Lee et al., 2015), and the ultrasound method is energy intensive (Sahinkaya and Sevimli, 2013). Therefore, an economical and effective sludge pretreatment method is required.

Microwave (MW) pretreatment is a new technology for the solubilization of wastewater sludge with rapid heating, environmental friendliness, and low cost, and has attracted attention for increasing the biodegradability of sludge and improving the digestion efficiency ((Lee et al., 2015; Appels et al., 2013). MW irradiation induces the disintegration of sludge flocs and releases intracellular molecules (Peng et al., 2018). MW technology has been used in various environmental, biological, chemical, and food industries owing to its rapid, efficient, and selective heating properties. Microwaves induce dielectric heating by high-frequency electromagnetic radiation because of two different effects: thermal and non-thermal. The electric field of the microwaves induces dipole rotation and ionic conduction of polar molecules that transforms the microwave energy into heat.

Despite these advantages, the use of electrical energy is a disadvantage of using MW. For this reason, alkaline or ceramic materials have been used to increase the solubilization of sludge by MW irradiation or reduce the MW energy (Banu et al., 2018, 2019). The application of MW technologies depends on the microwave energy absorbability of the reaction mixture (Moseley and Kappe, 2011). The heating ability of materials by microwave is defined by their dielectric loss factor tangent (tanδ). Some materials do not have a sufficiently high loss factor to allow dielectric heating. Other microwave absorbers, such as metal oxide and carbon materials, can heat these materials indirectly.

Carbon materials, such as carbon nanotubes (CNTs), graphite, and activated carbon, have high loss tangent values compared to water, which can be transformed easily by microwave energy to heat. The π-electrons in carbon materials are free to move in the delimited region of the materials, which induces the electromagnetic field by absorbing microwaves. CNTs possess the highest dielectric loss tangent at 298 K (Menedez et al., 2010).

In this study, a CNT-coated MW (quartz) vessel was developed to enhance the energy efficiency of MW irradiation in the solubilization pretreatment of waste-activated sludge (WAS). This CNT-coated MW vessel was developed using a silane-CNT mixture by a spray coating on a quartz vessel. To evaluate the enhancement of MW energy efficiency by the CNT-coated MW vessel, the solubilization of WAS was performed by MW irradiation at different initial solid concentrations, target temperatures, and reaction times in CNT-coated and uncoated MW vessels. Based on the solubilization results, reaction rate analyses were also performed to verify the increase in the energy efficiency of the CNT-coated MW vessel.

Section snippets

CNT-coating of the MW irradiation vessel

The quartz MW vessels were coated with CNT to evaluate the increase in MW efficiency and sludge solubilization with hotspot generation onto the CNT-coated surface. The CNT-coating method was modified from the method reported by Han et al. (2008). The CNTs used as the vessel-coating reagents were commercially available multi-walled CNTs (MWCNTs, Carbon Nanotec, South Korea). Prior to use, the raw MWCNTs were treated thermally in an oven at 350 °C for 1 h to remove any amorphous carbon and then

Characteristics of WAS solubilization on TSS concentration

In microwave-based pretreatment of WAS solubilization, the water content is the most important factor that affects the energy efficiency and promotes solubilization (Tang et al., 2010). Therefore, MW irradiation was carried out first at TSS concentrations of 3, 5, and 7% to optimize the TSS concentration. To evaluate the effect of the TSS concentration on sludge solubilization, the variation of VSS and sCOD concentrations were analyzed at different TSS and VSS concentrations of sludge at an

Conclusion

CNTs have great thermal conductivity and a dielectric loss tangent that increases the MW-based pre-treatment efficiency. The CNTs enhance the thermal effect of MW and have the potential to induce cell lysis. To increase the MW energy efficiency for sludge solubilization, a CNT-coated MW vessel was developed using a silane-CNT mixture by a spray coating on a quartz vessel. The average total energy consumption was 284.4 kJ for 10 min MW irradiation using the CNT-coated MW vessel to reach the

Author contribution statement

Kyeong Hwan Kang: Methodology, Data curation, Validation, Formal analysis, Resources, Visualization, Investigation, Writing-original draft; Writing-review & editing, Junghyeon Kim: Formal analysis, Investigation, Hyeonjin Jeon: Data curation, Investigation, Imgyu Byun: Conceptualization, Project administration, Supervision, Writing-review & editing, Funding acquisition.

Acknowledgement

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2017R1A2B4009087).

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