Deciphering the role of microplastics in the bioelectrochemical wastewater treatment process

Wastewater is produced in large quantities by human industrial and domestic activities, and the high content of undesirable nutrients and toxic contaminants in this wastewater harms humans, animals and ecosystems. Various technologies have been developed to treat this issue and prevent environmental pollution, such as microbial electrochemical systems (MES), which remove organic compounds with electricity, energy and value-added production using electroactive bacteria as biocatalysts. The organic matter could be oxidized easily in the anode with electrons transferring to the anode electrode. Reduction reaction happens on the cathode with hydrogen and methane generation. The application of microbial electrochemical methods has been promoted in recent years. However, in applying microbial electrochemical systems in the actual wastewater treatment process, toxic compounds in the wastewater might affect performance. Therefore, the presence of emerging pollutants should be considered to influence the system performance in line with the application of bioelectrochemical systems. In recent years, widely available microplastics has been regarded as an “emerging pollutant” because it has been commonly found in wastewater and is harmful to the environment. Thus, how microplastics influence microbial electrochemical system performance should be considered when undertaking actual wastewater treatment.

The aim of the present PhD project is to decipher the role of microplastics in the microbial electrochemical wastewater treatment process. Different types of bioelectrochemical reactors, such as two-chamber microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) and single-chamber MECs, and the primary target pollutant, including chemical oxygen demand (COD) and nitrogen, were included in this project. Besides, as another popular technology, MEC and anaerobic digestion were combined to remove organic matter and recover methane energy. The following specific investigations were performed to decipher the role of microplastics in anodic wastewater treatment in a two-chamber MFC and MEC, as well as nitrogen removal and methane recovery from wastewater in a single-chamber MEC.

First, polyethylene microplastic (PE-MP) was used to evaluate the effect of microplastics on two-chamber MFC and MEC anodic wastewater treatment performance. The results imply lower COD removal efficiency in MFCs and MECs in the presence of microplastics than without them. The maximum current density for MECs declined as a result of microplastic stress, but for exoelectrogenic bacteria-driven MFCs it was not affected. Microplastics inhibit anodic biofilm viability and increase the ratio of dead cells, which in turn increases bioanode resistance and leads to low MEC current density. Microbial community analysis indicated that microbial structure changed as a result of adding microplastics, and the abundance of electroactive bacteria in MECs decreased when PE-MP concentration increased. Moreover, EET-related genes and enzymes also declined following the exposure of PE-MP in MECs. This study provides us with knowledge about the influence of microplastics on microbial electrochemical system performance and reveals potential mechanisms at the gene and enzyme levels.

Subsequently, the effect of PE-MP on nitrogen removal in a single-chamber MEC was evaluated. Following the application of cathode potential, nitrogen removal was enhanced, with the highest result achieved with cathode potential at -123 mV vs SHE. Microplastics worsened nitrogen removal in microbial electrochemical systems following long-term exposure, and the higher the PE-MP, the more severe inhibition was observed. Meanwhile, PE-MP concentration at 1 mg/L caused a significant decline in biofilm viability and EPS content. Total denitrification bacteria and the genes involved in the denitrification process also shrank on exposure to PE-MP. In conclusion, this part of the study showed how PE-MP inhibits nitrogen removal in microbial electrochemical systems, the mechanism for which was revealed through microbial community analysis and functional gene quantification.

Furthermore, wastewater treatment, coupled with methane recovery, can be enhanced in a single-chamber MEC. Thus, in this part of the work, the effect of PE-MP on wastewater treatment and the methane recovery performance of an MEC was evaluated. PE-MP at a concentration of 10 mg/L reduced COD removal and methane production by 30.71%. In addition, the amount of fermentation, acetogenesis and MEC-enhanced hydrogenotrophic methanogenesis declined with the addition of PE-MP. Both EET-related genes and methanogenic genes also declined following exposure to microplastics. Furthermore, this inhibition mechanism was revealed by the functional enzymes predicted by PICRUSt2. This part of the work could guide the application of MEC-AD in treating real wastewater containing microplastics.