Effect of Cathode Material and Its Size on the Abundance of Nitrogen Removal Functional Genes in Microcosms of Integrated Bioelectrochemical-Wetland Systems

Abstract

Constructed wetland-microbial electrochemical snorkel (CW-MES) systems, which are short-circuited microbial fuel cells (MFC), have emerged as a novel tool for wastewater management, although the system mechanisms are insufficiently studied in process-based or environmental contexts. Based on quantitative polymerase chain reaction assays, we assessed the prevalence of different nitrogen removal processes for treating nitrate-rich waters with varying cathode materials (stainless steel, graphite felt, and copper) and sizes in the CW-MES systems and correlated them to the changes of N2O emissions. The nitrate and nitrite removal efficiencies were in range of 40% to 75% and over 98%, respectively. In response to the electrochemical manipulation, the abundances of most of the nitrogen-transforming microbial groups decreased in general. Graphite felt cathodes supported nitrifiers, but nirK-type denitrifiers were inhibited. Anaerobic ammonium oxidation (ANAMMOX) bacteria were less abundant in the electrochemically manipulated treatments compared to the controls. ANAMMOX and denitrification are the main nitrogen reducers in CW-MES systems. The treatments with 1:1 graphite felt, copper, plastic, and stainless-steel cathodes showed higher N2O emissions. nirS- and nosZI-type denitrifiers are mainly responsible for producing and reducing N2O emissions, respectively. Hence, electrochemical manipulation supported dissimilatory nitrate reduction to ammonium (DNRA) microbes may play a crucial role in producing N2O in CW-MES systems.

Single-chamber microbial electrosynthesis reactor for nitrate reduction from waters with a low-electron donors’ concentration: from design and set-up to the optimal operating potential

“…Microbes possessing nirK genes are naturally in high abundances in nature, and therefore, their presence on the WE should not always be correlated to any specific activity (Lam and Kuypers, 2011;Hallin et al, 2018). The previous study on the microbial electrochemical systems also showed increased abundances of nirS and nosZI gene-harboring microbes (Gadegaonkar et al, 2020), supporting our results and indicating probable complete denitrification.…”

Bioelectrochemical systems for enhanced nitrogen removal with minimal greenhouse gas emission from carbon-deficient wastewater: A review

“…However, this method may cause secondary contamination. Alternatively, studies have shown that a larger electrode size increases the conductivity of BES (Gadegaonkar et al, 2020). Therefore, the applied liquid electrolyte and dosage should be selected rationally according to the actual situation.…”