Current intensities altered the performance and microbial community structure of a bio-electrochemical system

Highlights

• Suitable currents could improve the nutrient removal efficiencies.

• Higher current intensity (600 mA) inhibited wastewater nutrient removal.

• Currents changed the microbial diversity in a bio-electrochemical system.

• Some genus involved in nitrate and phosphorus removal reduced under high currents.

• Key genus Thiobacillus contributed to better denitrification performances.

Abstract

A novel integrated bio-electrochemical system with sulfur autotrophic denitrification (SAD) and electrocoagulation (BESAD-EC) system was established to remove nitrate (NO₃⁻-N) and phosphorus from contaminated groundwater. The impacts of a current intensity gradient on the system’s performance and microbial community were investigated. The results showed that NO₃⁻-N and total phosphorus (TP) could be effectively removed with maximum NO₃⁻-N reduction and TP removal efficiencies of 94.2% and 75.8% at current intensities of 200 and 400 mA, respectively. Lower current intensities could improve the removal efficiencies of NO₃⁻-N (≤200 mA) and phosphorus (≤400 mA), while higher current intensity (600 mA) caused the inhibition of nutrients removal in the system. MiSeq sequencing analysis revealed that low electrical stimulation improved the diversity and richness of microbial community, while high electrical stimulation reduced their diversity and richness. The relative abundance of some genus involved in denitrification and phosphorus removal processes such as Rhizobium, Hydrogenophaga, Denitratisoma and Gemmobacter, significantly (P < 0.05) reduced under high current conditions. This could be one of the main reasons for the deterioration of denitrification and phosphorus removal performance. The results of this study could be helpful to enhance the nutrient removal performance of bio-electrochemical systems in groundwater treatment processes.

Introduction

Groundwater is one of the important sources of drinking water all over the world. However, the contamination of NO₃⁻-N and phosphorus in groundwater has become an increasingly serious problem in recent years, resulting from discharge of industrial effluents, over fertilization in farms, animal and human wastes. High NO₃⁻-N and phosphorus concentrations in drinking water could cause serious illnesses, such as blue baby syndrome, methemoglobinemia and gastric cancer (Zhao et al., 2012; Liu et al., 2019). Accordingly, the contamination of NO₃⁻-N and phosphorus in groundwater has received great attentions all over the world. For example, the World Health Organization stipulates that the maximum allowable concentration of NO₃⁻-N in drinking water is 10 mg L⁻¹ (Liu et al., 2015). The Standards for Drinking Water Quality (GB5749-2006) proposed by China is 20 mg L⁻¹ (Peng et al., 2018). Consequently, it’s necessary to develop effective technologies to remove NO₃⁻-N and phosphorus from polluted groundwater.

NO₃⁻-N removal technologies are classified into physical, chemical and biological approaches (Chen et al., 2017, 2019bib_Chen_et_al_2017; Qian et al., 2018bib_Chen_et_al_2019), among which biological denitrification is considered as low-cost and environmentally friendly. Heterotrophic denitrification is the most common biological nitrogen removal process, and has been widely used in municipal wastewater treatment plants (WWTPs) for nitrogen removal (Hao et al., 2013). However, the scarcity of carbon sources greatly circumscribed the performance of heterotrophic denitrification in WWTPs. To address this limitation, additional organic compounds like methanol and glucose are recurrently added as carbon sources (Zhai et al., 2018a). While this not only adds additional operating costs, but also increases the organic load of WWTPs. In contrary, autotrophic denitrifying microorganisms could utilize inorganic carbon source or some energy source like H₂, Fe²⁺, sulfur or sulfur-reduced compounds as electron donor to achieve NO₃⁻-N removal. Consequently, some researchers preferred to apply autotrophic denitrification instead of heterotrophic denitrification to treat low C/N wastewater.

The bio-electrochemical method is a relatively emerging technology for facilitating NO₃⁻-N removal by autotrophic denitrification process (Qian et al., 2018). On the one hand, the production of H₂ in the electrolysis process can be directly utilized by hydrogen autotrophic denitrifying microorganisms as electron donors to reduce NO₃⁻-N (Sultana et al., 2015). On the other hand, the electrical stimulation in bio-electrochemical system may improve the activity of microorganisms, promote the transfer of electron, and enhance the metabolism of microorganisms, so as to improve the NO₃⁻-N removal performance (Liu et al., 2015; Lin et al., 2019). Therefore, the bio-electrochemical system and their integrated processes have been widely studied and applied to removal NO₃⁻-N from groundwater (Cecconet et al., 2018; Liu et al., 2019), drinking water (Zhao et al., 2012), and treated effluent (Hao et al., 2016). However, there are still some limitations in the conventional bio-electrochemical systems. For example, it is difficult for conventional bio-electrochemical systems to achieve efficient denitrification and phosphorus removal simultaneously (Hao et al., 2018). Electrocoagulation process has been recognized as a high-efficiency and rapid phosphorus removal with lower sludge production (Deng et al., 2017; Bakshi et al., 2020). Recently, the removal of NO₃⁻-N and phosphorus simultaneously by bio-electrochemical system coupled with electrocoagulation process from wastewater has received increasing attention (Kłodowska et al., 2016; Deng et al., 2017; Ziouvelou et al., 2019). However, previous researches have focused on the construction of bio-electrochemical system and the optimization of operating parameters such as temperature, pH value and C/N ratio (Sultana et al., 2015; He et al., 2016; Jiang et al., 2018). Few studies reported the effect of wastewater nutrients removal efficiencies in bio-electrochemical system along with a current intensity gradient. Furthermore, the impacts of current intensity gradient on microbial communities in bio-electrochemical systems still remain unclear.

In this work, to achieve better simultaneous nitrogen and phosphorus removal performance, granular active carbon (GAC) and sponge iron (SI) were used as mixed fillers to construct a lab-scale BESAD-EC system. Our primary objectives of this research were to (i) investigate the performance of the integrated system for NO₃⁻-N and phosphorus removal under different current intensities, (ii) reveal the response of microbial communities to a current intensity gradients, and (iii) analysis the possible mechanism of NO₃⁻-N and phosphorus removal in the system.