In the constructed wetland (CW), the inherent top-down redox gradient provides the natural conditions for the cathode-anode potential difference to form a cell, and the use of particle conductive filler as electrodes accelerates the integration of microbial fuel cell and constructed wetland.    

The constructed wetland-microbial fuel cell (CW-MFC) combine system is formed by embedding electrodes in different vertical positions. It can not only enhance sewage purification, but also capture electrons, transmit electrical signals, convert chemical energy in organic matter into electrical energy, and thus realize the in-situ COD (Chemical oxygen demand) biosensing. There are many structural parameters that affect the biosensor performance, yet how to design a suitable biosensor needs further research.   

Recently, a research team led by Dr. XIAO Enrong from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences proposed a structural parameter S and revealed the biosensing mechanism of the vertical flow CW-MFC (VFCW-MFC) biosensors and realized the optimization of the biosensing signal by studying the influence of the structural parameter S on the output electrical signal, biosensing performance, electrochemically active bacteria (EAB) and another functional microbiome. The study was published online in the journal Water Research.   

In response to too many parameters that affect the biosensing performance, researchers first devised the structural parameter S, which integrated and simplified electrode position, electrode spacing, and electrode size.    

“The structural parameter S was defined as the ratio of the thickness of the anode conductive filler layer to the thickness of the non-conductive isolation layer, as well as the ratio of the volumes of the anode to the electrode spacing. This ratio can be interpreted as the ratio of the volume percent of anode conductive filler layer to the total device and the volume percent of electrode spacing to the total device,” said Dr. XIAO.   

They constructed three groups of VFCW-MFC experimental devices with corresponding S values of 0.75 (CM1), 1 (CM2), and 1.5 (CM3). The results showed that all the three biosensors could achieve good monitoring for COD (R² > 0.97), but their different biosensing properties were also displayed, including the detection signal, detection range, detection time, correlation fitting degree, and sensitivity.    

To shorten monitoring time, researchers optimized the response signal of coulombic yield (Q). “We proposed the stable voltage charge (Q_S) which was defined as the cumulative coulombic yield during the period from initial stability to initial decline in output voltage,” said XIAO. They found that the detection time could be shortened by 70% without affecting the detection effect when the stable voltage charge (Q_S) was used as the signal.   

Researchers then analyzed the electrode microbiome of the three biosensors. The nitrogen-transforming bacteria (NTB) and EAB, enriched on electrodes, were in a different competitive situation. Therefore, EAB/NTB was proposed as the steady-state coefficient to denote the competition between EAB and NTB. The results showed that the significant linear correlation between the S values and the ratios of EAB to NTB colonized both on anodes and on cathodes. This indicates that S may ultimately affect the biosensor performance by affecting the competitive relationship of functional microbiome on electrodes.    

In this study, a novel structural parameters S that affect the performance of CW-MFC biosensor is proposed and the stable voltage coulomb quantity (Qs) is used as the biosensing signal to optimize the sensing performance. In practical application, CW-MFC can also be used as an early warning device for COD exceedance, and the remote monitoring of the operation status of the CW will be even realized through electrical signal networking. 

(Editor: MA Yun)