Can microbial electrochemical sensors measure biochemical oxygen demand in saline wastewater?

Treatment of industrial high-salinity wastewater (1%~3.5% NaCl) typically involves integrated physicochemical and biological technologies, necessitating real-time monitoring of biochemical oxygen demand (BOD) before biological treatment to assess biodegradability. However, while  microbial electrochemical sensors (MESs) employing electroactive biofilms (EABs) as sensing elements can measure BOD in municipal wastewater, their performance in saline environments may be compromised due to biofilm damage under salt stress.

To that end, a Professor Xin Wang’s team investigated microbial responses across different levels of salinity. The study examined MES performance and long-term stability at different salinity levels, aiming to determine whether such sensors can rapidly measure BOD in saline wastewater. Furthermore, the team elucidated salt-tolerance mechanisms by analyzing EAB adaptations under salinity stress.

“Excessively high salinity impairs cellular growth and metabolism, compromising bioreactor performance,” explains Wang. “Operational testing at 2.5%, 3%, 3.5%, and 4% NaCl revealed progressive inhibition: adaptation and recovery diminished as salinity increased and peak/limiting current declined > 94%.”

In particularly, current density negatively correlated with salinity, indicating impaired electron transfer efficiency and suppressed electroactivity under high-salinity conditions. At 3.5% and 4% NaCl, it failed to exceed 20 A/m³, exceeding electroactive biofilms (EAB) tolerance and defining the upper threshold.

“Hence, the maximum recommended NaCl is tbelow 3%.,” shares Wang. “Continuous operation across 0%~3% NaCl maintained functionality, but BOD quantification (BOD_Q) progressively decreased, likely attributed to long-term accumulation of outer-layer biomass.”

    Notably, significant BOD_Q decrease (19%~27%) occurred only at 3% NaCl; lower salinity levels exhibited negligible impact. As salinity increased, substrate and proton transport across cell membranes became constrained. After 120 days, microbial communities exhibited adaptive evolution of EABs, enhancing collective salt tolerance. 

   “MES-measured BOD exhibits linear correlation with standard BOD₅, remaining unaffected by salinity. This confirms the feasibility of precise BOD quantification in wastewater with salinity (NaCl mass fraction) ≤3%. MESs demonstrate validated long-term stability and operational suitability under these condition,” says Wang.

Taken together, the team’s findings, published in Water & Ecology, offer a stable, accurate, and rapid method for BOD measurement in saline water bodies. It carries broad implications for real-time water quality monitoring and advancing industrial wastewater treatment technologies.

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Contact the author: Xin Wang

-MOE Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, College of Environmental Science and Engineering, Nankai University, Tianjin 300350, China

xinwang1@nankai.edu.cn

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