Compact fluidic electrochemical sensor platform for on-line monitoring of chemical oxygen demand in urban wastewater

The rapid expansion of urban areas has given rise to the generation of large amounts of wastewater that pose a growing pressure on the surrounding ecosystems. Effective water quality surveillance programs demand the implementation of in-field tests to monitor water safe discharge in the environment after being adequately treated. This paper describes the manufacturing and application of a miniaturized electrochemical sensor for measuring dissolved chemical oxygen demand (COD) in surface waters entering and exiting urban wastewater treatment plants (UWWTP). Thin-film carbon electrodes are produced on Si/SiO2 substrates by a combined sol–gel material synthesis and photolithography/dry etching process at the wafer level and show superior electrochemical performance compared with the glassy carbon standard electrode. Three-electrode electrochemical cells of planar configuration are produced and the COD sensor is manufactured by electrodepositing copper nanoparticles (Cu NPs) on the working electrode under controlled potentiostatic conditions. A simple fluidic electrochemical sensor device is fabricated and allows for the Cu NPs electrodeposition as well as analytical sensor performance in an automatic fashion. A linear range up to 670 mg·L-1 O2 and a limit of detection of 30 mg·L-1 O2 are achieved. The ability of this electrochemical sensor approach to monitoring the soluble organic load of urban wastewater in real-time is evaluated by analyzing three samples collected in different locations of an UWWTP. The COD values estimated with the sensor are in excellent agreement with those obtained using the standard dichromate method provided by an accredited laboratory. This analytical platform provides a robust, user-friendly low-maintenance flow sensor approach for deployed analysis of COD that could aid to control the effectiveness of water treatment processes, especially when facing water influents containing organic overloads due to changing weather conditions or any anthropogenic phenomena.
The Chinese Scholarship Council fellowship (201806220063) to W.C. Duan is acknowledged. WD is enrolled in the Materials Science PhD Program of the UAB (Universitat Autònoma de Barcelona). This work has also received funding from the EU Horizon 2020 framework programme under the Marie Skłodowska-Curie grant agreement No 801342 (Tecniospring INDUSTRY)” and the Government of Catalonia's Agency for Business Competitiveness (ACCIÓ). The Spanish Ministry of Science and Innovation is acknowledged for financial support through the Severo Ochoa programme for Centers of Excellence in R&D (CEX2019-000917-S). Some additional funding was also given by the government of Catalonia, with grants numbers 2017 SGR 1771 and 2017 SGR 765. The X-ray diffraction and thermal analysis laboratories of the ICMAB scientific services contributed to this work and are acknowledged. This work used the Spanish ICTS Network MICRONANOFABS which was partly supported by the Spanish Ministry of Science and Innovation. We are very grateful to Mercè Baldi and Josefina Torán for their help and for kindly providing us with the real water samples from the wastewater treatment plant.
With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).