Acetate Detection with a Living Biosensor – The Capability of Anodic Biofilms
Authors: Kretzschmar Jörg , Liebetrau Jan , Mertig Michael , Harnisch Falk 
Published: February 2020

Biosensors based on electroactive bacteria are one promising application of microbial electrochemical technologies (MET) beside, e.g., microbial fuels, electrolysis cells and desalination cells [1]. Here we present a microbial electrochemical sensor platform for online detection of acetate within the anaerobic digestion (AD) process.
Despite its wide distribution and existing practical experiences, the anaerobic digestion process is still low automated compared to other industrial processes. Acetate as highly process-relevant volatile organic acid (VFA) in AD is nowadays measured off-line with gas- or liquid chromatography. To establish real time process monitoring and control especially for industrial biogas production (e.g. requiring flexible feeding regimes), an online measurement of process relevant VFA is needed. Online control of VFA and subsequent adaption the feeding rate could prevent process disturbances or process breakdown especially in high-throughput or flexible biogas production. Biosensors based on microbial fuel cells were already proposed for VFA quantification in anaerobic digestion [2–4]. By advancing the idea of a microbial electrochemical sensor we propose the application of a membrane less sensor with a three electrode transducer and a microbial anode as sensing element for online VFA measurement. Thereby the sensor housing allows an implementation into existing AD-reactors.
The basic sensor parameters like measurement range and resolution, cross sensitivity and response time were examined in a non sterile, continuous stirred flow cell setup (100 mL, 1 ml min-1) using artificial wastewater as growth medium. The sensors measurement range for acetate could be determined with 0.5 to 5 mmol L-1 with a measurement resolution from 0.25 to 1 mmol L-1 acetate (see figure) [5]. Cross sensitivity was tested for propionate and butyrate and showed only a low signal for both VFA.
To provide proof of concept for real process application first experiments in a semi continuous lab scale biogas fermenter (10 L) with maize silage and cow manure as substrate were carried out. Therefore, an external grown biosensor was introduced into an existing biogas reactor. The sensor signal followed to a typical intraday VFA concentration profile being confirmed by GC-FID measurements.
So far, the achieved measurement range of the sensor is too low for application in AD. We discuss measures to pave the way to application, e.g., by optimized sensor architecture, tailored anode material´s or dilution of process fluids in a bypass.

Figure: Biofilm current response on changing acetate concentrations. Data summarized from CA and CV measurements. The grey boxes correspond to the middle 50% of the data, whiskers (error bars) indicate minimum and maximum, n= 12 (6 for CA and 6 for CV) [5].

Literature:
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[4] Z. Liu, J. Liu, B. Li, Y. Zhang, X.-H. Xing, Focusing on the process diagnosis of anaerobic fermentation by a novel sensor system combining microbial fuel cell, gas flow meter and pH meter, Int. J. Hydrog. Energy. 39 (2014) 13658–13664. doi:10.1016/j.ijhydene.2014.04.076.
[5] J. Kretzschmar, L.F.M. Rosa, J. Zosel, M. Mertig, J. Liebetrau, F. Harnisch, A microbial biosensor platform for in-line quantification of acetate in anaerobic digestion: potential and challenges, Chem. Eng. Technol. accepted

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