Understanding the Significance of Current Density in Microbial Electrochemical Cells

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Current microbial electrochemical cells (MxCs) do not meet requirements of anaerobic waste and wastewater treatment, mainly due to sluggish kinetics and small current density. Hence, improving current density is essential to deploy MxCs in the field as energy-efficient anaerobic wastewater treatment. To better understand the limiting parameters and operating conditions associated with current density, this review mechanistically analyzed the biological kinetic parameters in biofilm anodes in parallel with operating conditions related to the parameters. Six biological parameters that include electron fraction used for catabolism ( f e o https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429487118/b8b3b135-94fb-4bd6-a54f-c8a61850878b/content/eq130.tif/> ), apparent maximum specific substrate-utilization rate (qmax,app), biofilm density of active exoelectrogens (Xf), biofilm thickness (Lf), concentration of donor substrate (Sd) and apparent half-saturation concentration (Ksd,app) can influence current density in a biofilm anode given that extracellular electron transfer (EET) would not limit current density. The proliferation of exoelectrogens can increase f e o https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429487118/b8b3b135-94fb-4bd6-a54f-c8a61850878b/content/eq131.tif/> and Xf, but all exoelectrogens do not have high kinetic features, thus, indicating the importance of enriching kinetically efficient exoelectrogens (i.e., Geobacter), not just exoelectrogens. Thick Lf cannot simply increase current density due to acidification of inner biofilms in thick biofilms related to Xf term. It is detrimental to optimize qmax,app, Ksd,app, Xf and Lf in biofilm anodes to given wastewater to achieve current density required for field application along with engineering approach.