Assessing The Electroactivity Of Biofilms On Electrodes In Single Chamber, Membraneless Microbial Electrolysis Cells By Electrochemical Methods

In conventional anaerobic processing, organic mineralisation by producing CH₄ with H⁺ and CO₂ as electron acceptors has been utilized in wastewater treatment and bioenergy production with better prospective. However, the process complexity, slowness, higher COD in effluent and easy to failure due to environmental perturbation limit its engineering implementation. The performance improvement in producing CH₄ mineralisation with MECs was demonstrated in digestion rate, high COD removal ratio, process robustness and the biogas quality. This makes it possible to widen this process application.Although the low cost biocathode is promising for MECs implementation, its low electroactivity is another emerging problem. How to culture biocathode with higher activity using simple and quick method becomes a focus. Easily fermentable carbonhydrates in real waste streams, e.g. glucose, is very common. To our knowledge, there is no report on developing biocathode through glucose-fed single chamber, membraneless microbial electrolysis cell?SCMMEC?. For most of the biofilms in nature, they are not electroactive. Based on the above, we confirmed the electroactivity of cathodic biofilms from glucose-fed SCMMEC and evaluated the possibility to obtain biocathode for engineering application.1.It is unavoidable to develop biofilm on the electrode in SCMMEC. The cathodic biofilms in glucose-fed SCMMEC showed electroactivity. Two microbial biocathodes had current density 0.59 A/m² and 0.62 A/m² under 0.8 V applied cell voltage, respectively. The results suggested that wastewater containing easily fermentable carbohydrates, e.g. glucose, could directly be used to enrich biocathode. However, relative to the bioanode activity, their electroactivity was still low. They were the current-limiting electrodes in the system. The system current in fed-batch mode operation suggested that possible redox mediators or suspended cells did not affect the system's performance upon the medium complete replacement. With low buffer dose?6 mM phosphate?, the current showed no difference at day 62 and 153. However, the current got down at day 275. Ion concentrating and salt precipitation may be the main reason of the activity loss. It is impossible to avoid ions concentrating and even salt precipitation with DC power source due to one direction long-term polarization.2.There was no evident difference in activity with biocathodes under 0.2?0.3 and 0.5 V applied voltage after 76 days, 71 days and 66 days enrichment, respectively. This makes it possible to decrease SCMMEC's construction cost by the use of power source with lower stability and reduce SCMMEC's operation cost by the optimization of applied cell voltage. In addition, the applied voltage should also consider the influent COD according to the applied voltage-current density-influent COD test, lower applied voltage for lower COD concentration, and rather than vice versa.Compared to graphite biocathode, the current density generated on stainless steel 304 and graphite hybride electrode under 0.4 V applied voltage was lower than that on graphite biocathode at day 173. This suggested that there is no advantage using stainless steel 304 under such operation conditions. The impaired biocathodes could not naturally recover to its activity level previous to being damaged after 30 days remediation. This suggested the complexity on the biocathode application. Besides, we also found that current density parameter is more sensitive than electrode potential for operation monitoring. There existed activity difference between the upper half biocathode and the rest of biocathode. The activity on the upper half one is higher than that of the lower one.3.In glucose-fed SCMMEC, new bioanode under low applied voltage could be initiated by inversing the polarity of biocathode. This is a practical method for initiating new bioanode during engineering implementation. The activity of biocathode after 10 days or 30 days bioanode experience showed no difference relative to the one prior to bioanode experience.4. Bioanodes in glucose-fed SCMMEC could be inversed as biocathodes. Compared to bioanodes, they are still the current-limiting electrodes due to their low activity. Consecutive medium complete replacement showed that possible redox mediators or suspended cells in the medium did not affect the current generation. Prolonging the developing period of bioaondes from 60 days to 90 days could not increase the activity of biocathodes. Increasing the cathode/anode surface area ratios to between 2:1 and 3:1, the activity of bioanodes was no more limited.