Signal trends of microbial fuel cells fed with different food-industry residues

A microbial fuel cell (MFC) is an anaerobic bioreactor where soluble metabolites liberated by hydrolysis and fermentation of macromolecules are simultaneously available for anode respiring bacteria (ARB). ARB can be influenced by chemical imbalances in the liquid phase of the bioreactor. The objective of the work was to explore the trend of electric signals generated by MFCs, in relation to anaerobic biodegradation of four different solid food-industry residual substrates. Four sets of membraneless single-chamber MFCs were operated in batch mode, with solid waste substrates characterized by a different base component: i) mixed kitchen waste (fibers), ii) whey from dairy industries (sugar), iii) fisheries residues previously processed to recover oils (proteins), iv) pulp waste from citrus juice production (acidic). All the tested MFCs were able to produce an electric output with different trends, depending on the principal component of the solid substrate. MFC potential varied as function of the COD and the feeding cycle, as well as of the substrate. The pH variability during the fermentative process significantly affected the electric output. Citrus (acidic) pulp fed MFCs started to operate only when the pH raised up 6.5. MFCs fed with mix kitchen wastes had a relatively stable electric signal; fish based waste caused spiking in the MFC signal and an averaging in the COD degradation trend. This phenomenon was attributed to a pH instability induced by proteins degradation forming ammonia. The fermentation process was strongly predominant with respect the electrochemical process in MFCs and the coulombic efficiency (CE) was low, ranging between 2 and 10%. This result call for a deeper exploration of harvesting power from solid wastes and pointed also to the possibility of using a MFC to monitor important parameters of fermentation processes in biotech production plants.