Enhanced production of short-chain fatty acid by co-fermentation of waste activated sludge and kitchen waste under alkaline conditions and its application to microbial fuel cells

In a previous publication the conversion of waste activated sludge (WAS) to short-chain fatty acid (SCFA) was reported to be significantly enhanced by carbohydrate addition. Herein, the effect of carbon to nitrogen (C/N) ratio, pH (especially alkaline pH), temperature and hydraulic retention time (HRT) (or solid retention time (SRT)) on SCFA production and the related mechanisms were investigated when kitchen waste was added to WAS fermentation system. By response surface methodology, the conditions for maximal SCFA production were optimized, i.e. pH 8, C/N ratio 22, temperature 37 °C and time 6 d. Mechanism exploration revealed that under the optimum conditions the general activity of anaerobic microorganisms, the activities of key acid-forming enzymes, and the ratio of Bacteria to Archaea were improved remarkably, whereas the increase of methane production was negligible. With the fermentation liquid from WAS and kitchen waste as the fuel in microbial fuel cells (MFCs), the performance of electricity generation was enhanced significantly compared with those in MFCs fed with ultrasonic-pretreated WAS or ultrasonic-pretreated WAS plus smashed kitchen waste, due to the increase of SCFA content and the decrease of viscosity of the fuel.