Fig. 3

VFAs concentration in different MECs during a fed-batch cycle. RS-OCMEC: raw sludge fed open-circuit MEC; AS-OCMEC: alkali-pretreated sludge fed open-circuit MEC; RS-MEC: raw sludge fed MEC with an applied voltage of 0.6 V; AS-MEC: alkali-pretreated sludge fed MEC with an applied voltage of 0.6 V