A biofilm containing both hydrolytic fermentative bacteria and acidogenic bacteria (including acetogenic and acetoclastic bacteria) was developed for the treatment of plastic wastes in a two-phase, batch digester. The biotransformation and further degradation were electrochemically observed. It was found that the organic wastes were initially fermented in a single-phase (i.e. liquid phase) digester, where it entirely obeyed microbial growth kinetics in accumulating acetate. As the carbonates produced were vaporised, the single-phase became a two-phase fermentation (gas and liquid) accumulating volatile fatty acids (VFAs), where it obeyed a proton driving force based on Le Chatelier’s principle. Interestingly, as the digester was depressurised to the saturated vapour pressure of water, the accumulated VFAs were rapidly transformed into acetone via butanol, so that the VFAs forms were not observable. It was found that in extreme conditions, the organic feeds were converted and stored into acetone, via butanol.