During the Acidogenesis, the Hydrolysis products which are relatively small, soluble compounds are diffused inside the bacterial cells through the cell membrane and then are either fermented or anaerobically oxidized. These processes occur by a complex consortium of hydrolytic and non-hydrolytic microorganisms which are the source of energy for the acidifying population. The acidification products consist of a variety of small organic compounds, mainly so-called volatile fatty acids (VFA's) (acetate and higher organic acids (like propionate and butyrate), H2, CO2, some lactic acids, ethanol and ammonia (see figure 2). Given that VFA's are the main end products, fermentative organisms are usually designated as acidifying or acidogenic microorganisms.