Hydrolytic and Fermentative Bacteria- This group includes both obligate and facultative anaerobes, and may occur upto 108-109 cells/ml of sewage sludge digesters. They remove the small amounts of O₂ present and create anaerobic conditions.

These bacteria hydrolyze and ferment the organic materials, e.g., cellulose, starch, proteins, sugars, lipids, etc., and produce organic acids, CO₂ and H₂. But usually only 50% of the polysaccharides present in the waste may be digested.