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ess takes the conventional activated sludge process one step farther. NOTES The activated sludge, while in the aeration tank, enters into an ad- vanced aerobic stabilization mode that is often compared to the aerobic digestion process. This mode, called reaction, gives rise to the process term "batch reactor." The advantage of the reactor process is found in the enhanced reduction of volatile suspended solids without affecting the development of those organism types required for the basic process. Reactor sludges, unlike typical flowthrough-type processes, produce no odor. Sedimentation, or clarification, follows the aeration process. This stage allows the activated sludge to flocculate and settle out, produc- ing a clear effluent of low organic content. After transfer of the clarified supernatant, the settled sludge is returned to the aeration tank as seed for incoming waste. MICROORGANISMS Activated sludge is made up of bacteria, protozoa, and rotifers, as well as other natural strains and groups of microorganisms. (The exceptions are treatment plants which are artificially seeded with known strains of organisms,a process known as bioaugmentation.) The bacteria are the most important group of microorganisms, as they are responsible for stabilization of the organic matter and floc formation. Although all types of bacteria make up activated sludge, the nature of organic compounds in the wastes being stabilized will determine which bacterial genera predominate. Protozoa do not contribute directly to stabilization of organic matter in the wastes being treated,because the organic concentration is too low to support animal growth; however, the protozoa can feed from the bacteria which are utilizing the organic matter. With the advent of complete oxidation-type activated sludge systems, the rotifers have been the predominant animal form, and can utilize larger fragments of activated sludge floc than can the protozoa.They can survive after all the free-swimming bacteria have been consumed by the protozoa. The dominance of rotifers indicates an extremely stable biological system. Activated sludge can be formed from wastes high in colloidal solids, such as domestic sewage, or from a completely soluble waste, such as industrial waste from the manufacture of synthetic chemicals.The formation of activated sludge is exactly the same in both extremes as long as the wastes are considered nutritionally stable, thereby con- taining all the elements necessary for normal bacteria growth. Most wastes contain sufficient microorganisms and nutrients to pro- duce a quantity of activated sludge without seeding. The exception would be a treatment plant receiving high degrees of rainwater or other nonsewage-bearing waters. 7