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Technical Description NOTE: Most conventional municipal treatment plants, as well as most other small-flow packaged systems, are of the flowthrough design. The weakness of the flowthrough design is that, when fully loaded, it forces out only partially treated wastewater at the same rate that fresh wastewater is taken in, something that cannot happen with a batch reactor. Activated sludge is generated by aerating a biologically degradable waste until a large mass of set- tleable solids forms. These settleable solids are active masses of living organisms, redesignated as activated sludge. Activated sludge is not formed by special floc-forming bacteria, but rather is a natural phenomenon of all bacteria at a definite energy level. The aeration, mixing, and oxidation of fresh wastes with sludge that is recycled in the treatment process takes place in the aeration chamber. Microorganisms aerobically stabilize incoming or- ganic matter, which is stored as activated sludge in the aeration chamber until enough volume is available for transfer to the clarifier tank as a batch. This storing process takes the conventional activated sludge process one step farther. The activated sludge, while in the aeration tank, enters into an advanced aerobic stabilization mode, which is comparable to the aerobic digestion proc- ess. 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 and, combined with the multiple batch- return cycles, produce very little sludge in the secondary discharge. As a result, sludge wasting is dramatically minimized. Sedimentation, or clarification, follows the aeration process. This stage allows the activated sludge to flocculate and settle out, producing a clear effluent of low organic content. After trans- fer of the clarified supernatant, the settled sludge is returned to the aeration tank as food for the bacteria contained in the incoming waste and air and to provide the carbon source for the denitri- fication process. MICROORGANISMS Activated sludge is made up of essentially four kinds of organisms which do most of the digestion in an activated-sludge process. The bacteria which eat dissolved organic compounds; the ciliates (protozoa), both free-swimming and stalked, which eat the bacteria and thus are heavy enough to settle by gravity; and the suctoria, which feed on the ciliates and assist with the settling process. This chain of bug eating bug is known as endogenous respiration or, as a treatment mode, contact stabilization. There are other lessor, more natural strains and groups of microorganisms in the process, but these four are the primary strains. The exception would be treatment plants artifi- cially seeded with known strains of organisms, a process known as bioaugmentation (seeding). (NOTE: 7H does not recommend bioaugmentation of the Bio-Pure plants, as stabilization time is normally less than ten days after startup.) The bacteria are the most important group of microorganisms, as they are responsible for stabili- zation of the organic matter and early floc formation. Although all types of bacteria make up acti- vated 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 organic concentration is too low to support animal growth; however, the protozoa feed 2 1'