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Technical Description first and the proteins last. As the energy level drops, floc colony formation increases rapidly. Free- swimming ciliated protozoa cannot find enough bacteria to stay alive and they too begin to die. As the bacteria die, an enzyme dissolves a portion of the cell wall, allowing the remaining nutrient contents in the cell to escape and thus furnishing the remaining cells with a little more food. This process falls under the spectrum of phagocytosis, or more specifically, lysis, after lysin, a specific antibody that destroys blood cells, tissues, and microorganisms only, and is a natural process that allows living bacteria to obtain nutrients from their dead neighbors. If the aeration period were allowed to continue, the bacterial population would continue to de- crease. The free-swimming ciliates would die out completely and the stalked ciliates would start to decrease, but rotifers would start to increase. Rotifers eat small particles of the floc and do not depend upon individual cells, as do protozoa. Too long an aeration period would result in death of all biological forms; only the inert fraction of the cells would remain. Activated sludge is never allowed to go this far in the aeration cycle of the Bio-Pure system before being moved on to the next step in the process. During the endogenous phase, the floc colonies are separated from the liquid before clarification. This concentrated floc is fed by fresh batches of organic matter entering the treatment system. Since the quantity of microorganisms is higher than the first time around, the initial F:M ratio is lower and the bacteria start at a higher level in the growth cycle. A constant period of aeration allows the system to progress further into the endogenous phase with each cycle. This results in better flocculation and a clearer effluent. Thus, the rate of organic removal is most rapid in the growth phase, while floc formation is best in the endogenous phase. EXTENDED AERATION Sludge production is generally an inherent part of the biological digestion process. In typical mu- nicipal systems, there is so much food that the bacteria multiply exponentially with much of the food going into production of new cells, so sludge, which is made up almost entirely of biomass, production is high. Conversely, when food is scarce, as in the low F:M ratio Bio-Pure system, the bacteria become cannibalistic and cell production is low, so sludge production may actually cease as new cells reach equilibrium with those consumed. In the Bio-Pure process, the rate of sludge buildup is bal- anced by the rate of sludge destruction. Since this balancing is not precise, over extended periods of time the effluent discharge quality may ultimately be degraded as small quantities of sludge concentrate, thus periodic wasting of a small volume of sludge may become necessary. This is es- pecially true where a highly polished effluent is desired. Unlike a municipal flowthrough system, where sludge is continually drawn off, the Bio-Pure sys- tem retains and recycles the sludge until the one-liter settleometer test (SSV-60 test) indicates 500 ml of sludge volume. In each batch, 66% of the mixed liquor, the portion that remains in the clarifier after transfer of the supernatant, is returned to the aeration chamber. After settling, this 66% represents 100% of the activated sludge. (NOTE: The SSV-60 test, although somewhat similar, is not to be confused with the more com- monly used SW test, (sludge volume index). The SW is used to determine the volume of sludge compared to its weight, and thus its settleability, whereas the SSV-60 test determines only the settled-sludge-to-supernatant ratio.)