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Technical Description from the bacteria that utilize the organic matter and thus assist greatly in the settling stage in the clarifier. TECHNICAL DESCRIPTION With the advent of complete oxidation-type, activated-sludge systems, such as the Bio-Pure sys- tem, the rotifers have been the predominant animal form and can utilize larger fragments of acti- vated 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 bio- logical 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 the same in both extremes as long as the wastes are considered nutritionally stable, thereby containing all the elements necessary for normal bacte- ria growth. Most domestic sewage contains sufficient microorganisms and nutrients to produce a activated sludge without seeding. The exception would be a treatment plant receiving high degrees of rain- water or other nonsewage-bearing waters. As fresh waste enters, the food-to-microorganism (F:M) ratio is very low. Microorganisms are in excess of food supply and are thus competitive for the food. As the bacteria begin to grow and multiply, the protozoa begin to grow. During this log growth rate, organic matter in the waste is removed at its maximum rate with optimum conversion of organic matter into new cells. The en- ergy level is sufficiently high to keep microorganisms dispersed. It is impossible to get activated sludge to form as long as the microorganisms remain in the log phase. The high metabolic rate of the bacteria creates a constant need for additional oxygen. If aerobic conditions are not maintained by proper oxygen transfer, the rate of metabolism will not follow a log rate, but rather an arithmetical rate until oxygen is no longer the limiting factor and the protozoa would then be adversely affected. The F:M ratio drops rapidly as the food is consumed and new cells are produced, consuming more of the nitrogen and phosphorous which is present in the wastewater. A point is reached where the food becomes the limiting factor in further growth. The bacteria and protozoa begin to decline. The cells begin to die and floc begins to form. In the turbulent aeration tank, the bacteria are constantly being brought into contact with each other; as long as they have sufficient energy, they split apart and continue their normal metabolic function. As their energy content decreases, more and more bacteria lack the energy to overcome the forces of attraction between two colliding cells. The two cells then move as a unit and soon become three, then four, and so on, until a large floc colony has formed. The food concentration continues to drop and microorganisms continue to increase, but at an ever-slowing rate. The minimum F:M ratio is reached at the end of the declining growth phase and nitrogenous substances begin to form. When the bacteria are unable to obtain sufficient energy from the little remaining food, they begin to metabolize food reserves within their own cells. Excess fats and carbohydrates are consumed 3 1'