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When aeration is started, the food-to-microorganism (F:M) ratio is very low. Microorganisms are in excess of food and the initial growth NOTES follows a log rate.As the bacteria begin to grow, 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 energy level is sufficiently high to keep microorganisms dispersed. It is impossible to get activated sludge to form as long as the micro- organisms 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 protozoa will be adversely affected. The F:M ratio drops rapidly as the food is consumed and new cells are produced, consuming more nitrogen and phosphorous 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 the energy content of the system decreases, more and more bacteria lack the energy to overcome the forces of attraction be- tween two colliding cells. The two cells move as a unit and soon become three, then four, and so on, until a small floc colony has formed. The food concentration continues to drop and microorganisms con- tinue 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 fust and the proteins last. As the energy level drops, floc colony forma- tion 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, thus furnishing the remaining cells a little more food. This process is called lysin, a natural process which allows living bacteria to obtain nutrients from their dead neighbors. If the aeration period were allowed to continue, the bacterial popu- lation would continue to decrease. The free-swimming ciliates would die out completely and the stalked ciliates would start to decrease, 8