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1%W .we This method of mixing air and waste, under pressure, produces vig- orous agitation and excellent oxygen transfer as the bubbles rise to NOTES the surface through the mixed liquor.It is far superior to the conven- tional surface-effect aerators,as it ensures near-uniform aerobic con- ditions throughout the entire chamber. Aeration is automatically selected as continuous or intermittent, de- pending on the level and age of liquor in the tank. The amount of air added for complete digestion of sewage need not be high because of the elevated temperature of the mixed liquor, which may be as much as 5 degrees higher than the incoming waste. This higher temperature, plus the floc particle breakup due to the vigorous air circulation, renders the organic material more readily digestible by the ever-increasing number of organisms residing in the tank. On reaching a predetermined level in the aeration tank, a float switch initiates the microprocessor, starting the batch cycle which transfers the mixed liquor to the primary clarifier. During the later stage of this transfer, any floatables in the clarifier are returned to the aera- tion chamber via a dual-tube weir system which skims the top sur- face. The remaining liquid is allowed to settle for approximately 70 minutes under static, or quiet, conditions. This settling separates the liquid into two levels;the clear top level is called supernatant and the lower level is termed activated sludge. After separation, the supernatant is transferred through an auto- demand chlorinating line into the chlorine contact tank, which also acts as a secondary clarifier. Any remaining floatables, along with the settled sludge remaining in the primary clarifier, are returned via pump to the aeration tank for additional processing and recycling. It is this constant recycling that provides a mean-cell-residence-time (MCRT) of approximately 14 hours and causes the near-total de- struction of sludge. In the chlorine contact chamber,sometimes called a secondary clari- fier, the supernatant, now chlorinated, is held for another 70-minute period of quiescent settling. This second settling period allows any remaining solids to settle.After this 70-minute period,approximately 95%of the treated waste is pumped through optional tertiary filters, or it may be discharged as secondary effluent. The remaining 5%, which is inert or activated sludge, is returned to the aeration cham- ber for reprocessing. As incoming sewage flow slows, such as during midday, late-night and early morning hours, the level in the aeration chamber is low- ered by continued batching. At a predetermined lower level, a float switch is triggered, reducing the aeration to an intermittent schedule more suitable to existing sludge conditions and the reduced quantity of incoming waste. The system flow is automatically programmed and requires no man- ual assistance,although an operator override control is provided. 3