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the sludge spends in the treatment system. This cycle is referred to as mean-cell-residence-time(MCRT), and is a function of the amount NOTES of food provided to bacteria in the aeration reactor tank (the food- to-microorganism ratio, F:M). There are three basic classifications of the activated sludge process which are defined by the process loading range to the microorgan- isms: high, conventional, and low.The following shows typical ranges of the MCRT and F:M for each loading range: MCRT, F/M Process day Ib BODS/day/lb MLVSS High Rate 3-5 0.4-1.5 Conventional Rate 5-15 0.2-0.4 Low Rate 15-30 0S-0.2 Note: lbs/day x 0.01157 = g/kg In the complete-mix, activated-sludge process, the characteristics of the mixed liquor are similar throughout the aeration tank. That is, the influent waste is rapidly distributed and operating characteristics measured in terms of solids, oxygen uptake rate (OUR),MLSS, and soluble BOD concentration are nearly identical throughout the tank, with a very low level of food available to a large mass of microorgan- isms at any given time. This is the principal reason for the system's ability to handle surges in the organic loading without producing a change in effluent quality. Critical design factors are the tank's length-to-width ratio, amount of air introduced, and wastewater velocity throughout the tank. The system is operated at recommended design standards of MISS of about 3,000 to 6,000 mg/l. The quantity of sludge returning from the clarifier is 25 to 100 percent of design flow. This percentage is required in order to maintain the aeration solids. To ensure that a dissolved oxygen (DO) of 0.05 ppm minimum exists at all points, a minimum oxygen input of 2.0 mg/1 is maintained at the effluent end of the aeration chamber. Therefore, air is supplied at a rate of 0.8 to 115 cubic feet/gallon of wastewater with an addi- tional mixing factor of 1.5 to 2.0 cubic feet/gallon. r For siring the air delivery system, the cubic-feet-of-air requirement is expressed as cubic feet of air required per pound of BOD re- moved from the aeration reactor.The minimum air/organic loading recommended by most state agencies is 1,000 cubic feet/pound of BOD. Well-designed systems will sometimes expand this to 1,500 cubic feet/pound of BOD removed, allowing a residual airbase for com- pensation of higher organic loadings than anticipated and to support further processing of ammonium nitrogen (NH,). 12