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Technical Description steps are accomplished by microbiological action. However, the different steps require different microorganisms and different reactor conditions. Steps 1 and 2 both require greater than 0.5 g/m3 dissolved oxygen, take place in the aeration chamber, and occur as an ongoing process. Step 3 occurs in the absence of dissolved oxygen, and since the aeration chamber receives forced air 24 hours a day, takes place in the clarifier. Nitrogen removal in the Bio-Pure process is considerably greater than that of flowthrough and single-tank, sequencing-batch reactor systems. The Bio-Pure system proposed for the Castello Project achieves an average of 96.4% total nitrogen removal. The influent domestic raw sewage concentration of total nitrogen is estimated at 45 mg/1 for the Castello Project. With Bio-Pures's nitrogen removal capabilities, this will result in an effluent concentration of total nitrogen far less than the targeted 10 mg/l. AERATION DETENTION The Bio-Pure system's superior low-sludge-producing performance is obtained, in part, by com- bining the complete-mix hydraulic design with the detention times of the extended-aeration de- signs. This combination effectively produces a low F:M ratio and a subsequent high MCRT. This promotes the development of specific, high-process microorganisms. The growth curve of the cell life cycle is related to the effectiveness of BOD5 removal within the treatment process. In all cases, removal of BOD, is dependent upon aerator detention time. Growth and predominance of microorganisms are controlled by a variety of circumstances, includ- ing type of food matter, metabolic rate, and microorganism size. Because a definite type of mi- croorganism lives best under certain conditions, it is possible to relate treatment efficiency to mi- croorganism type. This is a determining factor for good batch process detention design. As the BOD remaining in the batch decreases, ciliated protozoa increase in number. A general guide as to relative predominance of protozoa and efficiency in the process may be characterized in the following chronological order: TYPE PROCESS EFFICIEENCY Sarcodina Plant startup or recovery Holophytic flagellates High organic overloading Holozoic flagellates Decreasing organic overloading Ciliates Lowering efficiency Stalked ciliates Elevating efficiency Rotifers High BOD efficiency, rapid oxidation High rotifer concentration will occur when the BOD5 removal efficiency is high; therefore, addi- tional detention capacity is required. The aerator size required to achieve this efficiency is based upon F:M ratio criteria between 0. 1 and 0.5 pound BOD/pound MLVSS. A second method of determining retention time is a microscopic examination of the stalked cili- ates. Groupings of 3 to 4 stalks indicate a healthy sludge; less or more groupings indicate sludge is either too young or too old. The quantity of microorganisms can be represented by the quantity of MLVS S. Ideally, the living or active microorganisms would simply be counted, but this is not feasible; studies show the 11 , J