The illustration of an SBR in Figure 11.18.1 includes five discrete periods of time, with more than one operating strategy possible during any time period. The five discrete periods are defined as:
1. Idle: waiting period
2. Fill: influent is added
3. React: biological reactions are completed
4. Settle: solids separate from treated effluent
5. Draw: treated effluent is removed
Each discrete period is detailed in the following sections. IDLE
Idle is considered the beginning of the cycle, although there exists no true beginning after the initial start-up. Idle occurs between draw, the removal of treated effluent, and fill, the beginning of influent addition. Idle time may be short or long depending on flow rate variation and operating strategy. When the influent flow rates are constant and predictable, and when flow equalization is provided upstream, idle is nearly eliminated. Idle time is long during periods of low influent flow, and short during periods of high influent flow. During operations with variable idle times, the SBR also provides flow equalization. Idle can also be used to accomplish other functions, such as sludge wasting, and mixing to condition the biomass to a low substrate concentration.
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