Figure 7.25.2 shows a conventional activated-sludge process flow diagram. This process is primarily used in the treatment of municipal wastewater. The process uses long, rectangular aeration tanks with minimal longitudinal mixing that creates plug-flow patterns (see Figure 7.25.3). The wastewater is mixed with the recycled sludge at the head end of the aeration tank and then flows through the tank where organic matter is progressively removed. As a result, a BOD concentration profile is established through the tank that can diminish when the recycled sludge flow is significant. Air application is generally uniform through the tank.
The conventional activated-sludge process is susceptible to shock and toxic loading conditions since longitudinal mixing is absent in aeration tanks. The tapered-aera-tion, activated-sludge process (see Figure 7.25.4) and the step-feed-aeration, activated-sludge process (see Figure 7.21.3) are two process variations of the conventional activated-sludge process. The aeration rate decreases along the tank length in the tapered-aeration, activated-sludge process and matches the BOD concentration profile to improve process economy. The aeration equipment is spaced unevenly through the tank.
Settled wastewater enters at several points in the aeration tank in the step-feed-aeration, activated-sludge process, equalizing loading and oxygen demand. This operation mode increases the flexibility of the process to handle shock and toxic loading conditions.
On the other hand, mixing intensity in the aeration tank of the completely-mixed, activated-sludge process (see Figure 7.21.5) is sufficiently high to yield a uniform mixed
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