Example $2$: An Activated Sludge Process

The activated sludge process, a two step process, is one of the most commonly used means of biological wastewater treatment. In the first step, which takes place in the aeration rank, dissolved organics are converted to cellular mass by microorganisms. In the second step, which takes place in the settling tank, the cellular mass is removed from the flow stream by gravitational settling. A portion of the microbial mass is returned to the aeration tank in the return activated sludge, and a portion of the microbial mass is removed from the system in the waste activated sludge.

Example $2$: An Activated Sludge Process

The activated sludge set$-$up shown above has an influent flow of $10$ MGD $($million gallons per day$)$ and a suspended solids concentration of $50m\frac{g}{L}.$ The waste$-$activated sludge flow rate is $0\mathrm{.}2$ MGD and has a solids concentration of $1\mathrm{.}2\%.$ The effluent $($discharge$)$ has a solids concentration of $20m\frac{g}{L}.$ What is the rate of solids production, called the yield, in units of pounds per day? Assume the system operates continuously at steady state