Example problem $2$:

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 problem $2($cont$.)$:

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 $20$

$m\frac{g}{L}.$ What is the rate of solids production, called the yield,

in units of pounds per day? Assume the system operates

continuously at steady state