Problem $2$

A primary sedimentation tank prior to an activated sludge tank removes $55\%$ of an influent wastewater containing $350m\frac{g}{L}$ of suspended solids yielding a sludge of $5\%$ solids. The same plant produces a $600\frac{m^3}{d}$ wasted activated sludge $($WAS$)$ of $0\mathrm{.}75$ solids. Both the primary sludge and the WAS are to be sent into a digester. What will be the volume of the digester required if the combined primary sludge and WAS is thickened in a gravity thickener to $6\%$ solids and the treatment plant has a capacity of $65,000\frac{m^3}{?}$ day? Will this volume be different if the PS and WAS are sent to the digester separately? State the pros and cons of these two different approaches