Figure $1.$ System Schematic.

PROBLEM STATEMENT

Growth of a Colorado ski resort requires expansion of the community's wastewater treatment plant. The expanded plant will have a design capacity of $4\mathrm{.}00$ MGD and, if the current level of treatment technology is maintained, the plant will achieve an effluent with a BODU concentration of $L_W=60m\frac{g}{l}.$ A schematic of this scenario with pertinent stream data is shown in Figure $1.$ Water quality standards for the mountain stream receiving the wastewater treatment plant effluent require a minimum DO concentration of $5\mathrm{.}0m\frac{g}{l}.$

$[45$ points$]$ Determine the minimum downstream DO concentration $(m\frac{g}{l}),$ and the distance $($mi$)$ below the wastewater outfall where it occurs, if the current level of treatment technology is maintained for the expanded plant. Comment on compliance with the water quality standard.

$[70$ points$]$ Determine the maximum allowable $BO{D}_U$ in the wastewater treatment plant effluent, with implementation of improved treatment technology, to achieve compliance with the $5\mathrm{.}0m\frac{g}{l}DO$ water quality standard. Conduct a waste load allocation by developing a spreadsheet water quality model for performing the necessary trial$-$and$-$error calculations, plotting a family of DO sag curves $($at least six$)$ for various $L_W$ values $(m\frac{g}{l}$BODU $)$ including the $L_W$ value that yields a minimum stream DO concentration at$,$ or slightly above $5\mathrm{.}000$ mg$/1$ DO$.$

$[15$ points$]$ Determine the additional BODu removal $(\%)$ required to meet the water quality standard.

$[40$ points$]$ The required improved treatment can be accomplished by adding primary sedimentation basins before the existing, secondary biological treatment system. For a design overflow rate, $V_O=800ga\frac{l}{?}$ day $\frac{?}{f{t}^2},$ determine the diameter, $D(ft),$ of an appropriate number of identical cylindrical, primary sedimentation basins to handle the design flow rate. If primary influent contains $205m\frac{g}{1}TSS,$ and the primary treatment system achieves $52\mathrm{.}5$ percent TSS removal yielding a $2\mathrm{.}80$ percent underflow sludge concentration, determine the daily volume $($gal$/$day$)$ and dry solids mass flow rate $($lb$/$day$)$ of primary sludge requiring further processing and disposal. Be sure to begin with the fundamental differential forms of the water balance and mass balance equations, showing appropriate assumptions.

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