Q$7.$

$($a$)$ A primary clarifier with a surface area overflow rate of $55\frac{m^3}{m^2}.$ day receives

a wastewater from an equivalent population of $45,000$ having a capita flow

rate of $550\frac{L}{d}$ and a per capita suspended solids $($SS$)$ of $32\frac{g}{d}.$

$($i$)$ Determine the clarifier diameter and depth based on a $2-$hour hydraulic

retention time.

$($ii$)$ Determine the volume of primary sludge produced daily if $75\%$ of the SS

are removed by the clarification process. Assume that the sludge contains

$1\mathrm{.}5\%$ SS$,$ the sludge specific gravity is $1\mathrm{.}005,$ and a water density of $1,000$

$k\frac{g}{m^3}.$

$(4$ marks$)$

$($b$)$ An activated sludge process is used to treat a combined wastewater from two

separate sources of seafood processing and raw sugar refining, respectively.

The seafood processing produces a wastewater stream of $200\frac{m^3}{d}$ with $550$

$BO\frac{D_5}{L},$ and the sugar refinery produces another wastewater stream of $320$

$\frac{m^3}{d}$ with $650BO\frac{D_5}{L}.$ The activated sludge reactor is operating at an organic

loading rate of $4$kgBOD$\frac{5}{m^3}*d.$

$($i$)$ Calculate the volume of the reactor and the hydraulic retention time

based on the combined wastewater flow and $BO{D}_5$ concentration.

$(6$ marks$)$

$($ii$)$ For a Food$/$Microorganism ratio of $0\mathrm{.}6$kgBO$\frac{D_5}{k}g$ MLVSS$.$d$,$ what MLVSS

concentration should be maintained in the aeration tank?

$(3$ marks$)$

$($iii$)$ Estimate the mean cell residence time, assuming an effluent volatile solids

concentration of $20m\frac{g}{L}$ and the daily amount of waste sludge is $95m^3$

that contains $1\%$ of volatile solids.

$(3$ marks$)$