YAB$4243$

YAB$4243$

$2.$ A schematic system to demonstrate a complete$-$mixed activated sludge process is shown in FIGURE Q$2.$

FIGURE Q$2$: Schematic diagram of complete$-$mixed activated sludge process.

A wastewater stream containing biochemical oxygen demand $(BO{D}_5)$ of $200m\frac{g}{L}$ is to be treated via an activated sludge process. The wastewater stream also has a flow rate of $3000\frac{m^3}{d}$ in addition to the following characteristics for aeration tank and secondary clarifier in order to achieve the desired treatment efficiency during the commencement of treatment system.

Aeration Tank:

Food$/$Microorganism $($F$/$M$)$ ratio $=0\mathrm{.}25-5\mathrm{.}00$kgBO$\frac{D_5}{?}$ MLVSS$.$d

Mean Cell Residence Time $($MCRT$)=5-10d$

Hydraulic retention time $=5-7h$

Mixed liquor suspended solids $($MLSS$)$ concentration $=4000-6000m\frac{g}{L}$ with $75\%$ consisted of volatile matter

Recirculation ratio $=0\mathrm{.}4-1\mathrm{.}0$

Required effluent $BO{D}_5=20-25m\frac{g}{L}$

Excess sludge wastage rate $=0\mathrm{.}5-0\mathrm{.}8k\frac{g}{k}gBO{D}_5$ removed

Secondary Clarifier:

Hydraulic loading $=35-50\frac{m^3}{m^2}.d$

Depth of clarifier $=3-6m$

Solids loading $=<mn>1</mn><mn>5</mn><mn>0</mn><mi>k</mi><mi>g</mi>$ VSS$/$m ${?}^2.d$

NOTE: All the ASSUMPTIONS made must be explicitly stipulated along the designing steps.

a$.$ Design a suitable volume of aeration tank by considering the F$/$M ratio.

$[10$ marks$]$

b$.$ Evaluate an appropriate $BO{D}_5$ removal efficiency.

$[10$ marks$]$

c$.$ Identify the MCRT value that fits the aeration tank.

$[10$ marks$]$

d$.$ Predict the diameter of secondary clarifier as calculated from recirculation ratio.

$[10$ marks$]$

e$.$ Formulate a suitable volume of secondary clarifier that satisfies the depth required.

$[10$ marks$]$

END OF PAPER $-$

$3$

$4$