Question $4$

a$.$ Explain the difference between bulking and rising sludge in the clarifier of a biological wastewater

treatment plant and suggest measures for controlling them.

$[4$ marks$]$

b$.$ A municipal wastewater with a flow rate of $15000\frac{m^3}{d}$ and a soluble $BO{D}_5$ of $150m\frac{g}{L}$ is to be

treated using a conventional activated$-$sludge process in a completely stirred tank reactor. Existing

regulations require that the effluent $BO{D}_5$ be limited to $20m\frac{g}{L}.$ It is expected that the effluent will

contain $10m\frac{g}{L}$ of suspended solids, $65\%$ of which is biodegradable. The tank reactor volume is

$1800m^3$ and the return sludge concentration is $4000m\frac{g}{L}$ as VSS$.$

i$.$ Sketch the process, showing the influent line, the return line, the aerator and the secondary

clarifier, for a system in which wasting is occurring along the return line. $[2$ marks$]$

ii$.$ If the wastewater treatment plant is to just meet the regulatory requirements, estimate the

effluent soluble $BO{D}_5.$

$[2$ marks$]$

iii. Determine the mixed liquor volatile suspended solids $($MLVSS$)$ concentration to be maintained

in the aerator.

$[6$ marks$]$

iv$.$ Estimate the daily sludge wastage from the return line to maintain the sludge age.

$[6$ marks$]$