Question $2[30$ marks$]$

$($a$)$ Discuss three physical processes for the natural purification of water. $[10]$

$($b$)$ A sample of wastewater in a glass beaker had a dissolved oxygen $($DO$)$ concentration of $\backslash (10\backslash$mathrm$\{$mg$\}/\backslash$mathrm$\{$L$\}\backslash )$ after it had been aerated overnight at $\backslash (6^\{\backslash$circ$\}\backslash$mathrm$\{$C$\}\backslash ).$ The sample was then de$-$aerated using an oxygen$-$reducing chemical. An aeration device was placed in the beaker and the DO was monitored for $30$ minutes. The water temperature remained constant at $\backslash (6^\{\backslash$circ$\}\backslash$mathrm$\{$C$\}\backslash )$ during the experiment. The results are displayed in Table Q$2.$

Table Q$2$

i$.$ Calculate the re$-$aeration coefficient, $\backslash (\backslash$mathrm$\{$K$\}\_\{\backslash$mathrm$\{$La$\}\}\backslash ),$ of the aeration device.

ii$.$ Interpret the re$-$aeration coefficient value you have calculated in $($i$)$ above $($i$.$e$.,$ is it high or low?$)$

iii. Write a brief note on the factors that may potentially impact the re$-$aeration coefficient.