B$2(25$ marks$)$

An activated sludge system treating a wastewater flow of $\backslash (20,000\backslash$mathrm$\{$~m$\}^\{3\}/\backslash$mathrm$\{$d$\}\backslash )$ has the following primary effluent characteristics:

a$.\backslash ($ B O D$\_\{5\}=120\backslash$mathrm$\{$mg$\}/\backslash$mathrm$\{$L$\}\backslash )$

b$.\backslash (\backslash$quad T K N$=25\backslash$mathrm$\{$mg$\}/\backslash$mathrm$\{$L$\}\backslash )$

I. For a VSS yield of $0\mathrm{.}65$ kg VSS$/$ kg BODS, calculate the volume of waste activated sludge per day for a secondary clarifier underflow sludge concentration of $\backslash (8,000\backslash$mathrm$\{$mg$\}/\backslash$mathrm$\{$L$\}\backslash )(10$ marks$)$

II$.$ Calculate the volume of aeration tank required. Choose an SRT with an assumption that system is required to consistently nitrify. $(15$ marks$)$

B$3(25$ marks$)$

i$.$ For the problem B$2$ above, calculate the total oxygen and air requirement per day $(10$ marks$)$

ii$.$ With the help of a neat diagram, explain the operation of a single stage anaerobic digester. Discuss briefly the two key parameters that define the digester efficiency, and comment on the operating parameters that dictate the digester efficiency. $(15$marks$)$