$($d$)$ A common septic tark is to be designed to reduce suspended solids and BOD from wastewater of

three adjoining houses, each with average oceupancy of seven users. Prior studies have revealed

average per capita water consumption of $100$ litres$/$day$,$ with $75\%$ turning into wastewater. The

sludge accumulation rate is $0\mathrm{.}04\frac{m^3}{?}$ person. year. Soll in this location was found to be $$itty loam with

a long$-$tern infiltration rate of $20\frac{U}{m^2}$ day

$($i$)$ Explain the processes through which suspended solids and BOD are reduced in a septic tank.

$[2$ marks$]$

$($ii$)$ Explain why a septic tank rectangular in plan is more effective in treating wastewater thnn a

square tank of similar capacity?

$[2$ marks$)$

$($iii$)$ Design a suitable septic tank for the households and show the dimensions on a two$-$chambered

sketch.

$($iv$)$ Design a scak away pit for the disposal of septic tank effluent.

$[4$ marks$)$