$1.$ If a filter is backwashed at $\backslash (36\backslash$mathrm$\{$~m$\}/\backslash$mathrm$\{$h$\}\backslash )$ at $\backslash (20^\{\backslash$circ$\}\backslash$mathrm$\{$C$\}\backslash ),$ what is the minimum size of the sand grain in the filter medium that is required to avoid being washed away?

$2.$ A deep bed granular filter media is made of $0\mathrm{.}5$ m of sand stacked on top of $1$ m of anthracite. ES of anthracite media and sand media is $1$ mm and $1\mathrm{.}2$ mm $,$ respectively. Calculate the clean bed head loss for a filtration rate of $\backslash (10\backslash$mathrm$\{$~m$\}/\backslash$mathrm$\{$h$\}\backslash )$ at temperature of $\backslash (20^\{\backslash$circ$\}\backslash$mathrm$\{$C$\}\backslash ).($hint: calculate the head loss separately for each filter bed and add them$)$