$1-1.$ Describe everything you know about the planning standards for sewers, catchment sewers, and wastewater treatment facilities, including primary settling tanks and aeration tanks, when establishing a combined sewer system plan.

$1-2.$ Present the material balance equation for a first$-$order dispersed flow reactor and use it to derive the expression for C$/$Ci in a plug flow reactor under steady$-$state conditions.

$1-3.$ A certain pollutant in the sewage is decomposing according to a first$-$order rate constant k $(=10$ hr$^-1).$ Calculate the necessary hydraulic retention times $($HRTs$)$ to achieve a $50\%$ removal rate in a single CFSTR$,$ a series of three completely mixed reactors connected in series, and a single PFR$.$