What design variable was used before SRT? Define this term (conceptually). Why did SRT replace this variable? When is specific growth rate higher: at low SRTs or at high SRTs? Solution c)The process loading factor (U) also called the Food to Microorganism(F/M) Ratio was used before SRT.It can be defined as the mass of substrate applied to the reactor per unit time divided by the mass of microbes in the reactor.Three major causes led to be the reason of replacement of F/R Ratio by SRT which are :- 1) Increased oxygen transfer efficiency 2) Improved Biomass Particle Size Distribution 3) Enhanced removal of many emerging contaminants such as pharmaceuticals,personal care products etc. d)The specific growth rate is higher at low SRT\'s because of a higher oxygen utilisation rate..