KLD- Kilo litres per day or cubic meters per day PE- Population or Persons Equivalent (150 lts/ day) MLD- million liters per day PE- Population equivalent or Person Equivalent BOD- Biological/ biochemical oxygen demand in mg/day (the weight of oxygen required to remove the organic pollutant in one litre of water) COD- Chemical oxygen demand in mg/day (the weight of oxygen required to remove the chemical pollutant in one litre of water) HRT- Hydraulic retention time in hours (the time the incoming sewage is retained so that there is enough microbes in the system available with the supply of adequate air to remove pollutants as expected) MLSS- Mixed liquid suspended solids in ml/l or mg/l (the amount of micro organisms in one litre that will be adequate to remove the pollutants in a given period of time) FM ratio- The amount of food coming as BOD and COD to the amount of microbes available in the reactor SVI- Suldge Volume Index- It is the measure of settle-ability of activated sludge in the reactor. It is an indirect measure of MLSS and performance of the activated sludge process.