a. Selection of process The selection of specific unit processes depends on the nature of the sewage water , including degradability and treatability by selected processes plus the selection also depends on the discharge requirements . Other important factors for selecting any stp design are environmental impact , land availability , projected life of plant design and cost. All the relevant factors in process selection should be considered , although their relative importance will vary due to social , environmental , political differences as well as technological availability and expertise. b. Design of Biological reactor . The rate at which components of wastewater , such as organic matter are removed and the rate at which biomass is produced within a reactor are important criteria in the design and calculation of the size of biological reactors . c. Environmental Factors Affection growth. A number of environmental factors affect the activity of wastewater microbial populations and the rate of biochemical reaction. The factors include temperature , pH , Dissolved Oxygen , nutrient concentration and inhibition by toxic compounds. It is possible to control all these factors within a biological treatment system , except temperature .