Performance analysis of the activated sludge model number 1 in a two reactor cascade

We provide theoretical insights into the activated sludge process by utilising the activated sludge model number 1 in a two reactor cascade with an ideal settling unit. The process configuration contains a settling unit and a recycle unit. We investigate how the removal of organic carbon and total nitrogen depend upon the hydraulic retention time as well as the process configuration, namely the impact of aeration, the presence of a settling unit, and recirculation of mixed liquor from the second reactor to the first reactor. As the hydraulic retention time increases from zero there are two critical values. Organic carbon can only be removed for values of the retention time higher than the first critical value. Removal of organic nitrogen occurs in conjunction with the removal of organic carbon. Heterotropic and autotrophic biomass are only viable for values of hydraulic retention larger than the first and second critical values, respectively. Inorganic nitrogen can only be significantly removed when autotrophic biomass are present. However, the removal of inorganic nitrogen is significantly influenced by the process configuration. • The performance of a reactor cascade is greatly affected whether the first reactor is aerated. • The performance is greatly improved through using a recycling unit from the settler unit to the first reactor. • The performance is not greatly affected through using an internal recycling unit from the second reactor back to the first reactor.