The paper presents a mathematical description of biological wastewater treatment processes, whereas the study develops more reliable methods for calculating the parameters of aerobic wastewater treatment from organic contaminants in aerotanks. A general mathematical model has been constructed for the evaluation and analysis of the joint removal of organic pollutants (OPs) by weighed and fixed biocenoses in plugflow aerotanks. This model takes into account the provision of the oxidation process with the required amount of oxygen and the peculiarities of the joint removal of organic pollutants by a biofilm, which is formed on the surface of additional loading, and a suspended biocenosis. Some simplifications of the model, which help obtain analytical dependencies, are suggested and proved. It is assumed that the process of OP oxidation with suspended active sludge occurs in a zero-order reaction, and in a biofilm, it occurs by a first-order reaction. The implementation of the proposed model helps estimate the influence of various purification factors in plugflow aerotanks and substantiate the most economical and effective parameters of OP removal in these structures. The study considers the possible technological and design schemes for plugflow aerotanks when a reactor containing elements with a fixed biocenosis is located first in the direction of the flow of sewage and vice versa. It is proposed to use such technological solutions to intensify the work of biological treatment plants such as aerotanks.