Mathematical Modeling and Design of Fiber-Packed Bioreactors in Plants for Advanced Treatment of Domestic Wastewater

Abstract

Processes of advanced treatment of domestic wastewater in fiber-packed bioreactors were studied. It was shown that, at the first stage of the operation of the bioreactors, such processes as adhesion, sorption, diffusion, decomposition, oxidation, etc. occur; and at the second stage, the biomass dies, and, together with mineral particles, is detached from the surface of Nylon 6 threads of the fiber packing, and is carried out of the bioreactors under the action of the hydrodynamic forces of the liquid flow. A mathematical model was constructed to describe the processes of the mass transfer between the wastewater flow and the surface of the biofilm formed on the threads of the fiber packing with taking into account the balance of the contaminant concentrations in the wastewater in the course of its downward movement. It was determined that, in designing the bioreactor, the distance between the axes of the threads should be chosen such that, on the one hand, the water velocity between the threads does not exceed a certain critical velocity, at which microorganisms attached to fibers are washed away, and on the other, the water velocity is sufficient for the rest of the biomass detached from the fibers to be carried out of the bioreactor. It was proposed to consider that the velocity of the water being treated in the bioreactor is in the range 1–5 m/h. It was studied how the degree of wastewater treatment in a plant depends on the design and process parameters of the bioreactor, namely, the contents of organic contaminants of wastewater, the length of threads of the fiber packing, the water velocity between the threads, the duration of the downward movement of wastewater in the bioreactor, and the biosorption parameter. For example, the biosorption parameter at a diameter of threads of the fiber packing of 2 mm and a distance between their axes of 10 mm is nine times higher than the biosorption parameter at values of the respective parameters of 1 and 20 mm, respectively. A procedure of engineering design of a bioreactor was developed, which enables one to determine the characteristics of the main design elements ensuring the standard quality of the treated water.