MBBR (Moving Bed Biofilm Reactor) technology in many respects is a flexible alternative to the traditional method of wastewater treatment, which is an activated sludge method. One of the key features of this solution is economy of the construction site, construction costs and the operation of the system. The benefits of this technology can be increased by improving parameters of the most important elements of the system, which are biofilm carriers. A multitude of carrier parameters and their impact on the growth of microorganisms, and also the effectiveness of treatment, allows for optimization of solutions, which means such a construction of a carrier, suiting the conditions in which the carriers will be used. Therefore, mathematical modelling for example using software COMSOL (R), can facilitate the selection of the optimum geometry of the carrier so that it was characterized by a sufficient capacity for mass transfer and transport oxygen,