The design of mixing technique is still empirical in biogas plants in spite of the large proportion of the electricity production costs. The effect of the flow pattern through the positioning of the agitators and consequently on the gas yield are presented in this article. To begin with, the rheology of various substrates of the biogas process is discussed. Precise measurements for organic waste, sewage sludge and corn silage can be made with a tube viscometer; these measurements will serve as the basis for further studies of the flow pattern. These studies demonstrate that the mixed volume depends on the positioning of the submersible mixers, as well as the substrate composition and their rheology.