Annually there is a lot of food waste in the form of kieselguhr and brewers' spent grain at breweries in Russia. In the pelleted form they are in use as components of compound feeds. Brewers' spent grain is a valuable brewing by-product with a limited shelf life (up to 3 days due to its organic composition) and high moisture content (80–85%). After the expiration of the shelf life, brewers' spent grain turns sour with the release of ammonia and carbon dioxide. The paper presents the results of studies on the granulation of brewers' spent grain and kieselguhr by pressing in a rotary granulator with a flat die and channels of variable cross-section. The research presents a developed physical and mathematical model of the rolling process. Equations have been obtained for determining the forces acting in the channel of variable cross-section of the flat die. For the pressed mixtures, diagrams of force distribution along the channel length in the flat die have been constructed. It allows us to forecast the change in pressure in the channel depending on the powder morphology. A schematic diagram of the technological line for the production of a nutritive feed additive using a modernized design of a rotary granulator with a flat die is presented. The developed technology allows raw brewers' spent grain to be converted into granules to reduce the environmental pollution