Composting can be defined as a method of biological utilization of sewage sludges resulting in changes of their properties and chemical composition. This process turns them into a fertilizer product safe for the environment. Sewage was composted together with varying proportions of straw, sawdust and a mixture of bark and sawdust, under conditions of an outdoor composting plant. The composting process lasted 110 days, which was followed by another 255 days of compost heap maturation. The addition of organic wastes did not induce a decrease in the concentration of organic carbon in mature composts, but favored increases in the quantities of total nitrogen. In addition, the share of non-hydrolyzing nitrogen in humus compounds also increased.