Sulfur compounds are found in the leachate resulting from the fermentation of excess sludge from sewage treatment plants. Most likely, these compounds have an effect on nitrogen reduction. Currently, the most popular way to manage leachate is to recycle it to the beginning of the mainstream wastewater treatment. This affects a small increase in flow rate within 1-2%. It also introduces a high nitrogen load, which can constitute 10-30% of the load supplied to the sewage treatment plant. Another method of leachate purification is their lateral purification, e.g. in SBR reactors, in which conventional processes can be carried out, i.e. nitrification, denitrification or increasingly implemented alternative methods based on the anammox process. One such process is deammonification, which combines the process of nitrification and anammox. The advantage of this method is high energy savings (up to 60%) associated with aeration costs, as well as no need to dispense an external carbon source. Preliminary microbiological tests showed the presence of sulfur bacteria in the sludge, which was used to purify the leachate in the deammonification process. Sulfur changes accompanying the removal of nitrogen from wastewater are of great importance both due to the inhibition of the anammox process with hydrogen sulfide, and also due to the autotrophic denitrification carried out by sulfur bacteria, which allows the reduction of nitrates remaining after the anammox process in the denitrification process. The research plans to measure the sulfide oxidation rate in aerobic processes - partial and full nitrification, and the sulfate reduction rate in anaerobic processes - anammox and denitrification. Based on the results obtained, the share of hydrogen sulfide will calculated in the abovementioned processes. The results of the research will enable the determination of sulfur changes in the deammonification process. Currently, this process causes the greatest interest among sewage treatment plant operators. In the long term, it is planned to adapt the sludge to the leachate in the deammonification process. These leachates will gradually increase the concentration of magnesium sulfate, which is one of the compounds in their composition. During the adaptation, a microbiological analysis determining the share of sulfur bacteria in the sludge is also expected.