Sewage sludge incineration in a fluidized bed is considered to be one of the most suitable ways of sewage sludge disposal. This process reduces the volume of the waste and causes the destruction of organic contaminants such as POPs, pharmaceuticals, and other compounds with endocrine-disrupting potential. Oxygen-enriched air combustion and oxy-fuel combustion can increase the combustion efficiency, reduce the amount of flue gas, and make possible CO2 capture more effective. However, the influence of incineration medium composition has not yet been thoroughly investigated in the case of sewage sludge incineration. In this paper, the incineration of sewage sludge in a bubbling fluidized bed reactor was studied at oxygen-enriched air conditions, oxy-fuel conditions, and oxy-fuel conditions with zero and nonzero concentrations of steam, CO, NO, N2O, and SO2 in the inlet combustion medium. Consequently, the effects of various operating parameters on pollutants formation were comprehensively described with emphasis on aforementioned sewage sludge incineration processes. An increase in combustion temperature resulted in an increase in NOx and SO2 emissions and in a decrease in N2O emissions. Increase in inlet oxygen concentration led to a decrease in NOx and N2O emissions. N2O and SO2 emissions were higher in CO2-rich atmosphere (oxy-fuel combustion conditions). The presence of water vapor in the inlet combustion medium resulted mainly in the reduction of NOx emissions. The presence of CO, NO, N2O, and SO2 in the dry inlet combustion medium reduced mainly overall nitrogen-to-NOx conversion, while the effect on SO2 removal efficiency was only marginal.