The objective of this work was to study the sulphide, especially), the dihydrogen sulphide formation process in selvage pressure pipes (aid the influence of sulphur rich wastewater on oxygen consumption of activated sludge of Kohtla-Jarve wastewater treatment plant (WWTP). In this work the equilibrium distribution between three different forms of sulphide was investigated by changing pH of the wastewater and Na2S solutions in the closed vessel. At the same time the content of H2S in the gas phase was measured. To estimate the influence of H2S on the aerobic microorganisms of activated sludge, the ISO 8192 standard test of inhibition of oxygen consumption by activated sludge has been used. At the same time, other inflows of Kohtla-Jarve WWTP have been investigated. The leachate from semicoke and ash heaps, rich in sulphates and sulphides (pH 10-12) did not cause any significant inhibition of oxygen consumption of activated sludge microorganisms. The wastewater front oil shale chemical industry containing sulphates and sulphides (pH around 6-6.5), and also gross inflow of Kohtla-Jarve WWTP, cause inhibition of oxygen consumption of activated sludge.