The objective of this work was to test the impact of coagulant and flocculant at the stage of mechanical wastewater treatment on the wastewater treatment plant operation, performed in A2O process. In this paper the principles of correct conduct of coagulation in wastewater treatment have been discussed. It appears from the research performed that significant elimination of BOD5 such as 20÷30%, total suspended matter up to 90%, COD up to 50 % and total nitrogen at 30 % level was achieved supported by coagulation process in the pre-settling tanks. Approximately 50 % of phosphorus was eliminated after the mechanical part. Additionally, significant impact of Superfloc flocculant on effectiveness of separation of the solid phase (activated sludge) in the secondary settling tank was noted under diversified flow conditions.