This paper discusses the results of laboratory analyses of the coagulation and flocculation of model wastewater. The investigated wastewater was susceptible to treatment by chemical coagulation. The effectiveness of two commercial coagulants, PAC produced at the DEMPOL-ECO Chemical Plant and PIX manufactured by KEMIPOL, was compared. A mathematical model relying on a second-degree polynomial was used to describe and analyze experimental data. In each case, the parabola minimum point was a precisely determined coagulant dose, regarded as the optimal dose. The application of a coagulant dose higher than the optimal dose reduced the effectiveness of wastewater treatment by coagulation. A detailed analysis of turbidity, suspended solids, total phosphorus and pollutant removal measured by the COD test revealed that PAC was a more effective and a more efficient coagulant than PIX. The risk of coagulant overdosing was greater with the use of PAC than PIX.