The goal of our research was to show efficiency of the micro-sieves during the water treatment process in the water treatment plant (WTP) in Zawada near the city of Zielona Góra, Poland. This paper presents changes in water quality after particle-filtration (filtration above 10 μm).The samples were collected in the Zawada WTP from January to December. The reduction of the amount of phytoplankton, including cyanobacteria, caused by micro-sieves varied throughout 2009 and was between 21.2 and 93.3%. The maximum concentration of cyanobacteria in raw water was observed May 6 and amounted to about 400,000 org. (organisms)/ L and microcystins (MCs) content in raw water was below 0.2 μg/L and were absent in potable water. That day we observed a reduction of cyanobacteria after particle-filtration amounted to 83% and reduction of phytoplankton 84%. The particle-filtration process reduced organoleptic water parameters: color, turbidity, and total suspended solids. As a result of the decrease of suspend content, decreases of iron, manganese, and chemical oxygen demand (COD) were observed.