Biofiltration is water treatment technique that uses microorganisms in order to intake pollutants from water. To increase biofiltration efficiency, solid substrates are used in biofilters. The substrates give the microorganisms a place to attach and grow up. Startup time of the biofilter can be up to 1 month, during which the water is aimlessly discharged into a sewage system. Therefore acceleration of microorganism adhesion is required. One of factors influencing adhesion is substrate surface charge that was modified by UV radiation in this research (LC6 Hamamatsu lamp). Intensity of UV light was 5 mW/cm2 and exposure time up to 90 minutes. Two types of polypropylene Bioflow 9 Media substrates were used – uncoated and with iron-containing coating. Photoelectron emission current from the substrates was registered before and after UV irradiation and photoelectron work function was calculated in order to evaluate changes of surface charge. Yeast cells were used in adhesion experiments. Adhesion was evaluated by measuring optical absorbance of yeast suspension at 600 nm. It was found that before the irradiation, yeast adhesion to the substrate with iron-containing coating was higher than to the non-coated substrate. UV irradiation decreased work function of the non-coated substrate and increased work function of the substrate with the coating. This showed that surface charge of both substrates was influenced by UV. After 90 min of UV irradiation, yeast adhesion improved only to the non-coated substrate and did not change to the substrate with the coating. Microorganism adhesion to the uncoated substrate exposed to 90 minutes of UV was found to be equally effective as the adhesion to the non-irradiated coated substrate. The research was funded by the grant of Latvian Council of Science No. 491/2012 “Smart Biofiltration Technology”.