Four biolog. activated carbons were used for treatment of Miedwie lake water (optionally polluted with humic substances) before and after preliminary ozonation (O-3 dosis 1.7 mg/mg of org. C, contact time 3 min) to prep. drinking water. The pollution with humic substances (addn. of water from swamp) resulted in an increase in the org. C content from 9 to 21 mg/L and in colour intensity from 22 to 71 mg Pt/L. The purified water was also analyzed for COD and UV absorbance at 254 nm. The biofiltration resulted in a decrease in total org. C by 63% and COD by 62%. No remarkable decrease in pH was obsd.