Three bioreactor-based domestic sewage treatment plants (capacities 1.8-12 m(3)/day) were evaluated in 2004-2006 for the efficiency of water purifn. under summer and winter conditions. In particular, the BOD5 COD, total N and phosphates were detd. in the raw and treated sewage. The plants were more efficient in summer than in winter. The treatment resulted in an av. BOD5 decrease by 84.3-90.0% in summer and by 70.9-81.9% in winter. The resp. values for COD, total N and phosphates were 79.2-86.1 and 58.2-78.6%, 47.7-78.6% and 28.4-52.0% and 44.3-72.5% and 21.0-48.%, resp.