The osmotic action of highly concentrated organic waste water through soil was investigated by a column experiment. In this experiment, 50cm high sand-filled columns were used. The experiment included to estimate the toxic effects of mercury on the biological degradation of highly concentrated organic waste water. By adding seed sludge to the top of the sand column, 90% of the TOC was removed. The TOC was removed within the first 20-30cm of the sand layer. Though the removal rate of T-N was low, the T-P removal rate was over 86%. The effect of mercury on the removal of TOC, T-N, and T-P was not marked in either case. Both the acidogenic activity and the methanogenic activity were measured, and they showed higher values in the upper layer compared with the lower layer. The values of these activities were low in the column experiment with mercury. (C) 1997 IAWQ. Published by Elsevier Science Ltd.