Process water and phenols' balances for the two processes of oil shale thermal treatment, Kiviter (in vertical retort) and Galoter (with solid heat carrier) were compiled Options of wastewater treatment in the Kiviter process were analyzed in more detail. Laboratory experiments of biological oxidation of the process water after the dephenolation stage without other effluents and municipal wastewater were carried out. Experiments indicated that the oil shale phenols are generally quite easily degradable when the need for phosphorus is covered by added reagents. The experiments indicated that, compared to conventional aerobic bio-oxidation, the combined process, namely aerobic bio-oxidation with ozonation in re-circulation system, enabled to increase the efficiency of purification at relatively low ozone dosages. Application of ozone at moderate doses (up to 30 mg/L) improved the rate of pollutant removal. Injection of ozone at small dose - 2 mg/L - into the activated sludge in the bioreactor increased the sludge activity: the specific oxygen uptake increased about 15-20%. The biologically treated wastewater can be used as some kind of technological water (cooling water) after chemical precipitation and reduction of sulphate content. It can also been discharged to nature, but only after additional treatment to reduce nitrogen content and after filtration to remove the residual suspended solids.