To investigate the effectiveness of a rotating biological contactor (RBC) and a two-stage membrane bioreactor (MBR) for the treatment of coke wastewater, samples were collected three times (Batches I, II, III) from the “Jadwiga” coke plant in Zabrze, Poland at two-week intervals. The wastewater was then diluted with tap water (1:3 ratio, wastewater : tap water) and then treated at retention times of 4.1 days (RBC) and 7 days (MBR). For phytotoxicity and genotoxicity tests, the wastewater was sampled from various points in the treatment systems and further diluted to produce a range of concentrations. In the phytotoxicity tests (growth inhibition), Lemna minor and Vicia faba were used. A low concentration of wastewater (6.25%) often stimulated growth. Higher concentrations, however, inhibited L. minor growth completely. These tests indicated that the MBR generally reduced growth inhibition more effectively than the RBC. In the genotoxicity tests (chromosome aberrations and micronuclei formation) root meristem cells of V. faba were examined. The genotoxicity of the different batches varied, and neither system was particularly effective for reducing genotoxicity. The results of this study indicate that, because its composition is so variable, coke wastewater should be constantly monitored. Also, because of its potentially high genotoxicity, the ecotoxicological characteristics of coke wastewater should be monitored in addition to basic indicators of wastewater quality, such as COD, BOD, and content of nitrogen compounds.