Two different coal mine-impacted water (MIW) treatments (biological via biostimulation of sulfate-reducing bacteria (SRB), and electrocoagulation (elC)) were proposed, reaching efficiencies of up to 99.79% in relation to SO42-, Fe, Mn, and Al ions, as well as acidity removals. Thus, toxicological assays with duckweed Landoltia punctata were performed, in order to verify the safeness and usability of the two treated waters. Therefore, duckweeds were exposed to different dilutions (0, 25, 50, 75, and 100% of samples) of the two treated waters, and the growth (r) and inhibition of growth (G) rates were calculated, based on 50% effect concentration (EC50). The water from the biological treatment (microcosm assay) presented the highest toxicity (EC50 = 33.42%), even higher when compared to the raw MIW (EC50 = 42.78%), probably due to the hydrogen sulfide, that even after a purge removal, remained in solution. The results showed that this water, despite being within the standards in physicochemical terms, demonstrated risks in terms of toxicity. The water from electrocoagulation (elC) treatment, in the opposite way, showed much less toxicity, even lower than the control, and therefore not reaching EC50, also suggesting a possible nutrient function of the treated water. Consequently, the treated water by elC could, for example, have a non-potable use. The study made it possible to prove the efficiency of elC treatment, the importance of post-treatment toxicological assessments, and the potential of the duckweeds as an option for a test organism in these types of evaluations.