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The aim of this study was to evaluate and compare the toxicity of spent metalworking fluids (MWFs) and two different effluents obtained by biologic and electrochemical treatment of spent MWFs toward aquatic organisms of different trophic levels. The obtained toxicity data was used to calculate safe concentrations of both effluents. The spent MWFs without treatment showed the highest toxicity among the tested samples and should be classified as "extremely toxic" (toward invertebrates) or "very toxic" (toward other test organisms). Both methods applied for MWFs treatment resulted in a significant decrease in toxicity of the treated MWFs, but the obtained effluents had still to be regarded as "toxic". The ranges of the values of acute toxic units for both effluents were significantly narrower than for the untreated spent MWFs. The values of the safe concentrations of the spent MWFs, the biologic effluent, and the water phase resulting from electrochemical emulsion breaking were equal to 0.013%, l.8%, and 1.3%, respectively, corresponding to chemical oxygen demand concentrations of 1.8%, 32, and 34 mg O(2)*L(-1). These values are far lower than the discharging limit of 125 mg O(2)*L(-1) for industrial effluents, according to the relevant Polish regulations. It is therefore recommended to include toxicity bioassay parameters into guidelines for wastewater discharges to surface waters because the existing chemical and physical parameters are not sufficient to describe the environmental impact of industrial wastewater. Toxicity bioassays can be a promising tool for evaluating the efficacy of unit operations in industrial wastewater treatment (toxicity reduction evaluation) as well as identification of toxic substances in effluents (toxicity identification evaluation).

A laboratory scale sequencing batch reactor (SBR), fed with synthetic wastewater containing a mixture of organic compounds, was operated for nearly six months. Despite maintaining the same operational conditions, a deterioration of enhanced biological phosphorus removal (EBPR) occurred after 40 days of SBR operation. The Prel/Cupt ratio decreased from 0.28 to 0.06 P-mol C-mol−1, and C requirements increased from 11 to 32 mg C h−1 g−1 of mixed liquor suspended solids. A FISH analysis showed that the percentage of Accumulibacter in an overall community of polyphosphate accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs) dropped from 93% to 13%. An increase in abundance of Gammaproteobacteria (from 2.6% to 22%) and Alphaproteobacteria (from 1.8% to 10%) was observed. The number of Competibacter increased from 0.5% to nearly 9%. Clusters 1 and 2 of Defluviicoccus-related GAOs, not detected before deterioration, constituted 35% and 27% of Alphaproteobacteria, respectively. We concluded that lab-scale experiments should not be extended implicitly to full-scale EBPR systems because some bacterial groups are detected mainlyin lab-scale reactors. Well-defined, lab-scale operational conditions reduce the number of ecological niches available to bacteria.