Toxicity of ionic liquid cations and anions towards activated sewage sludge organisms from different sources – Consequences for biodegradation testing and wastewater treatment plant operation

• We systematically investigated the influence of ILs on activated sewage sludge communities.
• ILs substituted with short alkyl chains and polar anions showed a low inhibitory potential.
• For highly hydrophobic ionic cations and anions low IC50s were found (<10 mg L−1).
• A reduced performance of domestic WWTPs is unlikely.

Ionic liquids (ILs) have attracted great interest in academia and industry during the last decade. So far, several ILs have been used in technological processes, from small scale to industrial applications, which makes it more and more likely that they will be released into the environment. Researchers have been actively studying the environmental and toxicological behaviour of ILs, but their influence on the activated sludge communities of wastewater treatment plants have yet to be investigated. This study aims to fill this knowledge gap by systematically investigating the influence of ILs on activated sewage sludge communities. We tested the inhibition of activated sludge respiration (according to OECD guideline 209) by a selection of 19 different compounds covering the chemical space of ILs as comprehensively as possible. To elicit the differences in sensitivities/tolerances towards ILs we investigated activated sludge from different domestic and industrial sources. Generally speaking, the structure activity relationships of IL toxicity towards activated sludge are in good agreement with those found for other organisms and test systems. The inhibitory potential of tested ILs substituted with short alkyl chains (≤4) and polar anions was low. On the other hand, the toxic effects of highly hydrophobic ionic cations and anions were greater – IC50 values were low, some < 50 μM (<10 mg L−1). We were able to demonstrate that the EC50 values from Vibrio fischeri can be used for a reliable assessment of the sludge inhibition potential of tested ILs. All the results are discussed in the context of their consequences for biodegradation processes and the performance of wastewater treatment plants.

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