## 1 Introduction
QACs can be measured spectrophotometrically using anionic dyes or chromogenic reagents [21; 22]. While quick and simple, this technique can be influenced by the presence of anionic surfactants as QACs have a higher affinity for them than the dyes [23]. Moreover, this technique is unable to identify individual QAC structures, limiting its widespread use [13]. High-performance liquid chromatography (HPLC) is promising for screening BAC, but this technique is limited by its inability to analyze non-chromatic surfactants, as they are unable to absorb ultra-violet (UV) [13; 24]. Gas chromatography/mass spectrometry (GC/MS) has been used for qualitative determination of BAC in river water and sewage effluent [24; 25]; however, a complex pre-treatment is required [13; 25]. Nuclear magnetic resonance (NMR) spectroscopy is a powerful non-targeted analytical tool used to obtain high-resolution molecular-level data relating to the makeup of complex mixtures of organic compounds with minimal sample preparation [26; 27; 28]. Compared to other conventional analytical tools, including HPLC, GC, and MS, NMR is able to unambiguously identify multiple organic structures in a complex sample while si multaneously providing the means to quantify those compounds through comparison with a common internal standard of known concentration. Compared to traditional approaches based on HPLC or mass spectrometry, an approach based on NMR is able to provide a non-biased, non-targeted and fully quantitative approach that has the potential to quickly and simultaneously monitor VFA development, additional metabolomic responses due to chemical stresses imposed from BAC, and the BAC itself without the need for individual standards. Considerable efforts are being undertaken to develop spectroscopic methods for monitoring and quantifying key parameters involved in wastewater treatment processes, including volatile fatty acid production, BOD\({}_{5}\), COD, and TOC [9]; however, the use of NMR, specifically in the field of bioprocess control, is relatively unexplored in comparison to other spectroscopic techniques [6]. This study is designed to explore the changes in the microbial metabolomic profile of an anaerobic system as part of efforts to improve our understanding of the effects QACs may have on bioreactor systems. This is performed by preparing microcosm bottles with sludge from a functioning anaerobic washwater digestor used to treat washwater produced during potato production. Here, we expose the microcosms to different levels of QAC, and compare changes in the metabolic signal to changes in the production of CO\({}_{2}\) and CH\({}_{4}\). Metabolomics is emerging as a powerful tool to study sub-lethal responses to toxic substances before typical endpoints can be observed. Concentrations of QAC used in this study are chosen to elicit a wide range of responses in the anaerobic metabolome rather than to specifically identify the inhibitory limits of QAC exposure. The overall goal of this research is to further develop the use of NMR spectroscopy as a tool to provide both a qualitative and quantitative analysis of the metabolic footprint generated from the interaction between BAC and the bioreactor samples and by doing so be able to recognize deviations away from normal operating procedures.