## 1 Introduction  
Dissolved organic matter (DOM) consists of a complex mixture of derivatives of aromatic and aliphatic hydrocarbons, mainly humic substances, arising from soil and aquatic sources. Its composition is highly variable and depends on countless factors, such as its origin or agglomeration [1]. It has a remarkable influence on aqueous ecosystems, i.e., affecting the regulation of the nutrients' cycle or the microbial loop [2]. Within drinking water treatment plants (DWTPs), DOM monitoring is essential due to its many detrimental associated issues. For example, we can cite the generation of genotoxic and carcinogenic disinfection by-products (e.g., trihalomethanes or haloacetic acids) occurring after oxidation by the chlorination process [3], microorganism proliferation [4], or even membrane fouling enhancement [5], all of which result in an overall increase in water production costs. Despite the paramount importance of DOM analysis, the most employed methods are based on non-specific measurements, such as non-purgeable organic carbon (NPOC) or ultraviolet absorbance at 254 nm (UVA). However, these methods provide partial information on the behaviour of the organic matter along the process [6]. Therefore, implementingnew analytical techniques able to provide deeper information on the composition of DOM whilst also being sensitive, fast, and easy to use, are necessary. In this sense, fluorescence spectroscopy meets all these requirements. Excitation-emission matrices (EEMs) are three-dimensional plots of fluorescence intensity for each combination of excitation and emission wavelengths. The use of tridimensional data is an essential advantage of EEMs against UVA when studying samples with DOM from different sources, since the latter cannot differentiate between compounds with similar absorption spectra, while this might be possible if they differ in their fluorescence fingerprint [7]. Since typical fluorescence spectrometers could take between 10 and 40 min to acquire an EEM, the use of a single excitation wavelength is more practical for the real time monitoring. The most typical dimensionless values obtained from single emission spectra are the fluorescence index (FI), biological index (BIX), and humification index (HIX) [8; 9; 10]. Fortunately, due to the constant advancements in fluorescence spectrometers (as the replacement of the Xe lamp and monochromator for multiple single-wavelength LEDs as the excitation source [11]), the development of economic equipment for the instantaneous acquisition of EEMs is granted. This is important as FI, BIX, or HIX values have similar limitations as UVA, providing only background information compared to EEMs [12]. Traditionally, different types of DOM in an EEM were identified and quantified based on the visible peak-picking method [6; 13]. Multivariate analysis methods, such as PARAFAC (parallel factor analysis), allow for the deconvolution of overlapping signals within an EEM, one being able to measure the individual fluorescence of the different DOM fractions on a set of samples, as well as elucidating their fingerprints [6; 14; 15]. Due to these reasons, PARAFAC is the most employed algorithm to analyse fluorescent organic matter within water treatment plants; the information about cases of application can be find elsewhere [16; 17; 18]. Currently, EEM-PARAFAC is still gaining momentum with its application with DOM real-time continuous monitoring [19; 20; 21], combination with machine learning [22; 23], or even its detection of fluorescent emerging contaminants in wastewater effluents [24]. In this sense, our group has thoroughly explored, with success, the use of EEM-PARAFAC to visualise oxidative changes in fluoroquinolones (antibiotics), also being able to track the formation of fluorescent oxidation by-products and estimate its molecular structure [25; 26; 27]. Despite its gratifying progress, the real use of EEM-PARAFAC for DOM monitoring in DWTPs is still infrequent. Therefore, in this work, we have proposed to the facility that supplies drinking water to ca.