The development of zero-waste biorefineries is a promising yet challenging pathway toward a sustainable bioeconomy. Deep insights on the composition of biorefineries side streams along with the development of green technologies are mandatory to achieve such a goal. In this Article, a sustainable pilot-scale membrane process was used to recover valuable organic matter dissolved in the main side stream of a thermomechanical pulping (TMP) plant. A great number of analytical tools were combined to get an extensive characterization of the generated fraction, including inductively coupled plasma atomic emission spectroscopy (ICP-AES), high-performance ion chromatography coupled with pulse amperometric detection (HPIC-PAD), Fourier transform infrared (FTIR), size-exclusion chromatography (SEC), and C-13 and 2D heteronuclear single quantum correlation (HSQC) NMR. The results revealed the presence of a considerable amount of native lignin-carbohydrate complexes (LCCs) in the fraction representing 40% of the dry weight. The LCCs are composed of partially acetylated galactoglucomannans (GGMs) covalently linked to low condensed and ) beta-O-4-rich lignin through phenyl glycosidic bonds (25.5 bonds/100 aromatic rings (ar)) and characterized by a relatively low molecular weight around 7 kDa. Such complexes present an interesting potential for valorization with unique properties resulting from the combination of their aromatic and sugar moieties. Different applications are discussed in regards to the generated data in this work as an industrial sourcing of LCC is reported.