Dewatering of Short-Fibre Digestates from Paper Recycling Mills: Liquid Fraction and Mass Distribution Profiles

The paper sector is characterised by high freshwater consumption and a strong need for improved resource efficiency. In this context, industrial digestates derived from short-fibre residues in paper recycling mills represent a promising substrate for water recovery within a circular economy framework. This study investigated the dewatering of short-fibre digestates as a pre-treatment for downstream membrane processes, aiming to maximise the liquid fraction (LF) recovery while minimising dry matter (DM) content. Seven scenarios were studied: sedimentation (S0); pre-sedimentation with chemical addition using iron(III) chloride (FeCl3) + polydiallyldimethylammonium chloride (polyDADMAC) (S1), FeCl3 + starch (S2), Nanofloc® (S3), and polyDADMAC (S4); and direct dewatering without pre-sedimentation using polyDADMAC with cloth filtration (S5) and centrifugation (S6). With reference to the sedimentation supernatant, S4 achieved the highest DM separation efficiency of 76%, followed by S1 (64%), whereas S2 and S3 were below 40%. However, LF recovery relative to the initial digestate was limited in scenarios S1–S4 to 17% (170 g/kgdigestate), with DM concentrations of 2.0–4.8 g/kgLF. In contrast, direct dewatering increased LF recovery substantially, with centrifugation (S6) achieving up to 690 gLF/kgdigestate and cloth filtration (S5) 420 g/kgdigestate, while maintaining a low DM (1.7 g/kgLF). Chemical oxygen demand (COD) and phosphorus (Ptot) were largely separated from the liquid fractions in all the scenarios. Nitrogen (Ntot) and ammonium (NH4-N) in the LF remained more variable, ranging from 22 to 153 and 5 to 22 mg/kgdigestate, respectively. These results indicate that centrifugation with polyDADMAC is the most effective approach, suggesting that mechanical force with a chemical additive can be used for the efficient dewatering of short-fibre digestates.