Recycled paper mill process water pre-treatment using ultrafiltration for water system closure

The use of membranes in integrated water systems has garnered increasing attention in paper mills operating with virgin fibres, with documented closed water loops in industry. However, limited research exists on the performance of ultrafiltration (UF) in mills operating with 100% recycled fibres (RCF) and closed water systems, as these result in the introduction and accumulation of insoluble contaminants in the effluent. This study investigated the effect of transmembrane pressure (TMP) and feed organic load on the flux, types of fouling, and organic separation with UF membranes in a recycled paper mill using 100% RCF. The high feed organic load caused inevitable fouling, limiting the highest achievable flux. However, the TMP could be manipulated to improve on or exacerbate the process performance, within the range that was set as a result of the feed's characteristics. Low TMP (1.35 bar) generated the highest permeability and chemical oxygen demand removal, while higher TMPs produced higher fluxes and recoveries. The process was characterised by high initial fluxes, followed by cake formation, which dominated, and pore blockage. The excess cake formation induced cake filtration. This study shows the potential of UF membranes to be used as pre-treatment in filtering the process water in a recycled paper mill operating with 100% RCF, a critical step forward in implementing an integrated water system, and eventually closing the water loop.