Removal of metal complexes by nanofiltration in a TCF pulp mill: technical and economic feasibility

The goal of this work is to remove complexed metals by nanofiltration (NF) from a chelate (Q) stage effluent in a totally chlorine-free (TCF) bleaching plant, to permit recycle of the permeate as make-up water to the same bleaching stage. There are two main industrial benefits: (i) decreased consumption of fresh water in the pulp mill and (ii) an important reduction in the amount of effluent discharged. Two composite aromatic polyamide membranes from different manufacturers and a ceramic membrane (titanium oxide supported on zirconium oxide) were tested on a pilot scale at the pulp mill to assess the viability of the nanofiltration process for the industrial application. The effluent was fed directly to the membrane installation at a temperature of 80 °C. The polymeric membranes exhibited better performance than the ceramic membrane in terms of rejection and fouling, but the permeation rates were comparable. Essentially complete rejection (99–100%) of iron and manganese was achieved for volume reduction factors up to 7. The recovery in the permeate stream is equivalent to 86% of the original wastewater. For a TCF plant with a capacity of 300,000 t air-dried pulp (AD)/year, the effluent that must be treated is ca. 300 m3/h. In order to achieve 90% recovery and produce a permeate stream free of metals, a nanofiltration plant with a membrane area of about 2400 m2 would be required. This plant implies a capital investment of 5.3 million € and a total cost (including capital and operating costs) of 6 €/t of pulp.