Purification of pulp and paper wastewater, with membrane technology, for water reuse in a closed loop

The wastewaters of an Italian paper mill (Lucart), employing recycled paper, news print and cardboard as raw materials, were purified with membrane technologies with the aim to reuse the purified water in a closed loop in the paper manufacturing process. The performances of membrane technologies such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) were tested in pilot plants. In order to optimise the purification process, different membranes and modules were tested to increase membrane productivity and to assure a chemical purification suitable for water recycling. The best filtration performances were obtained with a ceramic MF membrane having a cut-off of 0.14 μm, which assured a stable productivity of 150-200 L/m2h. On the contrary, spiral-wound polymeric modules, both of MF and UF, showed low productivity and high fouling indexes. The MF permeate, having an SDI less than 2, was treated in RO with a spiral-wound module, with high NaCl rejection. The RO permeate, obtained at 20 bar, showed very a low content of salts of 15 mg/L, conductivity of 70 μS/cm, COD less than 30 ppm of O2 and TOC of about 1 ppm. The MF process with a tubular ceramic membrane, followed by a RO filtration step on the MF permeate, allowed the recovery and reuse of more than the 80% of the original wastewater. This process was patented by ENEA-Lucart, deposit number RM2003A000603.