PURIFICATION OF PAPER MILL WASTEWATER WITH MEMBRANE FILTRATION PROCESS FOR WATER REUSE IN A CLOSED LOOP

In most Chinese pulp and paper mills, wastewater is fully discharged. after the purification cleaning plant, without any water reuse. In addition, it is well known that the paper industry has many problems in the purification cycle to reach the environmental regulations, which are always more restrictive in China. Membrane filtration processes, including microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO), can all be used in paper industry depending on the needs and objectives. In this contribution, the wastewater of a tissue paper mill, employing wood pulp as raw materials, was purified with membrane filtration technologies with the aim to reuse the purified water in a closed loop in the paper manufacturing process. The performances of membrane filtration processes such as MF, UF and RO were used. In order to optimize the purification process, different types of membranes and filtration conditions, including molecular weight cut-off, trans -membrane pressure (TMP), flow rate and volume concentrating ratio (VCR), were tested to increase membrane productivity and to assure a chemical purification suitable for water recycling. The best filtration performances were obtained with a polyvinylidene fluoride (PVDF) MF membrane with a pore size of 0.13 mu m, which assured a stable productivity of 210 L/m(2).h. The MF permeate, having a silt density index (SDI) less than 2, was treated in RO with high NaCl rejection. The RO permeate, obtained at 20 bar, showed a very low conductivity of 20.31 mu S/cm, COD less than 11.0 mg/L and BOD of trace amount. The MF process with a PVDF membrane, followed by a RO filtration step on the MF permeate, allowed the recovery and reuse of more than 80% of the original wastewater.