Removal of hardness from RO concentrate of paper mill effluents with NF membrane for water reuse

Nanofiltration (NF) membrane was used to eliminate hardness (Ca²⁺ and Mg²⁺) from reverse osmosis (RO) concentrate of paper mill effluents in order to realize the on-site water reuse. It has been found that cross-flow velocity had significant effect on the membrane separation efficiency. Total hardness in the RO concentrate was reduced from 1,047 to 433 mg/L and the removal ratio of hardness was as high as 84% in the concentration mode of filtration experimented in the lab. The NF permeate was collected as recovered water fulfilling the reuse standard on hardness, sulfates, chemical oxygen demands, total dissolved solid, etc. In another 114 h test carried out at the factory, the water flux of the membrane was kept constant as 15 L/m²/h and the rejection of hardness maintained as 76%. Volumetric charge density (X_d) of the membrane was intensified with increasing concentration of CaCl₂ from 1 to 10 mol/m³ in solution. It helps to clarify the enhanced desalination efficiency (in particular the scaling ions) by the membrane at higher concentration of feed. In a conclusion, NF membrane process has been investigated and proven as an effective water treatment approach following RO process to improve industrial water recovery without utilizing additional chemicals.