Study on concentrating primary reverse osmosis retentate by direct contact membrane distillation

The direct contact membrane distillation (DCMD) applied for the concentration of the primary reverse osmosis (PRO) retentate was presented in the paper. All the experiments were carried out using the self-made hydrophobic polyvinylidene fluoride (PVDF) membranes. The DCMD process (permeate flux and conductivity) and membrane fouling phenomenon were investigated. Three kinds of PRO retentate (direct PRO retentate, silica bearing PRO retentate, and iron and manganese bearing PRO retentate) were used in the experiments. During the whole period of investigation, the membranes were found to be thermally stable and good separation characteristics. The RO was fed with the tap water with the recovery of 50%. After DCMD process, the whole water recovery can be significantly enhanced to 98.8%. Membrane fouling was observed during the process. Membrane clogging caused by the formed deposit (CaCO3, CaSO4, and silicate) was the main reason of the membrane efficiency reduction. It was found that CaCO3 formed at first during the process, and can be alleviated even eliminated by acidification. When the PRO retentate was concentrated a high level, CaSO4 formed and caused a sharp decline of module efficiency. During the DCMD process of silica bearing PRO retentate, silica may co-precipitate with soluble metals to form silicate at alkaline solutions while colloid silica may form at acidic solutions. The influence of iron and manganese is not significant.

The results of the present work have suggested that the PRO retentate (with the recovery 50%) can be concentrated about 40 times by direct contact membrane distillation, and then the whole recovery has been significantly increased to 98.8%. During the whole period of investigation, the PVDF membranes were found to be thermally stable and showed good separation characteristics. Membrane fouling was investigated during the DCMD process. Feed flow decrease caused by the deposit clogging was the main reason for the reduction of module efficiency. A flow rate decrease would cause an unfavorable increase of the temperature polarization and lead to a reduction of the module efficiency. CaSO4 would lead to a sharper module efficiency decline than CaCO3 for their different structure and properties. Silica fouling was strongly influenced by the solution pH. Silica may co-precipitate with soluble metals (Ca2+ and Mg2+) to form silicate at alkaline solutions while colloid silica formed at acidic solutions. The influence of iron and manganese is not significant.