Reverse osmosis brine treatment using direct contact membrane distillation: Effects of feed temperature and velocity

Membrane distillation (MD) is a promising technology for reverse osmosis (RO) brine treatment due to its superb tolerance to high salinity. In this work, a hydrophobic PVDF membrane was applied to treat simulated RO brine under different feed temperatures and velocities with membrane flux, permeate conductivity and thermal performance monitored. The raw membrane was characterized with respect to contact angle, porosity, thickness, maximum pore sizes, liquid entry pressure and clean water flux. The fouled membrane was characterized by scanning electron microscope (SEM) coupled with an energy dispersive X-ray spectroscopy (EDX). The results showed that MD achieved excellent desalination performance with the permeate conductivity lower than 11 μS cm^{− 1} and recovery rate higher than 70% during the RO brine treatment. Almost no membrane scaling was observed in the initial stage, but significant scaling occurred due to overconcentration (concentration factor > 3.3) of RO brine, resulting in serious pore wetting and decreased thermal performance. Moreover, increasing feed velocity was helpful to alleviate the membrane scaling, but the desalination performance would be impaired to some extents with feed velocity exceeding 0.4 m s^{− 1}. The increasing feed temperature could significantly increase the membrane flux, but membrane scaling was accelerated resulting in a lower recovery.