Novel graphene nanoplatelets-coated polyethylene membrane for the treatment of reject brine by pilot-scale direct contact membrane distillation: An optimization study

In this research study, a direct contact membrane distillation (DCMD) system was used in the concentration of raw brine from a full-scale reverse osmosis desalination plant for further zero-liquid discharge (ZLD) applications. Several operating conditions such as feed inlet temperature (T_f), feed (Q_f) and permeate (Q_p) flow rate, and feed concentration (C_f) were tested to obtain the optimum permeate flux across the membrane, salt rejection and specific thermal energy consumption (STEC). The highest flux of 16.7 LMH, salt rejection of 99.5% and STEC of 152 kWh/m³ were achieved at T_f, Q_f, Q_p, and C_f of 85 °C, 75 L/h, 48 L/h and 57,500 ppm, respectively. Furthermore, this study was carried out for a long term operation of 77 h in which the membrane was found to be prone to significant fouling thereby resulting in a flux decline across the membrane of 69%. To mitigate fouling, novel membranes were prepared through the surface coating of commercial membranes with graphene nanoplatelets (GNPs) at 0.08, 0.16 and 0.2 wt%. Over a 10 h operation, membranes coated with 0.08 and 0.16 wt% GNP reduced membrane fouling by 59 and 78%, respectively.