Limits of membrane distillation for concentrating brines: how high can MD go?
Sprache des Vortragstitels:
Engineering with Membranes Conference 2019
Sprache des Tagungstitel:
Innovative methods aiming to reduce the quantity of wastewater and to separate dissolved salts from RO reject brines can improve sustainability, and spare costs. Nevertheless, the fundamental insufficiency of this method is distillate flux reduction due to the long-term scale disposition on brine -side of the membrane surface. We investigated this deficiency in this work by measuring the system performance, water recovery, and scale formation, of a lab- scale vacuum membrane distillation (VMD). We test various low salinity feeds, a NaCl simplified saline feed and synthetic brine discharged from a RO unit. The tests employed hot brine at 75°C and transmembrane pressure at 30 mbar. The synthetic brine was either untreated RO brine or treated synthetic RO brine with an antiscalant. The results revealed that average permeate flux and permeate electrical conductivity ranged between 1.59 to 3.98 kg/m2/h and 2.0 to 44.8?S/cm for different feed models. The highest permeate flux with the lowest permeate electrical conductivity was achieved for the system with low feed salinity. The application of VMD resulted in water recovery of up to 91%, corresponding to the brine concentration factor of up to 11.1. Nevertheless, the long-term operations caused the permeate flux reductions of 49% and 50% for high retentate concentrations corresponding to high recovery ratio of 91% and 88% for untreated and treated synthetic seawater RO brine feeds, respectively. Moreover, for high retentate concentrations, scaling occurred on the membrane surface; nonetheless, large areas of membrane remained free of visible scaling. In all the conditions, scaling was reversible, and membrane surface scaling could be washed simply with water.