Silica fouling during direct contact membrane distillation of coal seam gas brine with high sodium bicarbonate and low hardness

Abstract

The presence of silica in coal seam gas (CSG) reverse osmosis (RO) brine complicates the membrane distillation (MD) process due to silica polymerization and metal silicate formation at high water recovery. This study reported that direct contact MD (DCMD) of CSG RO brine at electrical conductivity (EC) of 17.8 mS/cm achieved 90% brine recovery (overall recovery of 95%) with distillate flux decline of 16%, and distillate EC of 15 μS/cm over 150 h of operation. A series of experiments were conducted using synthetic solutions to evaluate the effect of CSG RO brine water quality on silica fouling in DCMD process. Results showed that CaCO3 formation was fast and did not co-precipitate with dissolved silica, while magnesium and silica were co-precipitated as magnesium silicate compounds. Results also revealed that polymerized silica formed a non-porous layer on the membrane surface. This fouling layer not only significantly reduced distillate flux but also led to a dramatic increase of distillate EC. Further study on silica fouling mitigation suggested that rapid cooling and filtration/clarification could be adopted for removal of silica from MD brines containing high silica concentration. It also showed that disodium ethylenediaminetetraacetic acid (Na2EDTA) was effective to hinder the formation of CaCO3 and magnesium silicate in MD.