Zhang, Po (2019) Fouling and its control in membrane distillation for the treatment of reverse osmosis brine. PhD thesis, Victoria University.

Abstract

Membrane distillation (MD) technology is a promising technology for treating RO brine to enhance overall water recovery and extract valuable salts, thus approaching zeroliquid- discharge. The benefits of MD technology such as no hydraulic pressure restriction, compact design and small carbon footprint by utilising renewable energy or waste heat sources, have created exciting opportunity of integrating MD into desalination systems and ultimately delivering safer, cheaper, more efficient and sustainable desalination operations. However, the lack of understanding of MD membrane fouling and its control are the major obstacles that hamper this technology being realised by industry. The aim of the present study was to understand the behaviours of MD membrane fouling formation and fouling kinetics in RO brine treatment, and use the knowledge to propose feasible ways to manage fouling in MD industrial applications. The scope of this problem was narrowed to focus on three real RO brines, namely, seawater RO (SWRO) brine, coal seam gas (CSG) RO brine and copper barren RO brine. SWRO brine study was mainly focused on the inorganic scaling, such as CaCO3 and CaSO4. Silica fouling mechanisms were investigated during the study of CSG RO brine. In copper barren RO brine study, the impact of ammonia transfer on the precipitation of metal salts was evaluated. In addition, MD performance was assessed using a commercial oleophobic membrane. The experimental works were undertaken using a bench-scale direct contact membrane distillation (DCMD) apparatus which was specifically constructed for this study. The key findings from this study include: (1) for brine with the high potential for CaCO3 and CaSO4 precipitation, antiscalant prolonged MD operational time by mitigating salt precipitation. However, significant homogeneous nucleation of CaCO3 and CaSO4 occurred at a critical brine electrical conductivity (EC) level even in the presence of antiscalant. The rapid formation of crystal deposits was correlated to the pH variation in brine, where pH was an early indicator of rapid flux decline. Although distillate flushing was effective in removing inorganic scaling from membrane surface, organic fouling on the other hand, showed resistance to non-chemical flushing. (2) for brine containing dissolved silica, low hardness and high NaHCO3, Mg2+ demonstrated stronger bonding with dissolved silica as compared to Ca2+. The precipitation of magnesium silicates was strongly influenced by the concentration of Mg2+ and brine pH. HCO3 − / CO3 2− on the other hand, showed greater ability to stabilise silica in dissolved form compared with Cl−. In addition, polymeric silica fouling significantly affected MD performance, demonstrated by rapid flux decline and exponential increase of distillate EC. (3) MD achieved stable performance with negligible fouling over a relative long period of operation for concentrating brine with high dissolved ammonia and metal ions. At above neutral pH, dissolved ammonia reacted with metal ions and formed stable metal-ammonia complexes in soluble form which prevented membrane fouling from metal deposits. (4) antiscalant existed in all three brines did not demonstrate fouling potential in MD membrane in terms of membrane surface attachment nor potential for membrane wetting. Practical MD fouling management strategies were also evaluated in this study and the key recommendations include: (1) intermittent flushing with distillate water and raw RO brine is effective in removing inorganic salt nucleus from membrane surface. (2) inline cartridge filtration can assist the removal of salt crystals in bulk brine and thereby improve brine water recovery and MD operational time between cleaning events. (3) disodium ethylenediaminetetraacetic acid (Na2EDTA) treatment in brine can hinder the formation of Ca / Mg–silicates. However, the key is to avoid sudden temperature decline in MD process which could cause rapid formation of colloidal amorphous silica. (4) oleophobic membrane could be adopted to tackle the high organic containing brines. Overall, the feasibility of MD for concentrating RO brine was demonstrated in this study. The investigation of MD fouling management strategies, such as cartridge filtration, distillate / raw brine flushing, EDTA / antiscalant treatment and chemical cleaning showed practical implications of sustainable operation of MD process. This study also showed that drastic precipitation of CaCO3 / CaSO4 and polymerised silica have high tendency to cause membrane wetting. Therefore, it is important for industry to determine the critical brine EC level via bench scale / pilot study when concentrating RO brine that has high scaling potential of CaCO3 / CaSO4, and to avoid sudden temperature decline in MD brine recycling stream to mitigate the formation of colloidal amorphous silica.

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