Silica fouling in coal seam gas water reverse osmosis desalination

Abstract

i AbstractSilica precipitation and silica scale formation on membrane surfaces in RO desalination present a significant operational challenge in water purification. Silica scale formation on the membrane surface leads to flux decline, RO system productivity lost, membrane degradation, and increased cost of chemicals. In spite of the apparent simplicity of silica's composition and scale formation on the membrane surface, fundamental questions and empirical knowledge persist about the formation, solubility and behaviour of silica species during the silica polymerisation, precipitation and scale formation.The broad objective of the present study was to define conditions and factors affecting silica polymerisation and precipitation in coal seam gas (CSG) waters in Australia. The scope of the problem was narrowed to focus on dissolved silica species studied by 29 Si NMR spectroscopy, removal of silica by coagulation as pre-treatment step for RO desalination and silica fouling patterns in RO desalination for a range of salinities in both synthetic and CSG waters to develop a conceptual model of silica precipitation and deposition on the membrane surface.The CSG industry in Australia generates significant quantities of CSG water, especially during the first 3-5 years of reservoir development when the hydraulic pressure needs to be released to extract the gas from coal seam. To avoid high cost of brine treatment and residual disposal frequently requires high recovery RO desalination to treat CSG water to level acceptable for further re-use. Furthermore, CSG waters in Australia have all four critical parameters, which potentially lead to silica precipitation prior it reached the solubility limit. These parameters include medium to high salinity, medium silica concentrations, high alkalinity at pH9 and slightly elevated aluminium concentrations.Practical field, theoretical and laboratory research works have been undertaken to study industry's concerns and bring fundamental and practical solutions to the problem. In Sodium ions depress silica solubility at the same time preventing silica from deposition on the membrane surface. The majority of dissolved silica polymerised in the bulk solution and was discharged in the reject stream of the RO system. To conclude practical silica solubility or the solubility defined empirically is a key for prevention of silica polymerisation in RO desalination systems.iii

Aqueous Silica and Silica Polymerisation

“…In this chapter, the past and recent studies on various aqueous silica species and its impact are discussed. Behaviours of aqueous silica species were studies using reverse osmosis (RO) desalination systems and 29 Si NMR techniques and coagulation to gain better understanding of aqueous silica species polymerisation and practical implications of these techniques [1][2][3][4][5][6].…”

“…Practical silica solubility or the solubility defined empirically is a key for prevention of silica polymerisation in RO desalination systems [1][2][3][4][5]7]. Soluble or dissolved (reactive) silica contains different forms of silica or silica species ( Figure 1); monomer, dimers, trimmers and other polymers of silicic acid in different solutions [5,7,8,12].…”

“…Practical silica solubility or the solubility defined empirically is a key for prevention of silica polymerisation in RO desalination systems [1][2][3][4][5]7]. Soluble or dissolved (reactive) silica contains different forms of silica or silica species ( Figure 1); monomer, dimers, trimmers and other polymers of silicic acid in different solutions [5,7,8,12]. These dissolved silica species (Q 0 < Q 1 < Q 2 < Q 3 aggregation) (Figure 1) can be presented in various ionisation states which depend on the pH of solutions and silica concentrations and presence of other anions and cations [1,2,5,8,[10][11][12][13][14][15][16].…”

“…Soluble or dissolved (reactive) silica contains different forms of silica or silica species ( Figure 1); monomer, dimers, trimmers and other polymers of silicic acid in different solutions [5,7,8,12]. These dissolved silica species (Q 0 < Q 1 < Q 2 < Q 3 aggregation) (Figure 1) can be presented in various ionisation states which depend on the pH of solutions and silica concentrations and presence of other anions and cations [1,2,5,8,[10][11][12][13][14][15][16]. Chemical reactions between these silica species and cations and anions often present in the Concentration Polarisation (CP) layer in super-saturation conditions during the reverse osmosis chemical separation of water molecules are commonly lead to irreversible silica scale formation on the membrane surface [17][18][19][20].…”

“…Chemical reactions between these silica species and cations and anions often present in the Concentration Polarisation (CP) layer in super-saturation conditions during the reverse osmosis chemical separation of water molecules are commonly lead to irreversible silica scale formation on the membrane surface [17][18][19][20]. Contrarily, not all highly super-saturated silica streams lead to scaling of membrane surfaces as it was shown in the past research [1,5,9].…”