title: Chemistry of silica scale mitigation for RO desalination with particular reference to remote operations.
abstract: Silica scaling in reverse osmosis of groundwater is a significant issue in water stressed areas due to the limitations that scaling imposes on water recovery. While calcium and magnesium scaling potential can be significantly reduced by the use of ion exchange or other softening processes, the silica scaling potential typically remains. Improving the recovery of reverse osmosis by limiting the potential for silica scale is important in ensuring maximum water recovery. This is particularly important for mining and natural gas industries that are located in remote regions. The remote nature of these sites imposes three major restrictions on the silica scale mitigation process. Firstly, the generation of poorly dewaterable sludges must be avoided. Also, the quality of any reverse osmosis (RO) permeate must be able to meet the end use requirements, particularly for boilers. Finally, silica removal should not impact upon other potentially useful or valuable components within the brine, and should not make the disposal of the unusable waste brine components more difficult. Reduction of scaling potential can be achieved in three main ways: operating RO at high pH after hardness has been removed, operating at low pH, and reducing the silica concentration either in pretreatment or by using an interstage technique. Operating at high pH has the initial requirement of hardness removal to prevent scaling and this could be an issue on some sites. Hardness removal operations that use ion exchange resins may be challenged by water chemistry and the operational costs associated with high chemical regeneration costs. Operating at low pH may be more desirable than high pH operation as this can help to reduce the risk of scale formation from calcium or magnesium salts. The drawback comes from the cost of acid, particularly for high-alkalinity waters. There are numerous silica removal techniques including chemical dosing of lime, or aluminium or iron salts, electrocoagulation, adsorption, ion exchange and seeded precipitation. Of these, adsorption onto aluminium compounds appears to give the best results and have received the most attention where restrictions on sludge production and brine disposal common to operations in remote locations are in place. Adsorption onto iron compounds appears to occur more quickly, but leads to the formation of a hard, glass-like scale that may be more difficult to remove, making this process unattractive from the point of view of sorbent regeneration.
authors.affiliation: Institute for Sustainability and Innovation, College of Engineering and Science, Victoria University, PO Box 14428, Melbourne, Vic 8001, Australia. Electronic address: Nicholas.Milne@vu.edu.au.
authors.forename: Nicholas A
authors.initials: NA
authors.lastname: Milne
authors.affiliation: Institute for Sustainability and Innovation, College of Engineering and Science, Victoria University, PO Box 14428, Melbourne, Vic 8001, Australia.
authors.forename: Tom
authors.initials: T
authors.lastname: O'Reilly
authors.affiliation: Institute for Sustainability and Innovation, College of Engineering and Science, Victoria University, PO Box 14428, Melbourne, Vic 8001, Australia.
authors.forename: Peter
authors.initials: P
authors.lastname: Sanciolo
authors.affiliation: Australia Pacific LNG, PO Box 148, Brisbane, Qld 4001, Australia.
authors.forename: Eddy
authors.initials: E
authors.lastname: Ostarcevic
authors.affiliation: Australia Pacific LNG, PO Box 148, Brisbane, Qld 4001, Australia.
authors.forename: Mark
authors.initials: M
authors.lastname: Beighton
authors.affiliation: Hatch Associates, GPO Box L923, Perth, WA 6842, Australia.
authors.forename: Kelvin
authors.initials: K
authors.lastname: Taylor
authors.affiliation: Hatch Associates, GPO Box L923, Perth, WA 6842, Australia.
authors.forename: Mark
authors.initials: M
authors.lastname: Mullett
authors.affiliation: Department of Civil Engineering, College of Engineering, University of Texas at El Paso, El Paso, TX 79968, USA.
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Tarquin
authors.affiliation: Institute for Sustainability and Innovation, College of Engineering and Science, Victoria University, PO Box 14428, Melbourne, Vic 8001, Australia.
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Gray
mesh_terms: D060587:Groundwater / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D009995:Osmosis; D054712:Salinity; D012822:Silicon Dioxide / Q000737:chemistry*; D014867:Water / Q000737:chemistry*; D018508:Water Purification / Q000379:methods*
chemical_list: D014867:Water; D012822:Silicon Dioxide
keywords: Adsorption; Brine management; Desalination; Scaling; Silica