Integrated treatment of brackish groundwater

As freshwater resources become more limited, Australian coastal cities have begun building seawater desalination plants, and inland communities have begun investigating the option of treating brackish groundwater to supplement their water supply. Membrane reverse osmosis (RO) is the leading technology applied in municipal desalination. Despite the advances in technology, membrane scaling is a common problem causing membrane failure, decline in membrane flux and deterioration of product water quality. Since inland plants cannot dispose of RO concentrate into the ocean, they operate at high water recovery in order to minimize the volume of RO concentrate. Antiscalants (AS) are often added during RO pretreatment to prevent membrane scaling. Water recovery percentages (Rw) are then limited by AS efficacy and yet large volumes of RO concentrate are frequently disposed of in evaporation ponds. Therefore, it is important to find novel technologies to combat scaling issues. The integration of a ‘High-pH pretreatment’ in inland desalination plants is a promising choice for facilitating the removal of scale-forming precursors and other contaminants negatively affecting the desalination process. In a comprehensive project, this study investigated the efficacy of ‘High-pH pretreatment’ for membrane scale control and the removal of specific pollutants such as boron. The first phase of the project highlighted the differences between inland and seawater desalination and critically reviewed the existing strategies for RO concentrate minimization towards zero liquid discharge (ZLD) in inland desalination. In contrast to previous studies, the groundwater and RO concentrate collected for these experiments had a magnesium concentration higher than the calcium concentration. Furthermore, no previous studies evaluated the ‘High-pH pretreatment’ on magnesium-dominated water as this study does. The investigation continued further to assess the efficacy and utilization of two scale control technologies: acid/AS addition and ‘High-pH pretreatment’. Therefore, the second phase of this study evaluated ‘High-pH pretreatment’ of a RO concentrate followed by secondary RO to increase overall water Rw in an existing inland desalination system. The results showed that the lime and soda ash softening treatment followed by pH readjustment and AS addition, allowed the overall water Rw to increase from 80 to 97%. Experimental trials also confirmed CaCO3 and CaO recovery from the precipitated sludge through CO2 gas injection to selectively dissolve magnesium. This success provided a further opportunity to explore ‘High-pH pretreatment’ of RO concentrate followed by other advanced desalination technologies such as air-gap membrane distillation (AGMD). In the third phase of the study, two scale control strategies, ‘High-pH pretreatment’ and AS addition, for RO concentrate minimization were further investigated in a labscale AGMD system. The results indicated that the first option was more efficient in terms of preventing scale build up in the AGMD system. Following ‘High-pH pretreatment’, pH readjustment and AS addition, the use of AGMD minimized the existing RO concentrate with a TDS level of 10.8 g/L by a concentration factor of 3.2. In addition, the ‘High pH-pretreatment’, using lime and soda ash, facilitated the operation of the AGMD system at a higher temperature, thus permeate flux also increased. Boron can also be present in groundwater due to natural or anthropogenic sources. It can produce harmful effects on human health depending on both the frequency and extent of exposure. Boron removal is considered to be very complex. In fact, it is largely unclear whether softening pretreatments can enhance boron removal in groundwater desalination systems. Therefore, the final phase of this study investigated the feasibility of ‘High-pH pretreatment’ for boron removal from magnesiumdominated groundwater samples obtained from an existing inland desalination facility. Before commencing the experiments, the brackish groundwater was spiked with 5 mg/L of boron. The results revealed that the lime and soda ash softening treatment achieved 33% boron removal by sorption of hydroxyborate ions onto precipitated magnesium silicate. An additional 9% boron removal was achieved with magnesium chloride addition before the softening treatment, or by a secondary polishing treatment by means of adsorption with MgO. This solution can safely facilitate compliance with strict boron standards in inland desalination plants using RO or electrodialysis technology. This study evaluated the efficacy of integrating a ‘High-pH pretreatment’ in inland desalination plants treating magnesium-dominated groundwater. The novel approach overcame AS limitations and increased freshwater Rw in the inland desalination plant. It also enabled partial removal of other contaminants such as boron. Since groundwater quality is site-specific, selection and optimization of the most suitable treatment for every single process must be based on raw water characteristics