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

A review of strategies for RO brine minimization in inland desalination plants

Water scarcity in many inland areas is increasing the demand for new groundwater desalination plants. Co-produced coal seam gas (CSG) water (or coal bed methane as known in the USA), which is mostly brackish, is extracted in huge quantities during CSG production and requires advanced treatment. Reverse osmosis (RO) is the leading technology applied in municipal desalination and for treating CSG water in Australia and in some locations in the USA. Antiscalants are often dosed during RO pretreatment to prevent membrane scaling. Recovery rates are limited by antiscalant efficacy and large volumes of brine are frequently disposed of in evaporation ponds. The search for environmentally friendly methods for RO brine minimization is considered as a key global issue. In this paper, differences between inland and seawater desalination are highlighted. The existing technologies for RO brine minimization and zero liquid discharge (ZLD) for inland desalination are reviewed. The efficacy and application of two scaling reduction technologies for RO brine minimization: (i) acid/antiscalant addition and (ii) ‘high pH precipitation treatment’ are compared. Finally, more complex ZLD and volume reduction systems, such as the high efficiency RO (HERO™) and the SAL-PROC™, are analyzed as well

The effect of ‘High-pH pretreatment’ on RO concentrate minimization in a groundwater desalination facility using batch air gap membrane distillation

This study compared the efficacy of two different scale control strategies: (i) ‘High-pH pretreatment’; and (ii) antiscalant (AS) addition for reverse osmosis (RO) concentrate minimization, in a lab-scale air gap membrane distillation (AGMD) unit. In contrast with previous studies that used a direct contact membrane distillation set-up, we systematically investigated the performance of batch AGMD configuration with and without the preliminary reduction of scale-forming constituents, using high salinity, high alkalinity, high SiO2 and high magnesium hardness RO concentrate. Results indicated that ‘High-pH pretreatment’ by means of the cold lime-soda ash process was more productive than only AS addition to the RO concentrate, resulting in significant salt precipitation when the concentration factor (CF) in the AGMD system increased above 1.3. The High-pH precipitation process provided significant concentration reduction of SiO2 (96%), magnesium (96%) and calcium (86%). Following ‘High-pH pretreatment’, pH re-adjustment and final AS addition, the use of AGMD allowed us to minimize the existing RO concentrate with an initial total dissolved solids (TDS) level of 10.8 g/L by a CF of 3.2. In addition, this chemical demineralization process enabled operation of the AGMD unit at a higher temperature, thus increasing permeate flux

‘High-pH softening pretreatment’ for boron removal in inland desalination systems

Boron removal from water remains a challenge. In fact, it is largely unclear whether softening pretreatments enhance boron removal in groundwater desalination systems. We therefore investigated the feasibility of a high-pH softening pretreatment for boron removal from magnesium dominated groundwater samples obtained from an existing desalination facility. Different alkaline reagents were trialled with brackish groundwater initially containing 5 mg/L of boron. The results indicated that the lime and soda ash softening treatment was a better option than the caustic soda alternative, achieving 33% boron removal by sorption of hydroxyborate ions onto precipitated magnesium silicates. The process could be further optimized by the addition of MgCl2·6H2O before the softening process. In addition, a secondary polishing treatment by means of adsorption with MgO was investigated. A total of 9% extra boron removal was achieved in both cases. This ‘high-pH softening pretreatment’ could enhance compliance with strict boron standards in inland facilities using reverse osmosis or electrodialysis technology

Research on ‘high-pH precipitation treatment’ for RO concentrate minimization and salt recovery in a municipal groundwater desalination facility

This study evaluated ‘high-pH precipitation treatment’ of a reverse osmosis (RO) concentrate followed by secondary RO treatment to increase overall water recovery in an existing inland desalination system. In contrast with previous studies that used calcium dominated surface waters, this study dealt with magnesium dominated groundwater. The high-pH process removed scale-forming precursors including magnesium, calcium, strontium, barium and SiO2 from primary RO concentrate by precipitation and adsorption/enmeshment. Lime and soda ash were determined to be the superior caustic agents when compared to sodium hydroxide for the high pH demineralization process. Following ‘high-pH precipitation treatment’, pH readjustment and antiscalant addition, the use of secondary RO enabled the overall water recovery to be increased from 80 to 97%. In addition, this study evaluated CaCO3 and CaO recovery from the precipitated sludge through CO2 gas injection to selectively dissolve magnesium

SWRO concentrates for more efficient wastewater reclamation

Developing technical alternatives to increase the volume of remediated waters is a promising way to alleviate pressure on natural water basins. However, the extra energy consumed in Wastewater Treatment Plants (WWTPs), mainly powered by fossil fuels, hampers this strategy. This work focuses on promoting efficient upgrading alternatives of treated waters by recovering energy within the treatment process. The approach consists on the recovery and integration of Salinity Gradient Energy (SGE) generated from the contact between SWRO brines and reclaimed wastewaters in reverse electrodialysis modules. The analysis of opportunity of implementing integrated processes in Spanish WWTPs near SWRO desalination plants is tackled. 16 SWRO-WWTP pairs have been inventoried, 10 of them located in Jucar and Segura river basins, hot spot areas for water reclamation. A gross power density of 0.46 W/m2 (71 Wh/m3 reclaimed water) of SGE has been generated in the contact between SWRO brines and reclaimed wastewater, increased up to 0.23 kWh/m3 in the most favourable scenario. Estimates of freshwater withdrawals savings up to 434,387 m3/day within the selected installations are obtained. Decrease in water abstraction and integration of renewable source of energy in the remediation process will contribute to water sources protection and water industry decarbonisation.This work was supported by the European LIFE programme [LIFE19 ENV/ES/000143] and the Spanish Ministry of Science and Innovation [PDC2021-120786-I00 and PLEC2021-007718]