Precipitation softening: a pretreatment process for seawater desalination

Abstract

Reduction of membrane fouling in reverse osmosis systems and elimination of scaling of heat transfer surfaces in thermal plants are a major challenge in the desalination of seawater. Precipitation softening has the potential of eliminating the major fouling and scaling species in seawater desalination plants, thus allowing thermal plants to operate at higher top brine temperatures and membrane plants to operate at a reduced risk of fouling, leading to lower desalinated water costs. This work evaluated the use of precipitation softening as a pretreatment step for seawater desalination. The effectiveness of the process in removing several scale-inducing materials such as calcium, magnesium, silica, and boron was investigated under variable conditions of temperature and pH. The treatment process was also applied to seawater spiked with other known fouling species such as iron and bacteria to determine the efficiency of removal. The results of this work show that precipitation softening at a pH of 11 leads to complete elimination of calcium, silica, and bacteria; to very high removal efficiencies of magnesium and iron (99.6 and 99.2 %, respectively); and to a reasonably good removal efficiency of boron (61 %).

Viscoelasticity of Quartz and Kaolin Slurries in Seawater: Importance of Magnesium Precipitates

“…The amount of sodium hydroxide used (g/L) to raise the pH of seawater, quartz pulp and kaolin pulp, normalised by the amount required to obtain pH 12.2, is shown in Figure 5. A marked buffer effect was observed for pure seawater and quartz slurry between pH 10 and 11, which coincided with the formation of magnesium precipitates, where carbonate precipitation started at pH 9.3 and ended at pH 10, while hydroxide deposition started at pH 10.3 and completed at pH 11 [39][40][41]. This effect was not clear in the kaolin pulp, indicating a lower formation of magnesium precipitates, which is compatible with the loss of magnesium in solution (from 1380 to 659 mg/L).…”

“…For that matter, the pulp began to flow depending on both the applied stress and the time of stress application (Figure 9). Otherwise, for pulps in which there were no solid precipitates, the yield stress was a precise value: quartz pH 8, 55 Pa; kaolin pH 8, 38 Pa; and kaolin pH 10.7, 42 Pa. An exceptional contrast of the system that held precipitates (quartz at pH 10.7) with respect to the rest of the slurries is that the yield stress was displayed in a range of stress (40)(41)(42)(43)(44)(45)(46) Pa) instead of a single point. For that matter, the pulp began to flow depending on both the applied stress and the time of stress application (Figure 9).…”

Performance of Reverse Osmosis Membranes in the Treatment of Flue-Gas Desulfurization (FGD) Wastewaters

“…The pH of the FGD wastewaters increased from 9.2 to 10.6 in the range of investigated Na 2 CO 3 •H 2 O/Ca 2+ molar ratio due to the increased release of OH − ions in the solution. Ayoub et al [20] investigated the removal of inorganic compounds (including calcium and magnesium) in seawater using Na 2 CO 3 as alkalizing agents and obtained similar results. In particular, the removal efficiencies recorded were of 90 ± 2.5% for Ca 2+ and 15 ± 4.55% for Mg 2+ .…”

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

“…(Note: Quality data to complete Figs. 1 and 2 represent worldwide characteristics and was collected from different books/technical papers [3,9,11,23,[31][32][33][34][35][36][37][38][39][40][41][42][43][44] and analytical records supplied by the Western Downs Regional Council (Australia) from different municipal groundwater wells. )…”