Gypsum scaling during reverse osmosis desalination − characterization and effects of natural organic matter

Reverse osmosis (RO) desalination of brackish water and seawater has been increasingly applied to augment water supplies in regions where freshwater is scarce or polluted. However, a serious limitation during RO desalination at high water recovery is membrane scaling due to the concentration and subsequent crystallization of sparingly soluble salts in the feed water stream (bulk crystallization) or directly on the membrane surface (surface crystallization). To date, interactions between RO membrane scaling and natural organic matter (NOM) at concentrations commonly found in natural waters (few mg C∙l) have been rarely investigated. The objective of this thesis is the systematic characterization of scaling by gypsum (CaSO4∙2H2O), representing one of the most problematic scale-forming salts in natural waters, and its interactions with different NOM sources using laboratory cross-flow RO desalination and stirred beaker crystallization experiments. During RO desalination, the attained degree of supersaturation, the severity of concentration polarization and the hydraulic residence time determined the dominating scaling mechanism (bulk vs. surface crystallization). Complementary stirred beaker crystallization experiments demonstrated that induction and crystallization times of bulk crystallization are shorter at higher supersaturation. Investigations into the development and reproducibility of surface crystallization on RO membrane samples by real-time membrane surface imaging during desalination show that the availability and distribution of heteronucleation sites on the membrane surface are important parameters that affect the surface scaling behavior. Selected NOM sources (alginate, bovine serum albumin, two humic acid sources of terrestrial origin and two aquatic NOM sources) prolonged induction and crystallization times of bulk crystallization in stirred beaker crystallization experiments. Retardation was consistently pronounced at higher NOM concentration. It is assumed that NOM adsorption onto nulcei and crystal faces was the relevant mechanism of retardation given that crystals were smaller and exhibited modified habits in the presence of NOM. Based on NOM characterization and previous studies, a combination rather than individual NOM properties are suggested to determine the NOM-specific retardation capability. Bulk crystallization during RO desalination was effectively retarded by NOM, however, strong fouling stimulated surface crystallization assumingly due to cake-enhanced concentration polarization and calcium ion attraction by the fouling layer. Effects of NOM on surface crystallization were assessed by RO desalination experiments which enabled real-time membrane surface imaging and which were designed to discriminate between the effects of NOM fouling and those of dissolved NOM. The results show that surface crystal growth is significantly inhibited and distorted due to the adsorption of dissolved NOM onto crystal faces. Retardation was enhanced at increased NOM concentration and particularly strong for large humic substances with molecular weights between 5 kDa − 150 kDa. While moderate degrees of NOM fouling can affect heteronucleation and crystal growth, those effects are irrelevant in the presence of dissolved NOM. Only in cases of strong fouling layer formation by specific NOM fractions (e.g. soil-extracted humic acids with molecular sizes greater than 150 kDa), loss of RO performance is aggravated due to accelerated crystal growth assumingly due to enhanced supersaturation by cake-enhanced concentration polarization. Although inhibition of gypsum scaling by a commercial phosphonic and polyacrylic acid-based antiscalant was considerably stronger, the results demonstrate that NOM is an effective natural antiscalant. The thesis contributes to the better understanding of membrane scaling by gypsum due to bulk and surface crystallization and its complex interactions with NOM. It suggests that NOMsupported scale inhibition can be considered as an alternative measure for scale control during membrane-based desalination of natural waters.