Search

In the membrane-applied water treatment process, residual manganese often generates a significant problem because scaling of oxidized manganese can lead to irreversible membrane fouling. For the membrane process to operate effectively, advanced treatment process should be considered for manganese removal. In this study, an adsorption process utilizing a newly developed magnetite-coated adsorbent was adopted to overcome manganese fouling in ultrafiltration (UF) applications, particularly during chemically enhanced backwashing. Typical manganese-coated sand (i.e., Ferox) was also evaluated simultaneously for comparison. These adsorbents were assessed for basic performance in a fixed bed adsorption column test under various operating conditions. With the addition of chlorine dosing, the efficiency of manganese removal was improved dramatically. Specifically, magnetite column was effective to prevent flux decline in typical UF applications even with an empty bed contact times of 1 min, while severe fouling was observed with no such pretreatment, suggesting it as a promising technology for the control of fouling caused by manganese.

AbstractThis study investigated pressure-assisted osmosis (PAFO), which uses the external pressure together with osmotic gradient across a membrane, to improve flux through forward osmosis (FO) membranes. Experiments were performed in a laboratory-scale PAFO system, which allows the application of external pressure up to 13 bar on the feed solution side. Deionized water (D.I. water) and synthetic seawater (35,000 mg/L NaCl) were used as feed solutions, and MgCl2was used as a draw solution. Humic acid was used as a model foulant to examine the characteristics of membrane fouling. A theoretical model based on osmotic transport theory incorporating internal/external concentration polarization, and mass balance equations were used to analyze the performance of FO and PAFO system. Results indicated that the addition of external pressure to the osmotic pressure allowed higher flux in PAFO than FO. Nevertheless, the flux in PAFO was less than the sum of the flux in RO and that in FO, which attributed to the increase in the internal concentration polarization by the external pressure. Under the test conditions in this study, fouling by sodium alginate and humic acid was negligible in FO, RO, and PAFO.

ABSTRACTTransport and deposition characteristics of natural organic matter (NOM) are systematically investigated using flow field-flow fractionation (Fl-FFF) at various chemical and physical conditions. Humic acid (HA) was chosen as model organic foulants. Prior to Fl-FFF analysis, HA was fractionated by membranes with different molecular weight cut-offs. To elucidate physicochemical factors affecting the deposition and transport characteristics of organic foulants, various concentrations of NaCl (i.e. up to seawater level) and CaCl2were employed as carrier solutions in Fl-FFF. Each fractionated NOM showed different transport and deposition characteristics with respect to the chemical and physical conditions employed during Fl-FFF analysis. When the total dissolved solids (TDS) concentration increased, there was more significant variation in the retention time for large NOM fractions compared with small NOM fractions. This means that the transport and deposition tendency of the larger NOM fractions varied more significantly with the alteration of ionic strength in Fl-FFF channel than the smaller ones. However, the smaller NOM fractions showed more considerable variation in retention time with increasing cross-flow intensity (i.e. flow perpendicular to channel flow in Fl-FFF) in Fl-FFF channel. This also means that the variation of physical factor could affect the transport and deposition tendency of the smaller ones more influentially. Results also elucidated that the retention time and area of elution peak of fractionated NOM were directly related to the amount of organic foulants attached to the membrane in Fl-FFF channel. It has been demonstrated that the deposition tendency of organic foulants increased at the higher TDS concentration, calcium concentration, and cross-flow intensity. This has been quantitatively determined using fouling index, Qf, derived from the data obtained from Fl-FFF. Based on this study, it is implied that Fl-FFF can be a useful tool to characterize the transport and deposition behavior of organic foulants in the solid–water interface and optimize pretreatment options for reducing membrane fouling.

Feasibility of osmotic backwashing for cleaning fouled membranes during forward osmosis (FO) process was investigated focusing on the mechanisms and factors involved. Alginate and humic acids were used as model organic foulants; and colloidal silica particles with different sizes were used as model inorganic particulate foulants. Results showed that noticeable flux recovery was achieved by osmotic backwashing through the instantaneous replacement of the draw solution with the dilute solution that has much less osmotic pressure than that of the feed solution. The switch of water flow direction through the membrane from feed-to-draw to draw-to-feed allows the effective detachment of foulants from the membrane surface. It was found that the efficiency of osmotic backwashing was affected by several factors including foulant type, membrane orientation and backwashing conditions (i.e. initial flux and duration). In addition, concentration polarization was found to play an important role in determining fouling behaviour, and thus, the osmotic backwashing efficiency.

The chemical heterogeneity of reverse osmosis membrane surface and its impacts on membrane fouling were investigated using dynamic hysteresis, which is a newly developed surface analytical technique. Based on dynamic hysteresis measurements, it has been demonstrated that the chemical heterogeneity of membrane surface was greatly influenced by solution—pH and ionic strength. Significant variation of dynamic hysteresis was observed as solution pH changed, implying the alteration of membrane surface heterogeneity. Interestingly, there existed the interplay between chemical and physical surface heterogeneity with respect to solution ionic strength. At low ionic strength, dynamic hysteresis mostly reflected chemical surface heterogeneity, while physical surface heterogeneity played more dominant role in the change of dynamic hysteresis with increasing ionic strength. This implies that membrane fouling due to chemical surface heterogeneity of the membrane is less remarkable in seawater desalination compared to wastewater and brackish water treatments. In addition, mechanisms and factors affecting chemical and physical surface heterogeneity and their interplay with respect to solution chemistry including pH, ionic strength and divalent cation concentration are discussed and elucidated.

Physical aspects of dynamic hysteresis for characterizing reverse osmosis (RO) membranes have been investigated. Dynamic hysteresis was used as a parameter of showing physical surface characteristics of RO membranes. Automated microbalance was utilized to determine dynamic hysteresis based on the Wilhelmy plate method. Dynamic hysteresis determined with non-polar liquid was related to physical surface characteristics including surface roughness and heterogeneity determined by atomic force microscopy imaging and analysis. A remarkable correlation between dynamic hysteresis and surface heterogeneity was obtained when nonpolar liquid was used during the measurements. Dynamic hysteresis increased as the surface heterogeneity of RO membrane increased.