Identification of surface chemical functional groups correlated to failure of reverse osmosis polymeric membranes

The goal of this study is to identify the causes of membrane failure observed during a 15-month operation of a low pressure reverse osmosis (RO) membrane pilot plant to treat a highly organic surface water from the Hillsborough River in Tampa, Florida, using various surface analytical techniques. Three different commercial RO membranes, made of cellulose acetate or polyamide, were used in this pilot study, and all of these membranes showed performance deterioration presumably due to membrane fouling and degradation at given experimental conditions. In order to elucidate the mechanisms of membrane failure, scanning electron microscopy with energy dispersive spectrometry (SEM/EDS), x-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) were performed on the surface of the polymeric RO membranes used. More specifically, molecular composition including surface functional groups were identified from XPS analysis, confirmed by FTIR, and correlated to membrane failure. In addition, surface morphology and fouling layer composition were determined by SEM/EDS. The results indicated that the cellulose acetate membrane was biologically damaged, while the polyamide membrane was compromised by chlorine oxidation. The biodegradation of cellulose acetate was evidenced by the presence of nitrogen on XPS and FTIR scans. Chlorine uptake shown in XPS and FTIR scans of used polyamide membranes was a good indicator of chemical degradation. This study demonstrated that XPS, combined with FTIR and SEM/EDS, is a valuable diagnostic tool for failure analysis of polymeric RO membranes and provides valuable information to aid the manufacturers in designing better membranes for reverse osmosis

Variations In Backwash Efficiency During Colloidal Filtration Of Hollow-Fiber Microfiltration Membranes

A series of filtration experiments was performed systematically to investigate physical and chemical factors affecting the efficiency of backwashing during microfiltration of colloidal suspensions. In this study, all experiments were conducted in dead-end filtration mode utilizing an outside-in, hollow-fiber module with a nominal pore size of 0.1 μm. Silica particles (mean diameter = 0.14 μm) were used as model colloids. Using a flux decline model based on the Happel\u27s cell for the hydraulic resistance of the particle layer, the cake structure was determined from experimental fouling data and then correlated to backwash efficiency. Modeling of experimental data revealed no noticeable changes in cake layer structure when feed particle concentration and operating pressure increased. Specifically, the packing density of the cake layer (1-cake porosity) in the cake layer ranged from 0.66 to 0.67, which corresponds well to random packing density. However, the particle packing density increased drastically with ionic strength. The results of backwashing experiments demonstrated that the efficiency of backwashing decreased significantly with increasing solution ionic strength, while backwash efficiency did not vary when particle concentration and operating pressure increased. This finding suggests that backwash efficiency is closely related to the structure of the cake layer formed during particle filtration. More densely packed cake layers were formed under high ionic strength, and consequently less flux was recovered per given backwash volume during backwashing. © 2005 Elsevier B.V. All rights reserved

Fouling Behavior Of A Pilot Scale Inside-Out Hollow Fiber Uf Membrane During Dead-End Filtration Of Tertiary Wastewater

A series of pilot-scale filtration experiments were performed systematically under various operating conditions to investigate the fouling behavior of ultrafiltration (UF) membranes to treat tertiary wastewater for resuse. All experiments were conducted using a pilot system, which consisted of six inside-out capillary polyether sulfone UF membrane modules (molecular-weight cutoff = 150,000 Da), arranged in parallel configuration. The pilot unit was operated in dead-end filtration mode and the membranes were frequently backwashed with chlorinated water. Results of this research clearly indicated that the productivity of the UF membranes, measured by the specific water flux (Kw), declined much faster as operating flux increased. This observation was attributed to enhanced solid and organic loading to the membrane surface at higher operating fluxes. Furthermore, the analysis of Kw variation against filtrate volume showed larger productivity reduction per foulant mass loading during operation at high flux rates, suggesting the formation of more compact cake layers which were not easily removed during backwashing. Pilot study results also demonstrated that increasing backwashing with chlorine addition significantly improved membrane productivity, primarily due to enhanced foulant removal by organic oxidation and biogrowth control. In addition, flux enhancement per backwashing volume increased with decreasing time between backwashing events. Ferric chloride pretreatment also markedly enhanced membrane productivity by increasing particle floc size, which led to decreased pore plugging, reduced cake layer resistance, and enhanced backwashing efficiency. © 2001 Elsevier Science B.V. All rights reserved

Combined Influence Of Membrane Surface Properties And Feed Water Qualities On Ro/Nf Mass Transfer, A Pilot Study

The impact of membrane surface characteristics and NOM on membrane performance has been investigated for varying pretreatment and membranes in a field study. Surface charge, hydrophobicity and roughness varied significantly among the four membranes used in the study. The membranes were tested in parallel following two different pretreatment processes, an enhanced Zenon ultrafiltration process (ZN) and a compact CSF process (Superpulsator (SP)) prior to RO membrane treatment for a total of eight integrated membrane systems. All membrane systems were exposed to the similar temperature, recovery and flux as well as chemical dosage. The membrane feed water qualities were statistically equivalent following ZN pretreatment and SP pretreatment except for NOM and SUVA. Membrane surface characteristics, NOM and SUVA measurements were used to describe mass transfer in a low-pressure RO integrated membrane system. Solute and water mass transfer coefficients (MTCs) were investigated for dependence on membrane surface properties and NOM mass loading. Inorganic MTCs were accurately described by a Gaussian distribution curve. Water productivity decreased with NOM loading and increased with contact angle and roughness. The negative effects of NOM loading on productivity were reduced as the negative charge on the membrane surface increased. Inorganic MTCs were also correlated to surface hydrophobicity and surface roughness. The permeability change of identical membranes was related to NOM loading, hydrophobicity and roughness. Organic fouling as measured by water, organic and inorganic mass transfer was less for membranes with higher hydrophilicity and roughness. © 2005 Elsevier Ltd. All rights reserved

Influence Of Membrane Surface Properties On Initial Rate Of Colloidal Fouling Of Reverse Osmosis And Nanofiltration Membranes

Recent studies have shown that membrane surface morphology and structure influence permeability, rejection, and colloidal fouling behavior of reverse osmosis (RO) and nanofiltration (NF) membranes. This investigation attempts to identify the most influential membrane properties governing colloidal fouling rate of RO/NF membranes. Four aromatic polyamide thin-film composite membranes were characterized for physical surface morphology, surface chemical properties, surface zeta potential, and specific surface chemical structure. Membrane fouling data obtained in a laboratory-scale crossflow filtration unit were correlated to the measured membrane surface properties. Results show that colloidal fouling of RO and NF membranes is nearly perfectly correlated with membrane surface roughness, regardless of physical and chemical operating conditions. It is further demonstrated that atomic force microscope (AFM) images of fouled membranes yield valuable insights into the mechanisms governing colloidal fouling. At the initial stages of fouling, AFM images clearly show that more particles are deposited on rough membranes than on smooth membranes. Particles preferentially accumulate in the \u27valleys\u27 of rough membranes, resulting in \u27valley clogging\u27 which causes more severe flux decline than in smooth membranes. Copyright © 2001 Elsevier Science B.V

Effect Of Surface Roughness On Fouling Of Ro And Nf Membranes During Filtration Of A High Organic Surficial Groundwater

Fouling characteristics of various thin film composite polyamide reverse osmosis (RO) and nanofiltration (NF) membranes were systematically investigated using a high organic surficial groundwater obtained from the City of Plantation, Florida. Prior to bench-scale fouling experiments, surface properties of the selected RO and NF membranes were carefully analysed in order to correlate the rate and extent of fouling to membrane surface characteristics, such as roughness, charge and hydrophobicity. More specifically, the surface roughness was characterized by atomic force microscopy, while the surface charge and hydrophobicity of the membranes were evaluated through zeta potential and contact angle measurements, respectively. The results indicated that membrane fouling became more severe with increasing surface roughness, as measured by the surface area difference, which accounts for both magnitude and frequency of surface peaks. Surface roughness was correlated to flux decline; however, surface charge was not. The limited range of hydrophobicity of the flat sheet studies prohibited conclusions regarding the correlation of flux decline and hydrophobicity. © IWA Publishing 2006

Monitoring Of Distribution Water Qualities Under Various Source Water Blending

The main goal of this large-scale pilot distribution study was to systematically investigate the impacts of blending different source waters on distribution water qualities. The principal source waters investigated were conventionally treated ground water (G1), surface water processed by enhanced treatment (S1), and desalted seawater by reverse osmosis membranes (RO). Due to the nature of raw water quality and associated treatment processes, G1 water had high alkalinity, while S1 and RO sources were characterized as high sulfate and high chloride waters, respectively. One year of pilot pipe study demonstrated that water quality was significantly deteriorated by increased color when source water blends with characteristics different from historic groundwater were introduced to pipe distribution systems. Elevated color was associated with release of iron corrosion products, mainly from aged unlined cast iron pipes. Iron release increased significantly when exposed to RO and S1 waters: that is, the greater iron release was experienced with alkalinity reduced below the background of G1 water. Lead and copper release to water, on the other hand, enhanced with the application of RO and G1 waters, respectively. © Springer Science + Business Media, Inc

Removal Of Assimilable Organic Carbon And Biodegradable Dissolved Organic Carbon By Reverse Osmosis And Nanofiltration Membranes

The main objective of this study was to evaluate the effectiveness of reverse osmosis (RO) and nanofiltration (NF), under various solution chemistries, on bacterial regrowth potential as quantified by assimilable organic carbon (AOC) and biodegradable dissolved organic carbon (BDOC). The bench-scale experiments, using tap groundwater spiked with acetate as organic carbon, revealed that AOC removals by RO/NF membranes were strongly dependent on charge repulsion. AOC removals were greater at conditions of low ionic strength and low hardness, and were slightly higher at high pH values. BDOC removals by NF membrane also increased with decreasing hardness and ionic strength, and increasing pH. However, the RO membrane showed less dependence on feed solution chemistry for BDOC removal, suggesting that BDOC removal was determined by the combined effect of both size exclusion and charge repulsion. The bench-scale observations were compared to a full-scale drinking water treatment plant that used nanofiltration as a primary treatment process. From full-scale operation, it was observed that nanofiltration was a very effective means to reduce BDOC, but conversely, did not reject the bulk of raw water AOC. The high BDOC rejection by NF membranes at full scale can be explained by size exclusion, since a significant fraction of BDOC in raw surficial ground water consists of compounds, such as humic and fulvic acids, which are larger than the pores of NF membranes. The insignificant AOC rejection observed in the full-scale system was probably due to the low pH, high hardness, and high ionic strength (TDS) of the raw groundwater combined with acid addition during pretreatment. These solution environments repress the electrostatic interaction between charged organic compounds and membranes, allowing passage of small molecular weight compounds and thus reducing AOC rejection. (C) 2000 Elsevier Science B.V