Reverse osmosis membrane treatment of acidic etchant wastewater: Effect of neutralization and polyelectrolyte coating on nitrate removal

The removal of nitrate from mixed acid etchant (MAE) wastewater was investigated by neutralization, followed by reverse osmosis (RO) membrane filtration. The coating of a RO membrane was conducted using polyacrylic acid (PAA) in order to enhance the removal of nitrate from the MAE wastewater. The addition of KOH, for the neutralization of the MAE wastewater, was most effective in terms of solid–liquid separation. Double RO filtrations, with crossflow and stirred-flow units, were examined in terms of nitrate rejection and membrane permeability. The Donnan exclusion, due to change in the solution pH, played an important role in nitrate rejection. As a result, RO filtration, at a moderate acidic pH level (e.g., pH 4), provided greater nitrate rejection than that at neutral or alkaline pH levels. The Donnan effect was associated with acetic acid present in MAE wastewater, since it could deprotonate to acetate with a negative charge. Improvement in nitrate rejection occurred with the PAA coating of the original RO membrane. This is because of the enhanced electrostatic repulsion of the nitrate by the carboxyl groups on the coated membrane surface, although the flux declined with the PAA coatings. The effect of charge repulsion was more obvious in the second pass of RO filtration where the ionic strength was relatively low. The increase in nitrate rejection leveled off with a PAA dosage of 0.262 mg/cm² of the membrane, so further coating beyond this level should be prevented.     

Resistance analysis for enhanced wastewater membrane filtration

This study investigated enhancement techniques for synthetic wastewater filtration in a membrane bioreactor (MBR) at mixed liquor suspended solids concentrations (MLSS) of 12–18 g/L. Air sparging (AS), backflushing (BF) and a combined application of both (AS + BF) were applied to increase permeate flux compared to the conventional application (NON). Scanning electron microscope (SEM) measurements of cake thickness served for evaluating cleaning effectiveness and as input data for some of the model calculations. AS + BF showed the lowest overall resistance, and thus the highest permeate yield, for about 2 weeks of observation. The contribution of fouling resistance, cake resistance and membrane resistance to the overall resistance was evaluated, based on experimental data. Air sparging significantly lowered cake thickness and consequently cake resistance. The experimental cake resistance and the model resistances were compared. A model based on the measured cake thickness and literature values for the specific surface area proved most successful. Finally, a relationship between the backflush resistance and the permeate flow resistance was observed.     

Fouling of reverse osmosis membranes by biopolymers in wastewater secondary effluent: Role of membrane surface properties and initial permeate flux

Reverse osmosis (RO) is being increasingly used in treatment of domestic wastewater secondary effluent for potable and non-potable reuse. Among other solutes, dissolved biopolymers, i.e., proteins and polysaccharides, can lead to severe fouling of RO membranes. In this study, the roles of RO membrane surface properties in membrane fouling by two model biopolymers, bovine serum albumin (BSA) and sodium alginate, were investigated. Three commercial RO membranes with different surface properties were tested in a laboratory-scale cross-flow RO system. Membrane surface properties considered include surface roughness, zeta potential, and hydrophobicity. Experimental results revealed that membrane surface roughness had the greatest effect on fouling by the biopolymers tested. Accordingly, modified membranes with smoother surfaces showed significantly lower fouling rates. When Ca²⁺ was present, alginate fouled RO membranes much faster than BSA. Considerable synergistic effect was observed when both BSA and alginate were present. The larger foulant particle sizes measured in the co-existence of BSA and alginate indicate formation of BSA-alginate aggregates, which resulted in greater fouling rates. Faster initial flux decline was observed at higher initial permeate flux even when the flux was measured against accumulative permeate volume, indicating a negative impact of higher operating pressure.     

Optimisation of dead-end filtration conditions for an immersed anoxic membrane bioreactor

The optimisation of the energy demand in the application of dead-end filtration in an immersed membrane bioreactor applied to groundwater denitrification has been studied. Filtration cycle length was varied at a set flux to control the amount of foulant deposited at the membrane surface. Physical cleans comprising a simultaneous backflush and gas injection were subsequently instigated and the reversibility of the deposit determined by the residual resistance, R_res. Examination of R_res versus flux and cycle length variation indicated an operational envelope where limited fouling occurred. The transition from limited fouling to extensive fouling was indicated by a parameter based on the critical accumulated mass, indicating incipient deposit consolidation. The transition between regions became less severe when the solids retention time was increased from 10 to 25 and 40 days. This was apparently related to a shift in bulk physical characteristics. Nevertheless, low residual fouling was observed during long-term filtration when operating below the critical mass, resulting in a 20× reduction in energy demand over that of constant gas injection.     

Long-term operation of a domestic wastewater treatment plant with membrane filtration

Possibilities of membrane technology and the use of membrane processes in wastewater treatment were investigated. The main focus was the monitoring of the starting phase of a domestic wastewater treatment plant. Experimental part of the study was realized at the municipal wastewater treatment plant (WWTP) Devínska Nová Ves — Bratislava during the period from February 2005 to September 2006. The system was stable without any external chemical treatment of the membrane modules and the permeate quality was very high. Observed decrease of COD and BOD₅ values ranged between 91 % and 98 %. The process of nitrification was very successful considering its high efficiency (> 95 %). Presented at the 34th International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 21–25 May 2007.     

Impact of ultrafiltration operating conditions on membrane irreversible fouling

The main limitation of the ultrafiltration (UF) process identified in drinking water treatment is membrane fouling. Although adsorption of natural organic matter (NOM) is known to cause irreversible fouling, operating conditions also impact the degree of irreversible fouling. This study examined the impact of several operating parameters on fouling including flux, concentrate velocity in hollow fibers, backwash frequency, and transmembrane pressure. A hydrophilic cellulose derivative membrane and a hydrophobic acrylic polymer membrane were used to conduct these tests. Pilot testing showed that when short-term reversible fouling was limited during a filtration cycle by increasing the concentrate velocity, reducing the flux, and increasing the backwash frequency, the evolution of the membrane toward irreversible fouling could be controlled. It appeared that operating parameters should be adjusted to maintain the increase of transmembrane pressure below a certain limit, determined to be approximately 0.85 to 1.0 bar for the tested UF membrane, in order to minimize the rate of irreversible fouling. This threshold for transmembrane pressure was confirmed empirically by compiling data from over 36 pilot studies. Other testing results demonstrated that hydraulic backwash effectiveness decreased as the transmembrane pressure applied in the previous filtration cycle increased. Backwash efficiency in terms of membrane flux recovery after hydraulic backwash was reduced by 50% when the transmembrane pressure was increased from 0.4 bar to 1.4 bar.     

Electrochemical removal and recovery of phosphorus from wastewater using cathodic membrane filtration reactor

Removal and recovery of phosphorus (P) from wastewater is of great importance to addressing the challenges of eutrophication and phosphorus shortage. The P removal and recovery performance of conventional electrochemical precipitation approach was constrained by the limited mass transfer rate. Herein, a cathodic membrane filtration (CMF) reactor was developed using Ti/SnO₂-Sb anode and titanium mesh cathodic membrane module to achieve efficient removal and recovery of P in wastewater. Compared with the flow-by mode, the CMF system in the flow-through mode exhibited excellent P removal performance due to the markedly enhanced mass transfer. At the current density of 4 A/m², membrane flux of 16.6 L m^?2 h^?1, and Ca/P molar ratio of 1.67, the removal efficiency of P was 96.2% and the energy consumption was only 45.7 kWh/kg P. The local high pH of cathode surface played a vital role in P removal, which substantially accelerated the nucleation of calcium phosphate (CaP). Based on the crystalline and morphological characterization of the precipitates, the hydroxyapatite was the most stable crystalline phase of CaP, which was transformed from intermediate phases (such as dicalcium phosphate and amorphous calcium phosphate). This study paves the way for applying electrochemical membrane filtration system for P removal and recovery from wastewater.     

Permeate flow in hexagonal 19-channel inorganic membrane under filtration and backflush operating modes

Numerical simulations and experimental study of incompressible Newtonian permeate flow in porous support of hexagonal 19-channel inorganic membrane are presented both for filtration and backflush operating modes. Under several simplifying assumptions the problem could be treated as two-dimensional potential flow. The mathematical model was solved using finite element method. The results of numerical simulations show that the contributions of particular channels to the total permeate flux are not equal and depend on the ratio of skin layer to porous support permeability as well as on the distance of a channel from the membrane outer surface. For membranes with high permeability of skin layer there is an area of nearly constant pressure around inner channels and their contribution to total flux is negligible. This effect will probably be more pronounced in backflush operating mode while in filtration mode possible dynamic membrane adds a resistance to that of skin layer which leads to more uniform permeate flux distribution. Qualitative trends of the numerical simulations were verified by experiments with ceramic 19-channel membranes of Membralox® type in backflush operating mode.     

Retraction Note to: Secondary treated wastewater of latex processing: reusing for irrigation or treatment by membrane filtration

Research on Chemical Intermediates -     

Influence of operating conditions on performance of ceramic membrane used for water treatment

The removal of natural organic matter (NOM) is a critical aspect of potable water treatment because NOM compounds are precursors of harmful disinfection by-products, hence should be removed from water intended for human consumption. Ultrafiltration using ceramic membranes can be a suitable process for removal of natural substances. Previously reported experiments were dedicated to evaluating the suitability of ultrafiltration through ceramic membrane for water treatment with a focus on the separation of natural organic matter. The effects of the membrane operating time and linear flow velocity on transport and separation properties were also examined. The experiments, using a 7-channel 300 kDa MWCO ceramic membrane, were carried out with model solutions and surface water at trans-membrane pressure of 0.2–0.5 MPa. The results revealed that a loose UF ceramic membrane can successfully eliminate natural organic matter from water. The permeability of the membrane was strongly affected by the composition of the feed stream, i.e. the permeate flux decreased with an increase in the NOM concentration. The permeate flux also decreased over the period of the operation, while this parameter did not influence the effectiveness of separation, i.e. the removal of NOM. It was observed that the increased cross-flow velocity resulted in the decrease in the membrane-fouling intensity and slightly improved the retention of contaminants.     

Influence of synthesis and operating parameters on silicalite-1 membrane properties

The present study deals with the synthesis of nanostructured silicalite-1 membranes on porous α-Al₂O₃ supports by a hydrothermal method. Different parameters including the synthesis conditions (temperature and alkalinity) and operating conditions (temperature and pressure) were investigated. The membranes were characterized by X-ray diffraction and scanning electron microscopy techniques. The optimum synthesis temperature and alkalinity were determined to be 160 °C and pH = 11, respectively. The permeability of CO₂ and CH₄ through the optimized membrane was determined by the pressure drop method. The results revealed that the main effective separation mechanism was adsorption. The permeation of CO₂ and CH₄ declined with increasing temperature, whereas high feed pressures enhanced the single gas flux. The CO₂ and CH₄ permeability values at 30 °C and 2 bar were 1.62 × 10^?7 and 2.07 × 10^?7 mol m^?2 s^?1 Pa^?1, respectively. Furthermore, the response surface methodology analysis confirmed the significance of all the variables and the proposed model. Excellent correlation between the experimental and predicted data (R² = 0.99) was obtained, confirming that response surface methodology is a powerful tool for modeling nanostructured silicalite-1 membrane processes.     

Bipolar reverse osmosis membrane for separating mono-and divalent ions

Bipolar reverse osmosis membranes that have both negatively and positively charged layers have been prepared to enhance the selectivity towards mono- and divalent ions in respect of both cations and anions. Positively charged layers are formed on low pressure reverse osmosis membranes having negative charge (NTR-7410 and 7450) by an adsorption method using polyethyleneimine (PEI) or a quaternary ammonium polyelectrolyte (QAP). These layers attach to the membrane's dense layer, which is made of sulfonated polyether sulfone. The selectivity of mono- and divalent ions is proven by experimental results for single electrolytes (NaCl, Na₂SO₄ and MgCl₂). Although negatively charged membranes repulse divalent anions more strongly than cations and monovalent anions, bipolar reverse osmosis membranes reject both divalent cations and divalent anions better than monovalent ions. An optimal preparation method for bipolar membranes showing selectivity towards mono- and divalent ions were developed. The bipolar membranes showed good ion selectivity for artificial sea water.     

Preparation,structure characteristics and separation properties of thin-film composite polyamide-urethane seawater reverse osmosis membrane

A novel thin-film composite (TFC) seawater reverse osmosis membrane was developed by the interfacial polymerization of 5-chloroformyloxyisophthaloyl chloride (CFIC) and metaphenylenediamine (MPD) on the polysulphone supporting membrane. The performance of the TFC membrane was optimized by studying the preparation parameters, which included the reaction time, pH of the aqueous-MPD solution, monomer CFIC concentration, additive isopropyl alcohol content in aqueous solution, curing temperature and time. The reverse osmosis performance of the resulting membrane was evaluated through permeation experiment with synthetic seawater, and the structure of the novel membrane was characterized by using SEM, AFM and XPS. Furthermore, the separation properties of the TFC membrane were tested by examining the reverse osmosis performances of various conditions, the boron rejection performance and the long-term stability. The results show that the desired TFC seawater reverse osmosis membrane has a typical salt rejection of 99.4% and a flux of about 35 L/m² h for a feed aqueous solution containing 3.5 wt.% NaCl at 5.5 MPa, and an attractive boron rejection of more than 92% at natural pH of 7–8; that the novel seawater reverse osmosis membrane appears to comprise a thicker, smoother and less cross-linking film structure. Additionally, the TFC membrane exhibits good long-term stability.     

pH transients in hydroxyapatite chromatography columns-Effects of operating conditions and media properties

pH transitions occur in hydroxyapatite (HAP) columns that are subject to step changes in salt concentration, which have been shown to be controlled by proton exchange on the HAP surface. The pH temporarily decreases before gradually returning to the feed value when the salt concentration increases, potentially compromising the stability of the HAP when either the magnitude or duration of the pH drop is excessive. The opposite happens when the salt concentration decreases. In this work we address the effects of several key variables: the flow rate, the particle size, the use of salt gradients instead of steps, the use of different co-buffers, the surface area of the HAP, and the use of a slightly alkaline wash prior to increasing the salt concentration. Flow rate and particle size were found to have virtually no effect, demonstrating that the pH transitions are equilibrium rather than kinetically driven. Salt gradients resulted in smaller pH drops compared to steps since the exchanged protons are diluted over the gradient volume. MES and histidine used as co-buffers were effective at reducing the duration of the pH transitions but did not affect their magnitude. The same result was found when comparing HAP samples with different surface areas, with the lower surface area HAP yielding much shorter duration but similar pH drops and rises. Finally, washing the HAP column with a pH 7.5 buffer prior to the salt step was found to dramatically reduce the subsequent pH drop. In general, there was good agreement between these results and predictions based on our previously developed model.     

Industrial multi-stage continuous filtration process: influence of operating parameters

This paper presents a theoretical study of the effects of operating parameters on the behaviour of an industrial crossflow filtration unit. A continuous three stage unit and its application to yeast cell harvesting are considered. A model of standard industrial processing is developed, and the influence of dead volumes, start-up procedure, crossflow and permeation velocities and maximum transmembrane pressure are investigated. Concentration factors are discussed with respect to the effects due to dead volumes.     

Reverse osmosis as a concentration technique for soluble organic phosphorus in fresh water

The chemical nature and availability of soluble organic phosphorus for algal growth is largely unknown. A commercially available reverse osmosis water purification system was adapted for concentrating the soluble organic phosphorus fraction form 100-l volumes of drainage water collected from tile drains underlying an intensively managed grassland area in the Lough Neagh catchment. After membrane filtration the drainage water was recirculated through the reverse osmosis module while the permeate was removed from the system. During a single passage across the reverse osmosis membrane, 20% of the sample was discarded as pure water while the remaining 80% of the sample was pumped back to the reverse osmosis cartridge. Recirculation was continued, with the addition of an intermediate sodium ion-exchange step to prevent the precipitation of insoluble (largely calcium and magnesium) salts, until the volume was reduced to 2.5 l. The recovery of soluble organic phosphorus based on the original sample concentration was almost 93%. A further tenfold increase in concentration was achieved without salt precipitation or loss of soluble organic phosphorus by vacuum-assisted rotary evaporation. The mild, efficient concentration process developed a soluble organic phosphorus concentrate suitable for chemical fractionation and algal availability studies.     

Applicability of nanofiltration and reverse osmosis for the treatment of wastewater of different origin

Membrane separations are finding greater use in wastewater treatment because of their efficiency. In order to prove the effectiveness of membrane filtration an applicability study is carried out. Nanofiltration and reverse osmosis membranes are tested under quite different conditions to reduce the chemical oxygen demands (COD) of wastewaters to meet the Council Directive 76/464/EEC release limit. Two kinds of real wastewaters were selected for the investigation. The wastewaters represent extreme different circumstances since the difference between their COD is two orders of magnitude. All of the membranes tested can be applied either to the treatment of wastewater of high COD (pharmaceutical wastewater) or wastewater of low COD (dumpsite leachate), since the different conditions do not change the membrane characteristics. The experimental data show that none of the membranes can decrease the COD to the release limit in one step. However, if two-stage filtrations (nanofiltration followed by reverse osmosis) are accomplished for both of the wastewaters, a total COD reduction of 94% can be achieved. With the application of the two-stage filtration the COD of the wastewater of low COD can be decreased below the release limit but in case of wastewater of the high COD further treatment will be required.      

Ultrafiltration of hydrosoluble polymers. : Effect of operating conditions on the performance of the membrane

Two sulfonated polyvinylidene fluoride membranes of different porosity, prepared by the casting and gelation technique, were tested in an ultrafiltration laboratory-scale pilot plant with hydrosoluble polymer feed solutions. Polyethyleneglycols and dextrans of different average molecular weight were used as solutes for the feed solutions. Flux and rejection of the membranes were determined as functions of operating conditions (pressure, temperature and recirculation rate). The effect of properties of feed solutions (solute, concentration and molecular weight) on the performance of the membranes was also investigated.     

Convex hull: A new method to determine the separation space used and to optimize operating conditions for comprehensive two-dimensional gas chromatography

It is important to develop methods of optimizing the selection of column sets and operating conditions for comprehensive two-dimensional gas chromatography. A new method for the calculation of the percentage of separation space used was developed using Delaunay's triangulation algorithms (convex hull). This approach was compared with an existing method and showed better precision and accuracy. It was successfully applied to the selection of the most convenient column set and the geometrical parameters of second column for the analysis of 49 target compounds in wastewater.     

PEG-coated reverse osmosis membranes: Desalination properties and fouling resistance

This study focuses on the use of surface-coated reverse osmosis (RO) membranes to reduce membrane fouling in produced water purification. A series of crosslinked PEG-based hydrogels were synthesized using poly(ethylene glycol) diacrylate as the crosslinker and poly(ethylene glycol) acrylate, 2-hydroxyethyl acrylate, or acrylic acid as comonomers. The hydrogels were highly water permeable, with water permeabilities ranging from 10.0 to 17.8 (L μm)/(m² h bar). The hydrogels were applied to a commercial RO membrane (AG brackish water RO membrane from GE Water and Process Technologies). The water flux of coated membranes and a series-resistance model were used to estimate coating thickness; the coatings were approximately 2 μm thick. NaCl rejection for both uncoated and coated membranes was 99.0% or greater, and coating the membranes appeared to increase salt rejection, in contrast to predictions from the series-resistance model. Zeta potential measurements showed a small reduction in the negative charge of coated membranes relative to uncoated RO membranes. Model oil/water emulsions were used to probe membrane fouling. Emulsions were prepared with either a cationic or an anionic surfactant. Surfactant charge played a significant role in membrane fouling even in the absence of oil. A cationic surfactant, dodecyltrimethyl ammonium bromide (DTAB), caused a strong decline in water flux while an anionic surfactant, sodium dodecyl sulfate (SDS), resulted in little or no flux decline. In the presence of DTAB, the AG RO membrane water flux immediately dropped to 30% of its initial value, but in the presence of SDS, its water flux gradually decreased to 74% of its initial value after 24 h. DTAB-fouled membranes had lower salt rejection than membranes not exposed to DTAB. In contrast, SDS-fouled membranes had higher salt rejection than membranes not exposed to SDS, with rejection values increasing, in some cases, from 99.0 to 99.8% or higher. In both surfactant tests, coated membranes exhibited less flux decline than uncoated AG RO membranes. Additionally, coated membranes experienced little fouling in the presence of an oil/water emulsion prepared from DTAB and n-decane. For example, after 24 h the water flux of the AG RO membrane fell to 26% of its initial value, while the water flux of a PEGDA-coated AG RO membrane was 73% of its initial value.