Removal of natural organic matter by ultrafiltration and nanofiltration for drinking water production

Water from Lake Butoniga near the town of Buzet, Croatia, was used as a source for drinking water production. Since lake water has a high concentration of trihalomethane precursors, a treatment was necessary. A process including ozonation, flocculation and filtration was chosen on the basis of preliminary work in a trial pilot plant with a capacity of 10 m³ h⁻¹. Although the chosen process succeeded in producing water that met the demands for drinking water, the efficiency of the removal of natural organic matter was relatively low. Ultrafiltration (UF) and nanofiltration (NF) processes were investigated as alternatives and possible upgrades of the process. Experiments were conducted at pilot plants with the Mavibran SP 006A and Romicon PM 10, PM 50, GM 80 and PM 500 OF membranes as well as with the Filmtec NF 45 NF membrane. Since most of the organic matter in the lake water was smaller than 6-8 kD, the use of the NF process was proposed. To avoid fouling of the NF membrane, we used flocculated and filtrated water from the trial plant as NF feed water. This combination produced water of high quality while process parameters remained stable over the entire period of investigation.     

Characterization of anion exchange membranes with natural organic matter (NOM) during electrodialysis

Natural organic matter (NOM) is thought to be a major source of fouling during membrane filtration of natural waters. The organic matter present in surface waters was characterized in terms of its molecular weight distribution, acidity and electrokinetic properties. The fouling potentials of anion exchange membranes were predicted by the characterization. Changes in the physicochemical properties of anion exchange membranes were also examined during electrodialysis (ED) process of solutions containing NOM. The ED performances were evaluated for the three anion exchange membranes (AMX, AM-1 and ACM) in the presence of NOM. Fouling phenomena in terms of current efficiency and NaCl flux were in good agreement with the fouling potentials predicted by the characterization results. Observations of the molecular weight distribution and the constituents of NOM revealed that the hydrophobic NOM fraction with high molecular weights deposited mainly on the membrane surface, providing fouling effects on the anion exchange membrane.     

Interaction mechanisms associated with organic colloid fouling of ultrafiltration membrane in a drinking water treatment system

The Extended Derjaguin Landau Verwey Overbeek (XDLVO) approach was introduced to predict organic colloid fouling of membranes in an initial ultrafiltration (UF) phase. Two polymeric UF membranes, made of polyvinylchloride (PVC) and polyvinylidene fluoride (PVDF) respectively, were selected to investigate membrane fouling by filtering water samples with different organic colloid compositions. The experiment was performed to determine the fouling contributions of van der Waals (LW) interactions, electrostatic (EL) interactions, as well as double layer and short-range acid–base (AB) interactions, to the total interaction energy caused by organic colloids attaching to UF membrane surfaces. The results showed that LW interaction energy predominated when the distance between the membrane surface and organic colloid was > 5 nm, while AB accounted for a key contribution to total interaction energy over short distances (< 2.5 nm). The influence of EL interaction energy was ignored in the total interaction energy composition. The surface energy, among all characteristics of membrane material, was a dominant factor affecting membrane fouling. The experimental results of initial ultrafiltration of raw water from the actual water source were in accordance with the predictions based on XDLVO theory, indicating that it was a feasible option for predicting membrane fouling during the initial ultrafiltration phase.     

Natural organic matter and formation of trihalomethanes in two water treatment processes

The investigation involved the study of performance of two local plants' slightly different treatment processes for the removal of natural organic matter (NOM) which comprises trihalomethane (THM) precursors. Ultrafiltration separated the contained NOM into various apparent molecular weight (AMW) fractions to reveal the two processes NOM removal efficiencies, in terms of dissolved organic carbon (DOC), UV absorbance (UV254) and THM formation potential (THMFP). DOC and UV254 levels of source water at both plants tested low and they concentrated in the portion with AMW less than 3,000 Daltons. The combined expression of UV254 with DOC as specific ultraviolet light absorbance (SUVA) indicated that the source water of Macau had a value less than 3 L/mg-m. Therefore the main DOC content was fulvic in character, which is difficult to remove and favors the formation of brominated THMs. Low DOC removal percentages recorded in both processes confirm this observation. However, both processes showed similar removal efficiency of around 50% on humic substances and THM precursors quantified by measurements of UV254 and THMFP, respectively.     

Membrane fouling formation when treating effluent by ultrafiltration

The objective of this study is to investigate membrane fouling caused by ultrafiltration (UF) when reusing tertiary effluent in an industrial park. A bench-scale membrane system was performed. Experimental results showed that the removal of iron, manganese, and turbidity were 62%, 30%, and 77%, respectively. From the infrared ray analysis of membrane fouling, one can see that the organic functional groups were aromatic rings and a few linear chain compounds of chromophore and auxochrome found in dyestuff. Therefore, tertiary effluent may induce both colloidal and organic fouling into the UF system.     

Comparison of ceramic and polymeric membranes for natural organic matter (NOM) removal

Ceramic membranes were compared with polymeric membranes with respect to natural organic matter (NOM) removal using two removal mechanisms (i.e., size exclusion and charge repulsion). NOM properties including molecular weight and molecular structure, at different charge densities, were examined, along with membrane characteristics, including molecular weight cut-off (MWCO) and surface charge. Integrated analyses of both NOM and membrane characteristics provided information for membrane evaluation of different membrane materials and configurations (i.e., tubular vs. flat sheet type). A ceramic tight-ultrafiltration (UF) membrane showed the same potential as a similar nanofiltration (NF) polymeric membrane, in terms of the minimization of haloacetic acid (HAA) formation. Moreover, a ceramic OF membrane with a MWCO of 8000 Daltons showed almost the same behavior as an equitable polymeric UF membrane with a MW CO of 8000 Daltons in terms of NOM removal.     

Colloidal organic matter fouling of UF membranes: role of NOM composition & size

The aim of this study was to fractionate pre-filtered surface water using a 3.5 and a 10 kDa dialysis membrane, and to compare the rate of fouling and the fouling reversibility/irreversibility of the NOM fractions. Trial dialyses (3.5 and 10 kDa) were carried out for 6 and 21 days with pre-filtered surface water using synthetic surface water as dialysate. The aim of the trials was to optimize the dialysis process for NOM fractionation. DOC, Ca²⁺, Mg²⁺, soluble silica and bacteria were monitored at intervals during the dialysis process. Thereafter, the various NOM fractions (with low and high Ca²⁺) were fed to a miniature UF system operated at a constant flux of 138.5 L/m2 h, filtration cycle times of 31.5 min and backwash duration of 1.75 min. A PES/PSV hollow fiber UF membrane (MWCO 100 kDa) with a surface area of 0.0125 m² was employed for the filtration tests (X-Flow). Transmembrane pressure (TMP) and UF feed and permeate (LC-OCD) were monitored at regular intervals. For a dialysate recirculation of 95 L/h, sample to dialysate ratio of 5.2:80 L and a dialysate change frequency of 3 times per 24 h, the shortest duration of dialysis was about 6–7 days for both 3.5 and 10 kDa dialyses membranes. The removal of organic carbon (OC) increased with dialysis duration and MWCO of the bags. The biopolymer fraction increased from 120% to 240% when the duration of dialysis was increased from 6 days (1.1 mg DOC/L, 151 mg Ca/L) to 21 days (0.82 mg DOC/L, 133 mg Ca/L) with the 10 kDa dialysis membrane. The increased biopolymer fraction in the NOM sample that was dialyzed for 21 days resulted in a doubling of the fouling rate from 3.5 to 6.6 mbar/min per mg DOC/L. The other NOM fractions (humics and building blocks) and the Ca/DOC ratio was more or less the same in both NOM samples suggesting that biopolymers were the major cause of UF fouling.     

Dissolved organic matter and arsenic removal with coupled chitosan/UF operation

Long time uptake arsenic will cause cancers and blackfoot disease. There are still several million people suffering from drinking arsenic contaminated water. This work studied the performance of coupled chitosan/UF for arsenic removal and the influence of dissolved organic matter on arsenic removal with UF operation. Humic acid representing dissolved organic matter was fractionated into five groups of molecular sizes by gel filtration chromatography. Arsenic rejection by UF under the experimental condition is only 10%. In the presence of humic compounds, the arsenic removal of 22% is obtained. DOM with apparent molecular weight > 35,000 Da is the principle component responsible for chelating arsenic and thereafter being rejected by UF membrane. The combined interactions of humic compounds, chitosan, and arsenic enable a 65% arsenic rejection by UF. The results presented here enable our understanding of the complicated mechanisms involved in chitosan/UF/DOM/arsenic system.     

Investigation of the adsorption and transport of natural organic matter (NOM) in ion-exchange membranes

The adsorption and transport characteristics of natural organic matter (NOM) in an ion-exchange (IX) membrane were investigated and the various methods to characterize the properties of NOM and the IX membrane were collectively evaluated in this study. NOM adsorption by an IX membrane is affected by both pH and ion strength. Under alkaline pH and low ionic strength, greater NOM adsorption can be expected. A good relationship was obtained between the amount of adsorbed NOM and the zeta potential. The NOM acids constituents are expected to be transported preferentially through an IX membrane during the electrodialysis (ED) process because of their negative charge density. However, the molecular mass of the NOM acids was too high to allow them to pass through the IX membrane pores, and this caused an accumulation or adsorption of the solutes on the membrane surface. A fractional-rejection method was applied to determine the apparent pore size distribution of IX membranes and the selectivity coefficient was used to calculate apparent charges of NOM. The major apparent pore size distribution (PDS) of the IX membrane used in this study lay in the range 100-200 mass units. The apparent charge of the NOM used was 5.5 (dimensionless).     

Optimizing the coagulant dose to control membrane fouling in combined coagulation/ultrafiltration systems for textile wastewater reclamation

Optimal coagulation conditions need to be re-examined when coagulation is coupled to membrane filtration for wastewater treatment. This work focused on the optimization of coagulant dosing in order to control membrane fouling in ultrafiltration (UF), following coagulation for the reclamation of textile wastewater. The effects of pore size and coagulant types and dosages on flux decline were investigated using a stirred-cell UF unit. The flux was greatly enhanced for the UF membrane when a coagulant was added, whereas for the microfiltration (MF) membrane the flux decreased. This could be attributed to changes in the size of coagulated particles and their interaction with membrane pores. At a low dosage (e.g., 0.0371 mM as Al), the polyaluminum chloride (PACl) coagulant was found to control the flux decline most effectively for low ionic-strength wastewater. The optimal dose minimized the fouling and cake layer resistances, although it was sharp and dependent on influent composition. The cake layer protected the membrane from fouling, but it provided additional resistance to permeation. Analyses of turbidity, particle size, and membrane surface exhibited the characteristics of coagulated particles and their cake structures that are closely associated with flux behavior.     

An advanced treatment of high-strength opium alkaloid processing industry wastewaters with membrane technology: pretreatment, fouling and retention characteristics of membranes

This paper presents the results of the laboratory and pilot-scale membrane experiments of opium alkaloid processing industry effluents. Different types of ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes were evaluated for membrane fouling, permeate flux and their suitability in separating COD, color and conductivity. Experiments demonstrated that membrane treatment is a very promising advanced treatment option for pollution control for opium alkaloid processing industry effluents. Almost complete color removal was achieved with NF and RO membranes. COD and conductivity removals were also greater than 95% and met the current local standards. Nevertheless, pretreatment was an important factor for the NF and RO membrane applications. Membrane fouling occurred with direct NF membrane applications without UF pretreatment. The total estimated cost of the UF and NF treatment system was calculated as $0.96/m³, excluding the concentrate disposal cost.     

Chemical control of colloidal fouling of reverse osmosis systems

Fouling of membranes by colloidal organic and inorganic particles continues to be documented as the most common and challenging obstacle in attaining stable continuous operation of reverse osmosis (RO) and ultrafiltration (UF) systems. Much current research is being conducted on physical parameters to mitigate such fouling. The focus has been on membrane synthesis and element design; microfiltration and ultrafiltration pretreatment; electromagnetic devices; correlation with physical factors such as Silt Density Index, zeta potential and critical flux; technique of direct observation of fouling process through a membrane; and classification of macromolecular organics for correlation with fouling characteristics. We report initial successes with chemical control of colloidal fouling. Through screening with a large number of observable coagulations of natural colloids, we have developed a group of proprietary anticoagulants and dispersants that would, at less than 10 ppm dosage to the RO feedwater, control various classes of colloidal foulants. Case studies of the control of humic matter, elemental sulfur and colloidal silicate in problematic RO systems that became stabilized are briefly presented. We conclude that a great need and potential exists in economically controlling the myriads of fouling interactions of colloidal particles during concentration within the brine channels of RO membrane elements. Low dosages of antifoulants can in many cases obviate the need for installation and maintenance of pretreatment unit or operations designed to remove such colloidal foulants from the process stream.     

Catalytic ozonation of natural organic matter on alumina

The paper aims to show the potential of catalytic ozonation in the presence of alumina for the removal of natural organic mater from drinking water. An investigation into the efficiency of catalytic ozonation, ozonation by-products formation and their biodegradability was the main goal of the paper. Characterisation of fresh and worn alumina was also conducted. The results clearly indicated the high activity of alumina over a long period of time, which is crucial in water treatment technology. The application of alumina to the ozonation system doubled the efficiency of NOM removal from water when compared to ozonation alone. Furthermore, catalytic ozonation resulted in lower by-products and biodegradable organic carbon formation.     

An experimental study of UF membrane fouling by humic acid and sodium alginate solutions: the effect of backwashing on flux recovery

One of the critical issues for the successful application of ultrafitration in water treatment is membrane fouling due to dissolved organic matter, which negatively affects productivity, product quality and process cost. The aim of the present study is to contribute to the understanding of fouling phenomena by organic matter and the efficiency of the backwashing technique, which is applied in practice to restore membrane flux. In this experimental study commercial humic acid and sodium alginate have been used as model substances representative of natural organic matter and extracellular organic matter, respectively. All fouling experiments were carried out in a special single fiber apparatus. An important parameter considered in the study of both model substances is the concentration of calcium ions, which promote their aggregation and influence the rate of flux decline, the reversibility of fouling and rejection. Membrane fouling by humic acid appears to be the outcome of simultaneous action of several fouling mechanisms. Initially, a relatively rapid irreversible fouling takes place due to internal pore adsorption; however, progressively pore blocking becomes important and a fouling layer develops on the membrane. Sodium alginate fouling on the other hand is apparently due to two consecutive mechanisms; i.e. a rapid irreversible fouling due to internal pore constriction, followed by cake development on the membrane surface which becomes the dominant mechanism. Comparing fouling in both cases it can be inferred that even though sodium alginate fouling is more severe than the one caused by humic acids, it is to a large extent reversible by backwashing. On the contrary, fouling caused by humic acid is characterized by greater and increasing with calcium addition irreversibility, which is not remedied by the periodic backwashing. The different fouling propensity of the two types of macromolecules is apparently due to differences of their physical–chemical characteristics.     

Nanofiltration of river water — fouling, cleaning and micropollutant rejection

Four nanofiltration and low pressure reverse osmosis membranes were investigated concerning their retention and fouling behavior during filtration of a surface water (River Alb) spiked with four polar micropollutants. Filtration and cleaning experiments were also conducted in deionized water spiked with the model contaminants in order to evaluate the influence of the water matrix on retention. Steric and electrostatic effects were found to be the most important factors influencing retention of the organic substances. In the presented experiments, fouling had little influence on retention. In case of the DI water experiments, treatment of the membranes with NaOH solution considerably lowered the retention of the investigated micropollutants, whereas in the experiments with river water the effect of alkaline treatment on the rejection of the trace organic substances decreased to an insignificant amount already 1 h after membrane cleaning.     

Effect of algal extracellular polymer substances on UF membrane fouling

Low-pressure membrane processes, such as ultrafiltration (UF), are effective for treating algae-laden eutrophic source waters. However, fouling caused by algae excreted organics can be a major obstacle to the smooth operation of membrane processes. In this study, three algae species, with a similar cell size and shape, but a different amount of extracellular polymeric substances (EPS), were selected. The characteristics of the bound EPS, including its amount and constituents, mainly polysaccharide and protein, were analyzed. Cell suspensions were filtered through a dead-end stirred cell with a regenerated cellulose UF membrane. Based on the observations of permeate flux decline at higher transmembrane pressure (TMP) and critical flux values, it can be concluded that algae cell EPS do affect membrane fouling, and the constituents of EPS may also play a role.     

Effect of Preozonation on Flux and Water Quality in Ozonation-Ultrafiltration Hybrid System for Water Treatment

The effect of ozonation on membrane flux and water quality was investigated in an ozonation ultrafiltration (UF) hybrid system. Crossflow UF was performed in total recycle mode to study the effect of ozonation on membrane fouling and disinfection by-product formation potentials of organics. Total organic carbon (TOC), UV absorbance at 254 ran (UV254) and trihalomethane formation potential (THMFP) were measured as water quality parameters.

The effect of ozonation on membrane flux was found to be largely dependent on raw water quality as well as ozone dose. In case of upstream water (A), preozonation achieved significant flux enhancement regardless of ozone dose. Whereas, for the downstream water (B), the steady state flux was increased or decreased depending on ozone dose.

The analysis based on the resistance-in-series model provided the mechanistic interpretation on the membrane flux variation. Ozonation in an ozone-ultrafiltration system always brought about a decrease in cake resistance (R_c) and an increase in fouling resistance (Rf). Based on the measurement of particle size distribution and zeta potential, the reduction in cake resistance through ozonation was attributed to an increase in particle size due to “ozone-induced particle destabilization”. However, the increase in the fouling resistance seems to be caused partly by the microbial characteristics of raw water.

Although there was little effect on TOC, ozone-UF treatment could get much higher removal of UV 254, THMFP (lday) and THMPF/TOC ratio than UF treatment alone.     

Mechanisms underlying the effects of membrane fouling on the nanofiltration of trace organic contaminants

The influence of membrane fouling on the retention of four trace organic contaminants - namely sulfamethoxazole, ibuprofen, carbamazepine, and triclosan - by nanofiltration membranes was investigated in this study. Humic acid, alginate, bovine serum albumin, and silica colloids were selected as model foulants to simulate various organic fractions and colloidal matter that are found in secondary treated effluent and surface water. The effects of membrane fouling on the separation process was delineated by comparing retention values of clean and fouled membranes and relate them to the membrane properties (under both clean and fouled conditions) as well as physicochemical characteristics of the trace organic contaminants. Membrane fouling was dependent on the physicochemical properties of the model foulants. Initial foulant-membrane interaction could probably be a major factor governing the process of membrane fouling particularly by the organic foulants. Such membrane-foulant interaction was also a dominating factor governing the effects of membrane fouling on the membrane separation efficacy. In good agreement with our previous study (Nghiem and Hawkes, 2007 [1]), the effects of fouling on retention were found to be membrane pore size dependent. In addition, results reported here suggest that these effects could also be foulant dependent. It was probable that the influence of membrane fouling on trace organic retention could be governed by four distinctive mechanisms: modification of the membrane charge surface, pore blocking, cake enhanced concentration polarisation, and modification of the membrane hydrophobicity. The presence of the fouling layer could affect the retention behavior of charged solutes by altering the membrane surface charge density. While the effect of surface charge modification was clear for inorganic salts, it was less obvious for the negatively charged pharmaceutical species (sulfamethoxazole and ibuprofen) examined in this investigation, possibly due to the interference of the pore blocking mechanism. Evidence of the cake enhanced concentration polarisation effect was quite clear, particularly under colloidal fouling conditions. In addition, organic fouling could also interfere with the solute-membrane interaction, and therefore, exerted considerable influence on the separation process of the hydrophobic trace organic contaminant triclosan.     

自然有机物对地下水混凝除铁的影响试验研究

通过模拟试验,考察了自然有机物的浓度、分子质量以及溶液的pH、温度对地下水混凝除铁的影响,并研究了预氧化在混凝除铁中的作用.结果表明,有机物的浓度越高、分子质量越大,有机铁的去除率越高.当Fe2 与分子质量＞10 ku的有机物络合时,用混凝除铁能取得良好的效果;当Fe2 与分子质量＜4 ku的有机物络合时,采用预氧化混凝工艺能达到较好的除铁效果.在pH6.0～7.5的范围内,降低pH有利于混凝除铁.温度升高对混凝除铁有利.