Yongtao Lv

Fouling behavior of typical organic foulants in polyvinylidene fluoride ultrafiltration membranes: characterization from microforces.

To further unravel the organic fouling behavior of polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes, the adhesion forces of membrane-foulant and foulant-foulant were investigated by atomic force microscopy (AFM) in conjunction with self-made PVDF colloidal probe and foulant-coated colloidal probe, respectively. Fouling experiments with bovine serum albumin, sodium alginate, humic acid, and secondary wastewater effluent organic matter (EfOM) were carried out with PVDF UF membrane. Results showed a positive correlation between the membrane-foulant adhesion force and the flux decline rate and extent in the initial filtration stage, whereas the foulant-foulant interaction force was closely related to the pseudostable flux and the cake layer structure in the later filtration stage. For each type of foulant used, the membrane-foulant adhesion force was much stronger than the foulant-foulant interaction force, and membrane flux decline mainly occurred in the earlier filtration stage indicating that elimination of the membrane-foulant interaction force is important for the control of membrane fouling. Upon considering the foulant-foulant interaction force and the membrane flux recovery rate of fouled membranes, it was evident that the main contributor to physically irreversible fouling is the foulant-foulant interaction force.

Enhancement and Mitigation Mechanisms of Protein Fouling of Ultrafiltration Membranes under Different Ionic Strengths

To determine further the enhancement and mitigation mechanisms of protein fouling, filtration experiments were carried out with polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes and bovine serum albumin (BSA) over a range of ionic strengths. The interaction forces, the adsorption behavior of BSA on the membrane surface, and the structure of the BSA adsorbed layers at corresponding ionic strengths were investigated. Results indicate that when the ionic strength increased from 0 to 1 mM, there was a decrease in the PVDF-BSA and BSA-BSA electrostatic repulsion forces, resulting in a higher deposition rate of BSA onto the membrane surface, and the formation of a denser BSA layer; consequently, membrane fouling was enhanced. However, at ionic strengths of 10 and 100 mM, membrane fouling and the BSA removal rate decreased significantly. This was mainly due to the increased hydration repulsion forces, which caused a decrease in the PVDF-BSA and BSA-BSA interaction forces accompanied by a decreased hydrodynamic radius and increased diffusion coefficient of BSA. Consequently, BSA passed more easily through the membrane and into permeate. There was less accumulation of BSA on the membrane surface. A more nonrigid and open structure BSA layer was formed on the membrane surface.

Identifying polyvinylidene fluoride ultrafiltration membrane fouling behavior of different effluent organic matter fractions using colloidal probes

The interaction forces between effluent organic matter (EfOM) fractions and membrane were measured by atomic force microscopy in conjunction with self-made membrane material colloidal probes. The inter-EfOM-fraction and intra-EfOM-fraction interactions were investigated using corresponding EfOM-fraction-coated colloidal probe. We combined this analysis with corresponding fouling experiments to identify the EfOM fractions responsible for polyvinylidene fluoride (PVDF) ultrafiltration membrane fouling. Results show that hydrophilic and hydrophobic fractions were the dominant fractions responsible for membrane fouling and flux decline in the initial and later filtration stages, respectively, which was mainly attributed to the stronger PVDF-hydrophilic fraction and intra-hydrophobic-fraction interaction forces. This phenomenon, in conjunction with the fact that each interaction force of PVDF-EfOM fraction was stronger than corresponding intra-EfOM-fraction force, suggests that the elimination of the PVDF-hydrophilic fraction interaction force is the best strategy for controlling EfOM fouling. Moreover, the inter-EfOM-fraction interaction force was mainly controlled by the corresponding intra-EfOM-fraction interaction forces. And, while the membrane-EfOM fraction and intra-EfOM-fraction interactions for each type of EfOM fraction are equivalent, the EfOM fractions with the molecular weight smaller than the molecular weight cutoff of the membranes used were mainly responsible for membrane fouling rather than the relatively high-molecular-weight fractions.

Autotrophic nitrogen removal discovered in suspended nitritation system

A lab-scale sequencing batch reactor was operated in batch mode to treat ammonium-rich synthetic wastewater, and minor N loss (about 9%) was observed without seeding anaerobic ammonium oxidation bacteria (AnAOB). Around 4 wk after the dissolved oxygen concentration was decreased (from 0.9+/-0.1 to 0.4+/-0.1 mg L(-1)) and the hydraulic retention time was elevated (from 11.0 to 14.7 h), 80% of total nitrogen removal efficiency and 0.49 kg N m(-3) d(-1) of maximum nitrogen removal capacity were obtained. During a single cycle, a peak of NO2(-) concentration was observed at 60 min, and the pH first increased from 8.02 to 8.17 and then returned to 8.04. Fluorescence in situ hybridization analysis revealed that the aerobic ammonium-oxidizing genus Nitrosomonas dominated the community, while AnAOB and very small amounts of Nitrospira spp. were also detected. The ratio of nitrate produced to ammonia consumed (0.10+/-0.02) was lower than that achieved with the completely autotrophic nitrogen removal over nitrite process. This study revealed that nitritation, Anammox and autotrophic denitrification were responsible for the nitrogen removal. The nitrate production was caused by the co-action of nitrite-oxidizing bacteria and AnAOB.

Enhanced gypsum scaling by organic fouling layer on nanofiltration membrane: Characteristics and mechanisms

To investigate how the characteristics of pregenerated organic fouling layers on nanofiltration (NF) membranes influence the subsequent gypsum scaling behavior, filtration experiments with gypsum were carried out with organic-fouled poly(piperazineamide) NF membranes. Organic fouling layer on membrane was induced by bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA), respectively. The morphology and components of the scalants, the role of Ca(2+) adsorption on the organic fouling layer during gypsum crystallization, and the interaction forces of gypsum on the membrane surface were investigated. The results indicated that SA- and HA-fouled membranes had higher surface crystallization tendency along with more severe flux decline during gypsum scaling than BSA-fouled and virgin membranes because HA and SA macromolecules acted as nuclei for crystallization. Based on the analyses of Ca(2+) adsorption onto organic adlayers and adhesion forces, it was found that the flux decline rate and extent in the gypsum scaling experiment was positively related to the Ca(2+)-binding capacity of the organic matter. Although the dominant gypsum scaling mechanism was affected by coupling physicochemical effects, the controlling factors varied among foulants. Nevertheless, the carboxyl density of organic matter played an important role in determining surface crystallization on organic-fouled membrane.

Comparative study of the photocatalytic performance for the degradation of different dyes by ZnIn2S4: adsorption, active species, and pathways

A comparative study of the photocatalytic performance for the degradation of rhodamine B (RhB) and methyl orange (MO) by ZnIn2S4 under visible light irradiation was investigated based on the adsorption of dyes, the active species generated during the photocatalytic process and the degradation pathway. The results show that 97.8% of RhB and 5.6% of MO were degraded under the same conditions, respectively. Photocatalytic degradation of RhB was obviously superior to degradation of MO. The adsorption of MO was almost negligible and 56.8% of RhB was adsorbed on ZnIn2S4. Superoxide radical was the key active species, and hydroxyl radical played a supplementary role during photocatalytic process. Different chemical bonds with different bond energies were destroyed when degradation of RhB and MO. Photocatalytic degradation of RhB was superior to that of MO because the excellent adsorptivity of RhB than that of MO, and the CN bond of RhB was easier destroyed compared with azo bond of MO under the same conditions. The photocatalytic degradation of MO in a system in which superoxide radical was key active species can be improved by adding hole scavengers to inhibit recombination of holes and electrons, which would result in more electrons reducing oxygen to superoxide radical.

Microprofiles of activated sludge aggregates using microelectrodes in completely autotrophic nitrogen removal over nitrite (CANON) reactor

Microsensor measurements and fluorescence in situ hybridization (FISH) analysis were combined to investigate the microbial populations and activities in a laboratory-scale sequencing batch reactor (SBR) for completely autotrophic nitrogen removal over nitrite (CANON). Fed with synthetic wastewater rich in ammonia, the SBR removed 82.5±5.4% of influent nitrogen and a maximum nitrogen-removal rate of 0.52 kgN∙m–3∙d–1 was achieved. The FISH analysis revealed that aerobic ammonium-oxidizing bacteria (AerAOB) Nitrosomonas and anaerobic ammonium-oxidizing bacteria (AnAOB) dominated the community. To quantify the microbial activities inside the sludge aggregates, microprofiles were measured using pH, dissolved oxygen (DO), NH 4+, NO 2- and NO3- microelectrodes. In the outer layer of sludge aggregates (0–700 μm), nitrite-oxidizing bacteria (NOB) showed high activity with 4.1 μmol∙cm–3∙h–1 of maximum nitrate production rate under the condition of DO concentration higher than 3.3 mg∙L–1. Maximum AerAOB activity was detected in the middle layer (depths around 1700 μm) where DO concentration was 1.1 mg∙L–1. In the inner layer (2200–3500 μm), where DO concentration was below 0.9 mg∙L–1, AnAOB activity was detected. We thus showed that information obtained from microscopic views can be helpful in optimizing the SBR performance.

Polyvinylidene fluoride/poly(ethylene-co-vinyl alcohol) blended membranes and a systematic insight into their antifouling properties

PVDF/EVOH (P/E) membranes were prepared via immersion precipitation to realize an antifouling property better than that of PVDF membranes. To determine the optimum conditions for preparing a P/E blended membrane, the effects of the P/E weight ratio and temperature of the coagulation bath on the properties of the blended membrane were investigated. The fouling behaviors and membrane–foulant interaction force of the P/E blended membrane were compared with those of a PVDF membrane for unraveling its antifouling ability. Results show that the P/E membrane appeared to be more hydrophilic, having a higher pure-water flux and bovine serum albumin rejection rate than the PVDF membrane. The integrated performance of the P/E membrane was best when the P/E weight ratio and temperature of the coagulation bath were fixed at 9/1 and 20 °C, respectively. Analysis of the antifouling ability revealed that the flux decline rate of the P/E membrane was much less than that of the PVDF membrane. This phenomenon, combined with the measurements of the membrane–foulant interaction force, demonstrates that the membrane–fouling interaction force was reduced by the addition of EVOH, which could weaken the adsorption and accumulation of foulants on the membrane surface or pores and reduce the rate of membrane flux decline.

Effect of Hydration Forces on Protein Fouling of Ultrafiltration Membranes: The Role of Protein Charge, Hydrated Ion Species, and Membrane Hydrophilicity

To investigate the influence of hydration forces on the protein fouling of membranes and the major influence factors of hydration forces during the ultrafiltration process, bovine serum albumin (BSA) was chosen as model foulant. For various pH levels and hydrated ion and membrane species, the membrane-BSA and BSA-BSA interaction forces, and fouling experiments with BSA, as a function of ionic strength, were measured. Results showed that hydration forces were a universal phenomenon during the membrane filtration process, when the levels of pH, ion species, and membrane performances were appropriate. First, for the BSA negatively charged or neutral, hydration forces caused a decrease in the membrane fouling. Conversely, for the BSA positively charged, the hydration forces were absent because the counterions were not hydrated, and membrane fouling was enhanced. For different hydrated ions, the smaller the radii of the ions were, the stronger the hydration forces that were produced, and the membrane fouling observed was less, indicating that hydration forces are closely correlated with the size of the hydrated ions. Moreover, in comparison with a hydrophobic membrane, it is more difficult to observe hydrophilic membrane-BSA hydration forces because the hydrophilic membrane surface adsorbs water molecules, which weakens its binding efficiency to hydrated ions.

Understanding PVDF ultrafiltration membrane fouling behaviour through model solutions and secondary wastewater effluent

AbstractIn this work, we investigated the fouling behaviour of polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes. PVDF UF membranes prepared by the phase separation method were used to filter four sample solutions, namely bovine serum albumin (BSA), sodium alginate (SA), humic acid (HA) and secondary wastewater effluent organic matter (EfOM). Fouling experiments were carried out in a dead-end filtration set-up. Besides, the removal rate of dissolved organic carbon, the distribution of molecular weight and the permeability of feed water were inspected through direct comparison of the surface morphology of an uncontaminated and a contaminated membrane. The different fouling behaviours of BSA, SA, HA and EfOM were noted. It was found that the flux of the BSA-fouled membrane declined sharply at the initial filtration stage, but a more significant flux decline occurred for the SA-fouled membrane at the later filtration stage. For the HA-fouled membrane, a gradual flux decline was observed throughou...