Model assessment of the prevailing fouling mechanisms in a submerged membrane anaerobic reactor treating low-strength wastewater

Modeling and simulation-assisted strategies for effective membrane-fouling mitigation during membrane bioreactor operation

This research principally aimed to present a suitable strategy for membrane-fouling mitigation in membrane-bioreactors (MBRs). The current strategies for membrane-fouling mitigation before initiating the process in many cases, are unmodifiable for a specific MBR system along the operations. Thus, membrane-fouling strategies during filtration should be applied. To select the best and most economical method for controlling fouling during the operations, the quality (site and mechanism) as well as quantity (thickness, mass, and porosity of the cake layer, and pore resistances) of fouling should be predicted. Accordingly, in this research, two powerful tools, i.e. modeling and simulation, have been used for predicting the quality and quantity of fouling, respectively. Through modeling, the best model describing the site and mechanism of fouling was chosen. Through simulation, the thickness, mass and porosity of the cake layer, along with resistance of cake and pores were calculated. In addition, the match between the results of modeling, simulation, and experimental results confirmed the accuracy of the performed predictions. Ultimately, to achieve the minimum membrane-fouling during filtration, based on the modeling results, the general solution of washing (physical or chemical), and based on the simulation results, its intensity (low, medium, and high) were proposed.

Critical fractions in reclaimed water responsible for membrane fouling: Isolation, fouling characteristics, quantitative and qualitative variations in practical application

Considering the different fouling characteristics between model foulants and organic components in real reclaimed water, it is of great importance to identify the critical foulants responsible for membrane fouling. This study identified and isolated the fraction with molecular weight (MW) > 100 kDa as the critical foulant in secondary effluent by MW cut-off membrane of 100 kDa with high efficiency. This fraction accounted for 92.2% membrane fouling of raw water, including 28.7%, 29.7% and 33.8% fouling contribution by subfractions with MW between 100-300, 300-500 and > 500 kDa. Specifically, the critical fraction with MW > 100 kDa were mainly distributed in two parts: < 0.22 μm and > 0.45 μm, corresponding to 41.9% and 56.9% fouling contribution of this fraction. Furthermore, both total organic carbon (TOC) and fouling potential of fraction with MW > 100 kDa were monitored, presenting about threefold increase from September to January in next year. Membrane fouling contribution of this critical fraction in raw secondary effluent were mainly distributed in 85∼95% throughout the 5 months, demonstrating its predominant fouling propensity. Moreover, the TOC concentration of fraction with MW > 100 kDa presented distinct positive correlation with the fouling potential of raw secondary effluent (R2 = 0.947), which was promising to be a surrogate for predicting membrane fouling in practical application.

Alleviating Ultrafiltration Membrane Fouling Caused by Effluent Organic Matter Using Pre-Ozonation: A Perspective of EEM and Molecular Weight Distribution

Wastewater reclamation has gradually become an important way to cope with the global water crisis. Ultrafiltration plays an imperative part as a safeguard for the aim but is often limited by membrane fouling. Effluent organic matter (EfOM) has been known to be a major foulant during ultrafiltration. Hence, the primary aim of this study was to investigate the effects of pre-ozonation on the membrane fouling caused by EfOM in secondary wastewater effluents. In addition, the physicochemical property changes of EfOM during pre-ozonation and the subsequent influence on membrane fouling were systemically investigated. The combined fouling model and the morphology of fouled membrane were adopted to scrutinize the fouling alleviation mechanism by pre-ozonation. It was found that membrane fouling by EfOM was dominated by hydraulically reversible fouling. In addition, an obvious fouling reduction was achieved by pre-ozonation with 1.0 mg O₃/mg DOC. The resistance results showed that the normalized hydraulically reversible resistance was reduced by ~60%. The water quality analysis indicated that ozone degraded high molecular weight organics such as microbial metabolites and aromatic protein and medium molecular weight organics (humic acid-like) into smaller fractions and formed a looser fouling layer on the membrane surface. Furthermore, pre-ozonation made the cake layer foul towards pore blocking, thereby reducing fouling. In addition, there was a little degradation in the pollutant removal performance with pre-ozonation. The DOC removal rate decreased by more than 18%, while UV₂₅₄ decreased by more than 20%.

Critical minority fractions causing membrane fouling in reclaimed water: Fouling characteristics, mechanisms and control strategies

In regard to membrane-based technologies of wastewater reclamation, the reported key foulants were faced with dilemma that they could not be effectively separated and extracted from reclaimed water for thorough investigation. In this study, the crucial foulants were proposed as "critical minority fraction (F_CM)", representing the fraction with molecular weight (MW) > 100 kDa which could be easily separated by physical filtration using MW cut-off membrane of 100 kDa with fairly high recovery ratio. F_CM with low dissolved organic carbon (DOC) concentration (∼1 mg/L) accounted for less than 20% of the total DOC in reclaimed water, while contributed to more than 90% of the membrane fouling, and thus F_CM could be considered as a "perfect criminal" causing membrane fouling. Furthermore, pivotal fouling mechanism was attributed to the significant attractive force between F_CM and membranes, which led to severe fouling development due to the aggregation of F_CM on membrane surface. Fluorescent chromophores of F_CM were concentrated in regions of proteins and soluble microbial products, with proteins and polysaccharides accounted for 45.2% and 25.1% of the total DOC, specifically. F_CM was further fractionated into six fractions, among which hydrophobic acids and hydrophobic neutrals were the dominant components in terms of DOC content (∼80%) as well as fouling contribution. Regarding to these pronounced properties of F_CM, targeted fouling control strategies including ozonation and coagulation were applied and proved to achieve remarkable fouling control effect. High-performance size-exclusion chromatography results suggested that ozonation achieved distinct transformation of F_CM into low MW fractions, while coagulation removed F_CM directly, thus leading to effective fouling alleviation. Therefore, the investigation of the critical foulants was expected to help glean valuable insight into the fouling mechanism and develop targeted fouling control technologies in practical applications.

Fouling mechanisms in anoxic-aerobic sequencing batch membrane bioreactor based on adapted Hermia models and main foulant characteristics

Various derivatives of Hermia models (complete pore blocking, intermediate pore blocking, cake layer formation, and standard pore blocking) and different assessments of foulant characteristics have long been used to determine the membrane fouling mechanisms. Accordingly, this study aims to adapt Hermia models and their combination according to the operating conditions of an anoxic-aerobic sequencing batch membrane bioreactor (A/O-SBMBR). In addition, fouling mechanisms of the A/O-SBMBR were assessed using these models along with the main foulant characteristics. Models fitting with the transmembrane pressure (TMP) data indicated that the intermediate-standard model was accounting for the increased fouling during the whole regular operating period, with the residual sum of squares (RSS) of 58.3. A more detailed study on the distinct stages of TMP curve showed that the intermediate-standard model had the best fit in stages of 2 and 3, with the RSS equal to 2.6 and 2.8, respectively. Also, the complete-standard model provided the best description of the fouling mechanism in stage 4, with the RSS of 12.5. Different analyzes revealed how the main foulant characteristics affect the occurrence of intermediate, complete and standard fouling mechanisms in the A/O-SBMBR, which is consistent with the fitting results of the adapted Hermia models. The modeling and experimental methods used in the presented study provided a valuable basis to prevent and control membrane fouling in membrane bioreactors.

Fouling Behavior in a High-Rate Anaerobic Submerged Membrane Bioreactor (AnMBR) for Palm Oil Mill Effluent (POME) Treatment

The characteristics of foulant in the cake layer and bulk suspended solids of a 10 L submerged anaerobic membrane bioreactor (AnMBR) used for treatment of palm oil mill effluent (POME) were investigated in this study. Three different organic loading rates (OLRs) were applied with prolonged sludge retention time throughout a long operation time (270 days). The organic foulant was characterized by biomass concentration and concentration of extracellular polymeric substances (EPS). The thicknesses of the cake layer and foulant were analyzed by confocal laser scanning microscopy and Fourier transform infrared spectroscopy. The membrane morphology and inorganic elements were analyzed by field emission scanning electron microscope coupled with energy dispersive X-ray spectrometer. Roughness of membrane was analyzed by atomic force microscopy. The results showed that the formation and accumulation of protein EPS in the cake layer was the key contributor to most of the fouling. The transmembrane pressure evolution showed that attachment, adsorption, and entrapment of protein EPS occurred in the membrane pores. In addition, the hydrophilic charge of proteins and polysaccharides influenced the adsorption mechanism. The composition of the feed (including hydroxyl group and fatty acid compounds) and microbial metabolic products (protein) significantly affected membrane fouling in the high-rate operation.

Membrane processes

This literature review provides a review for publications in 2018 and 2019 and includes information membrane processes findings for municipal and industrial applications. This review is a subsection of the annual Water Environment Federation literature review for Treatment Systems section. The following topics are covered in this literature review: industrial wastewater and membrane. Bioreactor (MBR) configuration, membrane fouling, design, reuse, nutrient removal, operation, anaerobic membrane systems, microconstituents removal, membrane technology advances, and modeling. Other sub-sections of the Treatment Systems section that might relate to this literature review include the following: Biological Fixed-Film Systems, Activated Sludge, and Other Aerobic Suspended Culture Processes, Anaerobic Processes, and Water Reclamation and Reuse. This publication might also have related information on membrane processes: Industrial Wastes, Hazardous Wastes, and Fate and Effects of Pollutants.