Dissect the role of particle size through collision-attachment simulations for colloidal fouling of RO/NF membranes

Unraveling the mechanism of fouling mitigation in AGS-MBR system: From AGS properties to foulant interactions

Aerobic granular sludge (AGS) has demonstrated a lower fouling propensity than floc sludge in membrane bioreactors (MBRs) due to various hypotheses, including differences in particle size and the efficacy of physical scouring. However, controversy exists regarding the dominant cause of this lower fouling. Therefore, in this work, we systematically investigated the contribution of four potential mechanisms of AGS on membrane fouling alleviation in MBRs: 1) loosening cake layer; 2) scouring of the membrane surface; 3) regulating soluble microbial product (SMP) secretion; and 4) changing the rheology of the bulk solution. Our results showed that, regardless of granular size range, AGS hardly caused cake fouling due to its low hydraulic resistances (<0.8 × 1012 m-1) and limited accumulation on the membrane surface. Scouring by AGS was ineffective in reducing the thickness and hydraulic resistance of the fouling layer compared with granular activated carbon, a commonly used scouring material for MBRs. Furthermore, liquid chromatography-organic carbon detection-organic nitrogen detection (LC-OCD-OND) results indicated that the lower fouling was related to reduced SMP secretion by AGS, with an optimal particle size (800-1000 μm) at which SMP secretion was minimized. AGS with this optimal particle size secreted over 54 % less high-molecular-weight SMP compared to floc sludge. As granule size further increased, SMP secretion increased due to biomass decay and cell lysis resulting from substrate transfer limitations in granules. Moreover, compared to floc sludge, granular sludge bulk solution exhibited lower viscosity, particularly in the 450-1000 μm size range. This enhanced rheological behavior could potentially improve shear stress induced by aeration, thereby mitigating membrane fouling. These findings emphasize that the indirect effects of AGS, including reduced SMP secretion and improved rheological properties, played a crucial role in the lower membrane fouling in AGS-MBRs, while direct effects such as loosening cake layer and the scouring effect played minor roles.

Collision-attachment simulation of membrane fouling by oppositely and similarly charged colloids

The fouling propensity of oppositely charged colloids (OCC) and similarly charged colloids (SCC) on reverse osmosis (RO) and nanofiltration (NF) membranes are systematically investigated using a developed collision-attachment approach. The probability of successful colloidal attachment (i.e., attachment efficiency) is modelled by Boltzmann energy distribution, which captures the critical roles of colloid-colloid/membrane interaction and permeate drag. Our simulations highlight the important effects of ionic strength Is, colloidal size dp and initial flux J0 on combined fouling. In a moderate condition (e.g., Is =10 mM, dp=50 nm and J0= 100 L/m2h), OCC mixtures shows more severe fouling compared to the respective single foulant owing to electrostatic neutralization. In contrast, the flux loss of SCC species falls between those of the two single foulants but more closely resembles that of the single low-charged colloids due to its weak electrostatic repulsion. Increased ionic strength Is leads to less severe fouling for OCC but more severe fouling for SCC, as a result of the suppressed electrostatic attraction/repulsion. At a high Is (e.g., 3-5 M), all the single and mixed systems show the identical pseudo-stable flux Js. Small colloidal size leads to the drag-controlled condition, where severe fouling occurs for both single and mixed foulants. On the contrary, better flux stability appears at greater dp for both individual and mixed species, thanks to the increasingly dominated role of energy barrier and thus lowered attachment efficiency. Furthermore, higher J0 above limiting flux exerts greater permeate drag, leading to elevated attachment efficiency, and thus more flux losses for both OCC and SCC. Our modelling gains deep insights into the role of energy barrier, permeate drag, and attachment efficiency in governing combined fouling, which provides crucial guidelines for fouling reduction in practical engineering.

Relating critical and limiting fluxes to metastable and long-term stable fluxes in colloidal membrane filtration through collision-attachment theory

In membrane technology for water/wastewater treatment, the concepts of critical flux (J_C) and limiting flux (J_L) suggest the existence of a threshold flux below which no fouling occurs. However, their important roles on stable flux duration have not been sufficiently understood. This work adopts a collision-attachment approach to clarify the relationship of J_C, J_L to metastable (i.e., short-term stable) and long-term stable fluxes based on their dependence on initial flux (J₀), foulant-clean-membrane energy barrier (E_f-m), and foulant-fouled-membrane energy barrier (E_f-f). When J₀ is below J_L, water flux remains stable over a long time even for the case of J₀ over J_C, thanks to the strongly repulsive E_f-f. At J₀ > J_L and J₀ > J_C, the water flux is unstable at the beginning of filtration, and the flux ultimately decreases to J_L as the long-term stable flux. Under the condition of J_L < J₀ ≤ J_C, an initial metastable flux appears owing to the high E_f-m, with longer metastable period observed at lower J₀ and for more hydrophilic/charged membrane or colloids. Nevertheless, rapid flux decline occurs subsequently due to the energy barrier shifting to weak E_f-f, and the water flux eventually degenerates to J_L in long-term fouling duration. Our results provide significant guidelines for fouling control strategies with respect to membrane design, feedwater pretreatment, and operational optimization.

Impact of pre-coagulation on the ceramic membrane process during oil-water emulsion separation: Fouling behavior and mechanism

Coagulation has been evaluated as an economical and effective pre-treatment method for controlling membrane fouling. We investigated the influence of the pre-coagulation of oil-water (O/W) emulsions on the formation of membrane fouling in the ceramic membrane process. The results confirmed that pre-coagulation effectively mitigated the fouling formation on the ceramic membrane surface during the O/W emulsion separation. The mechanism of mitigating membrane fouling by pre-coagulation was proposed, owing to the reduction in the zeta potential value of oil droplets by pre-coagulation, resulting in weak electrostatic attraction between oil droplets and ceramic membrane surfaces, and an increase in the size of the oil droplets by pre-coagulation, leading the formation of a cake layer fouling. In addition, the decrease in the hydrophobicity of oil droplets by pre-coagulation resulted in alleviating the hydrophobic interaction between oil droplets and membrane surface. The proposed fouling mechanism was supported by the characterization of the virgin and fouled membrane surfaces and the analysis of the fouling resistance ability of the membranes. Our study could be indicative of mitigation protocols that can be used to alleviate membrane fouling on ceramic membranes during oily wastewater treatment.

Autopsy of Used Reverse Osmosis Membranes from the Largest Seawater Desalination Plant in Oman

The Barka desalination plant, commissioned in 2018, is the largest desalination plant in Oman. It has a capacity of 281 MLD with a reverse osmosis (RO) first-pass recovery rate of 46%. As part of the standard operator practice, a membrane autopsy was conducted to determine the cause of reductions in membrane performance. This study investigated fouled membranes (model No. SW30HRLE-440) from two different locations in the membrane rack. Various analytical methods were used to conduct the membrane autopsy. Field-emission scanning electron microscopy/energy-dispersive X-ray (FESEM/EDS) analyses of membrane samples showed major components of inorganic foulants. Moreover, black and salt-like crystals deposited on the membrane surface revealed significant carbon (C) components and oxygen (O), with a small amount of magnesium (Mg), chloride (Cl), sodium (Na), aluminium (Al), and calcium (Ca), respectively. A Fourier transform infrared (FTIR) analysis revealed the presence of long-chain hydrocarbons, carboxylic acids/esters, carbohydrates/polysaccharides, and inorganic foulants. Thermogravimetric analyses (TGA) of the membranes showed a high initial weight loss due to organic and inorganic fouling. X-ray photoelectron (XPS) analyses further confirmed the presence of inorganic and organic foulants on the membrane surfaces. Bacteria identification results showed the presence of Bacillus cereus and Bacillus marisflavi. This paper offers a detailed analysis of the foulants present on the reverse osmosis membrane surface and sub-surface before and after a cleaning process.

Membrane Photobioreactor Applied for Municipal Wastewater Treatment at a High Solids Retention Time: Effects of Microalgae Decay on Treatment Performance and Biomass Properties

Membrane photobioreactor (MPBR) technology is a microalgae-based system that can simultaneously realize nutrient recovery and microalgae cultivation in a single step. Current research is mainly focused on the operation of MPBR at a medium SRT. The operation of MPBR at a high SRT is rarely reported in MPBR studies. Therefore, this study conducted a submerged MPBR to treat synthetic municipal wastewater at a long solids retention time of 50 d. It was found that serious microalgae decay occurred on day 23. A series of characterizations, including the biomass concentration, chlorophyll-a content, nutrients removal, and physical-chemical properties of the microalgae, were conducted to evaluate how microalgae decay affects the treatment performance and biomass properties. The results showed that the biomass concentration and chlorophyll-a/MLSS dropped rapidly from 3.48 to 1.94 g/L and 34.56 to 10.71 mg/g, respectively, after the occurrence of decay. The effluent quality significantly deteriorated, corresponding to the total effluent nitrogen and total phosphorus concentration sharply rising and exceeding that of the feed. In addition, the particle became larger, the content of the extracellular polymeric substances (EPSs) decreased, and the soluble microbial products (SMPs) increased instantaneously. However, the filtration resistance had no significant increase because of the comprehensive interactions of the floc size, EPSs, and SMPs. The above results suggest that the MPBR system cannot maintain long-term operation under a high SRT for municipal wastewater treatment. In addition, the biological treatment performance of the MPBR deteriorated while the antifouling performance of the microalgae flocs improved after the occurrence of decay. The occurrence of microalgae decay was attributed to the double stresses from the light shading and intraspecific competition under high biomass concentration. Therefore, to avoid microalgae decay, periodic biomass removal is required to control the environmental stress within the tolerance range of the microalgae. Further studies are required to explore the underlying mechanism of the occurrence of decay.