Understanding the interaction mechanism of algal cells and soluble algal products foulants in forward osmosis dewatering

Unveiling the role of stratified extracellular polymeric substances in membrane-based microalgae harvesting: Thermodynamic and computational insights

Membrane separation technology has emerged as a highly energy-efficient method for microalgae enrichment and harvesting in wastewater treatment. However, membrane fouling caused by algal cells and stratified extracellular polymeric substances (EPS) remains a critical barrier to its industrial-scale application. This study meticulously investigates the micro process of algae-derived pollutants stacking to the membrane surface affected by stratified EPS. The fouling process resulting from algal cell particle deposition and cake layer formation are clearly simulated using a semi-coupled computational method of Computational Fluid Dynamics (CFD)-Discrete Element Method (DEM) for the first time. The results reveal that the hydrophilic component and spatial network structure of soluble EPS (S-EPS) effectively impede the algae-membrane adhesion, and enable the algal cake layer exhibit "dynamic membrane" characteristic to enhance the organic matter retention. In contrast, bound EPS (B-EPS) with higher protein content exhibits a stronger fouling potential and adhesion tendency of algal cells. The influence of stratified EPS on the variation of thermodynamic interaction with contact scale in the sphere-plane/sphere-sphere model is inventively conducted. Based on different algal cell filtration modes, a sequential increase in the eigenvalue n was observed by delaminating EPS layer by layer, indicative of a more severe membrane pore blockage. The semi-coupled CFD-DEM method provides a quantitative analysis of the deposition process, offering spatial resolution and force analysis for algal-derived pollutants. Additionally, we propose a novel calculation method to reverse the deposition process based on the particle stress, providing a valuable reference for simulating membrane-based microalgae harvesting under the influence of stratified EPS.

Application of microbubble air flotation to harvest Microcystis sp. from agriculture wastewater: The regulation and mechanisms

The harvesting of microalgae is the main bottleneck of its large-scale biomass production, and seeking an efficient, green, and low-cost microalgae harvesting technology is one of the urgent problems to be solved. Microbubble air flotation has been proven to be an effective measure, but the mechanisms of microbubbles-algal cell attachment are still unclear. In this study, microbubble air flotation was used as a harvesting method for Microcystis cultured in agricultural wastewater. The process mechanism of microbubble air flotation harvesting microalgae in wastewater was fully revealed from three aspects (the design of bubble formation, the adhesion law, and the recovery rate of microalgae under different working conditions). The results show that the length of the release pipe is the main factor affecting the proportion of microbubbles with a particle size of less than 50 μm. In the process of adhesion, when the particle size of microbubbles is 0.6-1.7 times the size of Microcystis, the adhesion efficiency of microbubbles to Microcystis is the highest. Under the conditions of pressure 0.45 MPa, gas-liquid ratio 5%, and release pipe length 100 cm, the harvesting performance of Microcystis was the best. Microbubble air flotation has better harvesting performance (63.5%, collection rate) of Microcystis with higher density. By understanding the mechanism of microbubble flotation, the technical parameters of microbubble flotation for harvesting energy microalgae are optimized to provide support for the development of efficient and low-cost devices and equipment for collecting microalgae.

Fouling evolution of extracellular polymeric substances in forward osmosis based microalgae dewatering

Membrane fouling was still a challenge for the potential application of forward osmosis (FO) in algae dewatering. In this study, the fouling behaviors of Chlorella vulgaris and Scenedesmus obliquus were compared in the FO membrane filtration process, and the roles of their soluble-extracellular polymeric substances (sEPS) and bound-EPS (bEPS) in fouling performance were investigated. The results showed that fouling behaviors could be divided into two stages including a quickly dropped and later a stable process. The bEPS of both species presented the highest flux decline (about 40.0%) by comparison with their sEPS, cells and broth. This performance was consistent with the largest dissolved organic carbon losses in feed solutions, and the highest interfacial free energy analyzed by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The chemical characterizations of algal foulants further showed that the severe fouling performance was also consistent with a proper ratio of carbohydrates and proteins contents in the cake layer, as well as the higher low molecular weight (LMW) components. Compared with the bEPS, the sEPS was crucial for the membrane fouling of S. obliquus, and an evolution of the membrane fouling structure was found in both species at the later filtration stage. This work clearly revealed the fundamental mechanism of FO membrane fouling caused by real microalgal suspension, and it will improve our understanding of the evolutionary fouling performances of algal EPS.

Effect of ferrous-activated calcium peroxide oxidation on forward osmosis treatment of algae-laden water: Membrane fouling mitigation and mechanism

Forward osmosis (FO) is a high-efficiency and low-energy consumption way for algae-laden water treatment, whereas membrane fouling is still an unavoidable problem in its practical application. In this work, a strategy of ferrous-activated calcium peroxide (Fe(II)/CaO₂) was proposed to control FO membrane fouling in the purification of algae-laden water. With the treatment of Fe(II)/CaO₂, the aggregation of algal contaminants was promoted, the cell viability and integrity were well preserved, and the fluorescent organics were efficiently removed. With respect to the fouling of FO membrane, the flux decline was generally alleviated, and the flux recovery was promoted to varying degrees under different process conditions. It could be revealed through the extended Derjaguin-Landau-Verwey-Overbeek theory that the adhesion of contaminants and membrane surfaces was reduced by Fe(II)/CaO₂ treatment. The interface morphologies and functional groups of membrane verified that Fe(II)/CaO₂ could mitigate the fouling by reducing the amount of algal contaminants adhering to the FO membrane. The co-coagulation of in-situ Fe(III) together with Ca(OH)₂, as well as the oxidation of ^•OH were the main mechanisms for fouling mitigation. In sum, the Fe(II)/CaO₂ process could effectively improve the efficiency of FO for algae-laden water treatment, and has broad application prospects.

Effect of EPS and its forms of aerobic granular sludge on sludge aggregation performance during granulation process based on XDLVO theory

Different forms of extracellular polymeric substances (EPS) play different roles in the formation process of aerobic granular sludge (AGS). This work focused on the contribution of loosely bound EPS (LB-EPS), tightly bound EPS (TB-EPS) and EPS to the aggregation between sludge cells during the start-up of aerobic granular sludge in a sequencing batch reactor. By analyzing the changes of sludge surface characteristics before and after the extraction of each layer of EPS, the contribution of LB-EPS, TB-EPS and EPS to the adhesion and aggregation of sludge cells in the granulation was calculated by surface thermodynamics and the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The experimental results showed that AGS reactor basically completed the granulation process and kept stable operation within 40 d. In the process of sludge granulation, the effect of LB-EPS on the aggregation of sludge cells shifted from attraction to repulsion. TB-EPS could improve the surface zeta potential and hydrophobicity and show an attractive effect in the granulation process, which was conducive to the adhesion between sludge cells and was the main contributor to the formation of granules. Additionally, EPS played an apparently positive role in sludge flocculation and could promoted cell aggregation in the whole granulation process.

Membrane Fouling in Algal Separation Processes: A Review of Influencing Factors and Mechanisms

The use of algal biotechnologies in the production of biofuels, food, and valuable products has gained momentum in recent years, owing to its distinctive rapid growth and compatibility to be coupled to wastewater treatment in membrane photobioreactors. However, membrane fouling is considered a main drawback that offsets the benefits of algal applications by heavily impacting the operation cost. Several fouling control strategies have been proposed, addressing aspects related to characteristics in the feed water and membranes, operational conditions, and biomass properties. However, the lack of understanding of the mechanisms behind algal biofouling and control challenges the development of cost-effective strategies needed for the long-term operation of membrane photobioreactors. This paper reviews the progress on algal membrane fouling and control strategies. Herein, we summarize information in the composition and characteristics of algal foulants, namely algal organic matter, cells, and transparent exopolymer particles; and review their dynamic responses to modifications in the feedwater, membrane surface, hydrodynamics, and cleaning methods. This review comparatively analyzes (i) efficiency in fouling control or mitigation, (ii) advantages and drawbacks, (iii) technological performance, and (iv) challenges and knowledge gaps. Ultimately, the article provides a primary reference of algal biofouling in membrane-based applications.