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Any single existing technique cannot monitor the overall membrane fouling behavior in heterotrophic microalgae harvesting. Herein, multi-dimensional methods, included deposition dynamic, foulants spatial distribution, chemical composition and interactive force features, were developed to dissect the fouling mechanisms. Findings indicate that particles deposition rate was the largest in the initial stage due to the high permeate flux. Protein deposition rate had greatest influence on the permeate flux decline. Cake layer condense process was confirmed by the rigid particles. And the cake layer with low porosity at the bottom and orderly deposition presented good resistance performance on algae-related fouling. 3D evolution of the cake layer was a Brownian-tree liked formation process, namely, rivet seed-cluster-monolayer-multilayers. Interaction force analysis confirmed that the interfaces with protein and the loose cake layer always presented strong attractive forces. Permeate drag force and electrical repulsion force played important role on the algae cake formation in the initial stage and cake layer formation stage, respectively. Multi-dimensional techniques shed light on foulants occurrence and accumulation as well as the cake layer evolution in the membrane microalgae harvesting process.

During algae harvesting using membrane technology, membrane fouling caused by the deposition of algae cells and extracellular organic matter (EOM) poses a major challenge. In this study, axial vibration membrane (AVM) filtration was conducted at 0, 5 and 10 Hz. As the frequency increased, AVM could effectively reduce the reversible fouling caused by the deposition of algae cells on the membranes. With the increase of frequency from 0 to 10 Hz in the 2-h filtration experiments, the amount of algae deposited on the membranes sharply decreased from 8.64 to 0.03 g/m2. For the reversible EOM on the membranes, with increasing frequency, the protein and polysaccharide contents exhibited declining trends, and no humic-like material was observed. It was also found that both low-MW ( 200 kDa) EOM easily caused reversible membrane fouling. The irreversible EOM adhesion to the membrane consisted of protein, polysaccharide and humic-like material. With the increase of frequency the MW peak of irreversible EOM had a right shift tendency. At 0, 5 and 10 Hz, EOM with MW of 3.5, 4 and 5 kDa, respectively, was more easily adsorbed on the membranes and caused irreversible membrane fouling.

Membrane fouling caused by algae cells and extracellular organic matter (EOM) is a major challenge in microalgae harvesting. In the present study, the axial vibration membrane (AVM) process can effectively reduce membrane fouling by increasing vibration frequency. An equation was obtained to explain why the vibration frequency has a more important impact on critical flux than the amplitude. During the continuous filtration, the flux decline rate was only 3.6% at the frequency of 10 Hz and amplitude of 1 cm. While at the frequency of 5 and 0 Hz the decline rates were 34% and 64.2%, respectively. At high frequency, AVM can not only prevent the deposition of algae cells on membrane, but also reduce the adsorption of EOM on membrane. Scanning electron microscope and fourier transform infrared spectroscopy analysis showed that AVM can effectively reduce EOM adsorbing on membrane at 10 Hz. AVM had better rejections of protein (28–39%) and polysaccharide (about 35%) at 10 Hz, compared with 0 and 5 Hz. Membranes had obvious rejection of low molecular weight EOM, regardless of at high or low frequency; however, compared to low frequency at high frequency AVM could reject more high molecular weight organics.

Membrane fouling by algae and extracellular organic matter (EOM) is a major problem in algae harvesting. In this study, the axial vibration ultrafiltration-membrane (AVM) is able to limit membrane fouling during filtration effectively. A membrane can achieve high critical flux at a high shear rate. During filtration, AVM is capable of operating with less fouling at a constant flux. The result from “extended Derjaguin, Landau, Verwey, Overbeek” (XDLVO) calculation indicates that with the increase of shear rate, it is more difficult for algae to foul the membrane. At a frequency of 5 Hz, the average inertial lift force is 0.024 nN, and the interaction force becomes a long-range attractive force that draws algae to the membrane; there are still certain smaller algae, algae debris and EOM that deposit on the membrane; leading to many algae depositing on the membrane. At a frequency of 10 Hz, the average inertial lift force is 0.12 nN, and there is a long-range repulsive region preventing algae from depositing on the membrane; however, the result shows that the mechanism of fouling mitigation by vibration is preventing algae from approaching the membrane, which reduces the deposition of algae on the membrane.

Microalgae harvesting has been a primary problem encountered during microalgae utilization because of the low biomass concentration of microalgae and the severe membrane fouling caused by extracellular organic matters (EOM). Our previous study verified the applicability of dynamic membrane technology for algae dewatering. The aims of this study are to investigate the performances of both the diatomite dynamic membrane (DDM) technology, used for algae dewatering during a long-term (96 h) operation period, and the membrane fouling mechanisms, as well as their relationship with algae growth phases. The stable filtration flux reached 12.8 L/m 2 h, and the ultimate biomass concentration reached 49 g/L. The DDM continued to have advantages in cost, energy consumption and ultimate algae concentration. However, the stability of the DDM was questionable for long-term operations. Algae growth phases had distinct impacts on the EOM concentrations and composition and, therefore, on the membrane fouling mechanisms. The osmotic pressure originating from the EOM might exist during the DDM filtration process and could be largely responsible for the high cake resistance. As the algae growth phase advanced from the logarithmic phase to the decline phase, membrane fouling became much more severe.

Membrane fouling is a main problem in algae harvesting and retarding membrane fouling is significant to increase filtration efficiency and reduce harvesting cost. The aeration membrane system could mitigate membrane fouling by the coarse bubbles and the vibration membrane system could produce high shear by the vibration of membrane to mitigate fouling. In the critical flux experiment, the aeration membrane had a low critical flux of 34 L/(m2h) compared with the vibration membrane (49 L/(m2h)). After experiencing 12-h continuous filtration, the flux decline rates of vibration and aeration membrane were 3.6% and 46%, respectively. Based on the Extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory, the free energy of fouled vibration and aeration membranes were −53.76 and −62.51 mJ/m2, respectively, indicating that vibration could better reduce the interaction of algae cells and membrane, compared with aeration. Moreover, vibration also had a better effect on the rejection of extracellular organic matter (EOM), which could reduce secondary pollution of the water body. The different working mechanism of vibration and aeration acting on membrane was draw, suggesting that the fluid velocity induced by vibration mainly acting on membrane surface could decrease membrane fouling, while the fluid velocity induced by aeration basically could not act on membrane.

This research investigated the characteristics of the dynamic membrane bioreactor (DMBR) for wastewater treatment, including operation performance, critical flux of dynamic membrane (DM), and structure of the cake layer. Various analytical methods were used. The DMBR exhibited excellent pollutant-removal efficiencies, and the critical flux of DM was in the range of 70–75 L/m 2 h. The DM operated at sub-critical flux exhibited longer filtration time and smaller flux decline in the filtration process. The DM formed by activated sludge exhibited a hierarchical structure, and the order of mean particle size of three cake layers was as follows: top cake layer

This research investigated the characteristics of bio-enhanced powder-activated carbon dynamic membrane (BPDM) reactor for municipal wastewater treatment in the laboratory-scale continuous-flow mode. The BPDM was amenable to continuous operation at high filtration fluxes in the range of 100–235 L/m 2 h. Mean particle sizes of the BPDM increased from the top cake layer to the middle cake layer, with the largest mean particle sizes found in the bottom cake layer. The quantity of polysaccharides was greater than the quantity of proteins in the extracellular polymeric substances in the bio-enhanced powder-activated carbon mixed liquor, and the quantity of polysaccharides attached to the threads was much greater than the quantity of proteins attached to the threads of both the surface support mesh and the inner support mesh. The polysaccharides might be primarily responsible for the attachment of the BPDM to the stainless steel support mesh. Experimental results obtained via three-dimensional excitation–emission matrix fluorescence spectroscopy analysis showed that the main protein-like substances fouling the BPDM and stainless steel mesh could be identified as simple aromatic proteins, soluble microbial by-product-like materials. The amino acids in a hydrolyzed sample from the BPDM were present in larger quantities than those in the influent or the bio-enhanced powder-activated carbon mixed liquor. The BPDM was found to effectively retain the proteins.