Combining patterned membrane filtration and flocculation for economical microalgae harvesting

Harvesting marine microalgae Tetraselmis sp. using cellulose acetate membrane

This study presents the development and application of a cellulose acetate phase-inversion membrane for the efficient harvesting of Tetraselmis sp., a promising alternative for aquaculture feedstock. Once fabricated, the cellulose acetate membrane was characterized, and its performance was evaluated through the filtration of Tetraselmis sp. broth. The results demonstrated that the developed membrane exhibited exceptional microalgae harvesting efficiency. It showed a low intrinsic resistance and a high clean water permeability of 1100 L/(m2·h·bar), enabling high-throughput filtration of Tetraselmis sp. culture with a permeability of 400 L/(m2·h·bar) and a volume reduction factor of 2.5 ×. The cellulose acetate -based membrane demonstrated robust filtration performance over a 7-day back concentration filtration with minimum irreversible fouling of only 22.5 % irreversibility even without any cleaning. These results highlighted the potential of cellulose acetate as a versatile base polymer for custom-membrane for microalgae harvesting.

Control of cyanobacterial bloom and purification of bloom-laden water by sequential electro-oxidation and electro-oxidation-coagulation

The outbreaks of cyanobacterial blooms have caused severe threat to aquatic ecosystem and public health. In this work, electrochemical technology with RuO2/IrO2/Ti (RIT) or/and Al as anode for cyanobacterial bloom control and simultaneous water purification were studied. Compared with RIT-Al and Al electrodes, RIT exhibited the highest effects on bloom algae inactivation and inhibition of algae regrowth. Live/dead analysis, SEM, intracellular reactive oxygen species (ROS) and antioxidant system activities revealed that RIT could disintegrate bloom flocs and damage embedded algal cells due to high intensity of oxidation. With the lysis of cyanobacterial bloom, high content of intracellular compounds containing organic carbon, nitrogen and phosphorus released, necessitating water quality restoration. In the subsequent water purification process, RIT-Al overtook RIT and Al in removal of organic and nutrient pollutants due to the complex effects of electro-oxidation, coagulation, co-precipitation, electro-nitrification and electro-denitrification. Therefore, sequential electro-oxidation and electro-oxidation-coagulation process was an effective method for control cyanobacteria bloom and simultaneous removal of DOM, microcystin-LR (MC-LR), nitrogen and phosphorus, which is a promising technology.

Microalgae harvesting for wastewater treatment and resources recovery: A review

Microalgae-based wastewater treatment has been conceived to obtain reclaimed water and produce microalgal biomass for bio-based products and biofuels generation. However, microalgal biomass harvesting is challenging and expensive, hence one of the main bottlenecks for full-scale implementation. Finding an integrated approach that covers concepts of engineering, green chemistry and the application of microbial anabolism driven towards the harvesting processes, is mandatory for the widespread establishment of full-scale microalgae wastewater treatment plants. By using nature-based substances and applying concepts of chemical functionalization in already established harvesting methods, the costs of harvesting processes could be reduced while preventing microalgae biomass contamination. Moreover, microalgae produced during wastewater treatment have unique culture characteristics, such as the consortia, which are primarily composed of microalgae and bacteria, that should be accounted for prior to downstream processing. The aim of this review is to examine recent advances in microalgal biomass harvesting and recovery in wastewater treatment systems, considering the impact of consortia variability. The costs of available harvesting technologies, such as coagulation/flocculation, coupled to sedimentation and differential air flotation, are provided. Additionally, promising technologies are discussed, including autoflocculation, bioflocculation, new filtration materials, nanotechnology, microfluidic and magnetic methods.

Locking Patterned Carbon Nanotube Cages by Nanofibrous Mats to Construct Cucurbituril[n]-Based Ultrapermselective Dye/Salt Separation Membranes

Surface patterning is a promising strategy to overcome the trade-off effect of separation membranes. Herein, a bottom-up patterning strategy of locking micron-sized carbon nanotube cages (CNCs) onto a nanofibrous substrate is developed. The strongly enhanced capillary force triggered by the abundant narrow channels in CNCs endows the precisely patterned substrate with excellent wettability and antigravity water transport. Both are crucial for the preloading of cucurbit[n]uril (CB6)-embeded amine solution to form an ultrathin (∼20 nm) polyamide selective layer clinging to CNCs-patterned substrate. The CNCs-patterning and CB6 modification result in a 40.2% increased transmission area, a reduced thickness, and a lowered cross-linking degree of selective layer, leading to a high water permeability of 124.9 L·m^-2 h^-1 bar^-1 and a rejection of 99.9% for Janus Green B (511.07 Da), an order of magnitude higher than that of commercial membranes. The new patterning strategy provides technical and theoretical guidance for designing next-generation dye/salt separation membranes.

Microalgal wastewater recycling: Suitability of harvesting methods and influence on growth mechanisms

Wastewater recycling helps address the challenge of microalgae biomass commercialization by allowing for efficient resource recovery. In this study, three conventional harvesting methods, including centrifugation, microfiltration, and flocculation sedimentation, were investigated to explore the effects of harvesting methods on the characteristics of recycled wastewater and the growth of microalgae to select a suitable harvesting method for the microalgal wastewater recycling system. During the wastewater recycling process, the least amount of accumulated substances was exhibited in the wastewater recycled by microfiltration, followed by centrifugation, and the most by flocculation sedimentation. After 4 batches of cultivation, microalgal biomass harvested from centrifugation wastewater and microfiltration wastewater was 21.26 % and 13.54 % higher than that from flocculation wastewater, respectively. Lipids, carbohydrates and pigments were all increased by varying degrees. Additionally, flocculation sedimentation was not suitable for the microalgal wastewater recycling process since the low residual nutrients, high salinity, and excessive algal organic matter severely inhibited the growth of microalgae. Under the regulation of phytohormones, microalgae increased their energy reserves, enhanced photosynthesis, and improved their defense capability to resist the increasing abiotic stress. This study provides scientific support for the selection of suitable harvesting technology during the microalgal wastewater recycling process.

Enhanced removal of trace pesticides and alleviation of membrane fouling using hydrophobic-modified inorganic-organic hybrid flocculants in the flocculation-sedimentation-ultrafiltration process for surface water treatment

Pesticide concentrations in surface water occasionally exceed regulated values due to seasonal events (rainy season in high intensity agricultural areas) or intermittent discharges (leakage, spillage, or other emergency events). The need to remove pesticide compounds in these situations poses a challenge for drinking water treatment plants (DWTPs). In this work, the performance of dosing hydrophobic-modified inorganic-organic hybrid flocculants (HOC-M; lower acute toxicity than corresponding metal salt coagulants; acceptable economic costs when M=Al or Fe; prepared in large-scale quantities), for the removal of four different pesticides (each initial concentration: 0.25 μg/L) from Yangtze River water, and in mitigating membrane fouling, by an integrated flocculation-sedimentation-ultrafiltration (FSUF) process, was evaluated over a period of 40 days; the FSUF is well-established in many DWTPs. The mechanisms underlying the treatment were unveiled by employing a combination of instrumental characterizations, chemical computations, material flow analyses, and statistical analyses. Efficient pesticide removal (80.3%∼94.3%) and membrane fouling reduction (26.6%∼37.3% and 28.3%∼57.6% for reversible and irreversible membrane resistance, respectively) in the FSUF process were achieved by dosing HOC-M, whereas conventional inorganic coagulants were substantially inferior for pesticide removal (< 50%) and displayed more severe fouling development. Hydrophobic association between the pesticides and the hydrophobic organic chain of HOC-M played a predominant role in the improvement in pesticide removal; coexisting particulate/colloid inorganic minerals and natural organic matter with HOC-M adsorbed on the surface, acting as floc building materials, provided sites for the indirect combination of pesticides into flocs. The observed fouling alleviation from dosing HOC-M was ascribed to both the pre-removal of fouling-causing materials in the flocculation-sedimentation prior to UF, and a stable hydrophilization modification effect of residual HOC-M in the UF unit. The latter effect resulted from a hydrophobic association between the PVDF substrate of the membranes and the hydrophobic organic chains of the HOC-M, causing the hydrophilic ends of the HOC-M to be exposed away from the membrane surface, thereby inhibiting foulant accumulation. This work has not only demonstrated the superior performance of dosing HOC-M in the FSUF process for trace pesticide removal in DWTPs, but also clarified the underlying mechanisms.

Comprehensive analysis of the combined flocculation and filtration process for microalgae harvesting at various operating parameters

The combined process of flocculation and filtration can improve algae harvesting performance by combining the benefits of both and overcoming the drawbacks. The entire process was thoroughly examined in this study, considering technical and economic feasibility under a variety of operating situations. Dead-end filtration was performed to evaluate the harvesting performance, the removal of extracellular organic matter and the changes of flocs. Cross-flow filtration was then carried out to explore the effect of operating parameters on permeate flux and assess the technical and economic feasibility. The optimum operating condition was to use 5 mg/L cationic polyacrylamide with 25 μm pore size and 0.1 m/s cross-flow velocity, under which a high harvesting efficiency of 95.2 %, a high average permeate flux of 55.5 m³/(m² h) and a volumetric reduction factor of 118.9 were achieved. Algal floc analysis revealed that flocs formed by ferric chloride and polyaluminium sulfate tended to partially deconstruct into smaller pieces during the filtration process. In contrast, flocs formed by cationic polyacrylamide tended to aggregate into bigger flocs, which, when paired with the effect of flocculant dosage and membrane pore size, could explain the difference in filtration performance and membrane permeance. No negative effect on downstream technology was observed for the combined process. A significantly lowered estimated total cost of 0.139 $/kg under optimum operating condition was obtained compared to filtration without flocculation assisted (0.206 $/kg).

Low energy harvesting of hydrophobic microalgae (Tribonema sp.) by electro-flotation without coagulation

Microalgae have great potential for biofuel production and wastewater treatment, but the high cost of harvesting hinders their practical application. In this study, economical harvesting of hydrophobic microalgae by electro-flotation without coagulation was assessed. The harvesting performance of this method for selected species of freshwater microalgae with different degrees of hydrophobicity (Tribonema sp., highly hydrophobic; Scenedesmus sp., moderately hydrophobic; and Pandorina sp., hydrophilic) were compared. It was found that microalgal hydrophobicity played a critical role in electro-flotation. Under the same condition (current 0.3 A, velocity gradient 200 s^-1, biomass concentration 1 g/L), Tribonema sp. could be effectively harvested (96.2 ± 0.4%) after 20 min of electro-flotation, while the harvesting efficiency decreased significantly with Scenedesmus sp. (70.1 ± 5.2%, 20 min) and Pandorina sp. (<10%, 1 h). The influences of current, electrolysis time, mixing intensity (velocity gradient) and biomass concentration on Tribonema sp. (hydrophobic) harvesting were further investigated. Increasing the current within a certain range (0.1 A-0.4 A) was beneficial to harvesting, while it's further increase decreased floating velocity, which was similar to the effect of the velocity gradient. Under the optimal condition, the harvesting efficiency of Tribonema sp. was 96.3% and the energy consumption (0.19 kWh/kg biomass) was much lower than other harvesting techniques, indicating that electro-flotation is a time-saving and economical approach for hydrophobic microalgae harvesting.

Gradient electro-processing strategy for efficient conversion of harmful algal blooms to biohythane with mechanisms insight

Globally eruptive harmful algal blooms (HABs) have caused numerous negative effects on aquatic ecosystem and human health. Conversion of HABs into biohythane via dark fermentation (DF) is a promising approach to simultaneously cope with environmental and energy issues, but low HABs harvesting efficiency and biohythane productivity severely hinder its application. Here we designed a gradient electro-processing strategy for efficient HABs harvesting and disruption, which had intrinsic advantages of no secondary pollution and high economic feasibility. Firstly, low current density (0.888-4.444 mA/cm²) was supplied to HABs suspension to harvest biomass via electro-flocculation, which achieved 98.59% harvesting efficiency. A mathematic model considering coupling effects of multi-influencing factors on HABs harvesting was constructed to guide large-scale application. Then, the harvested HABs biomass was disrupted via electro-oxidation under higher current density (44.44 mA/cm²) to improve bioavailability for DF. As results, hydrogen and methane yields of 64.46 mL/ (g VS) and 171.82 mL/(g VS) were obtained under 6 min electro-oxidation, along with the highest energy yield (50.1 kJ/L) and energy conversion efficiency (44.87%). Mechanisms of HABs harvesting and disruption under gradient electro-processing were revealed, along with the conversion pathways from HABs to biohythane. Together, this work provides a promising strategy for efficient disposal of HABs with extra benefit of biohythane production.

Biological-Based Produced Water Treatment Using Microalgae: Challenges and Efficiency

Produced water (PW) is the most significant waste stream generated in the oil and gas industries. The generated PW has the potential to be a useful water source rather than waste. While a variety of technologies can be used for the treatment of PW for reuse, biological-based technologies are an effective and sustainable remediation method. Specifically, microalgae, which are a cost-effective and sustainable process that use nutrients to eliminate organic pollutants from PW during the bioremediation process. In these treatment processes, microalgae grow in PW free of charge, eliminate pollutants, and generate clean water that can be recycled and reused. This helps to reduce CO2 levels in the atmosphere while simultaneously producing biofuels, other useful chemicals, and added-value products. As such, this review focuses on PW generation in the oil and gas industry, PW characteristics, and examines the available technologies that can be used for PW remediation, with specific attention to algal-based technologies. In addition, the various aspects of algae growth and cultivation in PW, the effect of growth conditions, water quality parameters, and the corresponding treatment performance are presented. Lastly, this review emphasizes the bioremediation of PW using algae and highlights how to harvest algae that can be processed to generate biofuels for added-value products as a sustainable approach.

Recycling spent water from microalgae harvesting by fungal pellets to re-cultivate Chlorella vulgaris under different nutrient loads for biodiesel production

Fungal pellet is an emerging material to collect oleaginous microalgae, but rare studies have noticed that harvested water is available resource for the next round of cultivation. To systematically optimize regrowth performances of microalgae Chlorella vulgaris, separated water after harvesting by fungi Aspergillus oryzae was prepared under different N/P ratios. The results showed that chlorophylls and enzymes were significantly affected by the proportion of N and P. Although nutrient deficiency was functioned as a stress factor to restrict carbohydrate and protein synthesis, lipid content was obviously increased by 12.69%. The percentage of saturated fatty acids associated with oxidation stability increased, while this part in fresh wastewater accounted for only 36.96%. The favorable biomass concentration (1.37 g/L) with the highest lipid yield (0.42 g/L) appeared in N/P of 6:1. More strikingly, suitable conditions could save 52.4% of cultivation costs. These experiments confirmed that reusing bioflocculated water could be effectively utilized for biodiesel production.

Membrane-Based Harvesting Processes for Microalgae and Their Valuable-Related Molecules: A Review

The interest in microalgae production deals with its role as the third generation of feedstock to recover renewable energy. Today, there is a need to analyze the ultimate research and advances in recovering the microalgae biomass from the culture medium. Therefore, this review brings the current research developments (over the last three years) in the field of harvesting microalgae using membrane-based technologies (including microfiltration, ultrafiltration and forward osmosis). Initially, the principles of membrane technologies are given to outline the main parameters influencing their operation. The main strategies adopted by the research community for the harvesting of microalgae using membranes are subsequently addressed, paying particular attention to the novel achievements made for improving filtration performance and alleviating fouling. Moreover, this contribution also gives an overview of the advantages of applying membrane technologies for the efficient extraction of the high added-value compounds in microalgae cells, such as lipids, proteins and carbohydrates, which together with the production of renewable biofuels could boost the development of more sustainable and cost-effective microalgae biorefineries.