Flocculation properties of eight microalgae induced by aluminum chloride, chitosan, amphoteric polyacrylamide, and alkaline: Life-cycle assessment for screening species and harvesting methods

Economic and demonstrative pilot-scale harvesting of microalgae biomass via novel combined process of dissolved air flotation and screw-press filtration

Microalgae, a promising sustainable biomass resource, lacks sufficient research for pilot-scale processes despite available technologies. Harvesting methods also pose challenges for large-scale applications. To address this, the economically viable large-scale microalgae harvesting system is here presented. The design integrates dissolved air flotation (5 m3/h) and screw-press filtration (10 kg/h), minimizing energy consumption suitable for industrial processes. This system efficiently harvests chlorella sp. (up to 4.1 m3) with a biomass harvest efficiency of 93 % and a dewatering rate of 11.9 %. Compared to centrifugation, the multi-stage system improves energy efficiency by 60.5 % with 1.7 kWh/m3 of energy consumption. This innovative approach demonstrates the potential for large-scale microalgae biomass 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.

Self-flocculation behaviour of cellulose-based bioflocculant synthesized from sewage water grown Chlorella sorokiniana and Scenedesmus abundans

The global energy crisis has spurred a shift from conventional to clean and sustainable energy sources. Biomass derived from microalgae is emerging as an alternative energy source with diverse applications. Despite the numerous advantages of microalgae, large-scale biomass harvesting is not economical and convenient. Self-flocculation is considered an effective phenomenon facilitated by extracting the flocculating substances from microalgae that assist aggregation of algal cells into flocs. A novel cellulose-based bioflocculant has been synthesized from sewage water grown Chlorella sorokiniana and Scenedesmus abundans for harvesting application. The produced bioflocculant amounted to 38.5% and 19.38% of the dry weight of S. abundans and C. sorokiniana, respectively. Analysis via FTIR, XRD, and FESEM-EDX revealed the presence of cellulose hydroxyapatite (HA) in algae-derived cellulose. Harvesting efficiencies of 95.3% and 89.16% were attained for S. abundans and C. sorokiniana, respectively, at a dosage of 0.5 g/L. Furthermore, the bioflocculant was recovered, enabling its reuse with recovery efficiencies of 52% and 10% for S. abundans and C. sorokiniana, respectively. This simple and efficient approach has the potential to replace other harvesting methods, thereby contributing to the economic algal biofuel production.

Spherical Polyelectrolyte Brushes as Flocculants and Retention Aids in Wet-End Papermaking

As the criteria of energy conservation, emission reduction, and environmental protection become more important, and with the development of wet-end papermaking, developing excellent retention aids is of great significance. Spherical polyelectrolyte brushes (SPBs) bearing polyelectrolyte chains grafted densely to the surface of core particle have the potential to be novel retention aids in wet-end papermaking not only because of their spherical structure, but also due to controllable grafting density and molecular weight. Such characteristics are crucial in order to design multi-functional retention aids in sophisticated papermaking systems. This review presents some important recent advances with respect to retention aids, including single-component system and dual-component systems. Then, basic theory in papermaking is also briefly reviewed. Based on these advances, it emphatically describes spherical polyelectrolyte brushes, focused on their preparation methods, characterization, conformation, and applications in papermaking. This work is expected to contribute to improve a comprehensive understanding on the composition, properties, and function mechanisms of retention aids, which helps in the further investigation on the design of novel retention aids with excellent performance.

Instantaneous and reversible flocculation of Scenedesmus via Chitosan and Xanthan Gum complexation

An instantaneous and reversible flocculation method for Scenedesmus harvesting was developed, based on the complexation of Chitosan (CTS) and Xanthan Gum (XG). Under rapid stirring, Scenedesmus cells formed centimeter-sized flocs within 20 s using binary flocculants of 4 mg/L CTS and 16 mg/L XG. These flocs exhibited a remarkable harvest efficiency exceeding 95 % when filtered through 500-μm-pore-sized sieves. Furthermore, the flocs could be completely disintegrated by using alkaline or NaCl solutions (pH > 11 or NaCl concentration > 1.5 mol/L). Adjusting pH allowed recovery of 50 % CTS and 75 % XG, resulting in microalgae biomass with lower flocculant content and reducing reagent costs. Electrostatic interaction of -COO- of XG and -NH3+ of CTS deduced the formation of polyelectrolyte complexes (PECs), which shrink and wrap the coexisting algal cells to form the flocs under stirring. CTS and XG complexation was instantaneous and reversible, explaining quick flocculation and disintegration.

Flocculation of Chlorella vulgaris-induced algal blooms: critical conditions and mechanisms

Algal blooms have posed great threats to livestocks and human health. Although flocculation is effective, its efficiency may hinder the direct application for algal blooms. In this study, critical (optimal) conditions and mechanisms for AlCl₃, FeCl₃, poly-aluminum chloride (PAC), chitosan, and polydimethyldiallylammonium chloride (PDADMAC)-induced flocculation of Chlorella vulgaris (C. vulgaris) were studied. Results identified the critical conditions which can cause flocculation efficiencies over 90% in 45 min for the five flocculants. Specifically, 4~10-mg/L doses of PDADMAC were proved to be appropriate for the treatment of C. vulgaris-induced algal blooms at pH 6.0~12.0. To probe the underlying mechanisms, functional groups involved in flocculation, zeta potential, and species distribution were analyzed during flocculation. FT-IR results indicated that N-H stretching in amine and C-H deformation in aliphatics were involved in algal flocculation with FeCl₃, and C-H deformation played an important role with PDADMAC, PAC, and chitosan. For AlCl₃, zeta potential and species distribution results suggested that charge neutralization and adsorption bridging were responsible for algal flocculation at pH 6~8. However, adsorption bridging and sweeping effects were the main mechanisms at pH >3 for FeCl₃. The flocculation mechanisms for the rest of the three polymers were charge neutralization, adsorption bridging, and sweeping. Meanwhile, all the flocculation processes followed second-order kinetics. Strong linkages were found between the rate constant, fractal dimension, and flocculation efficiency (P < 0.05). The results of critical flocculation conditions and mechanisms indicated that PDADMAC was an excellent flocculant for C. vulgaris removing and recycling, especially in water bloom treatment.

A Novel Salt-Bridge Electroflocculation Technology for Harvesting Microalgae

Microalgae biomass, as a promising alternative feedstock, can be refined into biodiesel, pharmaceutical, and food productions. However, the harvesting process for quality biomass still remains a main bottleneck due to its high energy demand. In this study, a novel technique integrating alkali-induced flocculation and electrolysis, named salt-bridge electroflocculation (SBEF) with non-sacrificial carbon electrodes is developed to promote recovery efficiency and cost savings. The results show that the energy consumption decreased to 1.50 Wh/g biomass with a high harvesting efficiency of 90.4% under 300 mA in 45 min. The mean particle size of algae flocs increased 3.85-fold from 2.75 to 10.59 µm, which was convenient to the follow-up processing. Another major advantage of this method is that the salt-bridge firmly prevented cells being destroyed by the anode's oxidation and did not bring any external contaminants to algal biomass and flocculated medium, which conquered the technical defects in electro-flocculation. The proposed SBEF technology could be used as a low cost process for efficient microalgae harvest with high quality biomass.

Co-culture of microalga Chlorella sorokiniana with syntrophic Streptomyces thermocarboxydus in cassava wastewater for wastewater treatment and biodiesel production

This study aimed to conduct a co-culture of a microalga and syntrophic actinomycetes to treat cassava a biogas effluent wastewater and enhance biodiesel production. Streptomyces thermocarboxydus strain BMI 10 produced the greatest effect on biomass production by Chlorella sorokiniana strain P21. Maximal algal biomass production and total lipid yield were increased when strain BMI 10 was co-cultured (by 21 and 25 %, respectively). Furthermore, the nutrient removal efficiency of P21 was not significantly different under sterilized and unsterilized conditions. Harvestability of the strain was also increased under both conditions. Analysis of the amount and composition of fatty acids from this co-culture biomass revealed that it was quite satisfactory for biodiesel production (54.11-61.52% saturated fatty acids with a 0.59-0.82 degree of unsaturation). Overall, the results showed the co-culture of the alga and bacterium is a holistic enhancement that couples wastewater treatment with biodiesel production.

Metal-based flocculation to harvest microalgae: a look beyond separation efficiency

Metal-based flocculants are commonly used for biomass harvesting in microalgae-based bio-refineries. Besides the high separation efficiency, additional aspects should be considered, related to the toxicity of metals for the algal biomass. Partitioning tests for commonly used flocculants (i.e., FeCl₃ and Al₂(SO₄)₃) showed that metals were mostly transferred to the solid phase with more than 95% of dosed metal ending up into the biomass, and low metal concentrations in the liquid effluent (lower than 0.4 mg L^-1 for both metals), thus allowing for water reuse. Photosynthesis inhibition was tested on microalgae and microalgae-bacteria cultures, using a standardized photo-respirometry protocol in which typical concentrations used during coagulation-flocculation were assessed. Modelling dose-response curves, concentrations corresponding to 50% inhibition (IC₅₀) were obtained, describing short-term effects. The obtained IC₅₀ ranged from 13.7 to 28.3 mg Al L^-1 for Al, and from 127.9 to 195.8 mg Fe L^-1 for Fe, showing a higher toxicity for the Al-based flocculant. The recovery of photosynthesis inhibition was also quantified, to evaluate the possibility of reusing/recycling the harvested biomass. The results highlighted that the residual photosynthetic activities, evaluated after 1 h and 24 h of exposure to metals were partially recovered, especially for Al, passing from 67.3% to 94.6% activity, respectively, while long-term Fe effects were stronger (passing from 64.9% to 77.6% activity). A non-toxic flocculant (cationic starch) was finally tested, excluding potential effects due to biomass aggregation, as the reduction of photosynthetic activity only reached 3.4%, compared to control. Relevant modifications to the light availability and the optical properties of algal suspensions were assessed, identifying a strong effect of iron which caused an increase of the light absorbance up to approximately 40% at high Fe concentrations. Possible implications of dosing metallic flocculants in MBWWT processes are discussed, and suggestions are given to perform inhibition tests on flocculating chemicals.

Sustainably Cultivating and Harvesting Microalgae through Sedimentation and Forward Osmosis Using Wastes

Cost-effective nutrient sources and dewatering are major obstacles to sustainable, scaled-up cultivation of microalgae. Employing waste resources as sources of nutrients offsets costs for nutrient supplies while adding value through simultaneous waste treatment. Forward osmosis (FO), using simulated reverse osmosis brine, is a low-energy membrane technology that can be employed to efficiently harvest microalgae from a dilute solution. In this study, Scenedesmus obliquus, a green microalga, was cultivated with a fertilizer plant wastewater formula and simulated coal-fired power plant flue gas and then separated through either FO, with reverse osmosis reject model water as the draw solution, or sedimentation. Microalgal batches grown with simulated wastewater removed NH₄ ⁺ within 2 days and reached nitrogen and phosphorus limitation simultaneously on Day 5. Sparging with the flue gas caused S. obliquus to produce significantly greater quantities of extracellular polymeric substances (30.7 ± 1.8 μg mL^-1), which caused flocculation and enhanced settling to an advantageous extent. Five-hour FO trials showed no statistically significant difference (p = 0.65) between water fluxes for cultures grown with simulated flue gas and CO₂-supplemented air (3.0 ± 0.1 and 3.0 ± 0.3 LMH, respectively). Reverse salt fluxes were low for all conditions and, remarkably, the rate of reverse salt flux was -1.9 ± 0.6 gMH when the FO feed was culture grown with simulated flue gas. In this work, S. obliquus was cultivated and harvested with potential waste resources.