Effect of eggshells for the harvesting of microalgae species

Microalgal co-cultivation -recent methods, trends in omic-studies, applications, and future challenges

The burgeoning human population has resulted in an augmented demand for raw materials and energy sources, which in turn has led to a deleterious environmental impact marked by elevated greenhouse gas (GHG) emissions, acidification of water bodies, and escalating global temperatures. Therefore, it is imperative that modern society develop sustainable technologies to avert future environmental degradation and generate alternative bioproduct-producing technologies. A promising approach to tackling this challenge involves utilizing natural microbial consortia or designing synthetic communities of microorganisms as a foundation to develop diverse and sustainable applications for bioproduct production, wastewater treatment, GHG emission reduction, energy crisis alleviation, and soil fertility enhancement. Microalgae, which are photosynthetic microorganisms that inhabit aquatic environments and exhibit a high capacity for CO2 fixation, are particularly appealing in this context. They can convert light energy and atmospheric CO2 or industrial flue gases into valuable biomass and organic chemicals, thereby contributing to GHG emission reduction. To date, most microalgae cultivation studies have focused on monoculture systems. However, maintaining a microalgae monoculture system can be challenging due to contamination by other microorganisms (e.g., yeasts, fungi, bacteria, and other microalgae species), which can lead to low productivity, culture collapse, and low-quality biomass. Co-culture systems, which produce robust microorganism consortia or communities, present a compelling strategy for addressing contamination problems. In recent years, research and development of innovative co-cultivation techniques have substantially increased. Nevertheless, many microalgae co-culturing technologies remain in the developmental phase and have yet to be scaled and commercialized. Accordingly, this review presents a thorough literature review of research conducted in the last few decades, exploring the advantages and disadvantages of microalgae co-cultivation systems that involve microalgae-bacteria, microalgae-fungi, and microalgae-microalgae/algae systems. The manuscript also addresses diverse uses of co-culture systems, and growing methods, and includes one of the most exciting research areas in co-culturing systems, which are omic studies that elucidate different interaction mechanisms among microbial communities. Finally, the manuscript discusses the economic viability, future challenges, and prospects of microalgal co-cultivation methods.

Potential of biogenic and non-biogenic waste materials as flocculant for algal biomass harvesting: Mechanism, parameters, challenges and future prospects

In this review article, waste materials (biogenic/non-biogenic) are focused as the flocculants for harvesting of algal biomass. Chemical flocculants are widely utilized for the effective harvesting of algal biomass at a commercial scale while the high cost is a major drawback. The waste materials-based flocculants (WMBF) are started to utilize as one of the cost-effective performance for dual benefits of waste minimization and reuse for sustainable recovery of biomass. The novelty of the article is articulated with the objective that presents an insight of WMBF, classification of WMBF, preparation methods of WMBF, mechanisms of flocculation, factors affecting flocculation-mechanism, challenges and future recommendations that are required for harvesting of algae. The WMBF are shown similar flocculation mechanisms and flocculation efficiencies as chemical flocculants. Thus, the utilization of waste material for the flocculation process of algal cells minimizes the waste load into the environment and transforms the waste materials into valuable resources.

Co-culture of fungi-microalgae consortium for wastewater treatment: A review

The treatment of wastewater by microalgae has been studied and proved to be effective through previous studies. Due to the small size of microalgae, how to efficiently harvest microalgae from wastewater is a crucial factor restricting the development of algal technologies. Fungi-assisted microalgae bio-flocculation for microalgae harvesting and wastewater treatment simultaneously, which was overlooked previously, has attracted increasing attention in the recent decade due to its low cost and high efficiency. This review found that fungal hyphae and microalgae can stick together due to electrostatic neutralization, surface protein interaction, and exopolysaccharide adhesion in the co-culture process, realizing co-pelletization of microalgae and fungi, which is conducive to microalgae harvesting. Besides, the combination of fungi and microalgae has a complementary effect on pollutant removal from wastewaters. The co-culture of fungi-microalgae has excellent development prospects with both environmental and economic benefits, and it is expected to be applied on an industrial scale.

Single-step dynamic dewatering of microalgae from dilute suspensions using flocculant assisted filtration

Background

Dewatering constitutes a major challenge to the production of microalgae, accounting for 20–30% of the product cost. This presents a setback for the applicability of microalgae in the development of several sustainable products. This study presents an investigation into the dynamic dewatering of microalgae in a combined flocculation-filtration process. The effect of process conditions on the performance of 12 flocculants and their mixtures was assessed.

Results

The mechanism of flocculation via the electrostatic path was dominated by charge neutralization and subsequently followed bridging in a ‘sweep flocculation’ process. Cationic polyacrylamide (CPAM) based flocculants recorded the highest biomass retention with PAM1 and PAM2 attaining 99 and 98% retention with flocculant dosages of 10 and 15 mg/L respectively. Polyvinylamine (PVAM) was also found to improve system stability across the pH range 4–10. Alum was observed to be only effective in charge neutralization, bringing the system close to its isoelectric point (IEP). Chemometric analysis using the multi-criteria decision methods, PROMETHEE and GAIA, was applied to provide a sequential performance ranking based on the net outranking flow (ф) from 207 observations. A graphical exploration of the flocculant performance pattern, grouping the observations into clusters in relation to the decision axis (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pi$$\end{document}π), which indicated the weighted resultant of most favorable performance for all criteria was explored.

Conclusion

CPAM based flocculants and their mixtures demonstrated superior performance due to their viscoelastic behaviour under turbulence. The use of PVAM or alum in mixtures with CPAM reduced the required doses of both flocculants, which will provide beneficial financial impact for largescale microalgae dewatering in a flocculant assisted dynamic filtration process. Chemometric analysis based on the physico-chemical properties of the system provides a time saving assessment of performance across several criteria. The study findings provide an important foundation for flocculant assisted dynamic filtration processes.

Valorization of waste eggshell-derived bioflocculant for harvesting T. obliquus: Process optimization, kinetic studies and recyclability of the spent medium for circular bioeconomy

Waste eggshell-derived bioflocculant was used for harvesting T. obliquus in a circular bioeconomy approach. It was found that 120 mg L^-1 bioflocculant can flocculate 98.62 ± 0.43% of T. obliquus cells within 25 min at optimal pH 4.0 and temperature 35 °C. The influence of bioflocculant concentration, pH and temperature on zeta potential was evaluated to understand the flocculation mechanism. Microscopic and FESEM-EDX images were analyzed to evaluate the microalgal structural changes. Adsorption mechanism of bioflocculant over the microalgal cells was determined by performing adsorption kinetic studies. Pseudo-second order kinetic model was a suitable fit for the data obtained from the experiments, which indicated chemisorption as the probable mechanism. The spent medium recovered after harvesting process was successfully recycled for subsequent cultivation of T. obliquus, thus reducing the dependency on fresh medium. The FAME composition of the biomass treated with bioflocculant was not altered.

Algal cells harvesting using cost-effective magnetic nano-particles

Innovative iron-based nanoparticles were synthesized, characterized and tested for the first time for harvesting single and mixed algal culture from real wastewater. The tailor-made magnetic nanoparticles (MNPs; Fe-MNP-I and Fe-MNP-II) achieved a percentage algae harvesting efficiency (%AHE) higher than 95% using a concentration of MNPs (C_MNP) of 25 ± 0.3 (std. dev = 0.08) mg.L^-1, mixing speed (M_speed) of 120 ± 2 (std. dev = 0.10) rpm, short contact time (C_t) of 7 ± 0.1 (std. dev = 0.05) min and separation time (SP_t) of 3 ± 0.1 (std. dev = 0.09) min. The optimum operational conditions for harvesting of Chlorella vulgaris (C.v) were determined at (C_MNP = 40 ± 0.4 (std. dev = 0.5) g_MNPs.L^-1, SP_t = 2.5 ± 0.4 (std. dev = 0.1) min, M_speed = 145 ± 3 (std. dev = 1.50) rpm and C_t = 5 ± 0.3 (std. dev = 0.10) min using surface response methodology. Langmuir model describes better the adsorption behavior of algae-Fe-MNP-I system, while both Langmuir and Freundlich fit well the adsorption behavior of algae-Fe-MNP-II. The maximum adsorption capacity of Spirulina platensis (SP.PL) (18.27 ± 0.07 (std. dev = 0.19) mg_DWC.mg_particles^-1) was higher than that for Chlorella vulgaris (C.v) (11.52 ± 0.01 (std. dev = 0.34) mg_DWC.mg_particles^-1) and mixed algal culture (M.X) (17.20 ± 0.07 (std. dev = 0.54) mg_DWC.mg_particles^-1) over Fe-MNP-I. Zeta potential measurements revealed that the adsorption mechanism between MNPs and algal strains is controlled by electrostatic interaction. The synthesized MNPs were recycled 10 times using alkaline-ultrasonic regeneration procedure.

Subtopic: Advances in water and wastewater treatment harvesting of Chlorella sp. microalgae using Aspergillus niger as bio-flocculant for aquaculture wastewater treatment

Microalgae have been increasingly used to generate biofuel, thus a sustainable technique should be implemented to harvest the biomass to ensure its existence in the environment. Aspergillus niger was used as bio-flocculant to harvest microalgae from aquaculture wastewater via flocculation technique over a range of pH and mixing rate. The bio-flocculant showed ability to adapt at a wide range of pH from 3.0 to 9.0 and at a mixing rate of 100-150 rpm, producing a harvesting efficiency of higher than 90%. The treated water possessed low concentration of chlorophyll-a (0.3-0.6 mg L^-1) and cell density (2 × 10⁶-3 × 10⁶ cell mL^-1). These indicate that Aspergillus niger is a promising bio-flocculant to be used in harvesting microalgae, thus promoting the use of flocculation as a green technology in aquaculture wastewater treatment.

Flocculation Harvesting Techniques for Microalgae: A Review

Microalgae have been considered as one of the most promising biomass feedstocks for various industrial applications such as biofuels, animal/aquaculture feeds, food supplements, nutraceuticals, and pharmaceuticals. Several biotechnological challenges associated with algae cultivation, including the small size and negative surface charge of algal cells as well as the dilution of its cultures, need to be circumvented, which increases the cost and labor. Therefore, efficient biomass recovery or harvesting of diverse algal species represents a critical bottleneck for large-scale algal biorefinery process. Among different algae harvesting techniques (e.g., centrifugation, gravity sedimentation, screening, filtration, and air flotation), the flocculation-based processes have acquired much attention due to their promising efficiency and scalability. This review covers the basics and recent research trends of various flocculation techniques, such as auto-flocculation, bio-flocculation, chemical flocculation, particle-based flocculation, and electrochemical flocculation, and also discusses their advantages and disadvantages. The challenges and prospects for the development of eco-friendly and economical algae harvesting processes have also been outlined here.

Harvesting of intact microalgae in single and sequential conditioning steps by chemical and biological based - flocculants: Effect on harvesting efficiency, water recovery and algal cell morphology

Quick algae harvesting methodologies relating optimum flocculent dose (D_Opt.), percentage harvesting efficiency (%HE) and percentage water recovery (%W_Recovery) to the in-situ hydrodynamic properties of water-algae systems are presented. Flocculation of three microalgae in single and sequential steps, using chemical (polymer and ferric chloride) and biological (egg shells) flocculants, was studied. Zeta potential and pH analysis were completed to further understand the flocculation mechanism. Polymer at D_Opt. of 7.0 g/kgDS resulted in W_Recovery of 90% and %HE of 96.7%. Lower %HE (92.1), %W_Recovery (79) and noticeable algal cells deformation was observed for ferric chloride at D_Opt. of 7.0 g/kg DS. Bio-flocculant conserved algal structure and resulted in %HE of 96.2 and %W_Recovery of 90 at D_Opt. of 5.4 g/kgDS. Significant % HE of 99.8, %W_Recovery of 99.8%, and up to 95% reduction in D_Opt. were achieved in sequential flocculation. The results established the effectiveness and suitability of sequential/ bio-flocculation for algae harvesting.

The potential of a natural biopolymeric flocculant, ε-poly-L-lysine, for harvesting Chlorella ellipsoidea and its sustainability perspectives for cost and toxicity

The successful production of microalgal biomass requires the precise coordination of many different steps. Cell harvesting is a central process in all methods currently used for the production of microalgal biomass. Therefore, improving the harvesting process itself, and using a harvesting method that is compatible with adjacent steps, is necessary to prevent problems that may occur during downstream processing. This study examined the potential of the cationic biopolymer ε-poly-L-lysine (ε-PLL) for use in the harvest of microalgae (Chlorella ellipsoidea). The effects of ε-PLL concentration and mixing intensity on flocculation efficiency and operating costs were determined. We found that ε-PLL was not toxic to microalgal cells at concentrations of up to 25 mg/L, based on the photosystem II quantum yield. A recovery rate of 95% was achieved using 19 mg/L ε-PLL, and the estimated harvest cost was 20 US$/ton of harvested biomass. Moreover, ε-PLL displayed antimicrobial properties, leaving the harvested biomass intact and pure. Therefore, the use of ε-PLL-induced flocculation appears to be an attractive option when harvesting microalgal biomass for use as low- and high-value commodities for humans or animals.

Experimental studies on zeta potential of flocculants for harvesting of algae

An experimental study was performed to evaluate the comparative efficiency of bio-flocculant (waste egg shell), laboratory available calcium carbonate (LACC) and alum (Al₂ (SO₄)₃) for harvesting of unicellular microalga, Chlorella pyrenoidosa. The influence of pH on zeta potential (ζ) was also studied to explain the chemistry of flocculation process. The maximum harvesting efficiency (99%) was obtained with alum with deformities in algal cell surfaces. Waste egg-shell material is developed as a low-cost bio-flocculant for harvesting of Chlorella pyrenoidosa using 100 mg egg-shell bio-flocculant/L and 100 mg LACC/L, zeta potential analysis was completed to further understand the chemistry of harvesting efficiency over the different ranges of pH (2.0, 4.0, 6.0, 8.0, and 10.0). The optimized range for harvesting efficiency (HE) of pH is 4.0-8.0 for both flocculants. Maximal harvesting efficiency was achieved at pH 4.0 (99%) and pH 8.0 (95%) with bio-flocculant and LACC respectively. Hence, bio-flocculant based harvesting method is found as the best way to dewatering the algal biomass from aqueous medium with entire and intact algal cell surface with environment friendly and cost-effective approach.

A novel method to harvest Chlorella sp. via low cost bioflocculant: Influence of temperature with kinetic and thermodynamic functions

In this study, harvesting efficiency (HE) of bioflocculant (egg shell) was observed with variation in flocculent concentrations (0-100mgL^-1), temperature (30°C, 35°C 40°C, 45°C and 50°C) and variable contact time (0-50min). It was found maximum (≈95.6%) with 100mgL^-1 bioflocculant concentration whereas influence of temperature was also observed with optimized concentration of bioflocculant (100mgL^-1) at 40°C (≈98.1%) and 50°C (≈99.3%), in 30min of contact time. Significant changes in algal cell structures were also analyzed after exposure to various temperatures with microscopy, SEM (Scanning electron microscopy) and EDS (Energy dispersive X-ray spectroscopy) images with and without bioflocculant. The experimental data was found to be a good fit with pseudo-second order kinetic model. The thermodynamic functions such as ΔG (Gibbs free energy), ΔH (enthalpy), ΔS (entropy) were also determined. The negative value of ΔG and positive value of ΔH and ΔS shows the spontaneous and endothermic nature of flocculation process.