Sedimentation of algae flocculated using naturally-available, magnesium-based flocculants

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.

Growth of Dunaliella viridis in multiple cycles of reclaimed media after repeated high pH-induced flocculation and harvesting

Minimizing the use of water for growing microalgae is crucial for lowering the energy and costs of animal feed, food, and biofuel production from microalgae. Dunaliella spp., a haloterant species that can accumulate high intracellular levels of lipids, carotenoids, or glycerol can be harvested effectively using low-cost and scalable high pH-induced flocculation. However, the growth of Dunaliella spp. in reclaimed media after flocculation and the impact of recycling on the flocculation efficiency have not been explored. In this study, repeated cycles of growth of Dunaliella viridis in repeatedly reclaimed media from high pH-induced flocculation were studied by evaluating cell concentrations, cellular components, dissolved organic matter (DOM), and bacterial community shifts in the reclaimed media. In reclaimed media, D. viridis grew to the same concentrations of cells and intracellular components as fresh media-107 cells/mL with cellular composition of 3 % lipids, 40 % proteins, and 15 % carbohydrates-even though DOM accumulated and the dominant bacterial populations changed. There was a decrease in the maximum specific growth rate and flocculation efficiency from 0.72 d-1 to 0.45 d-1 and from 60 % to 48 %, respectively. This study shows the potential of repeated (at least five times) flocculation and reuse of media as a possible way of reducing the costs of water and nutrients with some tradeoffs in growth rate and flocculation efficiency.

Cultivation of Spirulina platensis for nutrient removal from piggery wastewater

The discharge of livestock wastewater without appropriate treatment causes severe harm to the environment and human health. In the pursuit of finding solutions to this problem, the cultivation of microalgae as feedstock for biodiesel and animal feed additive using livestock wastewater coupled with the removal of nutrients from wastewater has become a hot research topic. In this study, the cultivation of Spirulina platensis using piggery wastewater for the production of biomass and the removal of nutrients were studied. The results of single factor experiments confirmed that Cu2+ seriously inhibit the growth of Spirulina platensis, while the influences of nitrogen, phosphorous, and zinc on the growth of Spirulina platensis can all be described as "low promotes high inhibits." Spirulina platensis grew well in the 4-fold dilution of piggery wastewater supplemented with moderate sodium bicarbonate, which indicated that it is the limiting nutrients for Spirulina platensis growth in piggery wastewater. The biomass concentration of Spirulina platensis reached 0.56 g/L after 8 days of culture at the optimal conditions proposed by the response surface method, which were as follows: 4-fold dilution of piggery wastewater, 7 g/L sodium bicarbonate, pH of 10.5, initial OD560 of 0.63, light intensity of 3030 lx, and light time/dark time of 16 h/8 h. Spirulina platensis cultured in the diluted piggery wastewater contained 43.89% protein, 9.4% crude lipid, 6.41 mg/g chlorophyll a, 4.18% total sugar, 27.7 mg/kg Cu, and 246.2 mg/kg Zn. The removal efficiency for TN, TP, COD, Zn, and Cu from the wastewater by Spirulina platensis was 76%, 72%, 93.1%, 93.5%, and 82.5%, respectively. These results demonstrated the feasibility of piggery wastewater treatment by the cultivation of Spirulina platensis.

A review on the sustainable procurement of microalgal biomass from wastewaters for the production of biofuels

The feasibility of microalgal biomass as one of the most promising and renewable sources for the production of biofuels is being studied extensively. Microalgal biomass can be cultivated under photoautotrophic, heterotrophic, photoheterotrophic, and mixotrophic cultivation conditions. Photoautotrophic cultivation is the most common way of microalgal biomass production. Under mixotrophic cultivation, microalgae can utilize both organic carbon and CO2 simultaneously. Mixotrophic cultivation depicts higher biomass productivity as compared to photoautotrophic cultivation. It is evident from the literature that mixotrophic cultivation yields higher quantities of polyunsaturated fatty acids as compared to that photoautotrophic cultivation. In this context, for economical biomass production, the organic carbon of industrial wastewaters can be valorized for the mixotrophic cultivation of microalgae. Following the way, contaminants' load of wastewaters can be reduced while concomitantly producing highly productive microalgal biomass. This review focuses on different aspects covering the sustainable cultivation of different microalgal species in different types of wastewaters.

Microalgae Harvesting after Tertiary Wastewater Treatment with White-Rot Fungi

Tertiary wastewater treatment with microalgae incorporates environmental sustainability with future technologies and high exploitation costs. Despite the apparent ecological benefits of microalgae-assisted wastewater treatment/biomass-based resource production, technological improvements are still essential to compete with other technologies. Bio-flocculation instead of mechanical harvesting has been demonstrated as an alternative cost-effective approach. So far, mostly filamentous fungi of genus Aspergillus have been used for this purpose. Within this study, we demonstrate a novel approach of using white-rot fungi, with especially high potential of algae-Irpex lacteus complex that demonstrates efficiency with various microalgae species at a broad range of temperatures (5-20 °C) and various pH levels. Harvesting of microalgae from primary and secondary wastewater resulted in 73-93% removal efficiencies within the first 24 h and up to 95% after 48 h. The apparent reuse potential of the algae-I. lacteus pellets further complements the reduced operating costs and environmental sustainability of bio-flocculation technology.

Amelioration for oxidative stability and bioavailability of N-3 PUFA enriched microalgae oil: an overview

Technological improvements in dietary supplements and nutraceuticals have highlighted the significance of bioactive molecules in a healthy lifestyle. Eicosapentaenoic acid and Cervonic acid (DHA), omega-3 polyunsaturated fatty acids seem to be famed for their ability to prevent diverse physiological abnormalities. Selection of appropriate pretreatments and extraction techniques for extraction of lipids from robust microalgae cell wall are very important to retain their stability and bioactivity. Therefore, extraction techniques with optimized extraction parameters offer an excellent approach for obtaining quality oil with a high yield. Oils enriched in omega-3 are particularly imperiled to oxidation which ultimately affects customer acceptance. Bio active encapsulation could be one of the effective approaches to overcome this dilemma. This review paper aims to give insight into the cultivation methods, and downstream processes, various lipid extraction approaches, techniques for retaining oxidative stability, bioavailability and food applications based on extracted or encapsulated omega-3.

Microalgal flocculation and sedimentation: spatiotemporal evaluation of the effects of the pH and calcium concentration

The high cost of harvesting microalgae is a major hurdle for the microalgae industry, and an efficient pre-concentration method is required. In this study, the effects of using different pH values (between pH 3 and 11) and calcium (Ca²⁺) concentrations (between 0 and 5 mM) on Chlorella vulgaris sedimentation were investigated by evaluating the spacio-temporal distributions of microalgae cells. Fast and efficient sedimentation occurred (within 10 min) at a high Ca²⁺ concentration (5 mM) at pH 9 and 11. However, the sediment volume was lower at a Ca²⁺ concentration of 3 mM than at a Ca²⁺ concentration of 5 mM. This indicated that the Ca²⁺ concentration strongly affected the sediment volume. Fast sedimentation and a low sediment volume were found at pH 7 and a Ca²⁺ concentration of 5 mM, probably because of the neutral charge in the system (adhesion to calcium precipitates would have occurred at a high pH). The highest Ca²⁺ recovery (82%) was achieved when sediment produced at pH 11 and a Ca²⁺ concentration of 5 mM was acidified to pH 3.

Recent Advances of Microalgae Exopolysaccharides for Application as Bioflocculants

Microalgae are used in flocculation processes because biopolymers are released into the culture medium. Microalgal cell growth under specific conditions (temperature, pH, luminosity, nutrients, and salinity) provides the production and release of exopolysaccharides (EPS). These biopolymers can be recovered from the medium for application as bioflocculants or used directly in cultivation as microalgae autoflocculants. The optimization of nutritional parameters, the control of process conditions, and the possibility of scaling up allow the production and industrial application of microalgal EPS. Therefore, this review addresses the potential use of EPS produced by microalgae in bioflocculation. The recovery, determination, and quantification techniques for these biopolymers are also addressed. Moreover, other technological applications of EPS are highlighted.

Novel approach for harvesting of microalgal biomass using electric geyser waste material deposit as flocculant in coupling with poultry excreta leachate

The aim of this work was to study the flocculation efficiency of algal biomass (Chlorella pyrenoidosa) in coupling with waste materials i.e. poultry excreta leachate by using other waste material which was obtained from deposition of scaling in electric geyser. Utilization of electric geyser waste material deposit (EGWMD) for flocculation is a novel approach because of various elements which are replica of chemical flocculants responsible for flocculation mechanism in culture medium. Flocculation process was optimized by response surface methodology and 98.21% flocculation efficiency was achieved with designed process parameters as temperature 32.5 °C, flocculant dose 275 mgL^-1, pH 5 and time 30 min. The reusability of spent medium was also analyzed at 70.2% and 32.5% flocculation efficiency with two successive steps. The cellular morphology of pre-harvested and post-harvested Chlorella pyrenoidosa was also observed. EGWMD is abundant and freely available that has no application till now and can alternate of chemical flocculants.

Evaluation of natural flocculant efficiency in the harvest of microalgae Monoraphidium contortum

This study assesses two parameters that can result in high efficiency in the recovery of the microalgae Monoraphidium contortum. The significant contribution of this paper is to test different coagulants in different conditions of concentration in the coagulation and flocculation processes followed by sedimentation to evidence the best coagulant and the best condition for harvesting of Monoraphidium contortum biomass. So the proposed methodology aimed to perform preliminary tests using a tannin-based cationic coagulant (TANFLOC SG®), FeCl2, and Al2 (SO4)3, where they were performed at concentrations of 0, 20, 40, 60, 80, and 100 mg L−1 at a fast mixing speed of 400 RPM. The tests determined 20 mg L−1 of Tanfloc SG® as the most efficient turbidity reduction in the preliminary test. The obtained results were used to construct a non-factorial central composite planning. Therefore, after a design of experiments, the study outcome shows the best turbidity removal range from the main tests came at 35 mg L−1 and 550 RPM of fast mixing speed.

Reducing self-shading effects in Botryococcus braunii cultures: effect of Mg2+ deficiency on optical and biochemical properties, photosynthesis and lipidomic profile

Microalgae biomass exploitation as a carbon-neutral energy source is currently limited by several factors, productivity being one of the most relevant. Due to the high absorption properties of light-harvesting antenna, photosynthetic cells tend to capture an excessive amount of energy that cannot be entirely channeled through the electron transfer chain that ends up dissipated as heat and fluorescence, reducing the overall light use efficiency. Aiming to minimize this hurdle, in this work we studied the effect of decreasing concentrations of Magnesium (Mg2+) on the chlorophyll a content, photosynthetic performance, biomass and lipid production of autotrophic cultures of Botryococcus braunii LB 572. We also performed, for the first time, a comparative lipidomic analysis to identify the influence of limited Mg2+ supply on the lipid profile of this algae. The results indicated that a level of 0.0037 g L-1 MgSO4 caused a significant decline on chlorophyll a content with a concomitant 2.3-fold reduction in the biomass absorption coefficient. In addition, the Mg2+ limitation caused a decrease in the total carbohydrate content and triggered lipid accumulation, achieving levels of up to 53% DCW, whereas the biomass productivity remained similar for all tested conditions. The lipidome analysis revealed that the lowest Mg2+ concentrations also caused a differential lipid profile distribution, with an enrichment of neutral lipids and an increase of structural lipids. In that sense, we showed that Mg2+ limitation represents an alternative optimization approach that not only enhances accumulation of neutral lipids in B. braunii cells but also may potentially lead to a better areal biomass productivity due to the reduction in the cellular light absorption properties of the cells.

Integration of enzymatic pretreatment and sludge co-digestion in biogas production from microalgae

Integration of microalgae-based systems with conventional wastewater treatment plants provides an effective alternative to waste stream management. In this work, alkaline and enzymatic pretreatments of a microalgal culture mainly constituted by Chlorella sp. and Scenedesmus sp. and cultivated in wastewater from an industrial winery wastewater treatment plant were assessed. Microalgal enzymatic pretreatments were expected to overcome algal recalcitrancy before anaerobic digestion. pH-induced flocculation at pH 10 and 11 did not enhance microalgal harvesting and solubilisation, achieving a performance similar to that of natural sedimentation. Enzymatic hydrolysis of algal biomass was carried out using three commercial enzymatic cocktails (A, B and C) at two enzymatic doses (1% and 2% (v/v)) over 3 h of exposure time at 37 °C. Since pretreatments at a 1% dose for 0.5 h and 2% dose for 2 h achieved higher solubilisation, they were selected to evaluate the influence of the pretreatment on microalgal anaerobic digestibility. Biochemical methane potential tests showed that the pretreatments increased the methane production of the raw algal biomass 3.6- to 5.3-fold. The methane yield was 9-27% higher at the lower enzyme dose. Hence, microalgae pretreated with enzymes B and C at a 1% dose were co-digested with waste activated sludge (WAS). Even when the enzyme increased the methane yield of the inoculum and the WAS, the methane yield of the raw microalgae and WAS mixture was not significantly different from that obtained when algae were enzymatically pretreated. Nonetheless, co-digestion may achieve the goals of a waste recycled bio-circular economy.

Development of integrated culture systems and harvesting methods for improved algal biomass productivity and wastewater resource recovery - A review

Microalgae biomass has been considered as a potential feedstock for the production of renewable chemicals and biofuels. Microalgae culture combined with wastewater treatment is a promising approach to improve the sustainability of the business model. However, algae culture and harvest account for the majority of the high costs, hindering the development of the microalgae-based wastewater utilization. Cost-effective culture systems and harvesting methods for enhancing biomass yield and reducing the cost of resource recovery have become extremely urgent and important. In this review, different commonly used culture systems for microalgae are discussed; the current harvesting methods with different culture systems have also been evaluated. Also, the inherent characteristics of inefficiency in algae wastewater treatment are elaborated. Current literature collectively supports that a biofilm type device is a system designed for higher biomass productivity, and offers ease of harvesting, in small-scale algae cultivation. Additionally, bio-flocculation, which uses one kind of flocculated microalgae to concentrate on another kind of non-flocculated microalgae is a low-cost and energy-saving alternative harvesting method. These findings provide insight into a comprehensive understanding of integrated culture systems and harvesting methods for microalgae-based wastewater treatment.

Effect of shear rate on floc characteristics and concentration factors for the harvesting of Chlorella vulgaris using coagulation-flocculation-sedimentation

Coagulation-based separation has been increasingly applied to microalgal harvesting because of its competitive cost and high scalability. The characteristics of flocs formed during coagulation/flocculation are critical for efficient harvesting. However, few studies have been devoted to systematically investigating the structural characteristics of microalgal flocs and their influences on subsequent settling performance. In this paper, the dynamic mean size and fractal dimension, strength, regrowth and settling performance of Al³⁺ coagulated Chlorella vulgaris flocs were characterized at various flocculation shear rates. The influence of shear rate on floc characteristics was revealed. An appropriate shear rate (9 s^-1) produced more desirable microalgal flocs (in terms of size and compactness), with better settling performance and a higher concentration factor, than higher or lower shear rates, favoring their separation and subsequent harvesting. At this condition, the concentration factor reached 13.50, which was a 177.21% improvement over the 4.87 reached at a low shear rate.

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.

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.

Urban wastewater photobiotreatment with microalgae in a continuously operated photobioreactor: growth, nutrient removal kinetics and biomass coagulation-flocculation

The aim of this study was to investigate the growth, nutrient removal and harvesting of a natural microalgae bloom cultivated in urban wastewater in a bubble column photobioreactor. Batch and continuous mode experiments were carried out with and without pH control by means of CO₂ dosage. Four coagulants (aluminium sulphate, ferric sulphate, ferric chloride and polyaluminium chloride (PAC)) and five flocculants (Chemifloc CM/25, FO 4498SH, cationic polymers Zetag (Z8165, Z7550 and Z8160)) were tested to determine the optimal dosage to reach 90% of biomass recovery. The maximum volumetric productivity obtained was 0.11 g SS L^-1d^-1 during the continuous mode. Results indicated that the removal of total dissolved nitrogen and total dissolved phosphorous under continuous operation were greater than 99%. PAC, Fe₂(SO₄)₃ and Al₂(SO₄)₃ were the best options from an economical point of view for microalgae harvesting.

The effect of heavy metals on the viability of Tetraselmis marina AC16-MESO and an evaluation of the potential use of this microalga in bioremediation

The use of microalgae in biotechnological processes has received much attention worldwide. This is primarily due to the fact that they are inexpensive to grow, requiring only sunlight and CO₂, whilst lending themselves to a range of uses, such as to reduce CO₂ levels, as fish feed, in biofuel production, for the generation of secondary metabolites of interest, and in bioremediation. These features mean that microalgae are excellent candidates for the implementation of a range of eco-friendly technologies. Here, we investigated the behavior and feasibility of the use of the microalgal strain Tetraselmis marina AC16-MESO against heavy metal contamination focused on potential use in bioremediation. The following key parameters were recorded: (i) the sedimentation efficiency, which reached 95.6% after five hours of decantation; (ii) the ion tolerance (Ca²⁺, Co²⁺, Cu²⁺, Fe³⁺, Mn²⁺ and Ni²⁺) at concentrations of 0.1, 1.0, 5.0, 10.0 and 20.0 mg*L⁻¹ and (iii) ion removal efficiency (Cu²⁺, Fe³⁺ and Mn²⁺). Our results indicated a higher tolerance for iron and calcium (20 ± 1.10 mg*L⁻¹; 100 ± 8.10 mg*L⁻¹), partial to nickel, manganese and copper (4.4 ± 0.10 mg*L⁻¹; 4.4 ± 0.15 mg*L⁻¹; 5 ± 1.25 mg*L⁻¹) and less for cobalt (0.1 ± 0.20 mg*L⁻¹). Moreover, removal efficiency of 40–90% for Cu²⁺, 100% for Fe³⁺, and 20–50% for Mn²⁺ over a 72 hours period, for ion concentrations of 1.0 and 5.0 mg*L⁻¹.

A recycling culture of Neochloris oleoabundans in a bicarbonate-based integrated carbon capture and algae production system with harvesting by auto-flocculation

BACKGROUND

A bicarbonate-based integrated carbon capture and algae production system (BICCAPS) uses carbonate to capture CO₂ and produce bicarbonate for alkalihalophilic microalgal cultivation. In this process, carbonate is regenerated and re-used for CO₂ capture. However, a practical example of a recycling culture to prove its feasibility is still absent.

RESULTS

To reach this goal, a recycling culture of Neochloris oleoabundans was created in this study. The effect of bicarbonate concentration on N. oleoabundans growth showed that the highest productivity was obtained at 0.3 mol L^-1, but the highest apparent carbon utilization efficiency was obtained at 0.1 mol L^-1. The harvest of algal biomass was tested with alkaline flocculation, which is induced by high pH due to bicarbonate consumption. The result showed that the maximum recovery rate of 97.7 ± 0.29% was reached with a supplement of 20 mM Ca²⁺. Compared with this, alkaline flocculation without Ca²⁺ also resulted in a high recovery rate of up to 9 7.4± 0.21% in culture with 0.7 mol L^-1 bicarbonate. In recycling culture, the spent medium was bubbled with CO₂ and re-used for algal culture. After eight times of recycling, biomass productivity in recycling culture with 0.1 and 0.3 mol L^-1 bicarbonate was 0.24 and 0.39 g L^-1 day^-1, respectively, higher than the 0.20 and 0.30 g L^-1 day^-1 in the control. The apparent carbon utilization efficiencies achieved in these semi-continuous cultures with 0.1 mol L^-1 bicarbonate were 242 ± 3.1 and 266 ± 11% for recycling and control culture, respectively, while those with 0.3 mol L^-1 bicarbonate were 98 ± 0.78 and 87 ± 3.6%, respectively.

CONCLUSIONS

This study proved the feasibility of BICCAPS recycling culture with the first practical example. More importantly, the produced algal biomass can be harvested without any flocculant supplement. Thus, this process can reduce both culturing and harvesting costs in algal biomass production.

Bioflocculation: An alternative strategy for harvesting of microalgae - An overview

Microalgae based research has been extensively progressed for the production of value added products and biofuels. Potential application of microalgae for biofuel is recently gained more attention for possibilities of biodiesel and other high value metabolites. However, high cost of production of biomass associated with harvesting technologies is one of the major bottleneck for commercialization of algae based industrial product. Based on the operation economics, harvesting efficiency, technological possibilities, flocculation of algal biomass is a superior method for harvesting microalgae from the growth medium. In this article, latest trends of microalgal cell harvesting through flocculation are reviewed with emphasis on current progress and prospect in environmental friendly bio-based flocculation approach. Bio-flocculation based microalgae harvesting technologies is a promising strategy for low cost microalgal biomass production for various applications.

Switching cultivation for enhancing biomass and lipid production with extracellular polymeric substance as co-products in Heynigia riparia SX01

Switching cultivation (mixotrophic-heterotrophic, 12h:12h) of Heynigia riparia SX01 was studied, the maximum biomass concentration of 3.55gL^-1 and lipid yield of 1.45gL^-1 were achieved after 8days cultivation. The extracellular polymeric substance (EPS) was developed as co-product. Addition of MgSO₄ could enhance the production of EPS. The highest amount of 0.60gL^-1 EPS was obtained with the addition of 2gL^-1 MgSO₄, the self-flocculation efficiency was as high as 83% at this condition. The total lipid and lipid fractions did not show differences with extra MgSO₄. Based on the above results, a new biodiesel production model was proposed: culturing Heynigia riparia SX01 with extra 2gL^-1 MgSO₄ by switching cultivation and using self flocculation to collect microalgae for biodiesel production, while EPS was collected as valuable co-products.

The impact of zeta potential changes on Ceratium hirundinella cell removal and the ability of cells to restore its natural surface charge during drinking water purification

The removal efficacy ofCeratiumcells from source water was evaluated. The best ZP for coagulation were achieved with organic polymer and Ca(OH)₂. Cells were able to restore their ZP after 120 and 240 minutes settling time.

Effect of different organic matters on flocculation of Chlorella sorokiniana and optimization of flocculation conditions in swine manure wastewater

In this study, flocculation of Chlorella sorokiniana cultivated in swine manure wastewater, BG-11 medium and BG-11 medium supplemented with different organic matters (glucose, urea and tryptone) was investigated. The results demonstrated that the minimum amount of Al(3+) required for complete flocculation in wastewater would increase substantially, and flocculation efficiency became highly sensitive to pH. Tryptone could cause similar extent of inhibition on flocculation as in wastewater. Meanwhile, glucose could increase concentrations of Algogenic Organic Matter (AOM), inhibiting flocculation strongly at higher pH, including flocculation induced by Al(3+) and autoflocculation. However, urea had little effect on flocculation of C. sorokiniana. Moreover, the major factors: dilution times, pH and flocculants dosage, which had significant impact on flocculation efficiency of C. sorokiniana in piggery wastewater, were optimized using response surface methodology (RSM). The optimal flocculation efficiency (100%) was achieved at pH 8.5, 7-folds of dilution and 52.14 mg L(-1) of Al(3+).

Current progress and future prospect of microalgal biomass harvest using various flocculation technologies

Microalgae have been extensively studied for the production of various valuable products. Application of microalgae for the production of renewable energy has also received increasing attention in recent years. However, high cost of microalgal biomass harvesting is one of the bottlenecks for commercialization of microalgae-based industrial processes. Considering harvesting efficiency, operation economics and technological feasibility, flocculation is a superior method to harvest microalgae from mass culture. In this article, the latest progress of various microalgal cell harvesting methods via flocculation is reviewed with the emphasis on the current progress and prospect in environmentally friendly bio-based flocculation. Harvesting microalgae through bio-based flocculation is a promising component of the low-cost microalgal biomass production technology.

Harvesting carbohydrate-rich Arthrospira platensis by spontaneous settling

The filamentous cyanobacterium Arthrospira platensis is an attractive feedstock for carbohydrate-based biofuels because it accumulated up to 74% of carbohydrates when nitrogen stressed. Nitrogen stressed A. platensis also settled spontaneously, and this occurred simultaneously with carbohydrates accumulation, suggesting a link between both phenomena. The increased settling velocity was neither due to production of extracellular carbohydrates, nor due to degradation of gas vacuoles, but was caused by an increase in the specific density of the filaments as a result of accumulation of carbohydrates under the form of glycogen. Settling velocities of carbohydrate-rich A. platensis reached 0.64mh(-1), which allowed the biomass to be harvested using a lamella separator. The biomass could be concentrated at least 15 times, allowing removal of 94% of the water using gravity settling, thus offering a potential application as a low-cost and high-throughput method for primary dewatering of carbohydrate-rich A. platensis.

Harvesting Chlorella vulgaris by natural increase in pH: effect of medium composition

The freshwater microalga Chlorella vulgaris was harvested by autoflocculation resulting from the precipitation of magnesium or calcium compounds induced by a slow increase in pH in the absence of CO2 input. Autoflocculation was tested in two culture media with, respectively, ammonium (NH4+) and nitrate (NO3-) ions as nitrogen source. The culture pH increased because of photosynthesis and CO2 stripping. pH rose to 11 after 8 h in the NO3- medium, but did not exceed 9 in the NH4+ medium. No flocculation took place in any of the media. Autoflocculation tests were repeated in the NO(3-)-based culture medium by progressively increasing the concentrations of Ca2+ and Mg2+ until inorganic compounds precipitated and flocculated microalgae. The minimal concentrations for flocculation were found to be 120 mg Ca2 L(-1) and 1000 mg Mg2+ L(-1). These values were, respectively, 3.5 times and 20 times higher than those allowing flocculation by NaOH addition. Energy-dispersive X-ray spectroscopy, zeta potential measurement, and ionic chromatography suggest that the mechanisms involved are different. The rate of cell removal was close to 90% in both cases, but cells were more concentrated in the aggregates obtained by magnesium compound precipitation, with an estimated concentration close to 33 g (dry matter) L(-1), against 19 g L(-1) for calcium phosphates.

Harvesting Nannochloris oculata by inorganic electrolyte flocculation: effect of initial cell density, ionic strength, coagulant dosage, and media pH

Process variables affecting harvesting efficiency of Nannochloris oculata by AlCl(3) flocculation such as, cell density, ionic strength, coagulant dosage, media pH, and cell surface charge were investigated. Initial cell density and coagulant dosage had a significant effect on the removal efficiency; however, levels of ionic strength tested were not significant. Best flocculation conditions of investigated variables were: 0.0016 ng of AlCl(3)/cell, 3.0×10(7) cell/mL, and pH 5.3. Removal efficiency at optimum conditions and salt concentrations of: 0, 15, and 30 g/L NaCl was 96, 98, and 97 %, respectively. Low cell density cultures ∼10(6) cell/mL, required five times greater AlCl(3) dosage to achieve the same removal efficiency. Destabilization of algal cultures using 0.0032 ng of AlCl(3)/cell was observed by reducing the zeta potential to -22 mV. Acidification with HCl for conducting flocculation at pH 5.3 could be a significant cost burden unless is mitigated by selecting a low-buffering-capacity media.